Author name code: solanki ADS astronomy entries on 2022-09-14 author:"Solanki, Sami K." ------------------------------------------------------------------------ Title: What drives decayless kink oscillations in active region coronal loops on the Sun? Authors: Mandal, Sudip; Chitta, Lakshmi P.; Antolin, Patrick; Peter, Hardi; Solanki, Sami K.; Auchère, Frédéric; Berghmans, David; Zhukov, Andrei N.; Teriaca, Luca; Cuadrado, Regina A.; Schühle, Udo; Parenti, Susanna; Buchlin, Éric; Harra, Louise; Verbeeck, Cis; Kraaikamp, Emil; Long, David M.; Rodriguez, Luciano; Pelouze, Gabriel; Schwanitz, Conrad; Barczynski, Krzysztof; Smith, Phil J. Bibcode: 2022arXiv220904251M Altcode: We study here the phenomena of decayless kink oscillations in a system of active region (AR) coronal loops. Using high resolution observations from two different instruments, namely the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we follow these AR loops for an hour each on three consecutive days. Our results show significantly more resolved decayless waves in the higher-resolution EUI data compared with the AIA data. Furthermore, the same system of loops exhibits many of these decayless oscillations on Day-2, while on Day-3, we detect very few oscillations and on Day-1, we find none at all. Analysis of photospheric magnetic field data reveals that at most times, these loops were rooted in sunspots, where supergranular flows are generally absent. This suggests that supergranular flows, which are often invoked as drivers of decayless waves, are not necessarily driving such oscillations in our observations. Similarly, our findings also cast doubt on other possible drivers of these waves, such as a transient driver or mode conversion of longitudinal waves near the loop footpoints. In conclusion, through our analysis we find that none of the commonly suspected sources proposed to drive decayless oscillations in active region loops seems to be operating in this event and hence, the search for that elusive wave driver needs to continue. Title: Erratum: "Faculae Cancel out on the Surfaces of Active Suns" (2022, ApJL, 934, L23) Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L. Bibcode: 2022ApJ...936L..17N Altcode: No abstract at ADS Title: Faculae Cancel out on the Surfaces of Active Suns Authors: Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Sowmya, K.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L. Bibcode: 2022ApJ...934L..23N Altcode: 2022arXiv220706816N Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterization of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance, they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots or faculae. Here, we utilize a surface flux transport model to show that more effective cancellation of diffuse magnetic flux associated with faculae leads to spot area coverages increasing faster with stellar magnetic activity than that by faculae. Our calculations explain the observed dependence between solar spot and facular area coverages and allow its extension to stars that are more active than the Sun. This extension enables anticipating the properties of stellar signal and its more reliable mitigation, leading to a more accurate characterization of exoplanets and their atmospheres. Title: Predictions of Astrometric Jitter for Sun-like Stars. III. Fast Rotators Authors: Sowmya, K.; Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Krivova, N. A.; Solanki, S. K. Bibcode: 2022ApJ...934..146S Altcode: 2022arXiv220607702S A breakthrough in exoplanet detections is foreseen with the unprecedented astrometric measurement capabilities offered by instrumentation aboard the Gaia space observatory. Besides, astrometric discoveries of exoplanets are expected from the planned space mission, Small-JASMINE. In this setting, the present series of papers focuses on estimating the effect of the magnetic activity of G2V-type host stars on the astrometric signal. This effect interferes with the astrometric detections of Earth-mass planets. While the first two papers considered stars rotating at the solar rotation rate, this paper focuses on stars having solar effective temperature and metallicity but rotating faster than the Sun, and consequently more active. By simulating the distribution of active regions on such stars using the Flux Emergence And Transport model, we show that the contribution of magnetic activity to the astrometric measurements becomes increasingly significant with increasing rotation rates. We further show that the jitter for the most variable periodic Kepler stars is high enough to be detected by Gaia. Furthermore, due to a decrease in the facula-to-spot area ratio for more active stars, the magnetic jitter is found to be spot dominated for rapid rotators. Our simulations of the astrometric jitter have the potential to aid the interpretation of data from Gaia and upcoming space astrometry missions. Title: A highly dynamic small-scale jet in a polar coronal hole Authors: Mandal, Sudip; Chitta, Lakshmi Pradeep; Peter, Hardi; Solanki, Sami K.; Cuadrado, Regina Aznar; Teriaca, Luca; Schühle, Udo; Berghmans, David; Auchère, Frédéric Bibcode: 2022A&A...664A..28M Altcode: 2022arXiv220602236M We present an observational study of the plasma dynamics at the base of a solar coronal jet, using high resolution extreme ultraviolet imaging data taken by the Extreme Ultraviolet Imager on board Solar Orbiter, and by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory. We observed multiple plasma ejection events over a period of ∼1 h from a dome-like base that is ca. 4 Mm wide and is embedded in a polar coronal hole. Within the dome below the jet spire, multiple plasma blobs with sizes around 1−2 Mm propagate upwards to the dome apex with speeds of the order of the sound speed (ca. 120 km s−1). Upon reaching the apex, some of these blobs initiate flows with similar speeds towards the other footpoint of the dome. At the same time, high speed super-sonic outflows (∼230 km s−1) are detected along the jet spire. These outflows as well as the intensity near the dome apex appear to be repetitive. Furthermore, during its evolution, the jet undergoes many complex morphological changes, including transitions between the standard and blowout type eruption. These new observational results highlight the underlying complexity of the reconnection process that powers these jets and they also provide insights into the plasma response when subjected to rapid energy injection.

Movies associated to Figs. 1, 2, and 4 are available at https://www.aanda.org Title: Linking Small-scale Solar Wind Properties with Large-scale Coronal Source Regions through Joint Parker Solar Probe-Metis/Solar Orbiter Observations Authors: Telloni, Daniele; Zank, Gary P.; Sorriso-Valvo, Luca; D'Amicis, Raffaella; Panasenco, Olga; Susino, Roberto; Bruno, Roberto; Perrone, Denise; Adhikari, Laxman; Liang, Haoming; Nakanotani, Masaru; Zhao, Lingling; Hadid, Lina Z.; Sánchez-Cano, Beatriz; Verscharen, Daniel; Velli, Marco; Grimani, Catia; Marino, Raffaele; Carbone, Francesco; Mancuso, Salvatore; Biondo, Ruggero; Pagano, Paolo; Reale, Fabio; Bale, Stuart D.; Kasper, Justin C.; Case, Anthony W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey, Peter R.; Korreck, Kelly E.; Larson, Davin; Livi, Roberto; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Stevens, Michael L.; Whittlesey, Phyllis; Romoli, Marco; Andretta, Vincenzo; Deppo, Vania Da; Fineschi, Silvano; Heinzel, Petr; Moses, John D.; Naletto, Giampiero; Nicolini, Gianalfredo; Spadaro, Daniele; Stangalini, Marco; Teriaca, Luca; Capobianco, Gerardo; Capuano, Giuseppe E.; Casini, Chiara; Casti, Marta; Chioetto, Paolo; Corso, Alain J.; Leo, Yara De; Fabi, Michele; Frassati, Federica; Frassetto, Fabio; Giordano, Silvio; Guglielmino, Salvo L.; Jerse, Giovanna; Landini, Federico; Liberatore, Alessandro; Magli, Enrico; Massone, Giuseppe; Messerotti, Mauro; Pancrazzi, Maurizio; Pelizzo, Maria G.; Romano, Paolo; Sasso, Clementina; Schühle, Udo; Slemer, Alessandra; Straus, Thomas; Uslenghi, Michela; Volpicelli, Cosimo A.; Zangrilli, Luca; Zuppella, Paola; Abbo, Lucia; Auchère, Frédéric; Cuadrado, Regina Aznar; Berlicki, Arkadiusz; Ciaravella, Angela; Lamy, Philippe; Lanzafame, Alessandro; Malvezzi, Marco; Nicolosi, Piergiorgio; Nisticò, Giuseppe; Peter, Hardi; Solanki, Sami K.; Strachan, Leonard; Tsinganos, Kanaris; Ventura, Rita; Vial, Jean-Claude; Woch, Joachim; Zimbardo, Gaetano Bibcode: 2022ApJ...935..112T Altcode: The solar wind measured in situ by Parker Solar Probe in the very inner heliosphere is studied in combination with the remote-sensing observation of the coronal source region provided by the METIS coronagraph aboard Solar Orbiter. The coronal outflows observed near the ecliptic by Metis on 2021 January 17 at 16:30 UT, between 3.5 and 6.3 R ⊙ above the eastern solar limb, can be associated with the streams sampled by PSP at 0.11 and 0.26 au from the Sun, in two time intervals almost 5 days apart. The two plasma flows come from two distinct source regions, characterized by different magnetic field polarity and intensity at the coronal base. It follows that both the global and local properties of the two streams are different. Specifically, the solar wind emanating from the stronger magnetic field region has a lower bulk flux density, as expected, and is in a state of well-developed Alfvénic turbulence, with low intermittency. This is interpreted in terms of slab turbulence in the context of nearly incompressible magnetohydrodynamics. Conversely, the highly intermittent and poorly developed turbulent behavior of the solar wind from the weaker magnetic field region is presumably due to large magnetic deflections most likely attributed to the presence of switchbacks of interchange reconnection origin. Title: Chromospheric extension of the MURaM code Authors: Przybylski, D.; Cameron, R.; Solanki, S. K.; Rempel, M.; Leenaarts, J.; Anusha, L. S.; Witzke, V.; Shapiro, A. I. Bibcode: 2022A&A...664A..91P Altcode: 2022arXiv220403126P Context. Detailed numerical models of the chromosphere and corona are required to understand the heating of the solar atmosphere. An accurate treatment of the solar chromosphere is complicated by the effects arising from non-local thermodynamic equilibrium (NLTE) radiative transfer. A small number of strong, highly scattering lines dominate the cooling and heating in the chromosphere. Additionally, the recombination times of ionised hydrogen are longer than the dynamical timescales, requiring a non-equilibrium (NE) treatment of hydrogen ionisation.
Aims: We describe a set of necessary additions to the MURaM code that allow it to handle some of the important NLTE effects. We investigate the impact on solar chromosphere models caused by NLTE and NE effects in radiation magnetohydrodynamic simulations of the solar atmosphere.
Methods: The MURaM code was extended to include the physical process required for an accurate simulation of the solar chromosphere, as implemented in the Bifrost code. This includes a time-dependent treatment of hydrogen ionisation, a scattering multi-group radiation transfer scheme, and approximations for NLTE radiative cooling.
Results: The inclusion of NE and NLTE physics has a large impact on the structure of the chromosphere; the NE treatment of hydrogen ionisation leads to a higher ionisation fraction and enhanced populations in the first excited state throughout cold inter-shock regions of the chromosphere. Additionally, this prevents hydrogen ionisation from buffering energy fluctuations, leading to hotter shocks and cooler inter-shock regions. The hydrogen populations in the ground and first excited state are enhanced by 102-103 in the upper chromosphere and by up to 109 near the transition region.
Conclusions: Including the necessary NLTE physics leads to significant differences in chromospheric structure and dynamics. The thermodynamics and hydrogen populations calculated using the extended version of the MURaM code are consistent with previous non-equilibrium simulations. The electron number and temperature calculated using the non-equilibrium treatment of the chromosphere are required to accurately synthesise chromospheric spectral lines.

Movies associated to Fig. 2 are only available at https://www.aanda.org Title: Solar cyclic activity over the last millennium reconstructedfrom annual 14C data (Corrigendum) Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2022A&A...664C...3U Altcode: No abstract at ADS Title: The on-ground data reduction and calibration pipeline for SO/PHI-HRT Authors: Sinjan, J.; Calchetti, D.; Hirzberger, J.; Orozco Suárez, D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutierrez Marquez, P.; Kahil, F.; Kolleck, M.; Solanki, S. K.; del Toro Iniesta, J. C.; Volkmer, R.; Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Carmona, M.; Deutsch, W.; Fernandez-Rico, G.; Fernández-Medina, A.; García Parejo, P.; Gasent Blesa, J. L.; Gizon, L.; Grauf, B.; Heerlein, K.; Korpi-Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.; Torralbo, I.; Valori, G. Bibcode: 2022arXiv220814904S Altcode: The ESA/NASA Solar Orbiter space mission has been successfully launched in February 2020. Onboard is the Polarimetric and Helioseismic Imager (SO/PHI), which has two telescopes, a High Resolution Telescope (HRT) and the Full Disc Telescope (FDT). The instrument is designed to infer the photospheric magnetic field and line-of-sight velocity through differential imaging of the polarised light emitted by the Sun. It calculates the full Stokes vector at 6 wavelength positions at the Fe I 617.3 nm absorption line. Due to telemetry constraints, the instrument nominally processes these Stokes profiles onboard, however when telemetry is available, the raw images are downlinked and reduced on ground. Here the architecture of the on-ground pipeline for HRT is presented, which also offers additional corrections not currently available on board the instrument. The pipeline can reduce raw images to the full Stokes vector with a polarimetric sensitivity of $10^{-3}\cdot I_{c}$ or better. Title: Polarimetric calibration of the Sunrise UV Spectropolarimeter and Imager Authors: Iglesias, F. A.; Feller, A.; Gandorfer, A.; Lagg, A.; Riethmüller, T. L.; Solanki, S. K.; Katsukawa, Y.; Kubo, M.; Zucarelli, G.; Sanchez, M.; Sunrise Team Bibcode: 2022BAAA...63..305I Altcode: Sunrise is an optical observatory mounted in a stratospheric balloon, developed to study magnetic fields in the solar atmosphere with very high resolution. In its third flight, Sunrise carry the Sunrise UV Spectropolarimeter and Imager (SUSI), that operates in the 313-430 nm range, covering thousands of spectral lines not accessible from the ground and thus largely unexplored. SUSI does not include a polarimetric calibration unit on board. We report about the development status of SUSI and the preliminary results of its calibration. Title: Small-scale dynamo in cool stars. I. Changes in stratification and near-surface convection for main-sequence spectral types Authors: Bhatia, Tanayveer S.; Cameron, Robert H.; Solanki, Sami K.; Peter, Hardi; Przybylski, Damien; Witzke, Veronika; Shapiro, Alexander Bibcode: 2022A&A...663A.166B Altcode: 2022arXiv220600064B Context. Some of the small-scale solar magnetic flux can be attributed to a small-scale dynamo (SSD) operating in the near-surface convection. The SSD fields have consequences for solar granular convection, basal flux, and chromospheric heating. A similar SSD mechanism is expected to be active in the near-surface convection of other cool main-sequence stars, but this has not been investigated thus far.
Aims: We aim to investigate changes in stratification and convection due to inclusion of SSD fields for F3V, G2V, K0V, and M0V spectral types in the near-surface convection.
Methods: We studied 3D magnetohydrodynamic (MHD) models of the four stellar boxes, covering the subsurface convection zone up to the lower photosphere in a small Cartesian box, based on the MURaM radiative-MHD simulation code. We compared the SSD runs against reference hydrodynamic runs.
Results: The SSD is found to efficiently produce magnetic field with energies ranging between 5% to 80% of the plasma kinetic energy at different depths. This ratio tends to be larger for larger Teff. The relative change in density and gas pressure stratification for the deeper convective layers due to SSD magnetic fields is negligible, except for the F-star. For the F-star, there is a substantial reduction in convective velocities due to Lorentz force feedback from magnetic fields, which, in turn, reduces the turbulent pressure.
Conclusions: The SSD in near-surface convection for cool main-sequence stars introduces small but significant changes in thermodynamic stratification (especially for the F-star) due to a reduction in the convective velocities. Title: Historical Ca II K observations for irradiance reconstructions Authors: Chatzistergos, Theodosios; Krivova, Natalie; Ermolli, Ilaria; Leng Yeo, Kok; Solanki, Sami; Mandal, Sudip Bibcode: 2022cosp...44.1551C Altcode: Solar irradiance has been measured by various instruments from space since 1978 and exhibits variations on all accessible timescales. Irradiance variations prior to satellite measurements, knowledge of which is important for climate studies, can only be recovered with models. Models have successfully been used to reconstruct solar irradiance variations by attributing the variability on time scales longer than about a day to the evolution of the solar surface magnetic field. However, to account for the different contributions of surface magnetic features, such as sunspots and faculae, models require appropriate input data. Unfortunately, due to the scarcity of suitable facular data prior to the period of direct irradiance measurements, most available irradiance reconstructions rely on sunspot data alone or on other, indirect data of solar magnetism. Historical Ca II K archives carry direct information on facular regions, but they have not been used to their full potential for irradiance reconstructions. The main reasons limiting their use were the difficulty to account for the non-linear response of the photographic plates along with plentiful large-scale artefacts affecting the images. We have extensively studied Ca II K data to overcome most issues affecting them thus paving the way for them to be used for accurate irradiance reconstructions. We will present our latest work on employment of Ca II K data for reconstructing past irradiance variations. Title: The essential role of Earth-Sun L4 in solar particle event forecasting for Lunar and Mars exploration Authors: Posner, Arik; Toit Strauss, Du; Solanki, Sami K.; Effenberger, Frederic; Gandorfer, Achim; Hirzberger, Johann; Kühl, Patrick; Heber, Bernd; Malandraki, Olga; Folta, David; Jones, Sarah; Arge, Charles; Sterken, Veerle; Henney, Carl J.; Staub, Jan; Hatten, Noble; Stcyr, O. Chris Bibcode: 2022cosp...44.1157P Altcode: We learned from the STEREO mission that solar particle events originating from behind the west limb of the Sun, i.e., out of view from Earth, make up about 30 percent of those significantly affecting Earth's vicinity and thus could endanger human exploration of the Moon. The Earth-Sun Lagrangian point 4 is a meta-stable location at 1 au from the Sun, 60° ahead of Earth's orbit. L4 has an uninterrupted view of the solar photosphere centered on W60, the Earth's nominal magnetic field connection to the Sun. The role of L4 observations for improving several existing short-term SEP forecasting techniques, including protons, ESPERTA, UMASEP and pps, for Lunar exploration will be highlighted. We can show that BFO dose savings from short-term solar energetic particle forecasts are critically important in a worst-case scenario. Placing a mission at L4 is even a precondition for any SEP all-clear forecasting for Lunar exploration. Furthermore, we analyzed example trajectories of short-term Mars round trips that may be considered for future human exploration of Mars and find that L4-based SWx observations would have relevance for protecting Mars explorers from radiation exposure. Title: Reconstruction of Coronal Magnetic Fields Using a Poloidal-Toroidal Representation Authors: Yi, Sibaek; Choe, G. S.; Cho, Kyung-Suk; Solanki, Sami K.; Büchner, Jörg Bibcode: 2022arXiv220607189Y Altcode: A new method for reconstruction of coronal magnetic fields as force-free fields (FFFs) is presented. Our method employs poloidal and toroidal functions to describe divergence-free magnetic fields. This magnetic field representation naturally enables us to implement the boundary conditions at the photospheric boundary, i.e., the normal magnetic field and the normal current density there, in a straightforward manner. At the upper boundary of the corona, a source-surface condition can be employed, which accommodates magnetic flux imbalance at the bottom boundary. Although our iteration algorithm is inspired by extant variational methods, it is non-variational and requires much less iteration steps than most of them. The computational code based on our new method is tested against the analytical FFF solutions by Titov & Démoulin (1999). It is found to excel in reproducing a tightly wound flux rope, a bald patch and quasi-separatrix layers with a hyperbolic flux tube. Title: Stellar limb darkening. A new MPS-ATLAS library for Kepler, TESS, CHEOPS, and PLATO passbands Authors: Kostogryz, N. M.; Witzke, V.; Shapiro, A. I.; Solanki, S. K.; Maxted, P. F. L.; Kurucz, R. L.; Gizon, L. Bibcode: 2022arXiv220606641K Altcode: The detection of the first exoplanet paved the way into the era of transit photometry space missions with a revolutionary photometric precision that aim at discovering new exoplanetary systems around different types of stars. With this high precision, it is possible to derive very accurately the radii of exoplanets which is crucial for constraining their type and composition. However, it requires an accurate description of host stars, especially their center-to-limb variation of intensities (so called limb darkening) as it affects the planet-to-star radius ratio determination. We aim at improving the accuracy of limb darkening calculations for stars with a wide range of fundamental parameters. We used the recently developed 1D MPS-ATLAS code to compute model atmosphere structures and to synthesize stellar limb darkening on a very fine grid of stellar parameters. For the computations we utilized the most accurate information on chemical element abundances and mixing length parameters including convective overshoot. The stellar limb darkening was fitted using the two most accurate limb darkening laws: the power-2 and 4-parameters non-linear laws. We present a new extensive library of stellar model atmospheric structures, the synthesized stellar limb darkening curves, and the coefficients of parameterized limb-darkening laws on a very fine grid of stellar parameters in the Kepler, TESS, CHEOPS, and PLATO passbands. The fine grid allows overcoming the sizable errors introduced by the need to interpolate. Our computations of solar limb darkening are in a good agreement with available solar measurements at different view angles and wavelengths. Our computations of stellar limb darkening agree well with available measurements of Kepler stars. A new grid of stellar model structures, limb darkening and their fitted coefficients in different broad filters is provided in CDS. Title: Making our star sparkle: The Sun's magnetic field, activity and variability Authors: Solanki, Sami Bibcode: 2022AAS...24040001S Altcode: The magnetic field of the Sun relieves our star from the monotony of an unchanging existence, at least unchanging on timescales between days and millennia. Instead, it leads the Sun to display a large variety of ever-changing features, such as sunspots and faculae at the solar surface, a bright network in the chromosphere, and loops and plumes in the corona, among many others. It also leads to variations of the total brightness of the Sun. From time to time, the incessantly evolving magnetic field causes great flashes of radiation in the form of flares, or outbursts of particles in the form of coronal mass ejections. In other words, the nuclear reactions in the Sun's core may make it shine, but it takes the magnetic field to make our star sparkle.

How the solar magnetic field is structured, how it produces aspects of solar activity and variability and how we learn more about it will be subjects of this talk. Title: Magnetized supersonic downflows in the chromosphere. A statistical study using the He I 10 830 Å lines Authors: Sowmya, K.; Lagg, A.; Solanki, S. K.; Castellanos Durán, J. S. Bibcode: 2022A&A...661A.122S Altcode: 2022arXiv220211679S The chromosphere above active regions (ARs) on the Sun hosts magnetized supersonic downflows. Studies of these supersonic downflows help to decipher the magnetic fine structure and dynamics of the chromosphere. We perform a statistical analysis of the magnetized supersonic downflows in a number of ARs at different evolutionary stages and survey their characteristics. We analyze spectro-polarimetric scans of parts of 13 ARs obtained in the infrared He I 10 830 Å triplet formed in the upper chromosphere recorded with the GREGOR Infrared Spectrograph mounted at the GREGOR solar telescope. We retrieve the line-of-sight velocities and the magnetic field vector using the HELIX+ inversion code that assumes Milne-Eddington atmospheres. We find magnetized supersonic downflows in all the ARs, with larger area coverage by such flows in ARs observed during their emerging phase. The fact that supersonic downflows were detected in all scans, albeit only covering a small fraction, 0.2-6.4%, of the observed field-of-view, suggests that they are a comparatively common phenomenon in the upper chromospheres of ARs. The supersonic downflows are found to be associated with many AR features, such as pores, sunspot umbrae, sunspot penumbrae, light bridges, plages, He I loops as part of arch filament systems characteristic of emerging fields, and filaments. Although several mechanisms are identified to be causing the supersonic downflows, by far the most common one appears to be the draining of plasma along the legs of rising magnetic loops. The loops mainly drain into forming pores. The line-of-sight velocities of the supersonic downflows reach values of up to 49 km s−1, and the velocity distribution shows multiple populations. Almost 92% of these supersonic downflows coexist with a subsonic flow component. The weaker, more horizontal fields associated with the supersonic component suggests that it is formed above the subsonic component. Title: Multiwavelength Mitigation of Stellar Activity in Astrometric Planet Detection Authors: Kaplan-Lipkin, Avi; Macintosh, Bruce; Madurowicz, Alexander; Sowmya, Krishnamurthy; Shapiro, Alexander; Krivova, Natalie; Solanki, Sami K. Bibcode: 2022AJ....163..205K Altcode: 2021arXiv211206383K Astrometry has long been a promising technique for exoplanet detection. At the theoretical limits, astrometry would allow for the detection of smaller planets than previously seen by current exoplanet search methods, but stellar activity may make these theoretical limits unreachable. Astrometric jitter of a Sun-like star due to magnetic activity in its photosphere induces apparent variability in the photocenter of order 0.5 mR ⊙. This jitter creates a fundamental astrophysical noise floor preventing detection of lower-mass planets in a single spectral band. By injecting planet orbits into simulated solar data at five different passbands, we investigate mitigation of this fundamental astrometric noise using correlations across passbands. For a true solar analog and a planet at 1 au semimajor axis, the 6σ detection limit set by stellar activity for an ideal telescope at the best single passband is 0.01 Earth masses. We found that pairs of passbands with highly correlated astrometric jitter due to stellar activity, but with less motion in the redder band, enable higher-precision measurements of the common signal from the planet. Using this method improves detectable planet masses at 1 au by up to a factor of 10, corresponding to at best 0.005 Earth masses for a Sun-like star with a perfect telescope. Given these results, we recommend that future astrometry missions consider proceeding with two or more passbands to reduce noise due to stellar activity. Title: The magnetic drivers of campfires seen by the Polarimetric and Helioseismic Imager (PHI) on Solar Orbiter Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.; Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert, K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.; Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti, D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina, A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.; Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.; Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai, E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.; Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans, D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S. Bibcode: 2022A&A...660A.143K Altcode: 2022arXiv220213859K Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter (SO) spacecraft observed small extreme ultraviolet (EUV) bursts, termed campfires, that have been proposed to be brightenings near the apexes of low-lying loops in the quiet-Sun atmosphere. The underlying magnetic processes driving these campfires are not understood.
Aims: During the cruise phase of SO and at a distance of 0.523 AU from the Sun, the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI, offering the possibility to investigate the surface magnetic field dynamics underlying campfires at a spatial resolution of about 380 km.
Methods: We used co-spatial and co-temporal data of the quiet-Sun network at disc centre acquired with the High Resolution Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5 min). Campfires that are within the SO/PHI−SO/EUI common field of view were isolated and categorised according to the underlying magnetic activity.
Results: In 71% of the 38 isolated events, campfires are confined between bipolar magnetic features, which seem to exhibit signatures of magnetic flux cancellation. The flux cancellation occurs either between the two main footpoints, or between one of the footpoints of the loop housing the campfire and a nearby opposite polarity patch. In one particularly clear-cut case, we detected the emergence of a small-scale magnetic loop in the internetwork followed soon afterwards by a campfire brightening adjacent to the location of the linear polarisation signal in the photosphere, that is to say near where the apex of the emerging loop lays. The rest of the events were observed over small scattered magnetic features, which could not be identified as magnetic footpoints of the campfire hosting loops.
Conclusions: The majority of campfires could be driven by magnetic reconnection triggered at the footpoints, similar to the physical processes occurring in the burst-like EUV events discussed in the literature. About a quarter of all analysed campfires, however, are not associated to such magnetic activity in the photosphere, which implies that other heating mechanisms are energising these small-scale EUV brightenings. Title: A solar coronal loop in a box: Energy generation and heating Authors: Breu, C.; Peter, H.; Cameron, R.; Solanki, S. K.; Przybylski, D.; Rempel, M.; Chitta, L. P. Bibcode: 2022A&A...658A..45B Altcode: 2021arXiv211211549B Context. Coronal loops are the basic building block of the upper solar atmosphere as seen in the extreme UV and X-rays. Comprehending how these are energized, structured, and evolve is key to understanding stellar coronae.
Aims: Here we investigate how the energy to heat the loop is generated by photospheric magneto-convection, transported into the upper atmosphere, and how the internal structure of a coronal magnetic loop forms.
Methods: In a 3D magnetohydrodynamics model, we study an isolated coronal loop rooted with both footpoints in a shallow layer within the convection zone using the MURaM code. To resolve its internal structure, we limited the computational domain to a rectangular box containing a single coronal loop as a straightened magnetic flux tube. Field-aligned heat conduction, gray radiative transfer in the photosphere and chromosphere, and optically thin radiative losses in the corona were taken into account. The footpoints were allowed to interact self-consistently with the granulation surrounding them.
Results: The loop is heated by a Poynting flux that is self-consistently generated through small-scale motions within individual magnetic concentrations in the photosphere. Turbulence develops in the upper layers of the atmosphere as a response to the footpoint motions. We see little sign of heating by large-scale braiding of magnetic flux tubes from different photospheric concentrations at a given footpoint. The synthesized emission, as it would be observed by the Atmospheric Imaging Assembly or the X-Ray Telescope, reveals transient bright strands that form in response to the heating events. Overall, our model roughly reproduces the properties and evolution of the plasma as observed within (the substructures of) coronal loops.
Conclusions: With this model we can build a coherent picture of how the energy flux to heat the upper atmosphere is generated near the solar surface and how this process drives and governs the heating and dynamics of a coronal loop.

Movie associated to Fig. 2 is available at https://www.aanda.org Title: Final Report for SAG 21: The Effect of Stellar Contamination on Space-based Transmission Spectroscopy Authors: Rackham, Benjamin V.; Espinoza, Néstor; Berdyugina, Svetlana V.; Korhonen, Heidi; MacDonald, Ryan J.; Montet, Benjamin T.; Morris, Brett M.; Oshagh, Mahmoudreza; Shapiro, Alexander I.; Unruh, Yvonne C.; Quintana, Elisa V.; Zellem, Robert T.; Apai, Dániel; Barclay, Thomas; Barstow, Joanna K.; Bruno, Giovanni; Carone, Ludmila; Casewell, Sarah L.; Cegla, Heather M.; Criscuoli, Serena; Fischer, Catherine; Fournier, Damien; Giampapa, Mark S.; Giles, Helen; Iyer, Aishwarya; Kopp, Greg; Kostogryz, Nadiia M.; Krivova, Natalie; Mallonn, Matthias; McGruder, Chima; Molaverdikhani, Karan; Newton, Elisabeth R.; Panja, Mayukh; Peacock, Sarah; Reardon, Kevin; Roettenbacher, Rachael M.; Scandariato, Gaetano; Solanki, Sami; Stassun, Keivan G.; Steiner, Oskar; Stevenson, Kevin B.; Tregloan-Reed, Jeremy; Valio, Adriana; Wedemeyer, Sven; Welbanks, Luis; Yu, Jie; Alam, Munazza K.; Davenport, James R. A.; Deming, Drake; Dong, Chuanfei; Ducrot, Elsa; Fisher, Chloe; Gilbert, Emily; Kostov, Veselin; López-Morales, Mercedes; Line, Mike; Močnik, Teo; Mullally, Susan; Paudel, Rishi R.; Ribas, Ignasi; Valenti, Jeff A. Bibcode: 2022arXiv220109905R Altcode: Study Analysis Group 21 (SAG21) of the Exoplanet Exploration Program Analysis Group (ExoPAG) was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, and plages. This SAG has brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current needs that can be addressed in this context to make the most of transit studies from current NASA facilities like HST and JWST. The analysis produced 14 findings, which fall into three Science Themes encompassing (1) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities ("The Sun as the Stellar Benchmark"), (2) how stars other than the Sun extend our knowledge of heterogeneities ("Surface Heterogeneities of Other Stars") and (3) how to incorporate information gathered for the Sun and other stars into transit studies ("Mapping Stellar Knowledge to Transit Studies"). Title: Solar Surface Stereoscopy with Solar Orbiter's Polarimetric Helioseismic Imager (SO/PHI) Authors: Romero Avila, Amanda; Inhester, Bernd; Hirzberger, Johann; Solanki, Sami Bibcode: 2021AGUFMSH25B2095R Altcode: A compound method for a stereoscopic analysis of the height variations in the solar photosphere is presented. This method allows to estimate relevant quantities (i.e. the Wilson depression) and to study structures in the solar photosphere and within sunspots. We will demonstrate the feasibility of the method using simulated Stokes I continuum observations derived from a radiative transfer model using the plasma properties of a MHD simulation of the solar surface. The large scale variations in our method are estimated by shifting and correlating two signals of the same region as observed from two different view directions. This result is then introduced as an initial height estimate in a least squares optimization algorithm in order to reproduce smaller scale structures. This method has been developed to be applied to the high resolution images of the PHI instrument on board Solar Orbiter or similar instruments on other Sun-observing spacecraft. It will allow to perform direct stereoscopic studies of solar surface observations in different wavelengths of the solar spectrum. Preliminary results, advantages and limitations, applications and particular considerations for PHI data will be discussed. Title: Propagating brightenings in small loop-like structures in the quiet-Sun corona: Observations from Solar Orbiter/EUI Authors: Mandal, Sudip; Peter, Hardi; Chitta, Lakshmi Pradeep; Solanki, Sami K.; Aznar Cuadrado, Regina; Teriaca, Luca; Schühle, Udo; Berghmans, David; Auchère, Frédéric Bibcode: 2021A&A...656L..16M Altcode: 2021arXiv211108106M Brightenings observed in solar extreme-ultraviolet images are generally interpreted as signatures of micro- or nanoflares occurring in the transition region or at coronal temperatures. Recent observations with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed the smallest of such brightenings (called campfires) in the quiet-Sun corona. Analyzing EUI 174 Å data obtained at a resolution of about 400 km on the Sun with a cadence of 5 s on 30 May 2020, we report here a number of cases in which these campfires exhibit propagating signatures along their apparently small (3-5 Mm) loop-like structures. The measured propagation speeds are generally between 25 km s−1 and 60 km s−1. If the loop plasma is assumed to be at a million Kelvin, these apparent motions would be slower than the local sound speed. Furthermore, these brightenings exhibit nontrivial propagation characteristics such as bifurcation, merging, reflection, and repeated plasma ejections. We suggest that these features are manifestations of the internal dynamics of these small-scale magnetic structures and could provide important insights into the dynamic response (∼40 s) of the loop plasma to the heating events and also into the locations of the heating events themselves.

Movies associated to Figs 2-5, A.1, and B.1 are available at https://www.aanda.org Title: The first coronal mass ejection observed in both visible-light and UV H I Ly-α channels of the Metis coronagraph on board Solar Orbiter Authors: Andretta, V.; Bemporad, A.; De Leo, Y.; Jerse, G.; Landini, F.; Mierla, M.; Naletto, G.; Romoli, M.; Sasso, C.; Slemer, A.; Spadaro, D.; Susino, R.; Talpeanu, D. -C.; Telloni, D.; Teriaca, L.; Uslenghi, M.; Antonucci, E.; Auchère, F.; Berghmans, D.; Berlicki, A.; Capobianco, G.; Capuano, G. E.; Casini, C.; Casti, M.; Chioetto, P.; Da Deppo, V.; Fabi, M.; Fineschi, S.; Frassati, F.; Frassetto, F.; Giordano, S.; Grimani, C.; Heinzel, P.; Liberatore, A.; Magli, E.; Massone, G.; Messerotti, M.; Moses, D.; Nicolini, G.; Pancrazzi, M.; Pelizzo, M. -G.; Romano, P.; Schühle, U.; Stangalini, M.; Straus, Th.; Volpicelli, C. A.; Zangrilli, L.; Zuppella, P.; Abbo, L.; Aznar Cuadrado, R.; Bruno, R.; Ciaravella, A.; D'Amicis, R.; Lamy, P.; Lanzafame, A.; Malvezzi, A. M.; Nicolosi, P.; Nisticò, G.; Peter, H.; Plainaki, C.; Poletto, L.; Reale, F.; Solanki, S. K.; Strachan, L.; Tondello, G.; Tsinganos, K.; Velli, M.; Ventura, R.; Vial, J. -C.; Woch, J.; Zimbardo, G. Bibcode: 2021A&A...656L..14A Altcode: Context. The Metis coronagraph on board Solar Orbiter offers a new view of coronal mass ejections (CMEs), observing them for the first time with simultaneous images acquired with a broad-band filter in the visible-light interval and with a narrow-band filter around the H I Ly-α line at 121.567 nm, the so-called Metis UV channel.
Aims: We show the first Metis observations of a CME, obtained on 16 and 17 January 2021. The event was also observed by the EUI/FSI imager on board Solar Orbiter, as well as by other space-based coronagraphs, such as STEREO-A/COR2 and SOHO/LASCO/C2, whose images are combined here with Metis data.
Methods: Different images are analysed here to reconstruct the 3D orientation of the expanding CME flux rope using the graduated cylindrical shell model. This also allows us to identify the possible location of the source region. Measurements of the CME kinematics allow us to quantify the expected Doppler dimming in the Ly-α channel.
Results: Observations show that most CME features seen in the visible-light images are also seen in the Ly-α images, although some features in the latter channel appear more structured than their visible-light counterparts. We estimated the expansion velocity of this event to be below 140 km s−1. Hence, these observations can be understood by assuming that Doppler dimming effects do not strongly reduce the Ly-α emission from the CME. These velocities are comparable with or smaller than the radial velocities inferred from the same data in a similar coronal structure on the east side of the Sun.
Conclusions: The first observations by Metis of a CME demonstrate the capability of the instrument to provide valuable and novel information on the structure and dynamics of these coronal events. Considering also its diagnostics capabilities regarding the conditions of the ambient corona, Metis promises to significantly advance our knowledge of such phenomena.

Movies are available at https://www.aanda.org Title: Capturing transient plasma flows and jets in the solar corona Authors: Chitta, L. P.; Solanki, S. K.; Peter, H.; Aznar Cuadrado, R.; Teriaca, L.; Schühle, U.; Auchère, F.; Berghmans, D.; Kraaikamp, E.; Gissot, S.; Verbeeck, C. Bibcode: 2021A&A...656L..13C Altcode: 2021arXiv210915106C Intensity bursts in ultraviolet (UV) to X-ray wavelengths and plasma jets are typical signatures of magnetic reconnection and the associated impulsive heating of the solar atmospheric plasma. To gain new insights into the process, high-cadence observations are required to capture the rapid response of plasma to magnetic reconnection as well as the highly dynamic evolution of jets. Here, we report the first 2 s cadence extreme-UV observations recorded by the 174 Å High Resolution Imager of the Extreme Ultraviolet Imager on board the Solar Orbiter mission. These observations, covering a quiet-Sun coronal region, reveal the onset signatures of magnetic reconnection as localized heating events. These localized sources then exhibit repeated plasma eruptions or jet activity. Our observations show that this spatial morphological change from localized sources to jet activity could occur rapidly on timescales of about 20 s. The jets themselves are intermittent and are produced from the source region on timescales of about 20 s. In the initial phases of these events, plasma jets are observed to exhibit speeds, as inferred from propagating intensity disturbances, in the range of 100 km s−1 to 150 km s−1. These jets then propagate to lengths of about 5 Mm. We discuss examples of bidirectional and unidirectional jet activity observed to have been initiated from the initially localized bursts in the corona. The transient nature of coronal bursts and the associated plasma flows or jets along with their dynamics could provide a benchmark for magnetic reconnection models of coronal bursts and jets.

Movies are available at https://www.aanda.org Title: Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli, Ilaria; Yeo, Kok Leng; Mandal, Sudip; Solanki, Sami K.; Kopp, Greg; Malherbe, Jean-Marie Bibcode: 2021A&A...656A.104C Altcode: 2021arXiv210905844C Context. Knowledge of solar irradiance variability is critical to Earth's climate models and understanding the solar influence on Earth's climate. Direct solar irradiance measurements have only been available since 1978. Reconstructions of past variability typically rely on sunspot data. However, sunspot records provide only indirect information on the facular and network regions, which are decisive contributors to irradiance variability on timescales of the solar cycle and longer.
Aims: Our ultimate goal is to reconstruct past solar irradiance variations using historical full-disc Ca II K observations to describe the facular contribution independently of sunspot observations. Here, we develop the method and test it extensively by using modern CCD-based (charge-coupled device) Ca II K observations. We also carry out initial tests on two photographic archives.
Methods: We employ carefully reduced and calibrated Ca II K images from 13 datasets, including some of the most prominent series, such as those from the Meudon, Mt Wilson, and Rome observatories. We convert them to unsigned magnetic field maps and then use them as input to the adapted Spectral and Total Irradiance Reconstruction (SATIRE) model to reconstruct total solar irradiance (TSI) variations over the period 1978-2019, for which direct irradiance measurements are available.
Results: The reconstructed irradiance from the analysed Ca II K archives agrees well with direct irradiance measurements and existing reconstructions. The model also returns good results on data taken with different bandpasses and images with low spatial resolution. Historical Ca II K archives suffer from numerous inconsistencies, but we show that these archives can still be used to reconstruct TSI with reasonable accuracy provided the observations are accurately processed and the effects of changes in instrumentation and instrumental parameters are identified and accounted for. The reconstructions are relatively insensitive to the TSI reference record used to fix the single free parameter of the model. Furthermore, even employment of a series, itself reconstructed from Ca II K data, as a reference for further reconstructions returns nearly equally accurate results. This will enable the Ca II K archives without an overlap with direct irradiance measurements to be used to reconstruct past irradiance.
Conclusions: By using the unsigned magnetic maps of the Sun reconstructed from modern high-quality Ca II K observations as input into the SATIRE model, we can reconstruct solar irradiance variations nearly as accurately as from directly recorded magnetograms. Historical Ca II K observations can also be used for past irradiance reconstructions but need additional care, for example identifying and accounting for discontinuities and changes in the quality of the data with time. Title: First observations from the SPICE EUV spectrometer on Solar Orbiter Authors: Fludra, A.; Caldwell, M.; Giunta, A.; Grundy, T.; Guest, S.; Leeks, S.; Sidher, S.; Auchère, F.; Carlsson, M.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, É.; Caminade, S.; DeForest, C.; Fredvik, T.; Haberreiter, M.; Harra, L.; Janvier, M.; Kucera, T.; Müller, D.; Parenti, S.; Schmutz, W.; Schühle, U.; Solanki, S. K.; Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R.; Chitta, L. P. Bibcode: 2021A&A...656A..38F Altcode: 2021arXiv211011252F
Aims: We present first science observations taken during the commissioning activities of the Spectral Imaging of the Coronal Environment (SPICE) instrument on the ESA/NASA Solar Orbiter mission. SPICE is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper we illustrate the possible types of observations to give prospective users a better understanding of the science capabilities of SPICE.
Methods: We have reviewed the data obtained by SPICE between April and June 2020 and selected representative results obtained with different slits and a range of exposure times between 5 s and 180 s. Standard instrumental corrections have been applied to the raw data.
Results: The paper discusses the first observations of the Sun on different targets and presents an example of the full spectra from the quiet Sun, identifying over 40 spectral lines from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur, magnesium, and iron. These lines cover the temperature range between 20 000 K and 1 million K (10 MK in flares), providing slices of the Sun's atmosphere in narrow temperature intervals. We provide a list of count rates for the 23 brightest spectral lines. We show examples of raster images of the quiet Sun in several strong transition region lines, where we have found unusually bright, compact structures in the quiet Sun network, with extreme intensities up to 25 times greater than the average intensity across the image. The lifetimes of these structures can exceed 2.5 hours. We identify them as a transition region signature of coronal bright points and compare their areas and intensity enhancements. We also show the first above-limb measurements with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and far off limb measurements in the equatorial plane in Mg IX, Ne VIII, and O VI lines. We discuss the potential to use abundance diagnostics methods to study the variability of the elemental composition that can be compared with in situ measurements to help confirm the magnetic connection between the spacecraft location and the Sun's surface, and locate the sources of the solar wind.
Conclusions: The SPICE instrument successfully performs measurements of EUV spectra and raster images that will make vital contributions to the scientific success of the Solar Orbiter mission. Title: First light observations of the solar wind in the outer corona with the Metis coronagraph Authors: Romoli, M.; Antonucci, E.; Andretta, V.; Capuano, G. E.; Da Deppo, V.; De Leo, Y.; Downs, C.; Fineschi, S.; Heinzel, P.; Landini, F.; Liberatore, A.; Naletto, G.; Nicolini, G.; Pancrazzi, M.; Sasso, C.; Spadaro, D.; Susino, R.; Telloni, D.; Teriaca, L.; Uslenghi, M.; Wang, Y. -M.; Bemporad, A.; Capobianco, G.; Casti, M.; Fabi, M.; Frassati, F.; Frassetto, F.; Giordano, S.; Grimani, C.; Jerse, G.; Magli, E.; Massone, G.; Messerotti, M.; Moses, D.; Pelizzo, M. -G.; Romano, P.; Schühle, U.; Slemer, A.; Stangalini, M.; Straus, T.; Volpicelli, C. A.; Zangrilli, L.; Zuppella, P.; Abbo, L.; Auchère, F.; Aznar Cuadrado, R.; Berlicki, A.; Bruno, R.; Ciaravella, A.; D'Amicis, R.; Lamy, P.; Lanzafame, A.; Malvezzi, A. M.; Nicolosi, P.; Nisticò, G.; Peter, H.; Plainaki, C.; Poletto, L.; Reale, F.; Solanki, S. K.; Strachan, L.; Tondello, G.; Tsinganos, K.; Velli, M.; Ventura, R.; Vial, J. -C.; Woch, J.; Zimbardo, G. Bibcode: 2021A&A...656A..32R Altcode: 2021arXiv210613344R In this work, we present an investigation of the wind in the solar corona that has been initiated by observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, designed to measure the wind outflow speed by applying Doppler dimming diagnostics. Metis on Solar Orbiter complements the UVCS spectroscopic observations that were performed during solar activity cycle 23 by simultaneously imaging the polarized visible light and the H I Lyman-α corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The Metis observations, taken on May 15, 2020, provide the first H I Lyman-α images of the extended corona and the first instantaneous map of the speed of the coronal plasma outflows during the minimum of solar activity and allow us to identify the layer where the slow wind flow is observed. The polarized visible light (580-640 nm) and the ultraviolet H I Lyα (121.6 nm) coronal emissions, obtained with the two Metis channels, were combined in order to measure the dimming of the UV emission relative to a static corona. This effect is caused by the outward motion of the coronal plasma along the direction of incidence of the chromospheric photons on the coronal neutral hydrogen. The plasma outflow velocity was then derived as a function of the measured Doppler dimming. The static corona UV emission was simulated on the basis of the plasma electron density inferred from the polarized visible light. This study leads to the identification, in the velocity maps of the solar corona, of the high-density layer about ±10° wide, centered on the extension of a quiet equatorial streamer present at the east limb - the coronal origin of the heliospheric current sheet - where the slowest wind flows at about 160 ± 18 km s−1 from 4 R⊙ to 6 R⊙. Beyond the boundaries of the high-density layer, the wind velocity rapidly increases, marking the transition between slow and fast wind in the corona. Title: Similarities of magnetoconvection in the umbra and in the penumbra of sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...655A..61L Altcode: 2021arXiv211001352L Context. It is unclear why there is a rather sharp boundary in sunspots between the umbra and the penumbra. Both regions exhibit magnetoconvection, which manifests in penumbral filaments in the penumbra and in umbral dots in the umbra.
Aims: Here we compare the physical properties of umbral dots and penumbral filaments. Our goal is to understand how the properties of these convective features change across the boundary between the umbra and the penumbra and how this is related to the rapid increase in brightness at the umbra-penumbra boundary.
Methods: We derived ensemble averages of the physical properties of different types of convective features based on observations of two sunspots with Hinode.
Results: There are strong similarities between the convective features in the outer parts of the umbra and the ones in the penumbra, with most physical parameters being smooth and continuous functions of the length of the features.
Conclusions: Our results indicate that the transition in brightness from the umbra to the penumbra is solely caused by an increased effectiveness of magnetoconvection within individual convective cells. There is no significant difference in the number density of convective elements between the outer umbra and the inner penumbra. Penumbral filaments exhibit a larger area and a higher brightness compared to umbral dots. It is still unclear how exactly the underlying magnetic field causes the increase in the size and brightness of convective features in the penumbra. Title: First results of the CAST-RADES haloscope search for axions at 34.67 μeV Authors: Álvarez Melcón, A.; Arguedas Cuendis, S.; Baier, J.; Barth, K.; Bräuninger, H.; Calatroni, S.; Cantatore, G.; Caspers, F.; Castel, J. F.; Cetin, S. A.; Cogollos, C.; Dafni, T.; Davenport, M.; Dermenev, A.; Desch, K.; Díaz-Morcillo, A.; Döbrich, B.; Fischer, H.; Funk, W.; Gallego, J. D.; García Barceló, J. M.; Gardikiotis, A.; Garza, J. G.; Gimeno, B.; Gninenko, S.; Golm, J.; Hasinoff, M. D.; Hoffmann, D. H. H.; Irastorza, I. G.; Jakovčić, K.; Kaminski, J.; Karuza, M.; Lakić, B.; Laurent, J. M.; Lozano-Guerrero, A. J.; Luzón, G.; Malbrunot, C.; Maroudas, M.; Miralda-Escudé, J.; Mirallas, H.; Miceli, L.; Navarro, P.; Ozbey, A.; Özbozduman, K.; Peña Garay, C.; Pivovaroff, M. J.; Redondo, J.; Ruz, J.; Ruiz Chóliz, E.; Schmidt, S.; Schumann, M.; Semertzidis, Y. K.; Solanki, S. K.; Stewart, L.; Tsagris, I.; Vafeiadis, T.; Vogel, J. K.; Widmann, E.; Wuensch, W.; Zioutas, K. Bibcode: 2021JHEP...10..075A Altcode: 2021arXiv210413798A We present results of the Relic Axion Dark-Matter Exploratory Setup (RADES), a detector which is part of the CERN Axion Solar Telescope (CAST), searching for axion dark matter in the 34.67 μeV mass range. A radio frequency cavity consisting of 5 sub-cavities coupled by inductive irises took physics data inside the CAST dipole magnet for the first time using this filter-like haloscope geometry. An exclusion limit with a 95% credibility level on the axion-photon coupling constant of gaγ ≳ 4 × 10−13 GeV−1 over a mass range of 34.6738 μeV < ma< 34.6771 μeV is set. This constitutes a significant improvement over the current strongest limit set by CAST at this mass and is at the same time one of the most sensitive direct searches for an axion dark matter candidate above the mass of 25 μeV. The results also demonstrate the feasibility of exploring a wider mass range around the value probed by CAST-RADES in this work using similar coherent resonant cavities. Title: Predictions of Astrometric Jitter for Sun-like Stars. II. Dependence on Inclination, Metallicity, and Active-region Nesting Authors: Sowmya, K.; Nèmec, N. -E.; Shapiro, A. I.; Işık, E.; Witzke, V.; Mints, A.; Krivova, N. A.; Solanki, S. K. Bibcode: 2021ApJ...919...94S Altcode: 2021arXiv210701493S Ultra-precise astrometry from the Gaia mission is expected to lead to astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the factors that could hamper such detections is the astrometric jitter caused by the magnetic activity of the planet host stars. In our previous study, we modeled astrometric jitter for the Sun observed equator-on. In this work, we generalize our model and calculate the photocenter jitter as it would be measured by the Gaia and Small-JASMINE missions for stars with solar rotation rate and effective temperature, but with various values of the inclination angle of the stellar rotation axis. In addition, we consider the effect of metallicity and of nesting of active regions (i.e., the tendency of active regions to emerge in the vicinity of each other). We find that, while the jitter of stars observed equator-on does not have any long-term trends and can be easily filtered out, the photocenters of stars observed out of their equatorial planes experience systematic shifts over the course of the activity cycle. Such trends allow the jitter to be detected with continuous measurements, in which case it can interfere with planet detectability. An increase in the metallicity is found to increase the jitter caused by stellar activity. Active-region nesting can further enhance the peak-to-peak amplitude of the photocenter jitter to a level that could be detected by Gaia. Title: The relationship between bipolar magnetic regions and their sunspots Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Jiang, J. Bibcode: 2021A&A...654A..28Y Altcode: 2021arXiv210914313Y Context. The relationship between bipolar magnetic regions (BMRs) and their sunspots is an important property of the solar magnetic field, but it is not well constrained. One consequence is that it is a challenge for surface flux transport models (SFTMs) based on sunspot observations to determine the details of BMR emergence, which they require as input, from such data.
Aims: We aimed to establish the relationship between the amount of magnetic flux in newly emerged BMRs and the area of the enclosed sunspots, and examine the results of its application to an established SFTM.
Methods: Earlier attempts to constrain BMR magnetic flux were hindered by the fact that there is no extensive and reliable record of the magnetic and physical properties of newly emerged BMRs currently available. We made use of the empirical model of the relationship between the disc-integrated facular and network magnetic flux and the total surface coverage by sunspots reported in a recent study. The structure of the model is such that it enabled us to establish, from these disc-integrated quantities, an empirical relationship between the magnetic flux and sunspot area of individual newly emerged BMRs, circumventing the lack of any proper BMR database.
Results: Applying the constraint on BMR magnetic flux derived here to an established SFTM retained its key features, in particular its ability to replicate various independent datasets and the correlation between the model output polar field at the end of each cycle and the observed strength of the following cycle. The SFTM output indicates that facular and network magnetic flux rises with increasing sunspot magnetic flux at a slowing rate such that it appears to gradually saturate. This is analogous to what earlier studies comparing disc-integrated quantities sensitive to the amount of faculae and network present to sunspot indices had reported. The activity dependence of the ratio of facular and network flux to sunspot flux is consistent with the findings of recent studies: although the Sun is faculae-dominated (such that its brightness is mostly positively correlated with activity), it is only marginally so as facular and network brightening and sunspot darkening appear to be closely balanced. Title: Correction of atmospheric stray light in restored slit spectra Authors: Saranathan, S.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...653A..17S Altcode: 2021arXiv211202648S Context. A long-standing issue in solar ground-based observations has been the contamination of data due to stray light, which is particularly relevant in inversions of spectropolarimetric data.
Aims: We aim to build on a statistical method of correcting stray-light contamination due to residual high-order aberrations and apply it to ground-based slit spectra.
Methods: The observations were obtained at the Swedish Solar Telescope, and restored using the multi-frame blind deconvolution restoration procedure. Using the statistical properties of seeing, we created artificially degraded synthetic images generated from magneto-hydrodynamic simulations. We then compared the synthetic data with the observations to derive estimates of the amount of the residual stray light in the observations. In the final step, the slit spectra were deconvolved with a stray-light point spread function to remove the residual stray light from the observations.
Results: The RMS granulation contrasts of the deconvolved spectra were found to increase to approximately 12.5%, from 9%. Spectral lines, on average, were found to become deeper in the granules and shallower in the inter-granular lanes, indicating systematic changes to gradients in temperature. The deconvolution was also found to increase the redshifts and blueshifts of spectral lines, suggesting that the velocities of granulation in the solar photosphere are higher than had previously been observed. Title: Ti I lines at 2.2 μm as probes of the cooler regions of sunspots Authors: Smitha, H. N.; Castellanos Durán, J. S.; Solanki, S. K.; Tiwari, S. K. Bibcode: 2021A&A...653A..91S Altcode: 2021arXiv210701247S Context. The sunspot umbra harbours the coolest plasma on the solar surface due to the presence of strong magnetic fields. The atomic lines that are routinely used to observe the photosphere have weak signals in the umbra and are often swamped by molecular lines. This makes it harder to infer the properties of the umbra, especially in the darkest regions.
Aims: The lines of the Ti I multiplet at 2.2 μm are formed mainly at temperatures ≤4500 K and are not known to be affected by molecular blends in sunspots. Since the first systematic observations in the 1990s, these lines have been seldom observed due to the instrumental challenges involved at these longer wavelengths. We revisit these lines and investigate their formation in different solar features.
Methods: We synthesized the Ti I multiplet using a snapshot from 3D magnetohydrodynamic (MHD) simulations of a sunspot and explored the properties of two of its lines in comparison with two commonly used iron lines, at 6302.5 Å and 1.5648 μm.
Results: We find that the Ti I lines have stronger signals than the Fe I lines in both intensity and polarization in the sunspot umbra and in penumbral spines. They have little to no signal in the penumbral filaments and the quiet Sun, at μ = 1. Their strong and well-split profiles in the dark umbra are less affected by stray light. Consequently, inside the sunspot, it is easier to invert these lines and to infer the atmospheric properties as compared to the iron lines.
Conclusions: The Cryo-NIRSP instrument at the DKIST will provide the first-ever high-resolution observations in this wavelength range. In this preparatory study, we demonstrate the unique temperature and magnetic sensitivities of the Ti multiplet by probing the Sun's coolest regions, which are not favourable for the formation of other commonly used spectral lines. We thus expect such observations to advance our understanding of sunspot properties. Title: MPS-ATLAS: A fast all-in-one code for synthesising stellar spectra Authors: Witzke, V.; Shapiro, A. I.; Cernetic, M.; Tagirov, R. V.; Kostogryz, N. M.; Anusha, L. S.; Unruh, Y. C.; Solanki, S. K.; Kurucz, R. L. Bibcode: 2021A&A...653A..65W Altcode: 2021arXiv210513611W Context. Stellar spectral synthesis is essential for various applications, ranging from determining stellar parameters to comprehensive stellar variability calculations. New observational resources as well as advanced stellar atmosphere modelling, taking three dimensional effects from radiative magnetohydrodynamics calculations into account, require a more efficient radiative transfer.
Aims: For accurate, fast and flexible calculations of opacity distribution functions (ODFs), stellar atmospheres, and stellar spectra, we developed an efficient code building on the well-established ATLAS9 code. The new code also paves the way for easy and fast access to different elemental compositions in stellar calculations.
Methods: For the generation of ODF tables, we further developed the well-established DFSYNTHE code by implementing additional functionality and a speed-up by employing a parallel computation scheme. In addition, the line lists used can be changed from Kurucz's recent lists. In particular, we implemented the VALD3 line list.
Results: A new code, the Merged Parallelised Simplified ATLAS, is presented. It combines the efficient generation of ODF, atmosphere modelling, and spectral synthesis in local thermodynamic equilibrium, therefore being an all-in-one code. This all-in-one code provides more numerical functionality and is substantially faster compared to other available codes. The fully portable MPS-ATLAS code is validated against previous ATLAS9 calculations, the PHOENIX code calculations, and high-quality observations. Title: A Multi-Purpose Heliophysics L4 Mission Authors: Posner, A.; Arge, C. N.; Staub, J.; StCyr, O. C.; Folta, D.; Solanki, S. K.; Strauss, R. D. T.; Effenberger, F.; Gandorfer, A.; Heber, B.; Henney, C. J.; Hirzberger, J.; Jones, S. I.; Kühl, P.; Malandraki, O.; Sterken, V. J. Bibcode: 2021SpWea..1902777P Altcode: The Earth-Sun Lagrangian point 4 is a meta-stable location at 1 AU from the Sun, 60° ahead of Earth's orbit. It has an uninterrupted view of the solar photosphere centered on W60, the Earth's nominal magnetic field connection to the Sun. Such a mission on its own would serve as a solar remote sensing observatory that would oversee the entire solar radiation hemisphere with significant relevance for protecting Moon and Mars explorers from radiation exposure. In combination with appropriately planned observatories at L1 and L5, the three spacecraft would provide 300° longitude coverage of photospheric magnetic field structure, and allow continuous viewing of both solar poles, with >3.6° elevation. Ideally, the L4 and L5 missions would orbit the Sun with a 7.2° inclination out of the heliographic equator, 14.5° out of the ecliptic plane. We discuss the impact of extending solar magnetic field observations in both longitude and latitude to improve global solar wind modeling and, with the development of local helioseismology, the potential for long-term solar activity forecasting. Such a mission would provide a unique opportunity for interplanetary and interstellar dust science. It would significantly add to reliability of operational observations on fast coronal mass ejections directed at Earth and for human Mars explorers on their round-trip journey. The L4 mission concept is technically feasible, and is scientifically compelling. Title: Magnetic imaging of the outer solar atmosphere (MImOSA) Authors: Peter, H.; Ballester, E. Alsina; Andretta, V.; Auchère, F.; Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.; Chitta, L. P.; Dalmasse, K.; Alemán, T. del Pino; Feller, A.; Froment, C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski, D.; Solanki, S. K.; Štěpán, J.; Teriaca, L.; Bueno, J. Trujillo Bibcode: 2021ExA...tmp...95P Altcode: The magnetic activity of the Sun directly impacts the Earth and human life. Likewise, other stars will have an impact on the habitability of planets orbiting these host stars. Although the magnetic field at the surface of the Sun is reasonably well characterised by observations, the information on the magnetic field in the higher atmospheric layers is mainly indirect. This lack of information hampers our progress in understanding solar magnetic activity. Overcoming this limitation would allow us to address four paramount long-standing questions: (1) How does the magnetic field couple the different layers of the atmosphere, and how does it transport energy? (2) How does the magnetic field structure, drive and interact with the plasma in the chromosphere and upper atmosphere? (3) How does the magnetic field destabilise the outer solar atmosphere and thus affect the interplanetary environment? (4) How do magnetic processes accelerate particles to high energies? New ground-breaking observations are needed to address these science questions. We suggest a suite of three instruments that far exceed current capabilities in terms of spatial resolution, light-gathering power, and polarimetric performance: (a) A large-aperture UV-to-IR telescope of the 1-3 m class aimed mainly to measure the magnetic field in the chromosphere by combining high spatial resolution and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to measure the large-scale magnetic field in the corona with an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30 cm telescope that combines high throughput in the extreme UV with polarimetry to connect the magnetic measurements of the other two instruments. Placed in a near-Earth orbit, the data downlink would be maximised, while a location at L4 or L5 would provide stereoscopic observations of the Sun in combination with Earth-based observatories. This mission to measure the magnetic field will finally unlock the driver of the dynamics in the outer solar atmosphere and thereby will greatly advance our understanding of the Sun and the heliosphere. Title: On the size distribution of spots within sunspot groups Authors: Mandal, Sudip; Krivova, Natalie A.; Cameron, Robert; Solanki, Sami K. Bibcode: 2021A&A...652A...9M Altcode: 2021arXiv210403534M The size distribution of sunspots provides key information about the generation and emergence processes of the solar magnetic field. Previous studies of size distribution have primarily focused on either the whole group or individual spot areas. In this paper we investigate the organisation of spot areas within sunspot groups. In particular, we analysed the ratio (R) of the area of the biggest spot (Abig_spot) inside a group, to the total area of that group (Agroup). We used sunspot observations from Kislovodsk, Pulkovo, and Debrecen observatories, together covering solar cycles 17-24. We find that at the time when the group area reaches its maximum, the single biggest spot in a group typically occupies about 60% of the group area. For half of all groups, R lies in the range between roughly 50% and 70%. We also find R to change with Agroup, such that R reaches a maximum of about 0.65 for groups with Agroup ≈ 200 μHem and then remains at about 0.6 for larger groups. Our findings imply a scale-invariant emergence pattern, providing an observational constraint on the emergence process. Furthermore, extrapolation of our results to larger sunspot groups may have a bearing on the giant unresolved starspot features found in Doppler images of highly active Sun-like stars. Our results suggest that such giant features are composed of multiple spots, with the largest spot occupying roughly 55-75% of the total group area (i.e., the area of the giant starspots seen in Doppler images). Title: Slow magneto-acoustic waves in simulations of a solar plage region carry enough energy to heat the chromosphere Authors: Yadav, N.; Cameron, R. H.; Solanki, S. K. Bibcode: 2021A&A...652A..43Y Altcode: 2021arXiv210502932Y
Aims: We study the properties of slow magneto-acoustic waves that are naturally excited as a result of turbulent convection and we investigate their role in the energy balance of a plage region using three dimensional radiation magnetohydrodynamic simulations.
Methods: To follow slow magneto-acoustic waves traveling along the magnetic field lines, we selected 25 seed locations inside a strong magnetic element and tracked the associated magnetic field lines both in space and time. We calculate the longitudinal component (i.e., parallel to the field) of velocity at each grid point along the field line and compute the temporal power spectra at various heights above the mean solar surface. Additionally, the horizontally-averaged (over the whole domain) frequency power spectra for both longitudinal and vertical (i.e., the component perpendicular to the surface) components of velocity are calculated using time series at fixed locations. To compare our results with the observations, we degrade the simulation data with Gaussian kernels having a full width at half maxium of 100 km and 200 km and calculate the horizontally-averaged power spectra for the vertical component of velocity using time series at fixed locations.
Results: The power spectra of the longitudinal component of velocity, averaged over 25 field lines in the core of a kG magnetic flux concentration reveal that the dominant period of oscillations shifts from ∼6.5 min in the photosphere to ∼4 min in the chromosphere. This behavior is consistent with earlier studies that were restricted to vertically propagating waves. At the same time, the velocity power spectra, averaged horizontally over the whole domain, show that low frequency waves (∼6.5 min period) may reach well into the chromosphere. In addition, the power spectra at high frequencies follow a power law with an exponent close to −5/3, suggestive of turbulent excitation. Moreover, waves with frequencies above 5 mHz propagating along different field lines are found to be out of phase with each other, even within a single magnetic concentration. The horizontally-averaged power spectra of the vertical component of velocity at various effective resolutions show that the observed acoustic wave energy fluxes are underestimated by a factor of three, even if determined from observations carried out at a high spatial resolution of 200 km. Since the waves propagate along the non-vertical field lines, measuring the velocity component along the line-of-sight, rather than along the field, contributes significantly to this underestimation. Moreover, this underestimation of the energy flux indirectly indicates the importance of high-frequency waves that are shown to have a smaller spatial coherence and are thus more strongly influenced by the spatial averaging effect compared to low-frequency waves.
Conclusions: Inside a plage region, there is on average a significant fraction of low frequency waves leaking into the chromosphere due to inclined magnetic field lines. Our results show that longitudinal waves carry (just) enough energy to heat the chromosphere in the solar plage. However, phase differences between waves traveling along different field lines within a single magnetic concentration can lead to underestimations of the wave energy flux due to averaging effects in degraded simulation data and, similarly, in observations with lower spatial resolution. We find that current observations (with spatial resolution around 200 km) underestimate the energy flux by roughly a factor of three - or more if the observations are carried out at a lower spatial resolution. We expect that even at a very high resolution, which is expected with the next generation of telescopes such as DKIST and the EST, less than half, on average, of the energy flux carried by such waves will be detected if only the line-of-sight component of the velocity is measured. Title: How rare are counter Evershed flows? Authors: Castellanos Durán, J. S.; Lagg, A.; Solanki, S. K. Bibcode: 2021A&A...651L...1C Altcode: 2021arXiv210605592S One of the main characteristics of sunspot penumbrae is the radially outward-directed Evershed flow. Only recently have penumbral regions been reported with similar characteristics to normal penumbral filaments but with an opposite direction of the flow. Such flows directed toward the umbra are known as counter Evershed flows (CEFs). We aim to determine the occurrence frequency of CEFs in active regions (ARs) and to characterize their lifetime and the prevailing conditions in the ARs. We analyzed the continuum images, Dopplergrams, and magnetograms recorded by SDO/HMI of 97 ARs that appeared from 2011 to 2017. We followed the ARs for 9.6 ± 1.4 days on average. We found 384 CEFs in total, with a median value of six CEFs per AR. Counter Evershed flows are a rather common feature, occurring in 83.5% of all ARs regardless of the magnetic complexity of the AR. However, CEFs were only observed, on average, during 5.9% of the mean total duration of all the observations analyzed here. The lifetime of CEFs follows a log-normal distribution with a median value of 10.6−6.0+12.4 h. In addition, we report two populations of CEFs, those that are associated with light bridges and those that are not. We explain that the rarity of reports of CEFs in the literature is a result of highly incomplete coverage of ARs with spectropolarimetric data. By using the continuous observations now routinely available from space, we are able to overcome this limitation. Title: Radiative Transfer with Opacity Distribution Functions: Application to Narrowband Filters Authors: Anusha, L. S.; Shapiro, A. I.; Witzke, V.; Cernetic, M.; Solanki, S. K.; Gizon, L. Bibcode: 2021ApJS..255....3A Altcode: 2021arXiv210413661A Modeling of stellar radiative intensities in various spectral passbands plays an important role in stellar physics. At the same time, direct calculation of the high-resolution spectrum and then integration of it over the given spectral passband is computationally demanding due to the vast number of atomic and molecular lines. This is particularly so when employing three-dimensional (3D) models of stellar atmospheres. To accelerate the calculations, one can employ approximate methods, e.g., the use of opacity distribution functions (ODFs). Generally, ODFs provide a good approximation of traditional spectral synthesis, i.e., computation of intensities through filters with strictly rectangular transmission functions. However, their performance strongly deteriorates when the filter transmission noticeably changes within its passband, which is the case for almost all filters routinely used in stellar physics. In this context, the aims of this paper are (a) to generalize the ODFs method for calculating intensities through filters with arbitrary transmission functions, and (b) to study the performance of the standard and generalized ODFs methods for calculating intensities emergent from 3D models of stellar atmospheres. For this purpose we use the newly developed MPS-ATLAS radiative transfer code to compute intensities emergent from 3D cubes simulated with the radiative magnetohydrodynamics code MURaM. The calculations are performed in the 1.5D regime, i.e., along many parallel rays passing through the simulated cube. We demonstrate that the generalized ODFs method allows accurate and fast syntheses of spectral intensities and their center-to-limb variations. Title: Forward modelling of Kepler-band variability due to faculae and spots Authors: Johnson, Luke J.; Norris, Charlotte M.; Unruh, Yvonne C.; Solanki, Sami K.; Krivova, Natalie; Witzke, Veronika; Shapiro, Alexander I. Bibcode: 2021MNRAS.504.4751J Altcode: 2021arXiv210411544J; 2021MNRAS.tmp.1190J Variability observed in photometric light curves of late-type stars (on time-scales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in light-curve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) light curves of active G2, K0, M0, and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of light-curve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density, and active band latitudes is explored. For our light-curve modelling approach we used ACTRESS, a geometrically accurate model for stellar variability. ACTRESS generates two-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, light curves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied ACTRESS in Monte Carlo simulations, calculating light-curve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of $1{{\ \rm per\ cent}}$, the typical variability of a solar-type star is around 2 parts per thousand. The presence of faculae clearly affects the mean brightness and light-curve shape, but has relatively little influence on the variability. Title: Coronal loops in a box: 3D models of their internal structure, dynamics and heating Authors: Breu, C. A.; Peter, H.; Cameron, R.; Solanki, S.; Przybylski, D.; Chitta, L. Bibcode: 2021AAS...23810606B Altcode: The corona of the Sun, and probably also of other stars, is built up by loops defined through the magnetic field. They vividly appear in solar observations in the extreme UV and X-rays. High-resolution observations show individual strands with diameters down to a few 100 km, and so far it remains open what defines these strands, in particular their width, and where the energy to heat them is generated.

The aim of our study is to understand how the magnetic field couples the different layers of the solar atmosphere, how the energy generated by magnetoconvection is transported into the upper atmosphere and dissipated, and how this process determines the scales of observed bright strands in the loop.

To this end, we conduct 3D resistive MHD simulations with the MURaM code. We include the effects of heat conduction, radiative transfer and optically thin radiative losses.We study an isolated coronal loop that is rooted with both footpoints in a shallow convection zone layer. To properly resolve the internal structure of the loop while limiting the size of the computational box, the coronal loop is modelled as a straightened magnetic flux tube. By including part of the convection zone, we drive the evolution of the corona self-consistently by magnetoconvection.

We find that the energy injected into the loop is generated by internal coherent motions within strong magnetic elements. The resulting Poynting flux is channelled into the loop in vortex tubes forming a magnetic connection between the photosphere and corona, where it is dissipated and heats the upper atmosphere.

The coronal emission as it would be observed in solar extreme UV or X-ray observations, e.g. with AIA or XRT, shows transient bright strands.The widths of these strands are consistent with observations. From our model we find that the width of the strands is governed by the size of the individual photospheric magnetic field concentrations where the field line through these strands are rooted. Essentially, each coronal strand is rooted in a single magnetic patch in the photosphere, and the energy to heat the strand is generated by internal motions within this magnetic concentration.

With this model we can build a coherent picture of how energy and matter are transported into the upper solar atmosphere and how these processes structure the interior of coronal loops. Title: How wrong are the results of inverting Fe I lines when NLTE and 3D radiative transfer effects are ignored? Authors: Narayanamurthy, S.; Holzreuter, R.; van Noort, M.; Solanki, S. Bibcode: 2021AAS...23811305N Altcode: The Fe I 6301.5 Å and 6302.5 Å lines are widely used to probe the solar photosphere. They are known to be affected by the non-local thermodynamic equilibrium (NLTE) conditions due to the ultraviolet overionisation of iron atoms in the solar atmosphere. This leads to deviations in their level populations based on Saha-Boltzmann statistics. When inverting their Stokes profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. In this work, we discuss how the routinely employed LTE inversion introduces errors in the derived atmospheric quantities. We show that when the NLTE effects are neglected, these errors can be as high as 13% in temperature, and in line-of-sight velocity and magnetic field strength the errors can even exceed 50%. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region with strong vertical gradients in the atmosphere. Similarly, strong horizontal gradients in temperature introduce 3D effects in these lines. We find that errors due to neglecting the 3D effects are more localised and are lower than 5% in temperature, and lower than 20% in both velocity and magnetic field strength. The NLTE and 3D effects are found to persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST). Our findings have wide-ranging consequences since many results derived in solar physics are based on inversions of these Fe I lines carried out in LTE. Title: Small-scale Dynamo in Cool Main-Sequence Stars: Effect on Stratification, Convection and Bolometric Intensity Authors: Bhatia, T.; Cameron, R.; Solanki, S.; Peter, H.; Przybylski, D.; Witzke, V.; Shapiro, A. Bibcode: 2021AAS...23830404B Altcode: In cool main-sequence stars, the near-surface convection has an impact on the center-to-limb variation of photospheric emission, with implications for stellar lightcurves during planetary transits. In the Sun, there is strong evidence for a small-scale dynamo (SSD) maintaining the small-scale magnetic flux. This field could affect the near-surface convection in other cool main-sequence stars.

An SSD could conceivably generate equipartition magnetic fields, which could lead to non-negligible changes not only in convection and intensity characteristics, but also in stratification. We aim to investigate these changes for F, G, K and M stars. 3D MHD models of the four stellar types covering the subsurface region to lower atmosphere in a small cartesian box are studied using the MURaM rMHD simulation code. The MHD runs are compared against a reference hydrodynamic (HD) run.

The deviations in stratification for the deeper convective layers is negligible, except for the F-star, where reduction in turbulent pressure due to magnetic fields is substantial. Convective velocities are reduced by a similar percentage for all the cases due to inhibitory effect of strong magnetic fields near the bottom boundary. All four cases show small-scale brightenings in intergranular lanes, corresponding to magnetic field concentrations, but overall effects on the r.m.s contrast and spatial powerspectra are varied. Title: First Results of the Chromospheric MURaM code Authors: Przybylski, D. F.; Cameron, R.; Solanki, S.; Rempel, M. Bibcode: 2021AAS...23810605P Altcode: The solar chromosphere, spanning the region between the photosphere and the transition to the corona, remains one of the least understood parts of the Sun. This is partly because observing the chromosphere and interpreting these observations is full of pitfalls. Also, the simulation of the chromosphere is complex, as the particle densities and collisional rates are too low to maintain local thermodynamic equilibrium (LTE). Additionally, the recombination rates of hydrogen are larger than the dynamical timescales and the populations must be solved in non-equilibrium (NE). Realistic simulations of the chromosphere must treat the magneto-hydrodynamics, time-dependant atomic and molecular chemistry, and radiation transfer simultaneously.

The MURaM radiation-MHD code has previously been used for investigation of the connection between the solar photosphere and corona, ranging from small-scale dynamo generated 'quiet' sun fields to sunspots and complex active regions. Until now these simulations have been performed in LTE, greatly limiting their realism in the solar chromosphere. We have extended MURaM to include NLTE effects following the prescriptions used in the Bifrost code. The low viscocity and resistivity of the MURaM code leads to turbulent convection in the photosphere with kilo-Gauss mixed-polarity magnetic fields. This results in a dynamic chromosphere with strong shocks and a finely structured magnetic field. We discuss the implications of this new model towards observations of chromospheric spectral lines. Title: Modeling Stellar Ca II H and K Emission Variations. I. Effect of Inclination on the S-index Authors: Sowmya, K.; Shapiro, A. I.; Witzke, V.; Nèmec, N. -E.; Chatzistergos, T.; Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2021ApJ...914...21S Altcode: 2021arXiv210313893S The emission in the near-ultraviolet Ca II H and K lines is modulated by stellar magnetic activity. Although this emission, quantified via the S-index, has been serving as a prime proxy of stellar magnetic activity for several decades, many aspects of the complex relation between stellar magnetism and Ca II H and K emission are still unclear. The amount of measured Ca II H and K emission is suspected to be affected not only by the stellar intrinsic properties but also by the inclination angle of the stellar rotation axis. Until now, such an inclination effect on the S-index has remained largely unexplored. To fill this gap, we develop a physics-based model to calculate S-index, focusing on the Sun. Using the distributions of solar magnetic features derived from observations together with Ca II H and K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of the solar S-index over four activity cycles. Further, using the distribution of magnetic features over the visible solar disk obtained from surface flux transport simulations, we obtain S-index time series dating back to 1700 and investigate the effect of inclination on S-index variability on both the magnetic activity cycle and the rotational timescales. We find that when going from an equatorial to a pole-on view, the amplitude of S-index variations decreases weakly on the activity cycle timescale and strongly on the rotational timescale (by about 22% and 81%, respectively, for a cycle of intermediate strength). The absolute value of the S-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies. Title: Modelling the evolution of the Sun's open and total magnetic flux Authors: Krivova, N. A.; Solanki, S. K.; Hofer, B.; Wu, C. -J.; Usoskin, I. G.; Cameron, R. Bibcode: 2021A&A...650A..70K Altcode: 2021arXiv210315603K Solar activity in all its varied manifestations is driven by the magnetic field. Two global quantities are particularly important for many purposes, the Sun's total and open magnetic flux, which can be computed from sunspot number records using models. Such sunspot-driven models, however, do not take into account the presence of magnetic flux during grand minima, such as the Maunder minimum. Here we present a major update of a widely used simple model, which now takes into account the observation that the distribution of all magnetic features on the Sun follows a single power law. The exponent of the power law changes over the solar cycle. This allows for the emergence of small-scale magnetic flux even when no sunspots have been present for multiple decades and leads to non-zero total and open magnetic flux also in the deepest grand minima, such as the Maunder minimum, thus overcoming a major shortcoming of the earlier models. The results of the updated model compare well with the available observations and reconstructions of the solar total and open magnetic flux. This opens up the possibility of improved reconstructions of the sunspot number from time series of the cosmogenic isotope production rate. Title: Solar cyclic activity over the last millennium reconstructed from annual 14C data Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N. A.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2021A&A...649A.141U Altcode: 2021arXiv210315112U
Aims: The 11-year solar cycle (Schwabe cycle) is the dominant pattern of solar magnetic activity reflecting the oscillatory dynamo mechanism in the Sun's convection zone. Solar cycles have been directly observed since 1700, while indirect proxy data suggest their existence over a much longer period of time, but generally without resolving individual cycles and their continuity. Here we reconstruct individual solar cycles for the last millennium using recently obtained 14C data and state-of-the-art models.
Methods: Starting with the 14C production rate determined from the so far most precise measurements of radiocarbon content in tree rings, solar activity was reconstructed in the following three physics-based steps: (1) correction of the 14C production rate for the changing geomagnetic field; (2) computation of the open solar magnetic flux; and (3) conversion into sunspot numbers outside of grand minima. All known uncertainties, including both measurement and model uncertainties, were straightforwardly accounted for by a Monte-Carlo method.
Results: Cyclic solar activity is reconstructed for the period 971-1900 (85 individual cycles) along with its uncertainties. This more than doubles the number of solar cycles known from direct solar observations. We found that the lengths and strengths of well-defined cycles outside grand minima are consistent with those obtained from the direct sunspot observations after 1750. The validity of the Waldmeier rule (cycles with fast-rising phase tend to be stronger) is confirmed at a highly significant level. Solar activity is found to be in a deep grand minimum when the activity is mostly below the sunspot formation threshold for about 250 years. Therefore, although considerable cyclic variability in 14C is seen even during grand minima, individual solar cycles can hardly be reliably resolved therein. Three potential solar particle events, ca. 994, 1052, and 1279 AD, are shown to occur around the maximum phases of solar cycles.
Conclusions: A new approximately 1000-year-long solar activity reconstruction, in the form of annual (pseudo) sunspot numbers with the full assessment of all known uncertainties, is presented based on new high-precision Δ14C measurements and state-of-the-art models, more than doubling the number of individually resolved solar cycles. This forms a solid basis for new, more detailed studies of solar variability.

Tabular data of the reconstructed activity are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/649/A141 Title: Signatures of coronal hole substructure in the solar wind: combined Solar Orbiter remote sensing and in situ measurements Authors: Horbury, T. S.; Laker, R.; Rodriguez, L.; Steinvall, K.; Maksimovic, M.; Livi, S.; Berghmans, D.; Auchere, F.; Zhukov, A. N.; Khotyaintsev, Yu. V.; Woodham, L.; Matteini, L.; Stawarz, J.; Woolley, T.; Bale, S. D.; Rouillard, A.; O'Brien, H.; Evans, V.; Angelini, V.; Owen, C.; Solanki, S. K.; Nicula, B.; Muller, D.; Zouganelis, I. Bibcode: 2021arXiv210414960H Altcode: Context. The Sun's complex corona is the source of the solar wind and interplanetary magnetic field. While the large scale morphology is well understood, the impact of variations in coronal properties on the scale of a few degrees on properties of the interplanetary medium is not known. Solar Orbiter, carrying both remote sensing and in situ instruments into the inner solar system, is intended to make these connections better than ever before. Aims. We combine remote sensing and in situ measurements from Solar Orbiter's first perihelion at 0.5 AU to study the fine scale structure of the solar wind from the equatorward edge of a polar coronal hole with the aim of identifying characteristics of the corona which can explain the in situ variations. Methods. We use in situ measurements of the magnetic field, density and solar wind speed to identify structures on scales of hours at the spacecraft. Using Potential Field Source Surface mapping we estimate the source locations of the measured solar wind as a function of time and use EUI images to characterise these solar sources. Results. We identify small scale stream interactions in the solar wind with compressed magnetic field and density along with speed variations which are associated with corrugations in the edge of the coronal hole on scales of several degrees, demonstrating that fine scale coronal structure can directly influence solar wind properties and drive variations within individual streams. Conclusions. This early analysis already demonstrates the power of Solar Orbiter's combined remote sensing and in situ payload and shows that with future, closer perihelia it will be possible dramatically to improve our knowledge of the coronal sources of fine scale solar wind structure, which is important both for understanding the phenomena driving the solar wind and predicting its impacts at the Earth and elsewhere. Title: Observing and modelling the young solar analogue EK Draconis: starspot distribution, elemental abundances, and evolutionary status Authors: Şenavcı, H. V.; Kılıçoğlu, T.; Işık, E.; Hussain, G. A. J.; Montes, D.; Bahar, E.; Solanki, S. K. Bibcode: 2021MNRAS.502.3343S Altcode: 2021MNRAS.tmp..235S; 2021arXiv210107248S Observations and modelling of stars with near-solar masses in their early phases of evolution are critical for a better understanding of how dynamos of solar-type stars evolve. We examine the chemical composition and the spot distribution of the pre-main-sequence solar analogue EK Dra. Using spectra from the HERMES Spectrograph (La Palma), we obtain the abundances of 23 elements with respect to the solar ones, which lead to a [Fe/H] = 0.03, with significant overabundance of Li and Ba. The s-process elements Sr, Y, and Ce are marginally overabundant, while Co, Ni, Cu, Zn are marginally deficient compared to solar abundances. The overabundance of Ba is most likely due to the assumption of depth-independent microturbulent velocity. Li abundance is consistent with the age and the other abundances may indicate distinct initial conditions of the pre-stellar nebula. We estimate a mass of 1.04 M⊙ and an age of $27^{+11}_{-8}$ Myr using various spectroscopic and photometric indicators. We study the surface distribution of dark spots, using 17 spectra collected during 15 nights using the CAFE Spectrograph (Calar Alto). We also conduct flux emergence and transport (FEAT) simulations for EK Dra's parameters and produce 15-d-averaged synoptic maps of the likely starspot distributions. Using Doppler imaging, we reconstruct the surface brightness distributions for the observed spectra and FEAT simulations, which show overall agreement for polar and mid-latitude spots, while in the simulations there is a lack of low-latitude spots compared to the observed image. We find indications that cross-equatorial extensions of mid-latitude spots can be artefacts of the less visible southern-hemisphere activity. Title: The influence of NLTE effects in Fe I lines on an inverted atmosphere. II. 6301 Å and 6302 Å lines formed in 3D NLTE Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...647A..46S Altcode: 2021arXiv210100506S Context. This paper forms the second part of our study of how neglecting non-local thermodynamic equilibrium (NLTE) conditions in the formation of Fe I 6301.5 Å and the 6302.5 Å lines affects the atmosphere that is obtained by inverting the Stokes profiles of these lines in LTE. The main cause of NLTE effects in these lines is the line opacity deficit that is due to the excess ionisation of Fe I atoms by ultraviolet (UV) photons in the Sun.
Aims: In the first paper, these photospheric lines were assumed to have formed in 1D NLTE and the effects of horizontal radiation transfer (RT) were neglected. In the present paper, the iron lines are computed by solving the RT in 3D. We investigate the effect of horizontal RT on the inverted atmosphere and how it can enhance or reduce the errors that are due to neglecting 1D NLTE effects.
Methods: The Stokes profiles of the iron lines were computed in LTE, 1D NLTE, and 3D NLTE. They were all inverted using an LTE inversion code. The atmosphere from the inversion of LTE profiles was taken as the reference model. The atmospheres from the inversion of 1D NLTE profiles (testmodel-1D) and 3D NLTE profiles (testmodel-3D) were compared with it. Differences between reference and testmodels were analysed and correspondingly attributed to NLTE and 3D effects.
Results: The effects of horizontal RT are evident in regions surrounded by strong horizontal temperature gradients. That is, along the granule boundaries, regions surrounding magnetic elements, and its boundaries with intergranular lanes. In some regions, the 3D effects enhance the 1D NLTE effects, and in some, they weaken these effects. In the small region analysed in this paper, the errors due to neglecting the 3D effects are lower than 5% in temperature. In most of the pixels, the errors are lower than 20% in both velocity and magnetic field strength. These errors also persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST).
Conclusions: Neglecting horizontal RT introduces errors not only in the derived temperature, but also in other atmospheric parameters. The error sizes depend on the strength of the local horizontal temperature gradients. Compared to the 1D NLTE effect, the 3D effects are more localised in specific regions in the atmosphere and are weaker overall. Title: VizieR Online Data Catalog: 1000-year sunspot series (Usoskin+, 2021) Authors: Usoskin, I. G.; Solanki, S. K.; Krivova, N.; Hofer, B.; Kovaltsov, G. A.; Wacker, L.; Brehm, N.; Kromer, B. Bibcode: 2021yCat..36490141U Altcode: A new quantitative reconstruction of annually resolved solar activity, in the form of SNs (at least outside grand minima) with a full uncertainty assessment, is presented for the period 971-1900. For the first time, individual solar cycles are presented for the whole of the last millennium, more than doubling the existing statistics of solar cycles.

Three tabular files are presented.

File 'osf.dat' contains annual reconstructions of the opens solar flux (OSF) [Fo] along with its 1-sigma uncertainties [sF] and the smoothed (22-yr SSA -- see text) values [<Fo>]. This table corresponds to Figure 8 in the text.

File 'osn.dat' contains annual reconstructions of the sunspot number (SN) [SN] along with its 1-sigma uncertainties [sSN] and the smoothed (22-yr SSA -- see text) values [<SN>]. This table corresponds to Figure 11 in the text.

File 'table1.dat' presents a textual tabular version of Table 1 in the text and contains the internal cycle number [n], years of minimum [Ymin] and maximum [Ymax] of each cycle, cycle-averaged sunspot number [<SN>], its 1-sigma uncertainty [sS], cycle length (min-to-min) [T], and quality flag [q].

(3 data files). Title: Amplifying variability of solar-like stars by active longitudes and nesting Authors: Isik, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2021csss.confE.279I Altcode: Many solar-type stars with near-solar rotation periods exhibit much stronger variability than the Sun (Reinhold et al. 2020). Some of these stars even show very regular, sine-like light curves. Motivated by solar activity complexes, we developed a numerical model to quantify the effect of active-region (AR) nesting and active longitudes on stellar brightness variations in the rotational time scale. Modelling ARs with facular and spot components, we simulated light curves covering four years and using the Kepler passband. We found that the combined effect of the degree of nesting and the activity level, both being somewhat higher than on the Sun, can explain the whole range of observed light-curve amplitudes of solar-like stars. While nesting at random longitudes can explain variability amplitudes and light-curve morphology in many cases, active-longitude-type nesting reproduces sine-like light curves and the highest amplitude variability. Title: Modelling Solar Ca II H&K Emission Variations Authors: Krishnamurthy, Sowmya; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina-E.; Chatzistergos, Theodosis; Yeo, Kok Leng; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2021csss.confE.154K Altcode: The emission in the near ultraviolet Ca II H&K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between the coverage of a stellar disk by magnetic features and Ca II H&K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we made use of the distributions of the solar magnetic features derived from the simulations of magnetic flux emergence and surface transport, together with the Ca II H&K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code.We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer's line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity. Title: Small-scale dynamo in an F-star: effects on near-surface stratification, convection and intensity Authors: Bhatia, Tanayveer; Cameron, Robert; Solanki, Sami; Peter, Hardi; Przybylski, Damien; Witzke, Veronika; Shapiro, Alexander Bibcode: 2021csss.confE..75B Altcode: The emission from the photosphere of stars shows a systematic center-to-limb variation. In cool main-sequence stars, the near-surface convection has an impact on this variation, with implications for lightcurves of stars during planetary transits. In the Sun, there is strong evidence for a small-scale dynamo (SSD) maintaining the small-scale magnetic flux. We aim to investigate what additional effects such a field would play for other cool main-sequence stars. In our work we first concentrate on F-stars. This is because they have sonic velocities near the surface, implying a rough equipartition between internal and kinetic energies. In addition, an SSD might create a significant magnetic energy density to impact the results. We investigate the interplay between internal, kinetic and magnetic energies in 3D cartesian box MHD models of a F3V-star in the near-surface convection, using the MURaM radiative-MHD simulation code. Along with a reference hydrodynamic run, two MHD models with self-consistently generated magnetic fields with two different lower boundary conditions are considered. We find that the SSD process creates a magnetic field with energy within an order of magnitude of the internal and the kinetic energy. Compared to the hydrodynamic run, we find slight (~1-3%) but significant deviations in density, gas pressure and temperature stratification. At the surface, this corresponds to a temperature difference of ~130 K. As expected, there is a significant reduction in kinetic energy flux once the SSD is operational. The changes in intensity are more subtle, both in total intensity and granulation pattern. From this we conclude that the presence of an SSD will have a significant impact on the atmospheric structure and intensity characteristics seen at the surface. This makes it clear that it would be important to consider the spatially and temporally averaged effects of the SSD also for global stellar models. Title: Sunspot Simulations: Penumbra Formation and the Fluting Instability Authors: Panja, Mayukh; Cameron, Robert H.; Solanki, Sami K. Bibcode: 2021ApJ...907..102P Altcode: 2020arXiv201111447P The fluting instability has been suggested as the driver of the subsurface structure of sunspot flux tubes. We conducted a series of numerical experiments where we used flux tubes with different initial curvatures to study the effect of the fluting instability on the subsurface structure of spots. We used the MURaM code, which has previously been used to simulate complete sunspots, to first compute four sunspots in the slab geometry and then two complete circular spots of opposite polarities. We find that the curvature of a flux tube indeed determines the degree of fluting the flux tube will undergo—the more curved a flux tube is, the more fluted it becomes. In addition, sunspots with strong curvature have strong horizontal fields at the surface and therefore readily form penumbral filaments. The fluted sunspots eventually break up from below, with lightbridges appearing at the surface several hours after fluting commences. Title: Predictions of Astrometric Jitter for Sun-like Stars. I. The Model and Its Application to the Sun as Seen from the Ecliptic Authors: Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2021ApJ...908..223S Altcode: 2020arXiv201212312S The advent of Gaia, capable of measuring stellar wobbles caused by orbiting planets, raised interest in the astrometric detection of exoplanets. Another source of such wobbles (often also called jitter) is stellar magnetic activity. A quantitative assessment of the stellar astrometric jitter is important for a more reliable astrometric detection and characterization of exoplanets. We calculate the displacement of the solar photocenter due to the magnetic activity for an almost 16 yr period (1999 February 2-2014 August 1). We also investigate how the displacement depends on the spectral passband chosen for observations, including the wavelength range to be covered by the upcoming Small-JASMINE mission of JAXA. This is done by extending the SATIRE-S model for solar irradiance variability to calculating the displacement of the solar photocenter caused by the magnetic features on the surface of the Sun. We found that the peak-to-peak amplitude of the solar photocenter displacement would reach 0.5 μas if the Sun were located 10 pc away from the observer and observed in the Gaia G filter. This is by far too small to be detected by the Gaia mission. However, the Sun is a relatively inactive star so one can expect significantly larger signals for younger, and, consequently, more active stars. The model developed in this study can be combined with the simulations of emergence and surface transport of magnetic flux which have recently become available to model the astrometric jitter over the broad range of magnetic activities. Title: Where Have All the Solar-like Stars Gone? Rotation Period Detectability at Various Inclinations and Metallicities Authors: Reinhold, Timo; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina-E.; Işık, Emre; Solanki, Sami K. Bibcode: 2021ApJ...908L..21R Altcode: 2021arXiv210111426R The plethora of photometric data collected by the Kepler space telescope has promoted the detection of tens of thousands of stellar rotation periods. However, these periods are not found to an equal extent among different spectral types. Interestingly, early G-type stars with near-solar rotation periods are strongly underrepresented among those stars with known rotation periods. In this study we investigate whether the small number of such stars can be explained by difficulties in the period determination from photometric time series. For that purpose, we generate model light curves of early G-type stars with solar rotation periods for different inclination angles, metallicities, and (magnitude-dependent) noise levels. We find that the detectability is determined by the predominant type of activity (i.e., spot or faculae domination) on the surface, which defines the degree of irregularity of the light curve, and further depends on the level of photometric noise. These two effects significantly complicate the period detection and explain the lack of solar-like stars with known near-solar rotation periods. We conclude that the rotation periods of the majority of solar-like stars with near-solar rotation periods remain undetected to date. Finally, we promote the use of new techniques to recover more periods of near-solar rotators. Title: Coronal Heating and Solar Wind Formation in Quiet Sun and Coronal Holes: A Unified Scenario Authors: Tripathi, Durgesh; Nived, V. N.; Solanki, Sami K. Bibcode: 2021ApJ...908...28T Altcode: 2020arXiv201109803T Coronal holes (CHs) are darker than the quiet Sun (QS) when observed in coronal channels. This study aims to understand the similarities and differences between CHs and QS in the transition region using the Si IV 1394 Å line, recorded by the Interface Region Imaging Spectrograph, by considering the distribution of magnetic field measured by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We find that Si IV intensities obtained in CHs are lower than those obtained in QS for regions with identical magnetic flux densities. Moreover, the difference in intensities between CHs and QS increases with increasing magnetic flux. For the regions with equal magnetic flux density, QS line profiles are more redshifted than those measured in CHs. Moreover, the blueshifts measured in CHs show an increase with increasing magnetic flux density unlike in the QS. The non-thermal velocities in QS, as well as in CHs, show an increase with increasing magnetic flux. However, no significant difference was observed in QS and CHs, albeit a small deviation at small flux densities. Using these results, we propose a unified model for the heating of the corona in the QS and in CHs and the formation of solar wind. Title: Vortex flow properties in simulations of solar plage region: Evidence for their role in chromospheric heating Authors: Yadav, N.; Cameron, R. H.; Solanki, S. K. Bibcode: 2021A&A...645A...3Y Altcode: 2020arXiv201014971Y Context. Vortex flows exist across a broad range of spatial and temporal scales in the solar atmosphere. Small-scale vortices are thought to play an important role in energy transport in the solar atmosphere. However, their physical properties remain poorly understood due to the limited spatial resolution of the observations.
Aims: We explore and analyze the physical properties of small-scale vortices inside magnetic flux tubes using numerical simulations, and investigate whether they contribute to heating the chromosphere in a plage region.
Methods: Using the three-dimensional radiative magnetohydrodynamic simulation code MURaM, we perform numerical simulations of a unipolar solar plage region. To detect and isolate vortices we use the swirling strength criterion and select the locations where the fluid is rotating with an angular velocity greater than a certain threshold. We concentrate on small-scale vortices as they are the strongest and carry most of the energy. We explore the spatial profiles of physical quantities such as density and horizontal velocity inside these vortices. Moreover, to learn their general characteristics, a statistical investigation is performed.
Results: Magnetic flux tubes have a complex filamentary substructure harboring an abundance of small-scale vortices. At the interfaces between vortices strong current sheets are formed that may dissipate and heat the solar chromosphere. Statistically, vortices have higher densities and higher temperatures than the average values at the same geometrical height in the chromosphere.
Conclusions: We conclude that small-scale vortices are ubiquitous in solar plage regions; they are denser and hotter structures that contribute to chromospheric heating, possibly by dissipation of the current sheets formed at their interfaces. Title: Magnetic Imaging of the Outer Solar Atmosphere (MImOSA): Unlocking the driver of the dynamics in the upper solar atmosphere Authors: Peter, H.; Alsina Ballester, E.; Andretta, V.; Auchere, F.; Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.; Chitta, L. P.; Dalmasse, K.; del Pino Aleman, T.; Feller, A.; Froment, C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski, D.; Solanki, S. K.; Stepan, J.; Teriaca, L.; Trujillo Bueno, J. Bibcode: 2021arXiv210101566P Altcode: The magnetic activity of the Sun directly impacts the Earth and human life. Likewise, other stars will have an impact on the habitability of planets orbiting these host stars. The lack of information on the magnetic field in the higher atmospheric layers hampers our progress in understanding solar magnetic activity. Overcoming this limitation would allow us to address four paramount long-standing questions: (1) How does the magnetic field couple the different layers of the atmosphere, and how does it transport energy? (2) How does the magnetic field structure, drive and interact with the plasma in the chromosphere and upper atmosphere? (3) How does the magnetic field destabilise the outer solar atmosphere and thus affect the interplanetary environment? (4) How do magnetic processes accelerate particles to high energies? New ground-breaking observations are needed to address these science questions. We suggest a suite of three instruments that far exceed current capabilities in terms of spatial resolution, light-gathering power, and polarimetric performance: (a) A large-aperture UV-to-IR telescope of the 1-3 m class aimed mainly to measure the magnetic field in the chromosphere by combining high spatial resolution and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to measure the large-scale magnetic field in the corona with an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30 cm telescope that combines high throughput in the extreme UV with polarimetry to connect the magnetic measurements of the other two instruments. This mission to measure the magnetic field will unlock the driver of the dynamics in the outer solar atmosphere and thereby greatly advance our understanding of the Sun and the heliosphere. Title: Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) for SUNRISE III: optical design and performance Authors: Tsuzuki, Toshihiro; Katsukawa, Yukio; Uraguchi, Fumihiro; Hara, Hirohisa; Kubo, Masahito; Nodomi, Yoshifumi; Suematsu, Yoshinori; Kawabata, Yusuke; Shimizu, Toshifumi; Gandorfer, Achim; Feller, Alex; Grauf, Bianca; Solanki, Sami; Carlos del Toro Iniesta, Jose Bibcode: 2020SPIE11447E..AJT Altcode: The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is a near-IR spectro-polarimeter instrument newly designed for Sunrise III, which is a balloon-borne solar observatory equipped with a 1 m optical telescope. To acquire high-quality 3D magnetic and velocity fields, SCIP selects the two wavelength bands centered at 850 nm and 770 nm, which contain many spectrum lines that are highly sensitive to magnetic fields permeating the photosphere and chromosphere. To achieve high spatial and spectral resolution (0.21 arcsec and 2 × 105), SCIP optics adopt a quasi-Littrow configuration based on an echelle grating and two high-order aspheric mirrors. Using different diffraction orders of the echelle grating, dichroic beam splitter, and polarizing beam-splitters, SCIP can obtain s- and p-polarization signals in the two wavelength bands simultaneously within a relatively small space. We established the wavefront error budget based on tolerance analysis, surface figure errors, alignment errors, and environmental changes. In addition, we performed stray light analysis, and designed light traps and baffles needed to suppress unwanted reflections and diffraction by the grating. In this paper, we present the details of this optical system and its performance. Title: Reconstructing solar irradiance from Ca II K observations Authors: Chatzistergos, T.; Krivova, N.; Ermolli, I.; Yeo, K. L.; Solanki, S. K.; Puiu, C. C.; Giorgi, F.; Mandal, S. Bibcode: 2020AGUFMA237...10C Altcode: To understand the influence of the Sun on Earth's system, long and accurate measurements of solar irradiance are a prerequisite. The available direct measurements of solar irradiance since 1978 are clearly not sufficient for this purpose. This stimulated development of models used to reconstruct past solar irradiance variations from alternative observations. The main driver of the irradiance variations on time scales of days to millennia is the evolution of the solar surface magnetic field in form of dark sunspots and bright faculae and network. Therefore, models require input data describing the contributions of these various magnetic regions on the Sun at earlier times. Unfortunately, records that can be used to describe the facular and network contributions are barely longer than the direct irradiance measurements. Thus, irradiance reconstructions to earlier periods have to rely on sunspot data alone. Data that have hardly been used for solar irradiance reconstructions until now are full-disc solar observations in the Ca II K line. Such data exist since 1892 from various observatories and include all the needed information describing faculae and the network. However, they are plagued by a bunch of various problems and artefacts, and recovering the non-linear response of the photographic material to the radiation is non-trivial since the required information is usually missing, too. We have developed a method to process Ca II K observations from various sources and demonstrated the higher accuracy achieved by our method compared to other techniques presented in the literature. Here we use the carefully reduced Ca II K observations from multiple archives to reconstruct solar irradiance variations. Title: Erratum: "Amplification of Brightness Variability by Active-region Nesting in Solar-like Stars" (2020, ApJL, 901, L12) Authors: Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2020ApJ...905L..36I Altcode: No abstract at ADS Title: The magnetic fine structure of the Sun's polar region as revealed by Sunrise Authors: Prabhu, A.; Lagg, A.; Hirzberger, J.; Solanki, S. K. Bibcode: 2020A&A...644A..86P Altcode: Context. Polar magnetic fields play a key role in the solar magnetic cycle and they are the source of a significant portion of the interplanetary magnetic field. However, observations of the poles are challenging and hence our understanding of the polar magnetic environment is incomplete.
Aims: We deduce properties of small-scale magnetic features in the polar region using high-resolution data and specifically aim to determine the flux per patch above which one magnetic polarity starts to dominate over the other.
Methods: We study the high spatial resolution, seeing-free observations of the north solar polar region, obtained with the IMaX instrument on-board the balloon-borne SUNRISE observatory during June 2009, at the solar activity minimum. We performed inversions of the full Stokes vector recorded by IMaX to retrieve atmospheric parameters of the Sun's polar region, mainly the temperature stratification and the magnetic field vector.
Results: We infer kilo-Gauss (kG) magnetic fields in patches harbouring polar faculae, without resorting to a magnetic filling factor. Within these patches we find the maxima of the magnetic field to be near the dark narrow lanes, which are shifted towards the disc centre side in comparison to the maxima in continuum intensity. In contrast, we did not find any fields parallel to the solar surface with kG strengths. In addition to the kG patches, we found the polar region to be covered in patches of both polarities, which have a range of sizes. We find the field strength of these patches to increase with increasing size and flux, with the smaller patches showing a significant dispersion in field strength. The dominating polarity of the north pole during this phase of the solar cycle is found to be maintained by the larger patches with fluxes above 2.3 × 1017 Mx. Title: Multi-view magnetic synoptic maps with SO/PHI and SDO/HMI Authors: Löschl, P.; Hirzberger, J.; Schou, J.; Solanki, S. K. Bibcode: 2020AGUFMSH0360028L Altcode: With the recent launch of Solar Orbiter (SO) and the first data slowly becoming available, it will soon be possible to simultaneously observe the Sun from additional vantage points off the Earth-Sun line. One of its instruments, the Polarimetric and Heliospheric Imager (PHI), is the first spectro-polarimeter to operate outside of this line of sight. This opens the possibility for joint observational campaigns with similar instruments, such as the Heliospheric and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). We utilise these new observational possibilities to produce combined magnetic synoptic maps from magnetograms of the PHI and HMI instruments. Building on the existing software infrastructure for HMI synoptic maps, we extended its current functionality to include PHI data and correct for the different and varying relative orbital characteristics of the two spacecraft. The result are joint magnetic synoptic maps, that can be produced significantly faster than the approximately 27 days of one solar rotation and therefore are less likely to suffer from the evolution of the magnetic field over the observation period. Once Solar Orbiter leaves the ecliptic plane, we will also be able to include observations of the polar magnetic field into our synoptic maps, which will give an unprecedented insight into the magnetic field of the Sun. This work presents our preparatory modelling efforts and gives an outlook for the future capabilities of this novel data product. Title: First results from SO/PHI's on-board data reduction Authors: Albert, K.; Hirzberger, J.; Kolleck, M.; Albelo Jorge, N.; Busse, D.; Blanco Rodriguez, J.; Cobos Carrascosa, J. P.; Fiethe, B.; Gandorfer, A.; Germerott, D.; Guan, Y.; Guerrero, L.; Gutierrez-Marques, P.; Hernández Expósito, D.; Lange, T.; Michalik, H.; Orozco Suárez, D.; Schou, J.; Solanki, S. K.; Woch, J. G. Bibcode: 2020AGUFMSH038..05A Altcode: The Polarimetric and Helioseismic Imager (PHI), on-board Solar Orbiter (SO), is a spectropolarimeter imaging the solar photosphere at the wavelengths of the Fe I 617.3 nm Zeeman sensitive absorption line. SO/PHI's aim is to provide data about the magnetic structures and the line-of-sight (LOS) velocity in the solar atmosphere. For this, it takes time series of data sets consisting of 2048 x 2048 pixel images of the Sun at 6 wavelengths, each in 4 different polarisation states. With the minimum necessary 17 bits pixel depth, one data set amounts to approx. 0.2 GB. The guaranteed data telemetry for PHI, in contrast, is only 50 GiB/orbit which would also need to contain any calibration data obtained on-board, i.e. our flat and dark fields. To cope with this discrepancy, SO/PHI is performing full data reduction on-board, including the inversion of the radiative transfer equation. The downloaded results are science ready data, containing 5 final images: a total intensity image from nearby the spectral line, the magnetic field strength, azimuth and inclination (describing the magnetic vector) and the LOS velocity. This process maximises the science return by reducing the number of necessary images in a data set, as well as rendering the download of calibration data unessential. In the commissioning phase of SO/PHI we used the on-board data reduction system successfully for the first time. We have calibrated the instrument to its optimal operational parameters (calculation of exposure time, focus, etc.), acquired and processed calibration data (dark and flat fields), removed the most significant instrumental artefacts from the data (dark field, flat field, polarimetric modulation and polarimetric cross-talk), and performed the inversion of the radiative transfer equation. The data have then been compressed to further maximise the use of our telemetry. This contribution presents and discusses the final results from this process. Title: Power spectrum of turbulent convection in the solar photosphere Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.; Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2020A&A...644A..44Y Altcode: 2020arXiv201009037Y The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two balloon-borne SUNRISE missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the photospheric overshooting flow with the help of its governing equations and simulations. We find that quiet-Sun observations and smeared simulations are consistent with each other and exhibit a power-law behavior in the subgranular range of their Doppler velocity power spectra with a power-law index of ≈ - 2. The unsmeared simulations exhibit a power law that extends over the full range between the integral and Taylor scales with a power-law index of ≈ - 2.25. The smearing, reminiscent of observational conditions, considerably reduces the extent of the power-law-like portion of the power spectra. This suggests that the limited spatial resolution in some observations might eventually result in larger uncertainties in the estimation of the power-law indices. The simulated vertical velocity power spectra as a function of height show a rapid change in the power-law index (at the subgranular range) from roughly the optical depth unity layer, that is, the solar surface, to 300 km above it. We propose that the cause of the steepening of the power-law index is the transition from a super- to a subadiabatic region, in which the dominant source of motions is overshooting convection. A scale-dependent transport of the vertical momentum occurs. At smaller scales, the vertical momentum is more efficiently transported sideways than at larger scales. This results in less vertical velocity power transported upward at small scales than at larger scales and produces a progressively steeper vertical velocity power law below 180 km. Above this height, the gravity work progressively gains importance at all relevant scales, making the atmosphere progressively more hydrostatic and resulting in a gradually less steep power law. Radiative heating and cooling of the plasma is shown to play a dominant role in the plasma energetics in this region, which is important in terms of nonadiabatic damping of the convective motions. Title: The SUNRISE UV Spectropolarimeter and imager for SUNRISE III Authors: Feller, Alex; Gandorfer, Achim; Iglesias, Francisco A.; Lagg, Andreas; Riethmüller, Tino L.; Solanki, Sami K.; Katsukawa, Yukio; Kubo, Masahito Bibcode: 2020SPIE11447E..AKF Altcode: Sunrise is a balloon-borne solar observatory dedicated to the investigation of key processes of the magnetic field and the plasma flows in the lower solar atmosphere. The observatory operates in the stratosphere at an altitude of around 37 km in order to avoid image degradation due to turbulence in the Earth's atmosphere and to access the UV range. The third science flight of Sunrise will carry new instrumentation which samples the solar spectrum over a broad wavelength domain from the UV to the near IR and covers an extended height range in the solar atmosphere. A key feature of the Sunrise UV Spectropolarimeter and Imager (SUSI) operating between 309 nm and 417 nm, is its capability to simultaneously record a large number of spectral lines. By combining the spectral and polarization information of many individual lines with different formation heights and sensitivities, the accuracy and the height resolution of the inferred atmospheric parameters can be significantly increased. The spectral bands of SUSI are selected one at a time by rotating a diffraction grating with respect to a fixed polarimetry unit. The spatial and spectral field of view on the 2k x 2k cameras is 59" and 2.0 - 2.3 nm, respectively. A further innovation is the numerical restoration of the spectrograph scans by means of synchronized 2D context imaging, a technique that has recently produced impressive results at ground-based solar observatories. Title: The Solar-C (EUVST) mission: the latest status Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko; Suematsu, Yoshinori; Hara, Hirohisa; Tsuzuki, Toshihiro; Katsukawa, Yukio; Kubo, Masahito; Ishikawa, Ryoko; Watanabe, Tetsuya; Toriumi, Shin; Ichimoto, Kiyoshi; Nagata, Shin'ichi; Hasegawa, Takahiro; Yokoyama, Takaaki; Watanabe, Kyoko; Tsuno, Katsuhiko; Korendyke, Clarence M.; Warren, Harry; De Pontieu, Bart; Boerner, Paul; Solanki, Sami K.; Teriaca, Luca; Schuehle, Udo; Matthews, Sarah; Long, David; Thomas, William; Hancock, Barry; Reid, Hamish; Fludra, Andrzej; Auchère, Frederic; Andretta, Vincenzo; Naletto, Giampiero; Poletto, Luca; Harra, Louise Bibcode: 2020SPIE11444E..0NS Altcode: Solar-C (EUVST) is the next Japanese solar physics mission to be developed with significant contributions from US and European countries. The mission carries an EUV imaging spectrometer with slit-jaw imaging system called EUVST (EUV High-Throughput Spectroscopic Telescope) as the mission payload, to take a fundamental step towards answering how the plasma universe is created and evolves and how the Sun influences the Earth and other planets in our solar system. In April 2020, ISAS (Institute of Space and Astronautical Science) of JAXA (Japan Aerospace Exploration Agency) has made the final down-selection for this mission as the 4th in the series of competitively chosen M-class mission to be launched with an Epsilon launch vehicle in mid 2020s. NASA (National Aeronautics and Space Administration) has selected this mission concept for Phase A concept study in September 2019 and is in the process leading to final selection. For European countries, the team has (or is in the process of confirming) confirmed endorsement for hardware contributions to the EUVST from the national agencies. A recent update to the mission instrumentation is to add a UV spectral irradiance monitor capability for EUVST calibration and scientific purpose. This presentation provides the latest status of the mission with an overall description of the mission concept emphasizing on key roles of the mission in heliophysics research from mid 2020s. Title: Sunrise Chromospheric Infrared SpectroPolarimeter (SCIP) for sunrise III: system design and capability Authors: Katsukawa, Y.; del Toro Iniesta, J. C.; Solanki, S. K.; Kubo, M.; Hara, H.; Shimizu, T.; Oba, T.; Kawabata, Y.; Tsuzuki, T.; Uraguchi, F.; Nodomi, Y.; Shinoda, K.; Tamura, T.; Suematsu, Y.; Ishikawa, R.; Kano, R.; Matsumoto, T.; Ichimoto, K.; Nagata, S.; Quintero Noda, C.; Anan, T.; Orozco Suárez, D.; Balaguer Jiménez, M.; López Jiménez, A. C.; Cobos Carrascosa, J. P.; Feller, A.; Riethmueller, T.; Gandorfer, A.; Lagg, A. Bibcode: 2020SPIE11447E..0YK Altcode: The Sunrise balloon-borne solar observatory carries a 1 m aperture optical telescope and provides us a unique platform to conduct continuous seeing-free observations at UV-visible-IR wavelengths from an altitude of higher than 35 km. For the next flight planned for 2022, the post-focus instrumentation is upgraded with new spectro- polarimeters for the near UV (SUSI) and the near-IR (SCIP), whereas the imaging spectro-polarimeter Tunable Magnetograph (TuMag) is capable of observing multiple spectral lines within the visible wavelength. A new spectro-polarimeter called the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is under development for observing near-IR wavelength ranges of around 770 nm and 850 nm. These wavelength ranges contain many spectral lines sensitive to solar magnetic fields and SCIP will be able to obtain magnetic and velocity structures in the solar atmosphere with a sufficient height resolution by combining spectro-polarimetric data of these lines. Polarimetric measurements are conducted using a rotating waveplate as a modulator and polarizing beam splitters in front of the cameras. The spatial and spectral resolutions are 0.2" and 2 105, respectively, and a polarimetric sensitivity of 0.03 % (1σ) is achieved within a 10 s integration time. To detect minute polarization signals with good precision, we carefully designed the opto-mechanical system, polarization optics and modulation, and onboard data processing. Title: Reconstructing solar irradiance from Ca II K observations Authors: Chatzistergos, Theodosios; Krivova, Natalie A.; Ermolli, Ilaria; Leng Yeo, Kok; Solanki, Sami K.; Puiu, Cosmin C.; Giorgi, Fabrizio; Mandal, Sudip Bibcode: 2020AGUFM.A237..10C Altcode: To understand the influence of the Sun on Earth's system, long and accurate measurements of solar irradiance are a prerequisite. The available direct measurements of solar irradiance since 1978 are clearly not sufficient for this purpose. This stimulated development of models used to reconstruct past solar irradiance variations from alternative observations. The main driver of the irradiance variations on time scales of days to millennia is the evolution of the solar surface magnetic field in form of dark sunspots and bright faculae and network. Therefore, models require input data describing the contributions of these various magnetic regions on the Sun at earlier times. Unfortunately, records that can be used to describe the facular and network contributions are barely longer than the direct irradiance measurements. Thus, irradiance reconstructions to earlier periods have to rely on sunspot data alone. Data that have hardly been used for solar irradiance reconstructions until now are full-disc solar observations in the Ca II K line. Such data exist since 1892 from various observatories and include all the needed information describing faculae and the network. However, they are plagued by a bunch of various problems and artefacts, and recovering the non-linear response of the photographic material to the radiation is non-trivial since the required information is usually missing, too. We have developed a method to process Ca II K observations from various sources and demonstrated the higher accuracy achieved by our method compared to other techniques presented in the literature. Here we use the carefully reduced Ca II K observations from multiple archives to reconstruct solar irradiance variations. Title: Solar Orbiter: connecting remote sensing and in situ measurements Authors: Horbury, T. S.; Auchere, F.; Antonucci, E.; Berghmans, D.; Bruno, R.; Carlsson, M.; del Toro Iniesta, J. C.; Fludra, A.; Harra, L.; Hassler, D.; Heinzel, P.; Howard, R. A.; Krucker, S.; Livi, S. A.; Long, D.; Louarn, P.; Maksimovic, M.; Mueller, D.; Owen, C. J.; Peter, H.; Rochus, P. L.; Rodriguez-Pacheco, J.; Romoli, M.; Schühle, U.; Solanki, S. K.; Teriaca, L.; Wimmer-Schweingruber, R. F.; Zouganelis, Y.; Laker, R. Bibcode: 2020AGUFMSH038..10H Altcode: A key science goal of the Solar Orbiter mission is to make connections between phenomena on the Sun and their manifestations in interplanetary space. To that end, the spacecraft carries a carefully tailored payload of six remote sensing instruments and four making in situ measurements. During June 2020, while the spacecraft was around 0.5 AU from the Sun, the remote sensing instruments operated for several days. While this was primarily an engineering activity, the resulting observations provided outstanding measurements and represent the ideal first opportunity to investigate the potential for making connections between the remote sensing and in situ payloads on Solar Orbiter.

We present a preliminary analysis of the available remote sensing and in situ observations, showing how connections can be made, and discuss the potential for further, more precise mapping to be performed as the mission progresses. Title: Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere Authors: Chitta, L. P.; Peter, H.; Priest, E. R.; Solanki, S. K. Bibcode: 2020A&A...644A.130C Altcode: 2020arXiv201012560C Coronal plasma in the cores of solar active regions is impulsively heated to more than 5 MK. The nature and location of the magnetic energy source responsible for such impulsive heating is poorly understood. Using observations of seven active regions from the Solar Dynamics Observatory, we found that a majority of coronal loops hosting hot plasma have at least one footpoint rooted in regions of interacting mixed magnetic polarity at the solar surface. In cases when co-temporal observations from the Interface Region Imaging Spectrograph space mission are available, we found spectroscopic evidence for magnetic reconnection at the base of the hot coronal loops. Our analysis suggests that interactions of magnetic patches of opposite polarity at the solar surface and the associated energy release during reconnection are key to impulsive coronal heating.

Movies are available at https://www.aanda.org Title: How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations(Corrigendum) Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2020A&A...642C...2Y Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Faculae-Spot dominance & rotation periods (Amazo-Gomez+, 2020) Authors: Amazo-Gomez, E. M.; Shapiro, A. I.; Solanki, S. K.; Kopp, G.; Oshagh, M.; Reinhold, T.; Reiners, A. Bibcode: 2020yCat..36420225A Altcode: This table contains an example of the GPS outputs, the compared rotation period values from GLS and ACF, and stellar parameters for Kepler stars.

In column 4 and 5 values of alpha-factor and its 2-sigma uncertainty are reported respectively. Prot GPS values in column 6, as result of applying Eq. 1 using the factor alpha=0.19. 2) Column 7 shows the Prot reported by Reinhold & Gizon (2015, Cat. J/A+A/583/A65). 3) Prot and variability values reported by McQuillan et al. (2014, Cat. J/ApJS/211/24) in column 8. 4) Columns 10, 11 and 12 show the logg, [Fe/H], and Teff respectively, taken from Huber et al. (2014, Cat J/ApJS/211/2).

(1 data file). Title: Coordination within the remote sensing payload on the Solar Orbiter mission Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.; Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade, S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.; Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy, S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.; Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey, B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.; Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi, M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.; Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.; Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.; Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.; Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams, D.; Woch, J.; Zhukov, A. N.; Zouganelis, I. Bibcode: 2020A&A...642A...6A Altcode: Context. To meet the scientific objectives of the mission, the Solar Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing (RS) instruments designed for joint operations with inter-instrument communication capabilities. Indeed, previous missions have shown that the Sun (imaged by the RS instruments) and the heliosphere (mainly sampled by the IS instruments) should be considered as an integrated system rather than separate entities. Many of the advances expected from Solar Orbiter rely on this synergistic approach between IS and RS measurements.
Aims: Many aspects of hardware development, integration, testing, and operations are common to two or more RS instruments. In this paper, we describe the coordination effort initiated from the early mission phases by the Remote Sensing Working Group. We review the scientific goals and challenges, and give an overview of the technical solutions devised to successfully operate these instruments together.
Methods: A major constraint for the RS instruments is the limited telemetry (TM) bandwidth of the Solar Orbiter deep-space mission compared to missions in Earth orbit. Hence, many of the strategies developed to maximise the scientific return from these instruments revolve around the optimisation of TM usage, relying for example on onboard autonomy for data processing, compression, and selection for downlink. The planning process itself has been optimised to alleviate the dynamic nature of the targets, and an inter-instrument communication scheme has been implemented which can be used to autonomously alter the observing modes. We also outline the plans for in-flight cross-calibration, which will be essential to the joint data reduction and analysis.
Results: The RS instrument package on Solar Orbiter will carry out comprehensive measurements from the solar interior to the inner heliosphere. Thanks to the close coordination between the instrument teams and the European Space Agency, several challenges specific to the RS suite were identified and addressed in a timely manner. Title: Models and data analysis tools for the Solar Orbiter mission Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.; Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.; Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.; Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi, N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla, T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.; Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.; Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.; Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.; Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot, V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.; Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.; Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp, E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.; Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.; Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti, S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.; Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez, L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca, L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.; Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov, A. N. Bibcode: 2020A&A...642A...2R Altcode: Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in situ (IS) instruments to record novel and unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional (3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk.
Aims: The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science. We describe the results of a community-led effort by European Space Agency's Modelling and Data Analysis Working Group (MADAWG) to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully, many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to various photospheric and coronal features in support of spacecraft operations and future scientific studies.
Methods: Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter.
Results: This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation of Solar Orbiter data in order to maximise the scientific output of the mission.
Conclusions: The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our procedure and methodology during the first year of operations of this highly promising mission. Title: The Dimmest State of the Sun Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A.; Rempel, M.; Anusha, L. S.; Shapiro, A. I.; Tagirov, R. V.; Witzke, V. Bibcode: 2020GeoRL..4790243Y Altcode: 2021arXiv210209487Y How the solar electromagnetic energy entering the Earth's atmosphere varied since preindustrial times is an important consideration in the climate change debate. Detrimental to this debate, estimates of the change in total solar irradiance (TSI) since the Maunder minimum, an extended period of weak solar activity preceding the industrial revolution, differ markedly, ranging from a drop of 0.75 W m-2 to a rise of 6.3 W m-2. Consequently, the exact contribution by solar forcing to the rise in global temperatures over the past centuries remains inconclusive. Adopting a novel approach based on state-of-the-art solar imagery and numerical simulations, we establish the TSI level of the Sun when it is in its least-active state to be 2.0 ± 0.7 W m-2 below the 2019 level. This means TSI could not have risen since the Maunder minimum by more than this amount, thus restricting the possible role of solar forcing in global warming. Title: The Solar Orbiter Science Activity Plan. Translating solar and heliospheric physics questions into action Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.; Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.; Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.; Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.; Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio, L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun, A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso, F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.; Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.; Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.; van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi, L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine, D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot, S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham, G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier, K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins, J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis, I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.; Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis, G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.; Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.; Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis, K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien, H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.; Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.; Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines, J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.; Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.; Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.; Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.; Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.; Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula, G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio, A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.; Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann, T.; Young, P. R.; Zhukov, A. N. Bibcode: 2020A&A...642A...3Z Altcode: 2020arXiv200910772Z Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed. Title: Inflection point in the power spectrum of stellar brightness variations. III. Facular versus spot dominance on stars with known rotation periods Authors: Amazo-Gómez, E. M.; Shapiro, A. I.; Solanki, S. K.; Kopp, G.; Oshagh, M.; Reinhold, T.; Reiners, A. Bibcode: 2020A&A...642A.225A Altcode: 2020arXiv200811492A Context. Stellar rotation periods can be determined by observing brightness variations caused by active magnetic regions transiting visible stellar disk as the star rotates. Successful stellar photometric surveys stemming from the Kepler and TESS observations have led to the determination of rotation periods in tens of thousands of young and active stars. However, there is still a lack of information on the rotation periods of older and less active stars like the Sun. The irregular temporal profiles of light curves caused by the decay times of active regions, which are comparable to, or even shorter than, stellar rotation periods, in combination with the random emergence of active regions make period determination for such stars very difficult.
Aims: We tested the performance of a new method for the determination of stellar rotation periods against stars with previously determined rotation periods. The method is based on calculating the gradient of the power spectrum (GPS) and identifying the position of the inflection point (i.e. point with the highest gradient). The GPS method is specifically aimed at determining rotation periods of low-activity stars like the Sun.
Methods: We applied the GPS method to 1047 Sun-like stars observed by the Kepler telescope. We considered two stellar samples individually: one with near-solar rotation periods (24-27.4 d) and a broad range of effective temperatures (5000-6000 K) and the other with near-solar effective temperatures (5700-5900 K) and a broad range of rotation periods (15-40 d).
Results: We show that the GPS method returns precise values for stellar rotation periods. Furthermore, it allows us to constrain the ratio between facular and spot areas of active regions at the moment of their emergence. We also show that the relative facular area decreases with the stellar rotation rate.
Conclusions: Our results suggest that the GPS method can be successfully applied to retrieve the periods of stars with both regular and non-regular light curves.

Full Table 2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A225 Title: Autonomous on-board data processing and instrument calibration software for the Polarimetric and Helioseismic Imager on-board the Solar Orbiter mission Authors: Albert, Kinga; Hirzberger, Johann; Kolleck, Martin; Jorge, Nestor Albelo; Busse, Dennis; Rodríguez, Julian Blanco; Carrascosa, Juan Pedro Cobos; Fiethe, Björn; Gandorfer, Achim; Germerott, Dietmar; Guan, Yejun; Guerrero, Lucas; Gutierrez-Marques, Pablo; Expósito, David Hernández; Lange, Tobias; Michalik, Harald; Suárez, David Orozco; Schou, Jesper; Solanki, Sami K.; del Toro Iniesta, José Carlos; Woch, Joachim Bibcode: 2020JATIS...6d8004A Altcode: A frequent problem arising for deep space missions is the discrepancy between the amount of data desired to be transmitted to the ground and the available telemetry bandwidth. A part of these data consists of scientific observations, being complemented by calibration data to help remove instrumental effects. We present our solution for this discrepancy, implemented for the Polarimetric and Helioseismic Imager on-board the Solar Orbiter mission, the first solar spectropolarimeter in deep space. We implemented an on-board data reduction system that processes calibration data, applies them to the raw science observables, and derives science-ready physical parameters. This process reduces the raw data for a single measurement from 24 images to five, thus reducing the amount of downlinked data, and in addition, renders the transmission of the calibration data unnecessary. Both these on-board actions are completed autonomously. Title: The Polarimetric and Helioseismic Imager on Solar Orbiter Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer, A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.; Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.; Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.; Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.; Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott, D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.; Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.; Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.; Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.; Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.; Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.; Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila, B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.; Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.; Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond, J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h, J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.; Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell, A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai, E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa, J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas, A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.; Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel, H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I. Bibcode: 2020A&A...642A..11S Altcode: 2019arXiv190311061S
Aims: This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.
Methods: SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I 617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.
Results: SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission's highly elliptical orbit. Title: The Solar Orbiter SPICE instrument. An extreme UV imaging spectrometer Authors: SPICE Consortium; Anderson, M.; Appourchaux, T.; Auchère, F.; Aznar Cuadrado, R.; Barbay, J.; Baudin, F.; Beardsley, S.; Bocchialini, K.; Borgo, B.; Bruzzi, D.; Buchlin, E.; Burton, G.; Büchel, V.; Caldwell, M.; Caminade, S.; Carlsson, M.; Curdt, W.; Davenne, J.; Davila, J.; Deforest, C. E.; Del Zanna, G.; Drummond, D.; Dubau, J.; Dumesnil, C.; Dunn, G.; Eccleston, P.; Fludra, A.; Fredvik, T.; Gabriel, A.; Giunta, A.; Gottwald, A.; Griffin, D.; Grundy, T.; Guest, S.; Gyo, M.; Haberreiter, M.; Hansteen, V.; Harrison, R.; Hassler, D. M.; Haugan, S. V. H.; Howe, C.; Janvier, M.; Klein, R.; Koller, S.; Kucera, T. A.; Kouliche, D.; Marsch, E.; Marshall, A.; Marshall, G.; Matthews, S. A.; McQuirk, C.; Meining, S.; Mercier, C.; Morris, N.; Morse, T.; Munro, G.; Parenti, S.; Pastor-Santos, C.; Peter, H.; Pfiffner, D.; Phelan, P.; Philippon, A.; Richards, A.; Rogers, K.; Sawyer, C.; Schlatter, P.; Schmutz, W.; Schühle, U.; Shaughnessy, B.; Sidher, S.; Solanki, S. K.; Speight, R.; Spescha, M.; Szwec, N.; Tamiatto, C.; Teriaca, L.; Thompson, W.; Tosh, I.; Tustain, S.; Vial, J. -C.; Walls, B.; Waltham, N.; Wimmer-Schweingruber, R.; Woodward, S.; Young, P.; de Groof, A.; Pacros, A.; Williams, D.; Müller, D. Bibcode: 2020A&A...642A..14S Altcode: 2019arXiv190901183A; 2019arXiv190901183S
Aims: The Spectral Imaging of the Coronal Environment (SPICE) instrument is a high-resolution imaging spectrometer operating at extreme ultraviolet wavelengths. In this paper, we present the concept, design, and pre-launch performance of this facility instrument on the ESA/NASA Solar Orbiter mission.
Methods: The goal of this paper is to give prospective users a better understanding of the possible types of observations, the data acquisition, and the sources that contribute to the instrument's signal.
Results: The paper discusses the science objectives, with a focus on the SPICE-specific aspects, before presenting the instrument's design, including optical, mechanical, thermal, and electronics aspects. This is followed by a characterisation and calibration of the instrument's performance. The paper concludes with descriptions of the operations concept and data processing.
Conclusions: The performance measurements of the various instrument parameters meet the requirements derived from the mission's science objectives. The SPICE instrument is ready to perform measurements that will provide vital contributions to the scientific success of the Solar Orbiter mission. Title: Understanding the origins of the heliosphere: integrating observations and measurements from Parker Solar Probe, Solar Orbiter, and other space- and ground-based observatories Authors: Velli, M.; Harra, L. K.; Vourlidas, A.; Schwadron, N.; Panasenco, O.; Liewer, P. C.; Müller, D.; Zouganelis, I.; St Cyr, O. C.; Gilbert, H.; Nieves-Chinchilla, T.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Wimmer-Schweingruber, R. F.; Bale, S.; Kasper, J.; McComas, D. J.; Raouafi, N.; Martinez-Pillet, V.; Walsh, A. P.; De Groof, A.; Williams, D. Bibcode: 2020A&A...642A...4V Altcode: Context. The launch of Parker Solar Probe (PSP) in 2018, followed by Solar Orbiter (SO) in February 2020, has opened a new window in the exploration of solar magnetic activity and the origin of the heliosphere. These missions, together with other space observatories dedicated to solar observations, such as the Solar Dynamics Observatory, Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations from WIND and ACE, and ground based multi-wavelength observations including the DKIST observatory that has just seen first light, promise to revolutionize our understanding of the solar atmosphere and of solar activity, from the generation and emergence of the Sun's magnetic field to the creation of the solar wind and the acceleration of solar energetic particles.
Aims: Here we describe the scientific objectives of the PSP and SO missions, and highlight the potential for discovery arising from synergistic observations. Here we put particular emphasis on how the combined remote sensing and in situ observations of SO, that bracket the outer coronal and inner heliospheric observations by PSP, may provide a reconstruction of the solar wind and magnetic field expansion from the Sun out to beyond the orbit of Mercury in the first phases of the mission. In the later, out-of-ecliptic portions of the SO mission, the solar surface magnetic field measurements from SO and the multi-point white-light observations from both PSP and SO will shed light on the dynamic, intermittent solar wind escaping from helmet streamers, pseudo-streamers, and the confined coronal plasma, and on solar energetic particle transport.
Methods: Joint measurements during PSP-SO alignments, and magnetic connections along the same flux tube complemented by alignments with Earth, dual PSP-Earth, and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will allow a better understanding of the in situ evolution of solar-wind plasma flows and the full three-dimensional distribution of the solar wind from a purely observational point of view. Spectroscopic observations of the corona, and optical and radio observations, combined with direct in situ observations of the accelerating solar wind will provide a new foundation for understanding the fundamental physical processes leading to the energy transformations from solar photospheric flows and magnetic fields into the hot coronal plasma and magnetic fields and finally into the bulk kinetic energy of the solar wind and solar energetic particles.
Results: We discuss the initial PSP observations, which already provide a compelling rationale for new measurement campaigns by SO, along with ground- and space-based assets within the synergistic context described above. Title: The Solar Orbiter mission. Science overview Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.; Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.; Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.; Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.; Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.; Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D. Bibcode: 2020A&A...642A...1M Altcode: 2020arXiv200900861M
Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.
Methods: The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.
Results: Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission's science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.

ARRAY(0x207ce98) Title: The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager Authors: Rochus, P.; Auchère, F.; Berghmans, D.; Harra, L.; Schmutz, W.; Schühle, U.; Addison, P.; Appourchaux, T.; Aznar Cuadrado, R.; Baker, D.; Barbay, J.; Bates, D.; BenMoussa, A.; Bergmann, M.; Beurthe, C.; Borgo, B.; Bonte, K.; Bouzit, M.; Bradley, L.; Büchel, V.; Buchlin, E.; Büchner, J.; Cabé, F.; Cadiergues, L.; Chaigneau, M.; Chares, B.; Choque Cortez, C.; Coker, P.; Condamin, M.; Coumar, S.; Curdt, W.; Cutler, J.; Davies, D.; Davison, G.; Defise, J. -M.; Del Zanna, G.; Delmotte, F.; Delouille, V.; Dolla, L.; Dumesnil, C.; Dürig, F.; Enge, R.; François, S.; Fourmond, J. -J.; Gillis, J. -M.; Giordanengo, B.; Gissot, S.; Green, L. M.; Guerreiro, N.; Guilbaud, A.; Gyo, M.; Haberreiter, M.; Hafiz, A.; Hailey, M.; Halain, J. -P.; Hansotte, J.; Hecquet, C.; Heerlein, K.; Hellin, M. -L.; Hemsley, S.; Hermans, A.; Hervier, V.; Hochedez, J. -F.; Houbrechts, Y.; Ihsan, K.; Jacques, L.; Jérôme, A.; Jones, J.; Kahle, M.; Kennedy, T.; Klaproth, M.; Kolleck, M.; Koller, S.; Kotsialos, E.; Kraaikamp, E.; Langer, P.; Lawrenson, A.; Le Clech', J. -C.; Lenaerts, C.; Liebecq, S.; Linder, D.; Long, D. M.; Mampaey, B.; Markiewicz-Innes, D.; Marquet, B.; Marsch, E.; Matthews, S.; Mazy, E.; Mazzoli, A.; Meining, S.; Meltchakov, E.; Mercier, R.; Meyer, S.; Monecke, M.; Monfort, F.; Morinaud, G.; Moron, F.; Mountney, L.; Müller, R.; Nicula, B.; Parenti, S.; Peter, H.; Pfiffner, D.; Philippon, A.; Phillips, I.; Plesseria, J. -Y.; Pylyser, E.; Rabecki, F.; Ravet-Krill, M. -F.; Rebellato, J.; Renotte, E.; Rodriguez, L.; Roose, S.; Rosin, J.; Rossi, L.; Roth, P.; Rouesnel, F.; Roulliay, M.; Rousseau, A.; Ruane, K.; Scanlan, J.; Schlatter, P.; Seaton, D. B.; Silliman, K.; Smit, S.; Smith, P. J.; Solanki, S. K.; Spescha, M.; Spencer, A.; Stegen, K.; Stockman, Y.; Szwec, N.; Tamiatto, C.; Tandy, J.; Teriaca, L.; Theobald, C.; Tychon, I.; van Driel-Gesztelyi, L.; Verbeeck, C.; Vial, J. -C.; Werner, S.; West, M. J.; Westwood, D.; Wiegelmann, T.; Willis, G.; Winter, B.; Zerr, A.; Zhang, X.; Zhukov, A. N. Bibcode: 2020A&A...642A...8R Altcode: Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the "connection science" between solar activity and the heliosphere.
Aims: With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives.
Methods: The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft's heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor.
Results: In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products.
Conclusions: The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs. Title: PMI: The Photospheric Magnetic Field Imager Authors: Staub, Jan; Fernandez-Rico, German; Gandorfer, Achim; Gizon, Laurent; Hirzberger, Johann; Kraft, Stefan; Lagg, Andreas; Schou, Jesper; Solanki, Sami K.; del Toro Iniesta, Jose Carlos; Wiegelmann, Thomas; Woch, Joachim Bibcode: 2020JSWSC..10...54S Altcode: We describe the design and the capabilities of the Photospheric Magnetic field Imager (PMI), a compact and lightweight vector magnetograph, which is being developed for ESA's Lagrange mission to the Lagrange L5 point. After listing the design requirements and give a scientific justification for them, we describe the technical implementation and the design solution capable of fulfilling these requirements. This is followed by a description of the hardware architecture as well as the operations principle. An outlook on the expected performance concludes the paper. Title: Metis: the Solar Orbiter visible light and ultraviolet coronal imager Authors: Antonucci, Ester; Romoli, Marco; Andretta, Vincenzo; Fineschi, Silvano; Heinzel, Petr; Moses, J. Daniel; Naletto, Giampiero; Nicolini, Gianalfredo; Spadaro, Daniele; Teriaca, Luca; Berlicki, Arkadiusz; Capobianco, Gerardo; Crescenzio, Giuseppe; Da Deppo, Vania; Focardi, Mauro; Frassetto, Fabio; Heerlein, Klaus; Landini, Federico; Magli, Enrico; Marco Malvezzi, Andrea; Massone, Giuseppe; Melich, Radek; Nicolosi, Piergiorgio; Noci, Giancarlo; Pancrazzi, Maurizio; Pelizzo, Maria G.; Poletto, Luca; Sasso, Clementina; Schühle, Udo; Solanki, Sami K.; Strachan, Leonard; Susino, Roberto; Tondello, Giuseppe; Uslenghi, Michela; Woch, Joachim; Abbo, Lucia; Bemporad, Alessandro; Casti, Marta; Dolei, Sergio; Grimani, Catia; Messerotti, Mauro; Ricci, Marco; Straus, Thomas; Telloni, Daniele; Zuppella, Paola; Auchère, Frederic; Bruno, Roberto; Ciaravella, Angela; Corso, Alain J.; Alvarez Copano, Miguel; Aznar Cuadrado, Regina; D'Amicis, Raffaella; Enge, Reiner; Gravina, Alessio; Jejčič, Sonja; Lamy, Philippe; Lanzafame, Alessandro; Meierdierks, Thimo; Papagiannaki, Ioanna; Peter, Hardi; Fernandez Rico, German; Giday Sertsu, Mewael; Staub, Jan; Tsinganos, Kanaris; Velli, Marco; Ventura, Rita; Verroi, Enrico; Vial, Jean-Claude; Vives, Sebastien; Volpicelli, Antonio; Werner, Stephan; Zerr, Andreas; Negri, Barbara; Castronuovo, Marco; Gabrielli, Alessandro; Bertacin, Roberto; Carpentiero, Rita; Natalucci, Silvia; Marliani, Filippo; Cesa, Marco; Laget, Philippe; Morea, Danilo; Pieraccini, Stefano; Radaelli, Paolo; Sandri, Paolo; Sarra, Paolo; Cesare, Stefano; Del Forno, Felice; Massa, Ernesto; Montabone, Mauro; Mottini, Sergio; Quattropani, Daniele; Schillaci, Tiziano; Boccardo, Roberto; Brando, Rosario; Pandi, Arianna; Baietto, Cristian; Bertone, Riccardo; Alvarez-Herrero, Alberto; García Parejo, Pilar; Cebollero, María; Amoruso, Mauro; Centonze, Vito Bibcode: 2020A&A...642A..10A Altcode: 2019arXiv191108462A
Aims: Metis is the first solar coronagraph designed for a space mission and is capable of performing simultaneous imaging of the off-limb solar corona in both visible and UV light. The observations obtained with Metis aboard the Solar Orbiter ESA-NASA observatory will enable us to diagnose, with unprecedented temporal coverage and spatial resolution, the structures and dynamics of the full corona in a square field of view (FoV) of ±2.9° in width, with an inner circular FoV at 1.6°, thus spanning the solar atmosphere from 1.7 R⊙ to about 9 R⊙, owing to the eccentricity of the spacecraft orbit. Due to the uniqueness of the Solar Orbiter mission profile, Metis will be able to observe the solar corona from a close (0.28 AU, at the closest perihelion) vantage point, achieving increasing out-of-ecliptic views with the increase of the orbit inclination over time. Moreover, observations near perihelion, during the phase of lower rotational velocity of the solar surface relative to the spacecraft, allow longer-term studies of the off-limb coronal features, thus finally disentangling their intrinsic evolution from effects due to solar rotation.
Methods: Thanks to a novel occultation design and a combination of a UV interference coating of the mirrors and a spectral bandpass filter, Metis images the solar corona simultaneously in the visible light band, between 580 and 640 nm, and in the UV H I Lyman-α line at 121.6 nm. The visible light channel also includes a broadband polarimeter able to observe the linearly polarised component of the K corona. The coronal images in both the UV H I Lyman-α and polarised visible light are obtained at high spatial resolution with a spatial scale down to about 2000 km and 15 000 km at perihelion, in the cases of the visible and UV light, respectively. A temporal resolution down to 1 s can be achieved when observing coronal fluctuations in visible light.
Results: The Metis measurements, obtained from different latitudes, will allow for complete characterisation of the main physical parameters and dynamics of the electron and neutral hydrogen/proton plasma components of the corona in the region where the solar wind undergoes the acceleration process and where the onset and initial propagation of coronal mass ejections (CMEs) take place. The near-Sun multi-wavelength coronal imaging performed with Metis, combined with the unique opportunities offered by the Solar Orbiter mission, can effectively address crucial issues of solar physics such as: the origin and heating/acceleration of the fast and slow solar wind streams; the origin, acceleration, and transport of the solar energetic particles; and the transient ejection of coronal mass and its evolution in the inner heliosphere, thus significantly improving our understanding of the region connecting the Sun to the heliosphere and of the processes generating and driving the solar wind and coronal mass ejections.
Conclusions: This paper presents the scientific objectives and requirements, the overall optical design of the Metis instrument, the thermo-mechanical design, and the processing and power unit; reports on the results of the campaigns dedicated to integration, alignment, and tests, and to the characterisation of the instrument performance; describes the operation concept, data handling, and software tools; and, finally, the diagnostic techniques to be applied to the data, as well as a brief description of the expected scientific products. The performance of the instrument measured during calibrations ensures that the scientific objectives of Metis can be pursued with success.

Metis website: http://metis.oato.inaf.it Title: Simulating Variability due to Faculae and Spots on GKM Stars Authors: Johnson, Luke; Unruh, Yvonne; Norris, Charlotte; Solanki, Sami; Krivova, Natalie; Witzke, Veronika; Shapiro, Alexander Bibcode: 2020EPSC...14..844J Altcode: Stellar variability is a dominant noise source in exoplanet surveys and results largely from the presence of photospheric faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We model the lightcurves of active late-type stars as they rotate, using emergent intensity spectra calculated from 3D magnetoconvection simulations of G, K and M-type stellar atmosphere regions at different viewing angles to reproduce centre-to-limb brightness variations. We present mean expected variability levels for several cases and compare with solar and stellar observations. We also investigate the wavelength dependence of variability. Fig. 1: Example of our geometrically accurate lightcurve modelling approach. Top: normalised intensity maps of a limb darkened, solar-type star viewed in the \textit{Kepler} band at rotational phase 0.5 with stellar inclinations 90 deg (left) and 30 deg (right). At 90 deg, the star is viewed equator-on. Middle: Corresponding lightcurves calculated at inclinations 90 deg (black line) and 30 deg (red line). Bottom: HealPix map representing the active stellar surface, cosine-scaled in latitude and flattened in longitude to resemble a solar synoptic map. The quiet photosphere is displayed in orange, facular regions are bright yellow and spot regions are dark blue. The crosses represent the centres of the stellar discs in the top panel. Fig. 2: Example showing simulated lightcurves calculated at different wavelengths. Rotational lightcurves are on the left, transit lightcurves on the right. In the centre, one hemisphere of the simulated stellar surface is shown, with a quarter of the disc shown in each wavelength band. 'Giant' spots and facular regions are used in this example. The transit path is highlighted in grey. Title: Amplification of Brightness Variability by Active-region Nesting in Solar-like Stars Authors: Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2020ApJ...901L..12I Altcode: 2020arXiv200900692I Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behavior of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR properties and differential rotation. We show that a moderate increase of the emergence frequency from the solar value combined with the increase of the degree of nesting can explain the full range of observed amplitudes of variability of Sun-like stars with nearly the solar rotation period. Furthermore, nesting in the form of active longitudes, in which ARs tend to emerge in the vicinity of two longitudes separated by 180°, leads to highly regular, almost sine-like variability patterns, rather similar to those observed in a number of solar-like stars. Title: Simulations Show that Vortex Flows Could Heat the Chromosphere in Solar Plage Authors: Yadav, N.; Cameron, R.; Solanki, S. Bibcode: 2020SPD....5120107Y Altcode: Recent advances in both, observational techniques and numerical simulations have enabled us to detect small-scale vortices in the solar atmosphere. Vortices are ubiquitous throughout the solar surface and at all layers of the solar atmosphere existing over a wide range of spatial and temporal scales. Small-scale vortices are suggested to play an important role in the energy transport of the solar atmosphere, however, their physical properties remain poorly understood due to limited resolution. We explored the relationship between vortex flows at different spatial scales, analyze their physical properties, and investigate their contribution to Poynting flux transport. Using three-dimensional (3D) radiative magnetohydrodynamic (MHD)simulation code 'MURaM', we perform numerical simulations of a unipolar solar plage region. For detecting and isolating vortices, we use the 'Swirling Strength' criterion. We explore the spatial profiles of physical quantities viz. density, horizontal velocity, etc. inside these vortices. Moreover, to apprehend their general characteristics, a statistical investigation is performed. We found that magnetic flux tubes have a complex filamentary substructure abundant of small-scale vortices. On their interfaces strong current sheets are formed that may dissipate and heat the solar chromosphere. Statistically, vortices have higher densities and higher temperatures than the average values at the same geometrical height. We also degrade our simulation data to get an effective spatial resolution of 50 km, 100 km, 250 km, and 500 km, respectively. Analyzing simulation data at different effective resolutions, we found vortex flows existing over various spatial scales. In high-resolution simulation data, we detect a large number of small-scale vortices. Whereas, in the degraded data with relatively poor resolutions, smaller vortices are averaged-out and larger vortices are detected. The Poynting flux over vortex locations is more than adequate to compensate for the radiative losses in the chromosphere indicating their possible role in the chromospheric heating. Title: A Coronal Loop in a Box: Energy Generation, Heating and Dynamics Authors: Breu, C.; Peter, H.; Cameron, R.; Solanki, S.; Chitta, P.; Przybylski, D. Bibcode: 2020SPD....5121008B Altcode: In our study we aim at an understanding how the energy to heat the upper atmosphere is generated by the photospheric magneto-convection, transported into the upper atmosphere, and how its dissipation governs the formation of the internal structure of a coronal magnetic loop. In a 3D MHD model we study a coronal loop that is rooted with both footpoints in a shallow convection zone layer. Therefore the driving at the coronal base arises self-consistently from magneto-convection in plage-type areas. To fit into a cartesian box, we straighten the coronal loop. This allows a high spatial resolution within the loop that cannot be achieved in a model of a whole active region. To conduct the numerical experiments we employ the MURaM code that includes heat conduction, radiative transfer and optically thin radiative losses. We find that the Poynting flux into the loop is generated by small-scale photospheric motions within strong magnetic flux concentrations. Turbulent behaviour develops in the upper layers of the atmosphere as a response to the footpoint motions. Vortex flows are found at various heights within the loop. These are organised in swirls that form coherent structures with a magnetic connection from the intergranular lanes in the photosphere through the chromosphere up to several megameters into the corona. In the coronal part of the loop plasma motions perpendicular to the magnetic axis of the swirl are associated with an increased heating rate and thus enhanced temperatures. At any given time, only part of the loop is filled with swirls which leads to a substructure of the loop in terms of temperature and density. Consequently the emission as it would be observed by AIA or XRT reveals transient bright strands that form in response to the heating events related to the swirls. With this model we can build a coherent picture of how the energy flux to heat the upper atmosphere is generated near the solar surface and how this process drives and governs the heating and dynamics of a coronal loop Title: Effects of inclusion of small-scale dynamo in near-surface structure of F-stars Authors: Bhatia, T. S.; Cameron, R.; Solanki, S.; Peter, H.; Przbylski, D.; Witzke, V. Bibcode: 2020SPD....5120704B Altcode: The presence of (unresolved) small-scale mixed polarity regions in the quiet Sun photosphere plays an important role in determining the basal magnetic flux. Observationally, the magnitude of the vertical component of this field is estimated to be ~50-100 G on the Sun. This field is important for determining the energy balance in the chromosphere and may also subtly affect the radiative properties of the photosphere. These fields are believed to be the result of a small-scale dynamo (SSD) operating near the surface. While significant progress has been made in investigating the role of the SSD in the Sun, it is unclear what effects SSDs have on other stars. In particular, for F-stars, the photosheric kinetic and internal energies seem to be of the same order of magnitude. Since there is a rough equipartition in energies for a saturated SSD, deviations from a pure hydrodynamic (HD) stratification are expected. We aim to characterize these deviations. Box simulations of the upper convection zone and the photosphere are carried out using the radiative MHD code MURaM. To obtain SSD simulations, we use initial HD simulations and seed a magnetic field of negligible strength and zero net flux, which we then run till the magnetic field reaches saturation. We consider two different lower boundary conditions (BCs) for the magnetic field to characterize BC-effects: a) only vertical magnetic field is allowed, b) both vertical and horizontal magnetic field is allowed. Both boundary conditions exhibit SSD action. We observe slight increase (fraction of a percent) in the horizontally-averaged temperature profile for both the cases. Other thermodynamic quantities exhibit deviations (~ a percent) depending on the boundary condition considered. In addition, the spatial power spectra of the bolometric intensity shows deviations from the corresponding HD (without magnetic field) run, implying larger power at smaller spatial scales for SSD case. The presence of a SSD results in a significant amount of "quiet"-star magnetic flux with associated changes in the stratification of the atmosphere and spatial distribution of the bolometric intensity. Title: Magnetohydrostatic modeling of AR11768 based on a SUNRISE/IMaX vector magnetogram Authors: Zhu, X.; Wiegelmann, T.; Solanki, S. K. Bibcode: 2020A&A...640A.103Z Altcode: 2020arXiv200514332Z Context. High-resolution magnetic field measurements are routinely only done in the solar photosphere. Higher layers, such as the chromosphere and corona, can be modeled by extrapolating these photospheric magnetic field vectors upward. In the solar corona, plasma forces can be neglected and the Lorentz force vanishes. This is not the case in the upper photosphere and chromosphere where magnetic and nonmagnetic forces are equally important. One way to deal with this problem is to compute the plasma and magnetic field self-consistently, in lowest order with a magnetohydrostatic (MHS) model. The non-force-free layer is rather thin and MHS models require high-resolution photospheric magnetic field measurements as the lower boundary condition.
Aims: We aim to derive the magnetic field, plasma pressure, and density of AR11768 by applying the newly developed extrapolation technique to the SUNRISE/IMaX data embedded in SDO/HMI magnetogram.
Methods: We used an optimization method for the MHS modeling. The initial conditions consist of a nonlinear force-free field (NLFFF) and a gravity-stratified atmosphere. During the optimization procedure, the magnetic field, plasma pressure, and density are computed self-consistently.
Results: In the non-force-free layer, which is spatially resolved by the new code, Lorentz forces are effectively balanced by the gas pressure gradient force and gravity force. The pressure and density are depleted in strong field regions, which is consistent with observations. Denser plasma, however, is also observed at some parts of the active region edges. In the chromosphere, the fibril-like plasma structures trace the magnetic field nicely. Bright points in SUNRISE/SuFI 3000 Å images are often accompanied by the plasma pressure and electric current concentrations. In addition, the average of angle between MHS field lines and the selected chromospheric fibrils is 11.8°, which is smaller than those computed from the NLFFF model (15.7°) and linear MHS model (20.9°). This indicates that the MHS solution provides a better representation of the magnetic field in the chromosphere. Title: Sunspot area catalog revisited: Daily cross-calibrated areas since 1874 Authors: Mandal, Sudip; Krivova, Natalie A.; Solanki, Sami K.; Sinha, Nimesh; Banerjee, Dipankar Bibcode: 2020A&A...640A..78M Altcode: 2020arXiv200414618M Context. Long and consistent sunspot area records are important for understanding long-term solar activity and variability. Multiple observatories around the globe have regularly recorded sunspot areas, but such individual records only cover restricted periods of time. Furthermore, there are systematic differences between these records and require cross-calibration before they can reliably be used for further studies.
Aims: We produce a cross-calibrated and homogeneous record of total daily sunspot areas, both projected and corrected, covering the period between 1874 and 2019. In addition, we generated a catalog of calibrated individual group areas for the same period.
Methods: We compared the data from nine archives: Royal Greenwich Observatory (RGO), Kislovodsk, Pulkovo, Debrecen, Kodaikanal, Solar Optical Observing Network (SOON), Rome, Catania, and Yunnan Observatories, covering the period between 1874 and 2019. Cross-comparisons of the individual records were done to produce homogeneous and inter-calibrated records of daily projected and corrected areas. As in earlier studies, the basis of the composite is formed by the data from RGO. After 1976, the only datasets used are those from Kislovodsk, Pulkovo, and Debrecen observatories. This choice was made based on the temporal coverage and the quality of the data. While there are still 776 days missing in the final composite, these remaining gaps could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the additional archives cover these periods.
Results: In contrast to the SOON data used in previous area composites for the post-RGO period, the properties of the data from Kislovodsk and Pulkovo are very similar to those from the RGO series. They also directly overlap the RGO data in time, which makes their cross-calibration with RGO much more reliable. Indeed, comparing our area catalog with previous such composites, we find improvements both in data quality and coverage. We also computed the daily Photometric Sunspot Index, which is widely used, for example, in empirical reconstructions of solar irradiance.

Generated composites are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/A78 and at http://www2.mps.mpg.de/projects/sun-climate/data.html Title: Radiative MHD Simulations of Starspots Authors: Panja, M.; Cameron, R.; Solanki, S. Bibcode: 2020SPD....5121117P Altcode: We have performed the first-ever, realistic 3D simulations of the photospheric structure of complete starspots, including their penumbrae, for a range of cool main-sequence stars, namely the spectral types M0V, K0V, and G2V. We used the MHD code MURaM which includes radiative energy transfer and the effects of partial ionization. We explore several fundamental properties like umbral intensity contrast, temperature, and magnetic field strength as functions of spectral type. Our simulations show that there is an increase in spot contrast with the increase in stellar surface temperature, which is consistent with observations. The umbral field strength is determined by the depth at which the optical surface forms and the surface pressures of the host stars. We will present our results and discuss the physics behind them. Title: Analysis of full-disc Ca II K spectroheliograms. III. Plage area composite series covering 1892-2019 Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K.; Banerjee, Dipankar; Barata, Teresa; Belik, Marcel; Gafeira, Ricardo; Garcia, Adriana; Hanaoka, Yoichiro; Hegde, Manjunath; Klimeš, Jan; Korokhin, Viktor V.; Lourenço, Ana; Malherbe, Jean-Marie; Marchenko, Gennady P.; Peixinho, Nuno; Sakurai, Takashi; Tlatov, Andrey G. Bibcode: 2020A&A...639A..88C Altcode: 2020arXiv200501435C Context. Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. The archives of full-disc Ca II K observations that have been performed more or less regularly at various sites since 1892 can serve as an important source of such information.
Aims: We derive the plage area evolution over the last 12 solar cycles by employing data from all Ca II K archives that are publicly available in digital form, including several as-yet-unexplored Ca II K archives.
Methods: We analysed more than 290 000 full-disc Ca II K observations from 43 datasets spanning the period between 1892-2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts affecting many of the images, including some very specific artefacts, such as bright arcs found in Kyoto and Yerkes data. Our employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here.
Results: We produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. The 'backbone' approach was used to bridge the series together. We have also shown that the selection of the backbone series has little effect on the final composite of the plage area. We quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during the activity maxima.
Conclusions: We provide the most complete so far time series of plage areas based on corrected and calibrated historical and modern Ca II K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards.

The whole series described in the paper are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/639/A88 Title: No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...639A.106L Altcode: 2020arXiv200602346L Context. It has been reported that the boundary between the umbra and the penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted to be the threshold for the onset of magnetoconvection.
Aims: Here we investigate the reasons why this criterion, also called the Jurčák criterion in the literature, does not always identify the boundary between the umbra and the penumbra.
Methods: We performed a statistical analysis of 23 sunspots observed with Hinode/SOT. We compared the properties of the continuum intensity and the vertical magnetic field between filaments and spines and how they vary between spots of different sizes.
Results: We find that the inner boundary of the penumbra is not related to a universal value of the vertical magnetic field. The properties of spines and filaments vary between spots of different sizes. Both components are darker in larger spots and the spines exhibit a stronger vertical magnetic field. These variations of the properties of filaments and spines with the spot size are also the reason for the reported invariance in the averaged vertical magnetic field at 50% of the mean continuum intensity.
Conclusions: The formation of filaments and the onset of magnetoconvection are not related to a canonical value of the strength of the vertical magnetic field. The seemingly unique magnetic field strength is rather an effect of the filling factor of spines and penumbral filaments. Title: How faculae and network relate to sunspots, and the implications for solar and stellar brightness variations Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2020A&A...639A.139Y Altcode: 2020arXiv200614274Y Context. How global faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars.
Aims: We aim to extend and improve on earlier studies that established that the facular-to-sunspot-area ratio diminishes with total sunspot coverage.
Methods: Chromospheric indices and the total magnetic flux enclosed in network and faculae, referred to here as "facular indices", are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model, taking into account how active regions evolve and the possible non-linear relationship between plage emission, facular magnetic flux, and sunspot area. This model was incorporated into a model of total solar irradiance (TSI) to elucidate the implications for solar and stellar brightness variations.
Results: The reconstruction of the facular indices from the sunspot indices with the model presented here replicates most of the observed variability, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. This explains why even though solar cycle 23 is significantly weaker than cycle 22, TSI rose to comparable levels over both cycles. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition. This happens as sunspot darkening starts to rise more rapidly with activity than facular and network brightening. This bolsters the interpretation of this behaviour of Sun-like stars as the transition from a faculae-dominated to a spot-dominated regime. Title: Reply to the comment of T. Metcalfe and J. van Saders on the Science report "The Sun is less active than other solar-like stars" Authors: Reinhold, T.; Shapiro, A. I.; Solanki, S. K.; Montet, B. T.; Krivova, N. A.; Cameron, R. H.; Amazo-Gómez, E. M. Bibcode: 2020arXiv200704817R Altcode: This is our reply to the comment of T. Metcalfe and J. van Saders on the Science report "The Sun is less active than other solar-like stars" by T. Reinhold, A. I. Shapiro, S. K. Solanki, B. T. Montet, N. A. Krivova, R. H. Cameron, E. M. Amazo-Gomez. We hope that both the comment and our reply lead to fruitful discussions which of the two presented scenarios is more likely. Title: Connecting measurements of solar and stellar brightness variations Authors: Nèmec, N. -E.; Işık, E.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Unruh, Y. Bibcode: 2020A&A...638A..56N Altcode: 2020arXiv200406974N Context. A comparison of solar and stellar brightness variations is hampered by the difference in spectral passbands that are used in observations, and also by the possible difference in the inclination of the solar and stellar rotation axes from the line of sight.
Aims: We calculate the rotational variability of the Sun as it would be measured in passbands used for stellar observations. In particular, we consider the filter systems used by the CoRoT, Kepler, TESS, and Gaia space missions. We also quantify the effect of the inclination of the rotation axis on the solar rotational variability.
Methods: We employed the spectral and total irradiance reconstruction (SATIRE) model to calculate solar brightness variations in different filter systems as observed from the ecliptic plane. We then combined the simulations of the surface distribution of the magnetic features at different inclinations using a surface flux transport model with the SATIRE calculations to compute the dependence of the variability on the inclination.
Results: For an ecliptic-bound observer, the amplitude of the solar rotational variability, as observed in the total solar irradiance (TSI), is 0.68 mmag (averaged over solar cycles 21-24). We obtained corresponding amplitudes in the Kepler (0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag), Gaia GRP (0.62 mmag), and Gaia GBP (0.86 mmag) passbands. Decreasing the inclination of the rotation axis decreases the rotational variability. For a sample of randomly inclined stars, the variability is on average 15% lower in all filter systems we considered. This almost compensates for the difference in amplitudes of the variability in TSI and Kepler passbands, making the amplitudes derived from the TSI records an ideal representation of the solar rotational variability for comparison to Kepler stars with unknown inclinations.
Conclusions: The TSI appears to be a relatively good measure of solar variability for comparisons with stellar measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia GRP filters. Whereas the correction factors can be used to convert the variability amplitude from solar measurements into the values expected for stellar missions, the inclination affects the shapes of the light curves so that a much more sophisticated correction than simple scaling is needed to obtain light curves out of the ecliptic for the Sun. Title: On the Magnetic Nature of an Exploding Granule as Revealed by Sunrise/IMaX Authors: Guglielmino, Salvo L.; Martínez Pillet, Valentín; Ruiz Cobo, Basilio; Bellot Rubio, Luis R.; del Toro Iniesta, José Carlos; Solanki, Sami K.; Riethmüller, Tino L.; Zuccarello, Francesca Bibcode: 2020ApJ...896...62G Altcode: 2020arXiv200503371G We study the photospheric evolution of an exploding granule observed in the quiet Sun at high spatial (∼0"3) and temporal (31.5 s) resolution by the imaging magnetograph Sunrise/IMaX in 2009 June. These observations show that the exploding granule is cospatial to a magnetic flux emergence event occurring at mesogranular scale (up to ∼12 Mm2 area). Using a modified version of the SIR code for inverting the IMaX spectropolarimetric measurements, we obtain information about the magnetic configuration of this photospheric feature. In particular, we find evidence of highly inclined emerging fields in the structure, carrying a magnetic flux content up to ∼4 × 1018 Mx. The balance between gas and magnetic pressure in the region of flux emergence, compared with a very quiet region of the Sun, indicates that the additional pressure carried by the emerging flux increases the total pressure by about 5% and appears to allow the granulation to be modified, as predicted by numerical simulations. The overall characteristics suggest that a multipolar structure emerges into the photosphere, resembling an almost horizontal flux sheet. This seems to be associated with exploding granules. Finally, we discuss the origin of such flux emergence events. Title: Detection of the Strongest Magnetic Field in a Sunspot Light Bridge Authors: Castellanos Durán, J. S.; Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel Bibcode: 2020ApJ...895..129C Altcode: 2020arXiv200312078C; 2020ApJ...895..129D Traditionally, the strongest magnetic fields on the Sun have been measured in sunspot umbrae. More recently, however, much stronger fields have been measured at the ends of penumbral filaments carrying the Evershed and counter-Evershed flows. Superstrong fields have also been reported within a light bridge separating two umbrae of opposite polarities. We aim to accurately determine the strengths of the strongest fields in a light bridge using an advanced inversion technique and to investigate their detailed structure. We analyze observations from the spectropolarimeter on board the Hinode spacecraft of the active region AR 11967. The thermodynamic and magnetic configurations are obtained by inverting the Stokes profiles using an inversion scheme that allows multiple height nodes. Both the traditional 1D inversion technique and the so-called 2D coupled inversions, which take into account the point-spread function of the Hinode telescope, are used. We find a compact structure with an area of 32.7 arcsec2 within a bipolar light bridge with field strengths exceeding 5 kG, confirming the strong fields in this light bridge reported in the literature. Two regions associated with downflows of ∼5 km s-1 harbor field strengths larger than 6.5 kG, covering a total area of 2.97 arcsec2. The maximum field strength found is 8.2 kG, which is the largest ever observed field in a bipolar light bridge up to now. Title: Simulations Show that Vortex Flows Could Heat the Chromosphere in Solar Plage Authors: Yadav, Nitin; Cameron, R. H.; Solanki, S. K. Bibcode: 2020ApJ...894L..17Y Altcode: 2020arXiv200413996Y The relationship between vortex flows at different spatial scales and their contribution to the energy balance in the chromosphere is not yet fully understood. We perform three-dimensional (3D) radiation-magnetohydrodynamic simulations of a unipolar solar plage region at a spatial resolution of 10 km using the MURaM code. We use the swirling-strength criterion that mainly detects the smallest vortices present in the simulation data. We additionally degrade our simulation data to smooth out the smaller vortices, so that also the vortices at larger spatial scales can be detected. Vortex flows at various spatial scales are found in our simulation data for different effective spatial resolutions. We conclude that the observed large vortices are likely clusters of much smaller ones that are not yet resolved by observations. We show that the vertical Poynting flux decreases rapidly with reduced effective spatial resolutions and is predominantly carried by the horizontal plasma motions rather than vertical flows. Since the small-scale horizontal motions or the smaller vortices carry most of the energy, the energy transported by vortices deduced from low-resolution data is grossly underestimated. In full-resolution simulation data, the Poynting flux contribution due to vortices is more than adequate to compensate for the radiative losses in plage, indicating their importance for chromospheric heating. Title: Optical design of the multi-wavelength imaging coronagraph Metis for the solar orbiter mission Authors: Fineschi, S.; Naletto, G.; Romoli, M.; Da Deppo, V.; Antonucci, E.; Moses, D.; Malvezzi, A. M.; Nicolini, G.; Spadaro, D.; Teriaca, L.; Andretta, V.; Capobianco, G.; Crescenzio, G.; Focardi, M.; Frassetto, F.; Landini, F.; Massone, G.; Melich, R.; Nicolosi, P.; Pancrazzi, M.; Pelizzo, M. G.; Poletto, L.; Schühle, U.; Uslenghi, M.; Vives, S.; Solanki, S. K.; Heinzel, P.; Berlicki, A.; Cesare, S.; Morea, D.; Mottini, S.; Sandri, P.; Alvarez-Herrero, A.; Castronuovo, M. Bibcode: 2020ExA....49..239F Altcode: 2020ExA...tmp...14F This paper describes the innovative optical design of the Metis coronagraph for the Solar Orbiter ESA-NASA mission. Metis is a multi-wavelength, externally occulted telescope for the imaging of the solar corona in both the visible and ultraviolet wavelength ranges. Metis adopts a novel occultation scheme for the solar disk, that we named "inverse external occulter", for reducing the extremely high thermal load on the instrument at the spacecraft perihelion. The core of the Metis optical design is an aplanatic Gregorian telescope common to both the visible and ultraviolet channels. A suitable dichroic beam-splitter, optimized for transmitting a narrow-band in the ultraviolet (121.6 nm, HI Lyman-α) and reflecting a broadband in the visible (580-640 nm) spectral range, is used to separate the two optical paths. Along the visible light optical path, a liquid crystal electro-optical modulator, used for the first time in space, allows making polarimetric measurements. Title: Historical solar Ca II K observations at the Kyoto and Sacramento Peak observatories Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020JPhCS1548a2007C Altcode: Archives of full-disc solar Ca II K observations covering even short periods of time can offer important data to fill observation gaps and to allow a better transition from historical to modern data. Two examples of such archives are those from the Kyoto and Sacramento Peak observatories. As most other historical Ca II K data, they suffer from artefacts that need to be accounted for to derive accurate plage areas. Here we present the results of our analysis of these archives of solar Ca II K images. Title: The SO/PHI instrument on Solar Orbiter and its data products Authors: Solanki, Sami K.; Hirzberger, Johann; Wiegelmann, Thomas; Gandorfer, Achim; Woch, Joachim; del Toro Iniesta, José Carlos Bibcode: 2020EGUGA..2217904S Altcode: A central instrument of Solar Orbiter is the Polarimetric and Helioseismic Imager, SO/PHI. It is a vector magnetograph that also provides data for helioseismology. SO/PHI is composed of two telescopes, a full-disk telescope (FDT) and a high-resolution telescope (HRT). The HRT will observe at a resolution as high as 200 km on the solar surface, while the FDT will obtain the magnetic field and velocity of the full solar disc whenever it observes. SO/PHI will be the first solar spectro-polarimeter to leave the Sun-Earth line, opening up some unique perspectives, such as the first detailed view of the solar poles. This will allow not just a more precise and exact mapping of the polar magnetic field than possible so far, but will also enable us to follow the dynamics of individual magnetic features at high latitudes and to determine solar surface and sub-surface flows right up to the poles. In addition to its standard data products (vector magnetograms, continuum images and maps of the line-of-sight velocity), SO/PHI will also provide higher-level data products. These will include synoptic charts, local magnetic field extrapolations starting from HRT data and global magnetic field extrapolations (from FDT data) with potential field source-surface (PFSS) models and possibly also non-potential models such as NLFFF (non-linear force-free fields), magnetostatics and MHD. The SO/PHI data products will usefully complement the data taken by other instruments on Solar Orbiter and on Solar Probe, as well as instruments on the ground or in Earth orbit. Combining with observations by Earth-based and near-Earth telescopes will enable new types of investigations, such as stereoscopic polarimetry and stereoscopic helioseismology. Title: VizieR Online Data Catalog: Rotation periods of 97 solar-like stars (Witzke+, 2020) Authors: Witzke, V.; Reinhold, T.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020yCat..36349009W Altcode: The Kepler field of view was selected in order to contain a large fraction of solar-like stars. Focusing on stars in the effective temperature range of 5600K-5900K, it is challenging to determine their rotational periods.

Stellar fundamental parameters and rotation periods of the 97 periodic stars are presented.

(1 data file). Title: The Sun is less active than other solar-like stars Authors: Reinhold, Timo; Shapiro, Alexander I.; Solanki, Sami K.; Montet, Benjamin T.; Krivova, Natalie A.; Cameron, Robert H.; Amazo-Gómez, Eliana M. Bibcode: 2020Sci...368..518R Altcode: 2020arXiv200501401R The magnetic activity of the Sun and other stars causes their brightness to vary. We investigated how typical the Sun’s variability is compared with other solar-like stars, i.e., those with near-solar effective temperatures and rotation periods. By combining 4 years of photometric observations from the Kepler space telescope with astrometric data from the Gaia spacecraft, we were able to measure photometric variabilities of 369 solar-like stars. Most of those with well-determined rotation periods showed higher variability than the Sun and are therefore considerably more active. These stars appear nearly identical to the Sun except for their higher variability. Therefore, we speculate that the Sun could potentially also go through epochs of such high variability. Title: VizieR Online Data Catalog: Plage area composite series (Chatzistergos+, 2020) Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K.; Banerjee, D.; Barata, T.; Belik, M.; Gafeira, R.; Garcia, A.; Hanaoka, Y.; Hegde, M.; Klimes, J.; Korokhin, V. V.; Lourenco, A.; Malherbe, J. -M.; Marchenko, G. P.; Peixinho, N.; Sakurai, T.; Tlatov, A. G. Bibcode: 2020yCat..36390088C Altcode: Annual, monthly, and daily values of the plage area composite series.

The archives included are those from the Arcetri (Ar), Baikal (Ba), Brussels (Br), Calern (CL), Catania (CT), Coimbra (Co), Kanzelhoehe (Ka), Kenwood (Ke), Kharkiv (Kh), Kislovodsk (Ki), Kodaikanal taken with the spectroheliograph (Ko), Kodaikanal taken with the Twin telescope (KT), Kodaikanal taken with the White-Light Active Region Monitor (WARM) telescope (KW), Kyoto (Ky), Manila (Ma), Mauna Loa (ML) taken with the Precision Solar Photometric Telescope (PSPT), McMath-Hulbert (MM), Mees (MS), Meudon taken with the spectroheliograph (MD1), Meudon taken with an interference filter (MD2), Mitaka taken with the spectroheliograph (Mi1), Mitaka taken with the Solar Flare Telescope with an interference filter (Mi2), Mt Wilson (MW), Pic du Midi (PM), SOlar Diameter Imager and Surface Mapper (SODISM) telescope on board the PICARD spacecraft (PS), Rome taken with the equatorial bar at Monte Mario (Ro), Rome taken with the PSPT (RP1), Rome taken with the PSPT with narrow bandwidth (RP2), Sacramento Peak (SP), San Fernando taken with the Cartesian Full-Disk Telescope (CFDT) 1 (SF1), San Fernando taken with the CFDT2 (SF2), Schauinsland (Sc), Teide (Te) taken with the Chromospheric Telescope (ChroTel), Upice (UP), Valasske Mezirici (VM), Wendelstein (WS), and Yerkes (YR) sites

(3 data files). Title: Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line Authors: Zeuner, Franziska; Manso Sainz, Rafael; Feller, Alex; van Noort, Michiel; Solanki, Sami K.; Iglesias, Francisco A.; Reardon, Kevin; Martínez Pillet, Valentín Bibcode: 2020ApJ...893L..44Z Altcode: 2020arXiv200403679Z Magnetic fields in turbulent, convective high-β plasma naturally develop highly tangled and complex topologies - the solar photosphere being the paradigmatic example. These fields are mostly undetectable by standard diagnostic techniques with finite spatio-temporal resolution due to cancellations of Zeeman polarization signals. Observations of resonance scattering polarization have been considered to overcome these problems. But up to now, observations of scattering polarization lack the necessary combination of high sensitivity and high spatial resolution in order to directly infer the turbulent magnetic structure at the resolution limit of solar telescopes. Here, we report the detection of clear spatial structuring of scattering polarization in a magnetically quiet solar region at disk center in the Sr I 4607 Å spectral line on granular scales, confirming theoretical expectations. We find that the linear polarization presents a strong spatial correlation with the local quadrupole of the radiation field. The result indicates that polarization survives the dynamic and turbulent magnetic environment of the middle photosphere and is thereby usable for spatially resolved Hanle observations. This is an important step toward the long-sought goal of directly observing turbulent solar magnetic fields at the resolution limit and investigating their spatial structure. Title: 3D Radiative MHD Simulations of Starspots Authors: Panja, Mayukh; Cameron, Robert; Solanki, Sami K. Bibcode: 2020ApJ...893..113P Altcode: 2020arXiv200309656P There are no direct spatially resolved observations of spots on stars other than the Sun, and starspot properties are inferred indirectly through lightcurves and spectropolarimetric data. We present the first self-consistent 3D radiative MHD computations of starspots on G2V, K0V, and M0V stars, which will help us to better understand observations of activity, variability, and magnetic fields in late-type main-sequence stars. We used the MURaM code, which has been extensively used to compute "realistic" sunspots, for our simulations. We aim to study how fundamental starspot properties such as intensity contrast, temperature, and magnetic field strength vary with spectral type. We first simulated in 2D multiple spots of each spectral type to find out appropriate initial conditions for our 3D runs. We find that with increasing stellar effective temperature, there is an increase in the temperature difference between the umbra of the spot and its surrounding photosphere, from 350 K on the M0V star to 1400 K on the G2V star. This trend in our simulated starspots is consistent with observations. The magnetic field strengths of all the starspot umbrae are in the 3-4.5 kG range. The G2V and K0V umbrae have comparable magnetic field strengths around 3.5 kG, while the M0V umbra has a relatively higher field strength around 4 kG. We discuss the physical reasons behind both these trends. All of the three starspots develop penumbral filament-like structures with Evershed flows. The average Evershed flow speed drops from 1.32 km s-1 in the G2V penumbra to 0.6 km s-1 in the M0V penumbra. Title: Power spectra of solar brightness variations at various inclinations Authors: Nèmec, N. -E.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K.; Tagirov, R. V.; Cameron, R. H.; Dreizler, S. Bibcode: 2020A&A...636A..43N Altcode: 2020arXiv200210895N Context. Magnetic features on the surfaces of cool stars lead to variations in their brightness. Such variations on the surface of the Sun have been studied extensively. Recent planet-hunting space telescopes have made it possible to measure brightness variations in hundred thousands of other stars. The new data may undermine the validity of setting the sun as a typical example of a variable star. Putting solar variability into the stellar context suffers, however, from a bias resulting from solar observations being carried out from its near-equatorial plane, whereas stars are generally observed at all possible inclinations.
Aims: We model solar brightness variations at timescales from days to years as they would be observed at different inclinations. In particular, we consider the effect of the inclination on the power spectrum of solar brightness variations. The variations are calculated in several passbands that are routinely used for stellar measurements.
Methods: We employ the surface flux transport model to simulate the time-dependent spatial distribution of magnetic features on both the near and far sides of the Sun. This distribution is then used to calculate solar brightness variations following the Spectral And Total Irradiance REconstruction approach.
Results: We have quantified the effect of the inclination on solar brightness variability at timescales down to a single day. Thus, our results allow for solar brightness records to be made directly comparable to those obtained by planet-hunting space telescopes. Furthermore, we decompose solar brightness variations into components originating from the solar rotation and from the evolution of magnetic features. Title: Inflection point in the power spectrum of stellar brightness variations. II. The Sun Authors: Amazo-Gómez, E. M.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Kopp, G.; Reinhold, T.; Oshagh, M.; Reiners, A. Bibcode: 2020A&A...636A..69A Altcode: 2020arXiv200203455A Context. Young and active stars generally have regular, almost sinusoidal, patterns of variability attributed to their rotation, while the majority of older and less active stars, including the Sun, have more complex and non-regular light curves, which do not have clear rotational-modulation signals. Consequently, the rotation periods have been successfully determined only for a small fraction of the Sun-like stars (mainly the active ones) observed by transit-based planet-hunting missions, such as CoRoT, Kepler, and TESS. This suggests that only a small fraction of such systems have been properly identified as solar-like analogues.
Aims: We aim to apply a new method of determining rotation periods of low-activity stars, such as the Sun. The method is based on calculating the gradient of the power spectrum (GPS) of stellar brightness variations and identifying a tell-tale inflection point in the spectrum. The rotation frequency is then proportional to the frequency of that inflection point. In this paper, we compare this GPS method to already-available photometric records of the Sun.
Methods: We applied GPS, auto-correlation functions, Lomb-Scargle periodograms, and wavelet analyses to the total solar irradiance (TSI) time series obtained from the Total Irradiance Monitor on the Solar Radiation and Climate Experiment and the Variability of solar IRradiance and Gravity Oscillations experiment on the SOlar and Heliospheric Observatory missions. We analysed the performance of all methods at various levels of solar activity.
Results: We show that the GPS method returns accurate values of solar rotation independently of the level of solar activity. In particular, it performs well during periods of high solar activity, when TSI variability displays an irregular pattern, and other methods fail. Furthermore, we show that the GPS and light curve skewness can give constraints on facular and spot contributions to brightness variability.
Conclusions: Our results suggest that the GPS method can successfully determine the rotational periods of stars with both regular and non-regular light curves.

The two movies are available at https://www.aanda.org Title: Solar-cycle irradiance variations over the last four billion years Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2020A&A...636A..83S Altcode: 2020arXiv200208806S Context. The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is strong evidence that the SSI variability has an effect on the Earth's atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could lead to a stronger response of the Earth's atmosphere as well as other solar system planets' atmospheres to the solar activity cycle.
Aims: We calculate the amplitude of the SSI and total solar irradiance (TSI) variability over the course of the solar activity cycle as a function of solar age.
Methods: We employed the relationship between the stellar magnetic activity and the age based on observations of solar twins. Using this relation, we reconstructed solar magnetic activity and the corresponding solar disk area coverages by magnetic features (i.e., spots and faculae) over the last four billion years. These disk coverages were then used to calculate the amplitude of the solar-cycle SSI variability as a function of wavelength and solar age.
Results: Our calculations show that the young Sun was significantly more variable than the present Sun. The amplitude of the solar-cycle TSI variability of the 600 Myr old Sun was about ten times larger than that of the present Sun. Furthermore, the variability of the young Sun was spot-dominated (the Sun being brighter at the activity minimum than in the maximum), that is, the Sun was overall brighter at activity minima than at maxima. The amplitude of the TSI variability decreased with solar age until it reached a minimum value at 2.8 Gyr. After this point, the TSI variability is faculae-dominated (the Sun is brighter at the activity maximum) and its amplitude increases with age. Title: Solar Photosphere Authors: Chitta, L. P.; Smitha, H. N.; Solanki, S. K. Bibcode: 2020orep.bookE...1C Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Sunspot area catalogue revisited (1874-2019) (Mandal+, 2020) Authors: Mandal, S.; Krivova, N.; Solanki, S. K.; Sinha, N.; Banerjee, D. Bibcode: 2020yCat..36400078M Altcode: A number of observatories around the globe have carried out measurements of sunspot areas and positions over the last century. RGO, the longest sunspot area database to date, started its campaign in 1874 and after continuing for a century, stopped it in 1976. Several other observatories from different parts of the world (e.g., Kodaikanal, Kislovodsk, Debrecen, Rome etc.) also carried out such observing programs throughout the 20th century. Sunspot area datasets are invaluable historical records of solar magnetic fields and are key to understanding the solar variability and its historical reconstructions.

In this work, we analyze and compare sunspot group areas from a total of nine observatories (RGO, Kislovodsk, Pulkovo, Debrecen, Kodaikanal, SOON, Rome, Catania, Yunnan). It turns out that data from only four observatories (RGO, Kislovodsk, Pulkovo, Debrecen) are sufficient to produce cross-calibrated, up-to-date (1874-2019) catalogs of daily total and individual group areas. The remaining gaps (776 days in total) could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the other archives cover these periods.

(2 data files). Title: Connecting the Wilson depression to the magnetic field of sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...635A.202L Altcode: 2020arXiv200207484L Context. In sunspots, the geometric height of continuum optical depth unity is depressed compared to the quiet Sun. This so-called Wilson depression is caused by the Lorentz force of the strong magnetic field inside the spots. However, it is not understood in detail yet how the Wilson depression is related to the strength and geometry of the magnetic field or to other properties of the sunspot.
Aims: We aim to study the dependence of the Wilson depression on the properties of the magnetic field of the sunspots and how exactly the magnetic field contributes to balancing the Wilson depression with respect to the gas pressure of the surroundings of the spots.
Methods: Our study is based on 24 spectropolarimetric scans of 12 individual sunspots performed with Hinode. We derived the Wilson depression for each spot using both a recently developed method that is based on minimizing the divergence of the magnetic field and an approach that was developed earlier, which enforces an equilibrium between the gas pressure and the magnetic pressure inside the spot and the gas pressure in the quiet Sun, thus neglecting the influence of the curvature force. We then performed a statistical analysis by comparing the Wilson depression resulting from the two techniques with each other and by relating them to various parameters of the sunspots, such as their size or the strength of the magnetic field.
Results: We find that the Wilson depression becomes larger for spots with a stronger magnetic field, but not as much as one would expect from the increased magnetic pressure. This suggests that the curvature integral provides an important contribution to the Wilson depression, particularly for spots with a weak magnetic field. Our results indicate that the geometry of the magnetic field in the penumbra is different between spots with different strengths of the average umbral magnetic field. Title: Effect of metallicity on the detectability of rotational periods in solar-like stars Authors: Witzke, V.; Reinhold, T.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020A&A...634L...9W Altcode: 2020arXiv200101934W The accurate determination of stellar rotation periods is important for estimating stellar ages and for understanding stellar activity and evolution. While rotation periods can be determined for about thirty thousand stars in the Kepler field, there are over one hundred thousand stars, especially with low photometric variability and irregular pattern of variations, for which rotational periods are unknown. Here we investigate the effect of metallicity on the detectability of rotation periods. This is done by synthesising light curves of hypothetical stars that are identical to our Sun with the exception of the metallicity. These light curves are then used as an input to the period determination algorithms. We find that the success rate for recovering the rotation signal has a minimum close to the solar metallicity value. This can be explained by the compensation effect of facular and spot contributions. In addition, selecting solar-like stars with near-solar effective temperature and photometric variability, and with metallicity between M/H = -0.35 and M/H = 0.35 from the Kepler sample, we analyse the fraction of stars for which rotational periods have been detected as a function of metallicity. In agreement with our theoretical estimate we find a local minimum for the detection fraction close to the solar metallicity. We further report rotation periods of 87 solar-like Kepler stars for the first time. Title: The influence of NLTE effects in Fe I lines on an inverted atmosphere. I. 6301 Å and 6302 Å lines formed in 1D NLTE Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...633A.157S Altcode: 2019arXiv191207007S Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE.
Aims: We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (vLOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE.
Methods: We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects.
Results: Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles.
Conclusions: We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account. Title: Inflection point in the power spectrum of stellar brightness variations. I. The model Authors: Shapiro, A. I.; Amazo-Gómez, E. M.; Krivova, N. A.; Solanki, S. K. Bibcode: 2020A&A...633A..32S Altcode: 2019arXiv191008351S Context. Considerable effort has gone into using light curves observed by such space telescopes as CoRoT, Kepler, and TESS for determining stellar rotation periods. While rotation periods of active stars can be reliably determined, the light curves of many older and less active stars, such as stars that are similar to the Sun, are quite irregular. This hampers the determination of their rotation periods.
Aims: We aim to examine the factors causing these irregularities in stellar brightness variations and to develop a method for determining rotation periods for low-activity stars with irregular light curves.
Methods: We extended the Spectral And Total Irradiance Reconstruction approach for modeling solar brightness variations to Sun-like stars. We calculated the power spectra of stellar brightness variations for various combinations of parameters that define the surface configuration and evolution of stellar magnetic features.
Results: The short lifetime of spots in comparison to the stellar rotation period, as well as the interplay between spot and facular contributions to brightness variations of stars with near solar activity, cause irregularities in their light curves. The power spectra of such stars often lack a peak associated with the rotation period. Nevertheless, the rotation period can still be determined by measuring the period where the concavity of the power spectrum plotted in the log-log scale changes its sign, that is, by identifying the position of the inflection point.
Conclusions: The inflection point of the (log-log) power spectrum is found to be a new diagnostic for stellar rotation periods which is shown to work even in cases where the power spectrum shows no peak at the rotation rate. Title: Fast downflows in a chromospheric filament Authors: Sowmya, K.; Lagg, A.; Solanki, S. K.; Castellanos Durán, J. S. Bibcode: 2020IAUS..354..454S Altcode: 2019arXiv191206586S An active region filament in the upper chromosphere is studied using spectropolarimetric data in He i 10830 Å from the GREGOR telescope. A Milne-Eddingon based inversion of the Unno-Rachkovsky equations is used to retrieve the velocity and the magnetic field vector of the region. The plasma velocity reaches supersonic values closer to the feet of the filament barbs and coexist with a slow velocity component. Such supersonic velocities result from the acceleration of the plasma as it drains from the filament spine through the barbs. The line-of-sight magnetic fields have strengths below 200 G in the filament spine and in the filament barbs where fast downflows are located, their strengths range between 100 - 700 G. Title: First results on the search for chameleons with the KWISP detector at CAST Authors: Arguedas Cuendis, S.; Baier, J.; Barth, K.; Baum, S.; Bayirli, A.; Belov, A.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Castel, J. F.; Cetin, S. A.; Dafni, T.; Davenport, M.; Dermenev, A.; Desch, K.; Döbrich, B.; Fischer, H.; Funk, W.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gninenko, S.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jakovčić, K.; Kaminski, J.; Karuza, M.; Krieger, C.; Lakić, B.; Laurent, J. M.; Luzón, G.; Maroudas, M.; Miceli, L.; Neff, S.; Ortega, I.; Ozbey, A.; Pivovaroff, M. J.; Rosu, M.; Ruz, J.; Chóliz, E. Ruiz; Schmidt, S.; Schumann, M.; Semertzidis, Y. K.; Solanki, S. K.; Stewart, L.; Tsagris, I.; Vafeiadis, T.; Vogel, J. K.; Vretenar, M.; Yildiz, S. C.; Zioutas, K. Bibcode: 2019PDU....2600367A Altcode: 2019arXiv190601084A We report on a first measurement with a sensitive opto-mechanical force sensor designed for the direct detection of coupling of real chameleons to matter. These dark energy candidates could be produced in the Sun and stream unimpeded to Earth. The KWISP detector installed on the CAST axion search experiment at CERN looks for tiny displacements of a thin membrane caused by the mechanical effect of solar chameleons. The displacements are detected by a Michelson interferometer with a homodyne readout scheme. The sensor benefits from the focusing action of the ABRIXAS X-ray telescope installed at CAST, which increases the chameleon flux on the membrane. A mechanical chopper placed between the telescope output and the detector modulates the incoming chameleon stream. We present the results of the solar chameleon measurements taken at CAST in July 2017, setting an upper bound on the force acting on the membrane of 80 pN at 95% confidence level. The detector is sensitive for direct coupling to matter 104 ≤βm ≤ 108 , where the coupling to photons is locally bound to βγ ≤ 1011 . Title: The Polarimetric and Helioseismic Imager on Solar Orbiter Authors: Mueller, D.; Solanki, S. K.; del Toro Iniesta, J. C. Bibcode: 2019AGUFMSH21D3292M Altcode: The Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI) is the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter. It will provide valuable supporting observations for Parker Solar Probe.

The instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3 Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders (LCVRs). The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k x 2k CMOS detector. To save valuable telemetry, the raw data are reduced already on board, including being inverted under the assumption of a Milne-Eddington atmosphere. SO/PHI is composed of two telescopes, the Full Disc Telescope (FDT), covers the full solar disc at all phases of the orbit, while the High Resolution Telescope (HRT), can resolve structures as small as 200 km on the Sun at closest perihelion.

The standard data products of SO/PHI are maps of the photospheric magnetic field vector, line-of-sight velocity and continuum intensity with a highest cadence of one minute. The operational modes of SO/PHI are kept highly flexible allowing to adjust to the actual science goal chosen for a Solar Orbiter operation window. Title: Readdressing the UV solar variability with SATIRE-S: non-LTE effects Authors: Tagirov, R. V.; Shapiro, A. I.; Krivova, N. A.; Unruh, Y. C.; Yeo, K. L.; Solanki, S. K. Bibcode: 2019A&A...631A.178T Altcode: 2019arXiv190911736T Context. Solar spectral irradiance (SSI) variability is one of the key inputs to models of the Earth's climate. Understanding solar irradiance fluctuations also helps to place the Sun among other stars in terms of their brightness variability patterns and to set detectability limits for terrestrial exoplanets.
Aims: One of the most successful and widely used models of solar irradiance variability is Spectral And Total Irradiance REconstruction model (SATIRE-S). It uses spectra of the magnetic features and surrounding quiet Sun that are computed with the ATLAS9 spectral synthesis code under the assumption of local thermodynamic equilibrium (LTE). SATIRE-S has been at the forefront of solar variability modelling, but due to the limitations of the LTE approximation its output SSI has to be empirically corrected below 300 nm, which reduces the physical consistency of its results. This shortcoming is addressed in the present paper.
Methods: We replaced the ATLAS9 spectra of all atmospheric components in SATIRE-S with spectra that were calculated using the Non-LTE Spectral SYnthesis (NESSY) code. To compute the spectrum of the quiet Sun and faculae, we used the temperature and density stratification models of the FAL set.
Results: We computed non-LTE contrasts of spots and faculae and combined them with the corresponding fractional disc coverages, or filling factors, to calculate the total and spectral irradiance variability during solar cycle 24. The filling factors have been derived from solar full-disc magnetograms and continuum images recorded by the Helioseismic and Magnetic Imager on Solar Dynamics Observatory (SDO/HMI).
Conclusions: The non-LTE contrasts yield total and spectral solar irradiance variations that are in good agreement with empirically corrected LTE irradiance calculations. This shows that the empirical correction applied to the SATIRE-S total and spectral solar irradiance is consistent with results from non-LTE computations. Title: Superstrong photospheric magnetic fields in sunspot penumbrae Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki, S. K. Bibcode: 2019A&A...631A..99S Altcode: 2019arXiv190913619S Context. Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed.
Aims: Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930.
Methods: We applied classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compared the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at τ = 1) associated with supersonic downflows.
Results: The different inversions provide different results: while the SPINOR 2D inversions consider a height-dependent single-component model and return B > 7 kG and supersonic positive vLOS (corresponding to a counter-Evershed flow), height-dependent two-component inversions suggest the presence of an umbral component (almost at rest) with field strengths ∼4 - 4.2 kG and a penumbral component with vLOS ∼ 16 - 18 km s-1 and field strengths up to ∼5.8 kG. Likewise, height-independent two-component inversions find a solution for an umbral component and a strongly redshifted (vLOS ∼ 15 - 17 km s-1) penumbral component with B ∼ 4 kG. According to a Bayesian information criterion, the inversions providing a better balance between the quality of the fits and the number of free parameters considered by the models are the height-independent two-component inversions, but they lie only slightly above the SPINOR 2D inversions. Since it is expected that the physical parameters all display considerable gradients with height, as supported by magnetohydrodynamic (MHD) sunspot simulations, the SPINOR 2D inversions are the preferred ones.
Conclusions: According to the MHD sunspot simulation analyzed here, the presence of counter-Evershed flows in the photospheric penumbra can lead to the necessary conditions for the observation of ∼5 kG fields at the inner penumbra. Although a definite conclusion about the potential existence of fields in excess of 7 kG cannot be given, their nature could be explained (based on the simulation results) as the consequence of the extreme dynamical effects introduced by highly supersonic counter-Evershed flows (vLOS > 10 km s-1 and up to ∼30 km s-1 according to SPINOR 2D). The latter are much faster and more compressive downflows than those found in the MHD simulations and therefore could lead to field intensification up to considerably stronger fields. Also, a lower gas density would lead to a deeper depression of the τ = 1 surface, making possible the observation of deeper-lying stronger fields. The superstrong magnetic fields are expected to be nearly force-free, meaning that they can attain much larger strengths than expected when considering only balance between magnetic pressure and the local gas pressure. Title: Delving into the Historical Ca II K Archive from the Kodaikanal Observatory: The Potential of the Most Recent Digitized Series Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A.; Banerjee, Dipankar; Jha, Bibhuti K.; Chatterjee, Subhamoy Bibcode: 2019SoPh..294..145C Altcode: 2019arXiv190805493C Full-disc Ca II K photographic observations of the Sun carry direct information as regards the evolution of solar-plage regions for more than a century and are therefore a unique dataset for solar-activity studies. For a long time Ca II K observations were barely explored, but recent digitizations of multiple archives have allowed their extensive analysis. However, various studies have reported diverse results partly due to the insufficient quality of the digitized data. Furthermore, inhomogeneities have been identified within the individual archives, which, at least partly, could be due to the digitization. As a result, some of the archives, e.g. that from the Kodaikanal observatory, were re-digitized. The results obtained by different authors who analyzed the data from the new digitization of the Kodaikanal archive differ from each other and from those derived from the old digitization. Since the data were processed and analyzed using different techniques, it is not clear, however, whether the differences are due to the digitization or the processing of the data. To understand the reasons for such discrepancies, we analyze here the data from the two most recent digitizations of this archive. We use the same techniques to consistently process the images from both archives and to derive the plage areas from them. Some issues have been identified in both digitizations, implying that they are intrinsic characteristics of the data. Moreover, errors in timing of the observations plague both digitizations. Overall, the most recent 16-bit digitization offers an improvement over the earlier 8-bit one. It also includes considerably more data and should be preferred. Title: A comparison between solar plage and network properties Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2019A&A...630A..86B Altcode: 2019arXiv190807464B
Aims: We compare the properties of kG magnetic structures in the solar network and in active region plage at high spatial resolution.
Methods: Our analysis used six SP scans of the solar disc centre aboard Hinode SOT and inverted the obtained spectra of the photospheric 6302 Å line pair using the 2D SPINOR code.
Results: Photospheric magnetic field concentrations in network and plage areas are on average 1.5 kG strong with inclinations of 10° -20°, and have < 400 m s-1 internal and 2-3 km s-1 external downflows. At the disc centre, the continuum intensity of magnetic field concentrations in the network are on average 10% brighter than the mean quiet Sun, whilst their plage counterparts are 3% darker. A more detailed analysis revealed that all sizes of individual kG patches in the network have 150 G higher field strengths on average, 5% higher continuum contrasts, and 800 m s-1 faster surrounding downflows than similarly sized patches in the plage. The speed of the surrounding downflows also correlates with the patch area, and patches containing pores can produce supersonic flows exceeding 11 km s-1 in individual pixels. Furthermore, the magnetic canopies of kG patches are on average 9° more horizontal in the plage compared to the network.
Conclusions: Most of the differences between the network and plage are due to their different patch size distributions, but the intrinsic differences between similarly sized patches likely results from the modification of the convection photospheric convection with increasing amounts of magnetic flux. Title: Performance Analysis of the SO/PHI Software Framework for On-board Data Reduction Authors: Albert, K.; Hirzberger, J.; Busse, D.; Rodríguez, J. Blanco; Castellanos Duran, J. S.; Cobos Carrascosa, J. P.; Fiethe, B.; Gandorfer, A.; Guan, Y.; Kolleck, M.; Lagg, A.; Lange, T.; Michalik, H.; Solanki, S. K.; del Toro Iniesta, J. C. Bibcode: 2019ASPC..523..151A Altcode: 2019arXiv190508690A The Polarimetric and Helioseismic Imager (PHI) is the first deep-space solar spectropolarimeter, on-board the Solar Orbiter (SO) space mission. It faces: stringent requirements on science data accuracy, a dynamic environment, and severe limitations on telemetry volume. SO/PHI overcomes these restrictions through on-board instrument calibration and science data reduction, using dedicated firmware in FPGAs. This contribution analyses the accuracy of a data processing pipeline by comparing the results obtained with SO/PHI hardware to a reference from a ground computer. The results show that for the analyzed pipeline the error introduced by the firmware implementation is well below the requirements of SO/PHI. Title: Achievements of Hinode in the first eleven years Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick; Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks, David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio; Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra, Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.; Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao, Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota, Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu, Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin; Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young, Peter R. Bibcode: 2019PASJ...71R...1H Altcode: Hinode is Japan's third solar mission following Hinotori (1981-1982) and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in operation currently. Hinode carries three instruments: the Solar Optical Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These instruments were built under international collaboration with the National Aeronautics and Space Administration and the UK Science and Technology Facilities Council, and its operation has been contributed to by the European Space Agency and the Norwegian Space Center. After describing the satellite operations and giving a performance evaluation of the three instruments, reviews are presented on major scientific discoveries by Hinode in the first eleven years (one solar cycle long) of its operation. This review article concludes with future prospects for solar physics research based on the achievements of Hinode. Title: Solar irradiance variability over last four billion years Authors: Shapiro, Anna V.; Shapiro, Alexander I.; Gizon, Laurent; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2019EPSC...13.2071S Altcode: The action of dynamo generates magnetic field in the solar interior. This field then travels through the convective zone and emerges on the solar surface, leading to a various manifestations of solar magnetic activity. One of the most appealing among them is the variations of Spectral Solar Irradiance (SSI). There is an evidence that these variations have substantial effect on the Earth's climate system. The faster rotation of the Sun in the past led to a more vigorous dynamo and consequently larger amplitude of solar spectral irradiance variability. This could led to a stronger effect of the SSI variability on the Earth. The main goal of our study is to calculate the amplitude of the SSI variability over the course of the solar activity cycle (which presently lasts 11 years but could have different duration in the past) as a function of solar age. We utilise recently published relation between the stellar chromospheric activity and stellar age to reconstruct solar chromospheric activity back in time. It is used to calculate solar disk coverages by magnetic features, i.e. solar spots and faculae. Corresponding brightness variations are then computed using the SATIRE (which stands for Spectral and Total Irradiance Reconstruction) approach. Our study shows that the facular component of the irradiance variability over the solar activity cycle decreases slower with the solar age than the spot component. This makes the dependence of the amplitude of the solar variability on the age non-monotonic. The am- plitude decreases for the young Sun till it reaches minimum value and then gradually increases again. The variability of the Total Solar Irradiance (TSI, i.e. irradiance integrated over the entire spectral domain) changes from being spot- to facular-dominated at the solar age of about 2.8 Gyr. Our calculations show that the amplitude of the TSI variability of 600-Myr Sun was one order of magnitude larger than the present-day value. We have found that the age of the transition between spot- and facular-dominated regimes of the variability depends on the wavelength. For example, it is about 1.3 Gyr for the 210-400 nm spectral domain and becomes approximately 3.7 Gyr for the 400-700 nm spectral range. Our calculations of the past solar irradiance variability on the activity cycle timescale might be of interest for paleoclimate researchers as well as for modelling of atmospheres of exoplanets. Title: The Solar-C_EUVST mission Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko; Ichimoto, Kiyoshi; Suematsu, Yoshinori; Hara, Hirohisa; Katsukawa, Yukio; Kubo, Masahito; Toriumi, Shin; Watanabe, Tetsuya; Yokoyama, Takaaki; Korendyke, Clarence M.; Warren, Harry P.; Tarbell, Ted; De Pontieu, Bart; Teriaca, Luca; Schühle, Udo H.; Solanki, Sami; Harra, Louise K.; Matthews, Sarah; Fludra, A.; Auchère, F.; Andretta, V.; Naletto, G.; Zhukov, A. Bibcode: 2019SPIE11118E..07S Altcode: Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a solar physics mission concept that was selected as a candidate for JAXA competitive M-class missions in July 2018. The onboard science instrument, EUVST, is an EUV spectrometer with slit-jaw imaging system that will simultaneously observe the solar atmosphere from the photosphere/chromosphere up to the corona with seamless temperature coverage, high spatial resolution, and high throughput for the first time. The mission is designed to provide a conclusive answer to the most fundamental questions in solar physics: how fundamental processes lead to the formation of the solar atmosphere and the solar wind, and how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. The entire instrument structure and the primary mirror assembly with scanning and tip-tilt fine pointing capability for the EUVST are being developed in Japan, with spectrograph and slit-jaw imaging hardware and science contributions from US and European countries. The mission will be launched and installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in 2025. ISAS/JAXA coordinates the conceptual study activities during the current mission definition phase in collaboration with NAOJ and other universities. The team is currently working towards the JAXA final down-selection expected at the end of 2019, with strong support from US and European colleagues. The paper provides an overall description of the mission concept, key technologies, and the latest status. Title: Stellar Physics with High-Resolution UV Spectropolarimetry Authors: Morin, Julien; Bouret, Jean-Claude; Neiner, Coralie; Aerts, Conny; Bagnulo, Stefano; Catala, Claude; Charbonnel, Corinne; Evans, Chris; Fossati, Luca; Garcia, Miriam; Gómez de Castro, Ana I; Herrero, Artemio; Hussain, Gaitee; Kaper, Lex; Kochukhov, Oleg; Konstantinova-Antova, Renada; de Koter, Alex; Kraus, Michaela; Jiří\K{r}tička; Lèbre, Agnes; Lueftinger, Theresa; Meynet, Georges; Petit, Pascal; Shore, Steve; Solanki, Sami; Stelzer, Beate; Strugarek, Antoine; Vidotto, Aline; Vink, Jorick S Bibcode: 2019arXiv190801545M Altcode: Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution. UV spectropolarimetry is proposed on missions of various sizes and scopes, from POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV spectropolarimetry. Title: Cosmic-ray variability on the multi-millennial time scale: A new multi-proxy reconstruction Authors: Usoskin, I.; Wu, C. J.; Krivova, N.; Solanki, S. K.; Kovaltsov, G.; Baroni, M.; Bard, E. Bibcode: 2019ICRC...36.1164U Altcode: 2019PoS...358.1164U No abstract at ADS Title: Opacity distribution functions for stellar spectra synthesis Authors: Cernetic, M.; Shapiro, A. I.; Witzke, V.; Krivova, N. A.; Solanki, S. K.; Tagirov, R. V. Bibcode: 2019A&A...627A.157C Altcode: 2019arXiv190603112C Context. Stellar spectra synthesis is essential for the characterization of potential planetary hosts. In addition, comprehensive stellar variability calculations with fast radiative transfer are needed to disentangle planetary transits from stellar magnetically driven variability. The planet-hunting space telescopes, such as CoRoT, Kepler, and TESS, bring vast quantities of data, rekindling the interest in fast calculations of the radiative transfer.
Aims: We revisit the opacity distribution functions (ODF) approach routinely applied to speed up stellar spectral synthesis. To achieve a considerable speedup relative to the state of the art, we further optimize the approach and search for the best ODF configuration. Furthermore, we generalize the ODF approach for fast calculations of flux in various filters often used in stellar observations.
Methods: In a parameter-sweep fashion, we generated ODF in the spectral range from UV to IR with different setups. The most accurate ODF configuration for each spectral interval was determined. We adapted the wavelength grid based on the transmission curve for calculations of the radiative fluxes through filters before performing the normal ODF procedure.
Results: Our optimum ODF configuration allows for a three-fold speedup, compared to the previously used ODF configurations. The ODF generalization to calculate fluxes through filters results in a speedup of more than two orders of magnitude. Title: ALMA detection of dark chromospheric holes Authors: White, Stephen M.; Loukitcheva, Maria A.; Solanki, Sami K. Bibcode: 2019AAS...23421704W Altcode: Atacama Large Millimeter/submillimeter Array (ALMA) observations of a quiet-Sun region at a wavelength of 3 mm are compared with available chromospheric observations in the UV and visible as well as with photospheric magnetograms. The ALMA images clearly reveal the presence of distinctive cold areas in the millimeter maps having temperatures of around 60% of the normal quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA is sensing cool chromospheric gas, whose presence had earlier been inferred from infrared CO spectra. Title: ALMA Detection of Dark Chromospheric Holes in the Quiet Sun Authors: Loukitcheva, Maria A.; White, Stephen M.; Solanki, Sami K. Bibcode: 2019ApJ...877L..26L Altcode: 2019arXiv190506763L We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of a quiet-Sun region at a wavelength of 3 mm, obtained during the first solar ALMA cycle on 2017 April 27, and compare them with available chromospheric observations in the UV and visible as well as with photospheric magnetograms. ALMA images clearly reveal the presence of distinct particularly dark/cool areas in the millimeter maps with temperatures as low as 60% of the normal quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA is sensing cool chromospheric gas, whose presence had earlier been inferred from infrared CO spectra. Title: Recovering the unsigned photospheric magnetic field from Ca II K observations Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A.; Giorgi, Fabrizio; Yeo, Kok Leng Bibcode: 2019A&A...626A.114C Altcode: 2019arXiv190503453C Context. A number of studies have aimed at defining the exact form of the relation between magnetic field strength and Ca II H and K core brightness. All previous studies have however been restricted to isolated regions on the solar disc or to a limited set of observations.
Aims: We reassess the relationship between the photospheric magnetic field strength and the Ca II K intensity for a variety of surface features as a function of the position on the disc and the solar activity level. This relationship can be used to recover the unsigned photospheric magnetic field from images recorded in the core of Ca II K line.
Methods: We have analysed 131 pairs of high-quality, full-disc, near-co-temporal observations from the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To analytically describe the observationally determined relation, we considered three different functions: a power law with an offset, a logarithmic function, and a power-law function of the logarithm of the magnetic flux density. We used the obtained relations to reconstruct maps of the line-of-sight component of the unsigned magnetic field (unsigned magnetograms) from Ca II K observations, which were then compared to the original magnetograms.
Results: We find that both power-law functions represent the data well, while the logarithmic function is good only for quiet periods. We see no significant variation over the solar cycle or over the disc in the derived fit parameters, independently of the function used. We find that errors in the independent variable, which are usually not accounted for, introduce attenuation bias. To address this, we binned the data with respect to the magnetic field strength and Ca II K contrast separately and derived the relation for the bisector of the two binned curves. The reconstructed unsigned magnetograms show good agreement with the original ones. Root mean square differences are less than 90 G. The results were unaffected by the stray-light correction of the SDO/HMI and Rome/PSPT data.
Conclusions: Our results imply that accurately processed and calibrated Ca II K observations can be used to reconstruct unsigned magnetograms by using the relations derived in our study. Title: Moving Magnetic Features around a Pore Authors: Kaithakkal, A.; Riethmueller, T.; Solanki, S. K.; Lagg, A. Bibcode: 2019ASPC..526..307K Altcode: Moving magnetic features (MMFs) are small-scale magnetic elements observed to move radially outward from sunspots. Some studies have reported the presence of MMFs around pores as well. We analyzed data from SunriseII/IMaX observations obtained on 2013 June 12 between 23:39:10 and 23:55:37 UT. IMaX scanned the Fe I 5250.225 Å spectral line at eight wavelength positions and recorded the full Stokes vector at each of these positions. The field of view covered a large pore (μ = 0.93) with pixel scale of 0″.055. MMFs of opposite (positive) and same (negative) polarity as the pore were observed to stream from the pore boundary. We carried out a statistical analysis of the physical properties of MMFs and the main results are: 1) the number of opposite polarity MMFs within 1.5 Mm from the pore border, when they were first identified, is twice that of the same polarity MMFs. 2) Only 11% of the chosen MMFs appear to be monopolar and they all have the same polarity as the pore. 3) Majority of MMFs of both polarities move away from the pore border with an average speed of 1.5 km/s. However, they do not always follow a smooth radial track and some of them even move in tangential direction to the pore. 4) MMFs of opposite polarity show a preferential up-flow whereas those of the same polarity do not show any preference. 5) MMFs of both polarities are characterized by inclined fields.

This work, presented in an oral contribution at this Workshop, has been published on The Astrophysical Journal Supplement Series (Kaithakkal et al. 2017). Title: Fast Solar Polarimeter (Prototype): Preliminary Results of Stokes Measurements in the Sr I 4607 Å Line at VTT/TESOS Authors: Zeuner, F.; Feller, A.; Iglesias, F. A.; Solanki, S. K. Bibcode: 2019ASPC..526..295Z Altcode: Scattering polarization measurements at spatial scales in the 0.1″-1″ regime are a very promising complementary diagnostic for the Sun's atmosphere and magnetism at small spatial scales (e.g., Trujillo Bueno et al. 2004). So far, for the Sr I line at 4607.3 Å, which exhibits a large scattering polarization signal (Stenflo et al. 1997; Gandorfer 2002), only theoretical predictions for the strength and spatial distribution of linear polarization signals at sub-arcsecond scales close to disk center are made available by Trujillo Bueno & Shchukina (2007), whereas observational feedback is still very rare.
Here, we present preliminary results of two observational campaigns with the prototype of the Fast Solar Polarimeter (FSP, Iglesias et al. 2016) attached to the TESOS filtergraph, which is located at the German Vacuum Tower Telescope (VTT) in Tenerife, Spain.
We measured the center-to-limb variation of spatially averaged Stokes Q/I profiles in the Sr I line at 11 wavelength positions to check for systematic errors and to compare with previous results by other instruments. We find that our results are in agreement with previous findings reported in the literature (i.e., Stenflo et al. 1997).
In May 2015, we observed the Sr I line in the quiet Sun at μ=0.6 from the north solar limb with a spatial sampling of 0.08″ pixel-1 and noise levels significantly below 1% per pixel for linear polarized signals with 1.25 s integration time. To obtain lower noise levels (< 0.1%), while conserving sufficient spatial resolution to resolve solar granulation, we carefully used spatial, spectral, and temporal averaging, as well as a MOMFBD restoration (see van Noort et al. 2005). We compare our findings in the Sr I line with the neighboring, but not scattering sensitive Fe I line at 4607.6 Å.
Our statistical analysis of Stokes Q/I signals in the line core of Sr I reveals that the polarization amplitude is correlating with the intensity of the continuum image. We find stronger linear polarimetric signals corresponding to intergranular lanes of the Stokes I image, which is in contrast to the findings of Malherbe et al. (2007) at μ=0.3.
The results have been published as an article entitled "Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter" (Zeuner et al. 2018). Title: On the Magnetic Nature of Solar Exploding Granules Authors: Guglielmino, S. L.; Martínez Pillet, V.; Ruiz Cobo, B.; Bellot Rubio, L. R.; del Toro Iniesta, J. C.; Solanki, S. K.; Zuccarello, F. Bibcode: 2019ASPC..526..299G Altcode: We report on spectropolarimetric observations acquired by the imaging magnetograph SUNRISE/IMaX at high spatial 0.''3 and temporal (31.5 s) resolution during the first science flight of this balloon-borne solar observatory. We describe the photospheric evolution of an exploding granule observed in the quiet Sun. This granule is cospatial with a magnetic flux emergence event occurring at mesogranular scales (up to ∼12 Mm2 area). Using a modified version of the SIR code, we show that we can estimate the longitudinal field also in the presence of a residual cross-talk in these IMaX longitudinal measurements. We determine the magnetic flux content of the structure (∼3 ×1018 Mx), which appears to have a multipolar configuration, and discuss the origin of such flux emergence events. Title: GPS, decrypting brightness variations of the Sun and Sun-like Authors: Amazo-Gómez, Eliana Maritza; Shapiro, Alexander I.; Solanki, Sami K.; Kopp, Greg; Oshagh, Mahmoudreza; Reinhold, Timo; Krivova, Natalie A.; Reiners, Ansgar Bibcode: 2019shin.confE.109A Altcode: The rotation period is in general detectable in the light curves of young and active stars. Even after successful stellar surveys stemmed from Kepler mission, there is still a lack of information in photometric records of rotation periods in Sun-like stars. Non-periodic light-curve profiles, low variability contrast -therefore low modulation amplitude- short lifetime evolution and random emergence of magnetic features (in comparison to the rotation time-scale) are the main reasons of unreliable determination of rotation periodicity in the Sun and its closer analogs. This indicates that only a small fraction of solar-like systems have been properly analyzed. We show that the rotation periods of those stars can be reliably determined from the profile of the gradient of the power spectra, GPS. By analysing periodic patterns in high-accuracy measurements of the total solar irradiance, TSI, by SORCE/TIM and SoHO/VIRGO missions, here we test and validate GPS, linking the variability by transits of magnetic features over the stellar surface with a clear and enhanced signal of the solar rotation. GPS method retrieves accurate and stable values of rotation period during different regimes of solar activity cycle and could be applied to stars of comparable and higher activity - where other methods underperform. Furthermore, GPS gives us constraints on the faculae to spot driver ratio and consequently help us to interpret the stellar surface. Title: Spectropolarimetric Observations of an Arch Filament System with GREGOR Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.; Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.; Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O. Bibcode: 2019ASPC..526..217B Altcode: 2018arXiv180401789B We observed an arch filament system (AFS) in a sunspot group with the GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The AFS was located between the leading sunspot of negative polarity and several pores of positive polarity forming the following part of the sunspot group. We recorded five spectro-polarimetric scans of this region. The spectral range included the spectral lines Si I 1082.7 nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate on the silicon line which is formed in the upper photosphere. The line profiles are inverted with the code 'Stokes Inversion based on Response functions' to obtain the magnetic field vector. The line-of-sight velocities are determined independently with a Fourier phase method. Maximum velocities are found close to the ends of AFS fibrils. These maximum values amount to 2.4 km s-1 next to the pores and to 4 km s-1 at the sunspot side. Between the following pores, we encounter an area of negative polarity that is decreasing during the five scans. We interpret this by new emerging positive flux in this area canceling out the negative flux. In summary, our findings confirm the scenario that rising magnetic flux tubes cause the AFS. Title: Photospheric Magnetic Fields of the Trailing Sunspots in Active Region NOAA 12396 Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer, C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2019ASPC..526..291V Altcode: 2018arXiv180507752V The solar magnetic field is responsible for all aspects of solar activity. Sunspots are the main manifestation of the ensuing solar activity. Combining high-resolution and synoptic observations has the ambition to provide a comprehensive description of the sunspot growth and decay processes. Active region NOAA 12396 emerged on 2015 August 3 and was observed three days later with the 1.5-meter GREGOR solar telescope on 2015 August 6. High-resolution spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together with the chromospheric He I λ1083.0 nm triplet. These near-infrared spectropolarimetric observations were complemented by synoptic line-of-sight magnetograms and continuum images of the Helioseismic and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Title: Formation of the Penumbra and Start of the Evershed Flow Authors: Murabito, M.; Romano, P.; Guglielmino, S. L.; Zuccarello, F.; Solanki, S. K. Bibcode: 2019ASPC..526..243M Altcode: We analyze high-resolution observations of Active Region NOAA 11490, acquired on 2012 May 28 and 29. Spectropolarimetric measurements of the photospheric lines of Fe I at 617.3 nm and 630.25 nm were taken with the Interferometric BIdimensional Spectrometer (IBIS), mounted on NSO/DST, during about 30 minutes for each day. To study the evolution of continuum intensity, LOS velocity, inclination and strength of the magnetic field during the entire time interval, we also used data taken by SDO/HMI. We used the SIR code to invert the Stokes profiles observed with IBIS, using different initial models to take into account the physical conditions of the plasma in the region of umbra, penumbra, and quiet Sun. From the analysis of the SIR results, we found that, before the formation of the penumbra, the annular zone is characterized by downflows in the inner part. Furthermore, we observed that the onset of the classical Evershed flow occurs on a short time scale, 1-3 hours, while the penumbra is forming. In order to investigate the conditions that lead to the establishment of the classical Evershed flow, we analyzed the evolution of the continuum intensity, LOS velocity, inclination and strength of the magnetic field in a segment in the north-western part of the leading spot. In about 1 hour, we noted a clear evolution from redshift to blueshift in the penumbral filaments along the selected segment. We propose a scenario in which the penumbra is formed by magnetic flux dragged down from the canopy surrounding the initial pore: the Evershed flow starts when the sinking magnetic field dips below the solar surface and magnetoconvection sets in. Title: Analysis of full disc Ca II K spectroheliograms. II. Towards an accurate assessment of long-term variations in plage areas Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2019A&A...625A..69C Altcode: 2019arXiv190207122C Context. Reconstructions of past irradiance variations require suitable data on solar activity. The longest direct proxy is the sunspot number, and it has been most widely employed for this purpose. These data, however, only provide information on the surface magnetic field emerging in sunspots, while a suitable proxy of the evolution of the bright magnetic features, specifically faculae/plage and network, is missing. This information can potentially be extracted from the historical full-disc observations in the Ca II K line.
Aims: We use several historical archives of full-disc Ca II K observations to derive plage areas over more than a century. Employment of different datasets allows the identification of systematic effects in the images, such as changes in instruments and procedures, as well as an assessment of the uncertainties in the results.
Methods: We have analysed over 100 000 historical images from eight digitised photographic archives of the Arcetri, Kodaikanal, McMath-Hulbert, Meudon, Mitaka, Mt Wilson, Schauinsland, and Wendelstein observatories, and one archive of modern observations from the Rome/PSPT. The analysed data cover the period 1893-2018. We first performed careful photometric calibration and compensation for the centre-to-limb variation, and then segmented the images to identify plage regions. This has been consistently applied to both historical and modern observations.
Results: The plage series derived from different archives are generally in good agreement with each other. However, there are also clear deviations that most likely hint at intrinsic differences in the data and their digitisation. We showed that accurate image processing significantly reduces errors in the plage area estimates. Accurate photometric calibration also allows precise plage identification on images from different archives without the need to arbitrarily adjust the segmentation parameters. Finally, by comparing the plage area series from the various records, we found the conversion laws between them. This allowed us to produce a preliminary composite of the plage areas obtained from all the datasets studied here. This is a first step towards an accurate assessment of the long-term variation of plage regions.

The data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/625/A69 Title: VizieR Online Data Catalog: Plage area composite series (Chatzistergos+, 2019) Authors: Chatzistergos, T.; Ermolli, I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2019yCat..36250069C Altcode: Annual, monthly, and daily values of the plage area composite series produced in the paper.

(3 data files). Title: The potential of many-line inversions of photospheric spectropolarimetric data in the visible and near UV Authors: Riethmüller, T. L.; Solanki, S. K. Bibcode: 2019A&A...622A..36R Altcode: 2018arXiv181203757R Our knowledge of the lower solar atmosphere is mainly obtained from spectropolarimetric observations, which are often carried out in the red or infrared spectral range and almost always cover only a single or a few spectral lines. Here we compare the quality of Stokes inversions of only a few spectral lines with many-line inversions. In connection with this, we have also investigated the feasibility of spectropolarimetry in the short-wavelength range, 3000 Å-4300 Å, where the line density but also the photon noise are considerably higher than in the red, so that many-line inversions could be particularly attractive in that wavelength range. This is also timely because this wavelength range will be the focus of a new spectropolarimeter in the third science flight of the balloon-borne solar observatory SUNRISE. For an ensemble of state-of-the-art magneto-hydrodynamical atmospheres we synthesize exemplarily spectral regions around 3140 Å (containing 371 identified spectral lines), around 4080 Å (328 lines), and around 6302 Å (110 lines). The spectral coverage is chosen such that at a spectral resolving power of 150 000 the spectra can be recorded by a 2K × 2K detector. The synthetic Stokes profiles are degraded with a typical photon noise and afterward inverted. The atmospheric parameters of the inversion of noisy profiles are compared with the inversion of noise-free spectra. We find that significantly more information can be obtained from many-line inversions than from a traditionally used inversion of only a few spectral lines. We further find that information on the upper photosphere can be significantly more reliably obtained at short wavelengths. In the mid and lower photosphere, the many-line approach at 4080 Å provides equally good results as the many-line approach at 6302 Å for the magnetic field strength and the line-of-sight (LOS) velocity, while the temperature determination is even more precise by a factor of three. We conclude from our results that many-line spectropolarimetry should be the preferred option in the future, and in particular at short wavelengths it offers a high potential in solar physics. Title: Cancelation of small-scale magnetic features Authors: Kaithakkal, A. J.; Solanki, S. K. Bibcode: 2019A&A...622A.200K Altcode: 2018arXiv181211496K
Aims: We investigate small-scale flux cancelations in a young active region observed with the high-resolution imaging magnetograph IMaX on the SUNRISE balloon-borne solar observatory.
Methods: The observed Stokes profiles of the photospheric Fe I 5250.2 Å line were inverted using the SPINOR code to obtain the atmospheric parameters, including magnetic field vector and the line-of-sight velocity. We then identified 11 opposite-polarity canceling pairs using an automatic detection code, studied their evolution in detail, and derived their statistical properties. We classified the cancelations into two groups. Class I events are those for which cancelation happens between a pre-existing large magnetic feature of one polarity and a smaller feature of the other polarity that emerged or appeared nearby. For Class II events cancelations occur between two pre-existing, previously unconnected features that converge.
Results: All studied events have an apparent cancelation time of less than ten minutes and display a significant transient linear polarization signal along the polarity inversion line. The cancelation events are characterized by a flux decay rate of about 1015 Mx s-1. For Class I events, the Doppler velocity of the disappearing patch gradually switches from blueshift during the initial phase of cancelation to redshift toward the end of the cancelation. For class II events, the Doppler velocity is consistently redshifted. Horizontal convergence speeds of Class II pairs fall between 0.3 and 1.22 km s-1. The elements often do not converge directly toward each other, so that the proper motion speeds of the individual elements is higher, in the range of 1-2.7 km s-1.
Conclusions: We propose that these cancelation events result from either field-line submergence (Class I), or reconnection followed by submergence (Class II and/or Class I). Ohmic dissipation of magnetic energy could also play a role for both classes. The dynamics and evolution of these events are influenced by neighboring granular motions. We also propose that, at least for the Class II events, the granular motions could possibly be driving magnetic reconnection, rather than the supergranular motions proposed for the larger cancelation events studied earlier. Specific flux cancelation rates of the Class II events seem to indicate that they belong to somewhat different category of cancelations when compared with those studied in SOT/Hinode and MDI/SOHO data.

A movie associated to Fig. 5 is available at https://www.aanda.org Title: Chapter 3 - The Sun's Atmospher Authors: Shapiro, Alexander I.; Peter, Hardi; Solanki, Sami K. Bibcode: 2019sgsp.book...59S Altcode: The solar atmosphere covers a broad range of temperatures and densities from the solar surface, via the chromosphere and transition region, and to the corona. Although one-dimensional (1D) models of the atmospheric structure have reached a high level of maturity, high-spatial resolution observations have cast some doubt on their validity. Thus, such observations have revealed a richness of highly variable spatial structure, often reaching down to the current resolution limit of 0.1 arcsec, or roughly 70 km on the Sun, in the photosphere and chromosphere. These observational advances have led to a new generation of models that describe the solar atmosphere self-consistently using 3D magnetohydrodynamic approximation simulations, including 3D radiative energy transport for those that cover the lower atmosphere, while simplistically taking into account the complex magnetic structure and energy dissipation processes in the upper atmosphere. These models have achieved considerable success in explaining the best observations, although there are still a number of open questions. Nonetheless, thanks to modern advances, the solar atmosphere now provides an excellent setting to test models of stellar atmospheres critically. Title: Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution Authors: Kahil, F.; Riethmüller, T. L.; Solanki, S. K. Bibcode: 2019A&A...621A..78K Altcode: 2018arXiv181105759K Magnetic elements have an intensity contrast that depends on the type of region they are located in (for example quiet Sun, or active region plage). Observed values also depend on the spatial resolution of the data. Here we investigate the contrast-magnetic field dependence in active region plage observed near disk center with SUNRISE during its second flight in 2013. The wavelengths under study range from the visible at 525 nm to the near ultraviolet (NUV) at 300 nm and 397 nm. We use quasi-simultaneous spectropolarimetric and photometric data from the Imaging Magnetograph eXperiment (IMaX) and the Sunrise Filter Imager (SuFI), respectively. We find that in all wavelength bands, the contrast exhibits a qualitatively similar dependence on the line-of-sight magnetic field, BLOS, as found in the quiet Sun, with the exception of the continuum at 525 nm. There, the contrast of plage magnetic elements peaks for intermediate values of BLOS and decreases at higher field strengths. By comparison, the contrast of magnetic elements in the quiet Sun saturates at its maximum value at large BLOS. We find that the explanation of the turnover in contrast in terms of the effect of finite spatial resolution of the data is incorrect with the evidence provided by the high-spatial resolution SUNRISE data, as the plage magnetic elements are larger than the quiet Sun magnetic elements and are well-resolved. The turnover comes from the fact that the core pixels of these larger magnetic elements are darker than the quiet Sun. We find that plages reach lower contrast than the quiet Sun at disk center at wavelength bands formed deep in the photosphere, such as the visible continuum and the 300 nm band. This difference decreases with formation height and disappears in the Ca II H core, in agreement with empirical models of magnetic element atmospheres. Title: Historical solar Ca II K observations at the Rome and Catania observatories Authors: Chatzistergos, T.; Ermolli, I.; Falco, M.; Giorgi, F.; Guglielmino, S. L.; Krivova, N. A.; Romano, P.; Solanki, S. K. Bibcode: 2019NCimC..42....5C Altcode: 2019arXiv190207483C Here we present the little explored Ca II K archives from the Rome and the Catania observatories and analyse the digitised images from these archives to derive plage areas. Title: Improved search for solar chameleons with a GridPix detector at CAST Authors: Anastassopoulos, V.; Aune, S.; Barth, K.; Belov, A.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Castel, J. F.; Cetin, S. A.; Christensen, F.; Dafni, T.; Davenport, M.; Dermenev, A.; Desch, K.; Döbrich, B.; Eleftheriadis, C.; Fanourakis, G.; Ferrer-Ribas, E.; Fischer, H.; Funk, W.; Garcimatha, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Gninenko, S.; Hailey, C. J.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jakobsen, A.; Jacoby, J.; Jakovčić, K.; Kaminski, J.; Karuza, M.; Kostoglou, S.; Kralj, N.; Krčmar, M.; Krieger, C.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Luzón, G.; Maroudas, M.; Miceli, L.; Neff, S.; Ortega, I.; Papaevangelou, T.; Paraschou, K.; Pivovaroff, M. J.; Raffelt, G.; Rosu, M.; Ruz, J.; Ruiz Chóliz, E.; Savvidis, I.; Schmidt, S.; Semertzidis, Y. K.; Solanki, S. K.; Stewart, L.; Vafeiadis, T.; Vogel, J. K.; Vretenar, M.; Wuensch, W.; Yildiz, S. C.; Zioutas, K.; Brax, P. Bibcode: 2019JCAP...01..032A Altcode: 2018arXiv180800066A We report on a new search for solar chameleons with the CERN Axion Solar Telescope (CAST). A GridPix detector was used to search for soft X-ray photons in the energy range from 200 eV to 10 keV from converted solar chameleons. No significant excess over the expected background has been observed in the data taken in 2014 and 2015. We set an improved limit on the chameleon photon coupling, βγlesssim#5.7×1010 for 1<βm<106 at 95% C.L. improving our previous results by a factor two and for the first time reaching sensitivity below the solar luminosity bound for tachocline magnetic fields up to 12.5 T. Title: Solar total and spectral irradiance reconstruction over the last 9000 years Authors: Wu, C. -J.; Krivova, N. A.; Solanki, S. K.; Usoskin, I. G. Bibcode: 2018A&A...620A.120W Altcode: 2018arXiv181103464W Context. Changes in solar irradiance and in its spectral distribution are among the main natural drivers of the climate on Earth. However, irradiance measurements are only available for less than four decades, while assessment of solar influence on Earth requires much longer records.
Aims: The aim of this work is to provide the most up-to-date physics-based reconstruction of the solar total and spectral irradiance (TSI/SSI) over the last nine millennia.
Methods: The concentrations of the cosmogenic isotopes 14C and 10Be in natural archives have been converted to decadally averaged sunspot numbers through a chain of physics-based models. TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are carried out for each isotope record separately, as well as for their composite.
Results: We present the first ever SSI reconstruction over the last 9000 years from the individual 14C and 10Be records as well as from their newest composite. The reconstruction employs physics-based models to describe the involved processes at each step of the procedure.
Conclusions: Irradiance reconstructions based on two different cosmogenic isotope records, those of 14C and 10Be, agree well with each other in their long-term trends despite their different geochemical paths in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular variability in TSI is of the order of 0.11%, or 1.5 W m-2. After the Maunder minimum, the reconstruction from the cosmogenic isotopes is consistent with that from the direct sunspot number observation. Furthermore, over the nineteenth century, the agreement of irradiance reconstructions using isotope records with the reconstruction from the sunspot number by Chatzistergos et al. (2017, A&A, 602, A69) is better than that with the reconstruction from the WDC-SILSO series (Clette et al. 2014, Space Sci. Rev., 186, 35), with a lower χ2-value. Title: Activity variation driven by flux emergence and transport on Sun-like stars Authors: Işık, Emre; Solanki, Sami K.; Krivova, Natalie A.; Shapiro, Alexander I. Bibcode: 2018arXiv181208976I Altcode: In G dwarfs, the surface distribution, coverage and lifetimes of starspots deviate from solar-like patterns as the rotation rate increases. We set up a numerical platform which includes the large-scale rotational and surface flow effects, aiming to simulate evolving surface patterns over an activity cycle for up to 8 times the solar rotation and flux emergence rates. At the base of the convection zone, we assume a solar projected butterfly diagram. We then follow the rotationally distorted trajectories of rising thin flux tubes to obtain latitudes and tilt angles. Using them as source distributions, we run a surface flux transport model with solar parameters. Our model predicts surface distributions of the signed radial fields and the starspots that qualitatively agree with observations. Title: Forward modelling of brightness variations in Sun-like stars. I. Emergence and surface transport of magnetic flux Authors: Işık, E.; Solanki, S. K.; Krivova, N. A.; Shapiro, A. I. Bibcode: 2018A&A...620A.177I Altcode: 2018arXiv181006728I Context. The latitudinal distribution of starspots deviates from the solar pattern with increasing rotation rate. Numerical simulations of magnetic flux emergence and transport can help model the observed stellar activity patterns and the associated brightness variations.
Aims: We set up a composite model for the processes of flux emergence and transport on Sun-like stars to simulate stellar brightness variations for various levels of magnetic activity and rotation rates.
Methods: Assuming that the distribution of magnetic flux at the base of the convection zone follows solar scaling relations, we calculate the emergence latitudes and tilt angles of bipolar regions at the surface for various rotation rates, using thin-flux-tube simulations. Taking these two quantities as input to a surface flux transport (SFT) model, we simulate the diffusive-advective evolution of the radial field at the stellar surface, including effects of active region nesting.
Results: As the rotation rate increases, (1) magnetic flux emerges at higher latitudes and an inactive gap opens around the equator, reaching a half-width of 20° for 8 Ω⊙; and (2) the tilt angles of freshly emerged bipolar regions show stronger variations with latitude. Polar spots can form at 8 Ω⊙ by accumulation of follower-polarity flux from decaying bipolar regions. From 4 Ω⊙ to 8 Ω⊙, the maximum spot coverage changes from 3 to 20%, respectively, compared to 0.4% in the solar model. Nesting of activity can lead to strongly non-axisymmetric spot distributions.
Conclusions: On Sun-like stars rotating at 8 Ω⊙ (Prot ≃ 3 days), polar spots can form, owing to higher levels of flux emergence rate and tilt angles. Defining spots by a threshold field strength yields global spot coverages that are roughly consistent with stellar observations. Title: Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter Authors: Zeuner, F.; Feller, A.; Iglesias, F. A.; Solanki, S. K. Bibcode: 2018A&A...619A.179Z Altcode: 2018arXiv180806539Z Context. Scattering polarization in the Sr I 4607.3 Å line observed with high resolution is an important diagnostic of the Sun's atmosphere and magnetism at small spatial scales. Investigating the scattering polarization altered by the Hanle effect is key to constraining the role of small-scale magnetic activity in solar atmospheric activity and energy balance. At present, spatially resolved observations of this diagnostic are rare and have not been reported as close to the disk center as for μ = 0.6.
Aims: Our aim is to measure the scattering polarization in the Sr I line at μ = 0.6 and to identify the spatial fluctuations with a statistical approach.
Methods: Using the Fast Solar Polarimeter (FSP) mounted on the TESOS filtergraph at the German Vacuum Tower Telescope (VTT) in Tenerife, Spain, we measured both the spatially resolved full Stokes parameters of the Sr I line at μ = 0.6 and the center-to-limb variation of the spatially averaged Stokes parameters.
Results: We find that the center-to-limb variation of the scattering polarization in the Sr I line measured with FSP is consistent with previous measurements. A statistical analysis of Stokes Q/I (i.e., the linear polarization component parallel to the solar limb), sampled with 0.16″ pixel-1 in the line core of Sr I reveals that the signal strength is inversely correlated with the intensity in the continuum. We find stronger linear polarimetric signals corresponding to dark areas in the Stokes I continuum image (intergranular lanes). In contrast, independent measurements at μ = 0.3 show a positive correlation of Q/I with respect to the continuum intensity. We estimate that the subregion diameter responsible for the excess Q/I signal is on the order of 0.5″-1″.
Conclusions: The presented observations and the statistical analysis of Q/I signals at μ = 0.6 complement reported scattering polarization observations as well as simulations. The FSP has proven to be a suitable instrument to measure spatially resolved scattering polarization signals. In the future, a systematic center-to-limb series of observations with subgranular spatial resolution and increased polarimetric sensitivity (<10-3) compared to that in the present study is needed in order to investigate the change in trend with μ that the comparison of our results with the literature suggests. Title: From solar to stellar brightness variations. The effect of metallicity Authors: Witzke, V.; Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Schmutz, W. Bibcode: 2018A&A...619A.146W Altcode: 2018arXiv180904360W Context. Comparison studies of Sun-like stars with the Sun suggest an anomalously low photometric variability of the Sun compared to Sun-like stars with similar magnetic activity. Comprehensive understanding of stellar variability is needed to find a physical reason for this observation.
Aims: We investigate the effect of metallicity and effective temperature on the photometric brightness change of Sun-like stars seen at different inclinations. The considered range of fundamental stellar parameters is sufficiently small so the stars investigated here still count as Sun-like or even as solar twins.
Methods: To model the brightness change of stars with solar magnetic activity, we extended a well-established model of solar brightness variations based on solar spectra, Spectral And Total Irradiance REconstruction (SATIRE), to stars with different fundamental parameters. For this we calculated stellar spectra for different metallicities and effective temperature using the radiative transfer code ATLAS9.
Results: We show that even a small change (e.g. within the observational error range) of metallicity or effective temperature significantly affects the photometric brightness change compared to the Sun. We find that for Sun-like stars, the amplitude of the brightness variations obtained for Strömgren (b + y)/2 reaches a local minimum for fundamental stellar parameters close to the solar metallicity and effective temperature. Moreover, our results show that the effect of inclination decreases for metallicity values greater than the solar metallicity. Overall, we find that an exact determination of fundamental stellar parameters is crucially important for understanding stellar brightness changes. Title: Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2018A&A...619A..42L Altcode: 2018arXiv180806867L Context. The Wilson depression is the difference in geometric height of unit continuum optical depth between the sunspot umbra and the quiet Sun. Measuring the Wilson depression is important for understanding the geometry of sunspots. Current methods suffer from systematic effects or need to make assumptions on the geometry of the magnetic field. This leads to large systematic uncertainties of the derived Wilson depressions.
Aims: We aim to develop a robust method for deriving the Wilson depression that only requires the information about the magnetic field that is accessible from spectropolarimetry, and that does not rely on assumptions on the geometry of sunspots or on their magnetic field.
Methods: Our method is based on minimizing the divergence of the magnetic field vector derived from spectropolarimetric observations. We have focused on large spatial scales only in order to reduce the number of free parameters.
Results: We tested the performance of our method using synthetic Hinode data derived from two sunspot simulations. We find that the maximum and the umbral averaged Wilson depression for both spots determined with our method typically lies within 100 km of the true value obtained from the simulations. In addition, we applied the method to Hinode observations of a sunspot. The derived Wilson depression (∼600 km) is consistent with results typically obtained from the Wilson effect. We also find that the Wilson depression obtained from using horizontal force balance gives 110-180 km smaller Wilson depressions than both, what we find and what we deduce directly from the simulations. This suggests that the magnetic pressure and the magnetic curvature force contribute to the Wilson depression by a similar amount. Title: Observations of solar chromospheric heating at sub-arcsec spatial resolution Authors: Smitha, H. N.; Chitta, L. P.; Wiegelmann, T.; Solanki, S. K. Bibcode: 2018A&A...617A.128S Altcode: 2018arXiv180701078S A wide variety of phenomena such as gentle but persistent brightening, dynamic slender features (∼100 km), and compact (∼1″) ultraviolet (UV) bursts are associated with the heating of the solar chromosphere. High spatio-temporal resolution is required to capture the finer details of the likely magnetic reconnection-driven, rapidly evolving bursts. Such observations are also needed to reveal their similarities to large-scale flares, which are also thought to be reconnection driven, and more generally their role in chromospheric heating. Here we report observations of chromospheric heating in the form of a UV burst obtained with the balloon-borne observatory SUNRISE. The observed burst displayed a spatial morphology similar to that of a large-scale solar flare with a circular ribbon. While the co-temporal UV observations at 1.5″ spatial resolution and 24 s cadence from the Solar Dynamics Observatory showed a compact brightening, the SUNRISE observations at diffraction-limited spatial resolution of 0.1″ at 7 s cadence revealed a dynamic substructure of the burst that it is composed of an extended ribbon-like feature and a rapidly evolving arcade of thin (∼0.1″) magnetic loop-like features, similar to post-flare loops. Such a dynamic substructure reveals the small-scale nature of chromospheric heating in these bursts. Furthermore, based on magnetic field extrapolations, this heating event is associated with a complex fan-spine magnetic topology. Our observations strongly hint at a unified picture of magnetic heating in the solar atmosphere from some large-scale flares to small-scale bursts, all associated with such a magnetic topology.

The movie associated to Fig. 2 is available at https://www.aanda.org/ Title: Emission of solar chromospheric and transition region features related to the underlying magnetic field Authors: Barczynski, K.; Peter, H.; Chitta, L. P.; Solanki, S. K. Bibcode: 2018A&A...619A...5B Altcode: 2018arXiv180702372B Context. The emission of the upper atmosphere of the Sun is closely related to magnetic field concentrations at the solar surface.
Aims: It is well established that this relation between chromospheric emission and magnetic field is nonlinear. Here we investigate systematically how this relation, characterised by the exponent of a power-law fit, changes through the atmosphere, from the upper photosphere through the temperature minimum region and chromosphere to the transition region.
Methods: We used spectral maps from the Interface Region Imaging Spectrograph (IRIS) covering Mg II and its wings, C II, and Si IV together with magnetograms and UV continuum images from the Solar Dynamics Observatory. After a careful alignment of the data we performed a power-law fit for the relation between each pair of observables and determine the power-law index (or exponent) for these. This was done for different spatial resolutions and different features on the Sun.
Results: While the correlation between emission and magnetic field drops monotonically with temperature, the power-law index shows a hockey-stick-type variation: from the upper photosphere to the temperature-minimum it drops sharply and then increases through the chromosphere into the transition region. This is even seen through the features of the Mg II line, this is, from k1 to k2 and k3. It is irrespective of spatial resolution or whether we investigate active regions, plage areas, quiet Sun, or coronal holes.
Conclusions: In accordance with the general picture of flux-flux relations from the chromosphere to the corona, above the temperature minimum the sensitivity of the emission to the plasma heating increases with temperature. Below the temperature minimum a different mechanism has to govern the opposite trend of the power-law index with temperature. We suggest four possibilities, in other words, a geometric effect of expanding flux tubes filling the available chromospheric volume, the height of formation of the emitted radiation, the dependence on wavelength of the intensity-temperature relationship, and the dependence of the heating of flux tubes on the magnetic flux density. Title: Quiet-Sun and Coronal Hole in Mg II k Line as Observed by IRIS Authors: Kayshap, Pradeep; Tripathi, Durgesh; Solanki, Sami K.; Peter, Hardi Bibcode: 2018ApJ...864...21K Altcode: 2018arXiv180703494K Coronal hole (CH) regions are dark in comparison to the quiet Sun (QS) at coronal temperatures. However, at chromospheric and transition region temperatures, the QS and CHs are hardly distinguishable. In this study, we have used the Mg II 2796.35 Å spectral line recorded by the Interface Region Imaging Spectrometer (IRIS) to understand the similarities and differences in the QS and CH at chromospheric levels. Our analysis reveals that the emission from Mg II k3 and k2v that originates in the chromosphere is significantly lower in CH than in QS for the regions with similar magnetic field strength. The wing emissions of Mg II k that originates from the photospheric layer, however, do not show any difference between QS and CH. The difference in Mg II k3 intensities between QS and CH increases with increasing magnetic field strength. We further studied the effects of spectral resolution on these differences and found that the difference in the intensities decreases with decreasing spectral resolution. For a resolution of 11 Å, the difference completely disappears. These findings are not only important for mass and energy supply from the chromosphere to the corona but also provides essential ingredients for the modeling of the solar spectral irradiance for the understanding of the Sun-climate relationships. Title: Dark Structures in Sunspot Light Bridges Authors: Zhang, Jingwen; Tian, Hui; Solanki, Sami K.; Wang, Haimin; Peter, Hardi; Ahn, Kwangsu; Xu, Yan; Zhu, Yingjie; Cao, Wenda; He, Jiansen; Wang, Linghua Bibcode: 2018ApJ...865...29Z Altcode: 2018arXiv180900146Z We present unprecedented high-resolution TiO images and Fe I 1565 nm spectropolarimetric data of two light bridges taken by the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory. In the first light bridge (LB1), we find striking knot-like dark structures within the central dark lane. Many dark knots show migration away from the penumbra along the light bridge. The sizes, intensity depressions, and apparent speeds of their proper motion along the light bridges of 33 dark knots identified from the TiO images are mainly in the ranges of 80 ∼ 200 km, 30% ∼ 50%, and 0.3 ∼ 1.2 km s-1, respectively. In the second light bridge (LB2), a faint central dark lane and striking transverse intergranular lanes were observed. These intergranular lanes have sizes and intensity depressions comparable to those of the dark knots in LB1 and also migrate away from the penumbra at similar speeds. Our observations reveal that LB2 is made up of a chain of evolving convection cells, as indicated by patches of blueshift surrounded by narrow lanes of redshift. The central dark lane generally corresponds to blueshifts, supporting the previous suggestion of central dark lanes being the top parts of convection upflows. In contrast, the intergranular lanes are associated with redshifts and located at two sides of each convection cell. The magnetic fields are stronger in intergranular lanes than in the central dark lane. These results suggest that these intergranular lanes are manifestations of convergent convective downflows in the light bridge. We also provide evidence that the dark knots observed in LB1 may have a similar origin. Title: Temporal evolution of arch filaments as seen in He I 10 830 Å Authors: González Manrique, S. J.; Kuckein, C.; Collados, M.; Denker, C.; Solanki, S. K.; Gömöry, P.; Verma, M.; Balthasar, H.; Lagg, A.; Diercke, A. Bibcode: 2018A&A...617A..55G Altcode: 2018arXiv180700728G
Aims: We study the evolution of an arch filament system (AFS) and of its individual arch filaments to learn about the processes occurring in them.
Methods: We observed the AFS at the GREGOR solar telescope on Tenerife at high cadence with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) in the He I 10 830 Å spectral range. The He I triplet profiles were fitted with analytic functions to infer line-of-sight (LOS) velocities to follow plasma motions within the AFS.
Results: We tracked the temporal evolution of an individual arch filament over its entire lifetime, as seen in the He I 10 830 Å triplet. The arch filament expanded in height and extended in length from 13″ to 21″. The lifetime of this arch filament is about 30 min. About 11 min after the arch filament is seen in He I, the loop top starts to rise with an average Doppler velocity of 6 km s-1. Only two minutes later, plasma drains down with supersonic velocities towards the footpoints reaching a peak velocity of up to 40 km s-1 in the chromosphere. The temporal evolution of He I 10 830 Å profiles near the leading pore showed almost ubiquitous dual red components of the He I triplet, indicating strong downflows, along with material nearly at rest within the same resolution element during the whole observing time.
Conclusions: We followed the arch filament as it carried plasma during its rise from the photosphere to the corona. The material then drained toward the photosphere, reaching supersonic velocities, along the legs of the arch filament. Our observational results support theoretical AFS models and aids in improving future models.

The movie associated to Fig. 3 is available at https://www.aanda.org/ Title: Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) for the SUNRISE balloon-borne solar observatory Authors: Suematsu, Yoshinori; Katsukawa, Yukio; Hara, Hirohisa; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Kubo, Masahito; Barthol, Peter; Riethmueller, Tino; Gandorfer, Achim; Feller, Alex; Orozco Suárez, David; Del Toro Iniesta, Jose Carlos; Kano, Ryouhei; Ishikawa, Shin-nosuke; Ishikawa, Ryohko; Tsuzuki, Toshihiro; Uraguchi, Fumihiro; Quintero Noda, Carlos; Tamura, Tomonori; Oba, Takayoshi; Kawabata, Yusuke; Nagata, Shinichi; Anan, Tetsu; Cobos Carrascosa, Juan Pedro; Lopez Jimenez, Antonio Carlos; Balaguer Jimenez, Maria; Solanki, Sami Bibcode: 2018cosp...42E3285S Altcode: The SUNRISE balloon-borne solar observatory carries a 1 m aperture optical telescope, and allows us to perform seeing-free continuous observations at visible-IR wavelengths from an altitude higher than 35 km. In the past two flights, in 2009 and 2013, observations mainly focused on fine structures of photospheric magnetic fields. For the third flight planned for 2021, we are developing a new instrument for conducting spectro-polarimetry of spectral lines formed over a larger height range in the solar atmosphere from the photosphere to the chromosphere. Targets of the spectro-polarimetric observation are (1) to determine 3D magnetic structure from the photosphere to the chromosphere, (2) to trace MHD waves from the photosphere to the chromosphere, and (3) to reveal the mechanism driving chromospheric jets, by measuring height- and time-dependent velocities and magnetic fields. To achieve these goals, a spectro-polarimeter called SCIP (Sunrise Chromospheric Infrared spectroPolarimeter) is designed to observe near-infrared spectrum lines sensitive to solar magnetic fields. The spatial and spectral resolutions are 0.2 arcsec and 200,000, respectively, while 0.03% polarimetric sensitivity is achieved within a 10 sec integration time. The optical system employs an Echelle grating and off-axis aspheric mirrors to observe the two wavelength ranges centered at 850 nm and 770 nm simultaneously by two cameras. Polarimetric measurements are performed using a rotating waveplate and polarization beam-splitters in front of the cameras. For detecting minute polarization signals with good precision, we carefully assess the temperature dependence of polarization optics, and make the opto-structural design that minimizes the thermal deformation of the spectrograph optics. Another key technique is to attain good (better than 30 msec) synchronization among the rotating phase of the waveplate, read-out timing of cameras, and step timing of a slit-scanning mirror. On-board accumulation and data processing are also critical because we cannot store all the raw data read-out from the cameras. We demonstrate that we can reduce the data down to almost 10% with loss-less image compression and without sacrificing polarimetric information in the data. The SCIP instrument is developed by internal collaboration among Japanese institutes including Japan Aerospace Exploration Agency (JAXA), the Spanish Sunrise consortium, and the German Max Planck Institute for Solar System Research (MPS) with a leadership of the National Astronomical Observatory of Japan (NAOJ). Title: ACTReSS: ACTive Rotating Star Simulator Authors: Johnson, Luke Jonathan; Norris, Charlotte; Unruh, Yvonne; Solanki, Sami; Krivova, Natalie Bibcode: 2018csss.confE..58J Altcode: Variability of late-type stars on timescales of days or longer is largely due to magnetic surface features (starspots and faculae). We present ACTReSS, a software tool to calculate rotational lightcurves of stars. The model uses limb-dependent intensities derived from MURaM 3-D magneto-convection simulations (Beeck et al. 2014) for quiet stellar photospheres and bright active regions. This allows us to investigate the dependence of the flux variation on spectral type, inclination and wavelength. Title: Solar irradiance variability on time scales of decades to millennia Authors: Krivova, Natalie; Solanki, Sami K.; Leng Yeo, Kok; Wu, Chi-Ju Bibcode: 2018cosp...42E1847K Altcode: Reconstructions of past solar irradiance changes over as long periodsof time as possible are crucial to our understanding of solarinfluence on climate. They are only possible with the help of suitablemodels, based on detailed understanding of the mechanisms of thevariability. With the advance of such models, also the pastreconstructions are becoming more reliable. Nevertheless, theremaining uncertainties spread out when extrapolating back over longperiods of time. They are further amplified by the increasingly pooreramount and quality of the available data that bear information on pastsolar activity. We will discuss the progress and the uncertainties ofirradiance reconstructions on time scales of decades to millennia. Title: Solar Activity and Irradiance Reconstruction over the Holocene Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami K.; Kovaltsov, Gennady A.; Baroni, Melanie; Bard, Edouard Bibcode: 2018cosp...42E3695W Altcode: Direct measurements of the solar irradiance are only available since 1978. To understand the solar influence on Earth's climate, longer records and thus reconstructions of the solar variability in the past are needed. The directly observed sunspot number allows going back to 1610 A.D, although with progressively increasing uncertainty. To assess solar variability at earlier times, one has to rely on indirect proxies of solar activity, such as concentrations of cosmogenic isotopes 10Be and 14C in terrestrial archives. They are produced mostly in the upper atmosphere by impinging galactic cosmic rays (GCRs). The flux of GCRs is modulated by both the heliospheric magnetic field and the geomagnetic field. Therefore, the isotope signals retrieved from various sites around the globe show a very high degree of similarity, reflecting changes in the solar activity. Still, short-and mid-term deviations can be observed due to various systematic effects, such as different geochemical production, atmospheric distribution processes and local climatic conditions. To account for these differences, we have constructed a state-of-the-art consistent multi-isotope composite from one global 14C and six regional 10Be data sets. This composite is then used to reconstruct decadal values of the total and spectral solar irradiance over the Holocene with the semi-empirical SATIRE-M model, while the quasi-11 year solar cycle has been simulated statistically. Title: Image compression on reconfigurable FPGA for the SO/PHI space instrument Authors: Hernández Expósito, D.; Cobos Carrascosa, J. P.; Ramos Mas, J. L.; Rodríguez Valido, M.; Orozco Suárez, D.; Hirzberger, J.; Woch, J.; Solanki, S.; del Toro Iniesta, J. C. Bibcode: 2018SPIE10707E..2FH Altcode: In this paper we present a novel FPGA implementation of the Consultative Committee for Space Data Systems Image Data Compression (CCSDS-IDC 122.0-B-1) for performing image compression aboard the Polarimetric Helioseismic Imager instrument of the ESA's Solar Orbiter mission. This is a System-On-Chip solution based on a light multicore architecture combined with an efficient ad-hoc Bit Plane Encoder core. This hardware architecture performs an acceleration of 30 times with respect to a software implementation running into space-qualified processors, like LEON3. The system stands out over other FPGA implementations because of the low resource usage, which does not use any external memory, and of its configurability. Title: Solar activity over nine millennia: A consistent multi-proxy reconstruction Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.; Baroni, M.; Bard, E.; Solanki, S. K. Bibcode: 2018A&A...615A..93W Altcode: 2018arXiv180401302W
Aims: The solar activity in the past millennia can only be reconstructed from cosmogenic radionuclide proxy records in terrestrial archives. However, because of the diversity of the proxy archives, it is difficult to build a homogeneous reconstruction. All previous studies were based on individual, sometimes statistically averaged, proxy datasets. Here we aim to provide a new consistent multi-proxy reconstruction of the solar activity over the last 9000 yr, using all available long-span datasets of 10Be and 14C in terrestrial archives.
Methods: A new method, based on a Bayesian approach, was applied for the first time to solar activity reconstruction. A Monte Carlo search (using the χ2 statistic) for the most probable value of the modulation potential was performed to match data from different datasets for a given time. This provides a straightforward estimate of the related uncertainties. We used six 10Be series of different lengths (from 500-10 000 yr) from Greenland and Antarctica, and the global 14C production series. The 10Be series were resampled to match wiggles related to the grand minima in the 14C reference dataset. The stability of the long data series was tested.
Results: The Greenland Ice-core Project (GRIP) and the Antarctic EDML (EPICA Dronning Maud Land) 10Be series diverge from each other during the second half of the Holocene, while the 14C series lies in between them. A likely reason for the discrepancy is the insufficiently precise beryllium transport and deposition model for Greenland, which leads to an undercorrection of the GRIP series for the geomagnetic shielding effect. A slow 6-7 millennia variability with lows at ca. 5500 BC and 1500 AD in the long-term evolution of solar activity is found. Two components of solar activity can be statistically distinguished: the main component, corresponding to the "normal" moderate level, and a component corresponding to grand minima. A possible existence of a component representing grand maxima is indicated, but it cannot be separated from the main component in a statistically significant manner.
Conclusions: A new consistent reconstruction of solar activity over the last nine millennia is presented with the most probable values of decadal sunspot numbers and their realistic uncertainties. Independent components of solar activity corresponding to the main moderate activity and the grand-minimum state are identified; they may be related to different operation modes of the dynamo.

A table with the reconstructed SN series is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/615/A93 Title: Getting Ready for the Third Science Flight of SUNRISE Authors: Barthol, Peter; Katsukawa, Yukio; Lagg, Andreas; Solanki, Sami K.; Kubo, Masahito; Riethmueller, Tino; Martínez Pillet, Valentin; Gandorfer, Achim; Feller, Alex; Berkefeld, . Thomas; Orozco Suárez, David; Del Toro Iniesta, Jose Carlos; Bernasconi, Pietro; Álvarez-Herrero, Alberto; Quintero Noda, Carlos Bibcode: 2018cosp...42E.215B Altcode: SUNRISE is a balloon-borne, stratospheric solar observatory dedicated to the investigation of the structure and dynamics of the Sun's magnetic field and its interaction with convective plasma flows and waves. The previous science flights of SUNRISE in 2009 and 2013 have led to many new scientific results, so far described in around 90 refereed publications. This success has shown the huge potential of the SUNRISE concept and the recovery of the largely intact payload offers the opportunity for a third flight.The scientific instrumentation of SUNRISE 3 will have extended capabilities in particular to measure magnetic fields, plasma velocities and temperatures with increased sensitivity and over a larger height range in the solar atmosphere, from the convectively dominated photosphere up to the still poorly understood chromosphere. The latter is the key interaction region between magnetic field, waves and radiation and plays a central role in transporting energy to the outer layers of the solar atmosphere including the corona.SUNRISE 3 will carry 2 new grating-based spectro-polarimeters with slit-scanning and context imaging with slitjaw cameras. The SUNRISE UV Spectro-polarimeter and Imager (SUSI) will explore the rich near-UV range between 300 nm and 430 nm which is poorly accessible from the ground. The SUNRISE Chromospheric Infrared spectro-Polarimeter (SCIP) will sample 2 spectral windows in the near-infrared, containing many spectral lines highly sensitive to magnetic fields at different formation heights. In addition to the two new instruments the Imaging Magnetograph eXperiment (IMaX), an etalon-based tunable filtergraph and spectro-polarimeter flown on both previous missions, will be upgraded to IMaX+, enhancing its cadence and giving access to 2 spectral lines in the visible spectral range. All three instruments will allow investigating both the photosphere and the chromosphere and will ideally complement each other in terms of sensitivity, height coverage and resolution.A new gondola with a sophisticated attitude control system including roll damping will provide improved pointing/tracking performance. Upgraded image stabilization with higher bandwidth will further reduce residual jitter, maximizing the quality of the science data.SUNRISE 3 is a joint project of the German Max-Planck-Institut für Sonnensystemforschung together with the Spanish SUNRISE consortium, the Johns Hopkins University Applied Physics Laboratory, USA, the German Kiepenheuer Institut für Sonnenphysik, the National Astronomical Observatory of Japan and the Japan Aerospace eXploraion Agency (JAXA). Title: Nature of the energy source powering solar coronal loops driven by nanoflares Authors: Chitta, L. P.; Peter, H.; Solanki, S. K. Bibcode: 2018A&A...615L...9C Altcode: 2018arXiv180611045C Context. Magnetic energy is required to heat the corona, the outer atmosphere of the Sun, to millions of degrees.
Aims: We study the nature of the magnetic energy source that is probably responsible for the brightening of coronal loops driven by nanoflares in the cores of solar active regions.
Methods: We consider observations of two active regions (ARs), 11890 and 12234, in which nanoflares have been detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These images are combined with the co-temporal line-of-sight magnetic field maps from the Helioseismic and Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal loops and their magnetic roots in the photosphere.
Results: The core of these ARs exhibit loop brightening in multiple EUV channels of AIA, particularly in its 9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex mixed polarity magnetic field distribution at the base of these loops. We detect the cancellation of photospheric magnetic flux at these locations at a rate of about 1015 Mx s-1. The associated compact coronal brightenings directly above the cancelling magnetic features are indicative of plasma heating due to chromospheric magnetic reconnection.
Conclusions: We suggest that the complex magnetic topology and the evolution of magnetic field, such as flux cancellation in the photosphere and the resulting chromospheric reconnection, can play an important role in energizing active region coronal loops driven by nanoflares. Our estimate of magnetic energy release during flux cancellation in the quiet Sun suggests that chromospheric reconnection can also power the quiet corona.

The movie associated to Fig. 1 is available at https://www.aanda.org Title: Autonomous on-board data processing and instrument calibration software for the SO/PHI Authors: Albert, K.; Hirzberger, J.; Busse, D.; Lange, T.; Kolleck, M.; Fiethe, B.; Orozco Suárez, D.; Woch, J.; Schou, J.; Blanco Rodriguez, J.; Gandorfer, A.; Guan, Y.; Cobos Carrascosa, J. P.; Hernández Expósito, D.; del Toro Iniesta, J. C.; Solanki, S. K.; Michalik, H. Bibcode: 2018SPIE10707E..0OA Altcode: 2018arXiv181003493A The extension of on-board data processing capabilities is an attractive option to reduce telemetry for scientific instruments on deep space missions. The challenges that this presents, however, require a comprehensive software system, which operates on the limited resources a data processing unit in space allows. We implemented such a system for the Polarimetric and Helioseismic Imager (PHI) on-board the Solar Orbiter (SO) spacecraft. It ensures autonomous operation to handle long command-response times, easy changing of the processes after new lessons have been learned and meticulous book-keeping of all operations to ensure scientific accuracy. This contribution presents the requirements and main aspects of the software implementation, followed by an example of a task implemented in the software frame, and results from running it on SO/PHI. The presented example shows that the different parts of the software framework work well together, and that the system processes data as we expect. The flexibility of the framework makes it possible to use it as a baseline for future applications with similar needs and limitations as SO/PHI. Title: The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (PHI) onboard Solar Orbiter Authors: Gandorfer, A.; Grauf, B.; Staub, J.; Bischoff, J.; Woch, J.; Hirzberger, J.; Solanki, S. K.; Álvarez-Herrero, A.; García Parejo, P.; Schmidt, W.; Volkmer, R.; Appourchaux, T.; del Toro Iniesta, J. C. Bibcode: 2018SPIE10698E..4NG Altcode: Solar Orbiter is a joint mission of ESA and NASA scheduled for launch in 2020. Solar Orbiter is a complete and unique heliophysics mission, combining remote sensing and in-situ analysis; its special elliptical orbit allows viewing the Sun from a distance of only 0.28 AU, and - leaving the ecliptic plane - to observe the solar poles from a hitherto unexplored vantage point. One of the key instruments for Solar Orbiter's science is the "Polarimetric and Helioseismic Imager" (PHI), which will provide maps of the solar surface magnetic fields and the gas flows on the visible solar surface. Two telescopes, a full disc imager, and a high resolution channel feed a common Fabry-Perot based tunable filter and thus allow sampling a single Fraunhofer line at 617.3 nm with high spectral resolution; a polarization modulation system makes the system sensitive to the full state of polarization. From the analysis of the Doppler shift and the magnetically induced Zeeman polarization in this line, the magnetic field and the line-of-sight gas motions can be detected for each point in the image. In this paper we describe the opto-mechanical system design of the high resolution telescope. It is based on a decentred Ritchey-Chrétien two-mirror telescope. The telescope includes a Barlow type magnifier lens group, which is used as in-orbit focus compensator, and a beam splitter, which sends a small fraction of the collected light onto a fast camera, which provides the error signals for the actively controlled secondary mirror compensating for spacecraft jitter and other disturbances. The elliptical orbit of the spacecraft poses high demands on the thermo-mechanical stability. The varying size of the solar disk image requires a special false-light suppression architecture, which is briefly described. In combination with a heat-rejecting entrance window, the optical energy impinging on the polarimetric and spectral analysis system is efficiently reduced. We show how the design can preserve the diffraction-limited imaging performance over the design temperature range of -20°C to +60°C. The decentred hyperbolical mirrors require special measures for the inter-alignment and their alignment with respect to the mechanical structure. A system of alignment flats and mechanical references is used for this purpose. We will describe the steps of the alignment procedure, and the dedicated optical ground support equipment, which are needed to reach the diffraction limited performance of the telescope. We will also report on the verification of the telescope performance, both - in ambient condition - and in vacuum at different temperatures. Title: Effect of Transport Coefficients on Excitation of Flare-induced Standing Slow-mode Waves in Coronal Loops Authors: Wang, Tongjiang; Ofman, Leon; Sun, Xudong; Solanki, Sami K.; Davila, Joseph M. Bibcode: 2018ApJ...860..107W Altcode: 2018arXiv180503282W Standing slow-mode waves have been recently observed in flaring loops by the Atmospheric Imaging Assembly of the Solar Dynamics Observatory. By means of the coronal seismology technique, transport coefficients in hot (∼10 MK) plasma were determined by Wang et al., revealing that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude. In this study, we use 1D nonlinear MHD simulations to validate the predicted results from the linear theory and investigate the standing slow-mode wave excitation mechanism. We first explore the wave trigger based on the magnetic field extrapolation and flare emission features. Using a flow pulse driven at one footpoint, we simulate the wave excitation in two types of loop models: Model 1 with the classical transport coefficients and Model 2 with the seismology-determined transport coefficients. We find that Model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than Model 1, in better agreement with the observations. Simulations of the harmonic waves and the Fourier decomposition analysis show that the scaling law between damping time (τ) and wave period (P) follows τ ∝ P 2 in Model 2, while τ ∝ P in Model 1. This indicates that the largely enhanced viscosity efficiently increases the dissipation of higher harmonic components, favoring the quick formation of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are important in understanding both the wave excitation and damping mechanisms. Title: Initial features of an X-class flare observed with SUMER and TRACE Authors: Wang, T. J.; Solanki, S. K.; Innes, D. E.; Curdt, W. Bibcode: 2018arXiv180510418W Altcode: A class X1.5 flare started on the solar limb at 00:43 UT on 21 April 2002, which was associated with a CME observed at 01:27 UT by LASCO C2. The coordinated analyses of this flare include TRACE 195 Å images and SUMER spectra in lines of Fe XXI, Fe XII, and C II. We find that: 1) The flare began with a jet seen by TRACE, which was detected by SUMER in the C II line as a strong brightening with blue shifts up to 170 km s$^{-1}$. At that time only weak emission was detected in Fe XII and Fe XXI. 2) Subsequently, a weak looplike brightening started south of the jet, moving outwards with an average speed of about 150 km s$^{-1}$. The SUMER spectra responded this moving loop as separatingly brightenings, visible only in the Fe XXI line. The southwards moving component contains red- and blue-shifted emission features and has an apparent speed of $\sim$120 km s$^{-1}$. The absence of signatures in Fe XII and C II lines indicates that the moving weak loop seen by TRACE corresponds to the emission from very hot plasma, in a blend line in the 195 Å bandpass due to Fe XXIV formed at T > 10 MK. 3) The trigger mechanism of the flare and associated CME can be interpreted in the same way as that proposed by Wang et al. (2002) for an event with similar initial features. Title: Excitation and Damping of Standing Slow-Mode Waves in Flaring Coronal Loops Authors: Wang, Tongjiang; Ofman, Leon; Sun, Xudong; Solanki, Sami K.; Davila, Joseph M. Bibcode: 2018tess.conf22204W Altcode: We analyze and model a flare-induced longitudinal oscillation event detected by SDO/AIA. The magnetic field extrapolation and flare emission features suggest that the wave event is generated by slipping and null-point-type reconnections in a closed fan-spine magnetic topology, and the large spine loop appears to be heated impulsively to the flare temperature before the wave disturbances travel along it. By means of the seismology technique, we determined the transport coefficients in hot (about 10 MK) plasma, and found that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude from the observed wave properties. Using a flow pulse injected at the loop's footpoint constrained by the observation, we simulate the excitation of slow-mode waves in two types of 1D loop models. We find that the models with the seismology-determined transport coefficients can excite the standing waves quickly as observed, while the models with the classical transport coefficients excites basically the reflecting propagating waves. Simulations of harmonic waves and the Fourier decomposition analysis reveal a scaling between damping time and wave period as Td ~ P in the former type of models, while Td ~ P2 in the latter type. This suggests that anomalously large viscosity can efficiently enhance the dissipation of higher harmonic components, favoring quick setup of the fundamental standing mode. Our study indicates that observational constraints on the transport coefficients are crucial in understanding both the wave excitation and damping mechanisms. Title: Reconstructing Total Solar Irradiance with the help of MHD simulations Authors: Leng Yeo, Kok; Solanki, Sami; Noris, Charlotte; Unruh, Yvonne; Krivova, Natalie; Beeck, Benjamin Bibcode: 2018EGUGA..20.8166L Altcode: The variation in the solar irradiance is an important input into the climate system. Whereas it is commonly thought that solar irradiance variability is driven by the evolving surface magnetism, verifying this assumption has been more difficult. Making use of 3D MHD simulations of the solar atmosphere layers and state-of-the-art solar magnetograms from the Solar Dynamics Observatory, we present a model of total solar irradiance (TSI) that does not require any calibration of the modeled TSI by the measured TSI. I.e., the modeled irradiance variability is entirely independent of the observational record, except for the absolute level. The model replicates 95% of the observed variability between April 2010 and July 2016, leaving little scope for alternative drivers of solar irradiance variability, at least over the time scales examined (days to years). Title: Physical Processes on The Sun's Surface Authors: Solanki, Sami Bibcode: 2018iss..confE..35S Altcode: The Sun, as our nearest star, allows us to probe the surface of a main sequence G star in great detail and to determine the features and processes acting there with the help of high-resolution observations and MHD simulations. These features (possibly in somewhat different guises) and processes are thought to be present/acting also on other cool stars, whose spectra they influence and whose variability and activity they are responsible for. In this presentation a brief introduction will be given to the main physical processes acting in the solar photosphere and the structures and dynamics that they produce. An attempt will also be made to work out what influence these processes and structures have on the Sun seen as an unresolved star. Title: VizieR Online Data Catalog: Reconstructed decadal sunspot numbers (Wu+, 2018) Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G. A.; Baroni, M.; Bard, E.; Solanki, S. K. Bibcode: 2018yCat..36150093W Altcode: The file contains the decadal numbers of sunspot groups including the decade central year [Y], the low [Sl], the mean [SM] and the upper [S_u] 1-sigma (68% bounds).

(1 data file). Title: Solar polarimetry in the K I D2 line : A novel possibility for a stratospheric balloon Authors: Quintero Noda, C.; Villanueva, G. L.; Katsukawa, Y.; Solanki, S. K.; Orozco Suárez, D.; Ruiz Cobo, B.; Shimizu, T.; Oba, T.; Kubo, M.; Anan, T.; Ichimoto, K.; Suematsu, Y. Bibcode: 2018A&A...610A..79Q Altcode: 2018arXiv180101655Q Of the two solar lines, K I D1 and D2, almost all attention so far has been devoted to the D1 line, as D2 is severely affected by an O2 atmospheric band. This, however, makes the latter appealing for balloon and space observations from above (most of) the Earth's atmosphere. We estimate the residual effect of the O2 band on the K I D2 line at altitudes typical for stratospheric balloons. Our aim is to study the feasibility of observing the 770 nm window. Specifically, this paper serves as a preparation for the third flight of the Sunrise balloon-borne observatory. The results indicate that the absorption by O2 is still present, albeit much weaker, at the expected balloon altitude. We applied the obtained O2 transmittance to K I D2 synthetic polarimetric spectra and found that in the absence of line-of-sight motions, the residual O2 has a negligible effect on the K I D2 line. On the other hand, for Doppler-shifted K I D2 data, the residual O2 might alter the shape of the Stokes profiles. However, the residual O2 absorption is sufficiently weak at stratospheric levels that it can be divided out if appropriate measurements are made, something that is impossible at ground level. Therefore, for the first time with Sunrise III, we will be able to perform polarimetric observations of the K I D2 line and, consequently, we will have improved access to the thermodynamics and magnetic properties of the upper photosphere from observations of the K I lines. Title: Long-term Datasets for the Understanding of Solar and Stellar Magnetic Cycles Authors: Banerjee, D.; Jiang, J.; Kusano, K.; Solanki, S. Bibcode: 2018IAUS..340.....B Altcode: No abstract at ADS Title: Ca II K spectroheliograms for studies of long-term changes in solar irradiance Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2018IAUS..340..125C Altcode: 2018arXiv180503928C We address the importance of historical full-disc Ca II K spectroheliograms for solar activity and irradiance reconstruction studies. We review our work on processing such data to enable them to be used in irradiance reconstructions. We also present our preliminary estimates of the plage areas from five of the longest available historical Ca II K archives. Title: The potential of Ca II K observations for solar activity and variability studies Authors: Ermolli, Ilaria; Chatzistergos, Theodosios; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2018IAUS..340..115E Altcode: 2018arXiv180504483E Several observatories around the globe started regular full-disc imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. These observations are continued today at a few sites with either old spectroheliographs or modern telescopes equipped with narrow-band filters. The Ca II K time series are unique in representing long-term variations of the Sun's chromospheric magnetic field. However, meaningful results from their analysis require accurate processing of the available data and robust merging of the information stored in different archives. This paper provides an overview of the historical and modern full-disc Ca II K observations, with focus on their quality and the main results obtained from their analysis over the last decade. Title: Frequently Occurring Reconnection Jets from Sunspot Light Bridges Authors: Tian, Hui; Yurchyshyn, Vasyl; Peter, Hardi; Solanki, Sami K.; Young, Peter R.; Ni, Lei; Cao, Wenda; Ji, Kaifan; Zhu, Yingjie; Zhang, Jingwen; Samanta, Tanmoy; Song, Yongliang; He, Jiansen; Wang, Linghua; Chen, Yajie Bibcode: 2018ApJ...854...92T Altcode: 2018arXiv180106802T Solid evidence of magnetic reconnection is rarely reported within sunspots, the darkest regions with the strongest magnetic fields and lowest temperatures in the solar atmosphere. Using the world’s largest solar telescope, the 1.6 m Goode Solar Telescope, we detect prevalent reconnection through frequently occurring fine-scale jets in the Hα line wings at light bridges, the bright lanes that may divide the dark sunspot core into multiple parts. Many jets have an inverted Y-shape, shown by models to be typical of reconnection in a unipolar field environment. Simultaneous spectral imaging data from the Interface Region Imaging Spectrograph show that the reconnection drives bidirectional flows up to 200 km s-1, and that the weakly ionized plasma is heated by at least an order of magnitude up to ∼80,000 K. Such highly dynamic reconnection jets and efficient heating should be properly accounted for in future modeling efforts of sunspots. Our observations also reveal that the surge-like activity previously reported above light bridges in some chromospheric passbands such as the Hα core has two components: the ever-present short surges likely to be related to the upward leakage of magnetoacoustic waves from the photosphere, and the occasionally occurring long and fast surges that are obviously caused by the intermittent reconnection jets. Title: Solar Magnetic Fields Authors: Balogh, André; Cliver, Edward; Petrie, Gordon; Solanki, Sami; Thompson, Michael; von Steiger, Rudolf Bibcode: 2018smf..book.....B Altcode: No abstract at ADS Title: Solar Magnetoconvection and Small-Scale Dynamo Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2018smf..book..275B Altcode: No abstract at ADS Title: The Influence of Metallicity on Stellar Differential Rotation and Magnetic Activity Authors: Karoff, Christoffer; Metcalfe, Travis S.; Santos, Ângela R. G.; Montet, Benjamin T.; Isaacson, Howard; Witzke, Veronika; Shapiro, Alexander I.; Mathur, Savita; Davies, Guy R.; Lund, Mikkel N.; Garcia, Rafael A.; Brun, Allan S.; Salabert, David; Avelino, Pedro P.; van Saders, Jennifer; Egeland, Ricky; Cunha, Margarida S.; Campante, Tiago L.; Chaplin, William J.; Krivova, Natalie; Solanki, Sami K.; Stritzinger, Maximilian; Knudsen, Mads F. Bibcode: 2018ApJ...852...46K Altcode: 2017arXiv171107716K Observations of Sun-like stars over the past half-century have improved our understanding of how magnetic dynamos, like that responsible for the 11 yr solar cycle, change with rotation, mass, and age. Here we show for the first time how metallicity can affect a stellar dynamo. Using the most complete set of observations of a stellar cycle ever obtained for a Sun-like star, we show how the solar analog HD 173701 exhibits solar-like differential rotation and a 7.4 yr activity cycle. While the duration of the cycle is comparable to that generated by the solar dynamo, the amplitude of the brightness variability is substantially stronger. The only significant difference between HD 173701 and the Sun is its metallicity, which is twice the solar value. Therefore, this provides a unique opportunity to study the effect of the higher metallicity on the dynamo acting in this star and to obtain a comprehensive understanding of the physical mechanisms responsible for the observed photometric variability. The observations can be explained by the higher metallicity of the star, which is predicted to foster a deeper outer convection zone and a higher facular contrast, resulting in stronger variability. Title: Analysis of full disc Ca II K spectroheliograms. I. Photometric calibration and centre-to-limb variation compensation Authors: Chatzistergos, Theodosios; Ermolli, Ilaria; Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2018A&A...609A..92C Altcode: 2017arXiv171004949C Context. Historical Ca II K spectroheliograms (SHG) are unique in representing long-term variations of the solar chromospheric magnetic field. They usually suffer from numerous problems and lack photometric calibration. Thus accurate processing of these data is required to get meaningful results from their analysis.
Aims: In this paper we aim at developing an automatic processing and photometric calibration method that provides precise and consistent results when applied to historical SHG.
Methods: The proposed method is based on the assumption that the centre-to-limb variation of the intensity in quiet Sun regions does not vary with time. We tested the accuracy of the proposed method on various sets of synthetic images that mimic problems encountered in historical observations. We also tested our approach on a large sample of images randomly extracted from seven different SHG archives.
Results: The tests carried out on the synthetic data show that the maximum relative errors of the method are generally <6.5%, while the average error is <1%, even if rather poor quality observations are considered. In the absence of strong artefacts the method returns images that differ from the ideal ones by <2% in any pixel. The method gives consistent values for both plage and network areas. We also show that our method returns consistent results for images from different SHG archives.
Conclusions: Our tests show that the proposed method is more accurate than other methods presented in the literature. Our method can also be applied to process images from photographic archives of solar observations at other wavelengths than Ca II K. Title: Evershed and Counter-Evershed Flows in Sunspot MHD Simulations Authors: Siu-Tapia, A. L.; Rempel, M.; Lagg, A.; Solanki, S. K. Bibcode: 2018ApJ...852...66S Altcode: 2017arXiv171201202S There have been a few reports in the literature of counter-Evershed flows observed in well-developed sunspot penumbrae, i.e., flows directed toward the umbra along penumbral filaments. Here, we investigate the driving forces of such counter-Evershed flows in a radiative magnetohydrodynamic simulation of a sunspot, and compare them to the forces acting on the normal Evershed flow. The simulation covers a timespan of 100 solar hours and generates an Evershed outflow exceeding 8 km s-1 in the penumbra along radially aligned filaments where the magnetic field is almost horizontal. Additionally, the simulation produces a fast counter-Evershed flow (i.e., an inflow near τ =1) in some regions within the penumbra, reaching peak flow speeds of ∼12 km s-1. The counter-Evershed flows are transient and typically last a few hours before they turn into outflows again. By using the kinetic energy equation and evaluating its various terms in the simulation box, we found that the Evershed flow occurs due to overturning convection in a strongly inclined magnetic field, while the counter-Evershed flows can be well-described as siphon flows. Title: Solar Spectral Irradiance Reconstruction over 9 Millennia from a Composite 14C and 10Be Series Authors: Wu, C. J.; Usoskin, I. G.; Krivova, N.; Kovaltsov, G.; Solanki, S. K. Bibcode: 2017AGUFMSH42A..03W Altcode: The Sun is the main external energy source to the Earth and thus the knowledge of solar variability on different time scales is important for understanding the solar influence on the terrestrial atmosphere and climate. The overall energy input and its spectral distribution are described by the total (TSI) and spectral (SSI) solar irradiance, respectively. Direct measurements of the solar irradiance provide information on solar variability on the decadal and shorter time scales, while the sunspot number record covers four centuries. On yet longer time scales only indirect proxies can be used, such as the concentrations of the cosmogenic isotopes 10Be and 14C in terrestrial archives. These isotopes are produced in the terrestrial atmosphere by impinging cosmic rays, whose flux is modulated by solar activity. Therefore the isotope data retrieved from various natural archives around the globe show a very high degree of similarity reflecting changes in the solar activity. Nevertheless, significant short-term deviations can be observed due to the different geochemical production processes and local climatic conditions. We will present the newest TSI/SSI reconstruction over the last 9000 years based on a new consistent composite multi-isotope proxy series. The solar irradiance reconstruction reveals the global and robust pattern of solar variability in the past. Title: Probing photospheric magnetic fields with new spectral line pairs Authors: Smitha, H. N.; Solanki, S. K. Bibcode: 2017A&A...608A.111S Altcode: 2017arXiv170908926S Context. The magnetic line ratio (MLR) method has been extensively used in the measurement of photospheric magnetic field strength. It was devised for the neutral iron line pair at 5247.1 Å and 5250.2 Å (5250 Å pair). Other line pairs as well-suited as this pair have not been reported in the literature.
Aims: The aim of the present work is to identify new line pairs useful for the MLR technique and to test their reliability.
Methods: We used a three-dimensional magnetohydrodynamic 3D MHD simulation representing the quiet Sun atmosphere to synthesize the Stokes profiles. Then, we applied the MLR technique to the Stokes V profiles to recover the fields in the MHD cube both at original resolution and after degrading with a point spread function. In both these cases, we aim to empirically represent the field strengths returned by the MLR method in terms of the field strengths in the MHD cube.
Results: We have identified two new line pairs that are very well adapted to be used for MLR measurements. The first pair is in the visible, Fe I 6820-6842 Å, whose intensity profiles have previously been used to measure stellar magnetic fields, and the other pair is in the infrared (IR), Fe I 15 534-15 542 Å. The lines in these pairs reproduce the magnetic fields in the MHD cube rather well and, in fact, somewhat better than the original 5250 Å pair.
Conclusions: The newly identified line pairs complement the old pairs. The lines in the new IR pair, because of their higher Zeeman sensitivity, are ideal for the measurement of weak fields. The new visible pair works best above 300 G. The new IR pair, due to its large Stokes V signal samples more fields in the MHD cube than the old IR pair at 1.56 μm, even in the presence of noise, and hence likely also on the real Sun. Owing to their low formation heights (100-200 km above τ5000 = 1), both the new line pairs are well suited for probing magnetic fields in the lower photosphere. Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm Authors: Iwai, K.; Loukitcheva, M.; Shimojo, M.; Solanki, S. K.; White, S. M. Bibcode: 2017AGUFMSH43A2806I Altcode: We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are among the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ = 3 mm (100 GHz) mosaic image obtained by ALMA that includes a large sunspot within the active region AR12470, on 2015 December 16. The 3 mm map has a 300''×300'' field of view and 4.9''×2.2'' spatial resolution, which is the highest spatial resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3 mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slit-jaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ = 3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume, since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å. Title: METIS, the Multi Element Telescope for Imaging and Spectroscopy: an instrument proposed for the solar orbiter mission Authors: Antonucci, E.; Andretta, V.; Cesare, S.; Ciaravella, A.; Doschek, G.; Fineschi, S.; Giordano, S.; Lamy, P.; Moses, D.; Naletto, G.; Newmark, J.; Poletto, L.; Romoli, M.; Solanki, S.; Spadaro, D.; Teriaca, L.; Zangrilli, L. Bibcode: 2017SPIE10566E..0LA Altcode: METIS, the Multi Element Telescope for Imaging and Spectroscopy, is an instrument proposed to the European Space Agency to be part of the payload of the Solar Orbiter mission. The instrument design has been conceived for performing extreme ultraviolet (EUV) spectroscopy both on the solar disk and off-limb, and near-Sun coronagraphy and spectroscopy. The proposed instrument suite consists of three different interconnected elements, COR, EUS and SOCS, sharing the same optical bench, electronics, and S/C heat shield aperture. COR is a visible-EUV multiband coronagraph based on a classical externally occulted design. EUS is the component of the METIS EUV disk spectrometer which includes the telescope and all the related mechanisms. Finally, SOCS is the METIS spectroscopic component including the dispersive system and the detectors. The capability of inserting a small telescope collecting coronal light has been added to perform also EUV coronal spectroscopy. METIS can simultaneously image the visible and ultraviolet emission of the solar corona and diagnose, with unprecedented temporal coverage and space resolution the structure and dynamics of the full corona in the range from 1.2 to 3.0 (1.6 to 4.1) solar radii (R⊙, measured from Sun centre) at minimum (maximum) perihelion during the nominal mission. It can also perform spectroscopic observations of the solar disk and out to 1.4 R⊙ within the 50-150 nm spectral region, and of the geo-effective coronal region 1.7-2.7 R⊙ within the 30-125 nm spectral band. Title: Normal and counter Evershed flows in the photospheric penumbra of a sunspot. SPINOR 2D inversions of Hinode-SOT/SP observations Authors: Siu-Tapia, A.; Lagg, A.; Solanki, S. K.; van Noort, M.; Jurčák, J. Bibcode: 2017A&A...607A..36S Altcode: 2017arXiv170907386S Context. The Evershed effect, a nearly horizontal outflow of material seen in the penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the penumbra, also known as counter Evershed flows.
Aims: Here we present a rare feature observed in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows.
Methods: We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated.
Results: In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5-2 kG to regions with stronger fields. The sources and sinks of both penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments).
Conclusions: The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows. Title: Solar ALMA Observations: Constraining the Chromosphere above Sunspots Authors: Loukitcheva, Maria A.; Iwai, Kazumasa; Solanki, Sami K.; White, Stephen M.; Shimojo, Masumi Bibcode: 2017ApJ...850...35L Altcode: 2017arXiv171003812L We present the first high-resolution Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a sunspot at wavelengths of 1.3 and 3 mm, obtained during the solar ALMA Science Verification campaign in 2015, and compare them with the predictions of semi-empirical sunspot umbral/penumbral atmosphere models. For the first time, millimeter observations of sunspots have resolved umbral/penumbral brightness structure at the chromospheric heights, where the emission at these wavelengths is formed. We find that the sunspot umbra exhibits a radically different appearance at 1.3 and 3 mm, whereas the penumbral brightness structure is similar at the two wavelengths. The inner part of the umbra is ∼600 K brighter than the surrounding quiet Sun (QS) at 3 mm and is ∼700 K cooler than the QS at 1.3 mm, being the coolest part of sunspot at this wavelength. On average, the brightness of the penumbra at 3 mm is comparable to the QS brightness, while at 1.3 mm it is ∼1000 K brighter than the QS. Penumbral brightness increases toward the outer boundary in both ALMA bands. Among the tested umbral models, that of Severino et al. provides the best fit to the observational data, including both the ALMA data analyzed in this study and data from earlier works. No penumbral model among those considered here gives a satisfactory fit to the currently available measurements. ALMA observations at multiple millimeter wavelengths can be used for testing existing sunspot models, and serve as an important input to constrain new empirical models. Title: The Small-scale Structure of Photospheric Convection Retrieved by a Deconvolution Technique Applied to Hinode/SP Data Authors: Oba, T.; Riethmüller, T. L.; Solanki, S. K.; Iida, Y.; Quintero Noda, C.; Shimizu, T. Bibcode: 2017ApJ...849....7O Altcode: 2017arXiv170906933O Solar granules are bright patterns surrounded by dark channels, called intergranular lanes, in the solar photosphere and are a manifestation of overshooting convection. Observational studies generally find stronger upflows in granules and weaker downflows in intergranular lanes. This trend is, however, inconsistent with the results of numerical simulations in which downflows are stronger than upflows through the joint action of gravitational acceleration/deceleration and pressure gradients. One cause of this discrepancy is the image degradation caused by optical distortion and light diffraction and scattering that takes place in an imaging instrument. We apply a deconvolution technique to Hinode/SP data in an attempt to recover the original solar scene. Our results show a significant enhancement in both the convective upflows and downflows but particularly for the latter. After deconvolution, the up- and downflows reach maximum amplitudes of -3.0 km s-1 and +3.0 km s-1 at an average geometrical height of roughly 50 km, respectively. We found that the velocity distributions after deconvolution match those derived from numerical simulations. After deconvolution, the net LOS velocity averaged over the whole field of view lies close to zero as expected in a rough sense from mass balance. Title: The Maximum Entropy Limit of Small-scale Magnetic Field Fluctuations in the Quiet Sun Authors: Gorobets, A. Y.; Berdyugina, S. V.; Riethmüller, T. L.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..233....5G Altcode: 2017arXiv171008361G The observed magnetic field on the solar surface is characterized by a very complex spatial and temporal behavior. Although feature-tracking algorithms have allowed us to deepen our understanding of this behavior, subjectivity plays an important role in the identification and tracking of such features. In this paper, we continue studies of the temporal stochasticity of the magnetic field on the solar surface without relying either on the concept of magnetic features or on subjective assumptions about their identification and interaction. We propose a data analysis method to quantify fluctuations of the line-of-sight magnetic field by means of reducing the temporal field’s evolution to the regular Markov process. We build a representative model of fluctuations converging to the unique stationary (equilibrium) distribution in the long time limit with maximum entropy. We obtained different rates of convergence to the equilibrium at fixed noise cutoff for two sets of data. This indicates a strong influence of the data spatial resolution and mixing-polarity fluctuations on the relaxation process. The analysis is applied to observations of magnetic fields of the relatively quiet areas around an active region carried out during the second flight of the Sunrise/IMaX and quiet Sun areas at the disk center from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory satellite. Title: Flows along arch filaments observed in the GRIS `very fast spectroscopic mode' Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.; Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier, R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2017IAUS..327...28G Altcode: 2017arXiv170102206G A new generation of solar instruments provides improved spectral, spatial, and temporal resolution, thus facilitating a better understanding of dynamic processes on the Sun. High-resolution observations often reveal multiple-component spectral line profiles, e.g., in the near-infrared He i 10830 Å triplet, which provides information about the chromospheric velocity and magnetic fine structure. We observed an emerging flux region, including two small pores and an arch filament system, on 2015 April 17 with the `very fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS) situated at the 1.5-meter GREGOR solar telescope at Observatorio del Teide, Tenerife, Spain. We discuss this method of obtaining fast (one per minute) spectral scans of the solar surface and its potential to follow dynamic processes on the Sun. We demonstrate the performance of the `very fast spectroscopic mode' by tracking chromospheric high-velocity features in the arch filament system. Title: Spectral variability of photospheric radiation due to faculae. I. The Sun and Sun-like stars Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne C.; Solanki, Sami K.; Krivova, Natalie A.; Yeo, Kok Leng Bibcode: 2017A&A...605A..45N Altcode: 2017arXiv170504455N Context. Stellar spectral variability on timescales of a day and longer, arising from magnetic surface features such as dark spots and bright faculae, is an important noise source when characterising extra-solar planets. Current 1D models of faculae do not capture the geometric properties and fail to reproduce observed solar facular contrasts. Magnetoconvection simulations provide facular contrasts accounting for geometry.
Aims: We calculate facular contrast spectra from magnetoconvection models of the solar photosphere with a view to improve (a) future parameter determinations for planets with early G type host stars and (b) reconstructions of solar spectral variability.
Methods: Regions of a solar twin (G2, log g = 4.44) atmosphere with a range of initial average vertical magnetic fields (100 to 500 G) were simulated using a 3D radiation-magnetohydrodynamics code, MURaM, and synthetic intensity spectra were calculated from the ultraviolet (149.5 nm) to the far infrared (160 000 nm) with the ATLAS9 radiative transfer code. Nine viewing angles were investigated to account for facular positions across most of the stellar disc.
Results: Contrasts of the radiation from simulation boxes with different levels of magnetic flux relative to an atmosphere with no magnetic field are a complicated function of position, wavelength and magnetic field strength that is not reproduced by 1D facular models. Generally, contrasts increase towards the limb, but at UV wavelengths a saturation and decrease are observed close to the limb. Contrasts also increase strongly from the visible to the UV; there is a rich spectral dependence, with marked peaks in molecular bands and strong spectral lines. At disc centre, a complex relationship with magnetic field was found and areas of strong magnetic field can appear either dark or bright, depending on wavelength. Spectra calculated for a wide variety of magnetic fluxes will also serve to improve total and spectral solar irradiance reconstructions. Title: Solar Irradiance Variability is Caused by the Magnetic Activity on the Solar Surface Authors: Yeo, K. L.; Solanki, S. K.; Norris, C. M.; Beeck, B.; Unruh, Y. C.; Krivova, N. A. Bibcode: 2017PhRvL.119i1102Y Altcode: 2017arXiv170900920Y The variation in the radiative output of the Sun, described in terms of solar irradiance, is important to climatology. A common assumption is that solar irradiance variability is driven by its surface magnetism. Verifying this assumption has, however, been hampered by the fact that models of solar irradiance variability based on solar surface magnetism have to be calibrated to observed variability. Making use of realistic three-dimensional magnetohydrodynamic simulations of the solar atmosphere and state-of-the-art solar magnetograms from the Solar Dynamics Observatory, we present a model of total solar irradiance (TSI) that does not require any such calibration. In doing so, the modeled irradiance variability is entirely independent of the observational record. (The absolute level is calibrated to the TSI record from the Total Irradiance Monitor.) The model replicates 95% of the observed variability between April 2010 and July 2016, leaving little scope for alternative drivers of solar irradiance variability at least over the time scales examined (days to years). Title: Solar Magnetoconvection and Small-Scale Dynamo. Recent Developments in Observation and Simulation Authors: Borrero, J. M.; Jafarzadeh, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2017SSRv..210..275B Altcode: 2015SSRv..tmp..113B; 2015arXiv151104214B A number of observational and theoretical aspects of solar magnetoconvection are considered in this review. We discuss recent developments in our understanding of the small-scale structure of the magnetic field on the solar surface and its interaction with convective flows, which is at the centre of current research. Topics range from plage areas in active regions over the magnetic network shaped by supergranulation to the ubiquituous `turbulent' internetwork fields. On the theoretical side, we focus upon magnetic field generation by small-scale dynamo action. Title: Three-dimensional magnetic structure of a sunspot: Comparison of the photosphere and upper chromosphere Authors: Joshi, Jayant; Lagg, Andreas; Hirzberger, Johann; Solanki, Sami K. Bibcode: 2017A&A...604A..98J Altcode: 2017arXiv170508404J
Aims: We investigate the magnetic field of a sunspot in the upper chromosphere and compare it to the photospheric properties of the field.
Methods: We observed the main leading sunspot of the active region NOAA 11124 during two days with the Tenerife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). Through inversion of Stokes spectra of the He I triplet at 10 830 Å, we obtained the magnetic field vector of the upper chromosphere. For comparison with the photosphere, we applied height-dependent inversions of the Si I 10 827.1 Å and Ca I 10 833.4 Å lines.
Results: We found that the umbral magnetic field strength in the upper chromosphere is lower by a factor of 1.30-1.65 compared to the photosphere. The magnetic field strength of the umbra decreases from the photosphere toward the upper chromosphere by an average rate of 0.5-0.9 G km-1. The difference in the magnetic field strength between both atmospheric layers steadily decreases from the sunspot center to the outer boundary of the sunspot; the field, in particular its horizontal component, is stronger in the chromopshere outside the spot and this is suggestive of a magnetic canopy. The sunspot displays a twist that on average is similar in the two layers. However, the differential twist between the photosphere and chromosphere increases rapidly toward the outer penumbral boundary. The magnetic field vector is more horizontal with respect to the solar surface by roughly 5-20° in the photosphere compared to the upper chromosphere. Above a lightbridge, the chromospheric magnetic field is equally strong as that in the umbra, whereas the field of the lightbridge is weaker than its surroundings in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field structure above the lightbridge. Title: The Solar Ultraviolet Imaging Telescope on-board Aditya-L1 Authors: Tripathi, Durgesh; Ramaprakash, A. N.; Khan, Aafaque; Ghosh, Avyarthana; Chatterjee, Subhamoy; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K. Bibcode: 2017CSci..113..616T Altcode: 2022arXiv220407732T The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 mission of ISRO that will measure and monitor the solar radiation emitted in the near-ultraviolet wavelength range (200-400 nm). SUIT will simultaneously map the photosphere and the chromosphere of the Sun using 11 filters sensitive to different wavelengths and covering different heights in the solar atmosphere and help us understand the processes involved in the transfer of mass and energy from one layer to the other. SUIT will also allow us to measure and monitor spatially resolved solar spectral irradiance that governs the chemistry of oxygen and ozone in the stratosphere of Earth's atmosphere. This is central to our understanding of the Sun climate relationship. Title: The nature of solar brightness variations Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Yeo, K. L.; Schmutz, W. K. Bibcode: 2017NatAs...1..612S Altcode: 2017arXiv171104156S Determining the sources of solar brightness variations1,2, often referred to as solar noise3, is important because solar noise limits the detection of solar oscillations3, is one of the drivers of the Earth's climate system4,5 and is a prototype of stellar variability6,7—an important limiting factor for the detection of extrasolar planets. Here, we model the magnetic contribution to solar brightness variability using high-cadence8,9 observations from the Solar Dynamics Observatory (SDO) and the Spectral And Total Irradiance REconstruction (SATIRE)10,11 model. The brightness variations caused by the constantly evolving cellular granulation pattern on the solar surface were computed with the Max Planck Institute for Solar System Research (MPS)/University of Chicago Radiative Magnetohydrodynamics (MURaM)12 code. We found that the surface magnetic field and granulation can together precisely explain solar noise (that is, solar variability excluding oscillations) on timescales from minutes to decades, accounting for all timescales that have so far been resolved or covered by irradiance measurements. We demonstrate that no other sources of variability are required to explain the data. Recent measurements of Sun-like stars by the COnvection ROtation and planetary Transits (CoRoT)13 and Kepler14 missions uncovered brightness variations similar to that of the Sun, but with a much wider variety of patterns15. Our finding that solar brightness variations can be replicated in detail with just two well-known sources will greatly simplify future modelling of existing CoRoT and Kepler as well as anticipated Transiting Exoplanet Survey Satellite16 and PLAnetary Transits and Oscillations of stars (PLATO)17 data. Title: The variability of magnetic activity in solar-type stars Authors: Fabbian, D.; Simoniello, R.; Collet, R.; Criscuoli, S.; Korhonen, H.; Krivova, N. A.; Oláh, K.; Jouve, L.; Solanki, S. K.; Alvarado-Gómez, J. D.; Booth, R.; García, R. A.; Lehtinen, J.; See, V. Bibcode: 2017AN....338..753F Altcode: No abstract at ADS Title: Association of solar coronal loops to photospheric magnetic field Authors: Pradeep Chitta, Lakshmi; Peter, Hardi; Solanki, Sami Bibcode: 2017SPD....4810630P Altcode: Magnetic connectivity and its evolution from the solar photosphere to the corona will play a crucial role in the energetics of the solar atmosphere. To explore this connectivity, we use high spatial resolution magnetic field observations of an active region from the balloon-borne SUNRISE telescope, in combination with the observations of coronal loops imaged in extreme ultraviolet by SDO/AIA. We show that photospheric magnetic field at the base of coronal loops is rapidly evolving through small-scale flux emergence and cancellation events with rates on the order of 10^15 Mx/s. When observed at high spatial resolution better than 0.5 arcsec, we find that basically all coronal loops considered so far are rooted in the photosphere above small-scale opposite polarity magnetic field patches. In the photosphere, the magnetic field threading coronal loops is interacting with opposite polarity parasitic magnetic concentrations leading to dynamic signatures in the upper atmosphere. Chromospheric small-scale jets aligned to coronal loops are observed at these locations. We will present preliminary results from 3D MHD simulations of coronal loops driven by realistic magneto-convection and discuss what role the magnetic interactions at coronal loop footpoints could play in the evolution of coronal loops and their heating. Title: The Sunrise balloon-borne observatory: Results from the second flight and outlook on the third flight Authors: Solanki, Sami K. Bibcode: 2017SPD....4810507S Altcode: The balloon-borne solar observatory Sunrise flew for a second time in June 2013 and provided seeing-free UV images and spectropolarimetric data at close to the diffraction limit of the 1m telescope. The data analysis has so far concentrated on a time series of the heart of an active region recorded in the Stokes vector of the Fe I 525.02 nm line and a time series of UV images of the same region. First results were published in a special issue of the ApJ Supplement, vol. 229, in April 2017. The data suggest the presence of very strong fields in pores, a low-lying canopy of slender fibrils and different types of waves propagating along these fibrils. Furthermore, properties of the complex emergence of magnetic flux, of moving magnetic features around a pore and of a siphon flow along a low-lying slender magnetic loop are determined. A novel technique for inversions of Stokes profiles including constraints to make the results more physically consistent has also been developed and for the first time applied to Sunrise II data. In addition, the presence of small-scale mixed polarities and chromospheric jets was detected at the footpoints of particularly bright coronal loops. These and more results will be briefly presented. Preparations for the next flight of Sunrise have started. Many new features are being designed for this flight, which will allow it to reach considerably extended science goals. Thus it will feature two new spectropolarimeters, one exploring the near UV (from MPS), the other concentrating on chromospheric fields and their connection to the photosphere (led by NAOJ). The IMaX vector-magnetograph (provided by a Spanish consortium) will also be updated to be able to sample multiple spectral lines. Finally, Sunrise will have a new gondola (coming from APL) and an improved image stabilization system (from KIS). Title: ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm Authors: Iwai, Kazumasa; Loukitcheva, Maria; Shimojo, Masumi; Solanki, Sami K.; White, Stephen M. Bibcode: 2017ApJ...841L..20I Altcode: 2017arXiv170503147I We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are among the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ = 3 mm (100 GHz) mosaic image obtained by ALMA that includes a large sunspot within the active region AR12470, on 2015 December 16. The 3 mm map has a 300″ × 300″ field of view and 4.″9 × 2.″2 spatial resolution, which is the highest spatial resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3 mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slit-jaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ = 3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume, since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å. Title: New reconstruction of the sunspot group numbers since 1739 using direct calibration and "backbone" methods Authors: Chatzistergos, Theodosios; Usoskin, Ilya G.; Kovaltsov, Gennady A.; Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2017A&A...602A..69C Altcode: 2017arXiv170206183C Context. The group sunspot number (GSN) series constitute the longest instrumental astronomical database providing information on solar activity. This database is a compilation of observations by many individual observers, and their inter-calibration has usually been performed using linear rescaling. There are multiple published series that show different long-term trends for solar activity.
Aims: We aim at producing a GSN series, with a non-linear non-parametric calibration. The only underlying assumptions are that the differences between the various series are due to different acuity thresholds of the observers, and that the threshold of each observer remains constant throughout the observing period.
Methods: We used a daisy chain process with backbone (BB) observers and calibrated all overlapping observers to them. We performed the calibration of each individual observer with a probability distribution function (PDF) matrix constructed considering all daily values for the overlapping period with the BB. The calibration of the BBs was carried out in a similar manner. The final series was constructed by merging different BB series. We modelled the propagation of errors straightforwardly with Monte Carlo simulations. A potential bias due to the selection of BBs was investigated and the effect was shown to lie within the 1σ interval of the produced series. The exact selection of the reference period was shown to have a rather small effect on our calibration as well.
Results: The final series extends back to 1739 and includes data from 314 observers. This series suggests moderate activity during the 18th and 19th century, which is significantly lower than the high level of solar activity predicted by other recent reconstructions applying linear regressions.
Conclusions: The new series provides a robust reconstruction, based on modern and non-parametric methods, of sunspot group numbers since 1739, and it confirms the existence of the modern grand maximum of solar activity in the second half of the 20th century.

Values of the group sunspot number series are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A69 Title: Millimeter radiation from a 3D model of the solar atmosphere. II. Chromospheric magnetic field Authors: Loukitcheva, M.; White, S. M.; Solanki, S. K.; Fleishman, G. D.; Carlsson, M. Bibcode: 2017A&A...601A..43L Altcode: 2017arXiv170206018L
Aims: We use state-of-the-art, three-dimensional non-local thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic simulations of the quiet solar atmosphere to carry out detailed tests of chromospheric magnetic field diagnostics from free-free radiation at millimeter and submillimeter wavelengths (mm/submm).
Methods: The vertical component of the magnetic field was deduced from the mm/submm brightness spectra and the degree of circular polarization synthesized at millimeter frequencies. We used the frequency bands observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) as a convenient reference. The magnetic field maps obtained describe the longitudinal magnetic field at the effective formation heights of the relevant wavelengths in the solar chromosphere.
Results: The comparison of the deduced and model chromospheric magnetic fields at the spatial resolution of both the model and current observations demonstrates a good correlation, but has a tendency to underestimate the model field. The systematic discrepancy of about 10% is probably due to averaging of the restored field over the heights contributing to the radiation, weighted by the strength of the contribution. On the whole, the method of probing the longitudinal component of the magnetic field with free-free emission at mm/submm wavelengths is found to be applicable to measurements of the weak quiet-Sun magnetic fields. However, successful exploitation of this technique requires very accurate measurements of the polarization properties (primary beam and receiver polarization response) of the antennas, which will be the principal factor that determines the level to which chromospheric magnetic fields can be measured.
Conclusions: Consequently, high-resolution and high-precision observations of circularly polarized radiation at millimeter wavelengths can be a powerful tool for producing chromospheric longitudinal magnetograms. Title: Erratum: Morphological Properties of Slender CaII H Fibrils Observed by sunrise II (ApJS 229, 1, 6) Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..230...11G Altcode: No abstract at ADS Title: High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI Authors: Jafarzadeh, S.; Solanki, S. K.; Stangalini, M.; Steiner, O.; Cameron, R. H.; Danilovic, S. Bibcode: 2017ApJS..229...10J Altcode: 2016arXiv161109302J We characterize waves in small magnetic elements and investigate their propagation in the lower solar atmosphere from observations at high spatial and temporal resolution. We use the wavelet transform to analyze oscillations of both horizontal displacement and intensity in magnetic bright points found in the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the Sunrise balloon-borne solar observatory. Phase differences between the oscillations at the two atmospheric layers corresponding to the two passbands reveal upward propagating waves at high frequencies (up to 30 mHz). Weak signatures of standing as well as downward propagating waves are also obtained. Both compressible and incompressible (kink) waves are found in the small-scale magnetic features. The two types of waves have different, though overlapping, period distributions. Two independent estimates give a height difference of approximately 450 ± 100 km between the two atmospheric layers sampled by the employed spectral bands. This value, together with the determined short travel times of the transverse and longitudinal waves provide us with phase speeds of 29 ± 2 km s-1 and 31 ± 2 km s-1, respectively. We speculate that these phase speeds may not reflect the true propagation speeds of the waves. Thus, effects such as the refraction of fast longitudinal waves may contribute to an overestimate of the phase speed. Title: Slender Ca II H Fibrils Mapping Magnetic Fields in the Low Solar Chromosphere Authors: Jafarzadeh, S.; Rutten, R. J.; Solanki, S. K.; Wiegelmann, T.; Riethmüller, T. L.; van Noort, M.; Szydlarski, M.; Blanco Rodríguez, J.; Barthol, P.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez, D.; Schmidt, W. Bibcode: 2017ApJS..229...11J Altcode: 2016arXiv161003104J A dense forest of slender bright fibrils near a small solar active region is seen in high-quality narrowband Ca II H images from the SuFI instrument onboard the Sunrise balloon-borne solar observatory. The orientation of these slender Ca II H fibrils (SCF) overlaps with the magnetic field configuration in the low solar chromosphere derived by magnetostatic extrapolation of the photospheric field observed with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are qualitatively aligned with small-scale loops computed from a novel inversion approach based on best-fit numerical MHD simulation. Such loops are organized in canopy-like arches over quiet areas that differ in height depending on the field strength near their roots. Title: EMPIRE: A robust empirical reconstruction of solar irradiance variability Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2017JGRA..122.3888Y Altcode: 2017arXiv170407652Y We present a new empirical model of total and spectral solar irradiance (TSI and SSI) variability entitled EMPirical Irradiance REconstruction (EMPIRE). As with existing empirical models, TSI and SSI variability is given by the linear combination of solar activity indices. In empirical models, UV SSI variability is usually determined by fitting the rotational variability in activity indices to that in measurements. Such models have to date relied on ordinary least squares regression, which ignores the uncertainty in the activity indices. In an advance from earlier efforts, the uncertainty in the activity indices is accounted for in EMPIRE by the application of an error-in-variables regression scheme, making the resultant UV SSI variability more robust. The result is consistent with observations and unprecedentedly, with that from other modeling approaches, resolving the long-standing controversy between existing empirical models and other types of models. We demonstrate that earlier empirical models, by neglecting the uncertainty in activity indices, underestimate UV SSI variability. The reconstruction of TSI and visible and IR SSI from EMPIRE is also shown to be consistent with observations. The EMPIRE reconstruction is of utility to climate studies as a more robust alternative to earlier empirical reconstructions. Title: Magneto-static Modeling from Sunrise/IMaX: Application to an Active Region Observed with Sunrise II Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...18W Altcode: 2017arXiv170101458N; 2017arXiv170101458W Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the Sunrise balloon-borne solar observatory in 2013 June as boundary conditions for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which was applied earlier to a quiet-Sun region observed with Sunrise I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet-Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110-130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid-chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower-resolution photospheric measurements in the past. The linear model does not, however, permit us to model intrinsic nonlinear structures like strongly localized electric currents. Title: The Second Flight of the Sunrise Balloon-borne Solar Observatory: Overview of Instrument Updates, the Flight, the Data, and First Results Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic, S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott, D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez, J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta, J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.; Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati, L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.; Knölker, M.; Lecinski, A. Bibcode: 2017ApJS..229....2S Altcode: 2017arXiv170101555S The Sunrise balloon-borne solar observatory, consisting of a 1 m aperture telescope that provides a stabilized image to a UV filter imager and an imaging vector polarimeter, carried out its second science flight in 2013 June. It provided observations of parts of active regions at high spatial resolution, including the first high-resolution images in the Mg II k line. The obtained data are of very high quality, with the best UV images reaching the diffraction limit of the telescope at 3000 Å after Multi-Frame Blind Deconvolution reconstruction accounting for phase-diversity information. Here a brief update is given of the instruments and the data reduction techniques, which includes an inversion of the polarimetric data. Mainly those aspects that evolved compared with the first flight are described. A tabular overview of the observations is given. In addition, an example time series of a part of the emerging active region NOAA AR 11768 observed relatively close to disk center is described and discussed in some detail. The observations cover the pores in the trailing polarity of the active region, as well as the polarity inversion line where flux emergence was ongoing and a small flare-like brightening occurred in the course of the time series. The pores are found to contain magnetic field strengths ranging up to 2500 G, and while large pores are clearly darker and cooler than the quiet Sun in all layers of the photosphere, the temperature and brightness of small pores approach or even exceed those of the quiet Sun in the upper photosphere. Title: Estimation of the Magnetic Flux Emergence Rate in the Quiet Sun from Sunrise Data Authors: Smitha, H. N.; Anusha, L. S.; Solanki, S. K.; Riethmüller, T. L. Bibcode: 2017ApJS..229...17S Altcode: 2016arXiv161106432S Small-scale internetwork (IN) features are thought to be the major source of fresh magnetic flux in the quiet Sun. During its first science flight in 2009, the balloon-borne observatory Sunrise captured images of the magnetic fields in the quiet Sun at a high spatial resolution. Using these data we measure the rate at which the IN features bring magnetic flux to the solar surface. In a previous paper it was found that the lowest magnetic flux in small-scale features detected using the Sunrise observations is 9 × 1014 Mx. This is nearly an order of magnitude smaller than the smallest fluxes of features detected in observations from the Hinode satellite. In this paper, we compute the flux emergence rate (FER) by accounting for such small fluxes, which was not possible before Sunrise. By tracking the features with fluxes in the range {10}15{--}{10}18 Mx, we measure an FER of 1100 {Mx} {{cm}}-2 {{day}}-1. The smaller features with fluxes ≤slant {10}16 Mx are found to be the dominant contributors to the solar magnetic flux. The FER found here is an order of magnitude higher than the rate from Hinode, obtained with a similar feature tracking technique. A wider comparison with the literature shows, however, that the exact technique of determining the rate of the appearance of new flux can lead to results that differ by up to two orders of magnitude, even when applied to similar data. The causes of this discrepancy are discussed and first qualitative explanations proposed. Title: Vertical magnetic field gradient in the photospheric layers of sunspots Authors: Joshi, Jayant; Lagg, Andreas; Hirzberger, Johann; Solanki, Sami K.; Tiwari, Sanjiv K. Bibcode: 2017A&A...599A..35J Altcode: 2016arXiv161000500J
Aims: We investigate the vertical gradient of the magnetic field of sunspots in the photospheric layer.
Methods: Independent observations were obtained with the Solar Optical Telescope/Spectropolarimeter (SOT/SP) on board the Hinode spacecraft and with the Tenrife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical gradient along with other atmospheric parameters in the solar photosphere.
Results: In the sunspot penumbrae we detected patches of negative vertical gradients of the magnetic field strength, I.e., the magnetic field strength decreases with optical depth in the photosphere. The negative gradient patches are located in the inner and partly in the middle penumbrae in both data sets. From the SOT/SP observations we found that the negative gradient patches are restricted mainly to the deep photospheric layers and are concentrated near the edges of the penumbral filaments. Magnetohydrodynamic (MHD) simulations also show negative gradients in the inner penumbrae, also at the locations of filaments. In the observations and the simulation negative gradients of the magnetic field vs. optical depth dominate at some radial distances in the penumbra. The negative gradient with respect to optical depth in the inner penumbrae persists even after averaging in the azimuthal direction in the observations and, to a lesser extent, in the MHD simulations. If the gradients in the MHD simulations are determined with respect to geometrical height, then the azimuthal averages are always positive within the sunspot (above log τ = 0), corresponding to magnetic field increasing with depth, as generally expected.
Conclusions: We interpret the observed localized presence of negative vertical gradient of the magnetic field strength in the observations as a consequence of stronger field from spines expanding with height and closing above the weaker field inter-spines. The presence of the negative gradients with respect to optical depth after azimuthal averaging can be explained by two different mechanisms: the high corrugation of equal optical depth surfaces and the cancellation of polarized signal due to the presence of unresolved opposite polarity patches in the deeper layers of the penumbra. Title: A Tale of Two Emergences: Sunrise II Observations of Emergence Sites in a Solar Active Region Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld, T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....3C Altcode: 2016arXiv161003531C In 2013 June, the two scientific instruments on board the second Sunrise mission witnessed, in detail, a small-scale magnetic flux emergence event as part of the birth of an active region. The Imaging Magnetograph Experiment (IMaX) recorded two small (∼ 5\prime\prime ) emerging flux patches in the polarized filtergrams of a photospheric Fe I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured the highly dynamic chromospheric response to the magnetic fields pushing their way through the lower solar atmosphere. The serendipitous capture of this event offers a closer look at the inner workings of active region emergence sites. In particular, it reveals in meticulous detail how the rising magnetic fields interact with the granulation as they push through the Sun’s surface, dragging photospheric plasma in their upward travel. The plasma that is burdening the rising field slides along the field lines, creating fast downflowing channels at the footpoints. The weight of this material anchors this field to the surface at semi-regular spatial intervals, shaping it in an undulatory fashion. Finally, magnetic reconnection enables the field to release itself from its photospheric anchors, allowing it to continue its voyage up to higher layers. This process releases energy that lights up the arch-filament systems and heats the surrounding chromosphere. Title: Photospheric Response to an Ellerman Bomb-like Event—An Analogy of Sunrise/IMaX Observations and MHD Simulations Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....5D Altcode: 2016arXiv160903817D Ellerman Bombs are signatures of magnetic reconnection, which is an important physical process in the solar atmosphere. How and where they occur is a subject of debate. In this paper, we analyze Sunrise/IMaX data, along with 3D MHD simulations that aim to reproduce the exact scenario proposed for the formation of these features. Although the observed event seems to be more dynamic and violent than the simulated one, simulations clearly confirm the basic scenario for the production of EBs. The simulations also reveal the full complexity of the underlying process. The simulated observations show that the Fe I 525.02 nm line gives no information on the height where reconnection takes place. It can only give clues about the heating in the aftermath of the reconnection. However, the information on the magnetic field vector and velocity at this spatial resolution is extremely valuable because it shows what numerical models miss and how they can be improved. Title: Transverse Oscillations in Slender Ca II H Fibrils Observed with Sunrise/SuFI Authors: Jafarzadeh, S.; Solanki, S. K.; Gafeira, R.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....9J Altcode: 2016arXiv161007449J We present observations of transverse oscillations in slender Ca II H fibrils (SCFs) in the lower solar chromosphere. We use a 1 hr long time series of high- (spatial and temporal-) resolution seeing-free observations in a 1.1 Å wide passband covering the line core of Ca II H 3969 Å from the second flight of the Sunrise balloon-borne solar observatory. The entire field of view, spanning the polarity inversion line of an active region close to the solar disk center, is covered with bright, thin, and very dynamic fine structures. Our analysis reveals the prevalence of transverse waves in SCFs with median amplitudes and periods on the order of 2.4 ± 0.8 km s-1 and 83 ± 29 s, respectively (with standard deviations given as uncertainties). We find that the transverse waves often propagate along (parts of) the SCFs with median phase speeds of 9 ± 14 km s-1. While the propagation is only in one direction along the axis in some of the SCFs, propagating waves in both directions, as well as standing waves are also observed. The transverse oscillations are likely Alfvénic and are thought to be representative of magnetohydrodynamic kink waves. The wave propagation suggests that the rapid high-frequency transverse waves, often produced in the lower photosphere, can penetrate into the chromosphere with an estimated energy flux of ≈15 kW m-2. Characteristics of these waves differ from those reported for other fibrillar structures, which, however, were observed mainly in the upper solar chromosphere. Title: Kinematics of Magnetic Bright Features in the Solar Photosphere Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W.; van Noort, M. Bibcode: 2017ApJS..229....8J Altcode: 2016arXiv161007634J Convective flows are known as the prime means of transporting magnetic fields on the solar surface. Thus, small magnetic structures are good tracers of turbulent flows. We study the migration and dispersal of magnetic bright features (MBFs) in intergranular areas observed at high spatial resolution with Sunrise/IMaX. We describe the flux dispersal of individual MBFs as a diffusion process whose parameters are computed for various areas in the quiet-Sun and the vicinity of active regions from seeing-free data. We find that magnetic concentrations are best described as random walkers close to network areas (diffusion index, γ =1.0), travelers with constant speeds over a supergranule (γ =1.9{--}2.0), and decelerating movers in the vicinity of flux emergence and/or within active regions (γ =1.4{--}1.5). The three types of regions host MBFs with mean diffusion coefficients of 130 km2 s-1, 80-90 km2 s-1, and 25-70 km2 s-1, respectively. The MBFs in these three types of regions are found to display a distinct kinematic behavior at a confidence level in excess of 95%. Title: Spectropolarimetric Evidence for a Siphon Flow along an Emerging Magnetic Flux Tube Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...15R Altcode: 2016arXiv161106732R We study the dynamics and topology of an emerging magnetic flux concentration using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board the sunrise balloon-borne solar observatory. We obtain the full vector magnetic field and the line of sight (LOS) velocity through inversions of the Fe I line at 525.02 nm with the SPINOR code. The derived vector magnetic field is used to trace magnetic field lines. Two magnetic flux concentrations with different polarities and LOS velocities are found to be connected by a group of arch-shaped magnetic field lines. The positive polarity footpoint is weaker (1100 G) and displays an upflow, while the negative polarity footpoint is stronger (2200 G) and shows a downflow. This configuration is naturally interpreted as a siphon flow along an arched magnetic flux tube. Title: Morphological Properties of Slender Ca II H Fibrils Observed by SUNRISE II Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....6G Altcode: 2016arXiv161200319G We use seeing-free high spatial resolution Ca II H data obtained by the SUNRISE observatory to determine properties of slender fibrils in the lower solar chromosphere. In this work we use intensity images taken with the SuFI instrument in the Ca II H line during the second scientific flight of the SUNRISE observatory to identify and track elongated bright structures. After identification, we analyze theses structures to extract their morphological properties. We identify 598 slender Ca II H fibrils (SCFs) with an average width of around 180 km, length between 500 and 4000 km, average lifetime of ≈400 s, and average curvature of 0.002 arcsec-1. The maximum lifetime of the SCFs within our time series of 57 minutes is ≈2000 s. We discuss similarities and differences of the SCFs with other small-scale, chromospheric structures such as spicules of type I and II, or Ca II K fibrils. Title: A New MHD-assisted Stokes Inversion Technique Authors: Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...16R Altcode: 2016arXiv161105175R We present a new method of Stokes inversion of spectropolarimetric data and evaluate it by taking the example of a Sunrise/IMaX observation. An archive of synthetic Stokes profiles is obtained by the spectral synthesis of state-of-the-art magnetohydrodynamics (MHD) simulations and a realistic degradation to the level of the observed data. The definition of a merit function allows the archive to be searched for the synthetic Stokes profiles that best match the observed profiles. In contrast to traditional Stokes inversion codes, which solve the Unno-Rachkovsky equations for the polarized radiative transfer numerically and fit the Stokes profiles iteratively, the new technique provides the full set of atmospheric parameters. This gives us the ability to start an MHD simulation that takes the inversion result as an initial condition. After a relaxation process of half an hour solar time we obtain physically consistent MHD data sets with a target similar to the observation. The new MHD simulation is used to repeat the method in a second iteration, which further improves the match between observation and simulation, resulting in a factor of 2.2 lower mean {χ }2 value. One advantage of the new technique is that it provides the physical parameters on a geometrical height scale. It constitutes a first step toward inversions that give results consistent with the MHD equations. Title: Oscillations on Width and Intensity of Slender Ca II H Fibrils from Sunrise/SuFI Authors: Gafeira, R.; Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....7G Altcode: 2017arXiv170102801G We report the detection of oscillations in slender Ca II H fibrils (SCFs) from high-resolution observations acquired with the Sunrise balloon-borne solar observatory. The SCFs show obvious oscillations in their intensity, but also their width. The oscillatory behaviors are investigated at several positions along the axes of the SCFs. A large majority of fibrils show signs of oscillations in intensity. Their periods and phase speeds are analyzed using a wavelet analysis. The width and intensity perturbations have overlapping distributions of the wave period. The obtained distributions have median values of the period of 32 ± 17 s and 36 ± 25 s, respectively. We find that the fluctuations of both parameters propagate in the SCFs with speeds of {11}-11+49 km s-1 and {15}-15+34 km s-1, respectively. Furthermore, the width and intensity oscillations have a strong tendency to be either in anti-phase or, to a smaller extent, in phase. This suggests that the oscillations of both parameters are caused by the same wave mode and that the waves are likely propagating. Taking all the evidence together, the most likely wave mode to explain all measurements and criteria is the fast sausage mode. Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity Surface Magnetic Fields Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....4C Altcode: 2016arXiv161007484C How and where are coronal loops rooted in the solar lower atmosphere? The details of the magnetic environment and its evolution at the footpoints of coronal loops are crucial to understanding the processes of mass and energy supply to the solar corona. To address the above question, we use high-resolution line-of-sight magnetic field data from the Imaging Magnetograph eXperiment instrument on the Sunrise balloon-borne observatory and coronal observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory of an emerging active region. We find that the coronal loops are often rooted at the locations with minor small-scale but persistent opposite-polarity magnetic elements very close to the larger dominant polarity. These opposite-polarity small-scale elements continually interact with the dominant polarity underlying the coronal loop through flux cancellation. At these locations we detect small inverse Y-shaped jets in chromospheric Ca II H images obtained from the Sunrise Filter Imager during the flux cancellation. Our results indicate that magnetic flux cancellation and reconnection at the base of coronal loops due to mixed polarity fields might be a crucial feature for the supply of mass and energy into the corona. Title: Moving Magnetic Features around a Pore Authors: Kaithakkal, A. J.; Riethmüller, T. L.; Solanki, S. K.; Lagg, A.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; vanNoort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...13K Altcode: 2016arXiv160905664K Spectropolarimetric observations from Sunrise/IMaX, obtained in 2013 June, are used for a statistical analysis to determine the physical properties of moving magnetic features (MMFs) observed near a pore. MMFs of the same and opposite polarity, with respect to the pore, are found to stream from its border at an average speed of 1.3 km s-1 and 1.2 km s-1, respectively, with mainly same-polarity MMFs found further away from the pore. MMFs of both polarities are found to harbor rather weak, inclined magnetic fields. Opposite-polarity MMFs are blueshifted, whereas same-polarity MMFs do not show any preference for up- or downflows. Most of the MMFs are found to be of sub-arcsecond size and carry a mean flux of ∼1.2 × 1017 Mx. Title: Brightness of Solar Magnetic Elements As a Function of Magnetic Flux at High Spatial Resolution Authors: Kahil, F.; Riethmüller, T. L.; Solanki, S. K. Bibcode: 2017ApJS..229...12K Altcode: 2017arXiv170100759K We investigate the relationship between the photospheric magnetic field of small-scale magnetic elements in the quiet-Sun (QS) at disk center and the brightness at 214, 300, 313, 388, 397, and 525.02 nm. To this end, we analyzed spectropolarimetric and imaging time series acquired simultaneously by the Imaging Magnetograph eXperiment magnetograph and the SuFI filter imager on board the balloon-borne observatory {{S}}{{UNRISE}} during its first science flight in 2009, with high spatial and temporal resolution. We find a clear dependence of the contrast in the near ultraviolet and the visible on the line-of-sight component of the magnetic field, B LOS, which is best described by a logarithmic model. This function effectively represents the relationship between the Ca II H-line emission and B LOS and works better than the power-law fit adopted by previous studies. This, along with the high contrast reached at these wavelengths, will help with determining the contribution of small-scale elements in the QS to the irradiance changes for wavelengths below 388 nm. At all wavelengths, including the continuum at 525.40 nm, the intensity contrast does not decrease with increasing B LOS. This result also strongly supports the fact that {{S}}{{UNRISE}} has resolved small strong magnetic field elements in the internetwork, resulting in constant contrasts for large magnetic fields in our continuum contrast at 525.40 nm versus the B LOS scatterplot, unlike the turnover obtained in previous observational studies. This turnover is due to the intermixing of the bright magnetic features with the dark intergranular lanes surrounding them. Title: Convectively Driven Sinks and Magnetic Fields in the Quiet-Sun Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot Rubio, Luis R.; Martínez Pillet, Valentín; Solanki, Sami K.; Schmidt, Wolfgang Bibcode: 2017ApJS..229...14R Altcode: 2016arXiv161007622R We study the relation between mesogranular flows, convectively driven sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We obtain the horizontal velocity flow fields of two quiet-Sun regions (31.2 × 31.2 Mm2) via local correlation tracking. Mesogranular lanes and the central position of sinks are identified using Lagrange tracers. We find 6.7× {10}-2 sinks per Mm2 in the two observed regions. The sinks are located at the mesogranular vertices and turn out to be associated with (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts. The spatial distribution of magnetic fields in the quiet-Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal components of the magnetic field stronger than 500 G are located in the close neighborhood of sinks. In contrast, the small-scale magnetic loops detected by Martínez González et al. in the same two observed areas do not show any preferential distribution at mesogranular scales. The study of individual examples reveals that sinks can play an important role in the evolution of quiet-Sun magnetic features. Title: Overview of the Special Issue on the First Science Results from the Second Flight of Sunrise Authors: Solanki, Sami K. Bibcode: 2017ApJS..229....1S Altcode: No abstract at ADS Title: Statistical evolution of quiet-Sun small-scale magnetic features using Sunrise observations Authors: Anusha, L. S.; Solanki, S. K.; Hirzberger, J.; Feller, A. Bibcode: 2017A&A...598A..47A Altcode: 2016arXiv160808499A The evolution of small magnetic features in quiet regions of the Sun provides a unique window for probing solar magneto-convection. Here we analyze small-scale magnetic features in the quiet Sun, using the high resolution, seeing-free observations from the Sunrise balloon borne solar observatory. Our aim is to understand the contribution of different physical processes, such as splitting, merging, emergence and cancellation of magnetic fields to the rearrangement, addition and removal of magnetic flux in the photosphere. We have employed a statistical approach for the analysis and the evolution studies are carried out using a feature-tracking technique. In this paper we provide a detailed description of the feature-tracking algorithm that we have newly developed and we present the results of a statistical study of several physical quantities. The results on the fractions of the flux in the emergence, appearance, splitting, merging, disappearance and cancellation qualitatively agrees with other recent studies. To summarize, the total flux gained in unipolar appearance is an order of magnitude larger than the total flux gained in emergence. On the other hand, the bipolar cancellation contributes nearly an equal amount to the loss of magnetic flux as unipolar disappearance. The total flux lost in cancellation is nearly six to eight times larger than the total flux gained in emergence. One big difference between our study and previous similar studies is that, thanks to the higher spatial resolution of Sunrise, we can track features with fluxes as low as 9 × 1014 Mx. This flux is nearly an order of magnitude lower than the smallest fluxes of the features tracked in the highest resolution previous studies based on Hinode data. The area and flux of the magnetic features follow power-law type distribution, while the lifetimes show either power-law or exponential type distribution depending on the exact definitions used to define various birth and death events. We have also statistically determined the evolution of the flux within the features in the course of their lifetime, finding that this evolution depends very strongly on the birth and death process that the features undergo. Title: VizieR Online Data Catalog: Group sunspot number series since 1739 (Chatzistergos+, 2017) Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G. A.; Krivova, N. A.; Solanki, S. K. Bibcode: 2017yCat..36020069C Altcode: Annual, monthly, and daily values of the Group sunspot number series produced in the paper.

(3 data files). Title: Fan Loops Observed by IRIS, EIS, and AIA Authors: Ghosh, Avyarthana; Tripathi, Durgesh; Gupta, G. R.; Polito, Vanessa; Mason, Helen E.; Solanki, Sami K. Bibcode: 2017ApJ...835..244G Altcode: 2017arXiv170101617G A comprehensive study of the physical parameters of active region fan loops is presented using the observations recorded with the Interface Region Imaging Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on board Hinode, and the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). The fan loops emerging from non-flaring AR 11899 (near the disk center) on 2013 November 19 are clearly discernible in AIA 171 Å images and in those obtained in Fe viii and Si vii images using EIS. Our measurements of electron densities reveal that the footpoints of these loops are at an approximately constant pressure with electron densities of {log} {N}e=10.1 cm-3 at {log} [T/K]=5.15 (O IV), and {log} {N}e=8.9 cm-3 at {log} [T/K]=6.15 (Si x). The electron temperature diagnosed across the fan loops by means of EM-Loci suggest that two temperature components exist at {log} [T/K]=4.95 and 5.95 at the footpoints. These components are picked up by IRIS lines and EIS lines, respectively. At higher heights, the loops are nearly isothermal at {log} [T/K]=5.95, which remained constant along the loop. The measurement of the Doppler shift using IRIS lines suggests that the plasma at the footpoints of these loops is predominantly redshifted by 2-3 km s-1 in C II, 10-15 km s-1 in Si IV, and 15-20 km s-1 in O IV, reflecting the increase in the speed of downflows with increasing temperature from {log} [T/K]=4.40 to 5.15. These observations can be explained by low-frequency nanoflares or impulsive heating, and provide further important constraints on the modeling of the dynamics of fan loops. Title: The dark side of solar photospheric G-band bright points Authors: Riethmüller, T. L.; Solanki, S. K. Bibcode: 2017A&A...598A.123R Altcode: 2016arXiv161207887R Bright, small-scale magnetic elements found mainly in intergranular lanes at the solar surface are named bright points (BPs). They show high contrasts in Fraunhofer G-band observations and are described by nearly vertical slender flux tubes or sheets. A recent comparison between BP observations in the ultraviolet (UV) and visible spectral range recorded with the balloon-borne observatory Sunrise and state-of-the-art magnetohydrodynamical (MHD) simulations revealed a kilogauss magnetic field for 98% of the synthetic BPs. Here we address the opposite question, namely which fraction of pixels hosting kilogauss fields coincides with an enhanced G-band brightness. We carried out 3D radiation MHD simulations for three magnetic activity levels (corresponding to the quiet Sun, weak and strong plage) and performed a full spectral line synthesis in the G-band. Only 7% of the kilogauss pixels in our quiet-Sun simulation coincide with a brightness lower than the mean quiet-Sun intensity, while 23% of the pixels in the weak-plage simulation and even 49% in the strong-plage simulation are associated with a local darkening. Dark strong-field regions are preferentially found in the cores of larger flux patches that are rare in the quiet Sun, but more common in plage regions, often in the vertices of granulation cells. The significant brightness shortfall in the core of larger flux patches coincides with a slight magnetic field weakening. Kilogauss elements in the quiet Sun are, on average, brighter than similar features in plage regions. Almost all strong-field pixels display a more or less vertical magnetic field orientation. Hence, in the quiet Sun, G-band BPs correspond almost one-to-one with kilogauss elements. In weak plage, the correspondence is still very good, but not perfect. Title: Flux emergence rate in the quiet Sun from Sunrise data Authors: Smitha, H. N.; Anusha, L. S.; Solanki, S. K.; Riethmüller, T. L. Bibcode: 2017psio.confE.106S Altcode: No abstract at ADS Title: Lambda-shaped jets from a penumbral intrusion into a sunspot umbra: a possibility for magnetic reconnection Authors: Bharti, L.; Solanki, S. K.; Hirzberger, J. Bibcode: 2017A&A...597A.127B Altcode: We present the results of high resolution co-temporal and co-spatial photospheric and chromospheric observations of sunspot penumbral intrusions. The data were taken with the Swedish Solar Telescope (SST) on the Canary Islands. Time series of Ca II H images show a series of transient jets extending roughly 3000 km above a penumbral intrusion into the umbra. For most of the time series, jets were seen along the whole length of the intruding bright filament. Some of these jets develop a clear λ-shaped structure, with a small loop appearing at their footpoint and lasting for around a minute. In the framework of earlier studies, the observed transient λ shape of these jets suggests that they could be caused by magnetic reconnection between a curved arcade-like or flux rope-like field in the lower part of the penumbral intrusion and the more vertical umbral magnetic field forming a cusp-shaped structure above the penumbral intrusion.

Movies associated to Figs. 1 and 2 are available at http://www.aanda.org Title: Helioseismology with Solar Orbiter Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou, Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally, Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank; Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K. Bibcode: 2017hdsi.book..257L Altcode: No abstract at ADS Title: Handling Historical Ca II K Spectroheliogram Observation Authors: Solanki, S. K.; Chatzistergos, T.; Ermolli, I.; Krivova, N. Bibcode: 2016AGUFMSA51B2429S Altcode: The total solar irradiance has been continuously monitored since 1978, but climate studies require time-series extending further back in time. Models assuming that the irradiance variations are due to the evolution of the solar surface magnetic field manifesting itself as bright faculae and dark sunspots have been quite successful in reproducing the measured changes. Unfortunately full-disk observations of the magnetic field exist only for the last four decades. Sunspot observations (areas or numbers) have been employed to reconstruct past irradiance changes, however they provide information about the plage regions only indirectly. Information about the evolution of the plage regions can potentially be obtained from Ca II K spectroheliograms that are available for the last century. Such observational programs have been carried out at several observatories and some of these photographic archives have been digitised. However, analysis of such data is plagued by numerous problems affecting the images and the lack of photometric calibration. Here we present a new method to process historical Ca II K spectroheliograms in order to perform the photometric calibration and compensate for the centre-to-limb variations and artefacts affecting the data. The method is tested on synthetic images with known artefacts and a sample of images from various historical archives. Title: Solar Total and Spectral Irradiance Reconstruction over Last 9000 Years Authors: Wu, C. J.; Krivova, N.; Solanki, S. K.; Usoskin, I. G. Bibcode: 2016AGUFMSA51B2430W Altcode: Although the mechanisms of solar influence on Earth climate system are not yet fully understood, solar total and spectral irradiance are considered to be among the main determinants. Solar total irradiance is the total flux of solar radiative energy entering Earth's climate system, whereas the spectral irradiance describes this energy is distributed over the spectrum. Solar irradiance in the UV band is of special importance since it governs chemical processes in the middle and upper atmosphere. On timescales of the 11-year solar cycle and shorter, solar irradiance is measured by space-based instruments while models are needed to reconstruct solar irradiance on longer timescale. The SATIRE-M model (Spectral And Total Irradiance Reconstruction over millennia) is employed in this study to reconstruct solar irradiance from decadal radionuclide isotope data such as 14C and 10Be stored in tree rings and ice cores, respectively. A reconstruction over the last 9000 years will be presented. Title: Non-linear re-calibration of group sunspot number series back to 1819 Authors: Chatzistergos, T.; Usoskin, I. G.; Kovaltsov, G.; Solanki, S. K.; Krivova, N. Bibcode: 2016AGUFMSH31B2547C Altcode: Sunspot numbers form the longest series of direct observations that give information about the activity of the Sun. It consists in numerous records from different observers with different instruments and techniques, and unavoidably requires a calibration of individual observers to a standard reference condition. Such calibrations pose a challenge and are still a subject of debate. Most of the calibration methods are based on a simple linear scaling of data from one observer to the other, proposed in the mid-19th century by Rudolf Wolf. This method is still used in many recent reconstructions. However, this linear method is shown, on both real and synthetic data, to be very rough and incorrect because of the essentially non-linearity of the relation.Here we present a new reconstruction of the group sunspot number series back to 1819 attempting to take into account the non-linearity of the relation between data of different observers. We use an approach incorporating several overlapping backbone observers (similar to Svalgaard and Schatten, 2016) and perform the straightforward non-linear calibration of each observer to the backbone (and the backbones with each other) based on the actual statistics of the common daily values. Title: Lower solar atmosphere and magnetism at ultra-high spatial resolution Authors: Collet, Remo; Criscuoli, Serena; Ermolli, Ilaria; Fabbian, Damian; Guerreiro, Nuno; Haberreiter, Margit; Peck, Courtney; Pereira, Tiago M. D.; Rempel, Matthias; Solanki, Sami K.; Wedemeyer-Boehm, Sven Bibcode: 2016arXiv161202348C Altcode: We present the scientific case for a future space-based telescope aimed at very high spatial and temporal resolution imaging of the solar photosphere and chromosphere. Previous missions (e.g., HINODE, SUNRISE) have demonstrated the power of observing the solar photosphere and chromosphere at high spatial resolution without contamination from Earth's atmosphere. We argue here that increased spatial resolution (from currently 70 km to 25 km in the future) and high temporal cadence of the observations will vastly improve our understanding of the physical processes controlling solar magnetism and its characteristic scales. This is particularly important as the Sun's magnetic field drives solar activity and can significantly influence the Sun-Earth system. At the same time a better knowledge of solar magnetism can greatly improve our understanding of other astrophysical objects. Title: Far side Helioseismology with Solar Orbiter Authors: Appourchaux, T.; Birch, A.; Gizon, L. C.; Löptien, B.; Schou, J.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.; Hirzberger, J.; Alvarez-Herrero, A.; Woch, J. G.; Schmidt, W. Bibcode: 2016AGUFMSH43A2554A Altcode: The Solar Orbiter mission, to be launched in October 2018, will carry a suite of remote sensing and in-situ instruments, including the Polarimetric and Helioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in intensity and Doppler velocity suitable for carrying out novel helioseismic studies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude up to 34 degrees by the end of the extended mission and thus will enable the first local helioseismology studies of the polar regions. The full range of Earth-Sun-spacecraft angles provided by the orbit will enable helioseismology from two vantage points by combining PHI with another instrument: stereoscopic helioseismology will allow the study of the deep solar interior and a better understanding of the physics of solar oscillations in both quiet Sun and sunspots. In this paper we will review the helioseismic objectives achievable with PHI, and will also give a short status report of the development of the Flight Model of PHI. Title: Relationship between supergranulation flows, magnetic cancellation and network flares Authors: Attie, R.; Innes, D. E.; Solanki, S. K.; Glassmeier, K. H. Bibcode: 2016A&A...596A..15A Altcode: 2017arXiv170510389A Context. Photospheric flows create a network of often mixed-polarity magnetic field in the quiet Sun, where small-scale eruptions and network flares are commonly seen.
Aims: The aim of this paper is (1) to describe the characteristics of the flows that lead to these energy releases, (2) to quantify the energy build up due to photospheric flows acting on the magnetic field, and (3) to assess its contribution to the energy of small-scale, short-lived X-ray flares in the quiet Sun.
Methods: We used photospheric and X-ray data from the SoHO and Hinode spacecraft combined with tracking algorithms to analyse the evolution of five network flares. The energy of the X-ray emitting thermal plasma is compared with an estimate of the energy built up due to converging and sheared flux.
Results: Quiet-Sun network flares occur above sites of converging opposite-polarity magnetic flux that are often found on the outskirts of network cell junctions, sometimes with observable vortex-like motion. In all studied flares the thermal energy was more than an order of magnitude higher than the magnetic free energy of the converging flux model. The energy in the sheared field was always higher than in the converging flux but still lower than the thermal energy.
Conclusions: X-ray network flares occur at sites of magnetic energy dissipation. The energy is probably built up by supergranular flows causing systematic shearing of the magnetic field. This process appears more efficient near the junction of the network lanes. Since this work relies on 11 case studies, our results call for a follow-up statistical analysis to test our hypothesis throughout the quiet Sun. Title: Slipping reconnection in a solar flare observed in high resolution with the GREGOR solar telescope Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák, J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann, A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.; Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann, T. Bibcode: 2016A&A...596A...1S Altcode: 2016arXiv160500464S A small flare ribbon above a sunspot umbra in active region 12205 was observed on November 7, 2014, at 12:00 UT in the blue imaging channel of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference filter. Context observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical Telescope (SOT) onboard Hinode, and the Interface Region Imaging Spectrograph (IRIS) show that this ribbon is part of a larger one that extends through the neighboring positive polarities and also participates in several other flares within the active region. We reconstructed a time series of 140 s of Ca II H images by means of the multiframe blind deconvolution method, which resulted in spatial and temporal resolutions of 0.1″ and 1 s. Light curves and horizontal velocities of small-scale bright knots in the observed flare ribbon were measured. Some knots are stationary, but three move along the ribbon with speeds of 7-11 km s-1. Two of them move in the opposite direction and exhibit highly correlated intensity changes, which provides evidence of a slipping reconnection at small spatial scales.

Movies associated to Figs. 1 and 2 are available at http://www.aanda.org Title: Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.; Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe, O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.; Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H. Bibcode: 2016A&A...596A...2B Altcode: 2016arXiv160708165B Context. Some models for the topology of the magnetic field in sunspot penumbrae predict regions free of magnetic fields or with only dynamically weak fields in the deep photosphere.
Aims: We aim to confirm or refute the existence of weak-field regions in the deepest photospheric layers of the penumbra.
Methods: We investigated the magnetic field at log τ5 = 0 is by inverting spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45''. The data have been recorded using the GRIS instrument attached to the 1.5-m solar telescope GREGOR at the El Teide observatory. The data include three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks half a pressure scale height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Before the inversion, the data were corrected for the effects of scattered light using a deconvolution method with several point spread functions.
Results: At log τ5 = 0 we find no evidence of regions with dynamically weak (B< 500 Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations made on Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, is independent of the point spread function used to deconvolve the data, and does not depend on the amount of stray light (I.e., wide-angle scattered light) considered. Title: Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.; Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.; Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1050B Altcode: 2016arXiv160901514B Arch filament systems occur in active sunspot groups, where a fibril structure connects areas of opposite magnetic polarity, in contrast to active region filaments that follow the polarity inversion line. We used the GREGOR Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0 nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line to investigate the photospheric magnetic field and velocities, and use the line core intensities and velocities of the helium line to study the chromospheric plasma. The individual fibrils of the arch filament system connect the sunspot with patches of magnetic polarity opposite to that of the spot. These patches do not necessarily coincide with pores, where the magnetic field is strongest. Instead, areas are preferred not far from the polarity inversion line. These areas exhibit photospheric downflows of moderate velocity, but significantly higher downflows of up to 30 km s-1 in the chromospheric helium line. Our findings can be explained with new emerging flux where the matter flows downward along the field lines of rising flux tubes, in agreement with earlier results. Title: Magnetic fields of opposite polarity in sunspot penumbrae Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.; Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld, T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.; Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar, A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier, K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M. Bibcode: 2016A&A...596A...4F Altcode: 2016arXiv160800513F Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface.
Aims: We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface.
Methods: We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models.
Results: Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude. Title: Horizontal flow fields in and around a small active region. The transition period between flux emergence and decay Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke, A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert, M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A...3V Altcode: 2016arXiv160507462V Context. The solar magnetic field is responsible for all aspects of solar activity. Thus, emergence of magnetic flux at the surface is the first manifestation of the ensuing solar activity.
Aims: Combining high-resolution and synoptic observations aims to provide a comprehensive description of flux emergence at photospheric level and of the growth process that eventually leads to a mature active region.
Methods: The small active region NOAA 12118 emerged on 2014 July 17 and was observed one day later with the 1.5-m GREGOR solar telescope on 2014 July 18. High-resolution time-series of blue continuum and G-band images acquired in the blue imaging channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were complemented by synoptic line-of-sight magnetograms and continuum images obtained with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Horizontal proper motions and horizontal plasma velocities were computed with local correlation tracking (LCT) and the differential affine velocity estimator (DAVE), respectively. Morphological image processing was employed to measure the photometric and magnetic area, magnetic flux, and the separation profile of the emerging flux region during its evolution.
Results: The computed growth rates for photometric area, magnetic area, and magnetic flux are about twice as high as the respective decay rates. The space-time diagram using HMI magnetograms of five days provides a comprehensive view of growth and decay. It traces a leaf-like structure, which is determined by the initial separation of the two polarities, a rapid expansion phase, a time when the spread stalls, and a period when the region slowly shrinks again. The separation rate of 0.26 km s-1 is highest in the initial stage, and it decreases when the separation comes to a halt. Horizontal plasma velocities computed at four evolutionary stages indicate a changing pattern of inflows. In LCT maps we find persistent flow patterns such as outward motions in the outer part of the two major pores, a diverging feature near the trailing pore marking the site of upwelling plasma and flux emergence, and low velocities in the interior of dark pores. We detected many elongated rapidly expanding granules between the two major polarities, with dimensions twice as large as the normal granules. Title: A New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions Authors: Usoskin, I. G.; Kovaltsov, G. A.; Lockwood, M.; Mursula, K.; Owens, M.; Solanki, S. K. Bibcode: 2016SoPh..291.2685U Altcode: 2016SoPh..tmp....6U; 2015arXiv151206421U Although sunspot-number series have existed since the mid-nineteenth century, they are still the subject of intense debate, with the largest uncertainty being related to the "calibration" of the visual acuity of individual observers in the past. A daisy-chain regression method is usually applied to inter-calibrate the observers, which may lead to significant bias and error accumulation. Here we present a novel method for calibrating the visual acuity of the key observers to the reference data set of Royal Greenwich Observatory sunspot groups for the period 1900 - 1976, using the statistics of the active-day fraction. For each observer we independently evaluate their observational thresholds [SS] defined such that the observer is assumed to miss all of the groups with an area smaller than SS and report all the groups larger than SS. Next, using a Monte-Carlo method, we construct a correction matrix for each observer from the reference data set. The correction matrices are significantly non-linear and cannot be approximated by a linear regression or proportionality. We emphasize that corrections based on a linear proportionality between annually averaged data lead to serious biases and distortions of the data. The correction matrices are applied to the original sunspot-group records reported by the observers for each day, and finally the composite corrected series is produced for the period since 1748. The corrected series is provided as supplementary material in electronic form and displays secular minima around 1800 (Dalton Minimum) and 1900 (Gleissberg Minimum), as well as the Modern Grand Maximum of activity in the second half of the twentieth century. The uniqueness of the grand maximum is confirmed for the last 250 years. We show that the adoption of a linear relationship between the data of Wolf and Wolfer results in grossly inflated group numbers in the eighteenth and nineteenth centuries in some reconstructions. Title: Upper chromospheric magnetic field of a sunspot penumbra: observations of fine structure Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados, M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar, H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.; Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A...8J Altcode: 2016arXiv160801988J
Aims: The fine-structure of the magnetic field in a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere.
Methods: Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic field.
Results: We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°-15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed penumbra does not vary on small spatial scales in the upper chromosphere.
Conclusions: Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate penumbral chromospheric dynamic phenomena, such as penumbral micro-jets or transient bright dots. Title: Active region fine structure observed at 0.08 arcsec resolution Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.; Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.; Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.; Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann, T.; Kneer, F.; Nicklas, H.; Sobotka, M. Bibcode: 2016A&A...596A...7S Altcode: 2016arXiv160707094S Context. The various mechanisms of magneto-convective energy transport determine the structure of sunspots and active regions.
Aims: We characterise the appearance of light bridges and other fine-structure details and elaborate on their magneto-convective nature.
Methods: We present speckle-reconstructed images taken with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the 486 nm and 589 nm bands. We estimate the spatial resolution from the noise characteristics of the image bursts and obtain 0.08″ at 589 nm. We describe structure details in individual best images as well as the temporal evolution of selected features.
Results: We find branched dark lanes extending along thin (≈1″) light bridges in sunspots at various heliocentric angles. In thick (≳ 2″) light bridges the branches are disconnected from the central lane and have a Y shape with a bright grain toward the umbra. The images reveal that light bridges exist on varying intensity levels and that their small-scale features evolve on timescales of minutes. Faint light bridges show dark lanes outlined by the surrounding bright features. Dark lanes are very common and are also found in the boundary of pores. They have a characteristic width of 0.1″ or smaller. Intergranular dark lanes of that width are seen in active region granulation.
Conclusions: We interpret our images in the context of magneto-convective simulations and findings: while central dark lanes in thin light bridges are elevated and associated with a density increase above upflows, the dark lane branches correspond to locations of downflows and are depressed relative to the adjacent bright plasma. Thick light bridges with central dark lanes show no projection effect. They have a flat elevated plateau that falls off steeply at the umbral boundary. There, Y-shaped filaments form as they do in the inner penumbra. This indicates the presence of inclined magnetic fields, meaning that the umbral magnetic field is wrapped around the convective light bridge. Title: Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González, M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.; Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.; Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.; Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier, K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.; Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.; Schmidt, D.; Kiess, C.; Nicklas, H. Bibcode: 2016A&A...596A...6L Altcode: 2016arXiv160506324L Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector.
Aims: The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account.
Methods: Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM).
Results: About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields.
Conclusions: The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. Title: Flow and magnetic field properties in the trailing sunspots of active region NOAA 12396 Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer, C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados, M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1090V Altcode: Improved measurements of the photospheric and chromospheric three-dimensional magnetic and flow fields are crucial for a precise determination of the origin and evolution of active regions. We present an illustrative sample of multi-instrument data acquired during a two-week coordinated observing campaign in August 2015 involving, among others, the GREGOR solar telescope (imaging and near-infrared spectroscopy) and the space missions Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS). The observations focused on the trailing part of active region NOAA 12396 with complex polarity inversion lines and strong intrusions of opposite polarity flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV spectral profiles in the photospheric Si I λ1082.7 nm line, the chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I λ1083.9 nm line. Carefully calibrated GRIS scans of the active region provided maps of Doppler velocity and magnetic field at different atmospheric heights. We compare quick-look maps with those obtained with the ``Stokes Inversions based on Response functions'' (SIR) code, which furnishes deeper insight into the magnetic properties of the region. We find supporting evidence that newly emerging flux and intruding opposite polarity flux are hampering the formation of penumbrae, i.e., a penumbra fully surrounding a sunspot is only expected after cessation of flux emergence in proximity to the sunspots. Title: Three-dimensional structure of a sunspot light bridge Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.; Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A..59F Altcode: 2016arXiv161104803F Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay.
Methods: We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory.
Results: The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution.

Movies associated to Figs. 2 and 13 are available at http://www.aanda.org Title: Inference of magnetic fields in the very quiet Sun Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.; Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.; González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.; Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman, T.; Volkmer, R. Bibcode: 2016A&A...596A...5M Altcode: 2018arXiv180410089M Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution.
Aims: We present high-precision spectro-polarimetric data with high spatial resolution (0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR telescope to shed some light on this complex magnetism.
Methods: We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component.
Results: Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak ( 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised. Title: Fitting peculiar spectral profiles in He I 10830Å absorption features Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.; Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.; Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1057G Altcode: 2016arXiv160300679G The new generation of solar instruments provides better spectral, spatial, and temporal resolution for a better understanding of the physical processes that take place on the Sun. Multiple-component profiles are more commonly observed with these instruments. Particularly, the He I 10830 Å triplet presents such peculiar spectral profiles, which give information on the velocity and magnetic fine structure of the upper chromosphere. The purpose of this investigation is to describe a technique to efficiently fit the two blended components of the He I 10830 Å triplet, which are commonly observed when two atmospheric components are located within the same resolution element. The observations used in this study were taken on 2015 April 17 with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar telescope, located at the Observatorio del Teide, Tenerife, Spain. We apply a double-Lorentzian fitting technique using Levenberg-Marquardt least-squares minimization. This technique is very simple and much faster than inversion codes. Line-of-sight Doppler velocities can be inferred for a whole map of pixels within just a few minutes. Our results show sub- and supersonic downflow velocities of up to 32 km s-1 for the fast component in the vicinity of footpoints of filamentary structures. The slow component presents velocities close to rest. Title: Variability Of The Sun And Sun-Like Stars On Different Timescales Authors: Solanki, Sami K. Bibcode: 2016csss.confE..16S Altcode: Although many features can be seen moving across it at various wavelengths, the Sun is an almost constant star with regard to its radiative output. Its radiative variability has generally only been discovered after the beginning of the space age. For Sun-like stars it is easier to measure their small levels of variability from the ground, leading to some unexpected discoveries that seemed to suggest a behaviour rather different than that displayed by the Sun. However, revisions of the measurements as well as advances in modelling have in recent years shown that some of the main differences between the Sun and stars can be reconciled. A much richer data set of short and medium-term variability has been provided by the COROT and Kepler missions. Although a number of exciting results have already been obtained from these data, many more are expected in the coming years as the data are more thoroughly analysed and models of solar variability are extended to describe stars which, while similar to the Sun, are not exact solar analogs. Title: The statistical study of global properties of sunspots observed by SoHO/MDI continuum images over solar cycle 23 Authors: Goel, Suruchi; Krivova, Natalie; Solanki, Sami K.; Mathew, Shibu K. Bibcode: 2016cosp...41E.687G Altcode: A better understanding of inter-dependency of various sunspot parameters such as magnetic field, intensity, temperature, size etc., and also their variation with strength of solar activity cycle is important to understand the magneto-convection process involved in sunspot formation and evolution and hence to develop a consistent sunspot model. We have investigated global sunspot properties using parameters of sunspots identified from stray-light-corrected continuum images from SoHO/MDI spanning from years 1996 to 2011. We find that the non-linear relation between umbral core (minimum) intensity and sunspot area is best represented by an exponential function, which reaches an asymptotic value at 600 MSH. For the first time we have also observed that the core intensity depends on shape of umbrae, i.e., circular umbrae are statistically darker compared to the elongated ones. The core intensity increases slightly towards the limb (by value of ~0.1 from disk center to the limb). From sunspots sampled over the complete solar cycle 23 and during the rising phase of cycle 24, we did not find any solar-cycle variation in umbral core intensity. The penumbra to umbra area ratio is found to be not a constant parameter, instead it shows a quadratic decrease with sunspot area. Leading and following sunspots usually have different morphological features, however in this study we did not observe significant differences in their core intensity and penumbra-umbra area ratio relation with the sunspot area. Title: Formation of the Penumbra and Start of the Evershed Flow Authors: Murabito, M.; Romano, P.; Guglielmino, S. L.; Zuccarello, F.; Solanki, S. K. Bibcode: 2016ApJ...825...75M Altcode: 2016arXiv160405610M We studied the variations of line of sight photospheric plasma flows during the formation phase of the penumbra around a pore in active region NOAA 11490. We used a high spatial, spectral, and temporal resolution data set acquired by the Interferometric BIdimensional Spectrometer operating at the NSO/Dunn Solar Telescope as well as data taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory satellite (SDO/HMI). Before the penumbra formed we observed a redshift of the spectral line in the inner part of the annular zone surrounding the pore as well as a blueshift of material associated with opposite magnetic polarity farther away from the pore. We found that the onset of the classical Evershed flow occurs on a very short timescale (1 to 3 hr) while the penumbra is forming. During the same time interval we found changes in the magnetic field inclination in the penumbra, with the vertical field actually changing sign near the penumbral edge, while the total magnetic field showed a significant increase, about 400 G. To explain these and other observations related to the formation of the penumbra and the onset of the Evershed flow we propose a scenario in which the penumbra is formed by magnetic flux dragged down from the canopy surrounding the initial pore. The Evershed flow starts when the sinking magnetic field dips below the solar surface and magnetoconvection sets in. Title: Solar total and spectral irradiance reconstruction over last 9000 years Authors: Wu, Chi-Ju; Usoskin, Ilya; Krivova, Natalie; Solanki, Sami K. Bibcode: 2016cosp...41E2072W Altcode: Although the mechanisms of solar influence on Earth climate system are not yet fully understood, solar total and spectral irradiance are considered to be among the main determinants. Solar total irradiance is the total flux of solar radiative energy entering Earth's climate system, whereas the spectral irradiance describes this energy is distributed over the spectrum. Solar irradiance in the UV band is of special importance since it governs chemical processes in the middle and upper atmosphere. On timescales of the 11-year solar cycle and shorter, solar irradiance is measured by space-based instruments while models are needed to reconstruct solar irradiance on longer timescale. The SATIRE-M model (Spectral And Total Irradiance Reconstruction over millennia) is employed in this study to reconstruct solar irradiance from decadal radionuclide isotope data such as 14C and 10Be stored in tree rings and ice cores, respectively. A reconstruction over the last 9000 years will be presented. Title: The Solar Ultraviolet Imaging Telescope onboard Aditya-L1 Authors: Ghosh, Avyarthana; Chatterjee, Subhamoy; Khan, Aafaque R.; Tripathi, Durgesh; Ramaprakash, A. N.; Banerjee, Dipankar; Chordia, Pravin; Gandorfer, Achim M.; Krivova, Natalie; Nandy, Dibyendu; Rajarshi, Chaitanya; Solanki, Sami K.; Sriram, S. Bibcode: 2016SPIE.9905E..03G Altcode: The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 spacecraft, the first dedicated solar mission of the Indian Space Research Organization (ISRO), which will be put in a halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis Ritchey-Chrétien configuration with a combination of 11 narrow and broad bandpass filters which will be used for full-disk solar imaging in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun's influence on climate. Title: Long-term solar irradiance variability: knowns and unknowns Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria; Leng Yeo, Kok; Wu, Chi-Ju Bibcode: 2016cosp...41E1066K Altcode: Long-term reconstructions of solar irradiance variability are crucial to our understanding of solar influence on climate. They are only possible with the help of suitable models, which in turn require a thorough understanding of the mechanisms of this variability. With the advance of such models, also the past reconstructions are becoming more reliable. Nevertheless, the remaining uncertainties spread out when extrapolating back over long periods of time, amplified by the increasingly poorer quality and reliability of the available data that bear information on past solar activity. We will discuss the progress and the reliability of irradiance reconstructions on time scales of decades to millennia. Title: EMPIRE reconstruction of TSI and SSI since 1947 Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K. Bibcode: 2016cosp...41E2107L Altcode: TSI and SSI, at least in the UV, have been monitored from space since 1978. This is accompanied by the development of models aimed at reproducing TSI and SSI by relating the variability to solar magnetism. We present a reconstruction of TSI and SSI, spanning 1947 to 2016, based on the EMPirical Irradiance REconstruction (EMPIRE) model. EMPIRE is what is termed a proxy model, based on the regression of solar activity indices to measured solar irradiance. One of the main open questions in our understanding of solar irradiance variability, and consequently its influence on the Earth's climate, is the variation in UV SSI over the solar cycle. Solar cycle variability in UV SSI in preceding proxy models, such as NRLSSI, NRLSSI2 and MGNM, is significantly weaker than that indicated by other types of models longwards of around 240 nm. We demonstrate here that this discrepancy arises from previous proxy models neglecting the influence of measurement uncertainty on regression analyses. EMPIRE is based on the same activity indices as NRLSSI(2) but by taking measurement uncertainty into account, reproduces variability consistent with other modelling approaches and measurements. As such, EMPIRE supersedes earlier proxy models, including NRLSSI(2) and MGNM. Title: Solar electromagnetic emission and climate Authors: Solanki, Sami K. Bibcode: 2016cosp...41E1831S Altcode: Ever since the first solar radiometer flew in space it has been known that the level of emission of electromagnetic radiation from the Sun (the Sun's total irradiance) varies with time. This record now covers roughly 3.5 solar cycles. With time the quality of the observations has improved and the set of quantities that are observed have increased (now including the spectral solar irradiance over a wide range of wavelengths). At the same time, our understanding of the processes leading to variations in the Sun's radiative output and our ability to model such variations quantitatively have steadily improved, so that we are at a stage where we can reconstruct solar total and spectral irradiance over most of the Holocene, with increasing accuracy the closer we come to recent decades. The variable solar radiative output drives changes in the Earth's atmosphere, with such changes being well-documented in the upper and middle atmosphere of the Earth and becoming increasingly more subtle with depth in the atmosphere. In particular, uncovering the influence of the variable Sun on the troposphere, where the Earth's climate change mainly plays out, is a challenging task. In this talk an overview will be given of the chain from the Sun to the Earth, with the emphasis lying more on the solar part. Title: The origin of Total Solar Irradiance variability on timescales less than a day Authors: Shapiro, Alexander; Krivova, Natalie; Schmutz, Werner; Solanki, Sami K.; Leng Yeo, Kok; Cameron, Robert; Beeck, Benjamin Bibcode: 2016cosp...41E1774S Altcode: Total Solar Irradiance (TSI) varies on timescales from minutes to decades. It is generally accepted that variability on timescales of a day and longer is dominated by solar surface magnetic fields. For shorter time scales, several additional sources of variability have been proposed, including convection and oscillation. However, available simplified and highly parameterised models could not accurately explain the observed variability in high-cadence TSI records. We employed the high-cadence solar imagery from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and the SATIRE (Spectral And Total Irradiance Reconstruction) model of solar irradiance variability to recreate the magnetic component of TSI variability. The recent 3D simulations of solar near-surface convection with MURAM code have been used to calculate the TSI variability caused by convection. This allowed us to determine the threshold timescale between TSI variability caused by the magnetic field and convection. Our model successfully replicates the TSI measurements by the PICARD/PREMOS radiometer which span the period of July 2010 to February 2014 at 2-minute cadence. Hence, we demonstrate that solar magnetism and convection can account for TSI variability at all timescale it has ever been measured (sans the 5-minute component from p-modes). Title: Modelling the Spectral Contrasts of Stellar Faculae. Authors: Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne; Solanki, Sami; Yeo, Kok Leng; Krivova, Natalie Bibcode: 2016csss.confE..63N Altcode: Facular contrasts are an important parameter in modelling stellar variability and exoplanet transits. The ultimate goal of this work will be to model the contrasts of faculae for different spectral types and thus improve the modelling of solar and stellar photospheric variability. This is done by using a radiative transfer algorithm (ATLAS9) on magneto-convection simulations. Starting with solar twins, we derive facular contrasts as a function of limb angle and discuss their wavelength dependence. Title: Diagnostics of Coronal Magnetic Fields Through the Hanle Effect in UV and IR Lines Authors: Raouafi, Nour E.; Riley, Pete; Gibson, Sarah; Fineschi, Silvano; Solanki, Sami K. Bibcode: 2016FrASS...3...20R Altcode: 2016arXiv160608493R The plasma thermodynamics in the solar upper atmosphere, particularly in the corona, are dominated by the magnetic field, which controls the flow and dissipation of energy. The relative lack of knowledge of the coronal vector magnetic field is a major handicap for progress in coronal physics. This makes the development of measurement methods of coronal magnetic fields a high priority in solar physics. The Hanle effect in the UV and IR spectral lines is a largely unexplored diagnostic. We use magnetohydrodynamic (MHD) simulations to study the magnitude of the signal to be expected for typical coronal magnetic fields for selected spectral lines in the UV and IR wavelength ranges, namely the HI Ly-α and the He I 10830 Å lines. We show that the selected lines are useful for reliable diagnosis of coronal magnetic fields. The results show that the combination of polarization measurements of spectral lines with different sensitivities to the Hanle effect may be most appropriate for deducing coronal magnetic properties from future observations. Title: Formation of a solar Hα filament from orphan penumbrae Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2016A&A...589A..31B Altcode: 2016arXiv160305899B
Aims: The formation and evolution of an Hα filament in active region (AR) 10953 is described.
Methods: Observations from the Solar Optical Telescope (SOT) aboard the Hinode satellite starting from UT 18:09 on 27th April 2007 until UT 06:08 on 1st May 2007 were analysed. 20 scans of the 6302 Å Fe I line pair recorded by SOT/SP were inverted using the spatially coupled version of the SPINOR code. The inversions were analysed together with co-spatial SOT/BFI G-band and Ca II H and SOT/NFI Hα observations.
Results: Following the disappearance of an initial Hα filament aligned along the polarity inversion line (PIL) of the AR, a new Hα filament formed in its place some 20 h later, which remained stable for, at least, another 1.5 days. The creation of the new Hα filament was driven by the ascent of horizontal magnetic fields from the photosphere into the chromosphere at three separate locations along the PIL. The magnetic fields at two of these locations were situated directly underneath the initial Hα filament and formed orphan penumbrae already aligned along the Hα filament channel. The 700 G orphan penumbrae were stable and trapped in the photosphere until the disappearance of the overlying initial Hα filament, after which they started to ascend into the chromosphere at 10 ± 5 m/s. Each ascent was associated with a simultaneous magnetic flux reduction of up to 50% in the photosphere. The ascended orphan penumbrae formed dark seed structures in Hα in parallel with the PIL, which elongated and merged to form an Hα filament. The filament channel featured horizontal magnetic fields of on average 260 G at log (τ) = -2 suspended above the nearly field-free lower photosphere. The fields took on an overall inverse configuration at log (τ) = -2 suggesting a flux rope topology for the new Hα filament. The destruction of the initial Hα filament was likely caused by the flux emergence at the third location along the PIL.
Conclusions: We present a new interpretation of the Hα filament formation in AR 10953 whereby the mainly horizontal fields of orphan penumbrae, aligned along the Hα filament channel, ascend into the chromosphere, forming seed fragments for a new, second Hα filament. The orphan penumbral fields ascend into the chromosphere ~9-24 h before the Hα filament is fully formed. Title: Reconstruction of spectral solar irradiance since 1700 from simulated magnetograms Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K.; Unruh, Y. C.; Yeo, K. L. Bibcode: 2016A&A...590A..63D Altcode: 2016arXiv160502039D
Aims: We present a reconstruction of the spectral solar irradiance since 1700 using the SATIRE-T2 (Spectral And Total Irradiance REconstructions for the Telescope era version 2) model. This model uses as input magnetograms simulated with a surface flux transport model fed with semi-synthetic records of emerging sunspot groups.
Methods: The record of sunspot group areas and positions from the Royal Greenwich Observatory (RGO) is only available since 1874. We used statistical relationships between the properties of sunspot group emergence, such as the latitude, area, and tilt angle, and the sunspot cycle strength and phase to produce semi-synthetic sunspot group records starting in the year 1700. The semi-synthetic records are fed into a surface flux transport model to obtain daily simulated magnetograms that map the distribution of the magnetic flux in active regions (sunspots and faculae) and their decay products on the solar surface. The magnetic flux emerging in ephemeral regions is accounted for separately based on the concept of extended cycles whose length and amplitude are linked to those of the sunspot cycles through the sunspot number. The magnetic flux in each surface component (sunspots, faculae and network, and ephemeral regions) was used to compute the spectral and total solar irradiance (TSI) between the years 1700 and 2009. This reconstruction is aimed at timescales of months or longer although the model returns daily values.
Results: We found that SATIRE-T2, besides reproducing other relevant observations such as the total magnetic flux, reconstructs the TSI on timescales of months or longer in good agreement with the PMOD composite of observations, as well as with the reconstruction starting in 1878 based on the RGO-SOON data. The model predicts an increase in the TSI of 1.2+0.2-0.3 Wm-2 between 1700 and the present. The spectral irradiance reconstruction is in good agreement with the UARS/SUSIM measurements as well as the Lyman-α composite.

The complete total and spectral (115 nm-160 μm) irradiance reconstructions since 1700 will be available from http://www2.mps.mpg.de/projects/sun-climate/data.html Title: High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter. I. Prototype characterization Authors: Iglesias, F. A.; Feller, A.; Nagaraju, K.; Solanki, S. K. Bibcode: 2016A&A...590A..89I Altcode: 2016arXiv160401521I Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance when attempting to respond to a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (~10-1 arcsec) and low noise (10-3 to 10-5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC).
Aims: We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC.
Methods: We describe the instrument in depth, including the fast pnCCD camera employed, the achromatic modulator package, the main calibration steps, the effects of the modulation frequency on the levels of seeing-induced spurious signals, and the effect of the camera properties on the image restoration quality.
Results: The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6%) and very low noise (4.94 e- rms). The modulator is optimized to have high (>80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals that are present in narrow-band, non-modulated, quiet-sun measurements are (a) lower than the noise (7 × 10-5) after integrating 7.66 min, (b) lower than the noise (2.3 × 10-4) after integrating 1.16 min and (c) slightly above the noise (4 × 10-3) after restoring case (b) by means of a multi-object multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low S/N of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s. Title: Are solar brightness variations faculae- or spot-dominated? Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Yeo, K. L.; Schmutz, W. K. Bibcode: 2016A&A...589A..46S Altcode: 2016arXiv160204447S Context. Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to a surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability.
Aims: To understand whether the broader pattern of stellar variations is consistent with the solar paradigm, we assess relative contributions of faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness variability and its facular and spot contributions depend on the wavelength, timescale of variability, and position of the observer relative to the ecliptic plane.
Methods: We performed calculations with the SATIRE model, which returns solar brightness with daily cadence from solar disc area coverages of various magnetic features. We took coverages as seen by an Earth-based observer from full-disc SoHO/MDI and SDO/HMI data and projected them to mimic out-of-ecliptic viewing by an appropriate transformation.
Results: Moving the observer away from the ecliptic plane increases the amplitude of 11-year variability as it would be seen in Strömgren (b + y)/2 photometry, but decreases the amplitude of the rotational brightness variations as it would appear in Kepler and CoRoT passbands. The spot and facular contributions to the 11-year solar variability in the Strömgren (b + y)/2 photometry almost fully compensate each other so that the Sun appears anomalously quiet with respect to its stellar cohort. Such a compensation does not occur on the rotational timescale.
Conclusions: The rotational solar brightness variability as it would appear in the Kepler and CoRoT passbands from the ecliptic plane is spot-dominated, but the relative contribution of faculae increases for out-of-ecliptic viewing so that the apparent brightness variations are faculae-dominated for inclinations less than about I = 45°. Over the course of the 11-year activity cycle, the solar brightness variability is faculae-dominated shortwards of 1.2 μm independently of the inclination. Title: An update on the Axion Helioscopes front: current activities at CAST and the IAXO project Authors: Dafni, T.; Arik, M.; Armengaud, E.; Aune, S.; Avignone, F. T.; Barth, K.; Belov, A.; Betz, M.; Bräuninger, H.; Brax, P.; Breijnholt, N.; Brun, P.; Cantatore, G.; Carmona, J. M.; Carosi, G. P.; Caspers, F.; Caspi, S.; Cetin, S. A.; Chelouche, D.; Christensen, F. E.; Collar, J. I.; Dael, A.; Davenport, M.; Derbin, A. V.; Desch, K.; Diago, A.; Döbrich, B.; Dratchnev, I.; Dudarev, A.; Eleftheriadis, C.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Georgiopoulou, E.; Geralis, T.; Gimeno, B.; Giomataris, I.; Gninenko, S.; Gómez, H.; González-Díaz, D.; Gruber, E.; Guendelman, E.; Guthörl, T.; Hailey, C. J.; Hartmann, R.; Hauf, S.; Haug, F.; Hasinoff, M. D.; Hiramatsu, T.; Hoffmann, D. H. H.; Horns, D.; Iguaz, F. J.; Irastorza, I. G.; Isern, J.; Imai, K.; Jacoby, J.; Jaeckel, J.; Jakobsen, A. C.; Jakovčić, K.; Kaminski, J.; Kawasaki, M.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kousouris, K.; Krieger, C.; Kuster, M.; Lakić, B.; Laurent, J. M.; Limousin, O.; Lindner, A.; Liolios, A.; Ljubičić, A.; Luzón, G.; Matsuki, S.; Muratova, V. N.; Neff, S.; Niinikoski, T.; Nones, C.; Ortega, I.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Redondo, J.; Riege, H.; Ringwald, A.; Rodríguez, A.; Rosu, M.; Russenschuck, S.; Ruz, J.; Saikawa, K.; Savvidis, I.; Sekiguchi, T.; Semertzidis, Y. K.; Shilon, I.; Sikivie, P.; Silva, H.; Solanki, S. K.; Stewart, L.; ten Kate, H. H. J.; Tomas, A.; Troitsky, S.; Vafeiadis, T.; van Bibber, K.; Vedrine, P.; Villar, J. A.; Vogel, J. K.; Walckiers, L.; Weltman, A.; Wester, W.; Yildiz, S. C.; Zioutas, K. Bibcode: 2016NPPP..273..244D Altcode: Although they have not yet been detected, axions and axion-like particles (ALPs) continue to maintain the interest (even increasingly so) of the rare-event searches community as viable candidates for the Dark Matter of the Universe but also as a solution for several other puzzles of astrophysics. Their property of coupling to photons has inspired different experimental methods for their detection, one of which is the helioscope technique. The CERN Axion Solar Telescope (CAST) is the most sensitive helioscope built up to date and has recently published part of the latest data taken with the magnet bores gradually filled with 3He, probing the mass range up to 1.17 eV. The International AXion Observatory (IAXO) is being proposed as a facility where different axion studies can be performed, with the primary goal to study axions coming from the Sun. Designed to maximize sensitivity, it will improve the levels reached by CAST by almost 5 orders of magnitude in signal detection, that is more than one order of magnitude in terms of gaγ. Here we will summarize the most important aspects of the helioscopes, and focus mainly on IAXO, based on the recent papers [1, 2]. Title: Modelling Solar and Stellar Brightness Variabilities Authors: Yeo, K. L.; Shapiro, A. I.; Krivova, N. A.; Solanki, S. K. Bibcode: 2016ASPC..504..273Y Altcode: Total and spectral solar irradiance, TSI and SSI, have been measured from space since 1978. This is accompanied by the development of models aimed at replicating the observed variability by relating it to solar surface magnetism. Despite significant progress, there remains persisting controversy over the secular change and the wavelength-dependence of the variation with impact on our understanding of the Sun's influence on the Earth's climate. We highlight the recent progress in TSI and SSI modelling with SATIRE. Brightness variations have also been observed for Sun-like stars. Their analysis can profit from knowledge of the solar case and provide additional constraints for solar modelling. We discuss the recent effort to extend SATIRE to Sun-like stars. Title: Flows in and around Active Region NOAA12118 Observed with the GREGOR Solar Telescope and SDO/HMI Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke, A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert, M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016ASPC..504...29V Altcode: 2016arXiv160301109V Accurate measurements of magnetic and velocity fields in and around solar active regions are key to unlocking the mysteries of the formation and the decay of sunspots. High spatial resolution images and spectral sequences with a high cadence obtained with the GREGOR solar telescope give us an opportunity to scrutinize 3-D flow fields with local correlation tracking and imaging spectroscopy. We present GREGOR early science data acquired in 2014 July - August with the GREGOR Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series of blue continuum (λ 450.6 nm) images of the small active region NOAA 12118 were restored with the speckle masking technique to derive horizontal proper motions and to track the evolution of morphological changes. In addition, high-resolution observations are discussed in the context of synoptic data from the Solar Dynamics Observatory. Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: Exploiting Four Historical Ca II K Spectroheliogram Archives Authors: Chatzistergos, T.; Ermolli, I.; Solanki, S. K.; Krivova, N. A. Bibcode: 2016ASPC..504..227C Altcode: Here we briefly review the status of the project aimed to analyse the potential of historical Ca II K archives for studies of long-term (decades to a century) solar variability. Title: Fast Solar Polarimeter: Prototype Characterization and First Results Authors: Iglesias, F. A.; Feller, A.; Krishnappa, N.; Solanki, S. K. Bibcode: 2016ASPC..504..325I Altcode: Due to the differential and non-simultaneous nature of polarization measurements, seeing induced crosstalk (SIC) and seeing limited spatial resolution can easily counterbalance the benefits of solar imaging polarimetry from the ground. The development of instrumental techniques to treat these issues is necessary to fully exploit the next generation of large-aperture solar facilities, and maintain ground-based data at a competitive level with respect to its space-based counterpart. In particular, considering that many open questions in modern solar physics demand data with challenging specifications of resolution and polarimetric sensitivity that can only be achieved with large telescope apertures (Stenflo 1999). Even if state-of-the-art adaptive optics systems greatly improve image quality, their limited correction —due to finite bandwidth, mode number and seeing anisoplanat- ism— produces large residual values of SIC (Krishnappa & Feller 2012). Dual beam polarimeters are commonly used to reduce SIC between the intensity and polarization signals, however, they cannot compensate for the SIC introduced between circular and linear polarization, which can be relevant for high-precision polarimetry. It is known that fast modulation effectively reduces SIC, but the demodulation of the corresponding intensity signals imposes hard requirements on the frame rate of the associated cameras. One way to avoid a fast sensor, is to decouple the camera readout from the intensity demodulation step. This concept is the cornerstone of the very successful Zurich Imaging Polarimeter (ZIMPOL). Even though the ZIMPOL solution allows the detection of very faint signals (∼10-5), its design is not suitable for high-spatial-resolution applications. We are developing a polarimeter that focuses on both spatial resolution (<0.5 arcsec) and polarimetric sensitivity (10-4). The prototype of this Fast Solar Polarimeter (FSP, see Feller et al. 2014), employs a high frame-rate (400 fps), low-noise (<4 e- RMS), pnCCD camera (Hartmann et al. 2006) that is read in synchronization with a polarization modulator based on ferroelectric liquid crystals. The modulator package is similar to the SOLIS (Keller et al. 2003) design and optimized to have an achromatic total polarimetric efficiency above 80 % in the 400-700 nm wavelength range. The fast modulation frequency of FSP, yielding up to 100 full-Stokes measurements per second, and high duty cycle (>95%), have the double benefit of reducing seeing induced artifacts and improving the final spatial resolution by providing an optimal regime for the application of post-facto image reconstruction techniques. In this poster we describe the FSP prototype, including the characterization results, a technique to correct image smearing due to the sensor frame transfer (Iglesias et al. 2015) and some of the first measurements obtained with the 68-cm Vacuum Tower Telescope located at the Observatorio del Teide, Spain. Title: Variation of the Mn I 539.4 nm line with the solar cycle Authors: Danilovic, S.; Solanki, S. K.; Livingston, W.; Krivova, N.; Vince, I. Bibcode: 2016A&A...587A..33D Altcode: 2015arXiv151101286D Context. As a part of the long-term program at Kitt Peak National Observatory (KPNO), the Mn I 539.4 nm line has been observed for nearly three solar cycles using the McMath telescope and the 13.5 m spectrograph in double-pass mode. These full-disk spectrophotometric observations revealed an unusually strong change of this line's parameters over the solar cycle.
Aims: Optical pumping by the Mg II k line was originally proposed to explain these variations. More recent studies have proposed that this is not required and that the magnetic variability (I.e., the changes in solar atmospheric structure due to faculae) might explain these changes. Magnetic variability is also the mechanism that drives the changes in total solar irradiance variations (TSI). With this work we investigate this proposition quantitatively by using the same model that was earlier successfully employed to reconstruct the irradiance.
Methods: We reconstructed the changes in the line parameters using the model SATIRE-S, which takes only variations of the daily surface distribution of the magnetic field into account. We applied exactly the same model atmospheres and value of the free parameter as were used in previous solar irradiance reconstructions to now model the variation in the Mn I 539.4 nm line profile and in neighboring Fe I lines. We compared the results of the theoretical model with KPNO observations.
Results: The changes in the Mn I 539.4 nm line and a neighbouring Fe I 539.52 nm line over approximately three solar cycles are reproduced well by the model without additionally tweaking the model parameters, if changes made to the instrument setup are taken into account. The model slightly overestimates the change for the strong Fe I 539.32 nm line.
Conclusions: Our result confirms that optical pumping of the Mn II 539.4 nm line by Mg II k is not the main cause of its solar cycle change. It also provides independent confirmation of solar irradiance models which are based on the assumption that irradiance variations are caused by the evolution of the solar surface magnetic flux. The result obtained here also supports the spectral irradiance variations computed by these models. Title: Probing the Sun with ALMA: Observations and Simulations Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M.; Carlsson, M. Bibcode: 2015ASPC..499..349L Altcode: 2015arXiv150805686L ALMA will open a new chapter in the study of the Sun by providing a leap in spatial resolution and sensitivity compared to currently available mm wavelength observations. In preparation of ALMA, we have carried out a large number of observational tests and state-of-the-art radiation MHD simulations. Here we review the best available observations of the Sun at millimeter wavelengths.Using state of the art radiation MHD simulations of the solar atmosphere we demonstrate the huge potential of ALMA observations for uncovering the nature of the solar chromosphere. We show that ALMA will not only provide a reliable probe of the thermal structure and dynamics of the chromosphere, it will also open up a powerful new diagnostic of magnetic field at chromospheric heights, a fundamentally important, but so far poorly known parameter. Title: Sunspot areas and tilt angles for solar cycles 7-10 Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova, N. A.; Solanki, S. K. Bibcode: 2015A&A...584A..73S Altcode: 2015arXiv150807849S
Aims: Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims to estimate areas of sunspots observed by Schwabe in 1825-1867 and to calculate the tilt angles of sunspot groups.
Methods: The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century.
Results: Umbral areas for about 130 000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, average to 5̊.85 ± 0, with the leading polarity located closer to the equator, in good agreement with tilt angles obtained from 20th century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided at the CDS.

The sunspot area and tilt angle data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/584/A73 Title: Helioseismology with Solar Orbiter Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou, Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally, Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank; Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K. Bibcode: 2015SSRv..196..251L Altcode: 2014arXiv1406.5435L; 2014SSRv..tmp...31L The Solar Orbiter mission, to be launched in July 2017, will carry a suite of remote sensing and in-situ instruments, including the Polarimetric and Helioseismic Imager (PHI). PHI will deliver high-cadence images of the Sun in intensity and Doppler velocity suitable for carrying out novel helioseismic studies. The orbit of the Solar Orbiter spacecraft will reach a solar latitude of up to 21∘ (up to 34∘ by the end of the extended mission) and thus will enable the first local helioseismology studies of the polar regions. Here we consider an array of science objectives to be addressed by helioseismology within the baseline telemetry allocation (51 Gbit per orbit, current baseline) and within the science observing windows (baseline 3×10 days per orbit). A particularly important objective is the measurement of large-scale flows at high latitudes (rotation and meridional flow), which are largely unknown but play an important role in flux transport dynamos. For both helioseismology and feature tracking methods convection is a source of noise in the measurement of longitudinally averaged large-scale flows, which decreases as T -1/2 where T is the total duration of the observations. Therefore, the detection of small amplitude signals (e.g., meridional circulation, flows in the deep solar interior) requires long observation times. As an example, one hundred days of observations at lower spatial resolution would provide a noise level of about three m/s on the meridional flow at 80∘ latitude. Longer time-series are also needed to study temporal variations with the solar cycle. The full range of Earth-Sun-spacecraft angles provided by the orbit will enable helioseismology from two vantage points by combining PHI with another instrument: stereoscopic helioseismology will allow the study of the deep solar interior and a better understanding of the physics of solar oscillations in both quiet Sun and sunspots. We have used a model of the PHI instrument to study its performance for helioseismology applications. As input we used a 6 hr time-series of realistic solar magneto-convection simulation (Stagger code) and the SPINOR radiative transfer code to synthesize the observables. The simulated power spectra of solar oscillations show that the instrument is suitable for helioseismology. In particular, the specified point spread function, image jitter, and photon noise are no obstacle to a successful mission. Title: Magneto-static Modeling of the Mixed Plasma Beta Solar Atmosphere Based on Sunrise/IMaX Data Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.; Martínez Pillet, V.; Borrero, J. M. Bibcode: 2015ApJ...815...10W Altcode: 2015arXiv151105568W Our aim is to model the three-dimensional magnetic field structure of the upper solar atmosphere, including regions of non-negligible plasma beta. We use high-resolution photospheric magnetic field measurements from SUNRISE/IMaX as the boundary condition for a magneto-static magnetic field model. The high resolution of IMaX allows us to resolve the interface region between the photosphere and corona, but modeling this region is challenging for the following reasons. While the coronal magnetic field is thought to be force-free (the Lorentz force vanishes), this is not the case in the mixed plasma β environment in the photosphere and lower chromosphere. In our model, pressure gradients and gravity forces are self-consistently taken into account and compensate for the non-vanishing Lorentz force. Above a certain height (about 2 Mm) the non-magnetic forces become very weak and consequently the magnetic field becomes almost force-free. Here, we apply a linear approach where the electric current density consists of a superposition of a field-line parallel current and a current perpendicular to the Sun's gravity field. We illustrate the prospects and limitations of this approach and give an outlook for an extension toward a nonlinear model. Title: Sunrise Mission Highlights Authors: Riethmüller, Tino L.; Solanki, Sami K. Bibcode: 2015arXiv151103487R Altcode: Solar activity is controlled by the magnetic field, which also causes the variability of the solar irradiance that in turn is thought to influence the climate on Earth. The magnetic field manifests itself in the form of structures of different sizes, starting with sunspots (10-50 Mm) down to the smallest known magnetic features that often have spatial extents of 100 km or less. The study of the fine scale structure of the Sun's magnetic field has been hampered by the limited spatial resolution of the available observations. This has recently changed thanks to new space and ground-based telescopes. A significant step forward has been taken by the Sunrise observatory, built around the largest solar telescope to leave the ground, and containing two science instruments. Sunrise had two successful long-duration science flights on a stratospheric balloon in June 2009 (solar activity minimum) and in June 2013 (at a high activity level) and a number of scientific results have been obtained that have greatly advanced our understanding of solar magnetism, with data analysis still ongoing. After a brief introduction to the Sunrise mission, an overview of a selection of these results will be given. Title: Depth-dependent global properties of a sunspot observed by Hinode using the Solar Optical Telescope/Spectropolarimeter Authors: Tiwari, Sanjiv K.; van Noort, Michiel; Solanki, Sami K.; Lagg, Andreas Bibcode: 2015A&A...583A.119T Altcode: 2015arXiv150804830T Context. For the past two decades, the three-dimensional structure of sunspots has been studied extensively. A recent improvement in the Stokes inversion technique prompts us to revisit the depth-dependent properties of sunspots.
Aims: In the present work, we aim to investigate the global depth-dependent thermal, velocity, and magnetic properties of a sunspot, as well as the interconnection between various local properties.
Methods: We analysed high-quality Stokes profiles of the disk-centred, regular, leading sunspot of NOAA AR 10933, acquired by the Solar Optical Telescope/Spectropolarimeter (SOT/SP) on board the Hinode spacecraft. To obtain depth-dependent stratification of the physical parameters, we used the recently developed, spatially coupled version of the SPINOR inversion code.
Results: First, we study the azimuthally averaged physical parameters of the sunspot. We find that the vertical temperature gradient in the lower- to mid-photosphere is at its weakest in the umbra, while it is considerably stronger in the penumbra, and stronger still in the spot's surroundings. The azimuthally averaged field becomes more horizontal with radial distance from the centre of the spot, but more vertical with height. At continuum optical depth unity, the line-of-sight velocity shows an average upflow of ~300 ms-1 in the inner penumbra and an average downflow of ~1300 ms-1 in the outer penumbra. The downflow continues outside the visible penumbral boundary. The sunspot shows, at most, a moderate negative twist of <5° at log (τ) = 0, which increases with height. The sunspot umbra and the spines of the penumbra show considerable similarity with regard to their physical properties, albeit with some quantitative differences (weaker, somewhat more horizontal fields in spines, commensurate with their location being further away from the sunspot's core). The temperature shows a general anti-correlation with the field strength, with the exception of the heads of penumbral filaments, where a weak positive correlation is found. The dependence of the physical parameters on each other over the full sunspot shows a qualitative similarity to that of a standard penumbral filament and its surrounding spines.
Conclusions: The large-scale variation in the physical parameters of a sunspot at various optical depths is presented. Our results suggest that the spines in the penumbra are basically the outward extension of the umbra. The spines and the penumbral filaments, together, are the basic elements that form a sunspot penumbra. Title: VizieR Online Data Catalog: Sunspot areas and tilt angles (Senthamizh Pavai+, 2015) Authors: Senthamizh Pavai, V.; Arlt, R.; Dasi-Espuig, M.; Krivova, N.; Solanki, S. Bibcode: 2015yCat..35840073S Altcode: We present sunspot positions and areas from historical observations of sunspots by Samuel Heinrich Schwabe from Dessau, Germany. He has recorded his observations of sunspots from 1825-1867 as drawings in small circles of about 5cm diameter (representing the solar disk). Even though he has used quite a number of telescopes for his observations, the majority of the full-disk drawings were made with a 3-1/2-foot telescope from Fraunhofer. His observing log books are stored in the library of the Royal Astronomical Society in London. Those drawings were digitized photographically with a resolution of 2912x4378 pixels per page. The sizes and positions of the sunspots were measured using a dozen of circular mouse cursor shapes with different diameters. The sunspot sizes in Schwabe's drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century. Umbral areas for about 130,000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar (two or more spots). There is, of course, no polarity information in the observations.

Both an updated sunspot database and a tilt angle database are available at http://www.aip.de/Members/rarlt/ sunspots for further study.

(2 data files). Title: Flarelike brightenings of active region loops observed with SUMER Authors: Wang, T. J.; Innes, D. E.; Solanki, S. K.; Curdt, W. Bibcode: 2015arXiv151000337W Altcode: Coronal loops on the east limb of the Sun were observed by SUMER on SOHO for several days. Small flare-like brightenings are detected very frequently in the hot flare line Fe~{\small XIX}. We find that the relatively intense events are in good coincidence with the transient brightenings seen by Yohkoh/SXT. A statistical analysis shows that these brightenings have durations of 5-84 min and extensions along the slit of 2-67 Mm. The integrated energy observed in Fe~{\small XIX} for each event is in the range of $3\times10^{18}-5\times10^{23}$ ergs, and the estimated thermal energy ranges from $10^{26}-10^{29}$ ergs. Application of the statistical method proposed by Parnell \& Jupp (2000) yields a value of 1.5 to 1.8 for the index of a power law relation between the frequency of the events and the radiated energy in Fe~{\small XIX}, and a value of 1.7 to 1.8 for the index of the frequency distribution of the thermal energy in the energy range $>10^{27}$ ergs. We examine the possibility that these small brightenings give a big contribution to heating of the active region corona. Title: Three-dimensional non-LTE radiative transfer effects in Fe i lines. III. Line formation in magneto-hydrodynamic atmospheres Authors: Holzreuter, R.; Solanki, S. K. Bibcode: 2015A&A...582A.101H Altcode: 2015arXiv150704224H Non-local thermodynamic equilibrium (NLTE) effects in diagnostically important solar Fe i lines are important because of the strong sensitivity of Fe i to ionizing UV radiation, which may lead to a considerable underpopulation of the Fe i levels in the solar atmosphere and, therefore, to a sizeable weakening of Fe i lines. These NLTE effects may be intensified or weakened by horizontal radiative transfer (RT) in a three-dimensionally (3D) structured atmosphere. We analyze the influence of horizontal RT on commonly used Fe i lines in a snapshot of a 3D radiation magneto-hydrodynamic (MHD) simulation of a plage region. NLTE and horizontal RT effects occur with considerable strength (up to 50% in line depth or equivalent width) in the analyzed snapshot. Because they may have either sign, and both signs occur with approximately the same frequency and strength, the net effects are small when considering spatially averaged quantities. The situation in the plage atmosphere turns out to be rather complex. Horizontal transfer leads to line weakening relative to 1D NLTE transfer near the boundaries of kG magnetic elements. Around the centers of these elements, however, we find line strengthening, which is often significant. This behavior contrasts with what is expected from previous 3D RT computations in idealized flux-tube models, which only display line weakening. The origin of this unexpected behavior lies in the fact that magnetic elements are surrounded by dense and relatively cool downflowing gas, which forms the walls of the magnetic elements. The continuum in these dense walls is often formed in colder gas than in the central part of the magnetic elements. Consequently, the central parts of the magnetic element experience a subaverage UV-irradiation leading to the observed 3D NLTE line strengthening. Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini, Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini, Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio, Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne; Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio; Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska, Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza, Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca; Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele; Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini, Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre, Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio; Zuccarello, Francesca Bibcode: 2015JATIS...1d4006B Altcode: Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept for a small solar and space weather mission with a budget compatible with an European Space Agency (ESA) S-class mission, including launch, and a fast development cycle. ADAHELI was submitted to the European Space Agency by a European-wide consortium of solar physics research institutes in response to the "Call for a small mission opportunity for a launch in 2017," of March 9, 2012. The ADAHELI project builds on the heritage of the former ADAHELI mission, which had successfully completed its phase-A study under the Italian Space Agency 2007 Small Mission Programme, thus proving the soundness and feasibility of its innovative low-budget design. ADAHELI is a solar space mission with two main instruments: ISODY: an imager, based on Fabry-Pérot interferometers, whose design is optimized to the acquisition of highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared region of the solar spectrum. XSPO: an x-ray polarimeter for solar flares in x-rays with energies in the 15 to 35 keV range. ADAHELI is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere. Title: The Polarimetric and Helioseismic Imager for Solar Orbiter: SO/PHI Authors: Solanki, Sami K.; del Toro Iniesta, Jose Carlos; Woch, Joachim; Gandorfer, Achim; Hirzberger, Johann; Schmidt, Wolfgang; Appourchaux, Thierry; Alvarez-Herrero, Alberto Bibcode: 2015IAUS..305..108S Altcode: 2015arXiv150203368S The Solar Orbiter is the next solar physics mission of the European Space Agency, ESA, in collaboration with NASA, with a launch planned in 2018. The spacecraft is designed to approach the Sun to within 0.28 AU at perihelion of a highly eccentric orbit. The proximity with the Sun will also allow its observation at uniformly high resolution at EUV and visible wavelengths. Such observations are central for learning more about the magnetic coupling of the solar atmosphere. At a later phase in the mission the spacecraft will leave the ecliptic and study the enigmatic poles of the Sun from a heliographic latitude of up to 33°. Title: Search for chameleons with CAST Authors: Anastassopoulos, V.; Arik, M.; Aune, S.; Barth, K.; Belov, A.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Christensen, F.; Collar, J. I.; Dafni, T.; Davenport, M.; Desch, K.; Dermenev, A.; Eleftheriadis, C.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Hailey, C.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakobsen, A.; Jakovčić, K.; Kaminski, J.; Karuza, M.; Kavuk, M.; Krčmar, M.; Krieger, C.; Krüger, A.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Luzón, G.; Neff, S.; Ortega, I.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Riege, H.; Rosu, M.; Ruz, J.; Savvidis, I.; Solanki, S. K.; Vafeiadis, T.; Villar, J. A.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K.; Brax, P.; Lavrentyev, I.; Upadhye, A. Bibcode: 2015PhLB..749..172A Altcode: 2015arXiv150304561A In this work we present a search for (solar) chameleons with the CERN Axion Solar Telescope (CAST). This novel experimental technique, in the field of dark energy research, exploits both the chameleon coupling to matter (βm) and to photons (βγ) via the Primakoff effect. By reducing the X-ray detection energy threshold used for axions from 1 keV to 400 eV CAST became sensitive to the converted solar chameleon spectrum which peaks around 600 eV. Even though we have not observed any excess above background, we can provide a 95% C.L. limit for the coupling strength of chameleons to photons of βγ ≲1011 for 1 <βm <106. Title: The Maunder minimum (1645-1715) was indeed a grand minimum: A reassessment of multiple datasets Authors: Usoskin, Ilya G.; Arlt, Rainer; Asvestari, Eleanna; Hawkins, Ed; Käpylä, Maarit; Kovaltsov, Gennady A.; Krivova, Natalie; Lockwood, Michael; Mursula, Kalevi; O'Reilly, Jezebel; Owens, Matthew; Scott, Chris J.; Sokoloff, Dmitry D.; Solanki, Sami K.; Soon, Willie; Vaquero, José M. Bibcode: 2015A&A...581A..95U Altcode: 2015arXiv150705191U
Aims: Although the time of the Maunder minimum (1645-1715) is widely known as a period of extremely low solar activity, it is still being debated whether solar activity during that period might have been moderate or even higher than the current solar cycle #24. We have revisited all existing evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum.
Methods: We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of the sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum.
Results: The level of solar activity during the Maunder minimum is reassessed on the basis of all available datasets.
Conclusions: We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely lower than during the Dalton minimum of around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected with a high confidence level. Title: The role of the Fraunhofer lines in solar brightness variability Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Tagirov, R. V.; Schmutz, W. K. Bibcode: 2015A&A...581A.116S Altcode: 2015arXiv150705437S Context. The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections.
Aims: We estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales.
Methods: Our approach is based on the assumption that variability of the solar brightness on timescales greater than a day is driven by the evolution of the solar surface magnetic field. We calculated the solar brightness variations employing the solar disc area coverage of magnetic features deduced from the MDI/SOHO observations. The brightness contrasts of magnetic features relative to the quiet Sun were calculated with a non-LTE radiative transfer code as functions of disc position and wavelength. By consecutive elimination of molecular and atomic lines from the radiative transfer calculations, we assessed the role of these lines in producing solar brightness variability.
Results: We show that the variations in Fraunhofer lines define the amplitude of the solar brightness variability on timescales greater than a day and even the phase of the total solar irradiance variability over the 11-year cycle. We also demonstrate that molecular lines make substantial contribution to solar brightness variability on the 11-year activity cycle and centennial timescales. In particular, our model indicates that roughly a quarter of the total solar irradiance variability over the 11-year cycle originates in molecular lines. The maximum of the absolute spectral brightness variability on timescales greater than a day is associated with the CN violet system between 380 and 390 nm. Title: Magnetic reconnection as a source of jets from a penumbral intrusion into a sunspot umbra Authors: Bharti, L.; Solanki, S. K.; Hirzberger, J. Bibcode: 2015arXiv150902123B Altcode: We present the results of high resolution co-temporal and co-spatial photospheric and chromospheric observations of sunspot penumbral intrusions. The data was taken with the Swedish Solar Telescope (SST) on the Canary Islands. Time series of Ca\,II H images show a series of transient jets extending roughly 3000 km above a penumbral intrusion into the umbra. For most of the time series jets were seen along the whole length of the intruding bright filament. Some of these jets develop a clear $\lambda$-shaped structure, with a small loop appearing at their footpoint and lasting for around a minute. In the framework of earlier studies, the observed transient $\lambda$ shape of these jets strongly suggests that they are caused by magnetic reconnection between a curved arcade-like or flux-rope like field in the lower part of the penumbral intrusion and the more vertical umbral magnetic field forming a cusp-shaped structure above the penumbral intrusion. Title: Dynamics of Multi-cored Magnetic Structures in the Quiet Sun Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot Rubio, Luis R.; Martínez Pillet, Valentín; Solanki, Sami K.; Schmidt, Wolfgang Bibcode: 2015ApJ...810...79R Altcode: 2015arXiv150806998R We report on the dynamical interaction of quiet-Sun magnetic fields and granular convection in the solar photosphere as seen by Sunrise. We use high spatial resolution (0.″15-0.″18) and temporal cadence (33 s) spectropolarimetric Imaging Magnetograph eXperiment data, together with simultaneous CN and Ca ii H filtergrams from Sunrise Filter Imager. We apply the SIR inversion code to the polarimetric data in order to infer the line of sight velocity and vector magnetic field in the photosphere. The analysis reveals bundles of individual flux tubes evolving as a single entity during the entire 23 minute data set. The group shares a common canopy in the upper photospheric layers, while the individual tubes continually intensify, fragment and merge in the same way that chains of bright points in photometric observations have been reported to do. The evolution of the tube cores are driven by the local granular convection flows. They intensify when they are “compressed” by surrounding granules and split when they are “squeezed” between two moving granules. The resulting fragments are usually later regrouped in intergranular lanes by the granular flows. The continual intensification, fragmentation and coalescence of flux results in magnetic field oscillations of the global entity. From the observations we conclude that the magnetic field oscillations first reported by Martínez González et al. correspond to the forcing by granular motions and not to characteristic oscillatory modes of thin flux tubes. Title: Solar extreme ultraviolet variability of the quiet Sun Authors: Shakeri, F.; Teriaca, L.; Solanki, S. K. Bibcode: 2015A&A...581A..51S Altcode: 2015arXiv150705786S The last solar minimum has been unusually quiet compared to the previous minima (since space-based radiometric measurements are available). The Sun's magnetic flux was substantially lower during this minimum. Some studies also show that the total solar irradiance during the minimum after cycle 23 may have dropped below the values known from the two minima prior to that. For chromospheric and coronal radiation, the situation is less clear-cut. The Sun's 10.7 cm flux shows a decrease of ~4% during the solar minimum in 2008 compared to the previous minimum, but Ca ii K does not. Here we consider additional wavelengths in the extreme ultraviolet (EUV), specifically transitions of He i at 584.3 Å and O v at 629.7 Å, of which the CDS spectrometer aboard SOHO has been taking regular scans along the solar central meridian since 1996. We analysed this unique dataset to verify if and how the radiance distribution undergoes measurable variations between cycle minima. To achieve this aim we determined the radiance distribution of quiet areas around the Sun centre. Concentrating on the last two solar minima, we found out that there is very little variation in the radiance distribution of the chromospheric spectral line He i between these minima. The same analysis shows a modest, although significant, 4% variation in the radiance distribution of the TR spectral line O v. These results are comparable to those obtained by earlier studies employing other spectral features, and they confirm that chromospheric indices display a small variation, whereas in the transition region a more significant reduction of the brighter features is visible. Title: Modelling stellar brightness variations Authors: Shapiro, Alexander; Solanki, Sami K.; Krivova, Natalie Bibcode: 2015IAUGA..2256741S Altcode: We develop a model that attributes the variability of the stellar brightness to the imbalance between starspot darkening and facular brightening. Our approach is based on the assumption that the photometric variability of the Sun and Sun-like stars have the same fundamental causes so that we can describe stellar variability by extrapolating the solar model. Our results suggest that the solar paradigm is remarkably successful in explaining the stellar variability on the activity cycle time-scale. In particular, the model reproduces the observed reversal of the in-phase activity-brightness relationship for low-activity stars to an anti-phase one for more active stars.We simulate the solar variability as it would be measured out-of-ecliptic by Kepler and CoRoT and discuss the relative contributions of spots and faculae to the photometric stellar variability. Title: Statistical analysis of supersonic downflows in sunspot penumbrae. Authors: Kim, Hyunnam; Lagg, Andreas; Solanki, Sami K.; Narayan, Gautam; van Noort, Michiel; Kim, Kap-Sung Bibcode: 2015IAUGA..2254868K Altcode: Supersonic downflow patches was found in the outer edge of sunspot penumbra. These patches are believed to be the return channels of the Evershed flow. There was previous study to investigate their structure in detail using Hinode SOT/SP observations (M. van Noort et al. 2013) but their data sample was only two sunspots. To make general description it needs to check more sunspot data sample.We selected 242 downflow patches of 16 sunspots using Hinode SOT/SP observations from 2006 to 2012. Height-dependent maps of atmospheric parameters of these downflows was produced by using HeLix which was height dependent LTE inversion code of Stokes profiles.Statistical analysis of magnetic field strength, inclination angle of field line, temperature and line-of-sight velocity are presented. The recovered atmospheric data tell us that downflow patches have different physical signatures comparing normal penumbra properties. Furthermore, our results of three height-dependent layer support that heating process should occur on the downflow patches in the middle of layer. Title: UV solar irradiance in observations and the NRLSSI and SATIRE-S models Authors: Yeo, K. L.; Ball, W. T.; Krivova, N. A.; Solanki, S. K.; Unruh, Y. C.; Morrill, J. Bibcode: 2015JGRA..120.6055Y Altcode: 2015arXiv150701224Y Total solar irradiance and UV spectral solar irradiance has been monitored since 1978 through a succession of space missions. This is accompanied by the development of models aimed at replicating solar irradiance by relating the variability to solar magnetic activity. The Naval Research Laboratory Solar Spectral Irradiance (NRLSSI) and Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE-S) models provide the most comprehensive reconstructions of total and spectral solar irradiance over the period of satellite observation currently available. There is persistent controversy between the various measurements and models in terms of the wavelength dependence of the variation over the solar cycle, with repercussions on our understanding of the influence of UV solar irradiance variability on the stratosphere. We review the measurement and modeling of UV solar irradiance variability over the period of satellite observation. The SATIRE-S reconstruction is consistent with spectral solar irradiance observations where they are reliable. It is also supported by an independent, empirical reconstruction of UV spectral solar irradiance based on Upper Atmosphere Research Satellite/Solar Ultraviolet Spectral Irradiance Monitor measurements from an earlier study. The weaker solar cycle variability produced by NRLSSI between 300 and 400 nm is not evident in any available record. We show that although the method employed to construct NRLSSI is principally sound, reconstructed solar cycle variability is detrimentally affected by the uncertainty in the SSI observations it draws upon in the derivation. Based on our findings, we recommend, when choosing between the two models, the use of SATIRE-S for climate studies. Title: Models of Solar Irradiance Variability Authors: Solanki, Sami K. Bibcode: 2015IAUGA..2250049S Altcode: Models of solar irradiance variability have an important role to play due to the relatively short (although steadily increasing) length of measured irradiance time series. Advanced models also allow identifying the source of solar irradiance variations and give insight into the variation of irradiance as a function of wavelength. The first generation of models of solar irradiance were proxy-based, i.e. purely empirical. These were followed by models that combine spectra computed from semi-empirical model atmospheres, with a measure of solar activity variations. In future, models will build increasingly on 3D MHD simulations instead of 1D model atmospheres to compute the spectra. On longer timescales models are generally simpler, although there too considerable progress has been made, with irradiance reconstructions now available for multiple millennia, albeit with lower resolution and accuracy than at shorter timescales. Title: The Maunder minimum: A reassessment from multiple dataset Authors: Usoskin, Ilya; Arlt, Rainer; Asvestari, Eleanna; Kovaltsov, Gennady; Krivova, Natalie; Lockwood, Michael; Käpylä, Maarit; Owens, Matthew; Sokoloff, Dmitry D.; Solanki, Sami; Soon, Willie; Vaquero, Jose; Scott, Chris Bibcode: 2015IAUGA..2253036U Altcode: The Maunder minimum (MM) in 1645-1715 was a period of the lowest ever known solar activity recorded via sunspot numbers since 1610. Since it is the only Grand minimum of solar activity directly observed, it forms a benchmark for the solar variability studies. Therefore, it is crucially important to assess the level and other features of temporal and spatial solar magnetic variability during that time. However, because of uncertainties related mostly to ambiguity of some historical sunspot observation records, the exact level of solar activity during the MM is somewhat unclear, leaving room for continuous discussions and speculations. Many of these issues have been addressed by Jack Eddy in his cornerstone papers of 1976 and 1983, but since then numerous new pieces of evidence and datasets have appeared, making it possible to verify the paradigm of the Maunder minimum with far greater certainty than before.Here we provide a full reassessment of the Maunder minimum using all the available datasets: augmented sunspot counts and drawings; revisited historical archives; both well-known and newly revealed records of auroral observations; cosmic ray variability via cosmogenic isotope records of 14C in tree trunks, 10Be in ice cores and 44Ti in fallen meteorites. We show that, while the exact level of the activity is not easy to determine, the Sun indeed exhibited exceptionally low magnetic activity during the MM, in comparison to other periods of moderate or decreased activity, such as the Dalton minimum (ca. 1800), the Gleissberg minimum (ca. 1900) and the present weak solar cycle # 24. We show that a scenario of moderate or strong activity during the MM contradicts all the available datasets.Thus, we confirm, using all the presently available datasets of different nature, that the period of the Maunder minimum in 1645-1715 was indeed a Grand minimum, with very low solar surface magnetic activity, low intensity of the interplanetary magnetic field, as well as lower frequency and higher geographical latitude of auroral occurrence. Meanwhile some indications of the continuation, but at a very low level, of the 11-year solar cycle can be found in the data. Title: Solar cycle variation in UV solar spectral irradiance Authors: Leng Yeo, Kok; Krivova, Natalie; Solanki, Sami K. Bibcode: 2015IAUGA..2253798L Altcode: Solar spectral irradiance, SSI, in the UV has been measured from space, almost without interruption, since 1978. This is accompanied by the development of models aimed at reconstructing SSI by relating its variability to solar magnetic activity. The various satellite records and model reconstructions differ significantly in terms of the variation over the solar cycle, with the consequence that their application to climate models yield qualitatively different results. Here, we highlight the key discrepancies between available records and reconstructions, and discuss the possible underlying causes. Title: New solar axion search using the CERN Axion Solar Telescope with 4He filling Authors: Arik, M.; Aune, S.; Barth, K.; Belov, A.; Bräuninger, H.; Bremer, J.; Burwitz, V.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Da Riva, E.; Dafni, T.; Davenport, M.; Dermenev, A.; Eleftheriadis, C.; Elias, N.; Fanourakis, G.; Ferrer-Ribas, E.; Galán, J.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Geralis, T.; Georgiopoulou, E.; Giomataris, I.; Gninenko, S.; Gómez Marzoa, M.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Kavuk, M.; Krčmar, M.; Kuster, M.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Luzón, G.; Neff, S.; Niinikoski, T.; Nordt, A.; Ortega, I.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Shilon, I.; Solanki, S. K.; Stewart, L.; Tomás, A.; Vafeiadis, T.; Villar, J.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K.; CAST Collaboration Bibcode: 2015PhRvD..92b1101A Altcode: 2015arXiv150300610A The CERN Axion Solar Telescope (CAST) searches for a →γ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the x-ray refractive mass mγ to the axion search mass ma. After the vacuum phase (2003-2004), which is optimal for ma≲0.02 eV , we used 4He in 2005-2007 to cover the mass range of 0.02-0.39 eV and 3He in 2009-2011 to scan from 0.39 to 1.17 eV. After improving the detectors and shielding, we returned to 4He in 2012 to investigate a narrow ma range around 0.2 eV ("candidate setting" of our earlier search) and 0.39-0.42 eV, the upper axion mass range reachable with 4He, to "cross the axion line" for the KSVZ model. We have improved the limit on the axion-photon coupling to ga γ<1.47 ×10-10 GeV-1 (95% C.L.), depending on the pressure settings. Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity. Title: Properties of solar plage from a spatially coupled inversion of Hinode SP data Authors: Buehler, D.; Lagg, A.; Solanki, S. K.; van Noort, M. Bibcode: 2015A&A...576A..27B Altcode: 2015arXiv150101151B
Aims: The properties of magnetic fields forming an extended plage region in AR 10953 were investigated.
Methods: Stokes spectra of the Fe I line pair at 6302 Å recorded by the spectropolarimeter aboard the Hinode satellite were inverted using the SPINOR code. The code performed a 2D spatially coupled inversion on the Stokes spectra, allowing the retrieval of gradients in optical depth within the atmosphere of each pixel, whilst accounting for the effects of the instrument's PSF. Consequently, no magnetic filling factor was needed.
Results: The inversion results reveal that plage is composed of magnetic flux concentrations (MFCs) with typical field strengths of 1520 G at log (τ) = -0.9 and inclinations of 10°-15°. The MFCs expand by forming magnetic canopies composed of weaker and more inclined magnetic fields. The expansion and average temperature stratification of isolated MFCs can be approximated well with an empirical plage thin flux tube model. The highest temperatures of MFCs are located at their edges in all log (τ) layers. Whilst the plasma inside MFCs is nearly at rest, each is surrounded by a ring of downflows of on average 2.4 km s-1 at log (τ) = 0 and peak velocities of up to 10 km s-1, which are supersonic. The downflow ring of an MFC weakens and shifts outwards with height, tracing the MFC's expansion. Such downflow rings often harbour magnetic patches of opposite polarity to that of the main MFC with typical field strengths below 300 G at log (τ) = 0. These opposite polarity patches are situated beneath the canopy of their main MFC. We found evidence of a strong broadening of the Stokes profiles in MFCs and particularly in the downflow rings surrounding MFCs (expressed by a microturbulence in the inversion). This indicates the presence of strong unresolved velocities. Larger magnetic structures such as sunspots cause the field of nearby MFCs to be more inclined. Title: Millimeter radiation from a 3D model of the solar atmosphere. I. Diagnosing chromospheric thermal structure Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; White, S. M. Bibcode: 2015A&A...575A..15L Altcode: 2015arXiv150102898L
Aims: We use advanced 3D non-local thermodynamic equilibrium radiative magnetohydrodynamic simulations of the solar atmosphere to carry out detailed tests of chromospheric diagnostics at millimeter and submillimeter wavelengths.
Methods: We focused on the diagnostics of the thermal structure of the chromosphere in the wavelength bands from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large Millimeter/Submillimeter Array (ALMA) and investigated how these diagnostics are affected by the instrumental resolution.
Results: We find that the formation height range of the millimeter radiation depends on the location in the simulation domain and is related to the underlying magnetic structure. Nonetheless, the brightness temperature is a reasonable measure of the gas temperature at the effective formation height at a given location on the solar surface. There is considerable scatter in this relationship, but this is significantly reduced when very weak magnetic fields are avoided. Our results indicate that although instrumental smearing reduces the correlation between brightness and temperature, millimeter brightness can still be used to reliably diagnose electron temperature up to a resolution of 1''. If the resolution is more degraded, then the value of the diagnostic diminishes rapidly.
Conclusions: We conclude that millimeter brightness can image the chromospheric thermal structure at the height at which the radiation is formed. Thus multiwavelength observations with ALMA with a narrow step in wavelength should provide sufficient information for a tomographic imaging of the chromosphere. Title: Simulated magnetic flows in the solar photosphere Authors: Danilovic, S.; Cameron, R. H.; Solanki, S. K. Bibcode: 2015A&A...574A..28D Altcode: 2014arXiv1408.6159D Context. Recent Sunrise/IMaX observations have revealed supersonic magnetic flows.
Aims: Our aim is to determine the origin of these flows by using realistic magnetohydrodynamics simulations.
Methods: We simulated cancellation and emergence of magnetic flux through the solar photosphere. Our first numerical experiment started with a magnetic field of both polarities. To simulate emergence into a region with pre-existing field, we introduced a large-scale horizontally uniform sheet of a horizontal field. We followed the subsequent evolution and created synthetic polarimetric observations, including known instrumental effects of the Sunrise/IMaX and Hinode/SP instruments. We compared the simulated and observed spectropolarimetric signals.
Results: Strongly blue- and redshifted Stokes V signals are produced in locations where strong line-of-sight velocities coincide with the strong line-of-sight component of the magnetic field. The size and strength of simulated events is smaller than observed, and they are mostly associated with downflows, contrary to observations. In a few cases where they appear above a granule, single blue-lobed Stokes V are produced by strong gradients in magnetic field and velocity. No change of magnetic field sign is detected along the line of sight in these instances. More high-speed magnetised flows occurred when an emergence was simulated than when no horizontal field was added.
Conclusions: The simulations indicate that the observed events result from magnetic flux emergences in which reconnection may take place, but does not seem to be necessary.

The movies are available in electronic form at http://www.aanda.org Title: Centre-to-limb properties of small, photospheric quiet-Sun jets Authors: Rubio da Costa, F.; Solanki, S. K.; Danilovic, S.; Hizberger, J.; Martínez-Pillet, V. Bibcode: 2015A&A...574A..95R Altcode: 2014arXiv1412.1620R Context. Strongly Doppler-shifted Stokes V profiles have been detected in the quiet Sun with the IMaX instrument on-board the SUNRISE stratospheric balloon-borne telescope. High velocities are required to produce such signals, hence these events have been interpreted as jets, although other sources are also possible.
Aims: We aim to characterize the variation of the main properties of these events (occurrence rate, lifetime, size, and velocities) with their position on the solar disk between disk centre and the solar limb.
Methods: These events were identified in SUNRISE/IMaX data according to the same objective criteria at all available positions on the solar disk. Their properties were determined using standard techniques.
Results: Our study yielded a number of new insights into this phenomenon. Most importantly, the number density of these events is independent of the heliocentric angle, meaning that the investigated supersonic flows are nearly isotropically distributed. Size and lifetime are also nearly independent of the heliocentric angle, while their intensity contrast increases towards the solar limb. The Stokes V jets are associated with upflow velocities deduced from Stokes I, which are stronger towards the limb. Their intensity decreases with time, while their line-of-sight velocity does not display a clear temporal evolution. Their association with linear polarization signals decreases towards the limb.
Conclusions: The density of events appears to be independent of heliocentric angle, establishing that they are directed nearly randomly. If these events are jets triggered by magnetic reconnection between emerging magnetic flux and the ambient field, then our results suggest that there is no preferred geometry for the reconnection process. Title: Magnetic Flux Transport at the Solar Surface Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.; Gizon, L.; Upton, L. Bibcode: 2015sac..book..491J Altcode: No abstract at ADS Title: Solar Cycle Variation in Solar Irradiance Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2015sac..book..137Y Altcode: No abstract at ADS Title: The Formation and Disintegration of Magnetic Bright Points Observed by Sunrise/IMaX Authors: Utz, D.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.; Jurčák, J.; Martínez Pillet, V.; Solanki, S. K.; Schmidt, W. Bibcode: 2014ApJ...796...79U Altcode: 2014arXiv1411.3240U The evolution of the physical parameters of magnetic bright points (MBPs) located in the quiet Sun (mainly in the interwork) during their lifetime is studied. First, we concentrate on the detailed description of the magnetic field evolution of three MBPs. This reveals that individual features follow different, generally complex, and rather dynamic scenarios of evolution. Next, we apply statistical methods on roughly 200 observed MBP evolutionary tracks. MBPs are found to be formed by the strengthening of an equipartition field patch, which initially exhibits a moderate downflow. During the evolution, strong downdrafts with an average velocity of 2.4 km s-1 set in. These flows, taken together with the concurrent strengthening of the field, suggest that we are witnessing the occurrence of convective collapses in these features, although only 30% of them reach kG field strengths. This fraction might turn out to be larger when the new 4 m class solar telescopes are operational as observations of MBPs with current state of the art instrumentation could still be suffering from resolution limitations. Finally, when the bright point disappears (although the magnetic field often continues to exist) the magnetic field strength has dropped to the equipartition level and is generally somewhat weaker than at the beginning of the MBP's evolution. Also, only relatively weak downflows are found on average at this stage of the evolution. Only 16% of the features display upflows at the time that the field weakens, or the MBP disappears. This speaks either for a very fast evolving dynamic process at the end of the lifetime, which could not be temporally resolved, or against strong upflows as the cause of the weakening of the field of these magnetic elements, as has been proposed based on simulation results. It is noteworthy that in about 10% of the cases, we observe in the vicinity of the downflows small-scale strong (exceeding 2 km s-1) intergranular upflows related spatially and temporally to these downflows. The paper is complemented by a detailed discussion of aspects regarding the applied methods, the complementary literature, and in depth analysis of parameters like magnetic field strength and velocity distributions. An important difference to magnetic elements and associated bright structures in active region plage is that most of the quiet Sun bright points display significant downflows over a large fraction of their lifetime (i.e., in more than 46% of time instances/measurements they show downflows exceeding 1 km s-1). Title: Magnetic Flux Transport at the Solar Surface Authors: Jiang, J.; Hathaway, D. H.; Cameron, R. H.; Solanki, S. K.; Gizon, L.; Upton, L. Bibcode: 2014SSRv..186..491J Altcode: 2014SSRv..tmp...43J; 2014arXiv1408.3186J After emerging to the solar surface, the Sun's magnetic field displays a complex and intricate evolution. The evolution of the surface field is important for several reasons. One is that the surface field, and its dynamics, sets the boundary condition for the coronal and heliospheric magnetic fields. Another is that the surface evolution gives us insight into the dynamo process. In particular, it plays an essential role in the Babcock-Leighton model of the solar dynamo. Describing this evolution is the aim of the surface flux transport model. The model starts from the emergence of magnetic bipoles. Thereafter, the model is based on the induction equation and the fact that after emergence the magnetic field is observed to evolve as if it were purely radial. The induction equation then describes how the surface flows—differential rotation, meridional circulation, granular, supergranular flows, and active region inflows—determine the evolution of the field (now taken to be purely radial). In this paper, we review the modeling of the various processes that determine the evolution of the surface field. We restrict our attention to their role in the surface flux transport model. We also discuss the success of the model and some of the results that have been obtained using this model. Title: Solar Cycle Variation in Solar Irradiance Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2014SSRv..186..137Y Altcode: 2014arXiv1407.4249Y; 2014SSRv..tmp...25Y The correlation between solar irradiance and the 11-year solar activity cycle is evident in the body of measurements made from space, which extend over the past four decades. Models relating variation in solar irradiance to photospheric magnetism have made significant progress in explaining most of the apparent trends in these observations. There are, however, persistent discrepancies between different measurements and models in terms of the absolute radiometry, secular variation and the spectral dependence of the solar cycle variability. We present an overview of solar irradiance measurements and models, and discuss the key challenges in reconciling the divergence between the two. Title: The magnetic field in the solar atmosphere Authors: Wiegelmann, Thomas; Thalmann, Julia K.; Solanki, Sami K. Bibcode: 2014A&ARv..22...78W Altcode: 2014arXiv1410.4214W This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes. Title: A New SATIRE-S Spectral Solar Irradiance Reconstruction for Solar Cycles 21-23 and Its Implications for Stratospheric Ozone* Authors: Ball, William T.; Krivova, Natalie A.; Unruh, Yvonne C.; Haigh, Joanna D.; Solanki, Sami K. Bibcode: 2014JAtS...71.4086B Altcode: 2014arXiv1408.0365B We present a revised and extended total and spectral solar irradiance (SSI) reconstruction, which includes a wavelength-dependent uncertainty estimate, spanning the last three solar cycles using the SATIRE-S model. The SSI reconstruction covers wavelengths between 115 and 160,000 nm and all dates between August 1974 and October 2009. This represents the first full-wavelength SATIRE-S reconstruction to cover the last three solar cycles without data gaps and with an uncertainty estimate. SATIRE-S is compared with the NRLSSI model and SORCE/SOLSTICE ultraviolet (UV) observations. SATIRE-S displays similar cycle behaviour to NRLSSI for wavelengths below 242 nm and almost twice the variability between 242 and 310 nm. During the decline of last solar cycle, between 2003 and 2008, SSI from SORCE/SOLSTICE version 12 and 10 typically displays more than three times the variability of SATIRE-S between 200 and 300 nm. All three datasets are used to model changes in stratospheric ozone within a 2D atmospheric model for a decline from high solar activity to solar minimum. The different flux changes result in different modelled ozone trends. Using NRLSSI leads to a decline in mesospheric ozone, while SATIRE-S and SORCE/SOLSTICE result in an increase. Recent publications have highlighted increases in mesospheric ozone when considering version 10 SORCE/SOLSTICE irradiances. The recalibrated SORCE/SOLSTICE version 12 irradiances result in a much smaller mesospheric ozone response than when using version 10 and now similar in magnitude to SATIRE-S. This shows that current knowledge of variations in spectral irradiance is not sufficient to warrant robust conclusions concerning the impact of solar variability on the atmosphere and climate. Title: Evolution of Small Scale Magnetic Structures from Sunrise Data Authors: Anusha, L. S.; Feller, A.; Hirzberger, J.; Solanki, S. K. Bibcode: 2014ASPC..489...83A Altcode: We present the results of an analysis of small scale magnetic features in the quiet Sun, observed with the Sunrise balloon borne telescope. Our aim is to understand the contribution of different physical processes that drive the evolution of magnetic features in quiet regions of the photosphere. To this end, we study the rearrangement, addition, and removal of magnetic flux through splitting, merging, cancellation, and emergence of magnetic fields. Title: Discriminant analysis of solar bright points and faculae II. Contrast and morphology analysis Authors: Kobel, P.; Hirzberger, J.; Solanki, S. K. Bibcode: 2014arXiv1410.5354K Altcode: Taken at a high spatial resolution of 0.1 arcsec, Bright Points (BPs) are found to coexist with faculae in images and the latter are often resolved as adjacent striations. Understanding the properties of these different features is fundamental to carrying out proxy magnetometry. To shed light on the relationship between BPs and faculae, we studied them separately after the application of a classification method, developed and described in a previous paper) on active region images at various heliocentric angles. In this Paper, we explore different aspects of the photometric properties of BPs and faculae, namely their G-band contrast profiles, their peak contrast in G-band and continuum, as well as morphological parameters. We find that: (1) the width of the contrast profiles of the classified BPs and faculae are consistent with studies of disk center BPs at and limb faculae, which indirectly confirms the validity of our classification, (2) the profiles of limb faculae are limbward skewed on average, while near disk center they exhibit both centerward and limbward skewnesses due to the distribution of orientations of the faculae, (3) the relation between the peak contrasts of BPs and faculae and their apparent area discloses a trend reminiscent of magnetogram studies. The skewness of facular profiles provides a novel constraint for 3D MHD models of faculae. As suggested by the asymmetry and orientation of their contrast profiles, faculae near disk center could be induced by inclined fields, while apparent BPs near the limb seem to be in fact small faculae misidentified. The apparent area of BPs and faculae could be possibly exploited for proxy magnetometry. Title: Modelling total solar irradiance since 1878 from simulated magnetograms Authors: Dasi-Espuig, M.; Jiang, J.; Krivova, N. A.; Solanki, S. K. Bibcode: 2014A&A...570A..23D Altcode: 2014arXiv1409.1941D
Aims: We present a new model of total solar irradiance (TSI) based on magnetograms simulated with a surface flux transport model (SFTM) and the Spectral And Total Irradiance REconstructions (SATIRE) model. Our model provides daily maps of the distribution of the photospheric field and the TSI starting from 1878.
Methods: The modelling is done in two main steps. We first calculate the magnetic flux on the solar surface emerging in active and ephemeral regions. The evolution of the magnetic flux in active regions (sunspots and faculae) is computed using a surface flux transport model fed with the observed record of sunspot group areas and positions. The magnetic flux in ephemeral regions is treated separately using the concept of overlapping cycles. We then use a version of the SATIRE model to compute the TSI. The area coverage and the distribution of different magnetic features as a function of time, which are required by SATIRE, are extracted from the simulated magnetograms and the modelled ephemeral region magnetic flux. Previously computed intensity spectra of the various types of magnetic features are employed.
Results: Our model reproduces the PMOD composite of TSI measurements starting from 1978 at daily and rotational timescales more accurately than the previous version of the SATIRE model computing TSI over this period of time. The simulated magnetograms provide a more realistic representation of the evolution of the magnetic field on the photosphere and also allow us to make use of information on the spatial distribution of the magnetic fields before the times when observed magnetograms were available. We find that the secular increase in TSI since 1878 is fairly stable to modifications of the treatment of the ephemeral region magnetic flux. Title: Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations Authors: Yeo, K. L.; Krivova, N. A.; Solanki, S. K.; Glassmeier, K. H. Bibcode: 2014A&A...570A..85Y Altcode: 2014arXiv1408.1229Y Context. Total and spectral solar irradiance are key parameters in the assessment of solar influence on changes in the Earth's climate.
Aims: We present a reconstruction of daily solar irradiance obtained using the SATIRE-S model spanning 1974 to 2013 based on full-disc observations from the KPVT, SoHO/MDI, and SDO/HMI.
Methods: SATIRE-S ascribes variation in solar irradiance on timescales greater than a day to photospheric magnetism. The solar spectrum is reconstructed from the apparent surface coverage of bright magnetic features and sunspots in the daily data using the modelled intensity spectra of these magnetic structures. We cross-calibrated the various data sets, harmonizing the model input so as to yield a single consistent time series as the output.
Results: The model replicates 92% (R2 = 0.916) of the variability in the PMOD TSI composite including the secular decline between the 1996 and 2008 solar cycle minima. The model also reproduces most of the variability in observed Lyman-α irradiance and the Mg II index. The ultraviolet solar irradiance measurements from the UARS and SORCE missions are mutually consistent up to about 180 nm before they start to exhibit discrepant rotational and cyclical variability, indicative of unresolved instrumental effects. As a result, the agreement between model and measurement, while relatively good below 180 nm, starts to deteriorate above this wavelength. As with earlier similar investigations, the reconstruction cannot reproduce the overall trends in SORCE/SIM SSI. We argue, from the lack of clear solar cycle modulation in the SIM record and the inconsistency between the total flux recorded by the instrument and TSI, that unaccounted instrumental trends are present.
Conclusions: The daily solar irradiance time series is consistent with observations from multiple sources, demonstrating its validity and utility for climate models. It also provides further evidence that photospheric magnetism is the prime driver of variation in solar irradiance on timescales greater than a day. Title: Nonlinear Force-free Field Modeling of the Solar Magnetic Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.; DeLuca, E. E.; Solanki, S. K. Bibcode: 2014ApJ...793..112C Altcode: 2014arXiv1408.0497C In the quiet solar photosphere, the mixed polarity fields form a magnetic carpet that continuously evolves due to dynamical interaction between the convective motions and magnetic field. This interplay is a viable source to heat the solar atmosphere. In this work, we used the line-of-sight (LOS) magnetograms obtained from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory, and the Imaging Magnetograph eXperiment instrument on the Sunrise balloon-borne observatory, as time-dependent lower boundary conditions, to study the evolution of the coronal magnetic field. We use a magneto-frictional relaxation method, including hyperdiffusion, to produce a time series of three-dimensional nonlinear force-free fields from a sequence of photospheric LOS magnetograms. Vertical flows are added up to a height of 0.7 Mm in the modeling to simulate the non-force-freeness at the photosphere-chromosphere layers. Among the derived quantities, we study the spatial and temporal variations of the energy dissipation rate and energy flux. Our results show that the energy deposited in the solar atmosphere is concentrated within 2 Mm of the photosphere and there is not sufficient energy flux at the base of the corona to cover radiative and conductive losses. Possible reasons and implications are discussed. Better observational constraints of the magnetic field in the chromosphere are crucial to understand the role of the magnetic carpet in coronal heating. Title: Fast Solar Polarimeter: Description and First Results Authors: Feller, A.; Iglesias, F. A.; Nagaraju, K.; Solanki, S. K.; Ihle, S. Bibcode: 2014ASPC..489..271F Altcode: We are developing a novel fast solar imaging polarimeter with an emphasis on significantly increased polarimetric accuracy and high spatial resolution. The instrument is based on a fast pnCCD sensor and shall work at frame rates of up to 400 fps, which suppresses spurious polarization signals induced by external disturbances such as atmospheric turbulence or jitter. The much higher polarimetric accuracy that can be achieved with the new instrument is in particular expected to extend studies of the enigmatic small-scale magnetic field in the quiet Sun, and of chromospheric magnetic fields. Here we will report on some key concepts of the polarimeter, and on first results obtained with an evaluation model at the spectrograph of the Vacuum Tower Telescope on Tenerife. Title: Inclinations of small quiet-Sun magnetic features based on a new geometric approach Authors: Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; Bellot Rubio, L. R.; van Noort, M.; Feller, A.; Danilovic, S. Bibcode: 2014A&A...569A.105J Altcode: 2014arXiv1408.2443J Context. High levels of horizontal magnetic flux have been reported in the quiet-Sun internetwork, often based on Stokes profile inversions.
Aims: Here we introduce a new method for deducing the inclination of magnetic elements and use it to test magnetic field inclinations from inversions.
Methods: We determine accurate positions of a set of small, bright magnetic elements in high spatial resolution images sampling different photospheric heights obtained by the Sunrise balloon-borne solar observatory. Together with estimates of the formation heights of the employed spectral bands, these provide us with the inclinations of the magnetic features. We also compute the magnetic inclination angle of the same magnetic features from the inversion of simultaneously recorded Stokes parameters.
Results: Our new, geometric method returns nearly vertical fields (average inclination of around 14° with a relatively narrow distribution having a standard deviation of 6°). In strong contrast to this, the traditionally used inversions give almost horizontal fields (average inclination of 75 ± 8°) for the same small magnetic features, whose linearly polarised Stokes profiles are adversely affected by noise. We show that for such magnetic features inversions overestimate the flux in horizontal magnetic fields by an order of magnitude.
Conclusions: The almost vertical field of bright magnetic features from our geometric method is clearly incompatible with the nearly horizontal magnetic fields obtained from the inversions. This indicates that the amount of magnetic flux in horizontal fields deduced from inversions is overestimated in the presence of weak Stokes signals, in particular if Stokes Q and U are close to or under the noise level. Inversions should be used with great caution when applied to data with no clear Stokes Q and no U signal. By combining the proposed method with inversions we are not just improving the inclination, but also the field strength. This technique allows us to analyse features that are not reliably treated by inversions, thus greatly extending our capability to study the complete magnetic field of the quiet Sun. Title: Variability of Sun-like stars: reproducing observed photometric trends Authors: Shapiro, A. I.; Solanki, S. K.; Krivova, N. A.; Schmutz, W. K.; Ball, W. T.; Knaack, R.; Rozanov, E. V.; Unruh, Y. C. Bibcode: 2014A&A...569A..38S Altcode: 2014arXiv1406.2383S Context. The Sun and stars with low magnetic activity levels become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behavior and become fainter when their activity increases.
Aims: We reproduce the observed photometric trends in stellar variations with a model thattreats stars as hypothetical suns with coverage by magnetic features different from that of the Sun.
Methods: The model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and bright faculae. A stellar spectrum is calculated from spectra of the individual components by weighting them with corresponding disk-area coverages. The latter are obtained by extrapolating the solar dependences of spot and facular disk-area coverages on chromospheric activity to stars with different levels of mean chromospheric activity.
Results: We find that the contribution by starspots to the variability increases faster with chromospheric activity than the facular contribution. This causes the transition from faculae-dominated variability and direct activity-brightness correlation to spot-dominated variability and inverse activity-brightness correlation with increasing chromospheric activity level. We show that the regime of the variability also depends on the angle between the stellar rotation axis and the line-of-sight and on the latitudinal distribution of active regions on the stellar surface. Our model can be used as a tool for extrapolating the observed photometric variability of the Sun to Sun-like stars at different activity levels, which makes a direct comparison between solar and stellar irradiance data possible.

Appendices are available in electronic form at http://www.aanda.org Title: Comparison of solar photospheric bright points between Sunrise observations and MHD simulations Authors: Riethmüller, T. L.; Solanki, S. K.; Berdyugina, S. V.; Schüssler, M.; Martínez Pillet, V.; Feller, A.; Gandorfer, A.; Hirzberger, J. Bibcode: 2014A&A...568A..13R Altcode: 2014arXiv1406.1387R Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17°, supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation. Title: Vigorous convection in a sunspot granular light bridge Authors: Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel; Danilovic, Sanja Bibcode: 2014A&A...568A..60L Altcode: 2014arXiv1407.1202L Context. Light bridges are the most prominent manifestation of convection in sunspots. The brightest representatives are granular light bridges composed of features that appear to be similar to granules.
Aims: An in-depth study of the convective motions, temperature stratification, and magnetic field vector in and around light bridge granules is presented with the aim of identifying similarities and differences to typical quiet-Sun granules.
Methods: Spectropolarimetric data from the Hinode Solar Optical Telescope were analyzed using a spatially coupled inversion technique to retrieve the stratified atmospheric parameters of light bridge and quiet-Sun granules.
Results: Central hot upflows surrounded by cooler fast downflows reaching 10 km s-1 clearly establish the convective nature of the light bridge granules. The inner part of these granules in the near surface layers is field free and is covered by a cusp-like magnetic field configuration. We observe hints of field reversals at the location of the fast downflows. The quiet-Sun granules in the vicinity of the sunspot are covered by a low-lying canopy field extending radially outward from the spot.
Conclusions: The similarities between quiet-Sun and light bridge granules point to the deep anchoring of granular light bridges in the underlying convection zone. The fast, supersonic downflows are most likely a result of a combination of invigorated convection in the light bridge granule due to radiative cooling into the neighboring umbra and the fact that we sample deeper layers, since the downflows are immediately adjacent to the slanted walls of the Wilson depression.

The two movies are available in electronic form at http://www.aanda.org Title: Scattered Lyman-α radiation of comet 2012/S1 (ISON) observed by SUMER/SOHO Authors: Curdt, W.; Boehnhardt, H.; Vincent, J. -B.; Solanki, S. K.; Schühle, U.; Teriaca, L. Bibcode: 2014A&A...567L...1C Altcode: 2014arXiv1406.4343C During its sungrazing perihelion passage, comet ISON appeared in the field of view of the SUMER spectrometer and allowed unique observations at far-ultraviolet wavelengths with high spatial and temporal resolution. We report results of these observations completed on November 28, 2013, when the comet was only 2.82 Rʘ away from the Sun. Our data show the arrow-shaped dust tail in Ly-α emission trailing behind the predicted position of the nucleus, but offset from the trajectory. We interpret the emission as sunlight that is scattered at μm-sized dust particles. We modeled the dust emission and dynamics to reproduce the appearance of the tail. We were unable to detect any signature of cometary gas or plasma around the expected position of the nucleus and conclude that the outgassing processes must have stopped before the observation started. Moreover, the model we used to reproduce the observed dust tail needs a sharp fall-off of the dust production hours before perihelion transit. We compare the radiances of the disk and the dust tail for an estimate of the dust column density and tail mass. Title: The History of a Quiet-Sun Magnetic Element Revealed by IMaX/SUNRISE Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot Rubio, Luis R.; Bonet, José A.; Martínez Pillet, Valentín; Solanki, Sami K.; Schmidt, Wolfgang Bibcode: 2014ApJ...789....6R Altcode: 2014arXiv1405.2837R Isolated flux tubes are considered to be fundamental magnetic building blocks of the solar photosphere. Their formation is usually attributed to the concentration of magnetic field to kG strengths by the convective collapse mechanism. However, the small size of the magnetic elements in quiet-Sun areas has prevented this scenario from being studied in fully resolved structures. Here, we report on the formation and subsequent evolution of one such photospheric magnetic flux tube, observed in the quiet Sun with unprecedented spatial resolution (0.''15-0.''18) and high temporal cadence (33 s). The observations were acquired by the Imaging Magnetograph eXperiment on board the SUNRISE balloon-borne solar observatory. The equipartition field strength magnetic element is the result of the merging of several same polarity magnetic flux patches, including a footpoint of a previously emerged loop. The magnetic structure is then further intensified to kG field strengths by convective collapse. The fine structure found within the flux concentration reveals that the scenario is more complex than can be described by a thin flux tube model with bright points and downflow plumes being established near the edges of the kG magnetic feature. We also observe a daisy-like alignment of surrounding granules and a long-lived inflow toward the magnetic feature. After a subsequent weakening process, the field is again intensified to kG strengths. The area of the magnetic feature is seen to change in anti-phase with the field strength, while the brightness of the bright points and the speed of the downflows varies in phase. We also find a relation between the brightness of the bright point and the presence of upflows within it. Title: Scattered Lyman-alpha radiation of comet 2012/S1 (ISON) observed by SUMER/SOHO Authors: Curdt, W.; Boehnhardt, H.; Germerott, D.; Schuehle, U.; Solanki, S.; Teriaca, L.; Vincent, J. Bibcode: 2014acm..conf..119C Altcode: During its recent perihelion passage, comet ISON came so close to the Sun that it appeared in the field of view (FOV) of the SUMER spectrometer on SOHO and allowed unique observations at far-UV wavelengths with high spatial and temporal resolution. We report results of these observations completed during the comet's encounter with the Sun on November 28.75, 2013. Our data show the dust tail trailing behind the predicted position of the nucleus seen in Lyman-alpha emission as light from the solar disk that is scattered by micron-sized dust particles. The arrow-shaped tail is offset from the trajectory and not aligned with it. We model the dust emission and dynamics to reproduce the appearance of the tail. We could not detect any signature of cometary gas or plasma around the expected position of the nucleus and conclude that the out-gassing processes must have stopped before the comet entered our FOV. Also the model we used to reproduce the observed dust tail needs a sharp fall-off of the dust production hours before perihelion. We compare the radiance of the dust tail to the Lyman-alpha emission of the disk for an estimate of the dust column density. After observing 18 years mostly solar targets, this was the first time that SUMER completed spectroscopic observations of a comet. Title: Search for Solar Axions by the CERN Axion Solar Telescope with He3 Buffer Gas: Closing the Hot Dark Matter Gap Authors: Arik, M.; Aune, S.; Barth, K.; Belov, A.; Borghi, S.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Da Riva, E.; Dafni, T.; Davenport, M.; Eleftheriadis, C.; Elias, N.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Geralis, T.; Georgiopoulou, E.; Giomataris, I.; Gninenko, S.; Gómez, H.; Gómez Marzoa, M.; Gruber, E.; Guthörl, T.; Hartmann, R.; Hauf, S.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kuster, M.; Lakić, B.; Lang, P. M.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Luzón, G.; Neff, S.; Niinikoski, T.; Nordt, A.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Riege, H.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Shilon, I.; Silva, P. S.; Solanki, S. K.; Stewart, L.; Tomás, A.; Tsagri, M.; van Bibber, K.; Vafeiadis, T.; Villar, J.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K.; CAST Collaboration Bibcode: 2014PhRvL.112i1302A Altcode: 2013arXiv1307.1985A The CERN Axion Solar Telescope has finished its search for solar axions with He3 buffer gas, covering the search range 0.64 eV≲ma≲1.17 eV. This closes the gap to the cosmological hot dark matter limit and actually overlaps with it. From the absence of excess x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of gaγ≲3.3×10-10 GeV-1 at 95% C.L., with the exact value depending on the pressure setting. Future direct solar axion searches will focus on increasing the sensitivity to smaller values of gaγ, for example by the currently discussed next generation helioscope International AXion Observatory. Title: Comparison between Mg II k and Ca II H Images Recorded by SUNRISE/SuFI Authors: Danilovic, S.; Hirzberger, J.; Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker, M.; Schmidt, W.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C. Bibcode: 2014ApJ...784...20D Altcode: We present a comparison of high-resolution images of the solar surface taken in the Mg II k and Ca II H channels of the Filter Imager on the balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled with 0.48 nm and 0.11 nm wide filters, respectively. The two channels show remarkable qualitative and quantitative similarities in the quiet Sun, in an active region plage and during a small flare. However, the Mg filtergrams display 1.4-1.7 times higher intensity contrast and appear more smeared and smoothed in the quiet Sun. In addition, the fibrils in a plage are wider. Although the exposure time is 100 times longer for Mg images, the evidence suggests that these differences cannot be explained only with instrumental effects or the evolution of the solar scene. The differences at least partially arise because of different line-formation heights, the stronger response of Mg k emission peaks to the higher temperatures, and the larger height range sampled by the broad Mg filter used here. This is evidently manifested during the flare when a surge in Mg evolves differently than in Ca. Title: Migration of Ca II H bright points in the internetwork Authors: Jafarzadeh, S.; Cameron, R. H.; Solanki, S. K.; Pietarila, A.; Feller, A.; Lagg, A.; Gandorfer, A. Bibcode: 2014A&A...563A.101J Altcode: 2014arXiv1401.7522J Context. The migration of magnetic bright point-like features (MBP) in the lower solar atmosphere reflects the dispersal of magnetic flux as well as the horizontal flows of the atmospheric layer they are embedded in.
Aims: We analyse trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca ii H MBPs (mean lifetime 461 ± 9 s) to obtain their diffusivity behaviour.
Methods: We use seeing-free high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca ii H 3968 Å passband of the Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory. Small MBPs in the internetwork are automatically tracked. The trajectory of each MBP is then calculated and described by a diffusion index (γ) and a diffusion coefficient (D). We also explore the distribution of the diffusion indices with the help of a Monte Carlo simulation.
Results: We find γ = 1.69 ± 0.08 and D = 257 ± 32 km2 s-1 averaged over all MBPs. Trajectories of most MBPs are classified as super-diffusive, i.e. γ > 1, with the determined γ being the largest obtained so far to our knowledge. A direct correlation between D and timescale (τ) determined from trajectories of all MBPs is also obtained. We discuss a simple scenario to explain the diffusivity of the observed, relatively short-lived MBPs while they migrate within a small area in a supergranule (i.e. an internetwork area). We show that the scatter in the γ values obtained for individual MBPs is due to their limited lifetimes.
Conclusions: The super-diffusive MBPs can be described as random walkers (due to granular evolution and intergranular turbulence) superposed on a large systematic (background) velocity, caused by granular, mesogranular, and supergranular flows. Title: Magnetic structure of an activated filament in a flaring active region Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2014A&A...561A..98S Altcode: 2013arXiv1312.2781S
Aims: While the magnetic field in quiescent prominences has been widely investigated, less is known about the field in activated prominences. We report observational results on the magnetic field structure of an activated filament in a flaring active region. In particular, we studied its magnetic structure and line-of-sight flows during its early activated phase, shortly before it displayed signs of rotation.
Methods: We inverted the Stokes profiles of the chromospheric He i 10 830 Å triplet and the photospheric Si i 10 827 Å line observed in this filament by the Vacuum Tower Telescope on Tenerife. Using these inversion results, we present and interpret the first maps of the velocity and magnetic field obtained in an activated filament, both in the photosphere and the chromosphere.
Results: Up to five different magnetic components are found in the chromospheric layers of the filament, while outside the filament a single component is sufficient to reproduce the observations. Magnetic components displaying an upflow are preferentially located towards the centre of the filament, while the downflows are concentrated along its periphery. Moreover, the upflowing gas is associated with an opposite-polarity magnetic configuration with respect to the photosphere, while the downflowing gas is associated with a same-polarity configuration.
Conclusions: The activated filament has a very complex structure. Nonetheless, it is compatible with a flux rope, albeit a distorted one, in the normal configuration. The observations are best explained by a rising flux rope in which part of the filament material is still stably stored (upflowing material, rising with the field), while the rest is no longer stably stored and flows down along the field lines.

The movie is available in electronic form at http://www.aanda.org Title: The chromosphere above sunspots at millimeter wavelengths Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2014A&A...561A.133L Altcode: 2014arXiv1403.3436L
Aims: The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy.
Methods: We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots.
Results: Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region.
Conclusions: A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae. Title: Time evolution of a single, quiet-Sun magnetic structure Authors: Requerey, Iker S.; Bonet, José Antonio; Solanki, Sami K.; Bellot Rubio, L. R.; Del Toro Iniesta, Jose Carlos Bibcode: 2014cosp...40E2828R Altcode: Isolated flux tubes are considered to be fundamental magnetic building blocks of the solar photosphere. Their formation is usually attributed to the concentration of magnetic field to kG strengths by the convective instability mechanism. However, the small size of the magnetic elements in quiet-Sun areas has prevented this scenario from being studied in fully resolved structures. Here we report on the formation and subsequent evolution of a photospheric magnetic flux tube, observed in the quiet Sun with unprecedented spatial resolution (0. ('') 15 - 0. ('') 18) and high temporal cadence (33 s). The observations were acquired by the Imaging Magnetograph Experiment (IMaX) aboard the textsc{Sunrise} balloon-borne solar observatory. The equipartition field strength magnetic element is reached from the merging of several magnetic flux patches in a mesogranule-sized sink. The magnetic structure is then further intensified to kG field strengths by convective collapse and granular compression. The fine structure found within the flux concentration reveal that the scenario is more complex than a canonical flux tube model. After a subsequent weakening process, the field is further intensified to kG strengths. Seen as a whole, the evolution of the magnetic structure is compatible with oscillations in all basic physical quantities. A discussion on whether this evolution fits to the current theoretical descriptions is also presented. Title: Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena Authors: Yeo, K. L.; Feller, A.; Solanki, S. K.; Couvidat, S.; Danilovic, S.; Krivova, N. A. Bibcode: 2014A&A...561A..22Y Altcode: 2013arXiv1310.4972Y
Aims: We present a point spread function (PSF) for the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data.
Methods: The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the Venusian disc and solar background with a guess PSF. We described the PSF as the sum of five Gaussian functions, the amplitudes of which vary sinusoidally with azimuth. This relatively complex functional form was required by the data. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. We also examined the variation in the shape of the solar aureole in daily data, as an indication of PSF changes over time.
Results: Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 6173 Å line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. The shape of the solar aureole varied, with time and between the two CCDs. There are also indications that the PSF varies across the FOV. However, all these variations were found to be relatively small, such that a single PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error.
Conclusions: Restoring HMI observations with the PSF presented here returns a reasonable estimate of the stray light-free intensity contrast. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation. Title: Reconstructions of solar irradiance on centennial time scales Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria; Kok Leng, Yeo Bibcode: 2014cosp...40E1664K Altcode: Solar irradiance is the main external source of energy to Earth's climate system. The record of direct measurements covering less than 40 years is too short to study solar influence on Earth's climate, which calls for reconstructions of solar irradiance into the past with the help of appropriate models. An obvious requirement to a competitive model is its ability to reproduce observed irradiance changes, and a successful example of such a model is presented by the SATIRE family of models. As most state-of-the-art models, SATIRE assumes that irradiance changes on time scales longer than approximately a day are caused by the evolving distribution of dark and bright magnetic features on the solar surface. The surface coverage by such features as a function of time is derived from solar observations. The choice of these depends on the time scale in question. Most accurate is the version of the model that employs full-disc spatially-resolved solar magnetograms and reproduces over 90% of the measured irradiance variation, including the overall decreasing trend in the total solar irradiance over the last four cycles. Since such magnetograms are only available for about four decades, reconstructions on time scales of centuries have to rely on disc-integrated proxies of solar magnetic activity, such as sunspot areas and numbers. Employing a surface flux transport model and sunspot observations as input, we have being able to produce synthetic magnetograms since 1700. This improves the temporal resolution of the irradiance reconstructions on centennial time scales. The most critical aspect of such reconstructions remains the uncertainty in the magnitude of the secular change. Title: Analysis of magnetic fields in a plage region using a spatially coupled 2D inversion technique Authors: Buehler, David; Lagg, Andreas; Solanki, Sami K.; Van Noort, Michiel Bibcode: 2014cosp...40E.427B Altcode: The properties of magnetic features (MFCs) within a plage region in the vicinity of a sunspot were investigated at high spatial resolution. Stokes spectra of the 630nm line pair recorded by the spectropolarimeter aboard Hinode were inverted using an extended version of the SPINOR code. The code preformed a spatially coupled inversion of the Stokes spectra using three log(tau) nodes in optical depth. No magnetic filling factors was employed. The analysis of the inversion results reveals that the MFCs have typical field strengths of 1500G at log(tau)=-0.9 and inclinations between 10-15 degrees in all three log(tau) nodes. The MFCs expand by forming magnetic canopies composed of weaker and more inclined magnetic fields. The expansion of the magnetic field and temperature stratification of MFCs with optical depth is in good agreement with a thin flux tube model. Whilst the gas inside magnetic flux concentrations is typically at rest, the majority of MFCs were surrounded by a ring of downflows with an average value of 2.5km/s at log(tau)=0. The ring gradually shifts outwards following the expansion of the MFC. Within the downflow rings of MFCs small magnetic patches of opposite polarity to that of the main MFC were identified, which are predominantly situated beneath the canopy of its main MFC. We found evidence for a strong broadening of the Stokes profiles within MFCs and their surrounding downflow rings (expressed by a microturbulence in the inversion). This indicates the presence of strong unresolved velocities. Title: Total solar irradiance reconstruction since 1700 using a flux transport model Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.; Jiang, Jie Bibcode: 2014cosp...40E.634D Altcode: Reconstructions of solar irradiance into the past are crucial for studies of solar influence on climate. Models based on the assumption that irradiance changes are caused by the evolution of the photospheric magnetic fields have been most successful in reproducing the measured irradiance variations. Daily magnetograms, such as those from MDI and HMI, provide the most detailed information on the changing distribution of the photospheric magnetic fields. Since such magnetograms are only available from 1974, we used a surface flux transport model to describe the evolution of the magnetic fields on the solar surface due to the effects of differential rotation, meridional circulation, and turbulent diffusivity, before 1974. In this model, the sources of magnetic flux are the active regions, which are introduced based on sunspot group areas, positions, and tilt angles. The RGO record is, however, only available since 1874. Here we present a model of solar irradiance since 1700, which is based on a semi-synthetic sunspot record. The semi-synthetic record was obtained using statistical relationships between sunspot group properties (areas, positions, tilt angles) derived from the RGO record on one hand, and the cycle strength and phase derived from the sunspot group number (Rg) on the other. These relationships were employed to produce daily records of sunspot group positions, areas, and tilt angles before 1874. The semi-synthetic records were fed into the surface flux transport model to simulate daily magnetograms since 1700. By combining the simulated magnetograms with a SATIRE-type model, we then reconstructed total solar irradiance since 1700. Title: Sunrise Mission Highlights Authors: Riethmüller, T.; Solanki, S. K.; Sunrise Team Bibcode: 2014frap.confE..34R Altcode: 2014PoS...237E..34R No abstract at ADS Title: Fast Solar Polarimeter: First Light Results Authors: Krishnappa, N.; Feller, A.; Iglesia, F. A.; Solanki, S. Bibcode: 2013AGUFMSM53A2213K Altcode: Accurate measurements of magnetic fields on the Sun are crucial to understand various physical processes that take place in the solar atmosphere such as solar eruptions, coronal heating, solar wind acceleration, etc. The Fast Solar Polarimeter (FSP) is a new instrument that is being developed to probe magnetic fields on the Sun. One of the main goals of this polarimeter is to carry out high precision spectropolarimetric observations with spatial resolution close to the telescope diffraction limit. The polarimeter is based on pnCCD technology with split frame transfer and simultaneous multi-channel readout, resulting in frame rate upto 1 kHz. The FSP prototype instrument uses a small format pnCCD of 264x264 pixels which has been developed by PNSensor and by the semiconductor lab of the Max Planck Society. The polarization modulator is based on two ferro-electric liquid crystals (FLCs) interlaced between two static retarders. The first solar observations have been carried out with this prototype during May-June, 2013 at German Vacuum Tower Telescope (VTT) on Tenerife, Canary Islands, Spain. Here we present the instrument performance assessments and the first results on the magnetic field measurements. Further, we briefly discuss about the next phase of FSP which will be a dual beam system with 1k x 1k CCDs. Title: Three-dimensional non-LTE radiative transfer effects in Fe i lines. II. Line formation in 3D radiation hydrodynamic simulations Authors: Holzreuter, R.; Solanki, S. K. Bibcode: 2013A&A...558A..20H Altcode: 2013arXiv1308.2490H Here we investigate the effects of horizontal radiative transfer (RT) in combination with non-local thermodynamic equilibrum (NLTE) on important diagnostic iron lines in a realistic atmosphere. Using a snapshot of a 3D radiation-hydrodynamic (HD) simulation and a multilevel iron atom, we computed widely used Fe i line profiles at three different levels of approximation of the RT (3D NLTE, 1D NLTE, LTE). By comparing the resulting line profiles and the circumstances of their formation, we gain new insight into the importance of horizontal RT. We find that the influence of horizontal RT is of the same order of magnitude as that of NLTE, although spatially more localized. Also, depending on the temperature of the surroundings, horizontal RT is found to either weaken or strengthen spectral lines. Line depths and equivalent width may differ by up to 20% from the corresponding LTE value if 3D RT is applied. Residual intensity contrasts in LTE are found to be larger than those in 3D NLTE by up to a factor of two. When compared to 1D NLTE, we find that horizontal RT weakens the contrast by up to 30% almost independently of the angle of line of sight. While the center-to-limb variation (CLV) of the 1D and 3D NLTE contrasts have a similar form, the LTE contrast CLV shows a different run. Determination of temperatures by 1D NLTE inversions of spatially resolved observations may produce errors of up to 200 K if one neglects 3D RT. We find a linear correlation between the intensity difference of 1D and 3D NLTE and a simple estimate of the temperature in the horizontal environment of the line formation region. This correlation could be used to coarsely correct inversions done in 1D NLTE for some of the effects of horizontal RT. Horizontal RT is less important if one considers spatially averaged line profiles because local line strengthening and weakening occur with similar frequency in our HD atmosphere. Thus, the iron abundance is underestimated by 0.012 dex if calculated using 1D NLTE RT. Since effects of horizontal RT are greatest for spatially resolved quantities, the use of 3D RT is particularly important for interpreting high spatial resolution observations. Title: Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE Authors: Quintero Noda, C.; Martínez Pillet, V.; Borrero, J. M.; Solanki, S. K. Bibcode: 2013A&A...558A..30Q Altcode: 2013arXiv1309.0627Q Context. Localized strongly Doppler-shifted Stokes V signals were detected by IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that are observed as linear polarization features.
Aims: We aim to set constraints on the physical nature and causes of these highly Doppler-shifted signals. In particular, the temporal relation between the appearance of transverse fields and the strong Doppler shifts is analyzed in some detail.
Methods: We calculated the time difference between the appearance of the strong flows and the linear polarization. We also obtained the distances from the center of various features to the nearest neutral lines and whether they overlap or not. These distances were compared with those obtained from randomly distributed points on observed magnetograms. Various cases of strong flows are described in some detail.
Results: The linear polarization signals precede the appearance of the strong flows by on average 84 ± 11 s. The strongly Doppler-shifted signals are closer (0.″19) to magnetic neutral lines than randomly distributed points (0.″5). Eighty percent of the strongly Doppler-shifted signals are close to a neutral line that is located between the emerging field and pre-existing fields. That the remaining 20% do not show a close-by pre-existing field could be explained by a lack of sensitivity or an unfavorable geometry of the pre-existing field, for instance, a canopy-like structure.
Conclusions: Transverse fields occurred before the observation of the strong Doppler shifts. The process is most naturally explained as the emergence of a granular-scale loop that first gives rise to the linear polarization signals, interacts with pre-existing fields (generating new neutral line configurations), and produces the observed strong flows. This explanation is indicative of frequent small-scale reconnection events in the quiet Sun. Title: First High-resolution Images of the Sun in the 2796 Å Mg II k Line Authors: Riethmüller, T. L.; Solanki, S. K.; Hirzberger, J.; Danilovic, S.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker, M.; Schmidt, W.; Del Toro Iniesta, J. C. Bibcode: 2013ApJ...776L..13R Altcode: 2013arXiv1309.5213R We present the first high-resolution solar images in the Mg II k 2796 Å line. The images, taken through a 4.8 Å broad interference filter, were obtained during the second science flight of Sunrise in 2013 June by the Sunrise Filter Imager (SuFI) instrument. The Mg II k images display structures that look qualitatively very similar to images taken in the core of Ca II H. The Mg II images exhibit reversed granulation (or shock waves) in the internetwork regions of the quiet Sun, at intensity contrasts that are similar to those found in Ca II H. Very prominent in Mg II are bright points, both in the quiet Sun and in plage regions, particularly near the disk center. These are much brighter than at other wavelengths sampled at similar resolution. Furthermore, Mg II k images also show fibril structures associated with plage regions. Again, the fibrils are similar to those seen in Ca II H images, but tend to be more pronounced, particularly in weak plage. Title: Peripheral downflows in sunspot penumbrae Authors: van Noort, M.; Lagg, A.; Tiwari, S. K.; Solanki, S. K. Bibcode: 2013A&A...557A..24V Altcode: 2013arXiv1308.0466V Context. Sunspot penumbrae show high-velocity patches along the periphery.
Aims: The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations.
Methods: We employ Fourier interpolation in combination with spatially coupled height dependent LTE inversions of Stokes profiles to produce high-resolution, height-dependent maps of atmospheric parameters of these downflows and investigate their properties.
Results: High-speed downflows are observed over a wide range of viewing angles. They have supersonic line-of-sight velocities, some in excess of 20 km s-1, and very high magnetic field strengths, reaching values of over 7 kG. A relation between the downflow velocities and the magnetic field strength is found, in good agreement with MHD simulations.
Conclusions: The coupled inversion at high resolution allows for the accurate determination of small-scale structures. The recovered atmospheric structure indicates that regions with very high downflow velocities contain some of the strongest magnetic fields that have ever been measured on the Sun.

Two movies are available in electronic form at http://www.aanda.org Title: Structure of sunspot penumbral filaments: a remarkable uniformity of properties Authors: Tiwari, Sanjiv Kumar; van Noort, Michiel; Lagg, Andreas; Solanki, Sami K. Bibcode: 2013A&A...557A..25T Altcode: 2013arXiv1307.3668T Context. The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear.
Aims: We aim to investigate the fine-scale structure of these penumbral filaments.
Methods: We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope.
Results: Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800 K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1 kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth.
Conclusions: We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature. Title: Solar Irradiance Variability and Climate Authors: Solanki, Sami K.; Krivova, Natalie A.; Haigh, Joanna D. Bibcode: 2013ARA&A..51..311S Altcode: 2013arXiv1306.2770S The brightness of the Sun varies on all timescales on which it has been observed, and there is increasing evidence that this has an influence on climate. The amplitudes of such variations depend on the wavelength and possibly the timescale. Although many aspects of this variability are well established, the exact magnitude of secular variations (going beyond a solar cycle) and the spectral dependence of variations are under discussion. The main drivers of solar variability are thought to be magnetic features at the solar surface. The climate response can be, on a global scale, largely accounted for by simple energetic considerations, but understanding the regional climate effects is more difficult. Promising mechanisms for such a driving have been identified, including through the influence of UV irradiance on the stratosphere and dynamical coupling to the surface. Here, we provide an overview of the current state of our knowledge, as well as of the main open questions. Title: Sunspot group tilt angles and the strength of the solar cycle (Corrigendum) Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron, R.; Peñuela, T. Bibcode: 2013A&A...556C...3D Altcode: No abstract at ADS Title: Quiet Sun magnetic fields observed by Hinode: Support for a local dynamo Authors: Buehler, D.; Lagg, A.; Solanki, S. K. Bibcode: 2013A&A...555A..33B Altcode: 2013arXiv1307.0789B Context. The Hinode mission has revealed copious amounts of horizontal flux covering the quiet Sun. Local dynamo action has been proposed to explain the presence of this flux.
Aims: We sought to test whether the quiet Sun flux detected by Hinode is due to a local or the global dynamo by studying long-term variations in the polarisation signals detectable at the disc centre of the quiet Sun between November 2006 and May 2012, with particular emphasis on weak signals in the internetwork.
Methods: The investigation focusses on line-integrated circular polarisation Vtot and linear polarisation LPtot profiles obtained from the Fe I 6302.5 Å absorption line in Hinode SOT/SP.
Results: Both circular and linear polarisation signals show no overall variation in the fraction of selected pixels from 2006 until 2012. There is also no variation in the magnetic flux in this interval of time. The probability density functions (PDF) of the line-of-sight magnetic flux can be fitted with a power law from 1.17 × 1017 Mx to 8.53 × 1018 Mx with index α = -1.82 ± 0.02 in 2007. The variation of α's across all years does not exceed a significance of 1σ. Linearly polarised features are also fitted with a power law, with index α = -2.60 ± 0.06 in 2007. Indices derived from linear polarisation PDFs of other years also show no significant variation.
Conclusions: Our results show that the ubiquitous horizontal polarisation on the edges of bright granules seen by Hinode are invariant during the minimum of cycle 23. This supports the notion that the weak circular and linear polarisation is primarily caused by an independent local dynamo. Title: Chromospheric magnetic field of an active region filament using the He I triplet and the primary observation of filaments (prominences) using New Vacuum Solar Tower of China Authors: Xu, Zhi; Lagg, A.; Solanki, S.; Liu, Z.; New Vacuum Solar Telescope Observers Bibcode: 2013SPD....4410504X Altcode: There are two parts in my presentation. In the first part I present the magnetic field measurement of an active region filament using the full Stokes profiles of He I 10830 and Si I 10827 band when the filament in its stable phase. This observation was fulfilled using German Vacuum Tower Telescope (VTT). The vector magnetic field and Doppler velocity map both in the photosphere and chromosphere were observed and analyzed co-temporally and co-spatially. The observation findings reveal that we were observing the emergence of a flux rope with a subsequent formation of a filament. In the second part, I would like to exhibit another ground-based observation facility, 1m New Vacuum Solar Telescope (NVST) located in Fu-Xian Lake Solar Observatory of China. After the basic introduction including the location and instrumentations, I give some high lights including granulation, faculae, micro-flares, jets, and filaments or prominence since the first running in 2010, showing our potential ability to do high-resolution solar observation from the ground. Observation proposals from the international solar community are well appreciated in future. Title: Vertical flows and mass flux balance of sunspot umbral dots Authors: Riethmüller, T. L.; Solanki, S. K.; van Noort, M.; Tiwari, S. K. Bibcode: 2013A&A...554A..53R Altcode: 2013arXiv1305.1164R A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and ≈600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background. Title: First evidence of interaction between longitudinal and transverse waves in solar magnetic elements Authors: Stangalini, M.; Solanki, S. K.; Cameron, R.; Martínez Pillet, V. Bibcode: 2013A&A...554A.115S Altcode: 2013arXiv1304.7088S Small-scale magnetic fields are thought to play an important role in the heating of the outer solar atmosphere. By taking advantage of the unprecedented high-spatial and temporal cadence of the Imaging Magnetograph eXperiment (IMaX), the filter vector polarimeter on board the Sunrise balloon-borne observatory, we study the transversal and longitudinal velocity oscillations in small magnetic elements. The results of this analysis are then compared to magnetohydrodynamic (MHD) simulations, showing excellent agreement. We found buffeting-induced transverse oscillations with velocity amplitudes of the order of 1-2 km s-1 to be common along with longitudinal oscillations with amplitudes ~0.4 km s-1. Moreover, we also found an interaction between transverse oscillations and longitudinal velocity oscillations, showing a ± 90° phase lag at the frequency at which they exhibit the maximum coherence in the power spectrum. Our results are consistent with the theoretical picture in which MHD longitudinal waves are excited inside small magnetic elements as a response of the flux tube to the forcing action of the granular flows. Title: Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations Authors: Wiegelmann, Thomas; Solanki, Sami; Borrero, Juan; Peter, Hardi; Sunrise Team Bibcode: 2013EGUGA..15.5251W Altcode: Observations with the balloon-borne Sunrise/ Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyse a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun's atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop footpoints is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3 ± 1 minutes in the upper solar atmosphere and 12 ± 4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun. Title: A Collaborative FP7 Effort towards the First European Comprehensive SOLar Irradiance Data Exploitation (SOLID) Authors: Haberreiter, Margit; Dasi, Maria; Delouille, Veronique; Del Zanna, Giulio; Dudok de Wit, Thierry; Ermolli, Ilaria; Kretzschmar, Matthieu; Krivova, Natalie; Mason, Helen; Qahwaji, Rami; Schmutz, Werner; Solanki, Sami; Thuillier, Gerard; Tourpali, Kleareti; Unruh, Yvonne; Verbeeck, Cis; Weber, Mark; Woods, Tom Bibcode: 2013EGUGA..1513079H Altcode: Variations of solar irradiance are the most important natural factor in the terrestrial climate and as such, the time dependent spectral solar irradiance is a crucial input to any climate modelling. There have been previous efforts to compile solar irradiance but it is still uncertain by how much the spectral and total solar irradiance changed on yearly, decadal and longer time scales. Observations of irradiance data exist in numerous disperse data sets. Therefore, it is important to bring together the European expertise in the field to analyse and merge the complete set of European irradiance data, complemented by archive data that include data from non-European missions. We report on the initiation of a collaborative effort to unify representatives from all European solar space experiments and European teams specialized in multi-wavelength solar image processing. It is intended to include the European groups involved in irradiance modelling and reconstruction. They will work with two different state of the art approaches to produce reconstructed spectral and total solar irradiance data as a function of time. These results will be used to bridge gaps in time and wavelength coverage of the observational data. This will allow the proposing SOLID team to reduce the uncertainties in the irradiance time series - an important requirement by the climate community - and to provide uniform data sets of modelled and observed solar irradiance data from the beginning of the space era to the present including proper error and uncertainty estimates. Climate research needs these data sets and therefore, the primary benefit is for the climate community, but the stellar community, planetary, lunar, and ionospheric researchers are also interested in having at their disposition incident radiation of the Sun. The proposing team plans to realize a wide international synergy in solar physics from 7 European countries, and collaborators from the US, complemented by representatives from the climate community, who will accompany their research work with wide dissemination activities. Title: The AD775 cosmic event revisited: the Sun is to blame Authors: Usoskin, I. G.; Kromer, B.; Ludlow, F.; Beer, J.; Friedrich, M.; Kovaltsov, G. A.; Solanki, S. K.; Wacker, L. Bibcode: 2013A&A...552L...3U Altcode: 2013arXiv1302.6897U
Aims: Miyake et al. (2012, Nature, 486, 240, henceforth M12) recently reported, based on 14C data, an extreme cosmic event in about AD775. Using a simple model, M12 claimed that the event was too strong to be caused by a solar flare within the standard theory. This implied a new paradigm of either an impossibly strong solar flare or a very strong cosmic ray event of unknown origin that occurred around AD775. However, as we show, the strength of the event was significantly overestimated by M12. Several subsequent works have attempted to find a possible exotic source for such an event, including a giant cometary impact upon the Sun or a gamma-ray burst, but they are all based on incorrect estimates by M12. We revisit this event with analysis of new datasets and consistent theoretical modelling.
Methods: We verified the experimental result for the AD775 cosmic ray event using independent datasets including 10Be series and newly measured 14C annual data. We surveyed available historical chronicles for astronomical observations for the period around the AD770s to identify potential sightings of aurorae borealis and supernovae. We interpreted the 14C measurements using an appropriate carbon cycle model.
Results: We show that: (1) The reality of the AD775 event is confirmed by new measurements of 14C in German oak; (2) by using an inappropriate carbon cycle model, M12 strongly overestimated the event's strength; (3) the revised magnitude of the event (the global 14C production Q = (1.1 - 1.5) × 108 atoms/cm2) is consistent with different independent datasets (14C, 10Be, 36Cl) and can be associated with a strong, but not inexplicably strong, solar energetic particle event (or a sequence of events), and provides the first definite evidence for an event of this magnitude (the fluence >30 MeV was about 4.5 × 1010 cm-2) in multiple datasets; (4) this interpretation is in agreement with increased auroral activity identified in historical chronicles.
Conclusions: The results point to the likely solar origin of the event, which is now identified as the greatest solar event on a multi-millennial time scale, placing a strong observational constraint on the theory of explosive energy releases on the Sun and cool stars. Title: Modeling the variability of Sun-like stars Authors: Shapiro, Alexander; Knaack, Reto; Krivova, Natalie; Schmutz, Werner; Solanki, Sami; Unruh, Yvonne Bibcode: 2013EGUGA..15.9981S Altcode: We present a model which attributes the variability of the stellar radiative energy flux to the imbalance between the contributions from dark starspots and bright faculae. The stellar radiative energy flux variations are modeled from the individual component's spectra, by weighting them with corresponding filling factors. The filling factors are deduced by extrapolating the sunspot and facular filling factors dependencies on solar CaII S-index to stars with different levels of the chromospheric activity. Our approach allows us to model the stellar photometric variability vs. activity dependency and reproduce the transition from spot-dominated to facula-dominated regimes of variability. We show how the effect of inclination (arising due to the random position of the Earth-bound observer relative to the directions of stellar rotational axis) can affect these dependencies and present the modeling of the individual stellar photometric light curves. Title: What can we learn about the Sun with PREMOS/PICARD? Authors: Cessateur, Gaël; Shapiro, Alexander; Schmutz, Werner; Krivova, Natalie; Solanki, Sami K.; Yeo, Kok Leng; Thuillier, Gérard Bibcode: 2013EGUGA..1511720C Altcode: Total and Spectral Solar Irradiance are key input parameters to atmospheric/oceanic and space weather models. We present here spectral solar irradiance data from the radiometer PREMOS onboard the PICARD satellite. This instrument convers the solar spectrum from the Ultraviolet to near-infrared, and provides valuable information and nourishes theoretical models. Based on redundancy strategies, instrumental degradation has been mostly corrected, revealing surprising behavior from the visible and near-infrared filters. We compare these data with those from the VIRGO/SOHO and SOLSTIC/SORCE experiments. Finally we use COSI to model the variability of the irradiance, assuming that the latter is determined by the evolution of the solar surface magnetic field as seen with SDO/HMI data. A direct comparison shows a very good correlation for most of channels from PREMOS. Title: Analysis of Ca II K images aiming to determine long-term trends in solar irradiance variability Authors: Kar, Anuradha; Ermolli, Ilaria; Krivova, Natalie; Solanki, Sami Bibcode: 2013EGUGA..15..330K Altcode: The change in radiative output of the Sun on time scales longer than a day is attributed to the variability in solar surface magnetic fields. Direct irradiance measurements are only available for less than four decades. To reconstruct long term trends in solar total and spectral irradiance, proxies of solar surface magnetism like sunspot, facular and network areas are needed. Currently, sunspot records alone are used for this purpose, from which the deduction of facular and network areas is rather indirect. Historical records of full disk images of the Sun taken in the Ca II K spectral line (393.3 nm) have the potential to provide far more direct information about the distribution and evolution of faculae and network elements. The latter appear as bright regions in the Ca II K spectroheliograms and their intensity is correlated with the magnetic field strength of the features on the solar surface. Solar full disk images in the Ca II K line have been recorded since the beginning of the 20th century at a number of solar observatories such as at Arcetri (Italy), Mount Wilson(California, US) and Kodaikanal (India). The images are available in digitized archives that contain the data processed for standard instrumental calibrations. To utilize these records for irradiance studies, the next step is to identify the bright magnetic features from the images using feature recognition techniques. We test different feature identification methods which are first applied to a set of recent images from the PSPT instrument at the Osservatorio Astronomico di Roma, taken during three periods characterized by high, medium and low levels of activity. Then the performance of these methods to historical images from Arcetri, Mt. Wilson and Kodaikanal archives is tested. The results will be presented and discussed here. Title: Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Peter, H.; Barthol, P.; Gandorfer, A.; Martínez Pillet, V.; Schmidt, W.; Knölker, M. Bibcode: 2013SoPh..283..253W Altcode: Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun's atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun. Title: Fine structures in the atmosphere above a sunspot umbra Authors: Bharti, L.; Hirzberger, J.; Solanki, S. K. Bibcode: 2013A&A...552L...1B Altcode: 2013arXiv1302.2772B We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904, obtained with the Swedish Solar Telescope (SST) La Palma, Canary Islands. Time series of high resolution Ca ii H images show transient jet-like structures in sunspot umbrae are elongated, which we call umbral microjets. These jets are directed roughly parallel to nearby penumbral microjets, suggesting that they are aligned with the background magnetic field. In general, first a bright dot-like structure appears, from which a jet later emerges, although some jets appear without an associated chromospheric dot. Bright photospheric umbral dots are associated with umbral microjets arising in the outer umbra. Nevertheless, a one-to-one correspondence between jet-like events and underlying umbral dots is not seen. They are typically less than 1'' long and less than 0.3 arcsec wide. The typical lifetime of umbral microjets is around one minute. The brightness of these structures increases from the center of the umbra toward the umbra-penumbra boundary along with the brightness of the local background. Title: Recent variability of the solar spectral irradiance and its impact on climate modelling Authors: Ermolli, I.; Matthes, K.; Dudok de Wit, T.; Krivova, N. A.; Tourpali, K.; Weber, M.; Unruh, Y. C.; Gray, L.; Langematz, U.; Pilewskie, P.; Rozanov, E.; Schmutz, W.; Shapiro, A.; Solanki, S. K.; Woods, T. N. Bibcode: 2013ACP....13.3945E Altcode: 2012ACPD...1224557E; 2013arXiv1303.5577E The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult. Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earth's atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earth's climate.

We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earth's atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks. We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities.

We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earth's climate in CCMs.

We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation.

The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response.

Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments. Title: Intensity contrast of solar network and faculae Authors: Yeo, K. L.; Solanki, S. K.; Krivova, N. A. Bibcode: 2013A&A...550A..95Y Altcode: 2013arXiv1302.1442Y
Aims: This study aims at setting observational constraints on the continuum and line core intensity contrast of network and faculae, specifically, their relationship with magnetic field and disc position.
Methods: Full-disc magnetograms and intensity images by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) were employed. Bright magnetic features, representing network and faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and faculae.
Results: In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core differ considerably, most notably, they exhibit opposite centre-to-limb variations. We found this difference in behaviour to likely be due to the different mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from differences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores.
Conclusions: HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account. Title: Solar irradiance variability Authors: Solanki, S. K.; Unruh, Y. C. Bibcode: 2013AN....334..145S Altcode: 2013csss...17..145S; 2012arXiv1210.5911S The Sun has long been considered a constant star, to the extent that its total irradiance was termed the solar constant. It required radiometers in space to detect the small variations in solar irradiance on timescales of the solar rotation and the solar cycle. A part of the difficulty is that there are no other constant natural daytime sources to which the Sun's brightness can be compared. The discovery of solar irradiance variability rekindled a long-running discussion on how strongly the Sun affects our climate. A non-negligible influence is suggested by correlation studies between solar variability and climate indicators. The mechanism for solar irradiance variations that fits the observations best is that magnetic features at the solar surface, i.e. sunspots, faculae and the magnetic network, are responsible for almost all variations (although on short timescales convection and p-mode oscillations also contribute). In spite of significant progress important questions are still open. Thus there is a debate on how strongly irradiance varies on timescales of centuries (i.e. how much darker the Sun was during the Maunder minimum than it is today). It is also not clear how the solar spectrum changes over the solar cycle. Both these questions are of fundamental importance for working out just how strongly the Sun influences our climate. Another interesting question is how solar irradiance variability compares with that of other cool dwarfs, particularly now that observations from space are available also for stars. Title: European Solar Physics: moving from SOHO to Solar Orbiter and beyond Authors: Solanki, S. K.; Teriaca, L.; Barthol, P.; Curdt, W.; Inhester, B. Bibcode: 2013MmSAI..84..286S Altcode: When ESA and NASA launched the Solar and Heliospheric Observatory (SOHO) to the Sun-Earth L1 point, they also launched European solar physics into a steep upward trajectory. Thanks to the battery of instruments on SOHO our picture of the Sun changed dramatically from that of a sedate, nearly static star into that of a highly structured, dynamic one. Subsequent solar missions have probed higher energy radiation, gazed at the Sun from new vantage points in the ecliptic, analysed the Sun at higher spatial resolution and imaged the whole Sun in many wavelengths at high cadence. Nonetheless, SOHO is still going strong and still delivering unique data. In the meantime European solar physicists are working hard on the next major mission, Solar Orbiter, now being implemented as the M1 mission of ESA's Cosmic Vision program. Solar Orbiter will leave the Earth's orbit and move ever closer to the Sun, reaching a perihelion inside the orbit of Mercury. This will allow it to sample the Sun's dynamic inner heliosphere in situ, while probing the source regions of the ambient solar wind with its remote sensing instruments, a unique combination. Solar Orbiter will also leave the ecliptic and, for the first time, image the Sun's poles. This will bring us closer to finding the missing pieces of the puzzle on how the solar dynamo works. Beyond Solar Orbiter are further exciting prospects, such as the Solar-C mission, or the large European Solar Telescope. Here an overview of solar missions and telescopes and the associated science is given from a European perspective. Title: Structure and dynamics of isolated internetwork Ca II H bright points observed by SUNRISE Authors: Jafarzadeh, S.; Solanki, S. K.; Feller, A.; Lagg, A.; Pietarila, A.; Danilovic, S.; Riethmüller, T. L.; Martínez Pillet, V. Bibcode: 2013A&A...549A.116J Altcode: 2012arXiv1211.4836J
Aims: We aim to improve our picture of the low chromosphere in the quiet-Sun internetwork by investigating the intensity, horizontal velocity, size and lifetime variations of small bright points (BPs; diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 Å passband along with their magnetic field parameters, derived from photospheric magnetograms.
Methods: Several high-quality time series of disc-centre, quiet-Sun observations from the Sunrise balloon-borne solar telescope, with spatial resolution of around 100 km on the solar surface, have been analysed to study the dynamics of BPs observed in the Ca II H passband and their dependence on the photospheric vector magnetogram signal.
Results: Parameters such as horizontal velocity, diameter, intensity and lifetime histograms of the isolated internetwork and magnetic Ca II H BPs were determined. Mean values were found to be 2.2 km s-1, 0.2 arcsec (≈150 km), 1.48 ⟨ ICa ⟩ and 673 s, respectively. Interestingly, the brightness and the horizontal velocity of BPs are anti-correlated. Large excursions (pulses) in horizontal velocity, up to 15 km s-1, are present in the trajectories of most BPs. These could excite kink waves travelling into the chromosphere and possibly the corona, which we estimate to carry an energy flux of 310 W m-2, sufficient to heat the upper layers, although only marginally.
Conclusions: The stable observing conditions of Sunrise and our technique for identifying and tracking BPs have allowed us to determine reliable parameters of these features in the internetwork. Thus we find, e.g., that they are considerably longer lived than previously thought. The large velocities are also reliable, and may excite kink waves. Although these wave are (marginally) energetic enough to heat the quiet corona, we expect a large additional contribution from larger magnetic elements populating the network and partly also the internetwork. Title: Pathway to the Square Kilometre Array - The German White Paper - Authors: Aharonian, F.; Arshakian, T. G.; Allen, B.; Banerjee, R.; Beck, R.; Becker, W.; Bomans, D. J.; Breitschwerdt, D.; Brüggen, M.; Brunthaler, A.; Catinella, B.; Champion, D.; Ciardi, B.; Crocker, R.; de Avillez, M. A.; Dettmar, R. J.; Engels, D.; Enßlin, T.; Enke, H.; Fieseler, T.; Gizon, L.; Hackmann, E.; Hartmann, B.; Henkel, C.; Hoeft, M.; Iapichino, L.; Innes, D.; James, C.; Jasche, J.; Jones, D.; Kagramanova, V.; Kauffmann, G.; Keane, E.; Kerp, J.; Klöckner, H. -R.; Kokkotas, K.; Kramer, M.; Krause, M.; Krause, M.; Krupp, N.; Kunz, J.; Lämmerzahl, C.; Lee, K. J.; List, M.; Liu, K.; Lobanov, A.; Mann, G.; Merloni, A.; Middelberg, E.; Niemeyer, J.; Noutsos, A.; Perlick, V.; Reich, W.; Richter, P.; Roy, A.; Saintonge, A.; Schäfer, G.; Schaffner-Bielich, J.; Schinnerer, E.; Schleicher, D.; Schneider, P.; Schwarz, D. J.; Sedrakian, A.; Sesana, A.; Smolčić, V.; Solanki, S.; Tuffs, R.; Vetter, M.; Weber, E.; Weller, J.; Wex, N.; Wucknitz, O.; Zwaan, M. Bibcode: 2013arXiv1301.4124A Altcode: The Square Kilometre Array (SKA) is the most ambitious radio telescope ever planned. With a collecting area of about a square kilometre, the SKA will be far superior in sensitivity and observing speed to all current radio facilities. The scientific capability promised by the SKA and its technological challenges provide an ideal base for interdisciplinary research, technology transfer, and collaboration between universities, research centres and industry. The SKA in the radio regime and the European Extreme Large Telescope (E-ELT) in the optical band are on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI) and have been recognised as the essential facilities for European research in astronomy. This "White Paper" outlines the German science and R&D interests in the SKA project and will provide the basis for future funding applications to secure German involvement in the Square Kilometre Array. Title: Inversions of L12-2 IMaX data of an emerging flux mantle Authors: Guglielmino, S. L.; Martínez Pillet, V.; Ruiz Cobo, B.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.; Solanki, S. K.; Sunrise/IMaX Team Bibcode: 2013MmSAI..84..355G Altcode: We present the analysis of a flux emergence event observed with the IMaX magnetograph flown aboard the SUNRISE balloon. IMaX took a 15' sequence with cadence of 31 s along the Fe I line at 525.0 nm, acquiring only Stokes I and V at 12 line positions (L12-2 mode). This sequence shows the emergence of a flux mantle at mesogranular scale, cospatial with a large exploding granule. An undesired cross-talk between Stokes U and V was found in such L12-2 data. We show that the use of a modified version of the SIR inversion code is able to remove such effect in inferring the physical quantities of interest. Title: MHD waves in small magnetic elements: comparing IMaX observations to simulations. Authors: Stangalini, M.; Solanki, S. K.; Cameron, R. Bibcode: 2013MmSAI..84..444S Altcode: Small-scale magnetic fields are thought to play an important role in the heating of the outer solar atmosphere. By exploiting the high-spatial and temporal resolution of IMaX, the bidimensional spectropolarimeter on board the Sunrise balloon-borne observatory, we study the excitation of MHD waves in small magnetic elements, providing clues on the interaction of the magnetic structures with the photospheric forcing and the ambient acoustic field. The large fraction of magnetic features observed by IMaX made it possible to study the interaction between the photospheric granulation and the flux tubes from a statistical point-of-view. In particular we find a 90 degree phase lag with an high confidence level between the horizontal displacements of the flux tubes and the velocity perturbations measured inside them. We also find that the observational results are in excellent agreement with MHD simulations. This result suggests that the horizontal displacement of small-scale magnetic features by the surrounding granulation excites longitudinal waves within the magnetic elements. Title: The GREGOR Solar Telescope on Tenerife Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.; Solanki, S. K.; Balthasar, H.; Bello González, N.; Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Sobotka, M.; Soltau, D.; Strassmeier, K. G. Bibcode: 2012ASPC..463..365S Altcode: 2012arXiv1202.4289S 2011 was a successful year for the GREGOR project. The telescope was finally completed in May with the installation of the 1.5-meter primary mirror. The installation of the first-light focal plane instruments was completed by the end of the year. At the same time, the preparations for the installation of the high-order adaptive optics were finished, its integration to the telescope is scheduled for early 2012. This paper describes the telescope and its instrumentation in their present first-light configuration, and provides a brief overview of the science goals of GREGOR. Title: The GREGOR Solar Telescope Authors: Denker, C.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.; Volkmer, R.; von der Luehe, O.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F. Bibcode: 2012IAUSS...6E.203D Altcode: The 1.5-meter GREGOR solar telescope is a new facility for high-resolution observations of the Sun. The telescope is located at the Spanish Observatorio del Teide on Tenerife. The telescope incorporates advanced designs for a foldable-tent dome, an open steel-truss telescope structure, and active and passive means to minimize telescope and mirror seeing. Solar fine structure can be observed with a dedicated suite of instruments: a broad-band imaging system, the "GREGOR Fabry-Perot Interferometer", and the "Grating Infrared Spectrograph". All post-focus instruments benefit from a high-order (multi-conjugate) adaptive optics system, which enables observations close to the diffraction limit of the telescope. The inclusion of a spectrograph for stellar activity studies and the search for solar twins expands the scientific usage of the GREGOR to the nighttime domain. We report on the successful commissioning of the telescope until the end of 2011 and the first steps towards science verification in 2012. Title: Doppler shift of hot coronal lines in a moss area of an active region Authors: Dadashi, N.; Teriaca, L.; Tripathi, D.; Solanki, S. K.; Wiegelmann, T. Bibcode: 2012A&A...548A.115D Altcode: 2012arXiv1211.5473D The moss is the area at the footpoint of the hot (3 to 5 MK) loops forming the core of the active region where emission is believed to result from the heat flux conducted down to the transition region from the hot loops. Studying the variation of Doppler shift as a function of line formation temperatures over the moss area can give clues on the heating mechanism in the hot loops in the core of the active regions. We investigate the absolute Doppler shift of lines formed at temperatures between 1 MK and 2 MK in a moss area within active region NOAA 11243 using a novel technique that allows determining the absolute Doppler shift of EUV lines by combining observations from the SUMER and EIS spectrometers. The inner (brighter and denser) part of the moss area shows roughly constant blue shift (upward motions) of 5 km s-1 in the temperature range of 1 MK to 1.6 MK. For hotter lines the blue shift decreases and reaches 1 km s-1 for Fe xv 284 Å (~2 MK). The measurements are discussed in relation to models of the heating of hot loops. The results for the hot coronal lines seem to support the quasi-steady heating models for nonsymmetric hot loops in the core of active regions. Title: A retrospective of the GREGOR solar telescope in scientific literature Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.; Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.; Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa, M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.; Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann, T.; Wiehr, E.; Wittmann, A. D.; Woche, M. Bibcode: 2012AN....333..810D Altcode: 2012arXiv1210.3167D In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post-focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer-reviewed journals and conference proceedings also provides the ``historical'' context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes. Title: Three-dimensional non-LTE radiative transfer effects in Fe I lines. I. Flux sheet and flux tube geometries Authors: Holzreuter, R.; Solanki, S. K. Bibcode: 2012A&A...547A..46H Altcode: 2012arXiv1209.0559H In network and active region plages, the magnetic field is concentrated into structures often described as flux tubes (FTs) and sheets (FSs). Three-dimensional (3D) radiative transfer is important for energy transport in these concentrations. It is also expected to be important for diagnostic purposes but has rarely been applied for that purpose. Using true 3D, non-local thermodynamic-equilibrium (non-LTE or NLTE) radiative transfer (RT) in FT and FS models, we compute iron line profiles commonly used to diagnose the Sun's magnetic field by using and comparing the results with those obtained from LTE or one-dimensional (1D) NLTE calculations. Employing a multilevel iron atom, we study the influence of several basic parameters such as either FS or FT Wilson depression, wall thickness, radius/width, thermal stratification or magnetic field strength on Stokes I and the polarized Stokes parameters in the thin-tube approximation. The use of different levels of approximations of RT (3D NLTE, 1D NLTE, LTE) may lead to considerable differences in profile shapes, intensity contrasts, equivalent widths, and the determination of magnetic field strengths. In particular, LTE, which often provides a good approach in planar 1D atmospheres, is a poor approximation in our flux sheet model for some of the most important diagnostic Fe i lines (524.7 nm, 525.0 nm, 630.1 nm, and 630.2 nm). The observed effects depend on parameters such as the height of line formation, field strength, and internal temperature stratification. Differences between the profile shapes may lead to errors in the determination of magnetic fields on the order of 10% to 20%, while errors in the determined temperature can reach 300-400 K. The empirical FT models NET and PLA turn out to minimize the effects of 3D RT, so that results obtained with these models by applying LTE may also remain valid for 3D NLTE calculations. Finally, horizontal RT is found to only insignificantly smear out structures such as the optically thick walls of flux tubes and sheets, allowing features as narrow as 10 km to remain visible.

Appendices are available in electronic form at http://www.aanda.org Title: The 1.5 meter solar telescope GREGOR Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.; Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.; Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Waldmann , T. A. Bibcode: 2012AN....333..796S Altcode: The 1.5 m telescope GREGOR opens a new window to the understanding of solar small-scale magnetism. The first light instrumentation includes the Gregor Fabry Pérot Interferometer (GFPI), a filter spectro-polarimeter for the visible wavelength range, the GRating Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The excellent performance of the first two instruments has already been demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest solar telescope and number 3 in the world. Its all-reflective Gregory design provides a large wavelength coverage from the near UV up to at least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR is equipped with a high-order adaptive optics system, with a subaperture size of 10 cm, and a deformable mirror with 256 actuators. The science goals are focused on, but not limited to, solar magnetism. GREGOR allows us to measure the emergence and disappearance of magnetic flux at the solar surface at spatial scales well below 100 km. Thanks to its spectro-polarimetric capabilities, GREGOR will measure the interaction between the plasma flows, different kinds of waves, and the magnetic field. This will foster our understanding of the processes that heat the chromosphere and the outer layers of the solar atmosphere. Observations of the surface magnetic field at very small spatial scales will shed light on the variability of the solar brightness. Title: GRIS: The GREGOR Infrared Spectrograph Authors: Collados, M.; López, R.; Páez, E.; Hernández, E.; Reyes, M.; Calcines, A.; Ballesteros, E.; Díaz, J. J.; Denker, C.; Lagg, A.; Schlichenmaier, R.; Schmidt, W.; Solanki, S. K.; Strassmeier, K. G.; von der Lühe, O.; Volkmer, R. Bibcode: 2012AN....333..872C Altcode: This paper describes the main characteristics of GRIS (GREGOR Infrared Spectrograph), the grating spectrograph installed in the recently inaugurated (May 2012) 1.5-meter GREGOR telescope located at the Observatorio del Teide in Tenerife. The spectrograph has a standard Czerny-Turner configuration with parabolic collimator and camera mirrors that belong to the same conic surface. Although nothing prevents its use at visible wavelengths, the spectrograph will be initially used in combination with the infrared detector of the Tenerife Infrared Polarimeter (TIP-II) in standard spectroscopic mode as well as for spectropolarimetric measurements. Title: LEMUR: Large European module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission Authors: Teriaca, Luca; Andretta, Vincenzo; Auchère, Frédéric; Brown, Charles M.; Buchlin, Eric; Cauzzi, Gianna; Culhane, J. Len; Curdt, Werner; Davila, Joseph M.; Del Zanna, Giulio; Doschek, George A.; Fineschi, Silvano; Fludra, Andrzej; Gallagher, Peter T.; Green, Lucie; Harra, Louise K.; Imada, Shinsuke; Innes, Davina; Kliem, Bernhard; Korendyke, Clarence; Mariska, John T.; Martínez-Pillet, Valentin; Parenti, Susanna; Patsourakos, Spiros; Peter, Hardi; Poletto, Luca; Rutten, Robert J.; Schühle, Udo; Siemer, Martin; Shimizu, Toshifumi; Socas-Navarro, Hector; Solanki, Sami K.; Spadaro, Daniele; Trujillo-Bueno, Javier; Tsuneta, Saku; Dominguez, Santiago Vargas; Vial, Jean-Claude; Walsh, Robert; Warren, Harry P.; Wiegelmann, Thomas; Winter, Berend; Young, Peter Bibcode: 2012ExA....34..273T Altcode: 2011ExA...tmp..135T; 2011arXiv1109.4301T The solar outer atmosphere is an extremely dynamic environment characterized by the continuous interplay between the plasma and the magnetic field that generates and permeates it. Such interactions play a fundamental role in hugely diverse astrophysical systems, but occur at scales that cannot be studied outside the solar system. Understanding this complex system requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0.1'' and 0.3''), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 170 Å and 1270 Å. The LEMUR slit covers 280'' on the Sun with 0.14'' per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km s - 1 or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission. Title: Spectroscopic observations of propagating disturbances in a polar coronal hole: evidence of slow magneto-acoustic waves Authors: Gupta, G. R.; Teriaca, L.; Marsch, E.; Solanki, S. K.; Banerjee, D. Bibcode: 2012A&A...546A..93G Altcode: 2012arXiv1209.3524G
Aims: We focus on detecting and studying quasi-periodic propagating features that have been interpreted in terms of both slow magneto-acoustic waves and of high-speed upflows.
Methods: We analyzed long-duration spectroscopic observations of the on-disk part of the south polar coronal hole taken on 1997 February 25 by the SUMER spectrometer onboard SOHO. We calibrated the velocity with respect to the off-limb region and obtained time-distance maps in intensity, Doppler velocity, and line width. We also performed a cross-correlation analysis on different time series curves at different latitudes. We studied average spectral line profiles at the roots of propagating disturbances and along the propagating ridges, and performed a red-blue asymmetry analysis.
Results: We clearly find propagating disturbances in intensity and Doppler velocity with a projected propagation speed of about 60 ± 4.8 km s-1 and a periodicity of ≈14.5 min. To our knowledge, this is the first simultaneous detection of propagating disturbances in intensity as well as in Doppler velocity in a coronal hole. During the propagation, an intensity enhancement is associated with a blueshifted Doppler velocity. These disturbances are clearly seen in intensity also at higher latitudes (i.e., closer to the limb), while disturbances in Doppler velocity become faint there. The spectral line profiles averaged along the propagating ridges are found to be symmetric, to be well fitted by a single Gaussian, and have no noticeable red-blue asymmetry.
Conclusions: Based on our analysis, we interpret these disturbances in terms of propagating slow magneto-acoustic waves. Title: Resolving the Internal Magnetic Structure of the Solar Network Authors: Martínez González, M. J.; Bellot Rubio, L. R.; Solanki, S. K.; Martínez Pillet, V.; Del Toro Iniesta, J. C.; Barthol, P.; Schmidt, W. Bibcode: 2012ApJ...758L..40M Altcode: 2012arXiv1209.2584M We analyze the spectral asymmetry of Stokes V (circularly polarized) profiles of an individual network patch in the quiet Sun observed by Sunrise/IMaX. At a spatial resolution of 0farcs15-0farcs18, the network elements contain substructure which is revealed by the spatial distribution of Stokes V asymmetries. The area asymmetry between the red and blue lobes of Stokes V increases from nearly zero at the core of the structure to values close to unity at its edges (single-lobed profiles). Such a distribution of the area asymmetry is consistent with magnetic fields expanding with height, i.e., an expanding magnetic canopy (which is required to fulfill pressure balance and flux conservation in the solar atmosphere). Inversion of the Stokes I and V profiles of the patch confirms this picture, revealing a decreasing field strength and increasing height of the canopy base from the core to the periphery of the network patch. However, the non-roundish shape of the structure and the presence of negative area and amplitude asymmetries reveal that the scenario is more complex than a canonical flux tube expanding with height surrounded by downflows. Title: Nitrate in Polar Ice: A New Tracer of Solar Variability Authors: Traversi, R.; Usoskin, I. G.; Solanki, S. K.; Becagli, S.; Frezzotti, M.; Severi, M.; Stenni, B.; Udisti, R. Bibcode: 2012SoPh..280..237T Altcode: 2012SoPh..tmp..169T Knowledge of the long-term variability of solar activity is of both astrophysical and geoscientific interest. Reconstructions of solar activity over multiple millennia are traditionally based on cosmogenic isotopes 14C or 10Be measured in natural terrestrial archives, but the two isotopes exhibit significant differences on millennial time scales, so that our knowledge of solar activity at this time scale remains somewhat uncertain. Here we present a new potential proxy of solar activity on the centennial-millennial time scale, based on a chemical tracer, viz. nitrate content in an ice core drilled at Talos Dome (Antarctica). We argue that this location is optimal for preserving the solar signal in the nitrate content during the Holocene. By using the firn core from the same location we show that the 11-year and Gleissberg cycles are present with the variability of 10 - 25 % in nitrate content in the pre-industrial epoch. This is consistent with the results of independent efforts of modeling HNO3 and NOy in Antarctic near surface air. However, meteorological noise on the interannual scale makes it impossible to resolve individual solar cycles. Based on different processes of formation and transport compared to cosmogenic isotopes, it provides new, independent insight into long-term solar activity and helps resolve the uncertainties related to cosmogenic isotopes as diagnostics of solar activity. Title: Results and perspectives of the solar axion search with the CAST experiment Authors: Ferrer-Ribas, E.; Arik, M.; Aune, S.; Barth, K.; Belov, A.; Borghi, S.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Dafni, T.; Davenport, M.; Eleftheriadis, C.; Elias, N.; Ezer, C.; Fanourakis, G.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Garza, J. G.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Gninenko, S.; Gómez, H.; Gruber, E.; Guthörl, T.; Hartmann, R.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kuster, M.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Lozza, V.; Lutz, G.; Luzón, G.; Morales, J.; Niinikoski, T.; Nordt, A.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Rashba, T.; Riege, H.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Silva, P. S.; Solanki, S. K.; Stewart, L.; Tomás, A.; Tsagri, M.; van Bibber, K.; Vafeiadis, T.; Villar, J.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K. Bibcode: 2012arXiv1209.6347F Altcode: The status of the solar axion search with the CERN Axion Solar Telescope (CAST) will be presented. Recent results obtained by the use of $^3$He as a buffer gas has allowed us to extend our sensitivity to higher axion masses than our previous measurements with $^4$He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV$ \le m_{a} \le $ 0.64 eV. From the absence of an excess of x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g$_{a\gamma} \le 2.3\times 10^{-10}$ GeV$^{-1}$ at 95% C.L., the exact value depending on the pressure setting. CAST published results represent the best experimental limit on the photon couplings to axions and other similar exotic particles dubbed WISPs (Weakly Interacting Slim Particles) in the considered mass range and for the first time the limit enters the region favored by QCD axion models. Preliminary sensitivities for axion masses up to 1.16 eV will also be shown reaching mean upper limits on the axion-photon coupling of g$_{a\gamma} \le 3.5\times 10^{-10}$ GeV$^{-1}$ at 95% C.L. Expected sensibilities for the extension of the CAST program up to 2014 will be presented. Moreover long term options for a new helioscope experiment will be evoked. Title: Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission Authors: Antonucci, Ester; Fineschi, Silvano; Naletto, Giampiero; Romoli, Marco; Spadaro, Daniele; Nicolini, Gianalfredo; Nicolosi, Piergiorgio; Abbo, Lucia; Andretta, Vincenzo; Bemporad, Alessandro; Auchère, Frédéric; Berlicki, Arkadiusz; Bruno, Roberto; Capobianco, Gerardo; Ciaravella, Angela; Crescenzio, Giuseppe; Da Deppo, Vania; D'Amicis, Raffaella; Focardi, Mauro; Frassetto, Fabio; Heinzel, Peter; Lamy, Philippe L.; Landini, Federico; Massone, Giuseppe; Malvezzi, Marco A.; Moses, J. Dan; Pancrazzi, Maurizio; Pelizzo, Maria-Guglielmina; Poletto, Luca; Schühle, Udo H.; Solanki, Sami K.; Telloni, Daniele; Teriaca, Luca; Uslenghi, Michela Bibcode: 2012SPIE.8443E..09A Altcode: METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements. Title: METIS: a novel coronagraph design for the Solar Orbiter mission Authors: Fineschi, Silvano; Antonucci, Ester; Naletto, Giampiero; Romoli, Marco; Spadaro, Daniele; Nicolini, Gianalfredo; Abbo, Lucia; Andretta, Vincenzo; Bemporad, Alessandro; Berlicki, Arkadiusz; Capobianco, Gerardo; Crescenzio, Giuseppe; Da Deppo, Vania; Focardi, Mauro; Landini, Federico; Massone, Giuseppe; Malvezzi, Marco A.; Moses, J. Dan; Nicolosi, Piergiorgio; Pancrazzi, Maurizio; Pelizzo, Maria-Guglielmina; Poletto, Luca; Schühle, Udo H.; Solanki, Sami K.; Telloni, Daniele; Teriaca, Luca; Uslenghi, Michela Bibcode: 2012SPIE.8443E..3HF Altcode: METIS (Multi Element Telescope for Imaging and Spectroscopy) METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The mission profile will bring the Solar Orbiter spacecraft as close to the Sun as 0.3 A.U., and up to 35° out-of-ecliptic providing a unique platform for helio-synchronous observations of the Sun and its polar regions. METIS coronagraph is designed for multi-wavelength imaging and spectroscopy of the solar corona. This presentation gives an overview of the innovative design elements of the METIS coronagraph. These elements include: i) multi-wavelength, reflecting Gregorian-telescope; ii) multilayer coating optimized for the extreme UV (30.4 nm, HeII Lyman-α) with a reflecting cap-layer for the UV (121.6 nm, HI Lyman-α) and visible-light (590-650); iii) inverse external-occulter scheme for reduced thermal load at spacecraft peri-helion; iv) EUV/UV spectrograph using the telescope primary mirror to feed a 1st and 4th-order spherical varied line-spaced (SVLS) grating placed on a section of the secondary mirror; v) liquid crystals electro-optic polarimeter for observations of the visible-light K-corona. The expected performances are also presented. Title: Grand minima of solar activity during the last millennia Authors: Usoskin, Ilya G.; Solanki, Sami K.; Kovaltsov, Gennady A. Bibcode: 2012IAUS..286..372U Altcode: In this review we discuss the occurrence and statistical properties of Grand minima based on the available data covering the last millennia. In particular, we consider the historical record of sunspot numbers covering the last 400 years as well as records of cosmogenic isotopes in natural terrestrial archives, used to reconstruct solar activity for up to the last 11.5 millennia, i.e. throughout the Holocene. Using a reconstruction of solar activity from cosmogenic isotope data, we analyze statistics of the occurrence of Grand minima. We find that: the Sun spends about most of the time at moderate activity, 1/6 in a Grand minimum and some time also in a Grand maximum state; Occurrence of Grand minima is not a result of long-term cyclic variations but is defined by stochastic/chaotic processes; There is a tendency for Grand minima to cluster with the recurrence rate of roughly 2000-3000 years, with a weak ~210-yr periodicity existing within the clusters. Grand minima occur of two different types: shorter than 100 years (Maunder-type) and long ~150 years (Spörer-type). It is also discussed that solar cycles (most possibly not sunspots cycle) could exist during the Grand minima, perhaps with stretched length and asymmetric sunspot latitudinal distribution.

These results set new observational constraints on long-term solar and stellar dynamo models. Title: A reconstruction of solar irradiance using a flux transport model Authors: Dasi Espuig, Maria; Krivova, Natalie; Solanki, Sami K.; Jiang, Jie Bibcode: 2012cosp...39..418D Altcode: 2012cosp.meet..418D Solar irradiance is one of the important drivers of the Earth's global climate, but it has only been measured for the past 33 years. Its reconstructions are therefore crucial to study longer term variations relevant to climate timescales. Most successful in reproducing the measured irradiance variations have being the models that are based on the assumption that irradiance changes are caused by the evolution of the photospheric magnetic field. Our SATIRE-S model is one of these, which uses solar full-disc magnetograms as an input, and these are available for less than four decades. To reconstruct the irradiance back to times when no observed magnetograms are available, we combine the SATIRE-S model with synthetic magnetograms, produced using a surface flux transport model. The model is fed with daily, observed or modelled statistically, records of sunspot positions, areas, and tilt angles. The concept of overlapping ephemeral region cycles is used to describe the secular change in the irradiance. Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and PREMOS/PICARD Authors: Cessateur, Gaël.; Kretzschmar, Matthieu; Krivova, Natalie; Schmutz, Werner; Solanki, Sami K.; Thuillier, Gerard; Shapiro, Alexander; Schoell, Micha; Shapiro, Anna; Dominique, Marie; Tagirov, Rinat; Wehrli, Christoph; Yeo, Kok Leng Bibcode: 2012cosp...39..287C Altcode: 2012cosp.meet..287C No abstract at ADS Title: Spectroscopic Observations of Propagating Disturbances in Polar Coronal Hole Authors: Gupta, Girjesh R.; Marsch, Eckart; Solanki, Sami K.; Banerjee, Dipankar; Teriaca, Luca Bibcode: 2012cosp...39..689G Altcode: 2012cosp.meet..689G We focus on long duration spectroscopic observations of the south polar coronal hole taken on 1997 February 25 by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer aboard SOHO. We analyze the data in the on-disk part of the coronal hole to find any signature of propagating waves or high speed up-flows. We find the clear presence of propagating disturbances in intensity and Doppler velocity with a projected propagation speed of about 60~km~s^{-1} and a periodicity of ≈14.5~min. During the propagation, the intensity enhancement is associated with a blue-shifted Doppler velocity. These disturbances are clearly seen in intensity at higher latitudes (i.e. closer to the limb), whereas disturbances in Doppler velocity becomes faint there. We study average spectral line profiles at the roots of these disturbances and along the propagating ridge. Based on our analysis, we interpret these disturbances in terms of propagating slow magneto-acoustic waves. Title: Long-term reconstructions of total solar irradiance Authors: Krivova, Natalie; Solanki, Sami K.; Dasi Espuig, Maria Bibcode: 2012cosp...39..985K Altcode: 2012cosp.meet..985K Solar irradiance is the main external driver of the Earth's climate, although its relative contribution compared to other internal and anthropogenic factors is not yet well determined. Variations of total solar irradiance have being measured for over three decades and are relatively well understood. Reconstructions of the irradiance into the past remain, however, rather uncertain. In particular, the magnitude of the secular change is highly debated. The reason is the lack of direct and well-sampled proxies of solar magnetic activity on time scales longer than a few decades. Reconstructions on time scales of centuries rely on sunspot observations available since 1610. Reconstructions on millennial time scales use concentrations of the cosmogenic isotopes in terrestrial archives. We will review long-term reconstructions of the solar irradiance using the SATIRE set of models, compare them with other recent models and discuss the remaining uncertainties. Title: Nitrate in polar ice as a new index of solar variability Authors: Traversi, Rita; Usoskin, Ilya; Solanki, Sami K.; Becagli, Silvia; Frezzotti, Massimo; Severi, Mirko; Stenni, Barbara; Udisti, Roberto Bibcode: 2012cosp...39.2001T Altcode: 2012cosp.meet.2001T The Sun affects Earth's climate and environment in multiple ways and on different time scales but understanding if and how the variations in solar radiative output might drive global climatic change has revealed to be a complex and controversial issue. In order to address the question about the Sun's role on the climate in the past, present and, eventually, in the future, a reliable reconstruction of solar activity is mandatory. Over millennia time scales, a quantitative information about past solar activity can only be obtained through proxies recorded in natural stratified archives (e.g., tree rings or ice cores), such as the cosmogenic isotopes 14C and 10Be. Nonetheless, the two isotopes exhibit significant differences on millennial time scales, making our knowledge of solar activity at this time scale less certain. Here we present a new potential proxy of solar activity on the centennial-millennial time scale, namely nitrate content in the TALDICE ice core (Talos Dome, placeEast Antarctica). Talos Dome site was shown to be favourably located to minimize possible local climate effects, which may be dominant in other coastal sites and, due to a sufficiently high snow accumulation rate throughout the Holocene, such a reversibly deposited species as nitrate is preserved here (in contrast to inland Antarctic sites) making it a potential paleomarker. Comparing the TALDICE nitrate record with the two traditionally used cosmogenic proxies of solar activity, we observed that a highly significant agreement on the millennial and multi-millennial time scales over the Holocene. The relation at centennial time scales is less significant, likely due to ice core dating uncertainties. No statistical relationship was found between the nitrate data series and other chemical markers of local climate/transport. Consequently, the coherence between nitrate and cosmogenic isotopes univocally suggests a common origin of their long-term variability. As concerning the more recent times, we show that the 11-year and Gleissberg cycles are present at a level of 10-25% in nitrate content in the pre-industrial epoch after the Maunder minimum, which is in agreement with model results. However, the ``meteorological noise'' on the interannual scale makes it impossible to resolve individual solar cycles. Since nitrate is characterised by different processes of formation and transport, compared to cosmogenic isotopes, we conclude that nitrate in this Antarctic ice core forms a novel proxy of solar activity on the centennial and longer time scales and could help in reconciling the discrepancies between the two cosmogenic isotopes as diagnostics of solar activity. Title: Status and perspectives of the CAST experiment Authors: Lakić, B.; Arik, M.; Aune, S.; Barth, K.; Belov, A.; Borghi, S.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Dafni, T.; Davenport, M.; Eleftheriadis, C.; Elias, N.; Ezer, C.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Gninenko, S.; Gómez, H.; Gruber, E.; Guthörl, T.; Hartmann, R.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kuster, M.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Lozza, V.; Lutz, G.; Luzón, G.; Morales, J.; Niinikoski, T.; Nordt, A.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Rashba, T.; Riege, H.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Silva, P. S.; Solanki, S. K.; Stewart, L.; Tomás, A.; Tsagri, M.; van Bibber, K.; Vafeiadis, T.; Villar, J.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K.; Cast Collaboration Bibcode: 2012JPhCS.375b2001L Altcode: The CERN Axion Solar Telescope (CAST) is currently the most sensitive axion helioscope designed to search for axions produced by the Primakoff process in the solar core. CAST is using a Large Hadron Collider (LHC) test magnet where axions could be converted into X-rays with energies up to 10 keV. During the phase I, the experiment operated with vacuum inside the magnet bores and covered axion masses up to 0.02 eV. In the phase II, the magnet bores were filled with a buffer gas (first 4He and later 3He) at various densities in order to extend the sensitivity to higher axion masses (up to f .18 eV). The phase II data taking was completed in 2011. So far, no evidence of axion signal has been found and CAST set the most restrictive experimental limit on the axion-photon coupling constant over a broad range of axion masses. The latest CAST results with 3He data in the mass range 0.39 eV < ma < 0.64 eV will be presented. Title: Waves as the Source of Apparent Twisting Motions in Sunspot Penumbrae Authors: Bharti, L.; Cameron, R. H.; Rempel, M.; Hirzberger, J.; Solanki, S. K. Bibcode: 2012ApJ...752..128B Altcode: 2012arXiv1204.2221B The motion of dark striations across bright filaments in a sunspot penumbra has become an important new diagnostic of convective gas flows in penumbral filaments. The nature of these striations has, however, remained unclear. Here, we present an analysis of small-scale motions in penumbral filaments in both simulations and observations. The simulations, when viewed from above, show fine structure with dark lanes running outward from the dark core of the penumbral filaments. The dark lanes either occur preferentially on one side or alternate between both sides of the filament. We identify this fine structure with transverse (kink) oscillations of the filament, corresponding to a sideways swaying of the filament. These oscillations have periods in the range of 5-7 minutes and propagate outward and downward along the filament. Similar features are found in observed G-band intensity time series of penumbral filaments in a sunspot located near disk center obtained by the Broadband Filter Imager on board the Hinode. We also find that some filaments show dark striations moving to both sides of the filaments. Based on the agreement between simulations and observations we conclude that the motions of these striations are caused by transverse oscillations of the underlying bright filaments. Title: Chromospheric magnetic fields of an active region filament Authors: Xu, Z.; Solanki, S.; Lagg, A. Bibcode: 2012EAS....55..163X Altcode: Vector magnetic fields of an active region filament are co-spatially and co-temporally mapped in photosphere and upper chromosphere, by using spectro-polarimetric observations made by Tenerife Infrared Polarimeter (TIP II) at the German Vacuum Tower Telescope (VTT). A Zeeman-based ME inversion is performed on the full Stokes vectors of both the photospheric Si I 1082.7 nm and the chromospheric He I 1083.0 nm lines. We found that the strong magnetic fields, with the field strength of 600 - 800 G in the He I line formation height, are not uncommon among AR filaments. But such strong magnetic field is not always found in AR filaments. Title: The continuum intensity as a function of magnetic field. II. Local magnetic flux and convective flows Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2012A&A...542A..96K Altcode: 2014arXiv1402.3474K Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained.
Aims: We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts.
Methods: The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box).
Results: Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases.
Conclusions: Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations. Title: First Results from the SUNRISE Mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Jafarzadeh, S.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; González, M. J. M.; Pillet, V. M.; Khomenko, E.; Yelles Chaouche, L.; Iniesta, J. C. d. T.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. B.; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2012ASPC..455..143S Altcode: The SUNRISE balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that reveal the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the SUNRISE data, which include a number of discoveries. Title: Detection of Vortex Tubes in Solar Granulation from Observations SUNRISE Authors: Steiner, O.; Franz, M.; González, N. B.; Nutto, C.; Rezaei, R.; Pillet, V. M.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2012ASPC..455...35S Altcode: We investigated a time series of continuum intensity maps and Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. We conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. This paper is a summary and update of the results previously presented in Steiner et al. (2010). Title: Supersonic Magnetic Flows in the Quiet Sun Observed with SUNRISE/IMaX Authors: Borrero, J. M.; Pillet, V. M.; Schlichenmaier, R.; Schmidt, W.; Berkefeld, T.; Solanki, S. K.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.; Barthol, P.; Gandorfer, A. Bibcode: 2012ASPC..455..155B Altcode: 2012arXiv1202.4354B In this contribution we describe some recent observations of high-speed magnetized flows in the quiet Sun granulation. These observations were carried out with the Imaging Magnetograph eXperiment (IMaX) onboard the stratospheric balloon SUNRISE, and possess an unprecedented spatial resolution and temporal cadence. These flows were identified as highly shifted circular polarization (Stokes V) signals. We estimate the LOS velocity responsible for these shifts to be larger than 6 km s-1, and therefore we refer to them as supersonic magnetic flows. The average lifetime of the detected events is 81.3 s and they occupy an average area of about 23 000 km2. Most of the events occur within granular cells and correspond therefore to upflows. However some others occur in intergranular lanes or bear no clear relation to the convective velocity pattern. We analyze a number of representative examples and discuss them in terms of magnetic loops, reconnection events, and convective collapse. Title: Reconstruction of total solar irradiance 1974-2009 Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.; Wenzler, T.; Mortlock, D. J.; Jaffe, A. H. Bibcode: 2012A&A...541A..27B Altcode: 2012arXiv1202.3554B Context. The study of variations in total solar irradiance (TSI) is important for understanding how the Sun affects the Earth's climate.
Aims: Full-disk continuum images and magnetograms are now available for three full solar cycles. We investigate how modelled TSI compares with direct observations by building a consistent modelled TSI dataset. The model, based only on changes in the photospheric magnetic flux can then be tested on rotational, cyclical and secular timescales.
Methods: We use Kitt Peak and SoHO/MDI continuum images and magnetograms in the SATIRE-S model to reconstruct TSI over cycles 21-23. To maximise independence from TSI composites, SORCE/TIM TSI data are used to fix the one free parameter of the model. We compare and combine the separate data sources for the model to estimate an uncertainty on the reconstruction and prevent any additional free parameters entering the model.
Results: The reconstruction supports the PMOD composite as being the best historical record of TSI observations, although on timescales of the solar rotation the IRMB composite provides somewhat better agreement. Further to this, the model is able to account for 92% of TSI variations from 1978 to 2009 in the PMOD composite and over 96% during cycle 23. The reconstruction also displays an inter-cycle, secular decline of 0.20+0.12-0.09 W m-2 between cycle 23 minima, in agreement with the PMOD composite.
Conclusions: SATIRE-S is able to recreate TSI observations on all timescales of a day and longer over 31 years from 1978. This is strong evidence that changes in photospheric magnetic flux alone are responsible for almost all solar irradiance variations over the last three solar cycles. Title: Solar Spectral Irradiance as observed by LYRA/PROBA2 and PREMOS/PICARD Authors: Cessateur, G.; Shapiro, A. I.; Dominique, M.; Kretzschmar, M.; Krivova, N.; Shapiro, A. V.; Schmutz, W.; Schoell, M.; Solanki, S.; Tagirov, R.; Thuillier, G.; Wehrli, C.; Yeo, K. L. Bibcode: 2012EGUGA..14.8254C Altcode: Total and Spectral Solar Irradiance are key input parameters to atmospheric/oceanic and space weather models. Both vary on time-scales ranging from days to millennia, although a complete picture of the solar irradiance variability is still missing. The recent launch of two European missions PROBA-2 with LYRA radiometer onboard and PICARD with PREMOS package onboard provides therefore valuable information and nourishes theoretical models. Both instruments covers the solar spectrum from the EUV to near-infrared. Here we present spectral solar irradiance data from these two recent missions. After a proper correction of the degradation and non-solar signatures, we compare these data with those from the VIRGO/SOHO and SOLSTICE+SIM/SORCE experiments. Both LYRA and PREMOS have also observed several solar eclipses, which allows us to accurately retrieve the center-to-limb variations (CLVs) of the quiet Sun's brightness at the wavelengths of the corresponding channels. CLVs play indeed an important role in modelling of the solar irradiance variability. We show that calculations with the published COde for Solar Irradiance (COSI) yield CLVs that are in good agreement with measurements. Finally we use COSI to model the variability of the irradiance, assuming that the latter is determined by the evolution of the solar surface magnetic field as seen with SDO/HMI data. These theoretical results are compared to PREMOS and LYRA measurements. Title: Break up of returning plasma after the 7 June 2011 filament eruption by Rayleigh-Taylor instabilities Authors: Innes, D. E.; Cameron, R. H.; Fletcher, L.; Inhester, B.; Solanki, S. K. Bibcode: 2012A&A...540L..10I Altcode: 2012arXiv1202.4981I Context. A prominence eruption on 7 June 2011 produced spectacular curtains of plasma falling through the lower corona. At the solar surface they created an incredible display of extreme ultraviolet brightenings.
Aims: To identify and analyze some of the local instabilities which produce structure in the falling plasma.
Methods: The structures were investigated using SDO/AIA 171 Å and 193 Å images in which the falling plasma appeared dark against the bright coronal emission.
Results: Several instances of the Rayleigh-Taylor instability were investigated. In two cases the Alfvén velocity associated with the dense plasma could be estimated from the separation of the Rayleigh-Taylor fingers. A second type of feature, which has the appearance of self-similar branching horns was discussed.

Appendix A and two movies are available in electronic form at http://www.aanda.org Title: Long-term magnetic field monitoring of the Sun-like star ξ Bootis A Authors: Morgenthaler, A.; Petit, P.; Saar, S.; Solanki, S. K.; Morin, J.; Marsden, S. C.; Aurière, M.; Dintrans, B.; Fares, R.; Gastine, T.; Lanoux, J.; Lignières, F.; Paletou, F.; Ramírez Vélez, J. C.; Théado, S.; Van Grootel, V. Bibcode: 2012A&A...540A.138M Altcode: 2011arXiv1109.5066M
Aims: We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star ξ Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators.
Methods: We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale photospheric magnetic field of the star was then modelled by means of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitored the width of several magnetically sensitive spectral lines, the radial velocity, the line asymmetry of intensity line profiles, and the chromospheric emission in the cores of the Ca II H and Hα lines.
Results: During the highest observed activity states, in 2007 and 2011, the large-scale field of ξ Bootis A is almost completely axisymmetric and is dominated by its toroidal component. The toroidal component persists with a constant polarity, containing a significant fraction of the magnetic energy of the large-scale surface field through all observing epochs. The magnetic topologies reconstructed for these activity maxima are very similar, suggesting a form of short cyclicity in the large-scale field distribution. The mean unsigned large-scale magnetic flux derived from the magnetic maps varies by a factor of about 2 between the lowest and highest observed magnetic states. The chromospheric flux is less affected and varies by a factor of 1.2. Correlated temporal evolution, due to both rotational modulation and seasonal variability, is observed between the Ca II emission, the Hα emission and the width of magnetically sensitive lines. The rotational dependence of polarimetric magnetic measurements displays a weak correlation with other activity proxies, presumably due to the different spatial scales and centre-to-limb darkening associated with polarimetric signatures, as compared to non-polarized activity indicators. Better agreement is observed on the longer term. When measurable, the differential rotation reveals a strong latitudinal shear in excess of 0.2 rad d-1.

Based on observations obtained at the Bernard Lyot Telescope (TBL, Pic du Midi, France) of the Midi-Pyrénées Observatory, which is operated by the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France.Tables 3-6 are available in electronic form at http://www.aanda.org Title: Solar magnetism eXplorer (SolmeX). Exploring the magnetic field in the upper atmosphere of our closest star Authors: Peter, Hardi; Abbo, L.; Andretta, V.; Auchère, F.; Bemporad, A.; Berrilli, F.; Bommier, V.; Braukhane, A.; Casini, R.; Curdt, W.; Davila, J.; Dittus, H.; Fineschi, S.; Fludra, A.; Gandorfer, A.; Griffin, D.; Inhester, B.; Lagg, A.; Landi Degl'Innocenti, E.; Maiwald, V.; Sainz, R. Manso; Martínez Pillet, V; Matthews, S.; Moses, D.; Parenti, S.; Pietarila, A.; Quantius, D.; Raouafi, N. -E.; Raymond, J.; Rochus, P.; Romberg, O.; Schlotterer, M.; Schühle, U.; Solanki, S.; Spadaro, D.; Teriaca, L.; Tomczyk, S.; Trujillo Bueno, J.; Vial, J. -C. Bibcode: 2012ExA....33..271P Altcode: 2011arXiv1108.5304P; 2011ExA...tmp..134P The magnetic field plays a pivotal role in many fields of Astrophysics. This is especially true for the physics of the solar atmosphere. Measuring the magnetic field in the upper solar atmosphere is crucial to understand the nature of the underlying physical processes that drive the violent dynamics of the solar corona—that can also affect life on Earth. SolmeX, a fully equipped solar space observatory for remote-sensing observations, will provide the first comprehensive measurements of the strength and direction of the magnetic field in the upper solar atmosphere. The mission consists of two spacecraft, one carrying the instruments, and another one in formation flight at a distance of about 200 m carrying the occulter to provide an artificial total solar eclipse. This will ensure high-quality coronagraphic observations above the solar limb. SolmeX integrates two spectro-polarimetric coronagraphs for off-limb observations, one in the EUV and one in the IR, and three instruments for observations on the disk. The latter comprises one imaging polarimeter in the EUV for coronal studies, a spectro-polarimeter in the EUV to investigate the low corona, and an imaging spectro-polarimeter in the UV for chromospheric studies. SOHO and other existing missions have investigated the emission of the upper atmosphere in detail (not considering polarization), and as this will be the case also for missions planned for the near future. Therefore it is timely that SolmeX provides the final piece of the observational quest by measuring the magnetic field in the upper atmosphere through polarimetric observations. Title: Magnetic Fields of an Active Region Filament from Full Stokes Analysis of Si I 1082.7 nm and He I 1083.0 nm Authors: Xu, Z.; Lagg, A.; Solanki, S.; Liu, Y. Bibcode: 2012ApJ...749..138X Altcode: 2012arXiv1202.4562X Vector magnetic fields of an active region filament in the photosphere and upper chromosphere are obtained from spectro-polarimetric observations recorded with the Tenerife Infrared Polarimeter (TIP II) at the German Vacuum Tower Telescope. We apply Milne-Eddington inversions on full Stokes vectors of the photospheric Si I 1082.7 nm and the upper chromospheric He I triplet at 1083.0 nm to obtain the magnetic field vector and velocity maps in two atmosphere layers. We find that (1) a complete filament was already present in Hα at the beginning of the TIP II data acquisition. Only a partially formed one, composed of multiple small threads, was present in He I. (2) The AR filament comprises two sections. One shows strong magnetic field intensities, about 600-800 G in the upper chromosphere and 800-1000 G in the photosphere. The other exhibits only comparatively weak magnetic field strengths in both layers. (3) The Stokes V signal is indicative of a dip in the magnetic field strength close to the chromospheric PIL. (4) In the chromosphere, consistent upflows are found along the PIL flanked by downflows. (5) The transversal magnetic field is nearly parallel to the PIL in the photosphere and inclined by 20°-30° in the chromosphere. (6) The chromospheric magnetic field around the filament is found to be in normal configuration, while the photospheric field presents a concave magnetic topology. The observations are consistent with the emergence of a flux rope with a subsequent formation of a filament. Title: Diffusivity of Isolated Internetwork Ca II H Bright Points Observed by SuFI/SUNRISE Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Feller, A.; Pietarila, A.; Lagg, A.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knoelker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A. Bibcode: 2012decs.confE..99J Altcode: We analyze trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca II H BPs (with mean lifetime of 461 sec) to obtain their diffusivity behaviors. We use high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca II H 397 nm passband of the Sunrise Filter Imager (SuFI) on board the SUNRISE balloon-borne solar observatory. In order to avoid misidentification, the BPs are semi-manually selected and then automatically tracked. The trajectory of each BP is then calculated and its diffusion index is described by a power law exponent, using which we classify the BPs' trajectories into sub-, normal and super- diffusive. In addition, the corresponding diffusion coefficients (D) based on the observed displacements are consequently computed. We find a strong super-diffusivity at a height sampled by the SuFI/SUNRISE Ca II H passband (i.e. a height corresponding roughly to the temperature minimum). We find that 74% of the identified tiny BPs are super-diffusive, 18% move randomly (i.e. their motion corresponds to normal diffusion) and only 8% belong to the sub-diffusion regime. In addition, we find that 53% of the super-diffusion regime (i.e. 39% of all BPs) have the diffusivity index of 2 which are termed as "Ballistic BPs". Finally, we explore the distribution of diffusion index with the help of a simple simulation. The results suggest that the BPs are random walkers superposed by a systematic (background) velocity in which the magnitude of each component (and hence their ratio) depends on the time and spatial scales. We further discuss a simple sketch to explain the diffusivity of observed BPs while they migrate within a supergranule (i.e. internetwork areas) or close to the network regions. Title: The Frontier between Small-scale Bipoles and Ephemeral Regions in the Solar Photosphere: Emergence and Decay of an Intermediate-scale Bipole Observed with SUNRISE/IMaX Authors: Guglielmino, S. L.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta, J. Carlos; Bellot Rubio, L. R.; Solanki, S. K.; Schmidt, W.; Gandorfer, A.; Barthol, P.; Knölker, M. Bibcode: 2012ApJ...745..160G Altcode: 2011arXiv1110.1405G We report on the photospheric evolution of an intermediate-scale (≈4 Mm footpoint separation) magnetic bipole, from emergence to decay, observed in the quiet Sun at high spatial (0farcs3) and temporal (33 s) resolution. The observations were acquired by the Imaging Magnetograph Experiment imaging magnetograph during the first science flight of the SUNRISE balloon-borne solar observatory. The bipole flux content is 6 × 1017 Mx, representing a structure bridging the gap between granular scale bipoles and the smaller ephemeral regions. Footpoints separate at a speed of 3.5 km s-1 and reach a maximum distance of 4.5 Mm before the field dissolves. The evolution of the bipole is revealed to be very dynamic: we found a proper motion of the bipole axis and detected a change of the azimuth angle of 90° in 300 s, which may indicate the presence of some writhe in the emerging structure. The overall morphology and behavior are in agreement with previous analyses of bipolar structures emerging at the granular scale, but we also found several similarities with emerging flux structures at larger scales. The flux growth rate is 2.6 × 1015 Mx s-1, while the mean decay rate is one order of magnitude smaller. We describe in some detail the decay phase of the bipole footpoints that includes break up into smaller structures, and interaction with preexisting fields leading to cancellation, but it appears to be dominated by an as-yet unidentified diffusive process that removes most of the flux with an exponential flux decay curve. The diffusion constant (8 × 102 km2 s-1) associated with this decay is similar to the values used to describe the large-scale diffusion in flux transport models. Title: The influence of spectral solar irradiance data on stratospheric heating rates during the 11 year solar cycle Authors: Oberländer, S.; Langematz, U.; Matthes, K.; Kunze, M.; Kubin, A.; Harder, J.; Krivova, N. A.; Solanki, S. K.; Pagaran, J.; Weber, M. Bibcode: 2012GeoRL..39.1801O Altcode: 2012GeoRL..3901801O Heating rate calculations with the FUBRad shortwave (SW) radiation parameterization have been performed to examine the effect of prescribed spectral solar fluxes from the NRLSSI, MPS and IUP data sets on SW heating rates over the 11 year solar cycle 22. The corresponding temperature response is derived from perpetual January General Circulation Model (GCM) simulations with prescribed ozone concentrations. The different solar flux input data sets induce clear differences in SW heating rates at solar minimum, with the established NRLSSI data set showing the smallest solar heating rates. The stronger SW heating in the middle and upper stratosphere in the MPS data warms the summer upper stratosphere by 2 K. Over the solar cycle, SW heating rate differences vary up to 40% between the irradiance data sets, but do not result in a significant change of the solar temperature signal. Lower solar fluxes in the newer SIM data lead to a significantly cooler stratosphere and mesosphere when compared to NRLSSI data for 2007. Changes in SW heating from 2004 to 2007 are however up to six times stronger than for the NRLSSI data. Title: Magnetic field emergence in mesogranular-sized exploding granules observed with sunrise/IMaX data Authors: Palacios, J.; Blanco Rodríguez, J.; Vargas Domínguez, S.; Domingo, V.; Martínez Pillet, V.; Bonet, J. A.; Bellot Rubio, L. R.; Del Toro Iniesta, J. C.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Berkefeld, T.; Schmidt, W.; Knölker, M. Bibcode: 2012A&A...537A..21P Altcode: 2011arXiv1110.4555P We report on magnetic field emergences covering significant areas of exploding granules. The balloon-borne mission Sunrise provided high spatial and temporal resolution images of the solar photosphere. Continuum images, longitudinal and transverse magnetic field maps and Dopplergrams obtained by IMaX onboard Sunrise are analyzed by local correlation traking (LCT), divergence calculation and time slices, Stokes inversions and numerical simulations are also employed. We characterize two mesogranular-scale exploding granules where ~1018 Mx of magnetic flux emerges. The emergence of weak unipolar longitudinal fields (~100 G) start with a single visible magnetic polarity, occupying their respective granules' top and following the granular splitting. After a while, mixed polarities start appearing, concentrated in downflow lanes. The events last around 20 min. LCT analyses confirm mesogranular scale expansion, displaying a similar pattern for all the physical properties, and divergence centers match between all of them. We found a similar behaviour with the emergence events in a numerical MHD simulation. Granule expansion velocities are around 1 kms-1 while magnetic patches expand at 0.65 kms-1. One of the analyzed events evidences the emergence of a loop-like structure. Advection of the emerging magnetic flux features is dominated by convective motion resulting from the exploding granule due to the magnetic field frozen in the granular plasma. Intensification of the magnetic field occurs in the intergranular lanes, probably because of being directed by the downflowing plasma.

Movies associated to Figs. 2-4 are available in electronic form at http://www.aanda.org Title: Search for Sub-eV Mass Solar Axions by the CERN Axion Solar Telescope with He3 Buffer Gas Authors: Arik, M.; Aune, S.; Barth, K.; Belov, A.; Borghi, S.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Dafni, T.; Davenport, M.; Eleftheriadis, C.; Elias, N.; Ezer, C.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; García, J. A.; Gardikiotis, A.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Gninenko, S.; Gómez, H.; Gruber, E.; Guthörl, T.; Hartmann, R.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Irastorza, I. G.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kuster, M.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Lozza, V.; Lutz, G.; Luzón, G.; Morales, J.; Niinikoski, T.; Nordt, A.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Rashba, T.; Riege, H.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Silva, P. S.; Solanki, S. K.; Stewart, L.; Tomás, A.; Tsagri, M.; van Bibber, K.; Vafeiadis, T.; Villar, J. A.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K. Bibcode: 2011PhRvL.107z1302A Altcode: 2011arXiv1106.3919A The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using He3 as a buffer gas. At T=1.8K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with He4. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39eV≲ma≲0.64eV. From the absence of excess x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of gaγ≲2.3×10-10GeV-1 at 95% C.L., the exact value depending on the pressure setting. Kim-Shifman-Vainshtein-Zakharov axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In the future we will extend our search to ma≲1.15eV, comfortably overlapping with cosmological hot dark matter bounds. Title: Fast Up-flows Observed on Granules with Sunrise Authors: Macdonald, G. A.; Hirzberger, J.; Solanki, S.; Choudhary, D. P. Bibcode: 2011AGUFMSH13B1985M Altcode: We present results from a rigorous study of fast granular up flows observed with the Imaging Magnetograph eXperiment (IMaX) aboard the SUNRISE balloon-borne observatory. A time series ∼ 23 minutes long made June 9, 2009 with a cadence of ∼ 33sec and resolution of 0.15--0.18'' was used. Our study concentrates on up flows with a LOS speeds in excess of 2km/s. These flows occur most frequently on the bright areas of the smallest granules, and less so on the bright edges of larger granules. The maximum up flow speed tends to scale with its area which scales with the size of the host granule. The longest-lived up flows are located on larger granules, while shorter-lived ones tend to be located on smaller granules. Results from simulations carried out in other studies suggest that the fastest granular up flows occur chiefly during two scenarios: 1) on the edges of granules when their centers cool, before the granules subsequently split and 2) when a granule is forced under the surface by powerful down flows from adjacent granules. Our observations provide evidence for these results. Title: Evolution of the fine structure of magnetic fields in the quiet Sun: Combining Sunrise observations and modelling Authors: Wiegelmann, T.; Solanki, S.; Borrero, J.; Martinez Pillet, V.; Sunrise Team Bibcode: 2011AGUFMSH41B..06W Altcode: Observations with the balloon borne SUNRISE/IMAX instrument provide us with unprecedented high spatial resolution (pixel size 40 km) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona we extrapolate these photospheric measurements into the upper solar atmosphere and analyse a timeseries with a cadence of 33s. We find that the majority of closed loops which reach into the chromosphere or corona have one foot point in strong photospheric magnetic field regions (B>300 G). Most loops are asymmetric and the weaker foot point is often located in the internetwork. We find that the magnetic connectivity of the loops changes rapidly with a typical recycling time of about 2 min in the upper solar atmosphere and 14 min in the photosphere. We discuss, to which extend the observed topological changes can be interpreted as evidence for magnetic reconnection and the relevance of these processes for coronal heating. Title: Chromospheric Observations of a Kink Wave in an On-disk Active Region Fibril Authors: Pietarila, A. M.; Aznar Cuadrado, R.; Hirzberger, J.; Solanki, S. Bibcode: 2011AGUFMSH13B1951P Altcode: Most observations of kink and Alfven waves in the chromosphere are made in off-limb spicules. Here we present observations of a kink wave in high spatial and temporal resolution Ca II 8542 data of an active region fibril on the solar disk. The properties of the observed wave are similar to kink waves in spicules. From the inferred wave phase and period we estimate the lower limit for the field strength in the chromospheric fibril to be a few hundred Gauss. The observations indicate that the event may have been triggered by a small-scale reconnection event higher up in the atmosphere. Title: Zeeman Broadening in Optical Stokes I of Solar-like Stars Authors: Anderson, R. I.; Reiners, A.; Solanki, S. K. Bibcode: 2011ASPC..448.1061A Altcode: 2010arXiv1012.2968A; 2011csss...16.1061A We present our analysis aimed at inferring average magnetic fields in slowly-rotating solar-like stars. Using the spectral line inversion code SPINOR, we perform high-accuracy line profile fitting and investigate whether Zeeman broadening can be reliably detected in optical data of unprecedented quality. We argue that our usage of both high- and low-geff lines does provide a certain sensitivity to magnetic fields that may, indeed, be detected. However, the measurement is subject to a model dependence and prone to ambiguities, e.g. due to spectral blends. Hence, while a field may be successfully recovered, the quantification of this field is subject to large uncertainties, even for the highest-quality optical data. Title: Long-term Magnetic Field Monitoring of the Sun-like Star ζ Bootis A Authors: Morgenthaler, A.; Petit, P.; Aurière, M.; Dintrans, B.; Fares, R.; Gastine, T.; Lanoux, J.; Lignières, F.; Morin, J.; Ramirez, J.; Saar, S.; Solanki, S. K.; Théado, S.; Van Grootel V., V. Bibcode: 2011ASPC..448.1203M Altcode: 2011csss...16.1203M Phase-resolved observations of the solar-type star ζ Bootis A were obtained using the NARVAL spectropolarimeter at the Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational modulation and the long-term evolution of various magnetic and activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and Hα emission. Title: Analyzing Solar Cycles Authors: Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2011Sci...334..916S Altcode: No abstract at ADS Title: The quiet Sun average Doppler shift of coronal lines up to 2 MK Authors: Dadashi, N.; Teriaca, L.; Solanki, S. K. Bibcode: 2011A&A...534A..90D Altcode: 2011arXiv1109.4493D Context. The average Doppler shift shown by spectral lines formed from the chromosphere to the corona reveals important information on the mass and energy balance of the solar atmosphere, providing an important observational constraint to any models of the solar corona. Previous spectroscopic observations of vacuum ultra-violet (VUV) lines have revealed a persistent average wavelength shift of lines formed at temperatures up to 1 MK. At higher temperatures, the behaviour is still essentially unknown.
Aims: Here we analyse combined SUMER (Solar Ultraviolet Measurements of Emitted Radiation)/SoHO (Solar and Heliospheric Observatory) and EIS (EUV Imaging Spectrometer)/Hinode observations of the quiet Sun around disk centre to determine, for the first time, the average Doppler shift of several spectral lines formed between 1 and 2 MK, where the largest part of the quiet coronal emission is formed.
Methods: The measurements are based on a novel technique applied to EIS spectra to measure the difference in Doppler shift between lines formed at different temperatures. Simultaneous wavelength-calibrated SUMER spectra allow establishing the absolute value at the reference temperature of T ≈ 1 MK.
Results: The average line shifts at 1 MK < T < 1.8 MK are modestly, but clearly bluer than those observed at 1 MK. By accepting an average blue shift of about (-1.8 ± 0.6) km s-1 at 1 MK (as provided by SUMER measurements), this translates into a maximum Doppler shift of (-4.4 ± 2.2) km s-1 around 1.8 MK. The measured value appears to decrease to about (-1.3 ± 2.6) km s-1 at the Fe xv formation temperature of 2.1 MK.
Conclusions: The measured average Doppler shift between 0.01 and 2.1 MK, for which we provide a parametrisation, appears to be qualitatively and roughly quantitatively consistent with what foreseen by 3D coronal models where heating is produced by dissipation of currents induced by photospheric motions and by reconnection with emerging magnetic flux. Title: Erratum:"Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere" (2011, ApJ, 734, L18) Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.; Aznar Cuadrado, R.; Merenda, L. Bibcode: 2011ApJ...740L..55J Altcode: No abstract at ADS Title: Kink Waves in an Active Region Dynamic Fibril Authors: Pietarila, A.; Aznar Cuadrado, R.; Hirzberger, J.; Solanki, S. K. Bibcode: 2011ApJ...739...92P Altcode: 2011arXiv1107.3113P We present high spatial and temporal resolution Ca II 8542 Å observations of a kink wave in an on-disk chromospheric active region fibril. The properties of the wave are similar to those observed in off-limb spicules. From the observed phase and period of the wave we determine a lower limit for the field strength in the chromospheric active region fibril located at the edge of a sunspot to be a few hundred gauss. We find indications that the event was triggered by a small-scale reconnection event higher up in the atmosphere. Title: Variation of quiet Sun magnetic elements between 2006 and 2011 using Hinode SOT/SP Authors: Buehler, D.; Lagg, A.; Solanki, S. K. Bibcode: 2011sdmi.confE..76B Altcode: The Hinode satellite has revealed copious amounts of horizontal flux covering the quiet Sun, nurturing the notion of local dynamo action operating close to the solar surface. We sought to investigate the variation in the occurrence as well as the strength of circular and linear polarisation on the quiet Sun during the minimum of cycle 23, covering a period from November 2006 until August 2011. This investigation used Hinode SOT/SP images of the disk centre and a large FOV and focussed on line-integrated linear and circular polarisation signals obtained from the Fe I 6302.5 Å absorption line. The circular polarisation showed an overall linear decline in occurrence from November 2006 until August 2011. By comparing PDFs we found that this decline is associated in particular with network elements. The internetwork on the other hand showed a 10% decrease in occurrence from November 2010 until June 2009, followed by an equal increase until August 2011. The investigation also revealed a reduction of 30% in the occurrence of linear polarisation signals between November 2006 and December 2009. From August 2010 until August 2011 the occurrence of linear polarisation was increasing again. Hence, our results show that the occurrence of the ubiquitous linear polarisation of the internetwork as seen by Hinode is measurably influenced by the solar cycle. This implies that an independent local dynamo process is unlikely to be the sole cause responsible for the generation of this magnetic flux. Title: Chromosphere above sunspots as seen at millimeter wavelengths Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M. Bibcode: 2011IAUS..273..408L Altcode: Millimeter emission is known to be a sensitive diagnostic of temperature and density in the solar chromosphere. In this work we use millimeter wave data to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. From mm brightness simulations we expect a radio umbra to change its appearance from dark to bright (compared to the Quiet Sun) at a given wavelength in the millimeter spectrum (depending on the exact temperature in the model used). Thereby the millimeter brightness observed above an umbra at several wavelengths imposes strong constraints on temperature and density stratification of the sunspot atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Current mm/submm observational data suggest that brightness observed at short wavelengths is unexpectedly low compared to the most widely used sunspot models such as of Maltby et al. (1986). A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K), such as in the models of Severino et al. (1994). However, we are not able to resolve the umbra cleanly with the presently available observations and better resolution as well as better wavelength coverage are needed for accurate diagnostics of umbral brightness at millimeter wavelengths. This adds one more scientific objective for the Atacama Large Millimeter/Submillimeter Array (ALMA). Title: The Sun at high resolution: first results from the Sunrise mission Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.; Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez; Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.; Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M. Bibcode: 2011IAUS..273..226S Altcode: The Sunrise balloon-borne solar observatory consists of a 1m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system and further infrastructure. The first science flight of Sunrise yielded high-quality data that reveal the structure, dynamics and evolution of solar convection, oscillations and magnetic fields at a resolution of around 100 km in the quiet Sun. Here we describe very briefly the mission and the first results obtained from the Sunrise data, which include a number of discoveries. Title: The height of chromospheric loops in an emerging flux region Authors: Merenda, L.; Lagg, A.; Solanki, S. K. Bibcode: 2011A&A...532A..63M Altcode: 2012arXiv1202.1113M Context. The chromospheric layer observable with the He i 10 830 Å triplet is strongly warped. The analysis of the magnetic morphology of this layer therefore requires a reliable technique to determine the height at which the He i absorption takes place.
Aims: The He i absorption signature connecting two pores of opposite polarity in an emerging flux region is investigated. This signature is suggestive of a loop system connecting the two pores. We aim to show that limits can be set on the height of this chromospheric loop system.
Methods: The increasing anisotropy in the illumination of a thin, magnetic structure intensifies the linear polarization signal observed in the He i triplet with height. This signal is altered by the Hanle effect. We apply an inversion technique incorporating the joint action of the Hanle and Zeeman effects, with the absorption layer height being one of the free parameters.
Results: The observed linear polarization signal can be explained only if the loop apex is higher than ≈ 5 Mm. Best agreement with the observations is achieved for a height of 6.3 Mm.
Conclusions: The strength of the linear polarization signal in the loop apex is inconsistent with the assumption of a He i absorption layer at a constant height level. The determined height supports the earlier conclusion that dark He 10 830 Å filaments in emerging flux regions trace emerging loops. Title: Latest results and prospects of the CERN Axion Solar Telescope Authors: Irastorza, I. G.; Aune, S.; Barth, K.; Belov, A.; Borghi, S.; Bräuninger, H.; Cantatore, G.; Carmona, J. M.; Cetin, S. A.; Collar, J. I.; Dafni, T.; Davenport, M.; Eleftheriadis, C.; Elias, N.; Ezer, C.; Fanourakis, G.; Ferrer-Ribas, E.; Friedrich, P.; Galán, J.; Gardikiotis, A.; Gazis, E. N.; Geralis, T.; Giomataris, I.; Gninenko, S.; Gómez, H.; Gruber, E.; Guthörl, T.; Hartmann, R.; Haug, F.; Hasinoff, M. D.; Hoffmann, D. H. H.; Iguaz, F. J.; Jacoby, J.; Jakovčić, K.; Karuza, M.; Königsmann, K.; Kotthaus, R.; Krčmar, M.; Kuster, M.; Lakić, B.; Laurent, J. M.; Liolios, A.; Ljubičić, A.; Lozza, V.; Lutz, G.; Luzón, G.; Morales, J.; Niinikoski, T.; Nordt, A.; Papaevangelou, T.; Pivovaroff, M. J.; Raffelt, G.; Rashba, T.; Riege, H.; Rodríguez, A.; Rosu, M.; Ruz, J.; Savvidis, I.; Silva, P. S.; Solanki, S. K.; Soufli, R.; Stewart, L.; Tomás, A.; Tsagri, M.; van Bibber, K.; Vafeiadis, T.; Villar, J.; Vogel, J. K.; Yildiz, S. C.; Zioutas, K. Bibcode: 2011JPhCS.309a2001I Altcode: The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into few keV photons via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) decommissioned test magnet. After results obtained with vacuum in the magnet pipes (phase I of the experiment) as well as with 4He the collaboration is now immersed in the data taking with 3He, to be finished in 2011. The status of the experiment will be presented, including a preliminary exclusion plot of the first 3He data. CAST is currently sensitive to realistic QCD axion models at the sub-eV scale, and with axion-photon couplings down to the ~ 2 × 10-10 GeV-1, compatible with solar life limits. Future plans include revisiting vaccuum and 4He configurations with improved sensitivity, as well as possible additional search for non-standard signals from chamaleons, paraphotons or other WISPs. For the longer term, we study the feasibility of an altogether improved version of the axion helioscope concept, with a jump in sensitivity of about one order of magnitude in gaγ beyond CAST. Title: Exploring the magnetic topologies of cool stars Authors: Morin, J.; Donati, J. -F.; Petit, P.; Albert, L.; Auriére, M.; Cabanac, R.; Catala, C.; Delfosse, X.; Dintrans, B.; Fares, R.; Forveille, T.; Gastine, T.; Jardine, M.; Konstantinova-Antova, R.; Lanoux, J.; Lignires, F.; Morgenthaler, A.; Paletou, F.; Velez, J. C. Ramirez; Solanki, S. K.; Thado, S.; Van Grootel, V. Bibcode: 2011IAUS..273..181M Altcode: 2010arXiv1009.2589M Magnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised light, the total magnetic flux averaged over the stellar disc can be derived but very little information on the field geometry is available. Spectropolarimetry provides a complementary information on the large-scale magnetic topology. With Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a map of the vector magnetic field at the surface of the star, and in particular to assess the relative importance of the poloidal and toroidal components as well as the degree of axisymmetry of the field distribution.

The development of high-performance spectropolarimeters associated with multi-lines techniques and ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel diagram, on stars spanning a wide range of mass, age and rotation period. These observations bring novel constraints on magnetic field generation by dynamo effect in cool stars. In particular, the study of solar twins brings new insight on the impact of rotation on the solar dynamo, whereas the detection of strong and stable dipolar magnetic fields on fully convective stars questions the precise role of the tachocline in this process. Title: Modelling solar irradiance variability on time scales from minutes to months Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A. Bibcode: 2011A&A...532A.108S Altcode: 2013arXiv1303.1961S We analyze and model total solar irradiance variability on time scales from minutes to months, excluding variations due to p-mode oscillations, using a combination of convective and magnetic components. These include granulation, the magnetic network, faculae and sunspots. Analysis of VIRGO data shows that on periods of a day or longer solar variability depends on magnetic activity, but is nearly independent at shorter periods. We assume that only granulation affects the solar irradiance variability on time scales from minutes to hours. Granulation is described as a large sample of bright cells and dark lanes that evolve according to rules deduced from observations and radiation hydrodynamic simulations. Comparison of this model combined with a high time resolution magnetic-field based irradiance reconstruction, with solar data reveals a good correspondence except at periods of 10 to 30 h. This suggests that the model is missing some power at these periods, which may be due to the absence of supergranulation or insufficient sensitivity of MDI magnetograms used for the reconstruction of the magnetic field-based irradiance reconstructions. Our model also shows that even for spatially unresolved data (such as those available for stars) the Fourier or wavelet transform of time series sampled at high cadence may allow properties of stellar granulation, in particular granule lifetimes to be determined. Title: Continuous upflows and sporadic downflows observed in active regions Authors: Kamio, S.; Peter, H.; Curdt, W.; Solanki, S. K. Bibcode: 2011A&A...532A..96K Altcode: 2011arXiv1107.1993K
Aims: We present a study of the temporal evolution of coronal loops in active regions and its implications for the dynamics in coronal loops.
Methods: We analyzed images of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) at multiple temperatures to detect apparent motions in the coronal loops.
Results: Quasi-periodic brightness fluctuations propagate upwards from the loop footpoint in hot emission at 1 MK, while sporadic downflows are seen in cool emission below 1 MK. The upward motion in hot emission increases just after the cool downflows.
Conclusions: The apparent propagating pattern suggests a hot upflow from the loop footpoints, and is considered to supply hot plasma into the coronal loop, but a wavelike phenomenon cannot be ruled out. Coronal condensation occasionally happens in the coronal loop, and the cool material flows down to the footpoint. Emission from cool plasma could have a significant contribution to hot AIA channels in the event of coronal condensation. Title: Evolution of the solar irradiance during the Holocene Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I. Bibcode: 2011A&A...531A...6V Altcode: 2011arXiv1103.4958V Context. Long-term records of solar radiative output are vital for understanding solar variability and past climate change. Measurements of solar irradiance are available for only the last three decades, which calls for reconstructions of this quantity over longer time scales using suitable models.
Aims: We present a physically consistent reconstruction of the total solar irradiance for the Holocene.
Methods: We extend the SATIRE (Spectral And Total Irradiance REconstruction) models to estimate the evolution of the total (and partly spectral) solar irradiance over the Holocene. The basic assumption is that the variations of the solar irradiance are due to the evolution of the dark and bright magnetic features on the solar surface. The evolution of the decadally averaged magnetic flux is computed from decadal values of cosmogenic isotope concentrations recorded in natural archives employing a series of physics-based models connecting the processes from the modulation of the cosmic ray flux in the heliosphere to their record in natural archives. We then compute the total solar irradiance (TSI) as a linear combination of the jth and jth + 1 decadal values of the open magnetic flux. In order to evaluate the uncertainties due to the evolution of the Earth's magnetic dipole moment, we employ four reconstructions of the open flux which are based on conceptually different paleomagnetic models.
Results: Reconstructions of the TSI over the Holocene, each valid for a different paleomagnetic time series, are presented. Our analysis suggests that major sources of uncertainty in the TSI in this model are the heritage of the uncertainty of the TSI since 1610 reconstructed from sunspot data and the uncertainty of the evolution of the Earth's magnetic dipole moment. The analysis of the distribution functions of the reconstructed irradiance for the last 3000 years, which is the period that the reconstructions overlap, indicates that the estimates based on the virtual axial dipole moment are significantly lower at earlier times than the reconstructions based on the virtual dipole moment. We also present a combined reconstruction, which represents our best estimate of total solar irradiance for any given time during the Holocene.
Conclusions: We present the first physics-based reconstruction of the total solar irradiance over the Holocene, which will be of interest for studies of climate change over the last 11 500 years. The reconstruction indicates that the decadally averaged total solar irradiance ranges over approximately 1.5 W/m2 from grand maxima to grand minima.

Appendix A is available in electronic form at http://www.aanda.orgThe TSI data is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A6 Title: The continuum intensity as a function of magnetic field. I. Active region and quiet Sun magnetic elements Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2011A&A...531A.112K Altcode: 2011arXiv1105.1958K Context. Small-scale magnetic fields are major contributors to the solar irradiance variations. Hence, the continuum intensity contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage is an essential quantity that needs to be measured reliably.
Aims: By using Hinode/SP disk center data at a constant, high spatial resolution, we aim at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network.
Methods: The field strength, filling factor and inclination of the field was retrieved by means of a Milne-Eddington inversion (VFISV code). As in earlier studies, we then performed a pixel-by-pixel study of 630.2 nm continuum contrast vs. apparent (i.e. averaged over a pixel) longitudinal magnetic field over large fields of view in ARs and in the QS.
Results: The continuum contrast of magnetic elements reaches larger values in the QS (on average 3.7%) than in ARs (on average 1.3%). This could not be attributed to any systematic difference in the chosen contrast references, so that it mainly reflects an intrinsic brightness difference. The larger contrasts in the QS are in agreement with earlier, lower resolution results, although our values are larger due to our better spatial resolution. At Hinode's spatial resolution, moreover, the relationship between contrast and apparent longitudinal field strength exhibits a peak at around 700 G in both the QS and ARs, whereas earlier lower resolution studies only found a peak in the QS and a monotonic decrease in ARs. We attribute this discrepancy both to our careful removal of the pores and their close surroundings affected by the telescope diffraction, as well as to the enhanced spatial resolution and very low scattered light of the Hinode Solar Optical Telescope. We verified that the magnetic elements producing the peak in the contrast curve are rather vertical in the AR and in the QS, so that the larger contrasts in the QS cannot be explained by larger inclinations, as had been proposed earlier. The opposite polarities in ARs do not exhibit any noticeable difference in inclination either, although they reach different contrasts when the amount of flux is significantly unbalanced between the polarities.
Conclusions: According to our inversions, the magnetic elements producing the peak of the contrast curves have similar properties (field strength, inclination, filling factor) in ARs and in the QS, so that the larger brightness of magnetic elements in the QS remains unexplained. Indirect evidence suggests that the contrast difference is not primarily due to any difference in average size of the magnetic elements. A possible explanation lies in the different efficiencies of convective energy transport in the QS and in ARs, which will be the topic of a second paper. Title: EUV jets, type III radio bursts and sunspot waves investigated using SDO/AIA observations Authors: Innes, D. E.; Cameron, R. H.; Solanki, S. K. Bibcode: 2011A&A...531L..13I Altcode: 2011arXiv1106.3417I Context. Quasi-periodic plasma jets are often ejected from the Sun into interplanetary space. The commonly observed signatures are day-long sequences of type III radio bursts.
Aims: The aim is to identify the source of quasi-periodic jets observed on 3 Aug. 2010 in the Sun's corona and in interplanetary space.
Methods: Images from the Solar Dynamics Observatory (SDO) at 211 Å are used to identify the solar source of the type III radio bursts seen in WIND/WAVES dynamic spectra. We analyse a 2.5 h period during which six strong bursts are seen. The radio signals are cross-correlated with emission from extreme ultraviolet (EUV) jets coming from the western side of a sunspot in AR 11092. The jets are further cross-correlated with brightening at a small site on the edge of the sunspot umbra, and the brightening with 3-min sunspot intensity oscillations.
Results: The radio bursts correlate very well with the EUV jets. The EUV jet emission also correlates well with brightening at what looks like their footpoint at the edge of the umbra. The jet emission lags the radio signals and the footpoint brightening by about 30 s because the EUV jets take time to develop. For 10-15 min after strong EUV jets are ejected, the footpoint brightens at roughly 3 min intervals. In both the EUV images and the extracted light curves, it looks as though the brightening is related to the 3-min sunspot oscillations, although the correlation coefficient is rather low. The only open field near the jets is rooted in the sunspot.
Conclusions: Active region EUV/X-ray jets and interplanetary electron streams originate on the edge of the sunspot umbra. They form along a current sheet between the sunspot open field and closed field connecting to underlying satellite flux. Sunspot running penumbral waves cause roughly 3-min jet footpoint brightening. The relationship between the waves and jets is less clear.

Movie is available in electronic form at http://www.aanda.org Title: Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.; Aznar Cuadrado, R.; Merenda, L. Bibcode: 2011ApJ...734L..18J Altcode: 2011arXiv1105.1877J We study the velocity structure of penumbral filaments in the deep photosphere to obtain direct evidence for the convective nature of sunspot penumbrae. A sunspot was observed at high spatial resolution with the 1 m Swedish Solar Telescope in the deep photospheric C I 5380 Å absorption line. The Multi-Object Multi-Frame Blind Deconvolution method is used for image restoration and straylight is filtered out. We report here the discovery of clear redshifts in the C I 5380 Å line at multiple locations in sunspot penumbral filaments. For example, bright head of filaments show larger concentrated blueshift and are surrounded by darker, redshifted regions, suggestive of overturning convection. Elongated downflow lanes are also located beside bright penumbral fibrils. Our results provide the strongest evidence yet for the presence of overturning convection in penumbral filaments and highlight the need to observe the deepest layers of the penumbra in order to uncover the energy transport processes taking place there. Title: Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model Authors: Ball, W. T.; Unruh, Y. C.; Krivova, N. A.; Solanki, S.; Harder, J. W. Bibcode: 2011A&A...530A..71B Altcode: 2011arXiv1104.0885B
Aims: We investigate how well modeled solar irradiances agree with measurements from the SORCE satellite, both for total solar irradiance and broken down into spectral regions on timescales of several years.
Methods: We use the SATIRE model and compare modeled total solar irradiance (TSI) with TSI measurements over the period 25 February 2003 to 1 November 2009. Spectral solar irradiance over 200-1630 nm is compared with the SIM instrument on SORCE over the period 21 April 2004 to 1 November 2009. We discuss the overall change in flux and the rotational and long-term trends during this period of decline from moderate activity to the recent solar minimum in ~10 nm bands and for three spectral regions of significant interest: the UV integrated over 200-300 nm, the visible over 400-691 nm and the IR between 972-1630 nm.
Results: The model captures 97% of the observed TSI variation. This is on the order at which TSI detectors agree with each other during the period considered. In the spectral comparison, rotational variability is well reproduced, especially between 400 and 1200 nm. The magnitude of change in the long-term trends is many times larger in SIM at almost all wavelengths while trends in SIM oppose SATIRE in the visible between 500 and 700 nm and again between 1000 and 1200 nm. We discuss the remaining issues with both SIM data and the identified limits of the model, particularly with the way facular contributions are dealt with, the limit of flux identification in MDI magnetograms during solar minimum and the model atmospheres in the IR employed by SATIRE. However, it is unlikely that improvements in these areas will significantly enhance the agreement in the long-term trends. This disagreement implies that some mechanism other than surface magnetism is causing SSI variations, in particular between 2004 and 2006, if the SIM data are correct. Since SATIRE was able to reproduce UV irradiance between 1991 and 2002 from UARS, either the solar mechanism for SSI variation fundamentally changed around the peak of cycle 23, or there is an inconsistency between UARS and SORCE UV measurements. We favour the second explanation. Title: Small-scale flux emergence events observed by Sunrise/IMaX Authors: Guglielmino, S. L.; Pillet, V. Martínez; del Toro Iniesta, J. C.; Rubio, L. R. Bellot; Zuccarello, F.; Solanki, S. K.; Solanki Bibcode: 2011IAUS..274..140G Altcode: Thanks to the unprecedented combination of high spatial resolution (0''.2) and high temporal cadence (33 s) spectropolarimetric measurements, the IMaX magnetograph aboard the Sunrise balloon-borne telescope is revealing new insights about the plasma dynamics of the all-pervasive small-scale flux concentrations in the quiet Sun. We present the result of a case study concerning the appearance of a bipole, with a size of about 4'' and a flux content of 5 × 1017 Mx, with strong signal of horizontal fields during the emergence. We analyze the data set using the SIR inversion code and obtain indications about the three-dimensional shape of the bipole and its evolution with time. Title: Solar total irradiance in cycle 23 Authors: Krivova, N. A.; Solanki, S. K.; Schmutz, W. Bibcode: 2011A&A...529A..81K Altcode: 2011arXiv1102.3077K Context. The most recent minimum of solar activity was deeper and longer than the previous two minima as indicated by different proxies of solar activity. This is also true for the total solar irradiance (TSI) according to the PMOD composite.
Aims: The apparently unusual behaviour of the TSI has been interpreted as evidence against solar surface magnetism as the main driver of the secular change in the TSI. We test claims that the evolution of the solar surface magnetic field does not reproduce the observed TSI in cycle 23.
Methods: We use sensitive, 60-min averaged MDI magnetograms and quasi-simultaneous continuum images as an input to our SATIRE-S model and calculate the TSI variation over cycle 23, sampled roughly every two weeks. The computed TSI is then compared with the PMOD composite of TSI measurements and with the data from two individual instruments, SORCE/TIM and UARS/ACRIM II, that monitored the TSI during the declining phase of cycle 23 and over the previous minimum in 1996, respectively.
Results: Excellent agreement is found between the trends shown by the model and almost all sets of measurements. The only exception is the early, i.e. 1996 to 1998, PMOD data. Whereas the agreement between the model and the PMOD composite over the period 1999-2009 is almost perfect, the modelled TSI shows a steeper increase between 1996 and 1999 than implied by the PMOD composite. On the other hand, the steeper trend in the model agrees remarkably well with the ACRIM II data. A closer look at the VIRGO data, which are the basis of the PMOD composite after 1996, reveals that only one of the two VIRGO instruments, the PMO6V, shows the shallower trend present in the composite, whereas the DIARAD measurements indicate a steeper trend.
Conclusions: Based on these results, we conclude that (1) the sensitivity changes of the PMO6V radiometers within VIRGO during the first two years have very likely not been correctly evaluated; and that (2) the TSI variations over cycle 23 and the change in the TSI levels between the minima in 1996 and 2008 are consistent with the solar surface magnetism mechanism. Title: VizieR Online Data Catalog: Evolution of solar irradiance during Holocene (Vieira+, 2011) Authors: Vieira, L. E. A.; Solanki, S. K.; Krivova, N. A.; Usoskin, I. Bibcode: 2011yCat..35310006V Altcode: 2011yCat..35319006V This is a composite total solar irradiance (TSI) time series for 9495BC to 2007AD constructed as described in Sect. 3.3 of the paper. Since the TSI is the main external heat input into the Earth's climate system, a consistent record covering as long period as possible is needed for climate models. This was our main motivation for constructing this composite TSI time series.

In order to produce a representative time series, we divided the Holocene into four periods according to the available data for each period. Table 4 (see below) summarizes the periods considered and the models available for each period. After the end of the Maunder Minimum we compute daily values, while prior to the end of the Maunder Minimum we compute 10-year averages. For the period for which both solar disk magnetograms and continuum images are available (period 1) we employ the SATIRE-S reconstruction (Krivova et al. 2003A&A...399L...1K; Wenzler et al. 2006A&A...460..583W). SATIRE-T (Krivova et al. 2010JGRA..11512112K) reconstruction is used from the beginning of the Maunder Minimum (approximately 1640AD) to 1977AD. Prior to 1640AD reconstructions are based on cosmogenic isotopes (this paper). Different models of the Earth's geomagnetic field are available before and after approximately 5000BC. Therefore we treat periods 3 and 4 (before and after 5000BC) separately. Further details can be found in the paper. We emphasize that the reconstructions based on different proxies have different time resolutions.

(1 data file). Title: Performance validation of phase diversity image reconstruction techniques Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Gandorfer, A.; Solanki, S. K. Bibcode: 2011A&A...529A.132H Altcode: We present a performance study of a phase diversity (PD) image reconstruction algorithm based on artificial solar images obtained from MHD simulations and on seeing-free data obtained with the SuFI instrument on the Sunrise balloon borne observatory. The artificial data were altered by applying different levels of degradation with synthesised wavefront errors and noise. The PD algorithm was modified by changing the number of fitted polynomials, the shape of the pupil and the applied noise filter. The obtained reconstructions are evaluated by means of the resulting rms intensity contrast and by the conspicuousness of appearing artifacts. The results show that PD is a robust method which consistently recovers the initial unaffected image contents. The efficiency of the reconstruction is, however, strongly dependent on the number of used fitting polynomials and the noise level of the images. If the maximum number of fitted polynomials is higher than 21, artifacts have to be accepted and for noise levels higher than 10-3 the commonly used noise filtering techniques are not able to avoid amplification of spurious structures. Title: The Continuum Contrast of Magnetic Elements as a Function of Magnetic Field (Disk Center): Early Studies and Hinode/SP Results Authors: Kobel, P.; Solanki, S. K.; Borrero, J. M. Bibcode: 2011ASPC..437..297K Altcode: To deepen our understanding of the role of small-scale magnetic fields on the solar irradiance, it is essential to quantify the continuum contrast of magnetic elements in the quiet Sun (QS) network and in active region (AR) plage. By using Hinode/SP disk center data at constant spatial resolution, we aimed at updating results of earlier ground-based studies of contrast vs. magnetogram signal, and to look for systematic differences between AR plages and QS network. We performed a pixel-per-pixel study of continuum contrast vs. longitudinal flux density over large fields of view in AR and in QS (as in earlier studies). Even at Hinode's resolution, the contrast of magnetic elements reaches larger values in the QS than in ARs. We show that this difference cannot be explained by different inclinations of magnetic elements in ARs and QS. We compared our contrast vs. magnetic flux density with earlier studies and attributed the differences both to our proper removal of the pores and their surrounding diffraction-spread radiation, as well as to our enhanced spatial resolution and quasi-absence of scattered light. At Hinode's resolution, the contrast of magnetic elements peaks on average at similar magnetic flux densities in ARs and in the QS, which indicates that the brightest flux tubes have similar sizes in ARs and QS. Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun Revealed by SUNRISE/IMaX Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz, R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste, A.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2011ApJ...730L..37M Altcode: 2011arXiv1103.0145M We present observational evidence for oscillations of magnetic flux density in the quiet areas of the Sun. The majority of magnetic fields on the solar surface have strengths of the order of or lower than the equipartition field (300-500 G). This results in a myriad of magnetic fields whose evolution is largely determined by the turbulent plasma motions. When granules evolve they squash the magnetic field lines together or pull them apart. Here, we report on the periodic deformation of the shapes of features in circular polarization observed at high resolution with SUNRISE. In particular, we note that the area of patches with a constant magnetic flux oscillates with time, which implies that the apparent magnetic field intensity oscillates in antiphase. The periods associated with this oscillatory pattern are compatible with the granular lifetime and change abruptly, which suggests that these oscillations might not correspond to characteristic oscillatory modes of magnetic structures, but to the forcing by granular motions. In one particular case, we find three patches around the same granule oscillating in phase, which means that the spatial coherence of these oscillations can reach 1600 km. Interestingly, the same kind of oscillatory phenomenon is also found in the upper photosphere. Title: Transport of Magnetic Flux from the Canopy to the Internetwork Authors: Pietarila, A.; Cameron, R. H.; Danilovic, S.; Solanki, S. K. Bibcode: 2011ApJ...729..136P Altcode: 2011arXiv1102.1397P Recent observations have revealed that 8% of linear polarization patches in the internetwork (INW) quiet Sun are fully embedded in downflows. These are not easily explained with the typical scenarios for the source of INW fields which rely on flux emergence from below. Using radiative MHD simulations, we explore a scenario where magnetic flux is transported from the magnetic canopy overlying the INW into the photosphere by means of downward plumes associated with convective overshoot. We find that if a canopy-like magnetic field is present in the simulation, the transport of flux from the canopy is an important process for seeding the photospheric layers of the INW with magnetic field. We propose that this mechanism is relevant for the Sun as well, and it could naturally explain the observed INW linear polarization patches entirely embedded in downflows. Title: Intensity contrast from MHD simulations and HINODE observations Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Lagg, A.; Vögler, A. Bibcode: 2011A&A...526A.120A Altcode: 2010arXiv1011.6102A Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate.
Aims: To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations.
Methods: To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system.
Results: We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres.
Conclusions: We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes. Title: The Role of Active Region Loop Geometry. II. Symmetry Breaking in Three-dimensional Active Region: Why are Vertical Kink Oscillations Observed so Rarely? Authors: Selwa, M.; Solanki, S. K.; Ofman, L. Bibcode: 2011ApJ...728...87S Altcode: We present numerical results of simulations of kink oscillations of coronal loops in an idealized active region (AR) that is initialized as a potential dipole magnetic configuration with gravitationally stratified density. We consider loops, with density higher than the surrounding plasma, embedded into the dipolar AR. We study the excitation of kink oscillations of such loops by velocity pulses at different positions, of a given duration and amplitude. The position of the pulse varies in the parametric studies. For a central (symmetric) loop within the AR, we find that the amplitude of vertical kink oscillations is significantly amplified in comparison to horizontal kink oscillations for exciters located centrally (symmetrically) below the loop. For pulses initiated further from such a symmetric loop a combination of vertical and horizontal oscillations is excited. The scenario changes significantly when we study an inclined loop (non-symmetric within a dipole field). In this case, we do not see vertical kink oscillations of any significant amplitude being excited, while horizontal ones can be easily detected. These results indicate that the reason why vertical kink oscillations are observed so rarely is that their excitation requires a set of conditions to occur simultaneously: the exciting pulse must be located roughly below the loop apex and the loop itself must be located symmetrically within the group of loops. The new findings of the present study show the importance of not only the position of the pulse, but mainly of the location of the loop within the set of field lines having the same magnetic connectivity. We find that the slow propagating wave is excited in all the studied loops and its excitation does not depend either on the geometry of the loop or the pulse. We discuss TRACE observations of coronal loop oscillations in view of our findings and find that our results can be used for identifying the polarization of the kink mode based on the location of the loop within the set of field lines of the same connectivity and the position of the flare. Title: Towards a long-term record of solar total and spectral irradiance Authors: Krivova, N. A.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2011JASTP..73..223K Altcode: 2009arXiv0911.4002K The variation of total solar irradiance (TSI) has been measured since 1978 and that of the spectral irradiance for an even shorter amount of time. Semi-empirical models are now available that reproduce over 80% of the measured irradiance variations. An extension of these models into the more distant past is needed in order to serve as input to climate simulations. Here we review our most recent efforts to model solar total and spectral irradiance on time scales from days to centuries and even longer. Solar spectral irradiance has been reconstructed since 1947. Reconstruction of solar total irradiance goes back to 1610 and suggests a value of about 1-1.5W/m2 for the increase in the cycle-averaged TSI since the end of the Maunder minimum, which is significantly lower than previously assumed but agrees with other modern models. First steps have also been made towards reconstructions of solar total and spectral irradiance on time scales of millennia. Title: Multicomponent He I 10 830 Å profiles in an active filament Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2011A&A...526A..42S Altcode: 2011arXiv1101.5563S
Aims: We present new spectropolarimetric observations of the chromospheric He i 10 830 Å multiplet observed in a filament during its phase of activity.
Methods: The data were recorded with the new Tenerife Infrared Polarimeter (TIP-II) at the German Vacuum Tower Telescope (VTT) on 2005 May 18. We inverted the He Stokes profiles using multiple atmospheric components.
Results: The observed He Stokes profiles display a remarkably wide variety of shapes. Most of the profiles show very broad Stokes I absorptions and complex and spatially variable Stokes V signatures. The inversion of the profiles shows evidence of different atmospheric blue- and redshifted components of the He i lines within the resolution element (~1 arcsec), with supersonic velocities of up to ~100 km s-1. Up to five different atmospheric components are found in the same profile. We show that even these complex profiles can be reliably inverted. Title: Mesogranulation and the Solar Surface Magnetic Field Distribution Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Martínez Pillet, V.; Wiegelmann, T.; Bonet, J. A.; Knölker, M.; Bellot Rubio, L. R.; del Toro Iniesta, J. C.; Barthol, P.; Gandorfer, A.; Schmidt, W.; Solanki, S. K. Bibcode: 2011ApJ...727L..30Y Altcode: 2010arXiv1012.4481Y The relation of the solar surface magnetic field with mesogranular cells is studied using high spatial (≈100 km) and temporal (≈30 s) resolution data obtained with the IMaX instrument on board SUNRISE. First, mesogranular cells are identified using Lagrange tracers (corks) based on horizontal velocity fields obtained through local correlation tracking. After ≈20 minutes of integration, the tracers delineate a sharp mesogranular network with lanes of width below about 280 km. The preferential location of magnetic elements in mesogranular cells is tested quantitatively. Roughly 85% of pixels with magnetic field higher than 100 G are located in the near neighborhood of mesogranular lanes. Magnetic flux is therefore concentrated in mesogranular lanes rather than intergranular ones. Second, magnetic field extrapolations are performed to obtain field lines anchored in the observed flux elements. This analysis, therefore, is independent of the horizontal flows determined in the first part. A probability density function (PDF) is calculated for the distribution of distances between the footpoints of individual magnetic field lines. The PDF has an exponential shape at scales between 1 and 10 Mm, with a constant characteristic decay distance, indicating the absence of preferred convection scales in the mesogranular range. Our results support the view that mesogranulation is not an intrinsic convective scale (in the sense that it is not a primary energy-injection scale of solar convection), but also give quantitative confirmation that, nevertheless, the magnetic elements are preferentially found along mesogranular lanes. Title: The Solar Orbiter Mission and its Polarimetric and Helioseismic Imager (SO/PHI) Authors: Gandorfer, Achim; Solanki, Sami K.; Woch, Joachim; Martínez Pillet, Valentin; Álvarez Herrero, Alberto; Appourchaux, Thierry Bibcode: 2011JPhCS.271a2086G Altcode: We briefly outline the scientific and instrumental aspects of ESA's Solar Orbiter mission. Special emphasis is given to the Polarimetric and Helioseismic Imager, the instrument with the highest relevance for helioseismology applications, which will observe gas motions and the vector magnetic field in the photosphere at high spatial and temporal resolution. Title: The Imaging Magnetograph eXperiment (IMaX) for the Sunrise Balloon-Borne Solar Observatory Authors: Martínez Pillet, V.; del Toro Iniesta, J. C.; Álvarez-Herrero, A.; Domingo, V.; Bonet, J. A.; González Fernández, L.; López Jiménez, A.; Pastor, C.; Gasent Blesa, J. L.; Mellado, P.; Piqueras, J.; Aparicio, B.; Balaguer, M.; Ballesteros, E.; Belenguer, T.; Bellot Rubio, L. R.; Berkefeld, T.; Collados, M.; Deutsch, W.; Feller, A.; Girela, F.; Grauf, B.; Heredero, R. L.; Herranz, M.; Jerónimo, J. M.; Laguna, H.; Meller, R.; Menéndez, M.; Morales, R.; Orozco Suárez, D.; Ramos, G.; Reina, M.; Ramos, J. L.; Rodríguez, P.; Sánchez, A.; Uribe-Patarroyo, N.; Barthol, P.; Gandorfer, A.; Knoelker, M.; Schmidt, W.; Solanki, S. K.; Vargas Domínguez, S. Bibcode: 2011SoPh..268...57M Altcode: 2010SoPh..tmp..181M; 2010arXiv1009.1095M The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by four institutions in Spain that flew on board the Sunrise balloon-borne solar observatory in June 2009 for almost six days over the Arctic Circle. As a polarimeter, IMaX uses fast polarization modulation (based on the use of two liquid crystal retarders), real-time image accumulation, and dual-beam polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to achieve a spectral resolution of 85 mÅ. IMaX uses the high-Zeeman-sensitive line of Fe I at 5250.2 Å and observes all four Stokes parameters at various points inside the spectral line. This allows vector magnetograms, Dopplergrams, and intensity frames to be produced that, after reconstruction, reach spatial resolutions in the 0.15 - 0.18 arcsec range over a 50×50 arcsec field of view. Time cadences vary between 10 and 33 s, although the shortest one only includes longitudinal polarimetry. The spectral line is sampled in various ways depending on the applied observing mode, from just two points inside the line to 11 of them. All observing modes include one extra wavelength point in the nearby continuum. Gauss equivalent sensitivities are 4 G for longitudinal fields and 80 G for transverse fields per wavelength sample. The line-of-sight velocities are estimated with statistical errors of the order of 5 - 40 m s−1. The design, calibration, and integration phases of the instrument, together with the implemented data reduction scheme, are described in some detail. Title: The Wave-Front Correction System for the Sunrise Balloon-Borne Solar Observatory Authors: Berkefeld, T.; Schmidt, W.; Soltau, D.; Bell, A.; Doerr, H. P.; Feger, B.; Friedlein, R.; Gerber, K.; Heidecke, F.; Kentischer, T.; v. d. Lühe, O.; Sigwarth, M.; Wälde, E.; Barthol, P.; Deutsch, W.; Gandorfer, A.; Germerott, D.; Grauf, B.; Meller, R.; Álvarez-Herrero, A.; Knölker, M.; Martínez Pillet, V.; Solanki, S. K.; Title, A. M. Bibcode: 2011SoPh..268..103B Altcode: 2010SoPh..tmp..236B; 2010arXiv1009.3196B This paper describes the wave-front correction system developed for the Sunrise balloon telescope, and it provides information about its in-flight performance. For the correction of low-order aberrations, a Correlating Wave-Front Sensor (CWS) was used. It consisted of a six-element Shack - Hartmann wave-front sensor (WFS), a fast tip-tilt mirror for the compensation of image motion, and an active telescope secondary mirror for focus correction. The CWS delivered a stabilized image with a precision of 0.04 arcsec (rms), whenever the coarse pointing was better than ± 45 arcsec peak-to-peak. The automatic focus adjustment maintained a focus stability of 0.01 waves in the focal plane of the CWS. During the 5.5 day flight, good image quality and stability were achieved during 33 hours, containing 45 sequences, which lasted between 10 and 45 min. Title: The Sunrise Mission Authors: Barthol, P.; Gandorfer, A.; Solanki, S. K.; Schüssler, M.; Chares, B.; Curdt, W.; Deutsch, W.; Feller, A.; Germerott, D.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Meller, R.; Müller, R.; Riethmüller, T. L.; Tomasch, G.; Knölker, M.; Lites, B. W.; Card, G.; Elmore, D.; Fox, J.; Lecinski, A.; Nelson, P.; Summers, R.; Watt, A.; Martínez Pillet, V.; Bonet, J. A.; Schmidt, W.; Berkefeld, T.; Title, A. M.; Domingo, V.; Gasent Blesa, J. L.; del Toro Iniesta, J. C.; López Jiménez, A.; Álvarez-Herrero, A.; Sabau-Graziati, L.; Widani, C.; Haberler, P.; Härtel, K.; Kampf, D.; Levin, T.; Pérez Grande, I.; Sanz-Andrés, A.; Schmidt, E. Bibcode: 2011SoPh..268....1B Altcode: 2010arXiv1009.2689B; 2010SoPh..tmp..224B The first science flight of the balloon-borne Sunrise telescope took place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern Canada. We describe the scientific aims and mission concept of the project and give an overview and a description of the various hardware components: the 1-m main telescope with its postfocus science instruments (the UV filter imager SuFI and the imaging vector magnetograph IMaX) and support instruments (image stabilizing and light distribution system ISLiD and correlating wavefront sensor CWS), the optomechanical support structure and the instrument mounting concept, the gondola structure and the power, pointing, and telemetry systems, and the general electronics architecture. We also explain the optimization of the structural and thermal design of the complete payload. The preparations for the science flight are described, including AIV and ground calibration of the instruments. The course of events during the science flight is outlined, up to the recovery activities. Finally, the in-flight performance of the instrumentation is discussed. Title: The Filter Imager SuFI and the Image Stabilization and Light Distribution System ISLiD of the Sunrise Balloon-Borne Observatory: Instrument Description Authors: Gandorfer, A.; Grauf, B.; Barthol, P.; Riethmüller, T. L.; Solanki, S. K.; Chares, B.; Deutsch, W.; Ebert, S.; Feller, A.; Germerott, D.; Heerlein, K.; Heinrichs, J.; Hirche, D.; Hirzberger, J.; Kolleck, M.; Meller, R.; Müller, R.; Schäfer, R.; Tomasch, G.; Knölker, M.; Martínez Pillet, V.; Bonet, J. A.; Schmidt, W.; Berkefeld, T.; Feger, B.; Heidecke, F.; Soltau, D.; Tischenberg, A.; Fischer, A.; Title, A.; Anwand, H.; Schmidt, E. Bibcode: 2011SoPh..268...35G Altcode: 2010SoPh..tmp..176G; 2010arXiv1009.1037G We describe the design of the Sunrise Filter Imager (SuFI) and the Image Stabilization and Light Distribution (ISLiD) unit onboard the Sunrise balloon borne solar observatory. This contribution provides the necessary information which is relevant to understand the instruments' working principles, the relevant technical data, and the necessary information about calibration issues directly related to the science data. Title: The Role of Active Region Loop Geometry. I. How Can it Affect Coronal Seismology? Authors: Selwa, M.; Ofman, L.; Solanki, S. K. Bibcode: 2011ApJ...726...42S Altcode: We present numerical results of coronal loop oscillation excitation using a three-dimensional (3D) MHD model of an idealized active region (AR) field. The AR is initialized as a potential dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. We study different ways of excitation of vertical kink oscillations of this loop by velocity: as an initial condition, and as an impulsive excitation with a pulse of a given position, duration, and amplitude. We vary the geometry of the loop in the 3D MHD model and find that it affects both the period of oscillations and the synthetic observations (difference images) that we get from oscillations. Due to the overestimated effective length of the loop in the case of loops which have maximum separation between their legs above the footpoints (>50% of observed loops), the magnetic field obtained from coronal seismology can also be overestimated. The 3D MHD model shows how the accuracy of magnetic field strength determined from coronal seismology can be improved. We study the damping mechanism of the oscillations and find that vertical kink waves in 3D stratified geometry are damped mainly due to wave leakage in the horizontal direction. Title: Reconstruction of solar spectral irradiance since the Maunder minimum Authors: Krivova, N. A.; Vieira, L. E. A.; Solanki, S. K. Bibcode: 2010JGRA..11512112K Altcode: Solar irradiance is the main external driver of the Earth's climate. Whereas the total solar irradiance is the main source of energy input into the climate system, solar UV irradiance exerts control over chemical and physical processes in the Earth's upper atmosphere. The time series of accurate irradiance measurements are, however, relatively short and limit the assessment of the solar contribution to the climate change. Here we reconstruct solar total and spectral irradiance in the range 115-160,000 nm since 1610. The evolution of the solar photospheric magnetic flux, which is a central input to the model, is appraised from the historical record of the sunspot number using a simple but consistent physical model. The model predicts an increase of 1.25 W/m2, or about 0.09%, in the 11-year averaged solar total irradiance since the Maunder minimum. Also, irradiance in individual spectral intervals has generally increased during the past four centuries, the magnitude of the trend being higher toward shorter wavelengths. In particular, the 11-year averaged Ly-α irradiance has increased by almost 50%. An exception is the spectral interval between about 1500 and 2500 nm, where irradiance has slightly decreased (by about 0.02%). Title: SUNRISE: Instrument, Mission, Data, and First Results Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.; Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.; Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.; Franz, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.127S Altcode: 2010arXiv1008.3460S The SUNRISE balloon-borne solar observatory consists of a 1 m aperture Gregory telescope, a UV filter imager, an imaging vector polarimeter, an image stabilization system, and further infrastructure. The first science flight of SUNRISE yielded high-quality data that revealed the structure, dynamics, and evolution of solar convection, oscillations, and magnetic fields at a resolution of around 100 km in the quiet Sun. After a brief description of instruments and data, the first qualitative results are presented. In contrast to earlier observations, we clearly see granulation at 214 nm. Images in Ca II H display narrow, short-lived dark intergranular lanes between the bright edges of granules. The very small-scale, mixed-polarity internetwork fields are found to be highly dynamic. A significant increase in detectable magnetic flux is found after phase-diversity-related reconstruction of polarization maps, indicating that the polarities are mixed right down to the spatial resolution limit and probably beyond. Title: Supersonic Magnetic Upflows in Granular Cells Observed with SUNRISE/IMAX Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.; Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.; Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M. Bibcode: 2010ApJ...723L.144B Altcode: 2010arXiv1009.1227B Using the IMaX instrument on board the SUNRISE stratospheric balloon telescope, we have detected extremely shifted polarization signals around the Fe I 5250.217 Å spectral line within granules in the solar photosphere. We interpret the velocities associated with these events as corresponding to supersonic and magnetic upflows. In addition, they are also related to the appearance of opposite polarities and highly inclined magnetic fields. This suggests that they are produced by the reconnection of emerging magnetic loops through granular upflows. The events occupy an average area of 0.046 arcsec2 and last for about 80 s, with larger events having longer lifetimes. These supersonic events occur at a rate of 1.3 × 10-5 occurrences per second per arcsec2. Title: Detection of Vortex Tubes in Solar Granulation from Observations with SUNRISE Authors: Steiner, O.; Franz, M.; Bello González, N.; Nutto, Ch.; Rezaei, R.; Martínez Pillet, V.; Bonet Navarro, J. A.; del Toro Iniesta, J. C.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2010ApJ...723L.180S Altcode: 2010arXiv1009.4723S We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface. Title: On detectability of Zeeman broadening in optical spectra of F- and G-dwarfs Authors: Anderson, R. I.; Reiners, A.; Solanki, S. K. Bibcode: 2010A&A...522A..81A Altcode: 2010arXiv1008.2213A We investigate the detectability of Zeeman broadening in optical Stokes I spectra of slowly rotating sun-like stars. To this end, we apply the LTE spectral line inversion package SPINOR to very-high quality CES data and explore how fit quality depends on the average magnetic field, Bf. One-component (OC) and two-component (TC) models are adopted. In OC models, the entire surface is assumed to be magnetic. Under this assumption, we determine formal 3σ upper limits on the average magnetic field of 200 G for the Sun, and 150 G for 61 Vir (G6V). Evidence for an average magnetic field of ~500 G is found for 59 Vir (G0V), and of ~ 1000 G for HD 68456 (F6V). A distinction between magnetic and non-magnetic regions is made in TC models, while assuming a homogeneous distribution of both components. In our TC inversions of 59 Vir, we investigate three cases: both components have equal temperatures; warm magnetic regions; cool magnetic regions. Our TC model with equal temperatures does not yield significant improvement over OC inversions for 59 Vir. The resulting Bf values are consistent for both. Fit quality is significantly improved, however, by using two components of different temperatures. The inversions for 59 Vir that assume different temperatures for the two components yield results consistent with 0-450 G at the formal 3σ confidence level. We thus find a model dependence of our analysis and demonstrate that the influence of an additional temperature component can dominate over the Zeeman broadening signature, at least in optical data. Previous comparable analyses that neglected effects due to multiple temperature components may be prone to the same ambiguities.

Based on observations collected at the European Southern Observatory, La Silla.Appendix A is only available in electronic form at http://www.aanda.org Title: Where the Granular Flows Bend Authors: Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.; Bonet, J. A.; Domingo, V.; Schmidt, W.; Barthol, P.; Knölker, M. Bibcode: 2010ApJ...723L.159K Altcode: 2010arXiv1008.0517K Based on IMaX/SUNRISE data, we report on a previously undetected phenomenon in solar granulation. We show that in a very narrow region separating granules and intergranular lanes, the spectral line width of the Fe I 5250.2 Å line becomes extremely small. We offer an explanation of this observation with the help of magneto-convection simulations. These regions with extremely small line widths correspond to the places where the granular flows bend from upflow in granules to downflow in intergranular lanes. We show that the resolution and image stability achieved by IMaX/SUNRISE are important requisites to detect this interesting phenomenon. Title: Bright Points in the Quiet Sun as Observed in the Visible and Near-UV by the Balloon-borne Observatory SUNRISE Authors: Riethmüller, T. L.; Solanki, S. K.; Martínez Pillet, V.; Hirzberger, J.; Feller, A.; Bonet, J. A.; Bello González, N.; Franz, M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; del Toro Iniesta, J. C.; Domingo, V.; Gandorfer, A.; Knölker, M.; Schmidt, W. Bibcode: 2010ApJ...723L.169R Altcode: 2010arXiv1009.1693R Bright points (BPs) are manifestations of small magnetic elements in the solar photosphere. Their brightness contrast not only gives insight into the thermal state of the photosphere (and chromosphere) in magnetic elements, but also plays an important role in modulating the solar total and spectral irradiance. Here, we report on simultaneous high-resolution imaging and spectropolarimetric observations of BPs using SUNRISE balloon-borne observatory data of the quiet Sun at the disk center. BP contrasts have been measured between 214 nm and 525 nm, including the first measurements at wavelengths below 388 nm. The histograms of the BP peak brightness show a clear trend toward broader contrast distributions and higher mean contrasts at shorter wavelengths. At 214 nm, we observe a peak brightness of up to five times the mean quiet-Sun value, the highest BP contrast so far observed. All BPs are associated with a magnetic signal, although in a number of cases it is surprisingly weak. Most of the BPs show only weak downflows, the mean value being 240 m s-1, but some display strong down- or upflows reaching a few km s-1. Title: Transverse Component of the Magnetic Field in the Solar Photosphere Observed by SUNRISE Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.; Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta, J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D We present the first observations of the transverse component of a photospheric magnetic field acquired by the imaging magnetograph SUNRISE/IMaX. Using an automated detection method, we obtain statistical properties of 4536 features with significant linear polarization signal. We obtain a rate of occurrence of 7 × 10-4 s-1 arcsec-2, which is 1-2 orders of magnitude larger than the values reported by previous studies. We show that these features have no characteristic size or lifetime. They appear preferentially at granule boundaries with most of them being caught in downflow lanes at some point. Only a small percentage are entirely and constantly embedded in upflows (16%) or downflows (8%). Title: Detection of Large Acoustic Energy Flux in the Solar Atmosphere Authors: Bello González, N.; Franz, M.; Martínez Pillet, V.; Bonet, J. A.; Solanki, S. K.; del Toro Iniesta, J. C.; Schmidt, W.; Gandorfer, A.; Domingo, V.; Barthol, P.; Berkefeld, T.; Knölker, M. Bibcode: 2010ApJ...723L.134B Altcode: 2010arXiv1009.4795B We study the energy flux carried by acoustic waves excited by convective motions at sub-photospheric levels. The analysis of high-resolution spectropolarimetric data taken with IMaX/SUNRISE provides a total energy flux of ~6400-7700 W m-2 at a height of ~250 km in the 5.2-10 mHz range, i.e., at least twice the largest energy flux found in previous works. Our estimate lies within a factor of two of the energy flux needed to balance radiative losses from the chromosphere according to the estimates of Anderson & Athay and revives interest in acoustic waves for transporting energy to the chromosphere. The acoustic flux is mainly found in the intergranular lanes but also in small rapidly evolving granules and at the bright borders, forming dark dots and lanes of splitting granules. Title: Magnetic Loops in the Quiet Sun Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Bonet, J. A.; Barthol, P.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.185W Altcode: 2010arXiv1009.4715W We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from SUNRISE/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and force-free extrapolation techniques. We find that most magnetic loops that reach into the chromosphere or higher have one footpoint in relatively strong magnetic field regions in the photosphere. Ninety-one percent of the magnetic energy in the mid-chromosphere (at a height of 1 Mm) is in field lines, whose stronger footpoint has a strength of more than 300 G, i.e., above the equipartition field strength with convection. The loops reaching into the chromosphere and corona are also found to be asymmetric in the sense that the weaker footpoint has a strength B < 300 G and is located in the internetwork (IN). Such loops are expected to be strongly dynamic and have short lifetimes, as dictated by the properties of the IN fields. Title: SUNRISE/IMaX Observations of Convectively Driven Vortex Flows in the Sun Authors: Bonet, J. A.; Márquez, I.; Sánchez Almeida, J.; Palacios, J.; Martínez Pillet, V.; Solanki, S. K.; del Toro Iniesta, J. C.; Domingo, V.; Berkefeld, T.; Schmidt, W.; Gandorfer, A.; Barthol, P.; Knölker, M. Bibcode: 2010ApJ...723L.139B Altcode: 2010arXiv1009.1992B We characterize the observational properties of the convectively driven vortex flows recently discovered on the quiet Sun, using magnetograms, Dopplergrams, and images obtained with the 1 m balloon-borne SUNRISE telescope. By visual inspection of time series, we find some 3.1 × 10-3 vortices Mm-2 minute-1, which is a factor of ~1.7 larger than previous estimates. The mean duration of the individual events turns out to be 7.9 minutes, with a standard deviation of 3.2 minutes. In addition, we find several events appearing at the same locations along the duration of the time series (31.6 minutes). Such recurrent vortices show up in the proper motion flow field map averaged over the time series. The typical vertical vorticities are lsim6 × 10-3 s-1, which corresponds to a period of rotation of some 35 minutes. The vortices show a preferred counterclockwise sense of rotation, which we conjecture may have to do with the preferred vorticity impinged by the solar differential rotation. Title: Radiative emission of solar features in the Ca II K line: comparison of measurements and models Authors: Ermolli, I.; Criscuoli, S.; Uitenbroek, H.; Giorgi, F.; Rast, M. P.; Solanki, S. K. Bibcode: 2010A&A...523A..55E Altcode: 2010arXiv1009.0227E Context. The intensity of the Ca II K resonance line observed with spectrographs and Lyot-type filters has long served as a diagnostic of the solar chromosphere. However, the literature contains a relative lack of photometric measurements of solar features observed at this spectral range.
Aims: We study the radiative emission of various types of solar features, such as quiet Sun, enhanced network, plage, and bright plage regions, identified on filtergrams taken in the Ca II K line.
Methods: We analysed full-disk images obtained with the PSPT, by using three interference filters that sample the Ca II K line with different bandpasses. We studied the dependence of the radiative emission of disk features on the filter bandpass. We also performed a non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II K line integrated over the bandpass of PSPT filters. The synthesis was carried out by utilizing the partial frequency redistribution (PRD) with the most recent set of semi-empirical atmosphere models in the literature and some earlier atmosphere models. As the studied models were computed by assuming the complete redistribution formalism (CRD), we also performed simulations with this approximation for comparison.
Results: We measured the center-to-limb variation of intensity values for various solar features identified on PSPT images and compared the results obtained with those derived from the synthesis. We find that CRD calculations derived using the most recent quiet Sun model, on average, reproduce the measured values of the quiet Sun regions slightly more accurately than PRD computations with the same model. This may reflect that the utilized atmospheric model was computed assuming CRD. Calculations with PRD on earlier quiet Sun model atmospheres reproduce measured quantities with a similar accuracy as to that achieved here by applying CRD to the recent model. We also find that the median contrast values measured for most of the identified bright features, disk positions, and filter bandpasses are, on average, a factor ≈1.9 lower than those derived from PRD simulations performed using the recent bright feature models. The discrepancy between measured and modeled values decreases by ≈12% after taking into account straylight effects on PSPT images. When moving towards the limb, PRD computations display closer agreement with the data than performed in CRD. Moreover, PRD computations on either the most recent or the earlier atmosphere models of bright features reproduce measurements from plage and bright plage regions with a similar accuracy.

Appendix A is only available in electronic form at http://www.aanda.org Title: Surface Waves in Solar Granulation Observed with SUNRISE Authors: Roth, M.; Franz, M.; Bello González, N.; Martínez Pillet, V.; Bonet, J. A.; Gandorfer, A.; Barthol, P.; Solanki, S. K.; Berkefeld, T.; Schmidt, W.; del Toro Iniesta, J. C.; Domingo, V.; Knölker, M. Bibcode: 2010ApJ...723L.175R Altcode: 2010arXiv1009.4790R Solar oscillations are expected to be excited by turbulent flows in the intergranular lanes near the solar surface. Time series recorded by the IMaX instrument on board the SUNRISE observatory reveal solar oscillations at high spatial resolution, which allow the study of the properties of oscillations with short wavelengths. We analyze two time series with synchronous recordings of Doppler velocity and continuum intensity images with durations of 32 minutes and 23 minutes, respectively, recorded close to the disk center of the Sun to study the propagation and excitation of solar acoustic oscillations. In the Doppler velocity data, both the standing acoustic waves and the short-lived, high-degree running waves are visible. The standing waves are visible as temporary enhancements of the amplitudes of the large-scale velocity field due to the stochastic superposition of the acoustic waves. We focus on the high-degree small-scale waves by suitable filtering in the Fourier domain. Investigating the propagation and excitation of f- and p 1-modes with wavenumbers k>1.4 Mm-1, we also find that exploding granules contribute to the excitation of solar p-modes in addition to the contribution of intergranular lanes. Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with the SUNRISE/IMAX Instrument Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero, J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.; Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M. Bibcode: 2010ApJ...723L.164L Altcode: 2010arXiv1009.0996L Until today, the small size of magnetic elements in quiet-Sun areas has required the application of indirect methods, such as the line-ratio technique or multi-component inversions, to infer their physical properties. A consistent match to the observed Stokes profiles could only be obtained by introducing a magnetic filling factor that specifies the fraction of the observed pixel filled with magnetic field. Here, we investigate the properties of a small magnetic patch in the quiet Sun observed with the IMaX magnetograph on board the balloon-borne telescope SUNRISE with unprecedented spatial resolution and low instrumental stray light. We apply an inversion technique based on the numerical solution of the radiative transfer equation to retrieve the temperature stratification and the field strength in the magnetic patch. The observations can be well reproduced with a one-component, fully magnetized atmosphere with a field strength exceeding 1 kG and a significantly enhanced temperature in the mid to upper photosphere with respect to its surroundings, consistent with semi-empirical flux tube models for plage regions. We therefore conclude that, within the framework of a simple atmospheric model, the IMaX measurements resolve the observed quiet-Sun flux tube. Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as Measured from SUNRISE Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler, M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.; Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M. Bibcode: 2010ApJ...723L.154H Altcode: We present high-resolution images of the Sun in the near-ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1 m SUNRISE balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures—up to 32.8% at a wavelength of 214 nm. We compare the rms contrasts obtained from the observational data with theoretical intensity contrasts obtained from numerical magnetohydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Evidence for Convection in Sunspot Penumbrae Authors: Bharti, L.; Solanki, S. K.; Hirzberger, J. Bibcode: 2010ApJ...722L.194B Altcode: 2010arXiv1009.2919B We present an analysis of twisting motions in penumbral filaments in sunspots located at heliocentric angles from 30° to 48° using three time series of blue continuum images obtained by the Broadband Filter Imager (BFI) on board Hinode. The relations of the twisting motions to the filament brightness and the position within the filament and within the penumbra, respectively, are investigated. Only certain portions of the filaments show twisting motions. In a statistical sense, the part of the twisting portion of a filament located closest to the umbra is brightest and possesses the fastest twisting motion, with a mean twisting velocity of 2.1 km s-1. The middle and outer sections of the twisting portion of the filament (lying increasingly further from the umbra), which are less bright, have mean velocities of 1.7 km s-1 and 1.35 km s-1, respectively. The observed reduction of brightness and twisting velocity toward the outer section of the filaments may be due to reducing upflow along the filament's long axis. No significant variation of twisting velocity as a function of viewing angles was found. The obtained correlation of brightness and velocity suggests that overturning convection causes the twisting motions observed in penumbral filament and may be the source of the energy needed to maintain the brightness of the filaments. Title: Reply to comment by P. Foukal on “A homogeneous database of sunspot areas covering more than 130 years” Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S. Bibcode: 2010JGRA..115.9103B Altcode: 2010JGRA..11509103B Abstract Available from http://www.agu.org Title: Magnetic structures of an emerging flux region in the solar photosphere and chromosphere Authors: Xu, Z.; Lagg, A.; Solanki, S. K. Bibcode: 2010A&A...520A..77X Altcode:
Aims: We investigate the vector magnetic field and Doppler velocity in the photosphere and upper chromosphere of a young emerging flux region of the sun close to disk center.
Methods: Spectropolarimetric scans of a young active region made using the second generation Tenerife Infrared Polarimeter (TIP II) on the German Vacuum Tower Telescope (VTT) are analyzed. The scanned area contained multiple sunspots and an emerging flux region. An inversion based on the Milne-Eddington approximation was performed on the full Stokes vector of the chromospheric He I 10 830 Å and the photospheric Si I 10 827.1 Å lines. This provided the magnetic vector and line-of-sight velocity at each spatial point in both atmospheric layers.
Results: A clear difference is seen between the complex magnetic structure of the emerging flux region (EFR) in the photosphere and the much simpler structure in the upper chromosphere. The upper chromospheric structure is consistent with a set of emerging loops following elongated dark structures seen in the He I 10 830 Å triplet, similar to arch filament systems (AFS), while in the photosphere we infer the presence of U-loops within the emergence zone. Nonetheless, in general the upper chromospheric field has a similar linear relationship between inclination angle and field strength as the photospheric field: the field is weak (≈300 G) and horizontal in the emergence zone, but strong (up to 850 G) and more vertical near its edges. The field strength decreases from the photosphere to the upper chromosphere by approximately 0.1-0.2 G km-1 (or even less) within the emergence zone and by 0.3-0.6 G km-1 in sunspots located at its edge. We reconstructed the magnetic field in 3D based on the chromospheric vector field under the assumption that the He I 10 830 Å triplet forms along the magnetic field loops. The reconstructed loops are quite flat with supersonic downflows at both footpoints. Arguments and evidence for an enhanced formation height of He I 10 830 Å in arch-filaments seen in this line are provided, which support the validity of the reconstructed loops.
Conclusions: The main chromospheric properties of EFRs previously deduced for a single region NOAA 9451 are shown to be valid for another region as well, suggesting that the main original results may have a wider application. The main exception is that only the first region displayed a current sheet in the chromosphere. We propose a scenario in which the relatively complex photospheric structure evolves into the simpler chromospheric one. Title: Quiet-Sun intensity contrasts in the near ultraviolet Authors: Hirzberger, Johann; Feller, Alex; Riethmüller, Tino L.; Schüssler, Manfred; Borrero, Juan M.; Afram, Nadine; Unruh, Yvonne C.; Berdyugina, Svetlana V.; Gandorfer, Achim; Solanki, Sami K.; Barthol, Peter; Bonet, Jose A.; Martínez Pillet, Valentin; Berkefeld, Thomas; Knölker, Michael; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2010arXiv1009.1050H Altcode: We present high-resolution images of the Sun in the near ultraviolet spectral range between 214 nm and 397 nm as obtained from the first science flight of the 1-m Sunrise balloon-borne solar telescope. The quiet-Sun rms intensity contrasts found in this wavelength range are among the highest values ever obtained for quiet-Sun solar surface structures - up to 32.8% at a wavelength of 214 nm. We compare with theoretical intensity contrasts obtained from numerical magneto-hydrodynamic simulations. For 388 nm and 312 nm the observations agree well with the numerical simulations whereas at shorter wavelengths discrepancies between observed and simulated contrasts remain. Title: Sunspot group tilt angles and the strength of the solar cycle Authors: Dasi-Espuig, M.; Solanki, S. K.; Krivova, N. A.; Cameron, R.; Peñuela, T. Bibcode: 2010A&A...518A...7D Altcode: 2010arXiv1005.1774D Context. It is well known that the tilt angles of active regions increase with their latitude (Joy's law). It has never been checked before, however, whether the average tilt angles change from one cycle to the next. Flux transport models show the importance of tilt angles for the reversal and build up of magnetic flux at the poles, which is in turn correlated to the strength of the next cycle.
Aims: Here we analyse time series of tilt angle measurements and look for a possible relationship of the tilt angles with other solar cycle parameters, in order to glean information on the solar dynamo and to estimate their potential for predicting solar activity.
Methods: We employed tilt angle data from Mount Wilson and Kodaikanal observatories covering solar cycles 15 to 21. We analyse the latitudinal distribution of the tilt angles (Joy's law), their variation from cycle to cycle, and their relationship to other solar cycle parameters, such as the strength (or total area covered by sunspots in a cycle), amplitude, and length.
Results: The two main results of our analysis follow. 1. We find an anti-correlation between the mean normalised tilt angle of a given cycle and the strength (or amplitude) of that cycle, with a correlation coefficient of rc = -0.95 (99.9% confidence level) and rc = -0.93 (99.76% confidence level) for Mount Wilson and Kodaikanal data, respectively. 2. The product of the cycle's averaged tilt angle and the strength of the same cycle displays a significant correlation with the strength of the next cycle (rc = 0.65 at 89% confidence level and rc = 0.70 at 92% confidence level for Mount Wilson and Kodaikanal data, respectively). An even better correlation is obtained between the source term of the poloidal flux in Babcock-Leighton-type dynamos (which contains the tilt angle) and the amplitude of the next cycle. Further we confirm the linear relationship (Joy's law) between the tilt angle and latitude with slopes of 0.26 and 0.28 for Mount Wilson and Kodaikanal data, respectively. In addition, we obtain good positive correlations between the normalised-area-weighted tilt angle and the length of the following cycle, whereas the strength or the amplitude of the next cycle does not appear to be correlated to the tilt angles of the current cycle alone.
Conclusions: The results of this study indicate that, in combination with the cycle strength, the active region tilt angles play an important role in building up the polar fields at cycle minimum. Title: GREGOR telescope: start of commissioning Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S.; Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvana, M.; Kneer, F.; Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. Bibcode: 2010SPIE.7733E..0KV Altcode: 2010SPIE.7733E..18V With the integration of a 1-meter Cesic primary mirror the GREGOR telescope pre-commissioning started. This is the first time, that the entire light path has seen sunlight. The pre-commissioning period includes testing of the main optics, adaptive optics, cooling system, and pointing system. This time was also used to install a near-infrared grating spectro-polarimeter and a 2D-spectropolarimeter for the visible range as first-light science instruments. As soon as the final 1.5 meter primary mirror is installed, commissioning will be completed, and an extended phase of science verification will follow. In the near future, GREGOR will be equipped with a multi-conjugate adaptive optics system that is presently under development at KIS. Title: Accelerating Waves in Polar Coronal Holes as Seen by EIS and SUMER Authors: Gupta, G. R.; Banerjee, D.; Teriaca, L.; Imada, S.; Solanki, S. Bibcode: 2010ApJ...718...11G Altcode: 2010arXiv1005.3453G We present EIS/Hinode and SUMER/SOHO observations of propagating disturbances detected in coronal lines in inter-plume and plume regions of a polar coronal hole. The observation was carried out on 2007 November 13 as part of the JOP196/HOP045 program. The SUMER spectroscopic observation gives information about fluctuations in radiance and on both resolved (Doppler shift) and unresolved (Doppler width) line-of-sight velocities, whereas EIS 40'' wide slot images detect fluctuations only in radiance but maximize the probability of overlapping field of view between the two instruments. From distance-time radiance maps, we detect the presence of propagating waves in a polar inter-plume region with a period of 15-20 minutes and a propagation speed increasing from 130 ± 14 km s-1 just above the limb to 330 ± 140 km s-1 around 160'' above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed is in the range of 25 ± 1.3 to 38 ± 4.5 km s-1, with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbances with the same range of periodicity but with propagation speeds in the range of 135 ± 18 to 165 ± 43 km s-1 only. A comparison between the distance-time radiance map of the two regions indicates that the waves within the plumes are not observable (may be getting dissipated) far off-limb, whereas this is not the case in the inter-plume region. A correlation analysis was also performed to find out the time delay between the oscillations at several heights in the off-limb region, finding results consistent with those from the analysis of the distance-time maps. To our knowledge, this result provides first spectroscopic evidence of the acceleration of propagating disturbances in the polar region close to the Sun (within 1.2 R/R sun), which provides clues to the understanding of the origin of these waves. We suggest that the waves are likely either Alfvénic or fast magnetoacoustic in the inter-plume region and slow magnetoacoustic in the plume region. This may lead to the conclusion that inter-plumes are a preferred channel for the acceleration of the fast solar wind. Title: Expansion of magnetic flux concentrations: a comparison of Hinode SOT data and models Authors: Pietarila, A.; Cameron, R.; Solanki, S. K. Bibcode: 2010A&A...518A..50P Altcode: 2010arXiv1005.3405P Context. The expansion of network magnetic fields with height is a fundamental property of flux tube models. A rapid expansion is required to form a magnetic canopy.
Aims: We characterize the observed expansion properties of magnetic network elements and compare them with the thin flux tube and sheet approximations, as well as with magnetoconvection simulations.
Methods: We used data from the Hinode SOT NFI NaD1 channel and spectropolarimeter to study the appearance of magnetic flux concentrations seen in circular polarization as a function of position on the solar disk. We compared the observations with synthetic observables from models based on the thin flux tube approximation and magnetoconvection simulations with two different upper boundary conditions for the magnetic field (potential and vertical).
Results: The observed circular polarization signal of magnetic flux concentrations changes from unipolar at disk center to bipolar near the limb, which implies an expanding magnetic field. The observed expansion agrees with expansion properties derived from the thin flux sheet and tube approximations. Magnetoconvection simulations with a potential field as the upper boundary condition for the magnetic field also produce bipolar features near the limb while a simulation with a vertical field boundary condition does not.
Conclusions: The near-limb apparent bipolar magnetic features seen in high-resolution Hinode observations can be interpreted using a simple flux sheet or tube model. This lends further support to the idea that magnetic features with vastly varying sizes have similar relative expansion rates. The numerical simulations presented here are less useful in interpreting the expansion since the diagnostics we are interested in are strongly influenced by the choice of the upper boundary condition for the magnetic field in the purely photospheric simulations. Title: Flight control software for the wave-front sensor of SUNRISE 1m balloon telescope Authors: Bell, Alexander; Barthol, Peter; Berkefeld, Thomas; Feger, Bernhard; Gandorfer, Achim M.; Heidecke, Frank; Knoelker, Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Sigwarth, Michael; Solanki, Sami K.; Soltau, Dirk; Title, Alan M. Bibcode: 2010SPIE.7740E..03B Altcode: 2010SPIE.7740E...2B This paper describes the flight control software of the wave-front correction system that flew on the 2009 science flight of the Sunrise balloon telescope. The software discussed here allowed fully automated operations of the wave-front sensor, communications with the adaptive optics sub-system, the pointing system, the instrument control unit and the main telescope controller. The software was developed using modern object oriented analysis and design techniques, and consists of roughly 13.000 lines of C++ code not counting code written for the on-board communication layer. The software operated error free during the 5.5 day flight. Title: SUNRISE Impressions from a successful science flight Authors: Schmidt, W.; Solanki, S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knölker, M.; Martínez Pillet, V.; Schüssler, M.; Title, A. Bibcode: 2010AN....331..601S Altcode: SUNRISE is a balloon-borne telescope with an aperture of one meter. It is equipped with a filter imager for the UV wavelength range between 214 nm and 400 nm (SUFI), and with a spectro-polarimeter that measures the magnetic field of the photosphere using the Fe I line at 525.02 nm that has a Landé factor of 3. SUNRISE performed its first science flight from 8 to 14 June 2009. It was launched at the Swedish ESRANGE Space Center and cruised at an altitude of about 36 km and geographic latitudes between 70 and 74 degrees to Somerset Island in northern Canada. There, all data, the telescope and the gondola were successfully recovered. During its flight, Sunrise achieved high pointing stability during 33 hours, and recorded about 1.8 TB of science data. Already at this early stage of data processing it is clear that SUNRISE recorded UV images of the solar photosphere, and spectropolarimetric measurements of the quiet Sun's magnetic field of unprecedented quality. Title: Spectropolarimetric analysis of 3D MHD sunspot simulations Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2010AN....331..567B Altcode: We have employed 3D non-grey MHD simulations of sunspots to compute theoretical Stokes profiles and compare the levels of circular and linear polarization in the simulations with those observed in a real sunspot. We find that the spatial distribution and average values of these quantities agree very well with the observations, although the polarization levels in the simulations are slightly larger. This can be explained by a slightly larger magnetic field strength or a larger temperature gradient in the simulated penumbra as compared to the observations. Title: Sideways displacement of penumbral fibrils by the solar flare on 2006 December 13 Authors: Zhang, Jun; Li, Leping; Solanki, S. K. Bibcode: 2010arXiv1007.0056Z Altcode: Flares are known to restructure the magnetic field in the corona and to accelerate the gas between the field lines, but their effect on the photosphere is less well studied. New data of the Solar Optical Telescope (SOT) onboard Hinode provide unprecedented opportunity to uncover the photospheric effect of a solar flare, which associates with an active region NOAA AR 10930 on 2006 December 13. We find a clear lateral displacement of sunspot penumbral regions scanned by two flare ribbons. In the impulsive phase of the flare, the flare ribbons scan the sunspot at a speed of around 18 km/s, derived from Ca II and G-band images. We find instantaneous horizontal shear of penumbral fibrils, with initial velocities of about 1.6 km/s, produced when a flare ribbon passes over them. This velocity decreases rapidly at first, then gradually decays, so that about one hour later, the fibrils return to a new equilibrium. During the one hour interval, the total displacement of these fibrils is around 2.0 Mm, with an average shear velocity of 0.55 km/s. This lateral motion of the penumbral fibrils indicates that the magnetic footpoints of these field lines being rearranged in the corona also move. Title: GREGOR solar telescope: Design and status Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.; Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G. Bibcode: 2010AN....331..624V Altcode: The integration and verification phase of the GREGOR telescope reached an important milestone with the installation of the interim 1 m SolarLite primary mirror. This was the first time that the entire light path had seen sunlight. Since then extensive testing of the telescope and its subsystems has been carried out. The integration and verification phase will culminate with the delivery and installation of the final 1.5 m Zerodur primary mirror in the summer of 2010. Observatory level tests and science verification will commence in the second half of 2010 and in 2011. This phase includes testing of the main optics, adaptive optics, cooling and pointing systems. In addition, assuming the viewpoint of a typical user, various observational modes of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared Spectrograph (GRIS), and high-speed camera systems will be tested to evaluate if they match the expectations and science requirements. This ensures that GREGOR will provide high-quality observations with its combination of (multi-conjugate) adaptive optics and advanced post-focus instruments. Routine observations are expected for 2012. Title: Are cold winters in Europe associated with low solar activity? Authors: Lockwood, M.; Harrison, R. G.; Woollings, T.; Solanki, S. K. Bibcode: 2010ERL.....5b4001L Altcode: Solar activity during the current sunspot minimum has fallen to levels unknown since the start of the 20th century. The Maunder minimum (about 1650-1700) was a prolonged episode of low solar activity which coincided with more severe winters in the United Kingdom and continental Europe. Motivated by recent relatively cold winters in the UK, we investigate the possible connection with solar activity. We identify regionally anomalous cold winters by detrending the Central England temperature (CET) record using reconstructions of the northern hemisphere mean temperature. We show that cold winter excursions from the hemispheric trend occur more commonly in the UK during low solar activity, consistent with the solar influence on the occurrence of persistent blocking events in the eastern Atlantic. We stress that this is a regional and seasonal effect relating to European winters and not a global effect. Average solar activity has declined rapidly since 1985 and cosmogenic isotopes suggest an 8% chance of a return to Maunder minimum conditions within the next 50 years (Lockwood 2010 Proc. R. Soc. A 466 303-29): the results presented here indicate that, despite hemispheric warming, the UK and Europe could experience more cold winters than during recent decades. Title: Probing quiet Sun magnetism using MURaM simulations and Hinode/SP results: support for a local dynamo Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2010A&A...513A...1D Altcode: 2010arXiv1001.2183D Context. Owing to the limited spatial resolution and the weak polarization signal coming from the quietest regions on the Sun, the organization of the magnetic field on the smallest scales is largely unknown.
Aims: We obtain information about the magnetic flux present in the quiet Sun by comparing radiative MHD simulations with observations, with particular emphasis on the role of surface dynamo action.
Methods: We synthesized Stokes profiles on the basis of the MHD simulation results. The profiles are degraded by taking the properties of the spectropolarimeter (SP) into account onboard the Hinode satellite. We used simulation runs with different magnetic Reynolds numbers (Rm) and observations at different heliocentric angles with different levels of noise.
Results: Simulations with an imposed mixed-polarity field and Rm below the threshold for dynamo action reproduce the observed vertical flux density, but do not display a high enough horizontal flux density. Surface dynamo simulations at the highest Rm feasible at the moment yield a ratio of the horizontal and vertical flux density consistent with observational results, but the overall amplitudes are too low. Based on the properties of the local dynamo simulations, a tentative scaling of the magnetic field strength by a factor 2-3 reproduces the signal observed in the internetwork regions.
Conclusions: We find agreement with observations at different heliocentric angles. The mean field strength in internetwork implied by our analysis is roughly 170 G at the optical depth unity. Our study shows that surface dynamo could be responsible for most of the magnetic flux in the quiet Sun outside the network, given that the extrapolation to higher Rm is valid. Title: Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver Authors: Selwa, M.; Murawski, K.; Solanki, S. K.; Ofman, L. Bibcode: 2010A&A...512A..76S Altcode:
Aims: We study an oscillatory driver as a possible excitation mechanism of vertical kink loop oscillations in the ideal MHD regime.
Methods: We consider a solar coronal magnetic arcade with a dense photospheric layer. The two-dimensional numerical model that we implement includes the effects of nonlinearity and line curvature on the excitation and attenuation of fast magnetosonic kink waves. We investigate the effects of a driven sinusoidal pressure pulse and compare it with the impulsive excitation by a pressure pulse that impacts the overlying loop.
Results: Our numerical simulations reveal wave signatures that are reminiscent of vertical loop oscillations seen in TRACE observational data.
Conclusions: We conclude that attenuation of vertical kink oscillations can be reduced to the value observed by adopting an oscillatory instead of an impulsive excitation. An oscillatory driver also naturally explains why only a small subset of all loops is excited to oscillate transversally in an active region. Title: A comparison of measured and simulated solar network contrast Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Mathew, S. K. Bibcode: 2010IAUS..264...63A Altcode: 2009arXiv0910.0976A Long-term trends in the solar spectral irradiance are important to determine the impact on Earth's climate. These long-term changes are thought to be caused mainly by changes in the surface area covered by small-scale magnetic elements. The direct measurement of the contrast to determine the impact of these small-scale magnetic elements is, however, limited to a few wavelengths, and is, even for space instruments, affected by scattered light and instrument defocus. In this work we calculate emergent intensities from 3-D simulations of solar magneto-convection and validate the outcome by comparing with observations from Hinode/SOT. In this manner we aim to construct the contrast at wavelengths ranging from the NUV to the FIR. Title: Observations of a rotating macrospicule associated with an X-ray jet Authors: Kamio, S.; Curdt, W.; Teriaca, L.; Inhester, B.; Solanki, S. K. Bibcode: 2010A&A...510L...1K Altcode: 2010arXiv1001.1924K
Aims: We attempt to understand the driving mechanism of a macrospicule and its relationship with a coronal jet.
Methods: We study the dynamics of a macrospicule and an associated coronal jet captured by multi-spacecraft observations. Doppler velocities in both the macrospicule and the coronal jet are determined by EIS and SUMER spectra. Their temporal evolution is studied using X-ray and He II λ304 images.
Results: A blueshift of -120 ± 15 km s-1 is detected on one side of the macrospicule, while a redshift of 50 ± 6 km s-1 is found at the base of the other side. The inclination angle of the macrospicule inferred from a stereoscopic analysis with STEREO suggests that the measured Doppler velocities can be attributed to a rotating motion of the macrospicule rather than a radial flow or an expansion.
Conclusions: The macrospicule is driven by the unfolding motion of a twisted magnetic flux rope, while the associated X-ray jet is a radial outflow. Title: Nonlinear force-free modelling: influence of inaccuracies in the measured magnetic vector Authors: Wiegelmann, T.; Yelles Chaouche, L.; Solanki, S. K.; Lagg, A. Bibcode: 2010A&A...511A...4W Altcode: 2009arXiv0912.3002W Context. Solar magnetic fields are regularly extrapolated into the corona starting from photospheric magnetic measurements that can be affected by significant uncertainty.
Aims: We study how inaccuracies introduced into the maps of the photospheric magnetic vector by the inversion of ideal and noisy Stokes parameters influence the extrapolation of nonlinear force-free magnetic fields.
Methods: We compute nonlinear force-free magnetic fields based on simulated vector magnetograms, by the inversion of Stokes profiles that were computed by a 3-D radiation MHD simulation snapshot. These extrapolations are compared with extrapolations that originate directly in the field in the MHD simulations, which is our reference. We investigate how line formation and instrumental effects such as noise, limited spatial resolution, and the effect of employing a filter instrument influence the resulting magnetic field structure. The comparison is performed qualitatively by visually inspecting the magnetic field distribution and quantitatively by different metrics.
Results: The reconstructed field is most accurate if ideal Stokes data are inverted and becomes less accurate if instrumental effects and noise are included. The results demonstrate that the nonlinear force-free field extrapolation method tested here is relatively insensitive to the effects of noise in measured polarization spectra at levels consistent with present-day instruments.
Conclusions: Our results show that we can reconstruct the coronal magnetic field as a nonlinear force-free field from realistic photospheric measurements with an accuracy of a few percent, at least in the absence of sunspots. Title: Convective Motions and Net Circular Polarization in Sunspot Penumbrae Authors: Borrero, J. M.; Solanki, S. K. Bibcode: 2010ApJ...709..349B Altcode: 2009arXiv0911.2570B We have employed a penumbral model, which includes the Evershed flow and convective motions inside penumbral filaments, to reproduce the azimuthal variation of the net circular polarization (NCP) in sunspot penumbrae at different heliocentric angles for two different spectral lines. The theoretical NCP fits the observations as satisfactorily as penumbral models based on flux tubes. The reason for this is that the effect of convective motions on the NCP is very small compared to the effect of the Evershed flow. In addition, the NCP generated by convective upflows cancels out the NCP generated by the downflows. We have also found that, in order to fit the observed NCP, the strength of the magnetic field inside penumbral filaments must be very close to 1000 G. In particular, field-free or weak-field filaments fail to reproduce both the correct sign of the NCP and its dependence on the azimuthal and heliocentric angles. Title: Evolution of the solar magnetic flux on time scales of years to millenia Authors: Vieira, L. E. A.; Solanki, S. K. Bibcode: 2010A&A...509A.100V Altcode: 2009arXiv0911.4396V
Aims: We improve the description of the evolution of the Sun's open and total magnetic flux on time scales of years to millenia.
Methods: In the model employed here the evolution of the solar total and open magnetic flux is computed from the flux emerging at the solar surface in the form of bipolar magnetic features, which is related to the sunspot number cycle parameters and can be estimated from historical records. Compared to earlier versions of the model in addition to the long-lived open flux, now also a more rapidly decaying component of the open flux is considered. The model parameters are constrained by comparing its output with observations of the total surface magnetic flux and with a reconstruction of the open magnetic flux based on the geomagnetic indexes. A method to compute the Sun's total magnetic flux and the sunspot number during the Holocene, starting from the open flux obtained from cosmogenic isotopes records, is also presented.
Results: By considering separately a rapidly evolving and a slowly evolving component of the open flux the model reproduces the Sun's open flux, as reconstructed based on the aa-index, much better and a reasonable description of the radial component of interplanetary magnetic field data is obtained. The greatest improvement is in the reproduction of the cyclic variation of the open flux, including the amplitudes of individual cycles. Furthermore, we found that approximately 25% of the modeled open flux values since the end of the Maunder minimum are lower than the averaged value over 2008, i.e. during the current low minimum. The same proportion is observed in reconstructions of the open flux during the Holocene based on cosmogenic isotopes, which suggests that the present solar minimum conditions are below average, but not exceptional in terms of the heliospheric magnetic flux. Title: Evolution of the solar radiative forcing on climate during the Holocene Authors: Vieira, Luis Eduardo; Solanki, Sami K.; Krivova, Natalie Bibcode: 2010cosp...38.1785V Altcode: 2010cosp.meet.1785V The main external heating source of the Earth's coupled atmosphere-ocean system is the solar radiative energy input. The variability of this energy source produces corresponding changes on the coupled system. However, there is still significant uncertainty on the level of changes. One way to distinguish the influence of the Sun on the climate from other sources is to search for its influence in the pre-industrial period, when the influence of human activities on the atmosphere composition and Earth's surface properties can be neglected. Such studies require long time series of solar and geophysical parameters, ideally covering the whole Holocene. Here, we compute the total and spectral irradiance for the Holocene employing the reconstructions of the open flux and sunspot number obtained from the cosmogenic isotope 14C. The model employed in this study is identical to the spectral and total irradiance reconstruction (SATIRE) models employed to study these parameters on time scales from days to centuries, but adapted to work with decadal averaged data. The model is tested by comparing to the total and spectral solar irradiance reconstructions from the sunspot number for the last 4 centuries. We also discuss limits and uncertainties of the model. Title: Is the current solar activity minimum special? Authors: Solanki, Sami K.; Krivova, Natalie; Antunes Vieira, Luis Eduardo Bibcode: 2010cosp...38.1693S Altcode: 2010cosp.meet.1693S Is the current solar activity minimum special? Sami K. Solanki(1,2), Natalie Krivova(1), Luis E. Vieira(1,3) (1) Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany (2) Kyung-Hee University, Yongin, Gyeonggi, Korea (3) Laboratory for Physics and Chemistry of the Terrestrial Environment/CNRS, Or-leans, France In many respects the current minimum of solar activity is rather different from other minima during the last half century, which has seen the Sun in a state of unusually high activity. This uncommon minimum has fascinated solar physicists. Of particular importance for our understanding is the behaviour of the magnetic field, which is the source of all activity observed in the solar atmosphere, including the heliosphere. Furthermore, the evolution of dark and bright magnetic features on the solar surface modulates the solar irradiance, which could directly influence the Earth's climate. In this presentation observations and models of the evolution of the Sun's magnetic field, sunspots and irradiance are reviewed. After discussing the variation of these quantities over the solar cycle and from cycle to cycle, the current minimum is considered in greater detail. It is compared to earlier minima going back to the Maunder minimum and, in a more averaged sense, to solar activity and irradiance during the whole Holocene. Questions that will be onsidered are: Are we leaving the period of high solar activity (a grand maximum) to enter into a phase of more moderate activity, or even into a grand minimum? How could such a change in the level of the magnetic field and irradiance aid us in gaining further insight into the nature of the Sun's influence on the Earth's climate? Title: Temperatures of small scale magnetic structures in deep solar photospheric layers Authors: Oklay, Nilda; Gandorfer, Achim; Lagg, Andreas; Solanki, Sami K.; Bianda, Michele; Ramelli, Renzo Bibcode: 2010cosp...38.2857O Altcode: 2010cosp.meet.2857O With current solar instrumentation, small scale magnetic structures still remain unresolved. Nevertheless, it is possible to retrieve information about these unresolved magnetic structures via spectropolarimetry. For this reason, IRSOL (Istituto Ricerche Solari Locarno) facility is used to obtain simultaneously recorded spectra of Stokes I and Stokes V/I of CI (5380.3 A), FeI (5379.6 A, 5383.4 A) and TiII (5381.0 A) lines on an active region using the ZIMPOL II (Zurich Imaging Polarimeter II) instrument. We used Stokes V amplitude ratios technique to investigate temperatures of unresolved magnetic features down to deep photospheric layers. Atmospheric parameters are obtained from the inversions done with the SPINOR code (Stokes-Profile-INversion-O-Routines). Then the results are compared with the results from the realistic MHD simulations obtained from MURaM code (MPS/University of Chicago radiative MHD). Comparisons of the results from observations, inversions and numerical simulations show a good agreement, which confirms the usage of this technique as a temperature diagnostic tool. Title: Observations of the solar chromosphere at millimeter wavelengths Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2010MmSAI..81..592L Altcode: Millimeter wavelengths provide a powerful tool to study the thermal structure of the solar chromosphere and its response to dynamic processes. We present initial studies of chromospheric fine structure and its dynamics obtained from observations of the quiet Sun with BIMA at 3.5 mm with a resolution of 12 arcsec. The two-dimensional millimeter maps of the solar chromosphere reveal brightness features corresponding to supergranular network boundaries and bright points within cells. Significant intensity oscillations with frequencies of 1.5 - 8.0 mHz with a tendency toward shorter-period oscillations in the internetwork and longer periods in network regions are found in the data. However, higher spatial resolution is required for a more detailed study. We discuss the requirements imposed on observations in the millimeter domain that might provide an insight into the fundamental questions of solar physics. We also review the capabilities of the current and future millimeter-wave interferometers, including the CARMA and ALMA arrays. Title: Solar Spectral Irradiance variations: the long-term view Authors: Solanki, Sami K. Bibcode: 2010cosp...38...19S Altcode: 2010cosp.meet...19S Solar Spectral Irradiance variations: the long-term view S.K. Solanki Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany The total and spectral irradiance of the Sun are prime candidates for external forcing of the Earth's climate system. Whereas variations of total solar irradiance have been observed and modelled with great success, our corresponding knowledge of the variation of spectral irradi-ance is, by comparison, still in its infancy. Although, on short time-scales multiple measured time series and models are now available, on longer time-scales models are the only source of information. Until recently even the models were rather restricted, being either based purely on proxies, or limited in spectral range. A new generation of models is now available, which goes beyond the limitations of the earlier approaches. An overview of the models and their results is given, with the emphasis being placed on the most recent ones. Title: How do inaccuracies and unresolved structures in the measured solar photospheric magnetic vector influence the accuracy of coronal magnetic field models? Authors: Wiegelmann, Thomas; Yelles, Lotfi; Solanki, Sami; Lagg, Andreas Bibcode: 2010cosp...38.2827W Altcode: 2010cosp.meet.2827W Solar magnetic fields are usually extrapolated into the corona starting from photospheric mag-netic measurements that can suffer from significant uncertainties caused by the presence of unresolved fine structures due to a limited spatial resolution and by the limited polarimet-ric accuracy and spectral resolution. We investigate how these inaccuracies in the measured photospheric magnetic field vector influence the accuracy of the extrapolated coronal mag-netic field. We compare non-linear force-free coronal magnetic field extrapolations from a 3-D radiation MHD simulation snapshot with corresponding extrapolations from synthetic vector magnetograms. The synthetic magnetograms contain instrumental effects such as noise, limited spatial and spectral resolution and the effect of employing a filter instrument. The parameters are chosen in particular to mimic the effects of the Hinode/SOT and the future Solar Orbiter PHI instruments, respectively. We discuss, how photospheric structures, which are inherent in the MHD snapshot, but not resolved in the photospheric magnetic field measurements, influence the quality of the computed coronal magnetic field model. Title: Accelerating disturbances in polar plume and inter-plume Authors: Gupta, Girjesh R.; Banerjee, Dipankar; Teriaca, Luca; Imada, Shinsuke; Solanki, Sami Bibcode: 2010cosp...38.2937G Altcode: 2010cosp.meet.2937G We present EIS/Hinode & SUMER/SoHO joint observations allowing the first spectroscopic detection of accelerating disturbances as recorded with coronal lines in inter-plume and plume regions of a polar coronal hole. From time-distance radiance maps, we detect the presence of propagating disturbances in a polar inter-plume region with a period of 15 to 20 min and a propagation speed increasing from 130±14 km/s just above the limb, to 330±140 km/s around 160" above the limb. These disturbances can also be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed was found to be in the range of 25±1.3 to 38±4.5 km/s, with the same periodicity. These on-disk bright regions can be vi-sualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbance with the same range of period but with propagation speeds in the range of 135±18 to 165±43 km/s only. A comparison between the time-distance radiance map of both regions, indicate that the disturbances within the plumes are not observable (may be getting dissipated) far off-limb whereas this is not the case in the inter-plume region. Conclu-sions drawn from these observations in terms of accelerating waves or high speed jets/upflows will be discussed. Title: High resolution imaging and polarimetry with SUNRISE, a balloon-borne stratospheric solar observatory Authors: Barthol, Peter; Chares, Bernd; Deutsch, Werner; Feller, Alex; Gandorfer, Achim; Grauf, Bianca; Hirzberger, Johann; Meller, Reinhard; Riethmueller, Tino; Schuessler, Manfred; Solanki, Sami K.; Knoelker, Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan Bibcode: 2010cosp...38.4063B Altcode: 2010cosp.meet.4063B SUNRISE is an international collaboration for the development and operation of a meter-class balloon-borne stratospheric solar observatory. Prime science goal is the study of structure and dynamics of the magnetic field in the solar atmosphere and the interaction of the magnetic field with convective plasma flows. These processes are studied by high resolution imaging in the UV and polarimetry at visible wavelengths. The instrument has been successfully launched on June 8, 2009 from ESRANGE, Kiruna, Northern Sweden. During the more than 5 days flight about 1.5 TByte of scientific data were collected. The paper gives an overview of the instrument and mission, examples of the scientific output will also be presented. SUNRISE is a joint project of the Max-Planck-Institut fuer Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut fuer Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Spanish IMaX consortium. Title: The Ultraviolet Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory: Instrument description and first results Authors: Gandorfer, Achim; Barthol, Peter; Feller, Alex; Grauf, Bianca; Hirzberger, Johann; Riethmueller, Tino; Solanki, Sami K.; Berkefeld, Thomas; Knoelker, Michael; Martinez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan Bibcode: 2010cosp...38.4064G Altcode: 2010cosp.meet.4064G We describe the design of the near UV filter imager SuFi onboard Sunrise, which was successfully flown in the stratosphere in June 2009. During its five days flight SuFI captured the highest contrast images of solar granulation ever. SuFI is a diffraction limited filter imager with an effective focal length of 121m, working in 5 distinct wavelength bands between 210nm and 397nm. It is based on a two mirror modified Schwarzschild microscope, which is integral part of the central Image stabilization and light Distribution unit (ISLiD) of Sunrise, which acts as the reimaging optics between the 1m telescope and the science instruments. The key technical features of the instrument are presented under the view of the specific demands of balloon-borne optical systems. First results obtained with the instrument are presented to demonstrate the capabilities of the instrument. Title: UV intensity distributions of the quiet Sun observed with Sunrise Authors: Hirzberger, Johann; Feller, A.; Riethmueller, T.; Borrero, J. M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knoelker, M.; Martínez Pillet, V.; Schmidt, W.; Solanki, S.; Title, A. Bibcode: 2010cosp...38.1735H Altcode: 2010cosp.meet.1735H High resolution solar images in the near UV have been obtained with the Solar UV Filtergraph (SUFI) onboard the Sunrise balloon borne observatory, amongst others in wavelength regions not accessible from the ground. We present intensity distributions of the quiet Sun at different heliocentric angles, from disk center to the solar limb. These results, obtained in spectral windows at 214 nm, 313 nm (OH band), 388 nm (CN band) and 396.7 nm (CaIIH), represent an important validation of numerical models of the solar photosphere and are, thus, fundamental ingredients for our understanding of the thermal processes in the solar surface region. Title: Magnetic field intensification: comparison of 3D MHD simulations with Hinode/SP results Authors: Danilovic, S.; Schüssler, M.; Solanki, S. K. Bibcode: 2010A&A...509A..76D Altcode: 2009arXiv0910.1211D Context. Recent spectro-polarimetric observations have provided detailed measurements of magnetic field, velocity and intensity during events of magnetic field intensification in the solar photosphere.
Aims: By comparing with synthetic observations derived from MHD simulations, we investigate the physical processes underlying the observations, as well as verify the simulations and the interpretation of the observations.
Methods: We consider the temporal evolution of the relevant physical quantities for three cases of magnetic field intensification in a numerical simulation. In order to compare with observations, we calculate Stokes profiles and take into account the spectral and spatial resolution of the spectropolarimeter (SP) on board Hinode. We determine the evolution of the intensity, magnetic flux density and zero-crossing velocity derived from the synthetic Stokes parameters, using the same methods as applied to the Hinode/SP observations to derive magnetic field and velocity information from the spectro-polarimetric data.
Results: The three events considered show a similar evolution: advection of magnetic flux to a granular vertex, development of a strong downflow, evacuation of the magnetic feature, increase of the field strength and the appearance of the bright point. The magnetic features formed have diameters of 0.1-0.2´´. The downflow velocities reach maximum values of 5-10 km s-1 at τ = 1. In the largest feature, the downflow reaches supersonic speed in the lower photosphere. In the same case, a supersonic upflow develops approximately 200 s after the formation of the flux concentration. We find that synthetic and real observations are qualitatively consistent and, for one of the cases considered, also agree very well quantitatively. The effect of finite resolution (spatial smearing) is most pronounced in the case of small features, for which the synthetic Hinode/SP observations miss the bright point formation and also the high-velocity downflows during the formation of the smaller magnetic features.
Conclusions: The observed events are consistent with the process of field intensification by flux advection, radiative cooling, and evacuation by strong downflow found in MHD simulations. The quantitative agreement of synthetic and real observations indicates the validity of both the simulations and the interpretations of the spectro-polarimetric observations. Title: Radiative emission of solar features in Ca II K Authors: Criscuoli, S.; Ermolli, I.; Fontenla, J.; Giorgi, F.; Rast, M.; Solanki, S. K.; Uitenbroek, H. Bibcode: 2010MmSAI..81..773C Altcode: 2010arXiv1002.0244C We investigated the radiative emission of different types of solar features in the spectral range of the Ca II K line. We analyzed full-disk 2k × 2k observations from the Precision Solar Photometric Telescope (PSPT). The data were obtained by using three narrow-band interference filters that sample the Ca II K line with different pass bands. Two filters are centered in the line core, the other in the red wing of the line. We measured the intensity and contrast of various solar features, specifically quiet Sun (inter-network), network, enhanced network, plage, and bright plage (facula) regions. Moreover, we compared the results obtained with those derived from the numerical synthesis performed for the three PSPT filters with a widely used radiative code on a set of reference semi-empirical atmosphere models. Title: Analysis of Ca II K spectroheliogram time-series for solar irradiance studies Authors: Ermolli, Ilaria; Krivova, Natalie; Solanki, Sami K. Bibcode: 2010cosp...38..133E Altcode: 2010cosp.meet..133E Historic Ca II K observations constitute an extremely valuable resource for many research topics, including investigations of solar activity and irradiance variations on century timescales. In fact, the intensities measured on these observations can serve as proxies for magnetic activity, due to the strong correlation found between the Ca II K emission and the line-of-sight magnetic flux density. We analyzed three time-series of historic Ca II K observations, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson spectroheliohram archives. We examined the quality of these data, in order to estimate their value for irradiance studies focusing on time-scales longer than the solar cycle. We employ different processing techniques in order to investigate whether uniform results on the evolution of the solar magnetic field, both in quiet and active regions, can be obtained for the three analyzed series. ——-Session A1.1 Atmosphere Chemistry and Physics (i) Solar Spectral Irradiance Variations and Their Influence on the Earth's Atmosphere Title: Relation between the Sunrise photospheric magnetic field and the Ca II H bright features Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.; Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A. Bibcode: 2010cosp...38.2856J Altcode: 2010cosp.meet.2856J Recent observations from the Sunrise balloon-borne solar telescope have enabled us to reach an unprecedented high spatial resolution on the solar surface with the near-ultraviolet photo-spheric and chromospheric images as well as the magnetograms. We use these high resolution observations to investigate the structure of the solar upper photosphere and lower chromosphere as well as their temporal evolutions. We study the relation between the inter-granular Ca II 397 nm bright structures in images obtained by the Sunrise Filter Imager (SuFI) and their corresponding photospheric vector magnetic field computed from the Imaging Magnetogram eXperiment (IMaX) observations. The targets under study are in a quiet Sun region and close to disc-centre. Title: Total and spectral irradiance comparisons between SIM and the SATIRE model in the declining phase of cycle 23 Authors: Ball, Will; Unruh, Yvonne; Krivova, Natalie; Solanki, Sami K.; Harder, Jerald Bibcode: 2010cosp...38..136B Altcode: 2010cosp.meet..136B Climate models rely on accurate total and spectral solar irradiance inputs, but until 2003 con-tinuous spectral irradiance information across a large portion of the solar spectrum was lacking. Since the launch of the Solar Radiation and Climate Experiment (SORCE), with the Spectral Irradiance Monitor (SIM) observing the UV, visible and IR, data have been accumulating and now cover a significant portion of a cycle. For the first time this allows spectral models to be tested over periods greater than a solar rotation. We present six years of total and spectral irradiance reconstructions using the SATIRE model that incorporates SOHO/MDI continuum and magnetogram images for the period April 2004 to November 2009 in the declining phase of cycle 23 and through the recent unusual minimum. We compare these results with the SIM instrument and so cover the spectral region 200 -1600 nm. While detrended, short-term, variation is recreated well by the model, there are discrepancies in longer-term trends between observations and the model. This may become important when considering the radiative forcing from the Sun used in climate research and so understanding why there is such a significant disagreement is an important area of investigation. Title: Center-to-Limb Variation of the continuum contrast of quiet Sun magnetic features from Hinode/SP Authors: Kobel, Philippe; Borrero Santiago, Juan Manuel; Solanki, Sami K. Bibcode: 2010cosp...38.1788K Altcode: 2010cosp.meet.1788K The center-to-limb variation (CLV) of the continuum contrast of magnetic features represents an important constraint for flux tube models, as well as a useful input to reproduce the total solar irradiance. However, precise measurements are difficult because the contrast depends on the atmospheric and magnetic parameters of those features, and is severely affected by the observing conditions (seeing). Here we present the first assessment of the dependence of the continuum contrast of magnetic features on both the heliocentric angle and on their magnetic properties, inferred from high spatial resolution spectro-polarimetric maps (Hinode/SP). At the same time, it allowed us to improve the results of previous lower resolution studies of contrast as a function of magnetogram signal. The magnetic parameters (field strength, filling factor and inclination) were inferred by Milne-Eddington inversions (VFISV) of spectro-polarimetric maps covering a continuous range of mu between 1 and 0.2 in the quiet Sun. Our contrasts reach positive values at disk center, and the peak of the contrast as a function of magnetogram signal at different are larger than in previous studies. The contrast was found to behave differently in two distinct regimes of "apparent flux density", characterizing fields that are tied to granulation and strong network-like magnetic features, respectively. The weak fields seem to be "wrapped" around granules and the contrasts reflects their location: pixels with horizontal fields are on average located within granules and are brighter, while pixels with more vertical fields are preferably located in the intergranular lanes and are thus darker. For the stronger fields, the contrast CLV was found to depend essentially on the apparent flux density, with a rather symmetric dependence on the field strength and on the filling factor, while the fields are quasi-vertical. Title: Long term variability of solar magnetic activity: the solar-stellar connection Authors: Solanki, Sami K. Bibcode: 2010cosp...38.2890S Altcode: 2010cosp.meet.2890S Long term variability of solar magnetic activity: the solar-stellar connection S.K. Solanki Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany The dominant time-scales of solar variability are the solar rotation period and in particular the length of the solar cycle, but variations of solar activity on longer time scales are also present. Variations of stellar activity are also well documented on time scales up to stellar activity cycles, although secular variations on longer time scales are somewhat less well established, due to the limited length of the available observational time series. Multi-wavelength observations of solar variability is mainly limited to the space age, while direct observations of sunspots are available back to the year 1610, i.e. over the whole era of telescopic astronomy. In the last decades considerable progress has been made in extending our knowledge of solar activity to pre-telescopic times, using records of the concentration of cosmogenic isotopes in terrestrial archives.Such studies have shown, e.g., that in addition to grand minima, the Sun has also enjoyed times of significantly anhanced activity, so called grand maxima. The recent decades formed a particularly prominent such grand maximum. The current peculiar minimum has shown strong indications that we are in the process of leaving this last grand maximum. There is also strong evidence that there are significant secular changes of the magnetic flux and other active phenomena. Although the period over which we have data on stellar activity is far shorter than for the Sun, these data allow stars of different types to be sampled (i.e. stars with different activity levels, of different spectral types, at different evolutionary stages, etc.) that display the full variety of active phenomena, including cycles, but also chaotic variability. After an overview of our knowledge and understanding of the longer-term evolution of solar activity, comparisons with observations of stellar activity time series are presented and some implications for our understanding of stellar activity variations are deduced. Title: Expansion of Magnetic Flux Concentrations with Height: A Comparison of Hinode SOT Data and MHD Simulations Authors: Pietarila, A.; Cameron, R.; Solanki, S. Bibcode: 2009ASPC..415...91P Altcode: The Hinode SOT (Tsuneta et al. 2008) NFI Na I D1 and SP Fe I data sampled at different positions on the solar disk provide a unique diagnostic for studying the expansion of magnetic flux concentrations with height. We make a comparative study of SOT observations and 2-dimensional (2D) radiative MHD-simulations to see how well the simulations capture the expansion properties. The expansion of flux concentrations is clearly seen in the SOT Na I D1 data, where most of the magnetic features appear unipolar at disk center while close to the limb bipolar appearance strongly dominates. This trend, albeit not as strong, is seen in the SP data as well. Some aspects of the observations are qualitatively reproduced by simulations with a potential (as opposed to vertical) upper boundary condition for the magnetic field. Title: Simulation of a flux emergence event and comparison with observations by Hinode Authors: Yelles Chaouche, L.; Cheung, M. C. M.; Solanki, S. K.; Schüssler, M.; Lagg, A. Bibcode: 2009A&A...507L..53Y Altcode: 2009arXiv0910.5737Y Aims: We study the observational signature of flux emergence in the photosphere using synthetic data from a 3D MHD simulation of the emergence of a twisted flux tube.
Methods: Several stages in the emergence process are considered. At every stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5 Å and Fe I 6302.5 Å and degrade the data to the spatial and spectral resolution of Hinode's SOT/SP. Then, following observational practice, we apply Milne-Eddington-type inversions to the synthetic spectra in order to retrieve various atmospheric parameters and compare the results with recent Hinode observations.
Results: During the emergence sequence, the spectral lines sample different parts of the rising flux tube, revealing its twisted structure. The horizontal component of the magnetic field retrieved from the simulations is close to the observed values. The flattening of the flux tube in the photosphere is caused by radiative cooling, which slows down the ascent of the tube to the upper solar atmosphere. Consistent with the observations, the rising magnetized plasma produces a blue shift of the spectral lines during a large part of the emergence sequence.

Figure 3 is only available in electronic form at http://www.aanda.org Title: Internetwork Horizontal Magnetic Fields in the Quiet Sun Chromosphere: Results from a Joint Hinode/VTT Study Authors: Lagg, A.; Ishikawa, R.; Merenda, L.; Wiegelmann, T.; Tsuneta, S.; Solanki, S. K. Bibcode: 2009ASPC..415..327L Altcode: We present results from a joint Hinode/VTT campaign (May 2008). Spectropolarimetric data of a quiet Sun super-granular network cell at a heliocentric angle of 28° in the He I 10830 Å line were analyzed using an inversion code incorporating Hanle and Zeeman effects (HeLIx^+) to retrieve magnetic field strength and direction in the upper chromosphere. Simultaneously recorded Hinode SOT/SP data reveal the photospheric magnetic field morphology, clearly showing magnetic flux concentrations in the internetwork. The photospheric magnetic field maps are used to feed potential field extrapolations similar to the work by Schrijver & Title (2003). The extrapolated magnetic field structure is compared with the magnetic field configuration resulting from the He 10830 inversions. These inversions also reveal horizontal magnetic structures extending over a length of up to 20 Mm above the internetwork, indicative of the presence of a magnetic canopy. The photospheric magnetic flux concentrations in the internetwork are obviously not sufficiently strong to prevent the formation of a canopy at chromospheric heights. Title: Solar magnetic field and irradiance: how unusual is the current minimum? (Invited) Authors: Solanki, S. K.; Krivova, N.; Vieira, L. A. Bibcode: 2009AGUFM.U34A..01S Altcode: In many respects the current minimum of solar activity is rather different from other minima during the last half century, which has seen the Sun in a state of unusually high activity. This uncommon minimum has fascinated solar physicists. Of particular importance for our understanding is the behaviour of the magnetic field, which is the source of all activity observed in the solar atmosphere, including the heliosphere. Furthermore, the evolution of dark and bright magnetic features on the solar surface modulates the solar irradiance, which could directly influence the Earth's climate. In this presentation observations and models of the evolution of the Sun's magnetic field and irradiance are reviewed. After discussing the variation of these quantities over the solar cycle and from cycle to cycle, the current minimum is considered in greater detail. It is compared to earlier minima going back to the Maunder minimum and, in a more averaged sense, to solar activity and irradiance during the whole Holocene. Questions that will be considered are: Are we leaving the period of high solar activity (a grand maximum) to enter into a phase of more moderate activity, or even into a grand minimum? How could such a change in the level of the magnetic field and irradiance aid us in gaining further insight into the nature of the Sun's influence on the Earth's climate? Title: The Local Dust Foregrounds in the Microwave Sky. I. Thermal Emission Spectra Authors: Dikarev, Valeri; Preuß, Oliver; Solanki, Sami; Krüger, Harald; Krivov, Alexander Bibcode: 2009ApJ...705..670D Altcode: 2009arXiv0910.1317D Analyses of the cosmic microwave background (CMB) radiation maps made by the Wilkinson Microwave Anisotropy Probe (WMAP) have revealed anomalies not predicted by the standard inflationary cosmology. In particular, the power of the quadrupole moment of the CMB fluctuations is remarkably low, and the quadrupole and octopole moments are aligned mutually and with the geometry of the solar system. It has been suggested in the literature that microwave sky pollution by an unidentified dust cloud in the vicinity of the solar system may be the cause for these anomalies. In this paper, we simulate the thermal emission by clouds of spherical homogeneous particles of several materials. Spectral constraints from the WMAP multi-wavelength data and earlier infrared observations on the hypothetical dust cloud are used to determine the dust cloud's physical characteristics. In order for its emissivity to demonstrate a flat, CMB-like wavelength dependence over the WMAP wavelengths (3 through 14 mm), and to be invisible in the infrared light, its particles must be macroscopic. Silicate spheres of several millimeters in size and carbonaceous particles an order of magnitude smaller will suffice. According to our estimates of the abundance of such particles in the zodiacal cloud and trans-Neptunian belt, yielding the optical depths of the order of 10-7 for each cloud, the solar system dust can well contribute 10 μK (within an order of magnitude) in the microwaves. This is not only intriguingly close to the magnitude of the anomalies (about 30 μK), but also alarmingly above the presently believed magnitude of systematic biases of the WMAP results (below 5 μK) and, to an even greater degree, of the future missions with higher sensitivities, e.g., Planck. Title: ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996? Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T. Bibcode: 2009GeoRL..3620101K Altcode: 2009arXiv0911.3817K A gap in the total solar irradiance (TSI) measurements between ACRIM-1 and ACRIM-2 led to the ongoing debate on the presence or not of a secular trend between the minima preceding cycles 22 (in 1986) and 23 (1996). It was recently proposed to use the SATIRE model of solar irradiance variations to bridge this gap. When doing this, it is important to use the appropriate SATIRE-based reconstruction, which we do here, employing a reconstruction based on magnetograms. The accuracy of this model on months to years timescales is significantly higher than that of a model developed for long-term reconstructions used by the ACRIM team for such an analysis. The constructed ‘mixed’ ACRIM — SATIRE composite shows no increase in the TSI from 1986 to 1996, in contrast to the ACRIM TSI composite. Title: High-resolution spectro-polarimetry of a flaring sunspot penumbra Authors: Hirzberger, J.; Riethmüller, T.; Lagg, A.; Solanki, S. K.; Kobel, P. Bibcode: 2009A&A...505..771H Altcode: 2009arXiv0908.3803H We present simultaneous photospheric and chromospheric observations of the trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary Islands. High-resolution Ca II H images show a typical two-ribbon structure that has been hitherto only known for larger flares, and the flare appears in a confined region that is discernible by a bright border. The underlying photosphere shows a disturbed penumbral structure with intersecting branches of penumbral filaments. High-resolution Doppler- and vector-magnetograms exhibit oppositely directed Evershed flows and magnetic field vectors in the individual penumbral branches, resulting in several regions of magnetic azimuth discontinuity and several islands where the vertical magnetic field is reversed. The discontinuity regions are co-spatial with the locations of the onset of the flare ribbons. From the results, we conclude that the confined flare region is detached from the global magnetic field structure by a separatrix marked by the bright border visible in C II H. We further conclude that the islands of reversed vertical field appear because of flux emergence and that the strong magnetic shear appearing in the regions of magnetic azimuth discontinuity triggers the flare.

Movies are only available in electronic form at http://www.aanda.org Title: Comparison of the thin flux tube approximation with 3D MHD simulations Authors: Yelles Chaouche, L.; Solanki, S. K.; Schüssler, M. Bibcode: 2009A&A...504..595Y Altcode: 2009arXiv0910.1796Y Context: The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation.
Aims: We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation.
Methods: We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box.
Results: Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures. Title: Reconstruction of solar UV irradiance since 1974 Authors: Krivova, N. A.; Solanki, S. K.; Wenzler, T.; Podlipnik, B. Bibcode: 2009JGRD..114.0I04K Altcode: 2009arXiv0907.1500K; 2009JGRD..11400I04K Variations of the solar UV irradiance are an important driver of chemical and physical processes in the Earth's upper atmosphere and may also influence global climate. Here we reconstruct solar UV irradiance in the range 115-400 nm over the period 1974-2007 by making use of the recently developed empirical extension of the Spectral And Total Irradiance Reconstruction (SATIRE) models employing Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) data. The evolution of the solar photospheric magnetic flux, which is a central input to the model, is described by the magnetograms and continuum images recorded at the Kitt Peak National Solar Observatory between 1974 and 2003 and by the Michelson Doppler Imager instrument on SOHO since 1996. The reconstruction extends the available observational record by 1.5 solar cycles. The reconstructed Ly-α irradiance agrees well with the composite time series by Woods et al. (2000). The amplitude of the irradiance variations grows with decreasing wavelength and in the wavelength regions of special interest for studies of the Earth's climate (Ly-α and oxygen absorption continuum and bands between 130 and 350 nm) is 1-2 orders of magnitude stronger than in the visible or if integrated over all wavelengths (total solar irradiance). Title: Bright fibrils in Ca II K Authors: Pietarila, A.; Hirzberger, J.; Zakharov, V.; Solanki, S. K. Bibcode: 2009A&A...502..647P Altcode: 2009arXiv0905.3124P Context: Except for the Ca II resonance lines, fibrils are ubiquitously present in most high-resolution observations of chromospheric lines.
Aims: We show that fibrils are also a prevailing feature in Ca II K, provided the spatial-resolution is sufficiently high.
Methods: We present high spatial resolution observations of an active region in the Ca II K line from the Swedish Solar Telescope. Through a comparison between photospheric intensity and magnetic field data, we study the connection between bright chromospheric fibrils and photospheric structures. Additionally, using Fourier analysis we study how the fibrils are linked to the observed dynamics.
Results: We find that very narrow, bright fibrils are a prevailing feature over large portions of the observed field. We also find a clear connection between the fibril footpoints and photospheric magnetic features. We show that the fibrils play two distinct roles in the observed dynamics: depending on their location they can act as a canopy suppressing oscillations or they can channel low-frequency oscillations into the chromosphere.
Conclusions: The Ca II K fibrils share many characteristics with fibrils observed in other chromospheric lines, but some features, such as the very small widths, are unique to these observations. Title: Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter, M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.; Unruh, Y.; Vögler, A. Bibcode: 2009SSRv..145..337D Altcode: The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we review the current status of solar irradiance measurements and modelling efforts based on solar photospheric magnetic fields. Thereby we restrict ourselves to the study of solar variations from days to the solar cycle. Phenomenological models of the solar atmosphere in combination with imaging observations of solar electromagnetic radiation and measurements of the photospheric magnetic field have reached high enough quality to show that a large fraction (at least, about 80%) of the solar irradiance variability can be explained by the radiative effects of the magnetic activity present in the photosphere. Also, significant progress has been made with magnetohydrodynamic simulations of convection that allow us to relate the radiance of the photospheric magnetic structures to the observations. Title: Stray light correction and contrast analysis of Hinode broad-band images Authors: Mathew, S. K.; Zakharov, V.; Solanki, S. K. Bibcode: 2009A&A...501L..19M Altcode: 2009arXiv0906.2451M The contrasts of features in the quiet Sun are studied using filtergrams recorded by the broad-band filter imager mounted on the Hinode/Solar Optical Telescope. In a first step, the scattered light originating in the instrument is modeled using Mercury transit data. Combinations of four two-dimensional Gaussians with different widths and weights were employed to retrieve the point-spread functions (PSF) of the instrument at different wavelengths, which also describe instrumental scattered light. The parameters of the PSFs at different wavelengths are tabulated. The observed images were then deconvolved using the PSFs. The corrected images were used to determine the contrasts of features such as bright points and granulation in different wavelength bands. After correction, rms contrasts of the granulation of between 0.11 (at 668 nm) and 0.22 (at 388 nm) were obtained. Similarly, bright point contrasts ranging from 0.07 (at 668 nm) to 0.78 (at 388 nm) are found, which are a factor of 1.8 to 2.8 higher than those obtained before PSF deconvolution. The mean contrast of the bright points is found to be somewhat higher in the CN-band than in the G-band, which confirms theoretical predictions.

Appendices are only available in electronic form at http://www.aanda.org Title: A homogeneous database of sunspot areas covering more than 130 years Authors: Balmaceda, L. A.; Solanki, S. K.; Krivova, N. A.; Foster, S. Bibcode: 2009JGRA..114.7104B Altcode: 2009JGRA..11407104B; 2009arXiv0906.0942B The historical record of sunspot areas is a valuable and widely used proxy of solar activity and variability. The Royal Greenwich Observatory regularly measured this and other parameters between 1874 and 1976. After that time records from a number of different observatories are available. These, however, show systematic differences and often have significant gaps. Our goal is to obtain a uniform and complete sunspot area time series by combining different data sets. A homogeneous composite of sunspot areas is essential for different applications in solar physics, among others for irradiance reconstructions. Data recorded simultaneously at different observatories are statistically compared in order to determine the intercalibration factors. Using these data we compile a complete and cross-calibrated time series. The Greenwich data set is used as a basis until 1976, the Russian data (a compilation of observations made at stations in the former USSR) are used between 1977 and 1985, and data compiled by the USAF network are used since 1986. Other data sets (Rome, Yunnan, and Catania) are used to fill up the remaining gaps. Using the final sunspot areas record the Photometric Sunspot Index is calculated. We also show that the use of uncalibrated sunspot areas data sets can seriously affect the estimate of irradiance variations. Our analysis implies that there is no basis for the claim that UV irradiance variations have a much smaller influence on climate than total solar irradiance variations. Title: Stereoscopic Polar Plume Reconstructions from STEREO/SECCHI Images Authors: Feng, L.; Inhester, B.; Solanki, S. K.; Wilhelm, K.; Wiegelmann, T.; Podlipnik, B.; Howard, R. A.; Plunkett, S. P.; Wuelser, J. P.; Gan, W. Q. Bibcode: 2009ApJ...700..292F Altcode: 2009arXiv0908.2365F We present stereoscopic reconstructions of the location and inclination of polar plumes of two data sets based on the two simultaneously recorded images taken by the EUVI telescopes in the SECCHI instrument package onboard the Solar TErrestrial RElations Observatory spacecraft. The 10 plumes investigated show a superradial expansion in the coronal hole in three dimensions (3D) which is consistent with the two-dimensional results. Their deviations from the local meridian planes are rather small with an average of 6fdg47. By comparing the reconstructed plumes with a dipole field with its axis along the solar rotation axis, it is found that plumes are inclined more horizontally than the dipole field. The lower the latitude is, the larger is the deviation from the dipole field. The relationship between plumes and bright points has been investigated and they are not always associated. For the first data set, based on the 3D height of plumes and the electron density derived from SUMER/SOHO Si VIII line pair, we found that electron densities along the plumes decrease with height above the solar surface. The temperature obtained from the density scale height is 1.6-1.8 times larger than the temperature obtained from Mg IX line ratios. We attribute this discrepancy to a deviation of the electron and the ion temperatures. Finally, we have found that the outflow speeds studied in the O VI line in the plumes corrected by the angle between the line of sight and the plume orientation are quite small with a maximum of 10 km s-1. It is unlikely that plumes are a dominant contributor to the fast solar wind. Title: Discriminant analysis of solar bright points and faculae. I. Classification method and center-to-limb distribution Authors: Kobel, P.; Hirzberger, J.; Solanki, S. K.; Gandorfer, A.; Zakharov, V. Bibcode: 2009A&A...502..303K Altcode: 2010arXiv1001.5143K Context: While photospheric magnetic elements appear mainly as Bright Points (BPs) at the disk center and as faculae near the limb, high-resolution images reveal the coexistence of BPs and faculae over a range of heliocentric angles. This is not explained by a “hot wall” effect through vertical flux tubes, and suggests that the transition from BPs to faculae needs to be quantitatively investigated.
Aims: To achieve this, we made the first recorded attempt to discriminate BPs and faculae, using a statistical classification approach based on Linear Discriminant Analysis (LDA). This paper gives a detailed description of our method, and shows its application on high-resolution images of active regions to retrieve a center-to-limb distribution of BPs and faculae.
Methods: Bright “magnetic” features were detected at various disk positions by a segmentation algorithm using simultaneous G-band and continuum information. By using a selected sample of those features to represent BPs and faculae, suitable photometric parameters were identified for their discrimination. We then carried out LDA to find a unique discriminant variable, defined as the linear combination of the parameters that best separates the BPs and faculae samples. By choosing an adequate threshold on that variable, the segmented features were finally classified as BPs and faculae at all the disk positions.
Results: We thus obtained a Center-to-Limb Variation (CLV) of the relative number of BPs and faculae, revealing the predominance of faculae at all disk positions except close to disk center (μ ≥ 0.9).
Conclusions: Although the present dataset suffers from limited statistics, our results are consistent with other observations of BPs and faculae at various disk positions. The retrieved CLV indicates that at high resolution, faculae are an essential constituent of active regions all across the solar disk. We speculate that the faculae near disk center as well as the BPs away from disk center are associated with inclined fields.

Figures 11-14 are only available in electronic form at http://www.aanda.org Title: Center to Limb Distribution of Bright Points and Faculae: First Results of an Automated Detection Algorithm Authors: Kobel, P.; Hirzberger, J.; Zakharov, V.; Gandorfer, A.; Solanki, S. K. Bibcode: 2009ASPC..405..211K Altcode: Center to limb variations (CLV) of photospheric Bright Points (BPs) and faculae are important to understand the fundamental relationship between these magnetic features. In this context, we present a statistical study of the center to limb distribution of BPs and faculae in active regions. Magnetic brightenings were detected at various disk positions by an automated segmentation algorithm based on joint G-band and continuum information. They were then classified as BPs or faculae according to a linear discriminant analysis, which allowed to determine the relative fraction of the two classes at each disk position. Title: Spectropolarimetric Investigations of the Deep Photospheric Layers of Solar Magnetic Structures Authors: Oklay, N.; Gandorfer, A.; Solanki, S. K.; Bianda, M.; Ramelli, R. Bibcode: 2009ASPC..405..233O Altcode: Solar surface magnetism manifests itself in a variety of structures with sizes often comparable or even below our spatial resolution capabilities. Nevertheless, sub-resolution information about the intrinsic atmospheric structure can be obtained via indirect techniques. We use state-of-the-art spectropolarimetric observations in carefully selected photospheric lines which include C~I~(5380.3~Å) as well as strong lines of Fe~I, Ti~I covering also the deep layers of the photosphere and obtain ratios of their Stokes V amplitudes. From there we deduce that the temperature within magnetic features is higher at locations of smaller magnetic flux. Title: Reconstructed and measured total solar irradiance: Is there a secular trend between 1978 and 2003? Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A. Bibcode: 2009GeoRL..3611102W Altcode: 2009arXiv0909.2975W Total solar irradiance reconstructed between 1978 and 2003 using solar surface magnetic field distributions is compared with three composites of total solar irradiance measurements. A good correspondence is found with the total solar irradiance composite from PMOD/WRC, with no bias between the three cycles. The agreement with the other composites (the ACRIM composite, mainly based on the Active Cavity Radiometer Irradiance Monitors I, II, and III, and the IRMB composite from the Institut Royal Meteorologique Belgique) is significantly poorer. In particular, a secular increase in the irradiance exhibited by these composites is not present in the reconstructions. Hence any secular trend in total solar irradiance between 1978 and 2003 is not due to magnetic fields at the solar surface. Title: Photospheric Magnetic Field: Quiet Sun Authors: Solanki, S. K. Bibcode: 2009ASPC..405..135S Altcode: The solar photosphere is the layer in which the magnetic field has been most reliably and most often measured. Zeeman- and Hanle-effect based probes have revealed many details of a rich variety of structures and dynamic processes, but the number of open and debated questions has remained large. The magnetic field in the quiet Sun has maintained a particularly large number of secrets and has been a topic of a particularly lively debate as new observations and analysis techniques have revealed new and often unexpected aspects of its organization, physical structure and origin. Title: Spectropolarimetric Diagnostics at the Solar Photosphere near the Limb Authors: Yelles Chaouche, L.; Solanki, S. K.; Rouppe van der Voort, L.; van Noort, M. Bibcode: 2009ASPC..405..189Y Altcode: 2009arXiv0910.2885Y In the present work, we investigate the formation of Stokes profiles and spectro-polarimetric diagnostics in an active region plage near the limb. We use 3-D radiation-MHD simulations with unipolar fields of an average strength of 400 G, which is largely concentrated in flux tubes in which the field reaches typical kilo-Gauss values. We generate synthetic Stokes spectra by radiative transfer calculations, then we degrade the simulated Stokes signal to account for observational conditions. The synthetic data treated in this manner are compared with and found to roughly reproduce spectro-polarimetric high-resolution observations at μ=0.39 obtained by the SOUP instrument with the Swedish 1-m Solar Telescope at the beginning of 2006. Title: Full Magnetic Field Vector of an Emerging Flux Region Authors: Xu, Z.; Lagg, A.; Solanki, S. K. Bibcode: 2009ASPC..405..223X Altcode: We present maps of the full magnetic field vector of an emerging flux region in both the photosphere and chromosphere. The magnetic and velocity structure of freshly emerged loops is determined. Title: Comparison Among Ca II K Spectroheliogram Time Series with an Application to Solar Activity Studies Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.; Ulrich, R. K.; Singh, J. Bibcode: 2009ApJ...698.1000E Altcode: Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line in the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image quality and contents of three Ca II K spectroheliogram time series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives, in order to estimate their value for studies focusing on timescales longer than the solar cycle. We analyze the quality of these data and compare the results obtained with those achieved for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We also investigate whether image-segmentation techniques, such as those developed for identification of plage regions on present-day Ca II K observations, can be used to process historic series. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. The segmentation technique tested in this study gives reasonably consistent results for the three analyzed series after application of a simple photographic calibration. Although the plage areas measured from the three analyzed series differ somewhat, the difference to previously published results is larger. Title: Multi-Channel Observations of a Solar Flare Authors: Hirzberger, J.; Riethmüller, T.; Solanki, S. K.; Kobel, P. Bibcode: 2009ASPC..405..125H Altcode: On August 13, 2006 we performed simultaneous observations in Ca IIH, G-band and Fe I 6303 Å of a complex sunspot in NOAA~10904 with the Swedish Solar Telescope (SST) on La Palma, Canary Islands. From spectro-polarimetric scans through the Fe I line with the tunable SOUP filter we computed the full Stokes vectors at each pixel of the field of view. At 8:47 UT a weak flare eruption (GOES class B7.8) was registered in the line core of Ca IIH. We present the changing magnetic field and flow topologies in the underneath penumbral photosphere during the flaring phase. The unmatched spatial resolution of SST observations allows detailed simultaneous mapping of chromospheric and photospheric events. Title: Hanle Effect Diagnostics of the Coronal Magnetic Field: A Test Using Realistic Magnetic Field Configurations Authors: Raouafi, N. -E.; Solanki, S. K.; Wiegelmann, T. Bibcode: 2009ASPC..405..429R Altcode: 2008arXiv0801.2202R Our understanding of coronal phenomena, such as coronal plasma thermodynamics, faces a major handicap caused by missing coronal magnetic field measurements. Several lines in the UV wavelength range present suitable sensitivity to determine the coronal magnetic field via the Hanle effect. The latter is a largely unexplored diagnostic of coronal magnetic fields with a very high potential. Here we study the magnitude of the Hanle-effect signal to be expected outside the solar limb due to the Hanle effect in polarized radiation from the H I Lyα and β lines, which are among the brightest lines in the off-limb coronal FUV spectrum. For this purpose we use a magnetic field structure obtained by extrapolating the magnetic field starting from photospheric magnetograms. The diagnostic potential of these lines for determining the coronal magnetic field, as well as their limitations are studied. We show that these lines, in particular H I Lyβ, are useful for such measurements. Title: Propagating waves in polar coronal holes as seen by SUMER & EIS Authors: Banerjee, D.; Teriaca, L.; Gupta, G. R.; Imada, S.; Stenborg, G.; Solanki, S. K. Bibcode: 2009A&A...499L..29B Altcode: 2009arXiv0905.1013B Context: To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode.
Aims: Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties.
Methods: The observations analysed here consist of SUMER spectra of the Ne viii 770 Å line (T = 0.6 MK) and EIS slot images in the Fe xii 195 Å line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions.
Results: We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km s-1 (Ne viii) to 125 km s-1 (Fe xii). These velocities are subsonic and roughly in the same ratio as the respective sound speeds.
Conclusions: We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind. Title: Constraints on O5+ Ions Kinetic Temperature Anisotropies in the Polar Coronal Holes Authors: Raouafi, Noureddine; Solanki, S. Bibcode: 2009SPD....40.1404R Altcode: We present new constraints on the kinetic temperature anisotropies of the O5+ ions in the polar coronal holes. Our analysis is based on the observations of the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) spacecraft. We find that the observations are consistently reproduced when considering anisotropies less than 5 between heliocentric distances 2.0 Rsun and 2.7 Rsun. Below 2.0 Rsun and above 2.7 Rsun no anisotropies or Tperpendicular/Tparallel significantly smaller than 5 are sufficient to obtain the observed spectral line profiles. Our analysis provides also height dependencies of Tperpendicular and Tparallel. We also discuss the possible existence of other mechanisms that may reduce kinetic temperature anisotropies in the polar coronal holes. Title: On the relation between photospheric magnetic field and chromospheric emission in the quiet Sun Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen M. Bibcode: 2009IAUS..259..185L Altcode: In this contribution we present an observational study of the interaction of the photosphere with different chromospheric layers. We study the correlations between emissions at varying temperature from the temperature minimum region (UV continuum at 1600 Å from TRACE) through the low chromosphere (CaII K-line from BBSO) to the middle chromosphere (continuum at 3.5 mm from BIMA) and photospheric magnetic field from MDI/SOHO. For the first time millimeter observational data are included in such analysis.

We report a high degree of correlation between considered emissions formed at different heights in the chromosphere. A power law is found to be a good representation for the relationship between photospheric magnetic field and chromospheric emissions at all considered wavelengths. Our analysis shows that the dependence of chromospheric intensities on magnetic field is different for the network and internetwork regions. In the network a power law provides the best fit with the exponent being close to 0.5-0.6, while almost no dependence of chromospheric intensity on magnetic flux is found for the cell interiors. The obtained results support the idea of different heating mechanisms acting in the network (magnetic) and cell interiors (acoustic). Title: The relationship between chromospheric emissions and magnetic field strength Authors: Loukitcheva, M.; Solanki, S. K.; White, S. M. Bibcode: 2009A&A...497..273L Altcode: 2009arXiv0910.1985L Aims: We analyze observational data from 4 instruments to study the correlations between chromospheric emission, spanning the heights from the temperature minimum region to the middle chromosphere, and photospheric magnetic field.
Methods: The data consist of radio images at 3.5 mm from the Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 Å from TRACE, Ca II K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric magnetograms. For the first time interferometric millimeter data with the highest currently available resolution are included in such an analysis. We determine various parameters of the intensity maps and correlate the intensities with each other and with the magnetic field.
Results: The chromospheric diagnostics studied here show a pronounced similarity in their brightness structures and map out the underlying photospheric magnetic field relatively well. We find a power law to be a good representation of the relationship between photospheric magnetic field and emission from chromospheric diagnostics at all wavelengths. The dependence of chromospheric brightness on magnetic field is found to be different for network and internetwork regions. Title: Small-Scale Solar Magnetic Fields Authors: de Wijn, A. G.; Stenflo, J. O.; Solanki, S. K.; Tsuneta, S. Bibcode: 2009SSRv..144..275D Altcode: 2008SSRv..tmp..190D; 2008SSRv..tmp..191D; 2008arXiv0812.4465D As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research. Title: Magnetic geometries of Sun-like stars: exploring the mass-rotation plane Authors: Petit, Pascal; Dintrans, B.; Aurière, M.; Catala, C.; Donati, J. -F.; Fares, R.; Gastine, T.; Lignières, F.; Morgenthaler, A.; Morin, J.; Paletou, F.; Ramirez Velez, J.; Solanki, S. K.; Théado, S. Bibcode: 2009IAUS..259..441P Altcode: Sun-like stars are able to continuously generate a large-scale magnetic field through the action of a dynamo. Various physical parameters of the star are able to affect the dynamo output, in particular the rotation and mass. Using the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France), it is now possible to measure the large-scale magnetic field of solar analogues (i.e. stars very close to the Sun in the mass-rotation plane, including strict solar twins). From spectropolarimetric time-series, tomographic inversion enables one to reconstruct the field geometry and its progressive distortion under the effect of surface differential rotation. We show the first results obtained on a sample of main-sequence dwarfs, probing masses between 0.7 and 1.4 solar mass and rotation rates between 1 and 3 solar rotation rate. Title: On the common solar signal in different cosmogenic isotope data sets Authors: Usoskin, Ilya G.; Horiuchi, Kazuho; Solanki, Sami; Kovaltsov, Gennady A.; Bard, Edouard Bibcode: 2009JGRA..114.3112U Altcode: 2009JGRA..11403112U In this article, we aim to determine frequency ranges and intervals of time in which the solar signal dominates in different cosmogenic isotope data. From a 14C-based reconstruction of cosmic ray intensity over the last millennia, we computed expected 10Be variations in two Antarctic sites (Dom Fuji and South Pole) and two Greenland sites (Dye-3 and GISP-2) and compared them with the actually measured 10Be abundance at the sites. By applying different methods of analysis, such as bivariate correlation, conventional FFT coherence, and wavelet coherence, we found the following: (1) The modeled series, on the basis of 14C data, are in good agreement with the measured 10Be data sets, on different timescales and at different locations, confirming the existence of a common solar signal in both isotope data. (2) The 10Be data are driven by the solar signal on timescales from about 100 years up to 1000 years or even to multimillennial scales (at the longer scales, paleomagnetism plays an increasingly important role). (3) The local climate dominates the 10Be data mostly on short (<100 years) timescales, but the solar signal becomes important even at short scales during periods of Grand minima of solar activity. (4) There is an indication of a possible systematic uncertainty in the early Holocene, likely due to a not-perfectly-stable thermohaline circulation, which requires additional studies. We have shown that both 14C- and 10Be-based records are consistent with each other over a wide range of timescales and time intervals. They form a robust basis for quantitative reconstructions of solar activity variations in the past. Title: Multiple-spacecraft study of an extended magnetic structure in the solar wind Authors: Ruan, P.; Korth, A.; Marsch, E.; Inhester, B.; Solanki, S.; Wiegelmann, T.; Zong, Q. -G.; Bucik, R.; Fornacon, K. -H. Bibcode: 2009JGRA..114.2108R Altcode: 2009JGRA..11402108R An extended magnetic structure was observed consecutively by five spacecraft (ACE, WIND, STEREO A and B, and CLUSTER) in the solar wind on 15 January 2007. The similar bipolar magnetic field variations from five spacecraft suggest that the magnetic structure is two-dimensional. The abrupt disappearance of the beam electrons in the core of the structure suggests that the core of the structure is magnetically isolated from the surrounding environment. Our analysis shows that this magnetic structure is a magnetic flux rope, which extends over at least 180 R E in space. The length and orientation of the flux rope were determined by a local minimum variance analysis (MVA) from individual spacecraft observations of the magnetic field and a timing analysis based on the joint observations by all five spacecraft. The results show that the orientation of the flux rope stays constant in space and time. The flux rope is embedded in a corotating interaction region (CIR), which followed a magnetic cloud. Title: Zeeman Broadening in Cool Stars Authors: Anderson, Richard I.; Reiners, Ansgar; Solanki, Sami K.; Lagg, Andreas Bibcode: 2009AIPC.1094..708A Altcode: 2009csss...15..708A We investigate detectability of magnetic fields by Zeeman broadening of well-isolated spectral lines in F, G and K type stars. Data of unprecedented quality were taken with CES1 mounted on the 3.6 m ESO telescope at La Silla, Chile, in three campaigns in the optical range between 5770 Å and 6280 Å, each with a wavelength coverage of roughly 40 Å. We use the SPINOR/STOPRO (cf. Frutiger et al. [1]) package developed by ETH2 and MPS3 to perform spectral line inversion via χ2 minimization. Starting from constraints given by previous measurements of stellar parameters, we fit a number of extracted spectral lines. Eventually, our goal is to determine the product of the magnetic field strength B and the surface filling factor, B×f.

Our work is in progress and thus no final measurements can be presented at this stage. Title: Slow magnetoacoustic standing waves in a curved solar coronal slab Authors: Ogrodowczyk, R.; Murawski, K.; Solanki, S. K. Bibcode: 2009A&A...495..313O Altcode: Aims: We consider a model of a two-dimensional solar coronal arcade to explore the effects of a curved magnetic field topology on excitation and attenuation of slow magnetoacoustic standing waves.
Methods: The time-dependent ideal magnetohydrodynamic equations are solved numerically to find the spatial and temporal signatures of these waves.
Results: A pulse in gas pressure initially launched at a loop footpoint excites the fundamental mode of slow magnetoacoustic standing waves. The typical excitation time of such a wave mode is 2.5 wave periods, with a similar attenuation timescale. These values are remarkably similar to those recovered from observations by SOHO/SUMER in an Fe XIX line.
Conclusions: Slow magnetoacoustic standing waves are excited and attenuated more efficiently in curved magnetic field lines than in a straight magnetic slab topology. The waves supported by the magnetic arcade are in far closer agreement with observations. Title: Towards Long-Term Solar Irradiance Modelling: Network Contrasts from Magneto-Convection Simulations Authors: Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Vögler, A.; Garcia-Alvarez, D. Bibcode: 2009AIPC.1094..768U Altcode: 2009csss...15..768U Solar irradiance changes on a wide range of time scales and is a key driver of the Earth's climate where secular variability in particular is relevant. This is, however, not well understood and our knowledge relies on reconstructions based on sunspot numbers and similar proxies.

The prime candidate to produce secular variability is a change in the surface coverage of small-scale magnetic elements. Direct observational determination of the flux emitted by these magnetic elements is difficult, especially as information covering a large spectral range is needed. Here we present a theoretical approach to this problem using intensity calculations from 3-D simulations of solar magneto-convection and compare these with the intensity calculations used in the successful semi-empirical S ATIRE models at disk centre. Eventually, such a comparison should lead to the removal of the last free parameter from S ATIRE-based irradiance reconstruction. Title: Faculae and Plague Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..153S Altcode: 2009LanB...4B.4124S No abstract at ADS Title: Sunspots Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..141S Altcode: 2009LanB...4B.4123S No abstract at ADS Title: The Origin and Dynamics of Solar Magnetism Authors: Thompson, M. J.; Balogh, A.; Culhane, J. L.; Nordlund, Å.; Solanki, S. K.; Zahn, J. -P. Bibcode: 2009odsm.book.....T Altcode: No abstract at ADS Title: Solar Space Missions: present and future Authors: Solanki, Sami K.; Marsch, Eckart Bibcode: 2009RvMA...21..229S Altcode: No abstract at ADS Title: The Solar Chromosphere: Old Challenges, New Frontiers Authors: Ayres, T.; Uitenbroek, H.; Cauzzi, G.; Reardon, K.; Berger, T.; Schrijver, C.; de Pontieu, B.; Judge, P.; McIntosh, S.; White, S.; Solanki, S. Bibcode: 2009astro2010S...9A Altcode: No abstract at ADS Title: Solar Activity Cycle Authors: Solanki, Sami; Krivova, Natalie Bibcode: 2009LanB...4B..124S Altcode: 2009LanB...4B.4122S No abstract at ADS Title: Dynamos and magnetic fields of the Sun and other cool stars, and their role in the formation and evolution of stars and in the habitability of planets Authors: Schrijver, Karel; Carpenter, Ken; Karovska, Margarita; Ayres, Tom; Basri, Gibor; Brown, Benjamin; Christensen-Dalsgaard, Joergen; Dupree, Andrea; Guinan, Ed; Jardine, Moira; Miesch, Mark; Pevtsov, Alexei; Rempel, Matthias; Scherrer, Phil; Solanki, Sami; Strassmeier, Klaus; Walter, Fred Bibcode: 2009astro2010S.262S Altcode: No abstract at ADS Title: Small-Scale Solar Magnetic Fields Authors: de Wijn, A. G.; Stenflo, J. O.; Solanki, S. K.; Tsuneta, S. Bibcode: 2009odsm.book..275D Altcode: As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research. Title: German Data Center for the Solar Dynamics Observatory: A model for the PLATO mission? Authors: Burston, R.; Gizon, L.; Saidi, Y.; Solanki, S. K. Bibcode: 2008CoAst.157..293B Altcode: The German Data Center for the Solar Dynamics Observatory (GDC-SDO), hosted by the Max Planck Institute for Solar System Research in Germany, will provide access to SDO data for the German solar physics community. The GDC-SDO will make available all the relevant Helioseismic and Magnetic Imager (HMI) data for helioseismology and smaller se- lected Atmospheric Imaging Assembly (AIA) data sets. This project commenced in August 2007 and is funded by the German Aerospace Center (Deutsches zentrum fuer Luft- und Raumfahrt or DLR) until December 2012. An important component of the GDC-SDO is the Data Record Management System (DRMS), developed in collaboration with the Stan- ford/Lockheed Joint Science Operations Center (JSOC). The PEGASUS workflow manage- ment system will be used to implement GDC-SDO data analysis pipelines. This makes use of the CONDOR High Throughput Computing Project for optimal job scheduling and also the GLOBUS Toolkit to enable grid technologies. Additional information about the GDC-SDO can be found at http://www.mps.mpg.de/projects/seismo/GDC1/index.html. Here, we sug- gest a similar structure and philosophy should be ideal for the PLATO mission, which looks for planetary transits and stellar oscillations and is being studied by ESA for an M-Mission slot in Cosmic Vision. Title: A nanoflare model for active region radiance: application of artificial neural networks Authors: Bazarghan, M.; Safari, H.; Innes, D. E.; Karami, E.; Solanki, S. K. Bibcode: 2008A&A...492L..13B Altcode: 2008arXiv0808.3925B Context: Nanoflares are small impulsive bursts of energy that blend with and possibly make up much of the solar background emission. Determining their frequency and energy input is central to understanding the heating of the solar corona. One method is to extrapolate the energy frequency distribution of larger individually observed flares to lower energies. Only if the power law exponent is greater than 2 is it considered possible that nanoflares contribute significantly to the energy input.
Aims: Time sequences of ultraviolet line radiances observed in the corona of an active region are modelled with the aim of determining the power law exponent of the nanoflare energy distribution.
Methods: A simple nanoflare model based on three key parameters (the flare rate, the flare duration, and the power law exponent of the flare energy frequency distribution) is used to simulate emission line radiances from the ions Fe XIX, Ca XIII, and Si III, observed by SUMER in the corona of an active region as it rotates around the east limb of the Sun. Light curve pattern recognition by an Artificial Neural Network (ANN) scheme is used to determine the values.
Results: The power law exponents, α≈2.8, 2.8, and 2.6 are obtained for Fe XIX, Ca XIII, and Si III respectively.
Conclusions: The light curve simulations imply a power law exponent greater than the critical value of 2 for all ion species. This implies that if the energy of flare-like events is extrapolated to low energies, nanoflares could provide a significant contribution to the heating of active region coronae. Title: Brightness, distribution, and evolution of sunspot umbral dots Authors: Riethmüller, T. L.; Solanki, S. K.; Zakharov, V.; Gandorfer, A. Bibcode: 2008A&A...492..233R Altcode: 2008arXiv0812.0477R Context: Umbral Dots (UDs) are thought to be manifestations of magnetoconvection in sunspot umbrae. Recent advances in their theoretical description point to the need for a thorough study of their properties and evolution based on data with the highest currently achievable resolution.
Aims: Our UD analysis aims to provide parameters such as lifetimes, diameters, horizontal velocities, and peak intensities, as well as the evolution of selected parameters.
Methods: We present a 106-min TiO (705.7 nm) time series of high spatial and temporal resolution that contains thousands of UDs in the umbra of a mature sunspot in the active region NOAA 10667 at μ = 0.95. The data were acquired with the 1-m Swedish Solar Telescope (SST) on La Palma. With the help of a multilevel tracking (MLT) algorithm the sizes, brightnesses, and trajectories of 12 836 umbral dots were found and extensively analyzed. The MLT allows UDs with very low contrast to be reliably identified.
Results: Inside the umbra we determine a UD filling factor of 11%. The histogram of UD lifetimes is monotonic, i.e. a UD does not have a typical lifetime. Three quarters of the UDs lived for less than 150 s and showed no or little motion. The histogram of the UD diameters exhibits a maximum at 225 km, i.e. most of the UDs are spatially resolved. UDs display a typical horizontal velocity of 420 m s-1 and a typical peak intensity of 51% of the mean intensity of the quiet photosphere, making them on average 20% brighter than the local umbral background. Almost all mobile UDs (large birth-death distance) were born close to the umbra-penumbra boundary, move towards the umbral center, and are brighter than average. Notably bright and mobile UDs were also observed along a prominent UD chain, both ends of which are located at the umbra-penumbra boundary. Their motion started primarily at either of the ends of the chain, continued along the chain, and ended near the chain's center. We observed the splitting and merging of UDs and the temporal succession of both. For the first time the evolution of brightness, size, and horizontal speed of a typical UD could be determined in a statistically significant way. Considerable differences between the evolution of central and peripheral UDs are found, which point to a difference in origin. Title: Future high-latitude observations anticipated from the Solar Orbiter mission Authors: Woch, J.; Solanki, S. K.; Marsch, E. Bibcode: 2008AGUFMSH44A..08W Altcode: As one of the cornerstones of the HELEX programme, the Solar Orbiter mission is currently scheduled for launch in 2015. After an initial cruise phase, Solar Orbiter will reach its science orbit in 2018. This orbit comprises initially a nearly Sun-synchronous phase at a distance of only 0.22 AU from Sun center. In a later stage, the orbital inclination will be raised, thus allowing Solar Orbiter to reach solar latitudes of about 35 degrees, and making it the first mission after Ulysses to study the Sun from a high-latitude vantage point. In contrast to Ulysses, however, Solar Orbiter will carry a complementary suite of both, in-situ and remote- sensing instruments, which will allow the study of the solar atmosphere to be extended to the largely unexplored polar regions of the Sun. The polar magnetic fields are responsible for the polar coronal holes driving the fast solar wind, but are poorly known. From its vantage point outside the ecliptic, Solar Orbiter will uncover the surface and sub-surface flows at the poles, the polar magnetic field structure and its evolution. It will provide new insights into the formation of the polar coronal holes, the nature of their boundaries and the acceleration of the fast solar wind emanating from the holes. The potential of Solar Orbiter for investigating the acceleration mechanism of the fast wind, the plumes and X-ray jets at the poles, and the high-latitude meridional circulation will be discussed. Title: Magnetic geometries of Sun-like stars : impact of rotation Authors: Petit, P.; Dintrans, B.; Aurière, M.; Catala, C.; Donati, J. -F.; Fares, R.; Gastine, T.; Lignières, F.; Morin, J.; Paletou, F.; Ramirez, J.; Solanki, S. K.; Théado, S. Bibcode: 2008sf2a.conf..523P Altcode: Sun-like stars are able to continuously generate a large-scale magnetic field through the action of a dynamo. Various physical parameters of the star are able to affect the dynamo output, in particular the rotation and mass. Using new generation stellar spectropolarimeters (ESPaDOnS@CFHT, NARVAL@TBL), it is now possible to measure the large-scale magnetic field of solar analogues (i.e. stars very close to the Sun in the stellar-parameter plane, including strict solar twins). From spectropolarimetric time-series, tomographic inversion of polarized Zeeman signatures allows us to reconstruct the field geometry and its progressive distortion under the effect of surface differential rotation. We detail the first results obtained on a sample of four main-sequence dwarfs, with masses close to 1 solar mass and rotation rates between 1 and 3 solar rotation rate. Title: Are There Field-Free Gaps near τ = 1 in Sunspot Penumbrae? Authors: Borrero, J. M.; Solanki, S. K. Bibcode: 2008ApJ...687..668B Altcode: 2008arXiv0806.4454B The vertical stratification of the magnetic field strength in sunspot penumbrae is investigated by means of spectropolarimetric observations at high spatial resolution from the Hinode spacecraft. Assuming that the magnetic field changes linearly with optical depth we find that, in those regions where the magnetic field is more inclined and the Evershed flow is strongest (penumbral intraspines), the magnetic field can either increase or decrease with depth. Allowing more degrees of freedom to the magnetic field stratification reveals that the magnetic field initially decreases from log τ5 = - 3 until log τ5simeq - 1.0, but increases again below that. The presence of strong magnetic fields near the continuum is at odds with the existence of regions void of magnetic fields at, or right below, the τ5 = 1 level in the penumbra. However, they are compatible with the presence of a horizontal flux-tube-like field embedded in a magnetic atmosphere. Title: SUMER observations of the inverse Evershed effect in the transition region above a sunspot Authors: Teriaca, L.; Curdt, W.; Solanki, S. K. Bibcode: 2008A&A...491L...5T Altcode: 2009arXiv0901.3317T Aims: We analyse SUMER spectral scans of a large sunspot within active region NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and dynamics of the sunspot atmosphere at different heights/temperatures.
Methods: The data analysed here consist of spectroheliograms in the continuum around 142.0 nm and in the Si iv 140.2 nm, O iii 70.3 nm, N iv 76.5 nm, and O iv 79.0 nm spectral lines. Gaussian-fitting of the observed profiles provides line-of-sight velocity and Doppler-width maps.
Results: The data show an asymmetric downflow pattern compatible with the presence of the inverse Evershed flow in a region within roughly twice the penumbral radius at transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous on small scales, seem to occur in a collar of radially directed filamentary structures, with an average width less than the 1 Mm spatial resolution of SUMER and characterised by different plasma speeds. Assuming that the flows are directed along the field lines, we deduce that such field lines are inclined by 10° to 25° with respect to the solar surface. Title: Identification of different types of kink modes in coronal loops: principles and application to TRACE results Authors: Wang, T. J.; Solanki, S. K.; Selwa, M. Bibcode: 2008A&A...489.1307W Altcode: 2008arXiv0808.0685W We explore the possible observational signatures of different types of kink modes (horizontal and vertical oscillations in their fundamental mode and second harmonic) that may arise in coronal loops, with the aim of determining how well the individual modes can be uniquely identified from time series of images. A simple, purely geometrical model is constructed to describe the different types of kink-mode oscillations. These are then “observed” from a given direction. In particular, we employ the 3D geometrical parameters of 14 TRACE loops of transverse oscillations to try to identify the correct observed wave mode. We find that for many combinations of viewing and loop geometry it is not straightforward to distinguish between at least two types of kink modes just using time series of images. We also considered Doppler signatures and find that these can help obtain unique identifications of the oscillation modes when employed in combination with imaging. We then compare the modeled spatial signatures with the observations of 14 TRACE loops. We find that out of three oscillations previously identified as fundamental horizontal mode oscillations, two cases appear to be fundamental vertical mode oscillations (but possibly combined with the fundamental horizontal mode), and one case appears to be a combination of the fundamental vertical and horizontal modes, while in three cases it is not possible to clearly distinguish between the fundamental mode and the second-harmonic of the horizontal oscillation. In five other cases it is not possible to clearly distinguish between a fundamental horizontal mode and the second-harmonic of a vertical mode. Title: Loop Morphology and Flows and their Relation to the Magnetic Field Authors: Teriaca, L.; Wiegelmann, T.; Lagg, A.; Solanki, S. K.; Curdt, W.; Sekii, T. Bibcode: 2008ASPC..397..196T Altcode: In November 2006 we obtained several rasters of a large sunspot and its trailing region using the SUMER spectrometer on SOHO. The observations consist of spectroheliograms in the continuum around 142 nm and in several spectral lines formed between 80000 K and 0.6 MK, covering the temperature range from the chromosphere to the lower corona. The observed profiles provide LOS velocity and Doppler width maps. TRACE images in the EUV passbands and in the 160 nm continuum provide a clear picture of the coronal loops and the chromosphere near their footpoints. The same target was also observed by all the instruments aboard Hinode and, in particular, by the SOT spectro-polarimeter measuring the photospheric magnetic vector. We combined SOT and MDI data (covering a larger FOV) to infer the coronal magnetic field of the active region by a nonlinear force-free field extrapolation. The observed radiance and velocity patterns at the various heights/temperatures throughout the solar atmosphere are compared with the field topology. Title: Fibrils in Ca II K Authors: Pietarila, A.; Solanki, S.; Hirzberger, J.; Zakharov, V. Bibcode: 2008ESPM...12.2.51P Altcode: High spatial resolution observations have revealed that fibril-like structures are a ubiquitous feature of the solar chromosphere. They are observed in most chromospheric lines, e.g., H-? and Ca II IRT. Until recently, there have been no reports of these structures in the Ca II H and K lines. Instead, these lines have revealed a hazy, uniform chromosphere and only in the highest resolution images have there been any indications of fibril structures.

We present high spatial resolution Ca II K observations from the Swedish Solar Telescope which show that fibrils are a prevailing feature in regions where the bulk of the signal is chromospheric. Based on the cotemporal continuum and nearly cotemporal magnetic field observations it is clear that the fibril foot points originate from photospheric magnetic concentrations. The fibrils share many characteristics, e.g. lifetime and dynamics, with fibrils observed in other spectral lines. They are also found to play an important role in the dynamics: in the plage they channel low frequency waves into the chromosphere while in the more quiet regions the highly inclined fibrils form a multi-layer canopy that suppresses oscillations from below. Title: The Magnetic Vector Structure of an Active Region Plage Field Authors: Meling, M. H. M.; Lagg, A.; Solanki, S. K. Bibcode: 2008ESPM...12.2.58M Altcode: We present scans of the active region plage associated with NOAA 10953, recorded with the Tenerife Infrared Polarimeter 2 (TIP-2) mounted behind the German Vacuum Tower Telescope (VTT) on Tenerife and the SOT-SP instrument on-board the HINODE spacecraft. TIP-2 recorded the full Stokes vector close to the diffraction limit of the telescope over a 20 angstroem wavelength range containing six magnetically sensitive FeI lines in the infrared including the g=3 line FeI 15648.5. With these lines we use the advantage that the Zeeman splitting dominates over the Doppler broadening for kG fields. The iron lines used by HINODE are FeI 6301.501and FeI 6302.494.

We analyzed the data by applying a Milne-Eddington type inversion to it. In the context of a two-component model we found a bimodal distribution of field strengths, strong fields whose field strength distribution peaks at 1400 [G] and a weak field distribution, which may be associated with quiet sun regions bordering on the plage. Title: Spectropolarimetric Investigations of the Deep Photospheric Layers of Magnetic Elements Authors: Oklay, N.; Gandorfer, A.; Solanki, S. K. Bibcode: 2008ESPM...12.2.49O Altcode: We observed simultaneously Stokes I and Stokes V/I profiles of Fe I (5379.574Å), C I(5380.332Å), Ti II(5381.021Å) and Fe I(5383.369Å) using the ZIMPOL II spectropolarimeter at the IRSOL (Istituto Ricerche Solari Locarno) facility. This set of spectral lines covers not only the mid-photosphere but also the deep photospheric layers, where the temperature sensitive C I line is formed. We analyzed ratios and asymmetries of their Stokes V amplitudes and areas. Further, the spectral profiles were analyzed using the SPINOR inversion code (Frutiger et al. 2000) to constrain the temperature structure of the magnetic elements down to deep photospheric layers. In this way, our understanding of the lowest photospheric layers of solar magnetic elements can be tested. Title: Structure and Evolution of Supergranulation from Local Helioseismology Authors: Hirzberger, Johann; Gizon, Laurent; Solanki, Sami K.; Duvall, Thomas L. Bibcode: 2008SoPh..251..417H Altcode: 2008SoPh..tmp..106H Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 105 supergranular cells and more than 103 lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment). Title: Evidence of convective rolls in a sunspot penumbra Authors: Zakharov, V.; Hirzberger, J.; Riethmüller, T. L.; Solanki, S. K.; Kobel, P. Bibcode: 2008A&A...488L..17Z Altcode: 2008arXiv0808.2317Z Aims: We study the recently discovered twisting motion of bright penumbral filaments with the aim of constraining their geometry and the associated magnetic field.
Methods: A large sunspot located 40° from disk center was observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of multi-wavelength polarimetric data and speckle reconstructed time series of continuum images were used to determine proper motions, as well as the velocity and magnetic structure in penumbral filaments.
Results: The continuum movie reveals apparent lateral motions of bright and dark structures inside bright filaments oriented parallel to the limb, confirming recent Hinode results. In these filaments we measure upflows of ≈1.1 km s-1 on their limbward side and weak downflows on their centerward side. The magnetic field in them is significantly weaker and more horizontal than in the adjacent dark filaments.
Conclusions: The data indicate the presence of vigorous convective rolls in filaments with a nearly horizontal magnetic field. These are separated by filaments harbouring stronger, more vertical fields. Because of reduced gas pressure, we see deeper into the latter. When observed near the limb, the disk-centerward side of the horizontal-field filaments appear bright due to the hot wall effect known from faculae. We estimate that the convective rolls transport most of the energy needed to explain the penumbral radiative flux. Title: Discriminant Analysis of Bright Points and Faculae: Center-to-Limb Distribution, Contrast and Morphology Authors: Kobel, P.; Hirzberger, J.; Gandorfer, A.; Solanki, S. K.; Zakharov, V. Bibcode: 2008ESPM...12.2.60K Altcode: High-resolution images of the solar photosphere reveal an intriguing mixture of Brights Points (BPs) and faculae at several disk positions, which is not explained by the conventional "hot wall'' model. Together with quantitative discrepancies between observations and simulations of faculae, it stresses that the fundamental relationship between BPs and faculae is not yet clear: How are BPs and faculae distributed on the solar disk? How do the photometric properties of BPs and faculae differ and vary with disk position?

To tackle these issues, a necessary step is to sort the BPs and faculae at various disk positions, in order to treat them separately. We present here the first attempt to discriminate BPs and faculae, using a statistical classification approach based on Linear Discriminant Analysis (LDA). This has never been done so far, presumably due to the lack of known automated methods to distinguish such features, and to the difficulty to obtain a coherent dataset of high-resolution images recorded in the same conditions. We applied our method to high-resolution G-band and continuum images of active regions recorded at the Swedish Solar Telescope, covering several disk positions where the transition from BPs to faculae is expected.

This allowed us to retrieve a first estimate of the center-to-limb variation of the relative distribution of both species. The center-to-limb distribution of BPs and faculae reveals the predominance of faculae at all disk positions except close to disk center. We argue that these ubiquitous faculae could be the transient signatures of swaying flux tubes with a wide range of inclination angles. Moreover, we statistically compared the G-band and continuum contrast of BPs and faculae, and characterized their morphology. Both the G-band and continuum contrast of BPs and faculae are found to similarly increase from center to limb. But when comparing G-band to continuum, BPs and faculae exhibit slightly different behaviours, which are related to radiative transfer processes. By orienting the features in local coordinate frames corresponding to the principal axes of their contrast moment of inertia, we could retrieve characteristic G-band contrast profiles exhibiting the typical predicted asymmetry for faculae. Finally, our BPs and faculae were found to have very similar morphological properties.

Although our study is essentially descriptive and based on purely photometric information, we hope that it will provide novel useful constraints for future BPs/faculae MHD models. Title: The Solar Chromosphere at Millimeter Wavelengths Authors: Loukitcheva, M.; Solanki, S. K.; White, S. Bibcode: 2008ESPM...12.2.18L Altcode: The solar chromosphere remains the least understood layer of the solar atmosphere. There is yet no answer to the question concerning its structure. Is it better described by the classical picture of a steady temperature rise as a function of height, with superposed weak oscillations, or does the temperature keep dropping outwards, with hot shocks producing strong localized heating? Observations in the UV and the IR give contrasting results, since they only sample either the hot or the cool parts of the chromosphere. Computations carried out with sophisticated dynamic models of the solar chromosphere demonstrate that millimeter emission is extremely sensitive to dynamic processes in the chromosphere and the appropriate wavelengths to look for dynamic signatures are in the range 0.8-5.0 mm. The models also suggest that high resolution observations at mm wavelengths have the unique property of reacting to both the hot and the cool gas, and thus have the potential of distinguishing between models.

In this contribution we use high-resolution millimeter-interferometer observations, obtained with the Berkeley-Illinois-Maryland Array at 3.5 mm (resolution of 12 arcsec), as a diagnostic tool to study the thermal structure of the solar chromosphere and its response to dynamic processes.

Our initial results obtained from the observations of the quiet Sun reveal brightness features corresponding to supergranular network boundaries and bright points within the cells. We found significant intensity oscillations with frequencies of 1.5-8 mHz with a tendency toward short-period oscillations in internetwork and longer periods in network regions. However higher spatial resolution is required for a clean separation between the brightness features and for an adequate comparison with the output of the comprehensive dynamic simulations. Title: Magnetic Structure of a Filament during its Phase of Activity Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2008ESPM...12.2.19S Altcode: We analyze and interpret spectropolarimetric observations of an active region filament located close to the solar disc center, during its phase of activity. The observations are obtained in the chromospheric He I lines at 1083.0 nm. We provide novel observational results on the magnetic field measurements in solar filaments to give constraints to the theoretical models of their support in the solar corona. Our main goal is to interpret the behavior of the atmospheric parameters retrieved from the spectropolarimetric data to give a picture of the magnetic structure of the observed filament. The analysis of the observed polarization of the He I 1083.0 nm multiplet in the filament, carried out by inverting the Stokes profiles, reveals the presence of different unresolved atmospheric components of the He lines, coexisting within the resolution element (1.2 arcsec). The different components, belonging to different magnetic field lines, show supersonic up- and downflows, sometimes within the same resolution element. The He blueshifted components belong to mostly transversal field lines in the body of the filament. These field lines are found to be curving upwards on both sides. This picture suggests the presence of dipped field lines that are moving upward, carrying with them the filament material. During this movement, we also observe filament material flowing down along field lines having the same polarity as the photospheric field (i.e. they have the opposite inclination with respect to the dipped field lines). These downflows are faster at the filament end points and can reach values close to 10 times the speed of sound. The field lines are found to be almost parallel to the filament axis in the plane perpendicular to the line of sight. We use the two main theoretical models of prominence support (dip or flux rope models) to interpret the results obtained. Title: Comparison of Magnetoconvection Simulations with the Approximation of Thin Flux Tubes Authors: Yelles Chaouche, L.; Solanki, S.; Schuessler, M. Bibcode: 2008ESPM...12..3.8Y Altcode: The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility to check the consistency of the approximation with the properties of the flux concentrations that form in the course of the simulation. We make a comparative analysis between the thin flux tube/sheet model and flux concentrations existing in a 3D radiation-MHD simulation. We have found that for flux concentration well above the equipartition distribution, the MHD magnetic structures are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures. Title: Evidence for Polar Jets as Precursors of Polar Plume Formation Authors: Raouafi, N. -E.; Petrie, G. J. D.; Norton, A. A.; Henney, C. J.; Solanki, S. K. Bibcode: 2008ApJ...682L.137R Altcode: 2008arXiv0806.3045R Observations from the Hinode/XRT telescope and STEREO/SECCHI/EUVI are utilized to study polar coronal jets and plumes. The study focuses on the temporal evolution of both structures and their relationship. The data sample, spanning 2007 April 7-8, shows that over 90% of the 28 observed jet events are associated with polar plumes. EUV images (STEREO/SECCHI) show plume haze rising from the location of approximately 70% of the polar X-ray (Hinode/XRT) and EUV jets, with the plume haze appearing minutes to hours after the jet was observed. The remaining jets occurred in areas where plume material previously existed, causing a brightness enhancement of the latter after the jet event. Short-lived, jetlike events and small transient bright points are seen (one at a time) at different locations within the base of preexisting long-lived plumes. X-ray images also show instances (at least two events) of collimated thin jets rapidly evolving into significantly wider plumelike structures that are followed by the delayed appearance of plume haze in the EUV. These observations provide evidence that X-ray jets are precursors of polar plumes and in some cases cause brightenings of plumes. Possible mechanisms to explain the observed jet and plume relationship are discussed. Title: SUNRISE: High resolution UV/VIS observations of the sun from the stratosphere Authors: Sunrise Team; Barthol, P.; Gandorfer, A. M.; Solanki, S. K.; Knölker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.; SUNRISE Team Bibcode: 2008AdSpR..42...70S Altcode: SUNRISE is an international project for the development, construction and operation of a balloon-borne solar telescope with an aperture of 1 m, working in the UV/VIS spectral domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide near diffraction-limited images of the photosphere and chromosphere with an unprecedented resolution down to 35 km on the solar surface at wavelengths around 220 nm. Active in-flight alignment and image stabilization techniques are used. The focal-plane instrumentation consists of a polarization sensitive spectrograph, a Fabry Perot filter magnetograph and a phase-diverse filter imager working in the near UV. The first stratospheric long-duration balloon flight of SUNRISE is planned in summer 2009 from the Swedish ESRANGE station. SUNRISE is a joint project of the German Max-Planck-Institut für Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, Germany, the High-Altitude Observatory (HAO), Boulder, USA, the Lockheed-Martin Solar and Astrophysics Laboratory (LMSAL), Palo Alto, USA, and the Spanish IMaX consortium. This paper will give an overview about the mission and a description of its scientific and technological aspects. Title: Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold Authors: Petit, P.; Dintrans, B.; Solanki, S. K.; Donati, J. -F.; Aurière, M.; Lignières, F.; Morin, J.; Paletou, F.; Ramirez Velez, J.; Catala, C.; Fares, R. Bibcode: 2008MNRAS.388...80P Altcode: 2008MNRAS.tmp..715P; 2008arXiv0804.1290P From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.

We reconstruct the large-scale magnetic geometry of the targets as a low-order (l < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ~12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component. Title: Spectral irradiance variations: comparison between observations and the SATIRE model on solar rotation time scales Authors: Unruh, Y. C.; Krivova, N. A.; Solanki, S. K.; Harder, J. W.; Kopp, G. Bibcode: 2008A&A...486..311U Altcode: 2008arXiv0802.4178U Aims: We test the reliability of the observed and calculated spectral irradiance variations between 200 and 1600 nm over a time span of three solar rotations in 2004.
Methods: We compare our model calculations to spectral irradiance observations taken with SORCE/SIM, SoHO/VIRGO, and UARS/SUSIM. The calculations assume LTE and are based on the SATIRE (Spectral And Total Irradiance REconstruction) model. We analyse the variability as a function of wavelength and present time series in a number of selected wavelength regions covering the UV to the NIR. We also show the facular and spot contributions to the total calculated variability.
Results: In most wavelength regions, the variability agrees well between all sets of observations and the model calculations. The model does particularly well between 400 and 1300 nm, but fails below 220 nm, as well as for some of the strong NUV lines. Our calculations clearly show the shift from faculae-dominated variability in the NUV to spot-dominated variability above approximately 400 nm. We also discuss some of the remaining problems, such as the low sensitivity of SUSIM and SORCE for wavelengths between approximately 310 and 350 nm, where currently the model calculations still provide the best estimates of solar variability. Title: SOHO/SUMER observations of prominence oscillation before eruption Authors: Chen, P. F.; Innes, D. E.; Solanki, S. K. Bibcode: 2008A&A...484..487C Altcode: 2008arXiv0802.1961C Context: Coronal mass ejections (CMEs), as a large-scale eruptive phenomenon, often reveal some precursors in the initiation phase, e.g., X-ray brightening, filament darkening, etc., which are useful for CME modelling and space weather forecasting.
Aims: With the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectroscopic observations of the 2000 September 26 CME, we propose another precursor for CMEs, namely, long-time prominence oscillations.
Methods: We observed the prominence oscillation-and-eruption event by ground-based Hα telescopes and space-borne white-light, EUV imaging, and spectroscopic instruments. In particular, the SUMER slit was observing the prominence in a sit-and-stare mode.
Results: The observations indicate that a siphon flow was moving from the proximity of the prominence to a site at a projected distance of 270'', which was followed by repetitive Hα surges and continual prominence oscillations. The oscillation lasted 4 hours before the prominence erupted as a blob-like CME. The analysis of the multiwavelength data indicates that the whole series of processes fits well into the emerging flux trigger mechanism for CMEs. In this mechanism, emerging magnetic flux drives a siphon flow due to increased gas pressure where the background polarity emerges. It also drives Hα surges through magnetic reconnection where the opposite polarity emerges. The magnetic reconnection triggers the prominence oscillations, as well as its loss of equilibrium, which finally leads to the eruption of the prominence. It is also found that the reconnection between the emerging flux and the pre-existing magnetic loop proceeds in an intermittent, probably quasi-periodic, way. Title: Understanding the WMAP Results: Low-Order Multipoles and Dust in the Vicinity of the Solar System Authors: Dikarev, Valeri; Preuß, Oliver; Solanki, Sami; Krüger, Harald; Krivov, Alexander Bibcode: 2008EM&P..102..555D Altcode: 2007EM&P..tmp...47D Analyses of the cosmic microwave background (CMB) radiation maps produced by the Wilkinson Microwave Anisotropy Probe (WMAP) have revealed anomalies not predicted by the standard cosmological theory. It has been suggested that a dust cloud in the vicinity of the Solar system may be the cause for these anomalies. In this paper, the thermal emission by particles from two known interplanetary meteoroid complexes is tested against the CMB maps. Conclusions are drawn based on the geometry of cloud projections onto the WMAP sky whether these clouds are likely to explain the observed anomaly. The smooth background Zodiacal cloud and one of the Taurid meteor complex branches do not explain the WMAP anomaly. Title: Models of solar irradiance variations: Current status Authors: Krivova, Natalie A.; Solanki, Sami K. Bibcode: 2008JApA...29..151K Altcode: Regular monitoring of solar irradiance has been carried out since 1978 to show that solar total and spectral irradiance varies at different time scales. Whereas variations on time scales of minutes to hours are due to solar oscillations and granulation, variations on longer time scales are driven by the evolution of the solar surface magnetic field. Here the most recent advances in modelling of solar irradiance variations on time scales longer than a day are briefly reviewed. Title: The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.; Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S. Bibcode: 2008A&A...484L..17D Altcode: 2008arXiv0804.4230D Context: The contrast of granulation is an important quantity characterizing solar surface convection.
Aims: We compare the intensity contrast at 630 nm, observed using the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations of Vögler & Schüssler (2007, A&A, 465, L43).
Methods: A synthetic image from the simulation is degraded using a theoretical point-spread function of the optical system, and by considering other important effects.
Results: The telescope aperture and the obscuration by the secondary mirror and its attachment spider, reduce the simulated contrast from 14.4% to 8.5%. A slight effective defocus of the instrument brings the simulated contrast down to 7.5%, close to the observed value of 7.0%.
Conclusions: A proper consideration of the effects of the optical system and a slight defocus, lead to sufficient degradation of the synthetic image from the MHD simulation, such that the contrast reaches almost the observed value. The remaining small discrepancy can be ascribed to straylight and slight imperfections of the instrument, which are difficult to model. Hence, Hinode SP data are consistent with a granulation contrast which is predicted by 3D radiation MHD simulations. Title: Moments of the latitudinal dependence of the sunspot cycle: a new diagnostic of dynamo models Authors: Solanki, S. K.; Wenzler, T.; Schmitt, D. Bibcode: 2008A&A...483..623S Altcode: Aims: The latitude-distribution of solar activity as represented by sunspots is studied.
Methods: We first determined the latitudinal distribution of a sunspot cycle by integrating the butterfly diagram at each latitude over the length of each cycle. We then formed the five lowest moments of the latitudinal distribution of all complete sunspot cycles since 1874 and compared these moments with each other.
Results: The three lowest moments correlate remarkably well with each other. For example, the mean latitude of the sunspots during a cycle and the latitude range are correlated at the 0.96 level. A clear asymmetry is seen between the two hemispheres, with the southern solar hemisphere showing consistently stronger and more positive correlations than the northern hemisphere. When applied to different simple dynamo models, the same analysis reveals significant differences between the models and demonstrates that such moments are a useful diagnostic in distinguishing between dynamo models. Remarkably, dynamos without a meridional flow provide results closer to those of the Sun's northern hemisphere, while a dynamo with a meridional flow produces fields more like those in the Sun's southern hemisphere. This may provide a clue to the cause of the well-known north-south asymmetry of solar activity. Title: Multiheight Analysis of Asymmetric Stokes Profiles in a Solar Active Region Authors: Deng, N.; Choudhary, D.; Solanki, S. K.; Lagg, A. Bibcode: 2008AGUSMSP51D..06D Altcode: Parameters characterizing Stokes asymmetries are derived from full Stokes I,Q,U,V spectra of FeI λλ 630.15, 630.25~nm line pair (formed at two different heights in the photosphere) and MgI b 517.27~nm line (formed at lower chromosphere) in a solar active region near disc center. The spectropolarimetric observations were taken with the National Solar Observatory/High Altitude Observatory Advanced Stokes Polarimeter. The observed active region consists of a α sunspot, a δ sunspot, several pores and granulation. The line center shifts and bi-sectors derived from Stokes-I profiles describe the line-of-sight Doppler velocity and Stokes-I asymmetry, respectively. Stokes-V amplitude and area asymmetries are defined by the normalized difference of respective quantities between blue and red lobes of circular polarization profiles. The same can be derived from linear polarization profiles ([Q2+U2]1/2). The Stokes asymmetries are compared for different regions and at multiple heights. Neutral line regions of the δ spot and outer penumbral regions show distinct large Stokes asymmetries. Both Stokes-V amplitude and area asymmetries become larger from lower to higher atmosphere in neutral line regions that have strong transverse field and mixed polarities. The Stokes-V area asymmetry of outer edge of penumbrae changes from positive in the photosphere to negative in lower chromosphere. Detailed results and interpretation will be presented. Title: The nature of running penumbral waves revealed Authors: Bloomfield, D. S.; Lagg, A.; Solanki, S. K. Bibcode: 2008IAUS..247...55B Altcode: 2007IAUS..247...55B We seek to clarify the nature of running penumbral (RP) waves: are they chromospheric trans-sunspot waves or a visual pattern of upward-propagating waves? Full Stokes spectropolarimetric time series of the photospheric Sii10827 Å line and the chromospheric Hei10830 Å multiplet were inverted using a Milne-Eddington code. Spatial pixels were paired between the outer umbral/inner penumbral photosphere and the penumbral chromosphere using inclinations retrieved by the inversion and the dual-height pairings of line-of-sight velocity time series were studied for signatures of wave propagation using a Fourier phase difference analysis. The dispersion relation for radiatively cooling acoustic waves, modified to incorporate an inclined propagation direction, fits well the observed phase differences between the pairs of photospheric and chromospheric pixels. We have thus demonstrated that RP waves are in effect low-β slow-mode waves propagating along the magnetic field. Title: Identification of types of kink modes in coronal loops: principles and application to TRACE results Authors: Wang, T.; Solanki, S.; Selwa, M.; Ofman, L. Bibcode: 2008AGUSMSP31C..08W Altcode: We explore the possible signatures of different types of kink modes (horizontal and vertical oscillations in their fundamental mode and second harmonic) which may arise in coronal loops. Based on the 3D geometrical parameters of 14 TRACE loops of transverse oscillations, we simulate qualitatively the loop displacements due to these types of kink mode oscillations. We find that for many combinations of viewing and loop geometry it is not straightforward to distinguish between the two types of kink modes. We have also considered Doppler signatures and found that these can in principle help to obtain unique identifications of the oscillation modes. We then compared the simulated spatial signatures with the observations for 14 TRACE loops. We find that two cases of loop oscillations previously identified as a fundamental horizontal mode appear to be a fundamental vertical mode, while in two other cases it is not possible to clearly distinguish between a horizontal oscillation of the fundamental mode and the second-harmonic, and in six cases it is not possible to clearly distinguish between a fundamental horizontal mode and a second-harmonic vertical mode. In addition, for the particular case that the oscillating loop has a S-shape, we find that the fundamental vertical oscillation can take on the appearance of the horizontal second harmonic due to projection effects. We also present numerical results of three dimensional MHD model of an idealized active region field with S-shaped field-lines. The active region is initialized as a force-free dipole magnetic configuration with uniform density and contains a loop with a higher density than its surroundings. We introduce a velocity pulse which models the impact of a flare on surrounding fields. Both the qualitative study and the MHD simulation support the conclusion of the presence of fundamental mode of vertical kink oscillations in an S-shaped loop. Our interpretation can naturally solve the puzzle of the absence of the fundamental mode and the apparent presence of second harmonic oscillations observed in a TRACE loop by De Moortel and Brady (2007). Title: Stratification of Sunspot Umbral Dots from Inversion of Stokes Profiles Recorded by Hinode Authors: Riethmüller, T. L.; Solanki, S. K.; Lagg, A. Bibcode: 2008ApJ...678L.157R Altcode: 2008arXiv0805.4324R This work aims to constrain the physical nature of umbral dots (UDs) using high-resolution spectropolarimetry. Full Stokes spectra recorded by the spectropolarimeter on Hinode of 51 UDs in a sunspot close to the disk center are analyzed. The height dependence of the temperature, magnetic field vector, and line-of-sight velocity across each UD is obtained from an inversion of the Stokes vectors of the two Fe I lines at 630 nm. No difference is found at higher altitudes [-3 <= log (τ500) <= - 2] between the UDs and the diffuse umbral background. Below that level the difference rapidly increases, so that at the continuum formation level [log (τ500) = 0] we find on average a temperature enhancement of 570 K, a magnetic field weakening of 510 G, and upflows of 800 m s-1 for peripheral UDs, whereas central UDs display an excess temperature of on average 550 K, a field weakening of 480 G, and no significant upflows. The results for, in particular, the peripheral UDs, including cuts of magnetic vector and velocity through them, look remarkably similar to the output of recent radiation MHD simulations. They strongly suggest that UDs are produced by convective upwellings. Title: First stereoscopic polar plume reconstructions from STEREO/SECCHI images Authors: Feng, L.; Inhester, B.; Solanki, S. K.; Wiegelmann, T.; Podlipnik, B.; Howard, R.; Plunkett, S.; Wuelser, J.; Gan, W. Bibcode: 2008AGUSMSH23A..01F Altcode: We present the first stereoscopic reconstruction of the three-dimensional structures of polar plumes based on the two simultaneously recorded images taken by the EUVI telescopes in the SECCHI instrument package onboard the recently launched STEREO mission. The reconstructed polar plumes were observed on April 7th, 2007 when the two spacecraft were well below the solar equatorial plane, an appropriate time for the observation of the plumes in the south polar coronal hole. The heliocentric separation of the two spacecraft was 3.6 degrees at that time. We determine locations of the footpoints of five EUV polar plumes on the solar surface as well as their inclinations relative to the line-of-sight and to their local radial directions. The five plumes are all within 21 degrees of the south pole and their inclinations to the line-of-sight of STEREO A(head) and radial directions are on average 107 degrees and 28 degrees, respectively. A simple dipole model for the south pole's magnetic field does not provide a good correspondence with the obtained inclinations. Of the three plumes in front of the limb only one is associated with an EUV bright point. Title: The FeH F4Δ-X4Δ system. Creating a valuable diagnostic tool to explore solar and stellar magnetic fields Authors: Afram, N.; Berdyugina, S. V.; Fluri, D. M.; Solanki, S. K.; Lagg, A. Bibcode: 2008A&A...482..387A Altcode: Context: Lines of diatomic molecules are ideal tools for studying cool stellar atmospheres and the internal structure of sunspots and starspots, given their temperature and pressure sensitivities, which are typically higher than in atomic lines. The Wing-Ford FeH F4Δ-X4Δ system represents such a diatomic molecule that is, in addition, highly sensitive to magnetic fields. The current theoretical description of those transitions that include the involved molecular constants, however, are only based on intensity measurements because polarimetric observations have not been available until now, which limits their diagnostic value. Furthermore, the theory has so far been optimized to reproduce energy levels and line strengths without taking magnetic sensitivities into account.
Aims: The FeH F4Δ-X4Δ system is produced by transitions between two electronic states with the coupling of the angular momenta that is intermediate between limiting Hund's cases (a) and (b). Our goal is to investigate the diagnostic capabilities of the current theoretical description of the molecule FeH.
Methods: Using the most precise available Hamiltonian, we carried out the perturbation calculation of the molecular Zeeman effect for this transition and computed the Landé factors of the energy levels and of transitions. We extracted Landé factors from a comparison of observed and calculated Stokes I and V profiles. Certain spectral lines, most frequently with high magnetic sensitivity, exhibited discrepancies between the theory and observations. We extended the theoretical model with a semi-empirical approach to obtain a diagnostic tool that is able to reproduce many of the interesting spectral lines.
Results: We find that the current theory successfully reproduces the magnetic properties of a large number of lines in the FeH F4Δ-X4Δ system and that the modified Hamiltonian allows us to synthesize and successfully reproduce the most sensitive lines. Thus, our observations have provided valuable constraints for determining empirical molecular constants and Landé factors.
Conclusions: The FeH F4Δ-X4Δ system is found to be a very sensitive magnetic diagnostic tool. Polarimetric data of these lines, in contrast to intensity measurements, provide us with more direct and detailed information to study the coolest parts of sunspot and starspot umbrae, as well as cool active dwarfs. Title: A first step in reconstructing the solar corona self-consistently with a magnetohydrostatic model during solar activity minimum Authors: Ruan, P.; Wiegelmann, T.; Inhester, B.; Neukirch, T.; Solanki, S. K.; Feng, L. Bibcode: 2008A&A...481..827R Altcode: Aims: We compute the distribution of the magnetic field and the plasma in the global corona with a self-consistent magnetohydrostatic (MHS) model.
Methods: Because direct measurements of the solar coronal magnetic field and plasma are extremely difficult and inaccurate, we use a modeling approach based on observational quantities, e.g. the measured photospheric magnetic field, to reconstruct the structure of the global solar corona. We take an analytic magnetohydrostatic model to extrapolate the magnetic field in the corona from photospheric magnetic field measurement. In the model, the electric current density can be decomposed into two components: one component is aligned with the magnetic field lines, whereas the other component flows in spherical shells. The second component of the current produces finite Lorentz forces that are balanced by the pressure gradient and the gravity force. We derive the 3D distribution of the magnetic field and plasma self-consistently in one model. The boundary conditions are given by a synoptic magnetogram on the inner boundary and by a source surface model at the outer boundary.
Results: The density in the model is higher in the equatorial plane than in the polar region. We compare the magnetic field distribution of our model with potential and force-free field models for the same boundary conditions and find that our model differs noticeably from both. We discuss how to apply the model and how to improve it. Title: Evidence of magnetic field wrapping around penumbral filaments Authors: Borrero, J. M.; Lites, B. W.; Solanki, S. K. Bibcode: 2008A&A...481L..13B Altcode: 2007arXiv0712.2548B We employ high-spatial resolution spectropolarimetric observations from the Solar Optical Telescope on-board the Hinode spacecraft to investigate the fine structure of the penumbral magnetic fields. The Stokes vector of two neutral iron lines at 630 nm is inverted at every spatial pixel to retrieve the depth-dependence of the magnetic field vector, line-of-sight velocity and thermodynamic parameters. We show that the azimuthal angle of the magnetic field vector has opposite sign on both sides above the penumbral filaments. This is consistent with the wrapping of an inclined field around the horizontal filaments. The wrapping effect is stronger for filaments with larger horizontal extensions. In addition, we find that the external magnetic field can penetrate into the intraspines, leading to non-radial magnetic fields inside them. These findings shed some light on the controversial small-scale structure of the sunspot penumbra. Title: Theoretical modeling for the stereo mission Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng, C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.; Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.; Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.; Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil, D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester, B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.; Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer, V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.; Wimmer-Schweingruber, R. F. Bibcode: 2008SSRv..136..565A Altcode: 2006SSRv..tmp...75A We summarize the theory and modeling efforts for the STEREO mission, which will be used to interpret the data of both the remote-sensing (SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The modeling includes the coronal plasma, in both open and closed magnetic structures, and the solar wind and its expansion outwards from the Sun, which defines the heliosphere. Particular emphasis is given to modeling of dynamic phenomena associated with the initiation and propagation of coronal mass ejections (CMEs). The modeling of the CME initiation includes magnetic shearing, kink instability, filament eruption, and magnetic reconnection in the flaring lower corona. The modeling of CME propagation entails interplanetary shocks, interplanetary particle beams, solar energetic particles (SEPs), geoeffective connections, and space weather. This review describes mostly existing models of groups that have committed their work to the STEREO mission, but is by no means exhaustive or comprehensive regarding alternative theoretical approaches. Title: On the potential value of Ca II K spectroheliogram time-series for solar activity and irradiance studies Authors: Ermolli, I.; Solanki, S. K.; Tlatov, A. G.; Krivova, N. A.; Ulrich, R. K.; Singh, J. Bibcode: 2008arXiv0802.3806E Altcode: Various observatories around the globe started regular full-disk imaging of the solar atmosphere in the Ca II K line since the early decades of the 20th century. The archives made by these observations have the potential of providing far more detailed information on solar magnetism than just the sunspot number and area records to which most studies of solar activity and irradiance changes are restricted. We evaluate the image contents of three Ca II K spectroheliogram time-series, specifically those obtained by the digitization of the Arcetri, Kodaikanal, and Mt Wilson photographic archives. We describe the main problems afflicting these data and analyze their quality by expressing the image contents through several quantities. We compare the results obtained with those for similar present-day observations taken with the Meudon spectroheliograph and with the Rome-PSPT. We show that historic data suffer from stronger geometrical distortions and photometric uncertainties than similar present-day observations. The latter uncertainties mostly originate from the photographic calibration of the original data and from stray-light effects. We also show that the image contents of the three analyzed series vary in time. These variations are probably due to instrument changes and aging of the spectrographs used, as well as changes of the observing programs. Our results imply that the main challenge for the analysis of historic data is their accurate photometric calibration. This problem must be solved before they can provide reliable information about solar magnetism and activity over the last century. Moreover, inter-calibration of results obtained from independent time-series is required to reliably trace changes of solar properties with time from the analysis of such data. Title: SOHO/SUMER observations of prominence oscillation before eruption Authors: Chen, P. F.; Innes, D. E.; Solanki, Sami Bibcode: 2008cosp...37..502C Altcode: 2008cosp.meet..502C Coronal mass ejections (CMEs) often reveal some precursors in the initiation phase, such as X- ray brightening and filament darkening, which are useful for CME modeling and space weather forecast. With the SOHO/SUMER spectroscopic observations of the 2000 September 26 event, we propose another precursor for CME eruptions, namely, long-time prominence oscillations. The observations indicate that a siphon flow was moving from the proximity of the prominence to a far site, which was followed by repetitive Hα surges and continual prominence oscillations. The oscillation lasted 4 hours before the prominence erupted as a blob-like CME. The analysis of the multiwavelength data indicates that the whole series of processes fits well into the emerging flux trigger mechanism for CMEs. In this mechanism, emerging magnetic flux drives a siphon flow due to increased gas pressure where the background polarity emerges. It also drives Hα surges through magnetic reconnection where the opposite polarity emerges. The magnetic reconnection triggers the prominence oscillations, as well as its loss of equilibrium, which finally leads to the eruption of the prominence. It is also found that the reconnection between the emerging flux and the pre-existing magnetic loop proceeds in an intermittent, probably quasiperiodic, way. Title: 3D MHD model of kink waves in a loop anchored in a realistic active region Authors: Selwa, Malgorzata; Ofman, Leon; Wang, Tongjiang; Solanki, Sami Bibcode: 2008cosp...37.2804S Altcode: 2008cosp.meet.2804S We present numerical results of three dimensional MHD model of the active region field. The active region is initialized using MDI data of 15 May 2001, 02:57 UT and potential extrapolation of the magnetic field with gravitationally stratified density and contains a loop with a higher density than its surroundings. The potential model imitates the original TRACE AR quite well, however, the choice of particular loop within AR is inaccurate due to limitation of the plane-of-the-sky view. This study represents an extension to the model of Ofman (2007). We introduce a velocity pulse based on TRACE observations to model the impact of a flare on surrounding fields, and study the resulting loop oscillations. The flare is initialized as a semispherical velocity pulse at the bottom of AR and corresponds to the event between loop's footpoints captured by TRACE. We investigate the influence of a realistic dense loop on the excitation and damping of the oscillations and compare our results with TRACE observations. By the means of 3D computer simulation we confirm that considering combination of viewing and loop geometry and Doppler signatures it is straightforward to distinguish between at least two types of kink modes: horizontal and vertical in the real loop observed by Aschwanden et al. (2002). We find that oscillation previously identified as a fundamental horizontal mode (Aschwanden et al. 2002) appears to be a fundamental vertical mode. As such vertical kink oscillations are not as rare compared to horizontal ones as previously thought. Title: Grand minima and maxima of solar activity on multi-millennial scale Authors: Usoskin, Ilya; Solanki, Sami; Kovaltsov, Gennady Bibcode: 2008cosp...37.3264U Altcode: 2008cosp.meet.3264U Using a reconstruction of sunspot numbers stretching over multiple millennia, we analyze the statistics of the occurrence of grand minima and maxima and set new observational constraints on long-term solar and stellar dynamo models. We present an updated reconstruction of sunspot number over multiple millennia, from 14C data by means of a physics-based model, using an updated model of the evolution of the solar open magnetic flux. A list of grand minima and maxima of solar activity is presented for the Holocene (since 9500 BC) and the statistics of both the length of individual events as well as the waiting time between them are analyzed. It is discussed that the occurrence of grand minima/maxima is driven not by long-term cyclic variability, but by a stochastic/chaotic process. The waiting time distribution of the occurrence of grand minima/maxima deviates from an exponential distribution, implying that these events tend to cluster together with long event-free periods between the clusters. Two different types of grand minima are observed: short (30-90 years) minima of Maunder type and long (>110 years) minima of Sp¨rer type, implying that a deterministic behaviour of the dynamo during o a grand minimum defines its length. The duration of grand maxima follows an exponential distribution, suggesting that the duration of a grand maximum is determined by a random process. Title: Variability of solar irradiance from the UV to the NIR from GOME and SCIAMACHY for use in atmospheric models Authors: Ambrose Pagaran, Joseph; Weber, Mark; Ambrose Pagaran, Joseph; Burrows, John P.; Krivova, Natalie; Solanki, Sami; Floyd, Linton Bibcode: 2008cosp...37.2329A Altcode: 2008cosp.meet.2329A The Sun is the primary energy source that drives the Earth's climate system. Its radiative output is known to vary in time, for instance, with the 11-year solar cycle and 27-day solar rotation period. Variations in the solar spectral irradiance (SSI) affect the thermal structure and chemical composition of the Earth's atmosphere. Although the largest solar variations are observed in the UV spectral region, a large fraction of the total solar irradiance (TSI, solar constant) variation over a solar cycle comes from the visible and near IR spectral range. In order to understand how SSI variations cause a detectable change in climate, we need to quantify UV, visible, and near IR variation in the solar spectral irradiance to a high level of certainty both over the short term solar rotation 27-day period and 11-year solar cycle. Using daily solar irradiance observations from SUSIM (1992-2005), GOME (1995-present), and SCIAMACHY (2002-present), we study solar variations over 27-day solar rotations from 120 nm to 1600 nm. The variability is modelled by parameterizing SSI in terms of faculae brightening (using the Mg II core-to-wing ratio proxy) and sunspot darkening (using the photospheric sunspot index). Since the variations in the visible and NIR are well below 1 Title: ALMA as the ideal probe of the solar chromosphere Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen Bibcode: 2008Ap&SS.313..197L Altcode: 2007arXiv0704.0023L; 2007Ap&SS.tmp..371L The very nature of the solar chromosphere, its structuring and dynamics, remains far from being properly understood, in spite of intensive research. Here we point out the potential of chromospheric observations at millimeter wavelengths to resolve this long-standing problem. Computations carried out with a sophisticated dynamic model of the solar chromosphere due to Carlsson and Stein demonstrate that millimeter emission is extremely sensitive to dynamic processes in the chromosphere and the appropriate wavelengths to look for dynamic signatures are in the range 0.8 5.0 mm. The model also suggests that high resolution observations at mm wavelengths, as will be provided by ALMA, will have the unique property of reacting to both the hot and the cool gas, and thus will have the potential of distinguishing between rival models of the solar atmosphere. Thus, initial results obtained from the observations of the quiet Sun at 3.5 mm with the BIMA array (resolution of 12″) reveal significant oscillations with amplitudes of 50 150 K and frequencies of 1.5 8 mHz with a tendency toward short-period oscillations in internetwork and longer periods in network regions. However higher spatial resolution, such as that provided by ALMA, is required for a clean separation between the features within the solar atmosphere and for an adequate comparison with the output of the comprehensive dynamic simulations. Title: The Nature of Running Penumbral Waves Revealed Authors: Bloomfield, D. Shaun; Lagg, Andreas; Solanki, Sami K. Bibcode: 2007ApJ...671.1005B Altcode: 2007arXiv0709.3731B We seek to clarify the nature of running penumbral (RP) waves: are they chromospheric trans-sunspot waves or a visual pattern of upward-propagating waves? Full Stokes spectropolarimetric time series of the photospheric Si I λ10827 line and the chromospheric He I λ10830 multiplet were inverted using a Milne-Eddington atmosphere. Spatial pixels were paired between the outer umbral/inner penumbral photosphere and the penumbral chromosphere using inclinations retrieved by the inversion and the dual-height pairings of line-of-sight velocity time series were studied for signatures of wave propagation using a Fourier phase difference analysis. The dispersion relation for radiatively cooling acoustic waves, modified to incorporate an inclined propagation direction, fits well the observed phase differences between the pairs of photospheric and chromospheric pixels. We have thus demonstrated that RP waves are in effect low-β slow-mode waves propagating along the magnetic field. Title: First Stereoscopic Coronal Loop Reconstructions from STEREO SECCHI Images Authors: Feng, L.; Inhester, B.; Solanki, S. K.; Wiegelmann, T.; Podlipnik, B.; Howard, R. A.; Wuelser, J. -P. Bibcode: 2007ApJ...671L.205F Altcode: 2008arXiv0802.0773F We present the first reconstruction of the three-dimensional shape of magnetic loops in an active region from two different vantage points based on simultaneously recorded images. The images were taken by the two EUVI telescopes of the SECCHI instrument on board the recently launched STEREO spacecraft when the heliocentric separation of the two space probes was 12°. We demonstrate that these data allow us to obtain a reliable three-dimensional reconstruction of sufficiently bright loops. The result is compared with field lines derived from a coronal magnetic field model extrapolated from a photospheric magnetogram recorded nearly simultaneously by SOHO MDI. We attribute discrepancies between reconstructed loops and extrapolated field lines to the inadequacy of the linear force-free field model used for the extrapolation. Title: First Stereoscopic Coronal Loop Reconstructions From STEREO/SECCHI Images Authors: Feng, L.; Inhester, B.; Solanki, S. K.; Wiegelmann, T.; Podlipnik, B.; Howard, R. A. Bibcode: 2007AGUFMSH41B..06F Altcode: We for the first time use simultaneously observed EUV images to reconstruct the 3D shape of magnetic loops which emerge from an active region. The images were taken by the two EUVI cameras of the SECCHI telescopes onboard the STEREO spacecraft. At the time the data was taken, the heliocentric separation of the two STEREO probes was 12 degrees. We show that under these conditions it is possible to obtain a reliable three- dimensional reconstruction of sufficiently bright loops as they usually emerge from an active region. The result is compared with field lines derived from a coronal magnetic field model extrapolated from a surface magnetogram. The magnetogram was abserved by SOHO/MDI only 9 seconds before the the EUV images were taken. Title: Discovery of inward moving magnetic enhancements in sunspot penumbrae Authors: Zhang, J.; Solanki, S. K.; Woch, J. Bibcode: 2007A&A...475..695Z Altcode: 2007arXiv0705.0604Z Context: Sunspot penumbrae show a fine structure in continuum intensity which displays considerable dynamics. The magnetic field, in contrast, although also highly structured, has been shown to be relatively static in earlier studies.
Aims: We report on the discovery of inward moving magnetic enhancements in the penumbrae of two regular sunspots, and characterize their fundamental properties.
Methods: Using continuum images, longitudinal magnetograms and Dopplergrams, recorded in high resolution mode by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have probed the evolution of magnetic features in the inner penumbra. The relationship between magnetic features, corresponding brightness in continuum images and Dopplergrams is also considered.
Results: Local enhancements of the line-of-sight (LOS) component of the magnetic field in the inner part of the penumbral region move inward towards the umbra-penumbra boundary with a radial speed of about 0.3 km s-1. These local inward-moving enhancements of the LOS component of the magnetic fields appear to be relatively common. They are associated with dark structures and tend to display downflows relative to the penumbral background. We also confirm the presence of outward moving magnetic enhancments in the outer half of the penumbra. Possible explanations are discussed. Title: The Fraction of DA White Dwarfs with Kilo--Gauss Magnetic Fields Authors: Jordan, S.; Aznar Cuadrado, R.; Napiwotzki, R.; Schmid, H. M.; Solanki, S. K. Bibcode: 2007ASPC..372..169J Altcode: 2006astro.ph.10881J Current estimates for white dwarfs with fields in excess of 1 MG are about 10%; according to our first high-precision circular-polarimetric study of 12 bright white dwarfs with the VLT tep{p26_Aznar-etal:04} this number increases up to about 25%\ in the kG regime. With our new sample of ten white dwarf observations (plus one sdO star) we wanted to improve the sample statistics to determine the incident of kG magnetic fields in white dwarfs. In one of our objects (LTT 7987) we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between (-1± 0.5) kG and (+1± 0.5) kG. This would be the weakest magnetic field ever found in a white dwarf, but at this level of accuracy, systematic errors cannot completely be ruled out. Together with previous investigations, the fraction of kG magnetic fields in white dwarfs amounts to about 11-15% , which is close to current estimates for highly magnetic white dwarfs (>1 MG). Title: A bright coronal downflow seen in multi-wavelength observations: evidence of a bifurcating flux-rope? Authors: Tripathi, D.; Solanki, S. K.; Mason, H. E.; Webb, D. F. Bibcode: 2007A&A...472..633T Altcode: 2008arXiv0802.3616T Aims:We study the origin and characteristics of a bright coronal downflow seen after a coronal mass ejection associated with erupting prominences on 5 March 2000.
Methods: This study extends that of Tripathi et al. (2006b, A&A, 449, 369) based on the Extreme-ultraviolet Imaging Telescope (EIT), the Soft X-ray Telescope (SXT) and the Large Angle Spectrometric Coronagraph (LASCO) observations. We combined those results with an analysis of the observations taken by the Hα and the Mk4 coronagraphs at the Mauna Loa Solar Observatory (MLSO). The combined data-set spans a broad range of temperature as well as continuous observations from the solar surface out to 30 R⊙.
Results: The downflow started at around 1.6 R⊙ and contained both hot and cold gas. The downflow was observed in the Hα and the Mk4 coronagraphs as well as the EIT and the SXT and was approximately co-spatial and co-temporal providing evidence of multi-thermal plasma. The Hα and Mk4 images show cusp-shaped structures close to the location where the downflow started. Mk4 observations reveal that the speed of the downflow in the early phase was substantially higher than the free-fall speed, implying a strong downward acceleration near the height at which the downflow started.
Conclusions: The origin of the downflow was likely to have been magnetic reconnection taking place inside the erupting flux rope that led to its bifurcation.

Movies are available in electronic form at http://www.aanda.org Title: The velocity structure of moving magnetic feature pairs around sunspots: support for the U-loop model Authors: Zhang, J.; Solanki, S. K.; Woch, J.; Wang, J. Bibcode: 2007A&A...471.1035Z Altcode: 2007arXiv0705.2507Z Context: Moving magnetic feature (MMF) pairs are among the most significant fine-scale structures around sunspots. Several models have been proposed to interpret the origin and evolution of MMF pairs. These models provide important clues to understanding MMF pairs.
Aims: We present an analysis of the velocity structure of MMF pairs in order to put further constrains on the MMF models.
Methods: Using continuum images, longitudinal magnetograms and Dopplergrams, recorded by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have traced 123 pairs of opposite magnetic polarity moving magnetic features (MMFs) in three active regions NOAA ARs 8375, 0330 and 9575. At the time of observation, AR 8375 was young, AR 0330 mature, and AR 9575 decaying.
Results: The vertical velocity, measured from MDI Dopplergrams for the three active regions, indicates that the elements of MMF pairs with polarity opposite to that of the sunspot support a downflow (Doppler redshift) of around 50-100 m s-1. The average Doppler shift difference between negative and positive elements of an MMF pair is about 150 m s-1 in AR 8375, 100 m s-1 in AR 0330, and 20 m s-1 in AR 9575. These observational results are in agreement with the model where MMF pairs are part of a U-loop emanating from the sunspot's magnetic canopy. According to this model, the downflow is caused by the Evershed flow returning below the solar surface. For AR 8375, the horizontal velocity of MMFs ranges from 0.1 km s-1 to 0.7 km s-1, and on average, the velocity of an MMF pair decreases significantly (from 0.6 km s-1 to 0.35 km s-1) with increasing distance from the MMF's birth place. In contrast, the decrease of the average velocity is far less obvious from 0.5 km s-1 to 0.4 km s-1 with increasing distance from the sunspot. This result suggests that the change in MMF flow speed does not reflect the radial structure of the moat flow, but rather is intrinsic to the evolution of the MMF pairs. This result is also in agreement with the U-loop model of MMF pairs. We also find that properties of MMF pairs, most strikingly the lifetime, depend on the evolution stages of the parent sunspot. The mean lifetimes of MMF pairs in ARs 9575 and 0330 are 0.7 h and 1.6 h, respectively, which is considerably shorter than the 4 h lifetime previously found for AR 8375. Title: Grand minima and maxima of solar activity: new observational constraints Authors: Usoskin, I. G.; Solanki, S. K.; Kovaltsov, G. A. Bibcode: 2007A&A...471..301U Altcode: 2007arXiv0706.0385U Aims:Using a reconstruction of sunspot numbers stretching over multiple millennia, we analyze the statistics of the occurrence of grand minima and maxima and set new observational constraints on long-term solar and stellar dynamo models.
Methods: We present an updated reconstruction of sunspot number over multiple millennia, from 14C data by means of a physics-based model, using an updated model of the evolution of the solar open magnetic flux. A list of grand minima and maxima of solar activity is presented for the Holocene (since 9500 BC) and the statistics of both the length of individual events as well as the waiting time between them are analyzed.
Results: The occurrence of grand minima/maxima is driven not by long-term cyclic variability, but by a stochastic/chaotic process. The waiting time distribution of the occurrence of grand minima/maxima deviates from an exponential distribution, implying that these events tend to cluster together with long event-free periods between the clusters. Two different types of grand minima are observed: short (30-90 years) minima of Maunder type and long (>110 years) minima of Spörer type, implying that a deterministic behaviour of the dynamo during a grand minimum defines its length. The duration of grand maxima follows an exponential distribution, suggesting that the duration of a grand maximum is determined by a random process.
Conclusions: These results set new observational constraints upon the long-term behaviour of the solar dynamo. Title: Solar irradiance variability Authors: Solanki, Sami K. Bibcode: 2007HiA....14..279S Altcode: We study solar irradiance variability. The current generation of models show that the irradiance since then has increased by between 0.9 and 1.5 W m-2. Title: Attenuation of Alfvén waves in straight and curved coronal slabs Authors: Gruszecki, M.; Murawski, K.; Solanki, S. K.; Ofman, L. Bibcode: 2007A&A...469.1117G Altcode: Aims:We consider impulsively generated Alfvén waves in coronal loops to investigate the role of energy leakage on wave attenuation, which includes lateral leakage, leakage into dense photospheric regions and nonlinear driving of magnetosonic waves.
Methods: A coronal loop is modelled either as a straight magnetic slab or as a curved slab of smooth mass density profiles. We perform numerical simulations of 2.5D ideal magnetohydrodynamic equations to determine the signatures of Alfvén waves.
Results: The numerical results show that lateral leakage of Alfvén waves is significant in comparison to leakage into the photospheric regions for realistic corona to photospheric density ratios. Energy leakage is enhanced by curvature of magnetic field lines and for large amplitude Alfvén waves for which nonlinear driving of magnetosonic waves is more significant than in the linear regime. Title: Low-lying magnetic loops in the solar internetwork Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.; Solanki, S. K. Bibcode: 2007A&A...469L..39M Altcode: 2007arXiv0705.1319M Aims:We study the structure of the magnetic field vector in the internetwork and search for the presence of small-scale loops.
Methods: We invert 1.56 μm spectropolarimetric observations of internetwork regions at disc centre by applying the SIR code. This allows us to recover the atmospheric parameters that play a role in the formation of these spectral lines. We are mainly interested in the structure of the magnetic field vector.
Results: We find that many opposite polarity elements of the internetwork are connected by short (2-6´´), low-lying (photospheric) loops. These loops connect at least the 10-20% of the internetwork flux visible in our data. Also we have some evidence that points towards a dynamic scenario that can be produced by the emergence of internetwork magnetic flux. Title: Modified p-modes in penumbral filaments? Authors: Bloomfield, D. S.; Solanki, S. K.; Lagg, A.; Borrero, J. M.; Cally, P. S. Bibcode: 2007A&A...469.1155B Altcode: 2007arXiv0705.0481B Aims:The primary objective of this study is to search for and identify wave modes within a sunspot penumbra.
Methods: Infrared spectropolarimetric time series data are inverted using a model comprising two atmospheric components in each spatial pixel. Fourier phase difference analysis is performed on the line-of-sight velocities retrieved from both components to determine time delays between the velocity signals. In addition, the vertical separation between the signals in the two components is calculated from the Stokes velocity response functions.
Results: The inversion yields two atmospheric components, one permeated by a nearly horizontal magnetic field, the other with a less-inclined magnetic field. Time delays between the oscillations in the two components in the frequency range 2.5-4.5 mHz are combined with speeds of atmospheric wave modes to determine wave travel distances. These are compared to expected path lengths obtained from response functions of the observed spectral lines in the different atmospheric components. Fast-mode (i.e., modified p-mode) waves exhibit the best agreement with the observations when propagating toward the sunspot at an angle ~50° to the vertical. Title: Stokes diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A. Bibcode: 2007A&A...469..731S Altcode: 2007astro.ph..3490S We present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow. Title: News and Views: Does the Sun affect the Earth's climate? Authors: Priest, Eric; Lockwood, Mike; Solanki, Sami; Wolfendale, Arnold; Coustenis, A. Bibcode: 2007A&G....48c...7P Altcode: 2007A&G....48c...7C Svensmark's article in the February issue (A&G 2007 48 1.18) presented a possible mechanism for the way the Sun could influence the Earth's climate. He suggested that water droplets condense in the ionization trail left by cosmic rays, whose flux varies inversely with solar activity: when the magnetic field of the solar wind is stronger, it shields the Earth from galactic cosmic rays and so decreases their flux on the Earth; according to Svensmark's ideas, this produces fewer clouds and thereby heats the Earth. Title: Observations of Running Waves in a Sunspot Chromosphere Authors: Bloomfield, D. S.; Lagg, A.; Solanki, S. K. Bibcode: 2007ASPC..368..239B Altcode: 2007astro.ph..2056B Spectropolarimetric time series data of the primary spot of active region NOAA 9448 were obtained in the Si I 10827 Å line and the He I 10830 Å multiplet with the Tenerife Infrared Polarimeter. Throughout the time series the spectrograph slit was fixed over a region covering umbra, a light bridge, penumbra, and quiet sun. We present speeds of running penumbral waves in the chromosphere, their relation to both photospheric and chromospheric umbral oscillations, and their dependence on the magnetic field topology. Title: Solar Cycle Variation of Chromospheric Radiation Authors: Solanki, S. K. Bibcode: 2007ASPC..368..481S Altcode: Radiation emitted by the Sun's chromospheric gas displays a significant cyclic variation. The magnitude of this variability and the shape of the light curve differs from that exhibited by photospheric radiation, or total solar irradiance. The amplitude of the cyclic variation of chromospheric radiation is larger and less affected by the contribution of sunspots. Consequently, the influence of small-scale magnetic features forming plage and the network dominates. Here a brief introduction is given to the solar cycle variation of chromospheric radiation, its connection with the magnetic field, its quantitative modeling and related questions, such as the solar cycle variability of the quiet Sun chromosphere and its cause. Finally, some thoughts on the possible secular change of chromospheric (and total) irradiance are presented. Title: Solar Activity and Irradiance Studies with Ca II Spectroheliograms: Potential and Problems Authors: Ermolli, I.; Tlatov, A.; Solanki, S. K.; Krivova, N. A.; Singh, J. Bibcode: 2007ASPC..368..533E Altcode: Various observatories around the globe carried out synoptic full-disk observations of the Sun since the beginning of the 20th century. The archives created by these observations, especially those including Ca II spectroheliograms, have the potential of providing far more detailed information on solar activity than the indices usually used to study activity variations, solar cycle and irradiance changes. However, these data suffer significantly from various problems including numerous defects in the photographic plates, missing or inaccurate calibration of the blackening curve, changes in the positioning of the exit slit with respect to the spectral line and variable seeing. Here we discuss the quality of images obtained by the digitization of three historic Ca II K time series, specifically those stored by the Arcetri, Kodaikanal and Mt Wilson Observatories. The aim of this work is to evaluate the potential value of these data for studies of solar activity and variability. It also shows the importance of the detailed and accurate image processing technique, in order to obtain uniform and trustable results from images coming from different historic archives. Title: Tenerife Infrared Polarimeter II Authors: Collados, M.; Lagg, A.; Díaz Garcí A, J. J.; Hernández Suárez, E.; López López, R.; Páez Mañá, E.; Solanki, S. K. Bibcode: 2007ASPC..368..611C Altcode: Since May 2005 the Tenerife Infrared Polarimeter II (TIP-II) has been operational at the Vacuum Tower Telescope on Tenerife. The core of the polarimeter is a 1024×1020 pixel infrared camera allowing for high precision measurements of the full Stokes vector with a pixel size of 0.18 arcsec, corresponding to the diffraction limit of the telescope at 1 μm. The polarimeter is able to reach a polarimetric accuracy of a few times 10-4, covering a wavelength range of 1 to 1.8 μm. With an upgrade in July 2006, the slit size has been increased to 77 arcsec allowing most active regions to be covered with a single scan. Here we present the technical details of the polarimeter and the camera. We also show some data illustrating the power of this new instrumentation. Title: 2d Simulations Of Excitation And Damping Of Vertical Kink Waves Authors: Selwa, Malgorzata; Murawski, K.; Solanki, S. K.; Ofman, L. Bibcode: 2007AAS...210.9115S Altcode: 2007BAAS...39S.206S We consider different kinds of excitation of fast vertical kink standing waves in a solar coronal loop that is embedded in a potential arcade. The two dimensional numerical model we implement includes the effects of field line curvature and nonlinearity on the excitation and damping of standing fast magnetosonic waves. We investigate the effects of a driven sinusodial pressure pulse and compare it with an impulsive excitation by a pressure pulse that impacts the overlaying loop. The results of the numerical simulations reveal wave signatures which are characteristic of vertical loop oscillations seen in recent TRACE observational data. Title: Reconstruction of the Long-Term Irradiance Variations Authors: Balmaceda, L.; Krivova, N.; Solanki, S. Bibcode: 2007AGUSMGP54A..05B Altcode: Solar irradiance variations have been recorded only since 1978. Clearly, there is a need to extend these records into the past in order to evaluate their possible influence on the Earth's climate. Here, a reconstruction of solar irradiance back to the Maunder minimum from the surface magnetic flux is presented. The reconstruction is based on a simple physical model that builds on the sunspot number records and sunspot areas where available. Since the sunspot area records generally consist of a compilation of data from multiple observatories, a proper cross-calibration is essential. The use of data of different sources directly combined can lead to errors in estimating the increase of solar irradiance during the past centuries. Thus, a brief description of the cross-calibration of sunspot areas is also presented. Title: Reconstruction of solar total irradiance since 1700 from the surface magnetic flux Authors: Krivova, N. A.; Balmaceda, L.; Solanki, S. K. Bibcode: 2007A&A...467..335K Altcode: Context: Total solar irradiance changes by about 0.1% between solar activity maximum and minimum. Accurate measurements of this quantity are only available since 1978 and do not provide information on longer-term secular trends.
Aims: In order to reliably evaluate the Sun's role in recent global climate change, longer time series are, however, needed. They can only be assessed with the help of suitable models.
Methods: The total solar irradiance is reconstructed from the end of the Maunder minimum to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the sunspot number using a simple but consistent physical model.
Results: Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux since 1974 and the open magnetic flux since 1868 empirically reconstructed using the geomagnetic aa-index. The model predicts an increase in the solar total irradiance since the Maunder minimum of 1.3^+0.2_-0.4 Wm-2. Title: Full-Stokes Observations and Analysis of He I 10830 Å in a Flaring Region Authors: Sasso, C.; Lagg, A.; Solanki, S. K.; Aznar Cuadrado, R.; Collados, M. Bibcode: 2007ASPC..368..467S Altcode: We present observations of the full Stokes vector in a flaring region, taken in the chromospheric He I 10830 Å multiplet. The data were recorded with the new Tenerife Infrared Polarimeter (TIP 2) at the German Vacuum Tower Telescope (VTT) during May 2005. The He profiles during the flare are extraordinary, showing extremely broad Stokes I absorption and very complex and spatially variable Stokes V signatures. We give first results on the line-of-sight velocities and the magnetic field vector values in the chromosphere for one observed Stokes profile by applying an inversion code to the He I lines. Title: Magnetic Stereoscopy of Coronal Loops in NOAA 8891 Authors: Feng, L.; Wiegelmann, T.; Inhester, B.; Solanki, S.; Gan, W. Q.; Ruan, P. Bibcode: 2007SoPh..241..235F Altcode: The Solar TErrestrial RElations Observatory (STEREO) requires powerful tools for the three-dimensional (3D) reconstruction of the solar corona. Here we test such a program with data from SOHO and TRACE. By taking advantage of solar rotation, a newly developed stereoscopy tool for the reconstruction of coronal loops is applied to the solar active region NOAA 8891 observed from 1 March to 2 March 2000. The stereoscopic reconstruction is composed of three steps. First, we identify loop structures in two TRACE images observed from two vantage viewpoints approximately 17 degrees apart, which corresponds to observations made about 30 hours apart. In the second step, we extrapolate the magnetic field in the corona with the linear force-free field model from the photospheric line-of-sight SOHO/MDI data. Finally, combining the extrapolated field lines and one-dimensional loop curves from two different viewpoints, we obtain the 3D loop structures with the magnetic stereoscopy tool. We demonstrate that by including the magnetic modeling this tool is more powerful than pure geometrical stereoscopy, especially in resolving the ambiguities generated by classical stereoscopy. This work will be applied to the STEREO mission in the near future. Title: Properties of sunspots in cycle 23. I. Dependence of brightness on sunspot size and cycle phase Authors: Mathew, S. K.; Martínez Pillet, V.; Solanki, S. K.; Krivova, N. A. Bibcode: 2007A&A...465..291M Altcode: 2007astro.ph..1401M Aims:In this paper we investigate the dependence of umbral core brightness, as well as the mean umbral and penumbral brightness on the phase of the solar cycle and on the size of the sunspot.
Methods: Albregtsen & Maltby (1978, Nature, 274, 41) reported an increase in umbral core brightness from the early to the late phase of solar cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here we revisit this topic by analysing continuum images of more than 160 sunspots observed by the MDI instrument on board the SOHO spacecraft for the period between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The advantage of this data set is its homogeneity, with no seeing fluctuations. A careful stray light correction, which is validated using the Mercury transit of 7th May, 2003, is carried out before the umbral and penumbral intensities are determined. The influence of the Zeeman splitting of the nearby Ni I spectral line on the measured "continuum" intensity is also taken into account.
Results: We did not observe any significant variation in umbral core, mean umbral and mean penumbral intensities with solar cycle, which is in contrast to earlier findings for the umbral core intensity. We do find a strong and clear dependence of the umbral brightness on sunspot size, however. The penumbral brightness also displays a weak dependence. The brightness-radius relationship has numerous implications, some of which, such as those for the energy transport in umbrae, are pointed out. Title: Properties of Solar Polar Coronal Plumes Constrained by Ultraviolet Coronagraph Spectrometer Data Authors: Raouafi, N. -E.; Harvey, J. W.; Solanki, S. K. Bibcode: 2007ApJ...658..643R Altcode: 2007astro.ph..3745R We investigate the plasma dynamics (outflow speed and turbulence) inside polar plumes. We compare line profiles (mainly of O VI) observed by the Ultraviolet Coronagraph Spectrometer (UVCS) instrument on SOHO at the minimum of solar cycle 22-23 with model calculations. We consider Maxwellian velocity distributions with different widths in plume and interplume regions. Electron densities are assumed to be enhanced in plumes and to approach interplume values with increasing height. Different combinations of the outflow and turbulence velocity in the plume regions are considered. We compute line profiles and total intensities of the H I Lyα and the O VI doublets. The observed profile shapes and intensities are reproduced best by a small solar wind speed at low altitudes in plumes that increases with height to reach ambient interplume values above roughly 3-4 Rsolar, combined with a similar variation of the width of the velocity distribution of the scattering atoms/ions. We also find that plumes very close to the pole give narrow profiles at heights above 2.5 Rsolar, which are not observed. This suggests a tendency for plumes to be located away from the pole. We find that the inclusion of plumes in the model computations provides an improved correspondence with the observations and confirms previous results showing that published UVCS observations in polar coronal holes can be roughly reproduced without the need for large temperature anisotropy. The latitude distributions of plumes and magnetic flux distributions are studied by analyzing data from different instruments on SOHO and with SOLIS. Title: Magnetic flux transport on active cool stars and starspot lifetimes Authors: Işik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2007A&A...464.1049I Altcode: 2006astro.ph.12399I Context: Many rapidly rotating cool stars show signatures of large magnetic regions at all latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes of 1 month or less, as indicated by observations using (Zeeman-) Doppler imaging techniques.
Aims: We aim to estimate the lifetimes of bipolar magnetic regions and starspots on the surfaces of cool stars. We consider different possible configurations for starspots and compare their flux variations and lifetimes based on a magnetic flux transport model.
Methods: We carry out numerical simulations of the surface evolution of bipolar magnetic regions (BMRs) and magnetic spots on stars, which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to convective flow patterns. We calculate the flux evolution and the lifetimes of BMRs and unipolar starspots, varying the emergence latitude, surface shear rate, and tilt angle.
Results: For BMRs comparable to the largest observed on the Sun, we find that varying the surface flows and the tilt angle modifies the lifetimes over a range of one month. For very large BMRs (area ~10% of the stellar surface) the assumption of a tilt angle increasing with latitude leads to a significant increase of lifetime, as compared to the case without tilt. Such regions can evolve to polar spots that live more than a year. Adopting the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. Random emergence of six additional tilted bipoles in an activity belt at 60° latitude enhanced the lifetimes of polar caps up to 7 years. We have also compared the evolution and lifetime of monolithic starspots with those of conglomerates of smaller spots of similar total area. We find similar decay patterns and lifetimes for both configurations. Title: Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides Authors: Selwa, M.; Murawski, K.; Solanki, S. K.; Wang, T. J. Bibcode: 2007A&A...462.1127S Altcode: Aims:We study wave leakage as a possible attenuation mechanism of coronal loop oscillations in the ideal MHD regime.
Methods: We consider impulsively generated oscillations in solar coronal magnetic wave guides such as a straight slab and a curved arcade loop. The two-dimensional numerical model we implement includes the effects of nonlinearity and line curvature on attenuation of fast magnetosonic kink waves.
Results: We show that these waves are more strongly attenuated in the arcade loop than in the slab and provide evidence that the curvature of magnetic field lines results in excess energy leakage. For parameters appropriate for a coronal loop the kink oscillation is too efficiently attenuated by energy leakage, suggesting that in the solar atmosphere wave leakage must be reduced compared to our simulations. We conclude that energy leakage is an efficient source of attenuation of coronal loop oscillations. Title: The fraction of DA white dwarfs with kilo-Gauss magnetic fields Authors: Jordan, S.; Aznar Cuadrado, R.; Napiwotzki, R.; Schmid, H. M.; Solanki, S. K. Bibcode: 2007A&A...462.1097J Altcode: 2006astro.ph.10875J Context: Weak magnetic fields have been searched for on only a small number of white dwarfs. Current estimates find that about 10% of all white dwarfs have fields in excess of 1 MG; according to previous studies this number increases up to about 25% in the kG regime.
Aims: Our aim is to improve on these statistics by a new sample of ten white dwarfs in order to determine the ratio of magnetic to field-free white dwarfs.
Methods: Mean longitudinal magnetic fields strengths were determined by means of high-precision circular polarimetry of Hβ and Hγ with the FORS1 spectrograph of the VLT "Kueyen" 8 m telescope.
Results: In one of our objects (LTT 7987), we detected a statistically significant (97% confidence level) longitudinal magnetic field varying between (-1± 0.5) kG and (+1± 0.5) kG. This would be the weakest magnetic field ever found in a white dwarf, but systematic errors cannot completely be ruled out at this level of accuracy. We also observed the sdO star EC 11481-2303 but could not detect a magnetic field.
Conclusions: . VLT observations with uncertainties typically of 1000 G or less suggest that 15-20% of WDs have kG fields. Together with previous investigations, the fraction of kG magnetic fields in white dwarfs amounts to about 11-15%, which is close to the current estimations for highly magnetic white dwarfs (>1 MG).

Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programme ID 073.D-0356. Figures A.1 and A.2 are only available in electronic form at http://www.aanda.org Title: Supersonic downflows in the vicinity of a growing pore. Evidence of unresolved magnetic fine structure at chromospheric heights Authors: Lagg, A.; Woch, J.; Solanki, S. K.; Krupp, N. Bibcode: 2007A&A...462.1147L Altcode: Aims:The velocity and magnetic fine structure of the chromosphere at the leg of an emerging magnetic loop is investigated at a location of supersonic downflows.
Methods: We analyze a time series of spectropolarimetric data in the He i 1083 nm triplet covering a time interval of ≈70 min. The temporal evolution as well as the topology of the magnetic field in the downflow region are investigated. We apply an inversion technique based on a genetic algorithm using the Milne-Eddington approach. The technique is very reliable and robust in retrieving maps of the velocity and the magnetic field vector for both atmospheric components separately.
Results: We observe redshifts corresponding to a downflow speed of up to 40 km s-1 in the vicinity of a growing pore. These supersonic downflows always coexist with a second atmospheric component almost at rest (slow component) within the same resolution element. The redshifted component is more inclined to the solar normal than the slow component and has a different field strength.
Conclusions: .We interpret this downflow as a consequence of the draining of the rising loops. The different magnetic field orientation of the redshifted and the slow component give rise to two possible interpretations: an uncombed structure of the chromosphere, similar to the differently inclined flux-tubes in the penumbra of a sunspot, or a cloud-like structure containing gas at different velocities in two separate height layers of the solar atmosphere. Title: T he Lower Transition Region As Seen In The H I Lyman-α Line Authors: Teriaca, L.; Schühle, U.; Solanki, S. K.; Curdt, W.; Marsch, E. Bibcode: 2007ESASP.641E..84T Altcode: The SUMER spectrometer aboard SOHO has been used to acquire several raster images and temporal series of quiet-Sun targets at both disk centre and the limb. Spectra have been recorded simultaneously in the H I Lyman α and the Si III 120.6 nm line. Both spatial and temporal maps of the integrated radiances appear very similar in the two lines, despite the huge difference in optical thickness, a result showing the H I Lyman α to be a good diagnostic of the dynamics and morphology of the lower transition region. Oscillations can be detected and studied at all observed locations. At disk centre, the 3 minute oscillations are sporadically observed in the inter-network but also at locations at the edges of network lanes, while 5 minute oscillations clearly dominate the network. At the limb, evidence of 3 to 5 minute oscillations is found at the base of spicules. Moreover, H I Lyman α spectra shows a high degree of variability, revealing also the signature of explosive events. The combination of high spectral purity images and slit spectra in the H I Lyman α line would therefore be an exceptional new tool to investigate the nature of the solar transition region. This line is therefore of interest for both, a high resolution channel in the EUI instrument and for the EUS spectrometer. Title: T he Lower Transitio n Region As Seen In The H I Lyman-α Line. Authors: Teriaca, L.; Schühle, U.; Solanki, S. K.; Curdt, W.; Marsch, E. Bibcode: 2007ESASP.641E..36T Altcode: The SUMER spectrometer aboard SOHO has been used to acquire several raster images and temporal series of quiet-Sun targets at both disk centre and the limb. Spectra have been recorded simultaneously in the H I Lyman α and the Si III 120.6 nm line. Both spatial and temporal maps of the integrated radiances appear very similar in the two lines, despite the huge difference in optical thickness, a result showing the H I Lyman α to be a good diagnostic of the dynamics and morphology of the lower transition region. Oscillations can be detected and studied at all observed locations. At disk centre, the 3 minute oscillations are sporadically observed in the inter-network but also at locations at the edges of network lanes, while 5 minute oscillations clearly dominate the network. At the limb, evidence of 3 to 5 minute oscillations is found at the base of spicules. Moreover, H I Lyman α spectra shows a high degree of variability, revealing also the signature of explosive events. The combination of high spectral purity images and slit spectra in the H I Lyman α line would therefore be an exceptional new tool to investigate the nature of the solar transition region. This line is therefore of interest for both, a high resolution channel in the EUI instrument and for the EUS spectrometer. Title: SUNRISE: High resolution UV/VIS observations of the Sun from the stratosphere Authors: Gandorfer, A. M.; Solanki, S. K.; Barthol, P.; Martínez Pillet, V.; Schmidt, W.; Title, A. M.; Knölker, M. Bibcode: 2007msfa.conf...69G Altcode: SUNRISE is an international project for the development, construction, and operation of a balloon-borne solar telescope with an aperture of 1 m, working in the UV/VIS spectral domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide near diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35 km on the solar surface at wavelengths around 220 nm. The focal-plane instrumentation consists of a polarization sensitive spectrograph, a Fabry-Perot filter magnetograph, and a phase-diverse filter imager working in the near UV. The first stratospheric long-duration balloon flight of SUNRISE is planned in summer 2009 from the Swedish ESRANGE station. SUNRISE is a joint project of the German Max-Planck-Institut für Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, Germany, the High-Altitude Observatory (HAO), Boulder, USA, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, USA, and the Spanish IMaX consortium. In this paper we will present a brief description of the scientific and technological aspects of SUNRISE. Title: Magnetic source of the solar cycle variation of the Mn I 539.4 nm line Authors: Danilović, S.; Solanki, S. K.; Livingston, W.; Krivova, N.; Vince, I. Bibcode: 2007msfa.conf..189D Altcode: As a part of the long term program at KPNO, the Mn I 539.4 nm line has been observed for nearly three solar cycles using the McMath telescope and the 13.5 m spectrograph in double pass mode. These full-disk spectrophotometric observations revealed an unusually large amplitude change of its parameters over the solar cycle and its correlation with Ca II K intensity. One of the proposed explanations for this phenomenon is the optical pumping by the Mg II k line. With this work we would like to show that this may not be the main mechanism behind the change. We reconstructed the changes of the line parameters using a model that takes into account only changes of the daily surface distributions of magnetic field. This model has already been used to successfully model total solar irradiance. We now apply it for modelling the Mn I line, as well as its neighboring Fe I line using exactly the same value of the free parameter as used for the reconstruction of total solar irradiance. We reproduce well the Mn I and Fe I line changes over the cycle purely with LTE modelling. This indicates that optical pumping of the Mn I line by Mg II k is not the main cause of its solar cycle change and sets an independent constraint on solar irradiance models. Title: Structure and evolution of supergranulation from local helioseismology Authors: Hirzberger, J.; Gizon, L.; Solanki, S. K.; Duvall, T. L. Bibcode: 2007msfa.conf..103H Altcode: Maps of the horizontal divergence of the near-surface velocity field have been calculated using local helioseismology and SOHO/MDI full-disk Dopplergrams. These maps provide a continuous coverage for two to three months each year with a cadence of 12 hours. Geometrical and evolutional properties of individual supergranular cells have been studied. Supergranular cells have sizes in a range around 650Mm2 (circular diameter of 28.77 Mm) with lifetimes of up to 4.5 days. We also observe a clear trend for larger cells to have stronger divergence values and larger lifetimes than smaller ones. Title: EUI, The Ultraviolet Imaging Telescopes Of Solar Orbiter Authors: Hochedez, J. -F.; Appourchaux, T.; Defise, J. -M.; Harra, L. K.; Schühle, U.; Auchère, F.; Curdt, W.; Hancock, B.; Kretzschmar, M.; Lawrence, G.; Leclec'h, J. -C.; Marsch, E.; Mercier, R.; Parenti, S.; Podladchikova, E.; Ravet, M. -F.; Rochus, P.; Rodriguez, L.; Rouesnel, F.; Solanki, S.; Teriaca, L.; Van Driel, L.; Vial, J. -C.; Winter, B.; Zhukov, A. Bibcode: 2007ESASP.641E..33H Altcode: The scientific objectives of Solar Orbiter rely ubiquitously on EUI, its suite of solar atmosphere imaging telescopes. In the configuration discussed here, EUI includes three co-aligned High Resolution Imagers (HRI) and one Full Sun Imager (FSI). FSI and two HRIs observe in extreme ultraviolet passbands, dominated by coronal emission. Another HRI is designed for the hydrogen Lyman α radiation in the far UV, imaging the Chromosphere and the lower Transition Region. The current EUI design and some of its development challenges are highlighted. EUI profits from co-rotation phases, solar proximity and departure from the ecliptic. In synergy with the other S.O. payload, EUI probes the dynamics of the solar atmosphere, provides context data for all investigations and helps to link in-situ and remote-sensing observations. In short, it serves all four top-level goals of the mission. For these reasons, the EUI suite is keenly anticipated in the European scientific community and beyond. Title: Nanoflare model of emission line radiance distributions in active region coronae Authors: Safari, H.; Innes, D. E.; Solanki, S. K.; Pauluhn, A. Bibcode: 2007msfa.conf..359S Altcode: Nanoflares are small impulsive bursts of energy that blend with and possibly make up much of the solar background emission. Determining their overall contribution is central to understanding the heating of the solar corona. Here, a simple nanoflare model based on three key parameters: the flare rate, the flare damping time, and the power-law slope of the flare energy frequency distribution has been used to simulate emission line radiances observed by SUMER in the corona above an active region. The three lines analysed, Fe XIX, Ca XIII, and Si III have very different formation temperatures, damping times and flare rates but all suggest a power-law slope greater than 2. Thus it is possible that nanoflares provide a significant fraction of the flare energy input to active region coronae. Title: A nanoflare model of quiet Sun EUV emission Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2007A&A...462..311P Altcode: 2006astro.ph.12585P Nanoflares have been proposed as the main source of heating of the solar corona. However, detecting them directly has so far proved elusive, and extrapolating to them from the properties of larger brightenings gives unreliable estimates of the power-law exponent α characterising their distribution. Here we take the approach of statistically modelling light curves representative of the quiet Sun as seen in EUV radiation. The basic assumption is that all quiet-Sun EUV emission is due to micro- and nanoflares, whose radiative energies display a power-law distribution. Radiance values in the quiet Sun follow a lognormal distribution. This is irrespective of whether the distribution is made over a spatial scan or over a time series. We show that these distributions can be reproduced by our simple model. By simultaneously fitting the radiance distribution function and the power spectrum obtained from the light curves emitted by transition region and coronal lines the power-law distribution of micro- and nanoflare brightenings is constrained. A good statistical match to the measurements is obtained for a steep power-law distribution of nanoflare energies, with power-law exponent α> 2. This is consistent with the dominant heat input to the corona being provided by nanoflares, i.e., by events with energies around 1023 erg. In order to reproduce the observed SUMER time series approximately 103 to 104 nanoflares are needed per second throughout the atmosphere of the quiet Sun (assuming the nanoflares to cover an average area of 1013 m2). Title: Reconstruction of solar irradiance using the Group sunspot number Authors: Balmaceda, L.; Krivova, N. A.; Solanki, S. K. Bibcode: 2007AdSpR..40..986B Altcode: 2007astro.ph..3147B We present a reconstruction of total solar irradiance since 1610 to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the Group sunspot number using a simple but consistent physical model. Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux from 1974 and the open magnetic flux since 1868 (as empirically reconstructed from the geomagnetic aa-index). The model predicts an increase in the total solar irradiance since the Maunder Minimum of about 1.3 Wm -2. Title: T he Performance Of The SOLO-VIM Instrument: Effects Of Instrumental Noise And Lossy Data Compression Authors: Lagg, A.; Yelles, L.; Hirzberger, J.; Woch, J.; Solanki, S. K. Bibcode: 2007ESASP.641E..69L Altcode: Spectropolarimetric observations in photospheric lines reveal a wealth of information on physical parameters of the solar atmosphere like magnetic field strength and di rection or the line-of sight velocity. These observations require the measurement of the four Stokes parameters at a sample of N wavelength positions around the core of the spectral line, resulting in 4N images for one observation. The Visible light Imager and Magnetograph (VIM) instrument on board Solar Orbiter is capable of performing these measurements. However, the data rate required to transfer all 4N images with the required cadence is well beyond the telemetry limit. Here we use realistic, three-dimensional MHD simulations in order to simulate science data provided by VIM which are then used to test various compression techniques. We conclude that lossy data compression and instrumental noise have similar effects on the output data. Title: S imulations Of Science Data Of The Solo-VIM Instrument Authors: Yelles, L.; Hirzberger, J.; Lagg, A.; Woch, J.; Solanki, S. K.; Vögler, A. Bibcode: 2007ESASP.641E..34Y Altcode: The SolO-VIM instrument will be a two-dimensional full-Stokes spectro-polarimeter which will provide diffraction-limited vector-magnetograms, Dopplergrams, and continuum images of the solar photosphere. The instrument's performance depends on various parameters such as aperture diameter, filter characteristics, spectral- line sampling, and orbital position. Here we compute Stokes profiles in realistic 3D MHD simulations. These synthetic data are then degraded to match the output ex- pected from the VIM instrument, and subsequently inverted using a Milne-Eddington atmosphere. We present parameter studies in order to set up minimum require- ments on limitations of VIM's capabilities. Title: Velocity distribution of chromospheric downflows Authors: Aznar Cuadrado, R.; Solanki, S. K.; Lagg, A. Bibcode: 2007msfa.conf..173A Altcode: Infrared spectropolarimetric observations were obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) of the Spanish observatory of Izana, Tenerife. We present the velocity distributions of a large dataset composed of maps of the Stokes I, Q, U, and V profiles of active and quiet sun regions obtained in the chromospheric He I 1083.0 nm triplet. The line-of-sight velocities were determined by applying a multi-Gaussian fit to the intensity profiles. Single and double component fits were carried out for all datasets. We find that 18.7% of all observed pixels show strong downflows as evidenced by a second line profile component, generally shifted by more than 8 km s-1 relative to the rest wavelegth. The distribution of these strong down-flows displays two distinct populations. The slower one (near sonic and weakly supersonic flows) has line-of-sight velocities up to 17 km s-1 and is associated with moderate to strong magnetic signal (up to √(Q2 + U2 + V2)/Ic = 0.08). Strongly supersonic downflows (reaching up to 60 km s-1) are found at places with weak to moderate magnetic signal, with √(Q2 + U2 + V2)/Ic values mainly between 0.01 and 0.03. Title: Spectropolarimetry in the chromospheric He I 1083.0 nm multiplet Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2007waas.work...43S Altcode: Spectropolarimetry in the He I 1083.0 nm multiplet has a great potential to obtain information on the magnetic field in the solar upper chromosphere. We apply an inversion technique to infrared spectropolarimetric observations, to retrieve the full magnetic vector and the line-of sight velocity. The observations, obtained with the Tenerife Infrared Polarimeter (TIP) II at the German Vacuum Tower Telescope (VTT), provide maps in the Stokes parameters I, Q, U and V. Title: A comparative study of the contrast of solar magnetic elements in CN and CH Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M. Bibcode: 2007A&A...461..695Z Altcode: No abstract at ADS Title: Magnetic flux transport and the lifetimes of spots on active cool stars Authors: Işik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2007msfa.conf..367I Altcode: We present results of numerical simulations of magnetic flux transport on the surfaces of cool stars with radii of 1 Rm and 3.3 Rm. The effects of differential rotation and the tilt angle on the lifetimes of stellar bipolar magnetic regions are discussed. The existence of long-lasting polar spots can be explained by high-latitude persistent emergence of bipolar regions. Title: High-resolution CN spectroscopy of small-scale solar magnetic features Authors: Zakharov, V. V.; Gandorfer, A.; Solanki, S. K. Bibcode: 2007msfa.conf..161Z Altcode: High-resolution spectroscopic observations of small-scale magnetic elements in the solar photosphere were carried out in the spectral region 387.5388.4 nm with the 1-m Swedish Solar Telescope (SST). This part of the spectrum covers not only the violet CN band-head, but also contains some lines of the CH molecule. The analysis of the line-core intensity contrasts of the CN and CH lines in bright points (BPs) yielded that on average the BPs appear brighter, thus providing a higher rms contrast, in the CN than in the CH lines in the same spectral band. Title: Wavelet-based method for coronal loop oscillations analysis Authors: Tothova, D.; Innes, D. E.; Solanki, S. K. Bibcode: 2007msfa.conf..265T Altcode: We study the properties of Doppler shift oscillations observed in hot coronal loops by SoHO/SUMER. These oscillations have been identified as magnetoacoustic slow mode standing waves and may be an important magnetic field diagnostic for the oscillating loops. Visual inspection of the SUMER data suggests that one in three microflares trigger oscillation. So for an average active region we can expect several oscillation events per hour. To study the statistics of the oscillations, automatic identification and classification of the events is required. We have performed a Wavelet analysis on a 12 hour time series of SUMER Fe XIX data and find that almost all of the oscillation events in the data set are identified by this technique, with very few false positives. The method is expected to be useful for the analysis of future Ca XVII and Fe XXIV spectra of microflares observed with HINODE/EIS. Title: Modified p-modes in penumbral filaments Authors: Bloomfield, D. S.; Lagg, A.; Solanki, S. K.; Borrero, J. M. Bibcode: 2007msfa.conf..241B Altcode: A time series analysis was performed on velocity signals in a sunspot penumbra to search for possible wave modes. The spectropolarimetric photospheric data obtained by the Tenerife Infrared Polarimeter were inverted using the SPINOR code. An atmospheric model comprising two magnetic components and one stray-light component gave an optimal fit to the data. Fourier phase difference analysis between line-of-sight velocities of both magnetic components provided time delays between the two atmospheres. These delays were combined with the speeds of atmospheric wave modes and compared to height separations derived from velocity response functions to determine the wave mode. Title: Nonlinear Force-Free Magnetic Field Modelling For VIM On SO Authors: Wiegelmann, T.; Solanki, S. K.; Yelles, L.; Lagg, A. Bibcode: 2007ESASP.641E..19W Altcode: The aim of this work is to investigate how photon noise and errors in the retrieval of solar magnetic parameters from measured Stokes profiles influences the extrapolartion of nonlinear force-free coronal magnetic fields from photospheric vector magnetograms. To do so we use a nonlinear force-free extrapolation code based on an optimization principle. The extrapolation methods has been extensively tested and applied to data from various telescopes. Here we apply the code artificial vector magnetograms obtained from 3-D radiation-MHD simulations. As a reference case we compute the coronal magnetic field from an ideal magnetogram and compare the result with more realistic magnetograms based on simulated Solar Orbiter/VIM-measurements. We investigate the effect of noise, ambiguities, spatial resolution, inversion mechanism, of Stokes profiles etc. We rate the quality of the reconstructed coronal magnetic field qualitatively by magnetic field line plots and quantitatively by a number of comparison metrices, e.g., the vector correlation with the exact solution and how accurate the free magnetic energy is computed. Not surprisingly, the instrument effects and noise influence the quality of the nonlinear force-free coronal magnetic field model. The extrapolations from realistic vector magnetograms show a reasonable agreement with the ideal reconstruction, however, and are in particular significantly better than extrapolations based on line-of-sight magnetograms only. High quality VIM data will thus allow reasonably accurate extrapolations that can serve as the basis for magnetic coupling science through a comparison with observations from EUS and EUI. Title: S pace Qualification Of A Thin Wafer Lithium Niobate Etalon For The Visible Light Imager And Magnetograph (Vim) Authors: Schühle, U.; Mathew, S. K.; Wedemeier, M.; Hartwig, H.; Ballesteros, E.; Martinez Pillet, V.; Solanki, S. K. Bibcode: 2007ESASP.641E..82S Altcode: For the Visible Light Imager and Magnetograph (VIM) a high-resolution filtergraph is under design. The system takes advantage of a lithium niobate (LiNbO3) crystal which can be used as a scanning filter using high voltage for tuning. We have undertaken first studies to qualify a lithium niobate wafer of 70 mm aperture size for deployment and use in space. We show the results of the mechanical mounting and vibration and thermal cycling tests as well as stability tests under fast voltage tuning in vacuum. Although these tests have all been very successful, further environmental testing is necessary to fully space-qualify the filter for the Solar Orbiter mission. Title: Spectro-Polarimetry of a Sunspot in Atomic and Molecular Lines with THEMIS Authors: Arnaud, J.; Berdyugina, S. V.; Fluri, D. M.; Afram, N.; Solanki, S. K.; Raouafi, N. -E. Bibcode: 2006ASPC..358..319A Altcode: We present spectro-polarimetric observations of a sunspot, which were recorded simultaneously with THEMIS in various atomic and molecular lines. These observations include the first full Stokes measurements of the band-head of TiO around 7055 Å. Title: Molecular Diagnostics of the Internal Structure of Starspots and Sunspots Authors: Afram, N.; Berdyugina, S. V.; Fluri, D. M.; Solanki, S. K.; Lagg, A.; Petit, P.; Arnaud, J. Bibcode: 2006ASPC..358..375A Altcode: We have analyzed the usefulness of molecules as a diagnostic tool for studying solar and stellar magnetism with the molecular Zeeman and Paschen-Back effects. In the first part we concentrate on molecules that are observed in sunspots such as MgH and TiO. We present calculated molecular line profiles obtained by assuming magnetic fields of 2-3 kG and compare these synthetic Stokes profiles with spectro-polarimetric observations in sunspots. The good agreement between the theory and observations allows us to turn our attention in the second part to starspots to gain insight into their internal structure. We investigate the temperature range in which the selected molecules can serve as indicators for magnetic fields on highly active cool stars and compare synthetic Stokes profiles with our recent observations. Title: The Molecular Paschen-Back Effect Authors: Berdyugina, S. V.; Fluri, D. M.; Solanki, S. K. Bibcode: 2006ASPC..358..329B Altcode: The molecular Paschen-Back effect (PBE) lacks a detailed description since the problem was first addressed by Hill in 1929. However, many diatomic molecules exhibit the PBE at field strengths typical of sunspots and active cool stars. Recently we have presented a complete theoretical description of the molecular PBE in Hund's cases (a), (b), and all intermediate cases. This description allows us to compute the splitting of levels of any multiplicity and the transitions between them. We find that in the partial PBE regime strongly asymmetric Stokes profiles are produced, whose strengths and asymmetries depend sensitively on the magnetic field. Also, the strength of the forbidden and satellite transitions increases rapidly with field strength, while the strength of the main branch transitions decreases. These signatures hold promise to form the basis of new diagnostics of solar and stellar magnetic fields. Title: Measuring the Magnetic Vector with the He I 10830 Å Line: A Rich New World Authors: Solanki, S. K.; Lagg, A.; Aznar Cuadrado, R.; Orozco Suárez, D.; Collados, M.; Wiegelmann, T.; Woch, J.; Sasso, C.; Krupp, N. Bibcode: 2006ASPC..358..431S Altcode: The triplet of the He I transitions around 10830 Å not only shows a rich variety of Stokes profiles, but also allows the full magnetic vector in the upper chromosphere to be probed, thus revealing the magnetic structure of loops, current sheets, finely structured supersonic downflows, the chromospheric layers of sunspots (supporting the presence of uncombed fields in the penumbra), flares, and the quiet Sun. A very brief overview of some of the observations and results obtained so far is given. Title: Reconstruction of solar irradiance variations in cycles 21-23 based on surface magnetic fields Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fröhlich, C. Bibcode: 2006A&A...460..583W Altcode: Aims.We present a reconstruction of total solar irradiance (TSI) back to 1974, i.e. from the minimum of cycle 21 to the declining phase of cycle 23. We also present a cross-calibration between the magnetograms obtained by the 512 channel magnetograph and the spectromagnetograph at Kitt Peak.
Methods: .The TSI reconstruction is carried out using data from the 512-channel Diode Array Magnetograph and the newer spectromagnetograph on Kitt Peak. The model is based on the assumption that all irradiance changes on time-scales of a day and longer are entirely due to the variations of the surface distribution of the solar magnetic field. The reconstructed irradiance is compared with the composite of total solar irradiance measurements from PMOD/WRC (version 41).
Results: .A good correspondence is found with the PMOD TSI composite, with no bias between the three cycles on time-scales longer than the solar rotation period, although the accuracy of the TSI reconstruction is somewhat lower when 512 channel magnetograph data are used. This suggests that the same driver of the irradiance variations, namely the evolution of the magnetic flux at the solar surface, is acting in cycles 21-23. Different methods of comparing the magnetograms obtained by the two Kitt Peak magnetographs give somewhat different results, with factors by which 512 channel data must be divided in the range 1.38-1.63 being found. This is due to the non-linearity of the relationship between the magnetic field measured by the two instruments.
Title: The Uncombed Penumbra Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2006ASPC..358...19B Altcode: 2006astro.ph..2130B The uncombed penumbral model explains the structure of the sunspot penumbra in terms of thick magnetic fibrils embedded in a surrounding, magnetic atmosphere. This model has been successfully applied to explain the polarization signals emerging from the sunspot penumbra. Thick penumbral fibrils face some physical problems, however. In this contribution we will offer possible solutions to these shortcomings. Title: Supersonic Downflows in the Vicinity of a Solar Pore Authors: Lagg, A.; Woch, J.; Solanki, S. K.; Gandorfer, A. Bibcode: 2006ASPC..358..437L Altcode: At the footpoints of magnetic arcades spanning over a site of flux emergence we observe strong redshifts in the He I triplet at 1083 nm. These redshifts are associated with downflow speeds of up to 40 km s-1. Within the spatial resolution of our data (1 arcsec-2 arcsec) obtained with the Tenerife Infrared Polarimeter at the VTT we find an almost unshifted atmospheric component coexisting with the redshifted component. We were able to retrieve the magnetic field configuration in both the unshifted and the redshifted component simultaneously and infer an uncombed, fibril-like structure of the upper chromosphere. The supersonic downflow speeds are interpreted as a consequence of a significantly reduced pressure scale height above the pore, where the magnetic arcades are rooted. A temporal series of the fast downflow region reveals that the supersonic flow is maintained for more than one hour. Making use of the increased spatial resolution of the new TIP2 instrument we are working on reducing the upper limit on the size of the fibril-like flux channels in the upper chromosphere. Title: Secular variations of solar activity and irradiance Authors: Solanki, S. Bibcode: 2006AGUFMSH41A..01S Altcode: Regular direct measurements of solar activity started around 1611 when sunspots detected by telescopic means began to be counted. Solar irradiance variations have been recorded for a much shorter time, only since 1978. Clearly, there is a need to extend both of these records into the past. In this talk reconstructions of solar irradiance back to the Maunder minimum and of sunspot number over multiple millenia are introduced. A comparison with climate reconstructions over the last millenium is shown and the next necessary steps in this line of research are considered. Title: Long-term solar activity reconstructions: direct test by cosmogenic 44Ti in meteorites Authors: Usoskin, I. G.; Solanki, S. K.; Taricco, C.; Bhandari, N.; Kovaltsov, G. A. Bibcode: 2006A&A...457L..25U Altcode: Aims.Long-term solar activity in the past is usually estimated from cosmogenic isotopes, 10Be or 14C, deposited in terrestrial archives such as ice cores and tree rings. A number of such reconstruction models have been proposed which differ from each other significantly. This approach suffers, however, from uncertainties due to the sensitivity of the data to several terrestrial processes. Here we propose a method to constrain these solar activity reconstructions using cosmogenic 44Ti activity in meteorites which is not affected by terrestrial processes.
Methods: .We test the veracity of recent solar activity reconstructions using the data on the activity of cosmogenic isotope 44Ti in meteorites which fell during the past 235 years, and provide an independent and direct measure of the cosmic ray flux near the Earth and allow decoupling of solar activity variations from terrestrial influences.
Results: .We demonstrate that the 44Ti data can distinguish between various reconstructions of past solar activity based on cosmogenic isotope data in terrestrial archives, allowing unrealistic models to be ruled out. We also show that a model based on the sunspot number record is consistent with the data on 44Ti activity in meteorites, thus confirming the validity of the method. In particular the 44Ti data confirm significant secular variations of the solar magnetic flux during the last century.
Title: High-resolution millimeter-interferometer observations of the solar chromosphere Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K. Bibcode: 2006A&A...456..697W Altcode: The use of millimeter-interferometer data for the study of chromospheric structure and dynamics is tested using 85 GHz observations with the 10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer data have the advantage over single-dish data that they allow both high spatial resolution and dense temporal sampling simultaneously. However, snapshot imaging of the quiet solar atmosphere with a small number of dishes is challenging. We demonstrate that techniques are available to carry out this task successfully using maximum entropy deconvolution from a default image constructed from the entire observation: one of our results is that the solar chromosphere at millimeter wavelengths exhibits features that are long-lasting and the map of the entire observation is significant provided that atmospheric phase errors do not prevent deconvolution. We compare observations of quiet Sun, active region and coronal hole targets. The interferometer is not sensitive to the disk emission and the positivity constraint of the maximum entropy algorithm used forces the zero level in the images to be at the temperature of the coolest feature in each field. The brightest features in the images are typically 1000-1500 K above the zero level, with a snapshot noise level of order 100 K. We use extensive tests to determine whether oscillation power can be recovered from sequences of snapshot images and show that individual sources can be down to quite weak levels at locations in the image where significant flux is present; oscillation power located in cool regions of the image is not well recovered due to the deconvolution method used and may be redistributed to brighter regions of the millimeter image. We then investigate whether the data do show oscillation power using uninterrupted 30-min scans of the target regions. Intensity oscillations with significant power in the frequency range 1.5-8.0 mHz are found in the quiet-Sun and active region targets. For the quiet-Sun region we compare the oscillation properties of network boundaries and cell interiors (internetwork) in the spatially-resolved time series. In agreement with investigations at other wavelengths, in the millimeter data the power in the network tends to be at periods of 5 min and longer while power in the internetwork is present also at shorter (3-min) periods. Title: The dynamics of the solar chromosphere: comparison of model predictions with millimeter-interferometer observations Authors: Loukitcheva, M.; Solanki, S. K.; White, S. Bibcode: 2006A&A...456..713L Altcode: We analyze the millimeter intensity spectrum expected from the dynamic model of Carlsson & Stein together with the interferometric observations of the quiet Sun obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array. The observational data products (Fourier and wavelet spectra, brightness histograms) are compared with the corresponding products obtained for the Carlsson & Stein (CS) models. We estimate how the limited spatial resolution of the observations influences the comparison with the predictions of chromospheric dynamic models and discuss the limitations of a one-dimensional non-magnetic modeling approach. In addition, we test the effect of the integration time of the BIMA observations on the dynamic signatures. The dependence of the observed brightness variations on spatial resolution is studied by employing artificial image degradation and approximating the obtained dependence by power laws. We are able to establish a correspondence between the CS model predictions and the observational data under assumptions on the horizontal coherence length of the oscillations. The reconstructed brightness rms values indicate that, assuming the coherence length of oscillating elements to be of order of 1'', the oscillation power in the observations recorded with 10'' resolution agrees within a factor of 2 with the power predicted by the CS model. We argue that millimeter continuum observations promise to be an important diagnostic of chromospheric structure and dynamics. Based on the analysis carried out in this work, the appropriate wavelengths to look for dynamic signatures are in the range 0.8-5.0 mm. Further millimeter interferometric observations with longer sequences and higher spatial resolution are highly desirable along with the development of realistic three-dimensional radiation magnetohydrodynamic simulations. Title: Fe XIX observations of active region brightenings in the corona Authors: Wang, T. J.; Innes, D. E.; Solanki, S. K. Bibcode: 2006A&A...455.1105W Altcode: Small flarelike brightenings seen in the hot flare line, Fe XIX, by the spectrometer SUMER on SOHO are analysed. We observe active region coronae about 30 Mm off the limb of the Sun for a period of several days. Brightenings are observed with a frequency 3-14 per hour and their lifetimes range from 5-150 min, with an average of about 25 min. The measured size of the events along the spectrometer slit range from 2-67 Mm, but most are around 7 Mm. Like soft X-ray active region transient brightenings, they range in estimated thermal energy from 1026 to 1029 erg with a power law index of 1.7 to 1.8, beyond 1027 erg. We conclude that they are the coronal parts of loops heated to > 6 MK by soft X-ray microflares. Title: Milne-Eddington inversions of the He <sf>I</sf> 10 830 Å Stokes profiles: influence of the Paschen-Back effect Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2006A&A...456..367S Altcode: 2011arXiv1102.0898S Context: .The Paschen-Back effect influences the Zeeman sublevels of the He I multiplet at 10 830 Å, leading to changes in strength and in position of the Zeeman components of these lines.
Aims: .We illustrate the relevance of this effect using synthetic Stokes profiles of the He I 10 830 Å multiplet lines and investigate its influence on the inversion of polarimetric data.
Methods: .We invert data obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). We compare the results of inversions based on synthetic profiles calculated with and without the Paschen-Back effect being included.
Results: .We find that when taking into account the incomplete Paschen-Back effect, on average 16% higher field strength values are obtained. We also show that this effect is not the main cause for the area asymmetry exhibited by many He I 10 830 Stokes V-profiles. This points to the importance of velocity and magnetic field gradients over the formation height range of these lines.
Title: Vector tomography for the coronal magnetic field. I. Longitudinal Zeeman effect measurements Authors: Kramar, M.; Inhester, B.; Solanki, S. K. Bibcode: 2006A&A...456..665K Altcode: Aims.We study the possibility of applying tomographic techniques in order to reconstruct the 3D magnetic field configuration in the solar corona. Our simulations are based on data which can be obtained from longitudinal Zeeman-effect (or from Faraday rotation) measurements obtained during a solar rotation.
Methods: .The Zeeman effect provides essentially the integrated line-of-sight component of the magnetic field. The reconstruction problem relates to a family of similar problems termed vector tomography. For inversion of this type problems it is known that the curl-free part of a vector field cannot be reconstructed from the integrated along line-of-sight data. To remove the resulting ambiguity of the reconstruction we include the additional constraint, ∇\cdotB=0, into the inversion similar to smoothing constraints in Tikhonov regularization problems.
Results: .It is shown that Zeeman data obtained from vantage points in the ecliptic plane alone is sensitive only to certain magnetic field structures. For a full reconstruction it is necessary either to have also observations from viewing directions at higher heliographic latitudes, or to combine the longitudinal Zeeman-effect data with observations that provide the transverse component of the field, e.g. from the Hanle effect.
Title: Solar Variability of Possible Relevance for Planetary Climates Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2006SSRv..125...25S Altcode: 2006SSRv..tmp...97S The global variability of the Sun of relevance for planetary climates has been directly measured for the past few decades. For longer stretches of time models are required. Semi-empirical models can now accurately reproduce the measured records of solar total and spectral irradiance, as well as of the magnetic flux. They can also provide reconstructions of these quantities on longer time scales. Here a summary is given of some of the modelling efforts and of the results achieved so far. Title: Multi-component analysis of a flaring region in the chromospheric He I 1083.0 nm triplet Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2006IAUJD...3E..31S Altcode: We present infrared spectropolarimetric observations of the solar active region NOAA 0763 obtained with the new Tenerife Infrared Polarimeter (TIP 2) at the German Vacuum Tower Telescope (VTT) during May 2005, taken in the chromospheric He I 1083.0 nm multiplet. The region was in the initial phase of a C2.0 flare. We observe up to 4 atmospheric components within the spatial resolution of our observations. The components are clearly separated in wavelength, the largest separation corresponding to downflow velocities of up to 64 km/s. We give preliminary results on the magnetic vector and the line-of sight velocity obtained by inverting the Stokes I, Q, U and V profiles of the He 1083.0 nm triplet. Title: High-Resolution CN Spectroscopy of Small-Scale Solar Magnetic Features Authors: Zakharov, V. V.; Gandorfer, A.; Solanki, S. K. Bibcode: 2006IAUJD...3E..87Z Altcode: High-resolution spectroscopic observations of the Sun have been carried out with the TRIPPEL spectrograph installed at the new 1-m Swedish Solar Telescope (SST) using realtime AO correction. A detailed spectroscopic analysis of individual photospheric bright points (BP) and faculae-like structures simultaneously in two spectral domains, i.e 387.588<λ<388.473 nm (violet CN band) and in a blue spectral band at 436.1<λ<436.9 nm, containing absorption lines of CH, obtained at the disc center and near the limb is presented. The estimated spatial resolution of the obtained spectra is around 0.25 arcsec while the spectral resolving power is around 130.000 in the first domain, and 76.000 in the second spectral region, respectivlely. The first spectral band covers absorption lines of both, CH and CN molecules, as well as many atomic lines. This enabled us to make a quantitative comparison of their absorption and Doppler shifts in the different photospheric features. The absorption lines of the CN molecule and many atoms are depressed in a BP's interior with respect to those in the quiet Sun. Our quantitative comparison of the relative line depression of CH lines with respect to CN lines showed that the latter have weaker absorption by a factor of 1.28 at the disc centre and 1.32 near the limb. The CN line-core intensity, at the disc centre, has higher BP contrast than the contrast in the CH line-core by a factor of 1.9, and the ratio of these contrasts is decreasing with increasing continuum intensity of the BPs. This trend is similar to that obtained from previous simultaneous G-band and violet CN-band imaging observations. Measurements of contrasts and rms contrasts of line-core, integrated and local continuum intensities are provided. Analysis of Doppler shifts and line broadening of an Fe I line at 387.777 nm revealed an increase of the FWHM in the BP's interior and in dark intergranular lanes and a decrease with increasing intensity of the granules. The first results of a direct comparison of observed CN spectra with those simulated in MHD models in different photospheric features is presented. Title: Flux Emergence In The Solar Photosphere - Diagnostics Based On 3-D Rradiation-MHD Simulations Authors: Yelles Chaouche, L.; Cheung, M.; Lagg, A.; Solanki, S. Bibcode: 2006IAUJD...3E..75Y Altcode: We investigate flux tube emergence in the solar photosphere using a diagnostic procedure based on analyzing Stokes signals from different spectral lines calculated in 3-D radiation-MHD simulations. The simulations include the effects of radiative transport and partial ionization and cover layers both above and below the solar surface. The simulations consider the emergence of a twisted magnetic flux tube through the solar surface. We consider different stages in the emergence process, starting from the early appearance of the flux tube at the solar surface, and following the emergence process until the emerged flux looks similar to a normal bipolar region. At every stage we compute line profiles by numerically solving the Unno-Rachkovsky equations at every horizontal grid point. Then, following observational practice, we apply Milne-Eddington-type inversions to the synthetic spectra in order to retrieve different atmospheric parameters. We include the influence of spatial smearing on the deduced atmospheric parameters to identify signatures of different stages of flux emergence in the solar photosphere. Title: Variations of solar irradiance Authors: Solanki, S. K. Bibcode: 2006IAUJD...8E..43S Altcode: Since 1978 satellite based radiometers have been measuring changes in solar irradiance, so that the available record now covers almost 3 solar cycles. In addition, spectral irradiance has been measured with increasing precision time has passed. These data have uncoverd a rich structure of the variable solar irradiance. In addition, the physical understanding of irradiance variations and the detailed reconstruction of this quantity (both for the time in which observations are available and for earlier times) have made great strides over the last years. It is now possible to reproduce both the total and spectral irradiance variations with very high precision. In this talk an overview of the measurements and the modelling of total and spectral irradiance variations is given, highlighting in particular the progress of the last few years. Title: Magnetic Flux Transport on Active Cool Stars and Starspot Lifetimes Authors: Isik, E.; Schüssler, M.; Solanki, S. K. Bibcode: 2006IAUJD...8E..21I Altcode: Rapidly rotating cool stars are known to have large magnetic regions at mid- to high-latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes on the order of one month as observed using (Zeeman-) Doppler imaging techniques. The structure and detailed morphology of starspots are not observable at present. In this study, we present numerical simulations of the surface transport of bipolar magnetic regions (BMRs) and magnetic spots on stars which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to supergranulation. We calculate flux evolution and lifetimes of BMRs with different emergence latitudes, surface shear rates, and tilt angles. For BMRs comparable to the largest ones on the Sun, we find that varying the surface flows and tilt angle modifies the lifetimes over a range of a month. For very large BMRs (area fraction ~ 0.1) the assumption of Joy's law for the tilt angle - as compared to the case with zero tilt - leads to a significant increase of lifetime. Such regions can evolve to form circumpolar spots that live more than a year. Taking the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. We have also considered the effect on decay and lifetimes of starspots if they are monolithic or a conglomerate of smaller spots of similar total size. We find these different configurations differ neither in their decay patterns, nor in their lifetimes. We also give an analytical explanation for the linear decay of magnetic flux in the monolithic-spot simulation. Title: Quiet Sun magnetic fields Authors: Solanki, S. K. Bibcode: 2006IAUJD...3E..69S Altcode: The amount of magnetic flux in the quiet Sun, its distribution on the solar surface, its origin and evolution are topics that have been strongly debated in recent years. Here a critical overview will be given of some of the results obtained from high-sensitivity polarimetric data and from numerical 3-D radiation-MHD simulations. Title: Numerical simulations of impulsively generated vertical oscillations in a solar coronal arcade loop Authors: Selwa, M.; Solanki, S. K.; Murawski, K.; Wang, T. J.; Shumlak, U. Bibcode: 2006A&A...454..653S Altcode: Aims.The main aims of the paper are to carry out numerical simulations of the vertical oscillations in a coronal loop in order to determine their dependence on various parameters and to compare them with recent TRACE observations.
Methods: .We consider impulsively generated oscillations in a solar coronal arcade loop. The two-dimensional numerical model we implement in the ideal MHD regime includes the effects of nonlinearity and line curvature. We perform parametric studies by varying both the position and the width/strength of the pulse.
Results: .A pulse launched below a loop is in general found to excite multiple wave modes, in particular a vertical oscillation with many properties of a kink mode, fast mode oscillations and a slow mode pulse (or two slow mode pulses, depending on the location of the original pulse). From our parametric studies we deduce that wave periods and attenuation times of the excited waves depend on the position below the loop summit, as well as on the width of the pulse. Wider pulses launched closer to a foot-point and to the loop's apex trigger wave packets of longer period waves which are more strongly attenuated. A perturbed loop does not return to its initial state but is instead stretched, with its apex shifted upwards. As a result the perturbations propagate along the stretched loop and consequently stronger and wider pulses which stretch a loop more lead to longer period oscillations. A pulse located near one of the foot-points is found to excite a distortion mode leading to asymmetric oscillations which are distinct from the vertical or horizontal kink modes that have been identified in TRACE data.
Title: Microflares and Loop Oscillations Authors: Innes, D. E.; Wang, T. J.; Solanki, S. K.; Tothova, D. Bibcode: 2006ESASP.617E.132I Altcode: 2006soho...17E.132I No abstract at ADS Title: Properties of Plasma Dynamics In The Polar Coronal Plumes Authors: Raouafi, N. -E.; Harvey, J. W.; Solanki, S. K. Bibcode: 2006ESASP.617E..16R Altcode: 2006soho...17E..16R No abstract at ADS Title: Large Temperature Anisotropies in Polar Coronal Holes: How Reliable are they? Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2006ESASP.617E.105R Altcode: 2006soho...17E.105R No abstract at ADS Title: Solar Coronal Magnetic Field Mapper Authors: Solanki, S. K.; Raouafi, N. -E.; Gandorfer, A.; Schühle, U.; Lagg, A. Bibcode: 2006ESASP.617E.160S Altcode: 2006soho...17E.160S No abstract at ADS Title: Solar Microscopy: Unveiling the Sun's Basic Physical Processes at their Intrinsic Scales Authors: Solanki, S. K.; Marsch, E. Bibcode: 2006ESASP.617E..34S Altcode: 2006soho...17E..34S No abstract at ADS Title: The Dynamic Nature of the Lower Transition Region as Revealed by Spectroscopy of the Hydrogen Lyman-α Line Authors: Teriaca, L.; Schühle, U.; Solanki, S. K.; Curdt, W.; Marsch, E. Bibcode: 2006ESASP.617E..77T Altcode: 2006soho...17E..77T No abstract at ADS Title: SUNRISE: high resolution UV/VIS observations of the Sun from the stratosphere Authors: Gandorfer, A. M.; Solanki, S. K.; Barthol, P.; Lites, B. W.; Martínez Pillet, V.; Schmidt, W.; Soltau, D.; Title, A. M. Bibcode: 2006SPIE.6267E..0SG Altcode: 2006SPIE.6267E..25G SUNRISE is an international project for the development, construction, and operation of a balloon-borne solar telescope with an aperture of 1 m, working in the UV/VIS spectral domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide near diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35 km on the solar surface at wavelengths around 220 nm. The focal-plane instrumentation consists of a polarization sensitive spectrograph, a Fabry-Perot filter magnetograph, and a phase-diverse filter imager working in the near UV. The first stratospheric long-duration balloon flight of SUNRISE is planned in Summer 2009 from the swedish ESRANGE station. SUNRISE is a joint project of the german Max-Planck-Institut fur Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut fur Sonnenphysik (KIS), Freiburg, Germany, the High-Altitude Observatory (HAO), Boulder, USA, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, USA, and the spanish IMaX consortium. In this paper we will present an actual update on the mission and give a brief description of its scientific and technological aspects. Title: Reconstruction of solar UV irradiance in cycle 23 Authors: Krivova, N. A.; Solanki, S. K.; Floyd, L. Bibcode: 2006A&A...452..631K Altcode: Solar irradiance variations show a strong wavelength dependence. Whereas the total solar irradiance varies by about 0.1% during the course of the solar cycle, variations at the wavelengths around the Ly-α emission line near 121.6 nm range up to 50-100%. These variations may have a significant impact on the Earth's climate system. Being almost completely absorbed in the upper atmosphere, solar UV radiation below 300 nm affects stratospheric chemistry and controls production and destruction of ozone. Models of the solar UV irradiance remain far from perfect, even though considerable progress has been made in modelling the irradiance variations longwards of about 200-300 nm. We show that after correcting for the exposure dependent degradation of the SUSIM channels sampling irradiance at λ >240 nm (making use of the Mg II core-to-wing ratio) the agreement between model and measurement is significantly improved. At shorter wavelengths the LTE approximation usually made in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. The model reproduces observed solar cycle variations of the irradiance at wavelengths down to 115 nm and indicates an important role of UV irradiance variability: up to 60% of the total irradiance variations over the solar cycle might be produced at wavelengths below 400 nm. Title: Observation of a bright coronal downflow by SOHO/EIT Authors: Tripathi, D.; Solanki, S. K.; Schwenn, R.; Bothmer, V.; Mierla, M.; Stenborg, G. Bibcode: 2006A&A...449..369T Altcode: A distinct coronal downflow has been discovered in the course of a prominence eruption associated coronal mass ejection (CME) imaged by EIT (Extreme ultraviolet Imaging Telescope) and LASCO (Large Angle Spectrometric Coronagraph) on board SOHO (Solar and Heliospheric Observatory) on 5-Mar.-2000. Evolution of the prominences seen by EIT was tracked into the LASCO/C2 and C3 field-of-view where they developed as the core of a typical three-part CME. In contrast to the inflow structures reported earlier in the literatures, which were dark and were interpreted as plasma voids moving down, the downflow reported here was bright. The downflow, which was only seen in EIT FOV had an onset time that coincided with the deceleration phase of the core of the CME. The downflow showed a rapid acceleration followed by a strong deceleration. The downflow followed a curved path which may be explained by material following the apex of a contracting magnetic loop sliding down along other field lines, although other explanations are also possible. Irrespective of the detailed geometry, this observation provides support for the pinching off of the field lines drawn-out by the erupting prominences and the contraction of the arcade formed by the reconnection. Title: On the fine structure of sunspot penumbrae. III. The vertical extension of penumbral filaments Authors: Borrero, J. M.; Solanki, S. K.; Lagg, A.; Socas-Navarro, H.; Lites, B. Bibcode: 2006A&A...450..383B Altcode: 2005astro.ph.10586B In this paper we study the fine structure of the penumbra as inferred from the uncombed model (flux tube embedded in a magnetic surrounding) when applied to penumbral spectropolarimetric data from the neutral iron lines at 6300 Å. The inversion infers very similar radial dependences in the physical quantities (LOS velocity, magnetic field strength etc.) as those obtained from the inversion of the Fe I 1.56 μm lines. In addition, the large Stokes V area asymmetry exhibited by the visible lines helps to constrain the size of the penumbral flux tubes. As we demonstrate here, the uncombed model is able to reproduce the area asymmetry with striking accuracy, returning flux tubes as thick as 100-300 kilometers in the vertical direction, in good agreement with previous investigations. Title: Solar proton events in cosmogenic isotope data Authors: Usoskin, Ilya G.; Solanki, Sami K.; Kovaltsov, Gennady A.; Beer, Jürg; Kromer, Bernd Bibcode: 2006GeoRL..33.8107U Altcode: A possible contribution of solar energetic particle events to the production of cosmogenic 10Be and 14C in the atmosphere is studied. The solar particle effect is negligible in the 14C data, but extreme events may be detectable in high-resolution 14C data. Although the overall effect is small in the 10Be data, strong events may contribute notably on the inter-annual time scale. In combination with the 11-year solar modulation of galactic cosmic rays, it may lead to an intermittent 5.5-year periodicity, which is seen in high resolution 10Be data. We have identified ten episodes during 1750-1950 when 10Be may hold signatures of strong solar proton events. This opens a new possibility to study extreme solar particle events in the past using high resolution cosmogenic isotope data. Title: Solar activity reconstructed over the last 7000 years: The influence of geomagnetic field changes Authors: Usoskin, I. G.; Solanki, S. K.; Korte, M. Bibcode: 2006GeoRL..33.8103U Altcode: The long-term solar activity, as manifested by sunspot number, has been recently reconstructed on multi-millennium time scales by S. K. Solanki et al. (2004) from the measured concentration of 14C in tree rings. The exact level of the reconstructed solar activity depends, however, on independently evaluated data of the geomagnetic dipole strength variations. Recently, a new series of the palaeomagnetic dipole moment reconstruction for the last 7000 years has been presented by M. Korte and C. G. Constable (2005a) on the basis of a thorough analysis of global samples. The new palaeomagnetic series yields a systematically lower dipole moment in the past, compared to the earlier geomagnetic reconstructions. We have revised the earlier sunspot activity reconstruction since 5000 BC, using the new geomagnetic data series, and found that it is roughly consistent with the previous results during most of the period, although the revised sunspot number values are in general higher. Nonetheless, it is confirmed with the new palaeomagnetic series that the Sun spends only 2-3% of the time in a state of high activity, similar to the modern episode. This strengthens the conclusion that the modern high activity level is very unusual during the last 7000 years. Title: The solar magnetic field Authors: Solanki, Sami K.; Inhester, Bernd; Schüssler, Manfred Bibcode: 2006RPPh...69..563S Altcode: 2010arXiv1008.0771S The magnetic field of the Sun is the underlying cause of the many diverse phenomena combined under the heading of solar activity. Here we describe the magnetic field as it threads its way from the bottom of the convection zone, where it is built up by the solar dynamo, to the solar surface, where it manifests itself in the form of sunspots and faculae, and beyond into the outer solar atmosphere and, finally, into the heliosphere. On the way it transports energy from the surface and the subsurface layers into the solar corona, where it heats the gas and accelerates the solar wind. Title: The uncombed penumbra Authors: Borrero, J. M.; Rempel, M.; Solanki, S. K. Bibcode: 2006astro.ph..2129B Altcode: The uncombed penumbral model explains the structure of the sunspot penumbra in terms of thick magnetic fibrils embedded in a magnetic surrounding atmosphere. This model has been successfully applied to explain the polarization signals emerging from the sunspot penumbra. Thick penumbral fibrils face some physical problems, however. In this contribution we will offer possible solutions to these shortcomings. Title: Plasma dynamics in the polar coronal plumes Authors: Raouafi, N. -E.; Harvey, J. W.; Solanki, S. K. Bibcode: 2006IAUS..233..193R Altcode: We use the spectral shapes of the EUV line profiles to study the plasma dynamics, acceleration and heating, in polar plumes (PP). We find that the observed profiles are reproduced fairly well when considering low plume wind speeds and velocity turbulence (α_S) at low altitudes followed by a rapid acceleration and heating of the plasma to reach the properties of inter-plumes (IP) by ≈3-4~R_⊙. We also find that plumes very close to the pole give narrow profiles at all heights that are not observed above ≈2.5~R_⊙. This suggests a tendency for plume footpoints to lie more than 10° away from the pole. High resolution magnetograms of SOLIS and EUV images support this hypothesis. Title: Acoustic shock waves in the solar chromosphere from millimeter observations Authors: Loukitcheva, Maria A.; Solanki, Sami K.; White, Stephen Bibcode: 2006IAUS..233..104L Altcode: We argue that millimeter continuum observations promise to be an important diagnostic of chromospheric dynamics and the appropriate wavelengths to look for dynamic signatures are in the range 0.8-5.0 mm. We have analyzed the millimeter intensity spectrum expected from the dynamic model of the solar non-magnetic atmosphere of Carlsson & Stein (1992, 1995, 1997, 2002, hereafter CS) together with the interferometric observations of the quiet Sun obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array. Model radio emission at millimeter wavelengths is found to be extremely sensitive to dynamic processes in the chromosphere, if these are spatially and temporally resolved. The estimated millimeter brightness temperatures are time-dependent, following changes in the atmospheric parameters, and result in clear signatures of waves with a period of 180 s seen in the radio intensity as a function of time. At the same time, the interferometric observations of the internetwork regions reveal significant oscillations with amplitudes of 50-150 K in the frequency range 1.5-8 mHz. We give an estimate of the influence of the limited available spatial resolution of observations on the comparison with the predictions of dynamic models. We are able to establish a correspondence between the CS model predictions and the observational data if we assume that the horizontal coherence length of the oscillations is on the order of 1 arcsec. Title: Multi-millennium changes of the geomagnetic field and solar activity Authors: Usoskin, I. G.; Solanki, S.; Korte, M. Bibcode: 2006cosp...36.2281U Altcode: 2006cosp.meet.2281U The long-term solar activity has been recently reconstructed on the multi-millennium time scale Solanki et al Nature 431 384 2004 from the measured concentration of radiocarbon 14 C in tree rings The exact level of the reconstructed solar activity depends however on independently evaluated data of the geomagnetic dipole strength variations Recently a new series of the palaeomagnetic dipole moment reconstruction for the last 7000 years has been presented by Korte and Constable Earth Plan Sci Lett 236 348 2005 on the basis of a thorough complex analysis of global samples The new palaeomagnetic series yields systematically lower dipole moment in the past compared to the earlier geomagnetic reconstructions We have revised the earlier sunspot activity reconstruction since 5000 BC using the new geomagnetic data and found that it is consistent with the previous results during most of the period although it yields a slightly higher level of the reconstructed sunspot activity The earlier finding on the very unusual level of the contemporary solar activity over the last millennia is confirmed with the new palaeomagnetic series The Sun spent only 2-3 of the time in a high activity state similar to the modern episode implying that the modern high activity level is very unusual during the last 7000 years On the other hand grand minima occupy about 12 of the time in the recent history of the Sun The new reconstruction allows for a comparative study of solar-terrestrial relations in the multi-millennium time scale Title: Influence of the Paschen-Back effect on the results of polarimetric inversions of the He I 10830 Å triplet Authors: Sasso, C.; Lagg, A.; Solanki, S. K.; Socas-Navarro, H. Bibcode: 2006MSAIS...9..126S Altcode: The He I triplet at 10830 Å has a great potential for determining the magnetic field vector in the upper chromosphere. The triplet is Zeeman sensitive (Landé factors 2.0, 1.75 and 1.25) and shows the signature of the Hanle effect under appropriate conditions. Additionally, the Zeeman sublevels are influenced by the Paschen-Back effect leading to changes in strength and in position of the Zeeman components of the transitions forming the triplet. In this work we calculate the influence of the Paschen-Back effect on the Stokes profiles and investigate its relevance to inversions on spectro-polarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). Title: SUNRISE: high-resolution UV/VIS observations of the Sun from the stratosphere Authors: Solanki, S. K.; Barthol, P.; Gandorfer, A.; Schüssler, M.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M. Bibcode: 2006cosp...36.2416S Altcode: 2006cosp.meet.2416S SUNRISE is a balloon-borne solar telescope with an aperture of 1m working in the UV VIS optical domain The main scientific goal of SUNRISE is to study the structure and dynamics of the magnetic field in the atmosphere of the Sun at high spatial resolution SUNRISE will provide diffraction-limited images of the photosphere and chromosphere with an unprecedented resolution down to 35km at wavelengths around 220nm Focal-plane instruments are a UV filter imager a Fabry-Perot filter magnetograph and a spectrograph polarimeter Stratospheric long-duration balloon flights of SUNRISE over the North Atlantic and or Antarctica are planned SUNRISE is a joint project of the Max-Planck-Institut fuer Sonnensystemforschung MPS Katlenburg-Lindau with the Kiepenheuer-Institut fuer Sonnenphysik KIS Freiburg the High-Altitude Observatory HAO Boulder the Lockheed-Martin Solar and Astrophysics Lab LMSAL Palo Alto and the spanish IMaX consortium The presentation will give an overview about the mission and a description of the instrumentation now at the beginning of the hardware construction phase Title: Sensitivity of solar off-limb line profiles to electron density stratification and the velocity distribution anisotropy Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2006A&A...445..735R Altcode: 2007arXiv0704.1127R The effect of the electron density stratification on the intensity profiles of the H I Ly-α line and the O VI and Mg X doublets formed in solar coronal holes is investigated. We employ an analytical 2-D model of the large scale coronal magnetic field that provides a good representation of the corona at the minimum of solar activity. We use the mass-flux conservation equation to determine the outflow speed of the solar wind at any location in the solar corona and take into account the integration along the line of sight (LOS). The main assumption we make is that no anisotropy in the kinetic temperature of the coronal species is considered. We find that at distances greater than 1 Rsun from the solar surface the widths of the emitted lines of O VI and Mg X are sensitive to the details of the adopted electron density stratification. However, Ly-α, which is a pure radiative line, is hardly affected. The calculated total intensities of Ly-α and the O VI doublet depend to a lesser degree on the density stratification and are comparable to the observed ones for most of the considered density models. The widths of the observed profiles of Ly-α and Mg X are well reproduced by most of the considered electron density stratifications, while for the O VI doublet only few stratifications give satisfying results. The densities deduced from SOHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS although only isotropic velocity distributions are employed. These density profiles also reproduce the other considered observables with good accuracy. Thus the need for a strong anisotropy of the velocity distribution (i.e. a temperature anisotropy) is not so clear cut as previous investigations of UVCS data suggested. However, these results do not rule completely out the existence of some degree of anisotropy in the corona. The results of the present computations also suggest that the data can also be reproduced if protons, heavy ions and electrons have a common temperature, if the hydrogen and heavy-ion spectral lines are also non-thermally broadened by a roughly equal amount. Title: Magnetic field measurements at different levels in the solar atmosphere and magnetic coupling Authors: Solanki, S. K.; Lagg, A. Bibcode: 2006cosp...36..916S Altcode: 2006cosp.meet..916S The magnetic field couples the different layers of the solar atmosphere with each other To unravel this coupling we need to be able to measure the field at all the relevant levels Most measurements of the magnetic field refer to the photosphere where magnetographs and spectropolarimetry using the Zeeman effect have allowed the magnetic structure and its evolution to be studied Observations of the field at higher layers in the atmosphere are more rare although a number of techniques are available These include Zeeman-effect and Hanle effect based measurements in the chromosphere and the corona as well as radio observations of coronal magnetic field These direct measurements are complemented by magnetic field extrapolations starting from the photosphere A brief overview of results obtained by various techniques of magnetic field measurements is given and a more detailed discussion of the main results of recent measurements of the field in the photosphere and the upper chromosphere are presented Title: How reliable are the large temperature anisotropies in polar coronal holes? Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2006IAUS..233..185R Altcode: We examine the influence of different electron density stratifications on the coronal LOS-integrated profiles of H I Ly-α and O VI lines at 103nm. We find that the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. Densities deduced from SOHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS although only isotropic kinetic temperatures are employed. Hence we expect the magnitude of the anisotropy to depend strongly on the density stratification adopted when analyzing the data. Title: Chromospheric Dynamics: Model Predictions and Comparison With Observations Authors: Loukitcheva, M.; Solanki, S. K.; White, S. Bibcode: 2006apri.meet...38L Altcode: No abstract at ADS Title: Fundamental parameters and granulation properties of Alpha Centauri A and B obtained from inversions of their spectra Authors: Frutiger, C.; Solanki, S. K.; Mathys, G. Bibcode: 2005A&A...444..549F Altcode: Properties of stellar granulation are obtained by inverting spectra of the late-type stars α Centauri A and B. Our inversions are based on a multi-component model of the stellar photosphere and take into account the center-to-limb variation and rotational broadening. The different atmospheric components describe the areas harboring up-, down- and horizontal flows. The inversions are constrained by fitting not only the flux profiles, but also their line bisectors, and by using a simple mass conservation scheme. The inversions return the properties of convection at the stellar surface, including the stratification of the thermodynamic parameters, as well as fundamental parameters such as the gravitational acceleration, v sin i and the element abundances. For α Cen A (G2V) the derived stratifications of the temperature and convective velocity are very similar to the Sun, while for α Cen B (K1V) we find similar up- and downflow velocities, but lower horizontal speeds and a reduced overshoot. The latter is consistent with the smaller scale height of the atmosphere, while mass conservation arguments taken with the lower horizontal speed imply that the granules on α Cen B are smaller than on the Sun. Both these properties are in good agreement with the hydrodynamic simulation of Nordlund & Dravins (1990, A&A, 228, 155). The inversions also return the fundamental parameters (T_eff, log g, abundances, v sin i, etc.) of the two stars. These values are on the whole in good agreement with literature values. Also, most of them do not strongly depend on the details of the inversion. However, importantly, the element abundances are 0.1 to 0.15 dex lower when a 2- or 3-component inversion is carried out than with a 1-component inversion.

Based on observations collected at the European Southern Observatories, La Silla, Chile. Title: Flarelike Brightenings of Active Region Loops Observed with SUMER Authors: Wang, T. J.; Innes, D. E.; Solanki, S. K.; Curdt, W. Bibcode: 2005ESASP.600E.105W Altcode: 2005ESPM...11..105W; 2005dysu.confE.105W No abstract at ADS Title: On the size distribution of sunspot groups in the Greenwich sunspot record 1874-1976 Authors: Baumann, I.; Solanki, S. K. Bibcode: 2005A&A...443.1061B Altcode: 2005astro.ph.10516B We investigate the size distribution of the maximum areas and the instantaneous distribution of areas of sunspot groups using the Greenwich sunspot group record spanning the interval 1874-1976. Both distributions are found to be well described by log-normal functions. Using a simple model we can transform the maximum area distribution into the instantaneous area distribution if the sunspot area decay rates are also distributed log-normally. For single-valued decay rates the resulting snapshot distribution is incompatible with the observations. The current analysis therefore supports the results of Howard (1992, Sol. Phys., 137, 51) and Martínez Pillet et al. (1993, A&A, 274, 521). It is not possible to distinguish between a linear and a quadratic decay law, however, with the employed data set. Title: The Dynamics of the Lower Transition Region as Inferred from Spectroscopy of the Hydrogen LYMAN-α Line Authors: Teriaca, L.; Schühle, U.; Solanki, S. K.; Curdt, W.; Marsch, E. Bibcode: 2005ESASP.600E.100T Altcode: 2005ESPM...11..100T; 2005dysu.confE.100T No abstract at ADS Title: The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields. III. Theoretical spectral patterns in the Paschen-Back regime Authors: Berdyugina, S. V.; Braun, P. A.; Fluri, D. M.; Solanki, S. K. Bibcode: 2005A&A...444..947B Altcode: Many diatomic molecules present in the atmospheres of the Sun and cool stars exhibit the Paschen-Back effect at field strengths typical of sunspots and active cool stars. Here we present a complete theoretical description of the molecular Paschen-Back efect in Hund's cases (a), (b) and all cases intermediate to them. This description allows us to compute the splitting of levels of any multiplicity and the transitions between them. We also introduce a generalized description of the effective magnetic Landé factor applicable not just in the Zeeman regime, but also in the Paschen-Back regime. We find that in the regime of the partial Paschen-Back effect strongly asymmetric Stokes profiles are produced, whose strengths and asymmetries depend sensitively on the magnetic field. In the regime of the complete Paschen-Back effect the profiles become symmetric again (although they may be strongly shifted). The strength of the forbidden and satellite transitions increases rapidly with field strength in the partial Paschen-Back regime, while the strength of the main branch transitions decreases. These signatures hold promise to form the basis of new diagnostics of solar and stellar magnetic fields. Title: Diagnostics of a Simulated Flux Tube Emergence Authors: Yelles Chaouche, L.; Cheung, M.; Lagg, A.; Solanki, S. Bibcode: 2005ESASP.600E..74Y Altcode: 2005ESASP.600E..74C; 2005ESPM...11...74C; 2005dysu.confE..74C No abstract at ADS Title: Impulsive Generation of Vertical Oscillations of a Solar Coronal Arcade Loop Authors: Selwa, M.; Murawski, K.; Solanki, S. K.; Wang, T. J.; Shumlak, U. Bibcode: 2005ESASP.596E..43S Altcode: 2005ccmf.confE..43S No abstract at ADS Title: Influence of the Paschen-Back Effect on the Stokes Profiles of the he 10830 Å Triplet Authors: Sasso, C.; Lagg, A.; Solanki, S. K. Bibcode: 2005ESASP.596E..64S Altcode: 2005ccmf.confE..64S No abstract at ADS Title: Stokes diagnostics of simulations of magnetoconvection of mixed-polarity quiet-Sun regions Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A. Bibcode: 2005A&A...442.1059K Altcode: Realistic solar magneto-convection simulations including the photospheric layers are used to study the polarization of the Fe i Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652 Å. The Stokes spectra are synthesized in a series of snapshots with a mixed-polarity magnetic field whose average unsigned strength varies from < B > = 10 to 140 G. The effects of spatial resolution and of the amount of magnetic flux in the simulation box on the profiles shapes, amplitudes and shifts are discussed. The synthetic spectra show many properties in common with those observed in quiet solar regions. In particular, the simulations reproduce the width and depth of spatially averaged Stokes I profiles, the basic classes of the Stokes V profiles and their amplitude and area asymmetries, as well as the abundance of the irregular-shaped Stokes V profiles. It is demonstrated that the amplitudes of the 1.56 μm lines observed in the inter-network are consistent with a "true" average unsigned magnetic field strength of 20 G. We show that observations using these and visible lines, carried out under different seeing conditions (e.g., simultaneous observations at different telescopes), may result in different asymmetries and even opposite polarities of the profiles in the two spectral regions observed at the same spatial point. Title: Photospheric and Chromospheric Magnetic Structure of a Sunspot Authors: Orozco Suarez, D.; Lagg, A.; Solanki, S. K. Bibcode: 2005ESASP.596E..59O Altcode: 2005ccmf.confE..59O No abstract at ADS Title: Magnetic Loops: a Comparison of Extrapolations from the Photosphere with Chromospheric Measurements Authors: Wiegelmann, T.; Lagg, A.; Solanki, S.; Inhester, B.; Woch, J. Bibcode: 2005ESASP.596E...7W Altcode: 2005ccmf.confE...7W No abstract at ADS Title: The Structure of the Lower Transition Region as Inferred from the Hydrogen LYMAN-α Line Radiance Authors: Teriaca, L.; Schühle, U.; Solanki, S. K.; Curdt, W.; Marsch, E. Bibcode: 2005ESASP.596E..66T Altcode: 2005ccmf.confE..66T No abstract at ADS Title: Supersonic Downflows in the Solar Chromosphere are Very Common Authors: Aznar Cuadrado, R.; Solanki, S. K.; Lagg, A. Bibcode: 2005ESASP.596E..49A Altcode: 2005ccmf.confE..49A No abstract at ADS Title: Polar Coronal Holes During Solar Cycles 22 and 23 Authors: Zhang, Jun; Woch, J.; Solanki, S. Bibcode: 2005ChJAA...5..531Z Altcode: Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 (from 1990 to end of 2003). In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes (PCHs) appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH. Title: Solar activity, cosmic rays, and Earth's temperature: A millennium-scale comparison Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K. Bibcode: 2005JGRA..11010102U Altcode: Previous studies of a solar influence on climate variations have often suffered from the relatively short length of continuous direct solar observations of less than 400 years. We use two recently reconstructed series of the sunspot number and the cosmic ray flux to study this question over time intervals of up to nearly 1800 years. Comparison of the Sun-related data sets with various reconstructions of terrestrial Northern Hemisphere mean surface temperatures reveals consistently positive correlation coefficients for the sunspot numbers and consistently negative correlation coefficients for the cosmic rays. The significance levels reach up to 99% but vary strongly for the different data sets. The major part of the correlation is due to the similarity of the long-term trends in the data sets. The trend of the cosmic ray flux correlates somewhat better with the terrestrial temperature than the sunspot numbers derived from the same cosmogenic isotope data. Title: Numerical simulations of vertical oscillations of a solar coronal loop Authors: Selwa, M.; Murawski, K.; Solanki, S. K.; Wang, T. J.; Tóth, G. Bibcode: 2005A&A...440..385S Altcode: We consider the impulsive excitation of fast vertical kink standing waves in a solar coronal loop that is embedded in a potential arcade. The two-dimensional numerical model we implement includes the effects of field line curvature and nonlinearity on the excitation and damping of standing fast magnetosonic waves. The results of the numerical simulations reveal wave signatures which are characteristic of vertical loop oscillations seen in recent TRACE observational data. Title: Detection of planetary transits using wavelet analysis and genetic algorithms. Authors: Husser, T. -O.; Dreizler, S.; Solanki, S.; Thomas, R. Bibcode: 2005AN....326R.628H Altcode: No abstract at ADS Title: Gravity-induced birefringence within the framework of Poincaré gauge theory Authors: Preuss, Oliver; Solanki, Sami K.; Haugan, Mark P.; Jordan, Stefan Bibcode: 2005PhRvD..72d2001P Altcode: 2005gr.qc.....7071P Gauge theories of gravity provide an elegant and promising extension of general relativity. In this paper we show that the Poincaré gauge theory exhibits gravity-induced birefringence under the assumption of a specific gauge invariant nonminimal coupling between torsion and Maxwell’s field. Furthermore we give for the first time an explicit expression for the induced phase shift between two orthogonal polarization modes within the Poincaré framework. Since such a phase shift can lead to a depolarization of light emitted from an extended source this effect is, in principle, observable. We use white dwarf polarimetric data to constrain the essential coupling constant responsible for this effect. Title: Kilo-Gauss Magnetic Fields in Three DA White Dwarfs Authors: Aznar Cuadrado, R.; Jordan, S.; Napiwotzki, R.; Schmid, H. M.; Solanki, S. K.; Mathys, G. Bibcode: 2005ASPC..334..159A Altcode: 2005astro.ph..1191A; 2005astro.ph..1191C We have detected longitudinal magnetic fields between 2 and 4 kG in three normal DA white dwarfs (WD 0446-790, WD 1105-048, WD 2359-434) out of a sample of 12 by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. A detection rate of 25 % (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have weak magnetic fields. This result, if confirmed by future observations, would form a cornerstone for our understanding of the evolution of stellar magnetic fields. Title: A comparative study of the contrast of solar magnetic elements in CN and CH Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M. Bibcode: 2005A&A...437L..43Z Altcode: Photospheric bright points were investigated in three different wavelength bands using interference filters centered at 436.5 nm (continuum), 430.5 nm (Fraunhofer's G-band dominated by absorption due to CH), and 388.7 nm (absorption band of CN). Such bright points serve as proxies of small-scale solar magnetic elements. Near diffraction limited imaging was achieved by real-time frame selection and subsequent joint phase diverse speckle reconstruction. Comparison of the filtergrams of NOAA0670 taken in CH and CN shows that the contrast of bright points is on average 1.4 times

higher in CN than in G-band, which is in good quantitative agreement with the predictions of Berdyugina et al. (2003, A&A, 412, 513) and Rutten et al. (2001, ASP Conf. Ser., 236, 445). Title: Constraining Gravitational Theories by Observing Magnetic White Dwarfs Authors: Preuss, O.; Jordan, S.; Haugan, M. P.; Solanki, S. K. Bibcode: 2005ASPC..334..265P Altcode: 2004astro.ph.11688P Under the assumption of a specific nonminimal coupling of torsion to electromagnetism, spacetime is birefringent in the presence of a gravitational field leading to depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to set for the very first time constraints on the essential coupling constant for this effect, giving k2 ∼ (22 m)2. Title: Climate: How unusual is today's solar activity? (reply) Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.; Beer, J. Bibcode: 2005Natur.436E...4S Altcode: Muscheler et al. claim that the solar activity affecting cosmic rays was much higher in the past than we deduced from 14C measurements. However, this claim is based on a problematic normalization and is in conflict with independent results, such as the 44Ti activity in meteorites and the 10Be concentration in ice cores. Title: Magnetic flux in the internetwork quiet Sun Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.; Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C. Bibcode: 2005A&A...436L..27K Altcode: We report a direct comparison of the amplitudes of Stokes spectra of the Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. A quiet inter-network region was observed at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot with < B > = 20 G. In observations with 1 resolution, up to 2/3 of the magnetic flux can remain undetected. Title: Excitation and damping of slow magnetosonic standing waves in a solar coronal loop Authors: Selwa, M.; Murawski, K.; Solanki, S. K. Bibcode: 2005A&A...436..701S Altcode: We consider slow magnetosonic standing waves that are impulsively excited in a solar coronal loop. The one-dimensional numerical model we implement includes the effects of nonlinearity, optionally thermal conduction, heating, and cooling of the solar plasma. We numerically evaluate excitation and damping times of a standing wave in hot coronal loops on the basis of a parametric study. Results of the numerical simulations reveal that initially launched impulses mainly trigger the fundamental mode and its first harmonic, depending on the location of these pulses in space. Parametric study shows that these standing waves are excited in a dozen or so wave periods corresponding roughly to 13 min and that they are strongly damped over a similar time-scale. Title: On the fine structure of sunspot penumbrae. II. The nature of the Evershed flow Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Collados, M. Bibcode: 2005A&A...436..333B Altcode: 2005astro.ph..3677B We investigate the fine structure of the sunspot penumbra by means of a model that allows for a flux tube in horizontal pressure balance with the magnetic background atmosphere in which it is embedded. We apply this model to spectropolarimetric observations of two neutral iron lines at 1.56 μm and invert several radial cuts in the penumbra of the same sunspot at two different heliocentric angles. In the inner part of the penumbra we find hot flux tubes that are somewhat inclined to the horizontal. They become gradually more horizontal and cooler with increasing radial distance. This is accompanied by an increase in the velocity of the plasma and a decrease of the gas pressure difference between flux tube and the background component. At large radial distances the flow speed exceeds the critical speed and evidence is found for the formation of a shock front. These results are in good agreement with simulations of the penumbral fine structure and provide strong support for the siphon flow as the physical mechanism driving the Evershed flow. Title: Millimeter-Interferometer Observations of Oscillations in the Solar Chromosphere Authors: White, S. M.; Loukitcheva, M.; Solanki, S. K. Bibcode: 2005AGUSMSH12A..03W Altcode: The use of millimeter-interferometer data for the study of chromospheric structure and dynamics is tested using 85 GHz observations with the 10-element Berkeley-Illinois-Maryland Array (BIMA). Interferometer data have the advantage over single-dish data that they allow both high spatial resolution and dense temporal sampling simultaneously. However, snapshot imaging of the quiet solar atmosphere with a small number of dishes is challenging. We demonstrate that techniques are available to carry out this task successfully. We compare observations of quiet Sun, active region and coronal hole targets using images with 10 arcsec resolution at 15 second cadence. The brightest features in the images are typically 1500 K above the background level, with a snapshot noise level of order 100 K. We use extensive tests to determine whether oscillation power can be recovered from sequences of snapshot images and show that they can down to quite weak levels. Intensity oscillations with significant power in the frequency range 1.5-8.0 mHz are found in the quiet-Sun and active region targets. For the quiet-Sun region we use the spatially-resolved time series to investigate the properties of network boundaries and cell interiors (internetwork) separately. In agreement with investigations at other wavelengths, the millimeter data show that power in the network tends to be at periods of 5 minutes and longer while power in the internetwork is present also at shorter (3-minute) periods. Title: Initiation of hot coronal loop oscillations: Spectral features Authors: Wang, T. J.; Solanki, S. K.; Innes, D. E.; Curdt, W. Bibcode: 2005A&A...435..753W Altcode: We explore the excitation of hot loop oscillations observed with the SUMER spectrograph on SOHO by analysing Fe XIX and Fe XXI spectral line profiles in the initial phase of the events. We investigate all 54 Doppler shift oscillations in 27 flare-like events, whose physical parameters have been measured so far. In nearly 50% of the cases, the spectral evolution reveals the presence of two spectral components, one of them almost undisturbed, the other highly shifted. We find that the shifted component reaches maximum Doppler shift (on the order of 100-300 km s-1) and peak intensity almost simultaneously. The velocity amplitude of the shifted component has no correlation with the oscillation amplitudes. These features imply that in these events the initial shifts are not caused by the locally oscillating plasma (or waves), but most likely by a pulse of hot plasma travelling along the loop through the slit position. This interpretation is also supported by several examples showing that standing slow mode waves are set up immediately after the initial line shift pulse (standing slow mode waves are inferred from the 1/4-period phase relationship between the velocity and intensity oscillations). We re-measure the physical parameters of the 54 Doppler oscillations by fitting the time profiles excluding the first peak, and find that the periods are almost unchanged, damping times are shorter by 5%, and amplitudes are smaller by 37% than measured when the first peak is included. We also measure the velocity of the net (background) flow during the oscillations, which is found to be nearly zero. Our result of initial hot flows supports the model of single footpoint (asymmetric) excitation, but contradicts chromospheric evaporation as the trigger. Title: How To Use Magnetic Field Information For Coronal Loop Identification Authors: Wiegelmann, T.; Inhester, B.; Lagg, A.; Solanki, S. K. Bibcode: 2005SoPh..228...67W Altcode: 2008arXiv0801.4573W The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT) outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed 3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field. Title: Comparing magnetic field extrapolations with measurements of magnetic loops Authors: Wiegelmann, T.; Lagg, A.; Solanki, S. K.; Inhester, B.; Woch, J. Bibcode: 2005A&A...433..701W Altcode: 2008arXiv0801.4519W We compare magnetic field extrapolations from a photospheric magnetogram with the observationally inferred structure of magnetic loops in a newly developed active region. This is the first time that the reconstructed 3D-topology of the magnetic field is available to test the extrapolations. We compare the observations with potential fields, linear force-free fields and non-linear force-free fields. This comparison reveals that a potential field extrapolation is not suitable for a reconstruction of the magnetic field in this young, developing active region. The inclusion of field-line-parallel electric currents, the so called force-free approach, gives much better results. Furthermore, a non-linear force-free computation reproduces the observations better than the linear force-free approximation, although no free parameters are available in the former case. Title: Can surface magnetic fields reproduce solar irradiance variations in cycles 22 and 23? Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A. Bibcode: 2005A&A...432.1057W Altcode: Total solar irradiance is reconstructed using data from the spectromagnetograph on Kitt Peak for 1992 2003 (covering parts of cycles 22 and 23) and compared with observational data. The model assumes that solar irradiance variations are caused by surface magnetism and its single free parameter is kept at the same value for both cycles. A good correspondence is found for the whole period of time, with no bias between the two cycles on time-scales longer than the solar rotation period. This is in agreement with the recent results of [CITE], but it is in contrast to the earlier analysis of [CITE], and suggests that the source of the irradiance variations is the same for cycles 22 and 23, namely the evolution of the magnetic flux at the solar surface. Title: Solar activity over the last 1150 years: does it correlate with climate? Authors: Usoskin, I. G.; Schüssler, M.; Solanki, S. K.; Mursula, K. Bibcode: 2005ESASP.560...19U Altcode: 2005csss...13...19U No abstract at ADS Title: SoHO/EIT Observation of a Coronal Inflow Authors: Tripathi, D.; Bothmer, V.; Solanki, S. K.; Schwenn, R.; Mierla, M.; Stenborg, G. Bibcode: 2005IAUS..226..133T Altcode: A distinct coronal inflow has been discovered after ∼90 min of prominence eruption associated coronal mass ejection (CME) on 05-Mar-2000 by EIT (Extreme ultraviolet Imaging Telescope) aboard SoHO (Solar and Heliospheric Observatory). Evolution of the prominence seen by EIT was tracked into the LASCO/C2 and C3 field-of-view (FOV; 4-10 R⊙) where it developed as the core of a typical three-part CME. The speed of the inflow, which was only seen in EIT FOV, was 70-80 km/s at a height between 1.5-1.2 R⊙ coinciding with the deceleration phase of the core of the CME in LASCO/C2. In contrast to dark inflow structures observed earlier and interpreted as plasma void moving down, the inflow reported here was bright. The inflow showed a constant deceleration and followed a curved path suggesting the apex of a contracting magnetic loop sliding down along other field lines. Title: Modelling of irradiance variations through atmosphere models . Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2005MmSAI..76..834K Altcode: Regular space-based measurements of solar total and spectral irradiance reveal its variations on time scales from minutes to decades. About 90% of these variations are reproduced by recent models assuming that the evolution of the solar surface magnetic fields is their main cause. Circumstantial evidence suggests that variations on yet longer time scales, which are of special interest for climate studies, are also possible. Once good understanding of the directly observed variations has been gained, we can then attempt to extend the models back in time. Here a brief overview of our recent efforts to reconstruct solar total and spectral irradiance on time scales of days to centuries is given. Title: A cross-calibrated sunspot areas time series since 1874 Authors: Balmaceda, L.; Solanki, S. K.; Krivova, N. Bibcode: 2005MmSAI..76..929B Altcode: A complete and homogeneous historical record of sunspot areas is a valuable proxy of solar variability, and is widely used, e.g., to understand the behaviour of total and spectral solar irradiance at earlier times. Since 1874, the Royal Greenwich Observatory (RGO) regularly carried out these and other measurements until December 1976. After that time the records from a number of different observatories are available. These, however, show some systematic differences and often have a lot of gaps. In order to compile a complete and cross-calibrated time series we compare the data from different observatories when they overlap and find the corresponding correction factors. The Greenwich data set is used as a basis until 1976, the Russian data (stations from the former USSR) between 1977 and 1985 and the Mt. Wilson data since 1986. Other data sets (Rome, Yunnan, Catania) are used to fill in the remaining gaps. Title: Reconstruction of solar UV irradiance Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2005AdSpR..35..361K Altcode: Understanding solar influence on the Earth’s climate requires a reconstruction of solar irradiance for the pre-satellite period. Considerable advances have been made in modelling the irradiance variations at wavelengths longer than 200 nm. At shorter wavelengths, however, the LTE approximation usually taken in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. Title: Irradiance models Authors: Solanki, S. K.; Krivova, N. A.; Wenzler, T. Bibcode: 2005AdSpR..35..376S Altcode: Measurements of solar irradiance have revealed variations at all the sampled time scales (ranging from minutes to the length of the solar cycle). One important task of models is to identify the causes of the observed (total and spectral) irradiance variations. Another major aim is to reconstruct irradiance over time scales longer than sampled by direct measurements in order to consider if and to what extent solar irradiance variations may be responsible for global climate change. Here, we describe recent efforts to model solar irradiance over the current and the previous two solar cycles. These irradiance models are remarkably successful in reproducing the observed total and spectral irradiance, although further improvements are still possible. Title: Shock Wave Driven by an Expanding System of Loops Authors: Raouafi, N. -E.; Mancuso, S.; Solanki, S. K.; Inhester, B.; Mierla, M.; Stenborg, G.; Delaboudinière, J. P.; Benna, C. Bibcode: 2005IAUS..226..127R Altcode: We report on a Coronal Mass Ejection (CME) observed on June 27 1999 by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board the SOHO spacecraft. The CME was also observed by LASCO (SOHO). Emission of hot material has been recorded by UVCS propagating in front of an opening system of loops generated by the CME. The evolution of the UVCS structure is highly correlated to the evolution of the opening loop. The data reveal excess broadening of the O VI doublet lines and an enhancement in the intensity of the Si XII λ 520.66 and λ499.37 lines due to the motion of the expanding hot gas. The hot gas emission seems to be due to a shock wave propagating in front of a very fast gas bubble traveling along the opening loop system. Title: Mechanisms of secular magnetic field variations. Authors: Solanki, S. K.; Schüssler, M. Bibcode: 2005MmSAI..76..781S Altcode: The variability of the solar magnetic field on time scales of decades and longer lies at the root of the various mechanisms by which the changing Sun could affect Earth's climate. We discuss the origin of the secular variability of both the open heliospheric flux and the total unsigned solar surface flux and review models that have been put forward to describe these variations. We propose that a combination of the effects of overlapping activity cycles and the long decay time of large-scale magnetic patterns is responsible for the secular variability of the solar magnetic field. Title: Excitation and Damping of Slow Magnetosonic Standing Waves in a Solar Coronal Loop Authors: Selwa, M.; Murawski, K.; Solanki, S. K. Bibcode: 2004ESASP.575...91S Altcode: 2004soho...15...91S No abstract at ADS Title: Signature of Current Sheets as Seen by Tip at VTT in the HeI Multiplet at 1083.0 nm Authors: Aznar Cuadrado, R.; Solanki, S. K.; Lagg, A.; Thomas, R. M. Bibcode: 2004ESASP.575..593A Altcode: 2004soho...15..593A No abstract at ADS Title: Small-Scale Flares as the Cause of Quiet Sun EUV Emission Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2004ESASP.575..501P Altcode: 2004soho...15..501P No abstract at ADS Title: Sensitivity of Solar Off-Limb Line Profiles to Electron Density Stratification and the Velocity Distribution Anisotropy Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2004ESASP.575..170R Altcode: 2004soho...15..170R No abstract at ADS Title: Why are Coronal Holes Indistinguishable from the Quiet Sun in Transition Region Radiation? Authors: Wiegelmann, T.; Solanki, S. K. Bibcode: 2004ESASP.575...35W Altcode: 2004soho...15...35W No abstract at ADS Title: Similarities and Differences between Coronal Holes and the Quiet Sun: Are Loop Statistics the Key? Authors: Wiegelmann, T.; Solanki, S. K. Bibcode: 2004SoPh..225..227W Altcode: 2008arXiv0802.0120W Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun. Title: Comparison between KPVT/SPM and SoHO/MDI magnetograms with an application to solar irradiance reconstructions Authors: Wenzler, T.; Solanki, S. K.; Krivova, N. A.; Fluri, D. M. Bibcode: 2004A&A...427.1031W Altcode: To be able to use both space- and ground-based solar magnetograms and construct long time series of derived parameters it is important to cross-calibrate them so that we can estimate their reliability and combine them. Using two different techniques, we compare magnetograms as well as continuum images recorded by the Spectropolarimeter (SPM) on Kitt Peak and the Michelson Doppler Interferometer (MDI) on board SoHO. We find that the result obtained depends on the method used. The method we favour gives almost identical umbral and penumbral areas and very similar total magnetic fluxes in faculae. The magnetic fluxes in umbrae and penumbrae returned by the two instruments, however, differ considerably. We also demonstrate that SPM data can be employed to reconstruct total solar irradiance variations with almost the same accuracy as recently shown for MDI data. Title: Small-Scale Solar Magnetic Elements: Simulations and Observations Authors: Solanki, S. K.; Schüssler, M. Bibcode: 2004ASPC..325..105S Altcode: Both the small-scale and large-scale properties of solar features, such as sunspots and the solar corona, are influenced strongly by the small-scale structure of the underlying magnetic field. Even some global properties of the Sun, such as variations of the Sun's irradiance, depend on the local properties of small-scale magnetic features. We briefly describe these dependences, as well as recent results concerning the small-scale magnetic elements deduced from radiation MHD simulations and spectropolarimetric observations. The simulations reproduce a number of sensitive observational tests and explain, e.g. why G-band images allow only a part of the magnetic flux to be identified. Title: The Magnetic Field from the Solar Interior to the Heliosphere Authors: Solanki, Sami K. Bibcode: 2004ASSL..317..373S Altcode: 2004shis.conf..373S No abstract at ADS Title: G-band spectral synthesis and diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Berdyugina, S. V.; Vögler, A. Bibcode: 2004A&A...427..335S Altcode: Realistic simulations of radiative magneto-convection in the solar (sub)photosphere are used for a spectral synthesis of Fraunhofer's G band, which is dominated by spectral lines from the CH molecule. It is found that the spatial pattern of integrated G-band brightness closely matches the spatial structure of magnetic flux concentrations in the convective downflow regions. The brightness contrast is mainly caused by the weakening of CH lines due to the reduced CH abundance and the resulting shift of the optical depth scale in the hot and tenuous magnetic flux concentrations. Various properties of the synthetic brightness images agree well with G-band observations. These results lends credit to the observational usage of G-band bright features as proxies for magnetic flux concentrations in the solar photosphere. However, the converse is only correct in a limited sense: only a fraction of the magnetic flux concentrations turn out to be bright in the G band. Title: Apparent solar radius variations. The influence of magnetic network and plage Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 2004A&A...427..735B Altcode: Solar radius measurements, a by-product of the magnetograms recorded several times daily at Mt. Wilson Observatory over a period of a few decades, have revealed apparent variations of about 0.4 arcsec that are correlated with the solar cycle. We note that the radius definition used for the analysis of those magnetograms automatically converts intensity variations near the limb into apparent radius variations. A change in the average temperature structure of the quiet Sun can be ruled out as the source of these variations, since such a change would need to be very significant and would lead to other easily measurable consequences that are not observed. We show that plage emission near the solar limb associated with the magnetic activity variation during a solar cycle produces apparent radius changes of the correct sign. The use of plane-parallel or spherically-symmetric models to describe the faculae gives apparent radius variations that are a factor of 4-10 too small in magnitude. If the Mt. Wilson results are correct, then this implies that the small-scale structure of faculae produces limb extensions that are considerably larger than those returned by a plane-parallel or spherically-symmetric model. Title: Effect of the electron density stratification on off-limb O VI line profiles: How large is the velocity distribution anisotropy in the solar corona? Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2004A&A...427..725R Altcode: Coronagraphic spectral observations carried out with UVCS on SoHO have shown that the velocity distribution in the solar corona is highly anisotropic. Here we examine the influence of the density stratification on the interpretation of such observations. In particular, we investigate the profiles of O VI lines emitted in the corona by employing an analytical 2-D model of the large scale coronal magnetic field and solar wind. We concentrate on the polar coronal holes and take into account the integration along the line of sight. We find that at distances greater than 1 R⊙ from the solar surface the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. In particular, the densities deduced by Doyle et al. (\cite{Doyle1999a},b) from SoHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS/SoHO although only isotropic velocity distributions are employed. Hence we expect that the magnitude of anisotropy of the velocity distribution deduced from UVCS data depends strongly on the adopted density profile. Title: Evolution of the large-scale magnetic field on the solar surface: A parameter study Authors: Baumann, I.; Schmitt, D.; Schüssler, M.; Solanki, S. K. Bibcode: 2004A&A...426.1075B Altcode: Magnetic flux emerging on the Sun's surface in the form of bipolar magnetic regions is redistributed by supergranular diffusion, a poleward meridional flow and differential rotation. We perform a systematic and extensive parameter study of the influence of various parameters on the large-scale field, in particular the total unsigned surface flux and the flux in the polar caps, using a flux transport model. We investigate both, model parameters and source term properties. We identify the average tilt angle of the emerging bipolar regions, the diffusion coefficient (below a critical value), the total emergent flux and, for the polar field, the meridional flow velocity and the cycle length as parameters with a particularly large effect. Of special interest is the influence of the overlap between successive cycles. With increasing overlap, an increasing background field (minimum flux at cycle minimum) is built up, which is of potential relevance for secular trends of solar activity and total irradiance. Title: SUNRISE: high-resolution UV/VIS observations of the Sun from the stratosphere Authors: Gandorfer, Achim M.; Solanki, Sami K.; Schüssler, Manfred; Curdt, Werner; Lites, Bruce W.; Martínez Pillet, Valentin; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2004SPIE.5489..732G Altcode: SUNRISE is a balloon-borne solar telescope with an aperture of 1m, working in the UV/VIS optical domain. The main scientific goal of SUNRISE is to understand the structure and dynamics of the magnetic field in the atmosphere of the Sun. SUNRISE will provide diffraction-limited images of the photosphere and chromosphere with an unpredecented resolution down to 35km at wavelengths around 220nm. Focal-plane instruments are a spectrograph/polarimeter, a Fabry-Perot filter magnetograph, and a filter imager. The first stratospheric long-duration balloon flight of SUNRISE over Antarctica is planned in winter 2006/2007. SUNRISE is a joint project of the Max-Planck-Institut fur Sonnensystemforschung (MPS), Katlenburg-Lindau, with the Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Instituto de Astrofisica de Canarias, La Laguna, Tenerife. In this paper we will present an overview on the mission and give a description of the instrumentation, now, at the beginning of the hardware construction phase. Title: Solar Irradiance Variations: From Current Measurements to Long-Term Estimates Authors: Solanki, Sami K.; Krivova, Natalie A. Bibcode: 2004SoPh..224..197S Altcode: 2005SoPh..224..197S Variations of solar total and spectral irradiance are prime solar quantities purported to have an influence on the Earth's climate. Quantitative estimates of irradiance over as long a time as possible are needed to judge their effectiveness in forcing the climate. In order to do this reliably, first the measured record must be reproduced and a feeling for the physics underlying the irradiance variations must be developed. With the help of this knowledge combined with the available proxy data, reconstructions of irradiance in the past, generally since the Maunder minimum, are attempted. Here a brief introduction to some of the irradiance reconstruction work aiming at irradiance on time scales of days to the solar cycle is given, followed by a brief and incomplete overview of the longer-term reconstructions. Title: Unusual activity of the Sun during recent decades compared to the previous 11,000 years Authors: Solanki, S. K.; Usoskin, I. G.; Kromer, B.; Schüssler, M.; Beer, J. Bibcode: 2004Natur.431.1084S Altcode: Direct observations of sunspot numbers are available for the past four centuries, but longer time series are required, for example, for the identification of a possible solar influence on climate and for testing models of the solar dynamo. Here we report a reconstruction of the sunspot number covering the past 11,400 years, based on dendrochronologically dated radiocarbon concentrations. We combine physics-based models for each of the processes connecting the radiocarbon concentration with sunspot number. According to our reconstruction, the level of solar activity during the past 70 years is exceptional, and the previous period of equally high activity occurred more than 8,000 years ago. We find that during the past 11,400 years the Sun spent only of the order of 10% of the time at a similarly high level of magnetic activity and almost all of the earlier high-activity periods were shorter than the present episode. Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades. Title: Shock wave driven by an expanding system of loops Authors: Raouafi, N. -E.; Mancuso, S.; Solanki, S. K.; Inhester, B.; Mierla, M.; Stenborg, G.; Delaboudinière, J. P.; Benna, C. Bibcode: 2004A&A...424.1039R Altcode: We report on a Coronal Mass Ejection (CME) observed on June 27, 1999 by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board the SOHO spacecraft. The CME was also observed by the Large Angle Spectroscopic Coronagraph (LASCO). Emission of hot material has been recorded by UVCS propagating in front of an opening system of loops generated by the CME. The evolution of the UVCS structure is highly correlated with the evolution of the opening loop. The data reveal excess broadening of the O VI doublet lines and an enhancement in the intensity of the Si XII λ520.66 and λ499.37 lines due to the motion of the expanding hot gas. The hot gas emission seems to be due to a shock wave propagating in front of a very fast gas bubble traveling along the opening loop system. Title: Discovery of kilogauss magnetic fields in three DA white dwarfs Authors: Aznar Cuadrado, R.; Jordan, S.; Napiwotzki, R.; Schmid, H. M.; Solanki, S. K.; Mathys, G. Bibcode: 2004A&A...423.1081A Altcode: 2004astro.ph..5308C; 2004astro.ph..5308A We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446-790, WD 1105-048, WD 2359-434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25% (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding of the evolution of stellar magnetic fields.

Based on observations collected at the European Southern Observatory, Paranal, Chile, under programme ID 70.D-0259. Title: An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf REJ0317-853 Authors: Preuss, Oliver; Haugan, Mark P.; Solanki, Sami K.; Jordan, Stefan Bibcode: 2004PhRvD..70f7101P Altcode: 2004gr.qc.....5068P The coupling of the electromagnetic field directly with gravitational gauge fields leads to new physical effects that can be tested using astronomical data. Here we consider a particular case for closer scrutiny, a specific nonminimal coupling of torsion to electromagnetism, which enters into a metric-affine geometry of space-time. We show that under the assumption of this nonminimal coupling, space-time is birefringent in the presence of such a gravitational field. This leads to the depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf REJ0317-853 to set, for the very first time, constraints on the essential coupling constant for this effect, giving k2≲(19 m)2. Title: Thermal-magnetic relation in a sunspot and a map of its Wilson depression Authors: Mathew, S. K.; Solanki, S. K.; Lagg, A.; Collados, M.; Borrero, J. M.; Berdyugina, S. Bibcode: 2004A&A...422..693M Altcode: We present relations between thermal and magnetic quantities in a simple, isolated sunspot, as deduced from the inversion of 1.56 μm spectropolarimetric data. We used a combination of two infrared Fe I lines at 15 648.5 Å and 15 652.8 Å/ in the inversions. Due to the high Zeeman sensitivity of these lines, we can study this relationship in the entire sunspot. The relevant parameters were derived both as a function of location within the sunspot and of height in the atmosphere using an inversion technique based on response functions. In this paper we relate the magnetic vector with temperature. We find a non-linear relationship between the various components of the magnetic vector and temperature, which confirm the results from earlier investigations. We also computed the Wilson depression and the plasma β for the observed sunspot and compare our results with earlier findings. Title: On the fine structure of sunspot penumbrae. I. A quantitative comparison of two semiempirical models with implications for the Evershed effect Authors: Borrero, J. M.; Solanki, S. K.; Bellot Rubio, L. R.; Lagg, A.; Mathew, S. K. Bibcode: 2004A&A...422.1093B Altcode: Sunspot penumbrae exhibit prominent fine structure. Different interpretations of spectropolarimetric observations suggest different, sometimes contradictory, properties of this fine structure. In this paper we show that the results of inversions of penumbral infrared profiles based on one-component models with gradients of the atmospheric parameters and two-component models without gradients are compatible with each other. Our analysis reconciles the results of previous investigations and provides further support for the picture that sunspot penumbrae are composed of penumbral flux tubes embedded in a magnetic background. The magnetic field in the tubes is more horizontal and weaker than that of the background atmosphere. While the tubes carry most of the Evershed flow, the background is essentially at rest. We notice also that the magnetic field strength in the flux tubes drops much more slowly with radial distance than the background field. This finding is discussed as a possible driver for the Evershed flow. Title: Vertical oscillations of a coronal loop observed by TRACE Authors: Wang, T. J.; Solanki, S. K. Bibcode: 2004A&A...421L..33W Altcode: We report on a loop oscillation event observed by TRACE in the 195 Åbandpass at the solar limb. The difference images reveal the first evidence for vertical kink oscillations of the loop, i.e., alternately expanding and shrinking motions, in contrast to horizontal transverse loop oscillations reported before, which exhibit swaying motions. Based on the 3D geometry of the oscillating loop derived from the observation by fitting with a circular or elliptical loop model, we simulate these two kinds of global kink modes and find that only the vertical oscillations produce a signature in the difference images in agreement with the observations. We also find that the oscillating loop is associated with intensity variations. Based on the measured displacement amplitude, the simulation predicts an intensity variation of about 13% due to density changes produced by the change of the loop length. The observed intensity changes have the same sign but are considerably larger than the predictions although the error bars are also large. This suggests that these oscillations are compressible. Title: Line profile characteristics of solar explosive event bursts Authors: Ning, Z.; Innes, D. E.; Solanki, S. K. Bibcode: 2004A&A...419.1141N Altcode: The spatial structure and temporal evolution of explosive events are explored using spectral observations of the Si IV 1393 Å line obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER). Three areas of quiet Sun near disk center, 8 arcsec×120 arcsec each, were rastered with a cadence of less than 1 min for a period of over half an hour. Events were identified by a non-Gaussian excess in the blue and/or red wings of the line. We found 21 sites where events re-occur. The sites are clustered near regions of evolving network fields seen in low resolution Michelson Doppler Imager (MDI) magnetic field images. Events tend to expand and shrink across the surface with a speed ∼ 25 km s-1. Individual events were also seen to move at this speed across the solar surface. Consecutive events often have different line profile characteristics and sizes, suggesting changes in the structure of the accelerated plasma from event to event. In the majority of events, blue and red wing brightenings do not produce a simultaneous line intensity increase. Also, the red and blue wing emission is mostly co-spatial implying either non-directed flow or jets orientated along the line-of-sight (i.e. out from and towards the solar surface). The explosive events within a burst are in some cases separated by 3-5 min, suggesting that oscillations, which are known to have such periods, may play a role in triggering the individual events of a burst. Title: Millimeter observations and chromospheric dynamics Authors: Loukitcheva, M.; Solanki, S. K.; Carlsson, M.; Stein, R. F. Bibcode: 2004A&A...419..747L Altcode: The intensities of submillimeter and millimeter continua, which are formed in LTE and depend linearly on temperature, may be able to provide a test of models of the Solar chromosphere. We have taken a collection of submillimeter and millimeter wave observed brightness temperatures Tb of the quiet Sun from the literature and compared it with brightness temperatures computed from the standard static models of Fontenla, Avrett and Loeser (FAL) and the dynamic simulations of Carlsson & Stein (CS). The analysis of the dynamic simulations of Carlsson & Stein reveals that radio emission at millimeter wavelengths is extremely sensitive to dynamic processes in the chromosphere, if these are spatially and temporally resolved. The most striking result is that the dynamic picture of the solar internetwork chromosphere is consistent with currently available millimeter and submillimeter brightness observations. The spectrum obtained by averaging over the spectra from all time-steps of CS simulations provides a good fit to observed temporally and spatially averaged millimeter data in spite of the absence of a permanent temperature rise at low chromospheric heights in the simulations. This does not by itself rule out the presence of a chromospheric temperature rise as present in the FAL models, since a combination of such models also reproduces the (low resolution) data relatively well. Millimeter observations indicate that using radio techniques it is possible to extend observations of the solar oscillatory component to the heights above those previously observed in the photospheric and low chromospheric spectral lines and submillimeter continuum. For more precise diagnostics of chromospheric dynamics, high temporal and spatial resolution interferometric observations in the millimeter-wavelength region would be particularly useful.

Table \ref{tab:table} is only available in electronic form at http://www.edpsciences.org Title: Usoskin et al. Reply: Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred; Mursula, Kalevi Bibcode: 2004PhRvL..92s9002U Altcode: A Reply to the Comment by G. M. Raisbeck and F. Yiou. Title: Measuring Stellar Differential rotation with asteroseismology Authors: Gizon, Laurent; Solanki, Sami K. Bibcode: 2004SoPh..220..169G Altcode: The variation of rotation with latitude is poorly known on stars other than the Sun. Several indirect techniques, photometric and spectroscopic, have been used to search for departure from rigid rotation for sufficiently fast rotators. Here we investigate the possibility of measuring stellar differential rotation for solar-type stars through asteroseismology. Rotationally split frequencies of global oscillation provide information about rotation at different latitudes depending on the azimuthal order, m, of the mode of pulsation. We present a method to estimate differential rotation based on the realization that the m = ±1 and m = ±2 components of quadrupole oscillations can be observed simultaneously in asteroseismology. Rotational frequency splittings can be inverted to provide an estimate of the difference in stellar angular velocity between the equator and 45° latitude. The precision of the method, assessed through Monte Carlo simulations, depends on the value of the mean rotation and on the inclination angle between the rotation axis and the line of sight. Title: Effect of spatial resolution on estimating the Sun's magnetic flux Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004A&A...417.1125K Altcode: A critical question related to a possible secular trend in the Sun's total magnetic flux and consequently in solar irradiance is the total amount of magnetic flux present on the Sun and how it is distributed between active regions and the quiet Sun. NSO/Kitt Peak synoptic charts have in the past been used to estimate the total flux and the fraction of the flux in active regions and in the quiet Sun. Since a single pixel of these synoptic charts is much bigger than individual small-scale magnetic elements and opposite polarities may be present within the same pixel, some magnetic flux escaped notice. Here we estimate the fraction of the magnetic flux escaping detection in Kitt Peak synoptic charts. By artificially reducing the spatial resolution of MDI full-disc and high-resolution magnetograms we study the influence of the resolution on the measured total magnetic flux. Noise in the data poses the main difficulty to this approach and is carefully studied. It is concluded that at least half of the magnetic flux in the quiet Sun remains undetected in Kitt Peak synoptic charts and that the total flux present on the solar surface at maxima of activity is around twice the flux present at activity minima. Title: Distribution of magnetically confined circumstellar matter in oblique rotators Authors: Preuss, O.; Schüssler, M.; Holzwarth, V.; Solanki, S. K. Bibcode: 2004A&A...417..987P Altcode: 2004astro.ph..1599P We consider the mechanical equilibrium and stability of matter trapped in the magnetosphere of a rapidly rotating star. Assuming a dipolar magnetic field and arbitrary inclination of the magnetic axis with respect to the axis of rotation we find stable equilibrium positions a) in a (warped) disk roughly aligned with the magnetic equatorial plane and b) at two locations above and below the disk, whose distance from the star increases with decreasing inclination angle between dipole and rotation axis. The distribution of matter is not strongly affected by allowing for a spatial offset of the magnetic dipole. These results provide a possible explanation for some observations of corotating localized mass concentrations in hot magnetic stars. Title: Towards understanding the β Pictoris dust stream Authors: Krivov, A. V.; Krivova, N. A.; Solanki, S. K.; Titov, V. B. Bibcode: 2004A&A...417..341K Altcode: The recent radar detection by \citet{baggaley-2000} of a collimated stream of interstellar meteoroids postulated to be sourced at β Pictoris, a nearby star with a prominent dust disk, presents a challenge to theoreticians. Two mechanisms of possible dust ejection from β Pic have been proposed: ejection of dust by radiation pressure from comets in eccentric orbits and by gravity of a hypothetical planet in the disk. Here we re-examine observational data and reconsider theoretical scenarios, substantiating them with detailed modeling to test whether they can explain quantitatively and simultaneously the masses, speeds, and fluxes. Our analysis of the stream geometry and kinematics confirms that β Pic is the most likely source of the stream and suggests that an intensive dust ejection phase took place ∼0.7 Myr ago. Our dynamical simulations show that high ejection speeds retrieved from the observations can be explained by both planetary ejection and radiation pressure mechanisms, providing, however, several important constraints. In the planetary ejection scenario, only a ``hot Jupiter''-type planet with a semimajor axis of less than 1 AU can be responsible for the stream, and only if the disk was dynamically ``heated'' by a more distant massive planet. The radiation pressure scenario also requires the presence of a relatively massive planet at several AU or more, that had heated the cometesimal disk before the ejection occurred. Finally, the dust flux measured at Earth can be brought into reasonable agreement with both scenarios, provided that β Pic's protoplanetary disk recently passed through an intensive short-lasting (∼0.1 Myr) clearance stage by nascent giant planets, similar to what took place in the early solar system. Title: Solar constraints on new couplings between electromagnetism and gravity Authors: Solanki, S. K.; Preuss, O.; Haugan, M. P.; Gandorfer, A.; Povel, H. P.; Steiner, P.; Stucki, K.; Bernasconi, P. N.; Soltau, D. Bibcode: 2004PhRvD..69f2001S Altcode: 2004gr.qc.....2055S The unification of quantum field theory and general relativity is a fundamental goal of modern physics. In many cases, theoretical efforts to achieve this goal introduce auxiliary gravitational fields, ones in addition to the familiar symmetric second-rank tensor potential of general relativity, and lead to nonmetric theories because of direct couplings between these auxiliary fields and matter. Here, we consider an example of a metric-affine gauge theory of gravity in which torsion couples nonminimally to the electromagnetic field. This coupling causes a phase difference to accumulate between different polarization states of light as they propagate through the metric-affine gravitational field. Solar spectropolarimetric observations are reported and used to set strong constraints on the relevant coupling constant k: k2<(2.5 km)2. Title: Spot sizes on Sun-like stars Authors: Solanki, S. K.; Unruh, Y. C. Bibcode: 2004MNRAS.348..307S Altcode: 2003astro.ph.11310S The total area coverage by starspots is of interest for a variety of reasons, but direct techniques only provide estimates of this important quantity. Sunspot areas exhibit a lognormal size distribution irrespective of the phase of the activity cycle, implying that most sunspots are small. Here we explore the consequences if starspot areas were similarly distributed. The solar data allow for an increase in the fraction of larger sunspots with increasing activity. Taking this difference between the size distribution at sunspot maximum and minimum, we extrapolate to higher activity levels, assuming different dependences of the parameters of the lognormal distribution on total spot coverage. We find that, even for very heavily spotted (hypothetical) stars, a large fraction of the spots are smaller than the current resolution limit of Doppler images and hence might be missed on traditional Doppler maps. Title: Retrieval of the full magnetic vector with the He I multiplet at 1083 nm. Maps of an emerging flux region Authors: Lagg, A.; Woch, J.; Krupp, N.; Solanki, S. K. Bibcode: 2004A&A...414.1109L Altcode: A technique is presented to invert Stokes profiles of the He I 1083 nm multiplet lines in order to obtain the full magnetic vector and the line-of-sight velocity. The technique makes use of spectropolarimetry connected with the Zeeman effect supplemented by a simple Hanle effect based diagnostic when appropriate. It takes into account effects like line saturation, magnetooptieffects, etc. and is coupled with a genetic algorithm, which ensures that the global minimum in a goodness of fit hypersurface is found. Tests using both artificial and real data demonstrated the robustness of the method. As an illustration maps of deduced parameters of an emerging flux region are shown and briefly discussed. Title: Dynamic Behaviors of the Quiet Sun Seen by SUMER Authors: Ning, Z.; Innes, D. E.; Solanki, S. K. Bibcode: 2004ESASP.547..297N Altcode: 2004soho...13..297N No abstract at ADS Title: Radiance Emission by Flaring Activity Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2004ESASP.547..323P Altcode: 2004soho...13..323P Radiance values in the quiet Sun follow a lognormal distribution, with shape and scaling parameters varying significantly over the temperature range from chromosphere to corona. We show that these distributions can be reproduced by a simple model, which assumes that the radiance is produced by a stochastic (micro-, nano-) flaring process. This allows the diagnostic capabilities of the radiance distribution to be judged, performing, e.g., estimates of the true damping times of the flares. Several energy distributions are tested for the flaring process, like a Gaussian and a power law. The resulting time series are compared with SUMER time series of equivalent sampling, after adjustment of the parameters of the simulation. A good statistical match of the measurements is obtained for a steep power law distribution of nanoflare energies. Title: Magnetic Coupling and Topological Change Authors: Solanki, S. K. Bibcode: 2004ESASP.547..155S Altcode: 2004soho...13..155S Magnetic field lines thread the convective layers of the Sun's interior and its atmosphere. They couple these parts of the Sun in the sense that energy is transported from the Sun's interior and surface into its atmosphere, where it is deposited, leading to a heating of the gas present there. One way of energy release is through magnetic reconnection, which leads to a change of the magnetic topology. Both these topics are briefly discussed and some recent results are reviewed, including, new measurements of the magnetic vector near the base of the corona which reveal the magnetic structure of loops and have led to the first detection of a magnetic current sheet in the solar atmosphere. Title: Reconstruction of solar UV irradiance Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004cosp...35..504K Altcode: 2004cosp.meet..504K Understanding solar influence on the Earth's climate requires a reconstruction of solar irradiance for the pre-satellite period. Considerable advances have been made in modelling the irradiance variations at wavelengths longer than 300 nm. At shorter wavelengths, however, the LTE approximation usually taken in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. Title: Effect of the electron density stratification on the off-limb O VI line profiles Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2004IAUS..223..481R Altcode: 2005IAUS..223..481R Coronagraphic spectral observations carried out with UVCS on SoHO above polar coronal holes have shown that the velocity distribution is highly anisotropic. Here we examine the influence of the density stratification on the interpretation of such observations. In particular, we investigate the profiles of O VI spectral lines emitted in polar coronal holes. We find that at distances greater than 1 R_{odot} from the solar surface the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. In particular, the densities deduced by Doyle et al. (1999) from SoHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS although only isotropic velocity distributions are employed. Hence we expect that the magnitude of the anisotropy of the velocity distribution deduced from UVCS data depends strongly on the adopted density profile. Title: Stokes diagnostics of magneto-convection. Profile shapes and asymmetries Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.; Schüssler, M. Bibcode: 2004IAUS..223..635K Altcode: 2005IAUS..223..635K We discuss the polarization signals produced in recent realistic 3D simulations of solar magnetoconvection. The Stokes profiles of the Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive spectral lines are synthesised and smeared to simulate the image degradation caused by the Earth's atmosphere and finite telescope resolution. A Principal Component Analysis approach is applied to classify the profiles. We find that the classes of Stokes V profiles as well as their amplitude and area asymmetries are very close to the observations in the network and inter-network regions. Title: Structure of the solar chromosphere Authors: Solanki, Sami K. Bibcode: 2004IAUS..223..195S Altcode: 2005IAUS..223..195S The chromosphere is an intriguing part of the Sun that has stubbornly resisted all attempts at a comprehensive description. Thus, observations carried out in different wavelength bands reveal very different, seemingly incompatible properties. Not surprisingly, a debate is raging between supporters of the classical picture of the chromosphere as a nearly plane parallel layer exhibiting a gentle temperature rise from the photosphere to the transition region and proponents of a highly dynamical atmosphere that includes extremely cool gas. New data are required in order settle this issue. Here a brief overview of the structure and dynamics of the solar chromosphere is given, with particular emphasis on the chromospheric structure of the quiet Sun. The structure of the magnetic field is also briefly discussed, although filaments and prominences are not considered. Besides the observations, contrasting models are also critically discussed. Title: Temporal evolution of chromospheric downflows Authors: Lagg, Andreas; Woch, J.; Krupp, N.; Gandorfer, A.; Solanki, S. K. Bibcode: 2004IAUS..223..279L Altcode: 2005IAUS..223..279L At the footpoints of loops spanning a site of flux emergence, earlier investigated in the papers by Solanki et al. (2003) and Lagg et al. (2004), we find large redshifts in the He 1083 nm line coexisting with an almost unshifted component. The speed associated with these redshifts reaches values as high as 40 km/s. We interpret these downflows in the context of several models: the free-fall downflow of matter along vertical field lines (Schmidt et al. 2000), the redshift by downward propagating acoustic waves (Hansteen 1993) and the motion of condensation regions to either side of loop footpoints (Müller et al. 2003). We present the temporal evolution of these redshifts and reconstruct the magnetic field vector in these regions for both the redshifted and the unshifted atmospheric component. Title: Overview of Slow Mode Oscillations in Hot Coronal Loops Observed by SUMER Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E. Bibcode: 2004IAUS..219..712W Altcode: 2003IAUS..219E..70W No abstract at ADS Title: Understanding Solar Variability as a Groundwork for Planet Transit Detection Authors: Seleznyov, A. D.; Krivova, N. A.; Solanki, S. K. Bibcode: 2004IAUS..219..815S Altcode: 2003IAUS..219E...7K Detection of planetary transits holds the greatest promise for the search of terrestrial planets. However intrinsic stellar variability can mask real transits or lead to 'false' planet transit detections. Understanding the origin of stellar variability can help to estimate the minimum sizes of planets detectable with this technique around different types of stars and to identify the best wavelength range for such measurements. The only star for which data with sufficient photometric accuracy and temporal sampling exist is the Sun. We analyze and model solar variability on timescales relevant for planetary transits (hours to several days) using a variety of components such as granulation network (supergranulation) faculae and sunspots. This study extends our successful work to model solar irradiance variations on days to years timescales (Fligge et al. 2000; Krivova et al. 2003). Title: Plasma dynamics of a prominence associated coronal mass ejection Authors: Tripathi, D.; Bothmer, V.; Solanki, S. K.; Schwenn, R.; Mierla, M.; Stenborg, G. Bibcode: 2004IAUS..223..401T Altcode: 2005IAUS..223..401T An erupting prominence seen by SOHO/EIT was tracked into the field of view of the LASCO C2 and C3 coronagraphs where it developed into the core of a structured CME. The erupting prominence was deflected by an angle of sim 20^{circ} towards the north pole whereas the consequent core of the CME and it's leading edge propagated in the outer corona at constant position angle. The prominence material underwent a constant acceleration phase until a height of sim1.5 solar radii before it started to decelerate up to a distance of 5.0 solar radii. An inflow of plasma with a speed of about 70-80 km/s was discovered in the EIT observations at a height of 1.5-1.2 solar radii in the course of the prominence eruption, matching in time the prominence deceleration phase. The downflowing material followed a curved path, suggestive of the apex of a contracting magnetic loop sliding down along other field lines. Title: The solar chromosphere as seen in high-resolution millimeter observations Authors: Loukitcheva, M. A.; Solanki, S. K.; White, S. Bibcode: 2004IAUS..223..643L Altcode: 2005IAUS..223..643L We report on chromospheric oscillations studied with 15 sec cadence observations obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array (BIMA). Different solar structures, including active and quiet-Sun regions, are analyzed. We compare the high-resolution millimeter 2-D images with Ca II K line images from Big Bear Solar Observatory. Using Fourier and wavelet time series analysis techniques we find signatures of intensity oscillations with periods in the range of 2-7 minutes. We discuss the results in the context of the dynamical model of the solar atmosphere by Carlsson & Stein (1995, 1997). Title: Stokes Diagnostics of Magnetoconvection. Profile shapes and asymmetries. Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.; Schussler, M. Bibcode: 2004cosp...35.2131K Altcode: 2004cosp.meet.2131K Stokes profiles observed in the quiet Sun have a broad range of asymmetries and show a variety of shapes. These asymmetries are the result of the velocity and magnetic field gradients both in horizontal and vertical directions. We use the most recent realistic 3D simulations of magnetoconvection at the solar surface to synthesize Stokes profiles of some photospheric lines and to compare them with observations. Such comparison provides an important constrains on the MHD models allowing to conclude about their realism and, thus, to understand the nature of solar magnetoconvection. The following Zeeman-sensitive spectral lines are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are extensively used in observations. The computed Stokes profiles of these lines were spatially smeared to simulate the effects of a telescope and atmospheric seeing. A Principal Component Analysis approach is applied to classify the profiles. The effects of spatial resolution and the amount of the magnetic flux in the MHD model on the profile shapes are discussed. The profiles of different classes are clustered together and form patches on the surface. The size of these patches decreases with increasing spatial resolution. The distributions of the amplitude and area asymmetries of Stokes V profiles are very close to the observations in network and inter-network regions. Some 15% of the profiles smeared with a 0.''5 seeing have irregular shape with 3 or more lobes. Finally, we show that simultaneous observations of the same area of the solar disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5, 6302.5 Å lines done under different seeing conditions (for example in the case of simultaneous observations at different telescopes) may result in different asymmetries and even different polarities of the profiles in two spectral regions observed at the same spatial point. This work was partially supported by INTAS grant 00-00084. Title: Reconstruction of solar activity for the last millennium using 10Be data Authors: Usoskin, I. G.; Mursula, K.; Solanki, S.; Schüssler, M.; Alanko, K. Bibcode: 2004A&A...413..745U Altcode: 2003astro.ph..9556U In a recent paper (Usoskin et al. 2002a), we have reconstructed the concentration of the cosmogenic 10Be isotope in ice cores from the measured sunspot numbers by using physical models for 10Be production in the Earth's atmosphere, cosmic ray transport in the heliosphere, and evolution of the Sun's open magnetic flux. Here we take the opposite route: starting from the 10Be concentration measured in ice cores from Antarctica and Greenland, we invert the models in order to reconstruct the 11-year averaged sunspot numbers since 850 AD. The inversion method is validated by comparing the reconstructed sunspot numbers with the directly observed sunspot record since 1610. The reconstructed sunspot record exhibits a prominent period of about 600 years, in agreement with earlier observations based on cosmogenic isotopes. Also, there is evidence for the century scale Gleissberg cycle and a number of shorter quasi-periodicities whose periods seem to fluctuate in the millennium time scale. This invalidates the earlier extrapolation of multi-harmonic representation of sunspot activity over extended time intervals. Title: Solar variability and global warming: a statistical comparison since 1850 Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2004AdSpR..34..361K Altcode: The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations entering climate modelling. Given the complexity of the climate system, however, such modelling is perforce based on simplifying assumptions, which leaves it open to criticism. We take a complementary approach. We assume that the Sun has been responsible for climate change prior to 1970 and that their interrelation remained unchanged afterwards. Then, employing reconstructions and measured records of relevant solar quantities as well as of the cosmic-ray flux, we estimate statistically which fraction of the dramatic temperature rise after that date could be due to the influence of the Sun. We show that at least in the most recent past (since about 1970) the solar influence on climate cannot have been significant. Title: Irradiance models Authors: Solanki, S. K. Bibcode: 2004cosp...35..318S Altcode: 2004cosp.meet..318S Measurements of solar irradiance have revealed variations at all the sampled time scales (ranging from minutes to the length of the solar cycle). One important task of models is to identify the causes of the observed (total and spectral) irradiance variations. Another major aim is to reconstruct irradiance over time scales longer than sampled by direct measurements in order to consider if and to what extent solar irradiance variations may be responsible for global climate change. Here I describe recent efforts to model solar irradiance both on the short-term and at longer time scales. On time scales up to the solar cycle the irradiance models are remarkably successful in reproducing the observed total and spectral irradiance, although further improvements are still possible. On longer time scales, in spite of considerable advances, some uncertainties still remain, although there is hope of reducing these in the near future. Title: Solar activity and climate during the last millennium Authors: Solanki, S. K.; Usoskin, I.; Schüssler, M. Bibcode: 2004cosp...35.2535S Altcode: 2004cosp.meet.2535S The sunspot number is the longest running direct index of solar activity, with direct measurements starting in 1610. For many purposes, e.g., for comparisons with climate indices, it is still too short. We present a reconstruction of the cycle-averaged sunspot number over the last millennium based on 10Be concentrations in Greenland and Antarctic ice cores. As intermediate steps of the method, we also reconstruct the cosmic ray flux at Earth and the Sun's open magnetic flux. The reconstructions are validated by comparison with direct measurements or independent reconstructions. We also compare with records of global climate, in particular with the global temperature ("hockey stick") curve of Mann et al (1998). A reasonable agreement is found for the entire millennium, excluding only the last decades, when the two curves start diverging from each other. Title: Effect of the Line-of-Sight Integration on the Profiles of Coronal Lines Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2004ESASP.547..401R Altcode: 2004soho...13..401R We reexamine the interpretation of coronal observations aiming at the determination of the coronal magnetic field and/or the solar wind velocity vectors. In particular we investigate the effect of the integration along the line-of-sight on the profiles of O vi spectral lines emitted in the corona employing a simple model of the large scale coronal magnetic field and solar wind. Here we present preliminary results. We concentrate on the polar coronal holes and find that although the line-of-sight integration is not important at small heights, the width of the emitted line is increasingly affected when moving outwards from the solar disk. At distances of 2.5 R and greater the width of the profile integrated along the LOS is more than 2 times as larger as the line profile obtained from an elementary emitting volume on the polar axis. Title: Interpretation of the Coronal EUV Signature of the CME Event on June 27 1999 Authors: Raouafi, N. -E.; Mancuso, S.; Solanki, S. K.; Inhester, B.; Benna, C.; Delaboudinière, J. P.; Stenborg, G.; Mierla, M. Bibcode: 2004ESASP.547..317R Altcode: 2004soho...13..317R We report the observation of a Coronal Mass Ejection (CME) detected on June 27 1999 by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board the SOHO spacecraft. The CME, whose leading edge was expanding at a projected speed of about 1200 km s, was observed in white light by the Large Angle Spectroscopic Coronagraph (LASCO). The UVCS spectra reveal excess broadening of the O VI doublet lines and enhancement in the intensity of the Si XII lines due to the motion of expanding hot material. The evolution of the UVCS structure is highly correlated to the evolution of the CME observed by LASCO in white light, so that the hot gas emission could be attributable to the passage of a shock wave propagating just in front of the fast CME. Title: Effect of anisotropic velocity distribution on the linear polarization of coronal lines. Does the ion cyclotron exist in the inner corona? Authors: Raouafi, N. -E.; Solanki, S. K. Bibcode: 2003A&A...412..271R Altcode: The effect of an anisotropic velocity field distribution of scattering ions on the polarization parameters of a spectral line emitted by resonance scattering is considered. The anisotropy of the velocity field distribution can be interpreted in terms of the ion-cyclotron effect that is believed to influence some heavy ions in the solar corona. We present a theoretical study of the Stokes parameters of a spectral line emitted by atoms or ions in the presence of a bi-Maxwellian velocity field distribution. It is found from test calculations that such a distribution measurably changes the polarization properties of the \ion {O}{vi} D_2 coronal line. Consequently, measurements of the linear polarization of this line may serve as a new diagnostic of a possible bi-Maxwellian velocity distribution. As a preliminary application, the obtained theoretical results are used to interpret the polarization parameters of the O Vi D_2 coronal line (lambda1031 .92) measured using SUMER/SoHO observations. The obtained results are compatible with SUMER's observations for more reasonable solar wind parameters than for an isotropic velocity field distribution of the scattering ions. Thus, the outflow velocity field vector of the emitting ions is less inclined with respect to the polar axis of the Sun (the minimal value of the polar angle eta is 8deg for an anisotropic velocity distribution versus 17deg for an isotropic velocity field distribution). These results are obtained assuming that the re-emitted photons come from a small area in the center of the coronal polar hole, with zero magnetic field. Since SUMER/SoHO observations integrate over the line of sight, the results of the current analysis must be considered preliminary pending computations including an integration along the line of sight. Title: Models of Solar Irradiance on Time Scales of Minutes to Centuries Authors: Solanki, S. K. Bibcode: 2003AGUFMSH31C..01S Altcode: Solar irradiance varies on all time scales accessible to observations from minutes to the solar cycle. For the Earth's climate even longer time scales are of relevance, from centuries to millennia. A series of models have been or are being constructed to account for the irradiance variations at these different time scales, which are dominated by different physical processes. The results of the modelling are described and, where possible, are compared with the data and are discussed. Title: The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields. II. Synthetic Stokes profiles in the Zeeman regime Authors: Berdyugina, S. V.; Solanki, S. K.; Frutiger, C. Bibcode: 2003A&A...412..513B Altcode: Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes V profiles may require very high S/N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots. Title: Successful Measurement of the Full Magnetic Vector Near the Base of the Solar Corona Authors: Solanki, S. K.; Lagg, A.; Woch, J.; Krupp, N.; Landi Degl'Innocenti, E.; Collados, M. Bibcode: 2003AGUFMSH41D..05S Altcode: The measurement of coronal fields has in the past generally been restricted to the field strength or to only some of the components of the magnetic vector. We present here a technique for measuring the full magnetic vector near the base of the solar corona. As an application we report on observations of a developing active region with ongoing magnetic flux emergence. The data allow the first measurement of the 3-D structure of magnetic loops. They also provide the first detection of an electric current sheet located near the base of the solar corona. Such current sheets or tangential discontinuities of the coronal magnetic field have long been thought to be a major source of coronal heating. Title: Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands Authors: Schüssler, M.; Shelyag, S.; Berdyugina, S.; Vögler, A.; Solanki, S. K. Bibcode: 2003ApJ...597L.173S Altcode: Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the ``G band,'' a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic structures, thus confirming the model of radiatively heated magnetic flux concentrations. These results provide a firm basis for observational studies of the evolution and dynamics of the small-scale solar magnetic field derived through ``proxy magnetometry'' with G-band images. Title: Millennium-Scale Sunspot Number Reconstruction: Evidence for an Unusually Active Sun since the 1940s Authors: Usoskin, Ilya G.; Solanki, Sami K.; Schüssler, Manfred; Mursula, Kalevi; Alanko, Katja Bibcode: 2003PhRvL..91u1101U Altcode: 2003astro.ph.10823U The extension of the sunspot number series backward in time is of considerable interest for dynamo theory, solar, stellar, and climate research. We have used records of the 10Be concentration in polar ice to reconstruct the average sunspot activity level for the period between the year 850 to the present. Our method uses physical models for processes connecting the 10Be concentration with the sunspot number. The reconstruction shows reliably that the period of high solar activity during the last 60years is unique throughout the past 1150years. This nearly triples the time interval for which such a statement could be made previously. Title: Spatial and temporal fluctuations in sunspots derived from MDI data Authors: Solanki, S. K.; Rüedi, I. Bibcode: 2003A&A...411..249S Altcode: The penumbra radiates an energy flux that is 0.75-0.8 times the photospheric value. One mechanism proposed to bring this flux to the surface is interchange convection according to which hot flux tubes rise to the surface, lie horizontally there while they cool and finally sink down again. We search for possible signatures of such a process using time series of magnetograms and continuum images recorded by the Michelson Doppler Imager (MDI) in its high resolution mode (0.6\arcsec pixels). The data reveal that at the spatial scales accessible to MDI, magnetic structures are on average smaller in the azimuthal direction than brightness features. The small-scale magnetic pattern resolvable by MDI lives for well over two hours, i.e. longer than the brightness pattern. As shown in a parallel paper (Schlichenmaier & Solanki \cite{Sch03}) this result, taken together with theoretical predictions, suggests that interchange convection is unable to account for the observed penumbral radiative flux. The need for higher resolution data obtained under stable conditions is pointed out. Title: On the heat transport in a sunspot penumbra Authors: Schlichenmaier, R.; Solanki, S. K. Bibcode: 2003A&A...411..257S Altcode: The penumbra radiates an energy flux that is roughly 75% of the quiet-sun value. One mechanism proposed to bring this flux to the surface is interchange convection of magnetic flux tubes according to which hot flux tubes rise to the surface, cool off their heat by radiation and sink down again. Another way to deposit heat in the penumbral photosphere is by steady upflows along magnetic flux tubes. We discuss these two mechanisms and elaborate on consequences that can be compared with and constrained by observations. We estimate the time scales for variations of the intensity and the magnetic field pattern. By comparing them with the corresponding observed time scales, we find that pure interchange convection is unable to account for the observed penumbral heat flux. Heating the penumbra by steady upflows along magnetic flux tubes, however, turns out to be sufficient to explain the penumbral brightness, under the condition that significant magnetic return flux is present within the penumbra. Associated with the magnetic return flux, downflows within the penumbra should be present, in accordance with recent observational findings of such downflows. Exploring other possible heating mechanisms, we find that dissipation of magnetic energy is negligible, while dissipation of the kinetic energy of the Evershed flow could contribute significantly to the brightness of the penumbra. Title: Three dimensional structure of a regular sunspot from the inversion of IR Stokes profiles Authors: Mathew, S. K.; Lagg, A.; Solanki, S. K.; Collados, M.; Borrero, J. M.; Berdyugina, S.; Krupp, N.; Woch, J.; Frutiger, C. Bibcode: 2003A&A...410..695M Altcode: The magnetic, thermal and velocity structure of a regular sunspot, observed close to solar disk center is presented. Spectropolarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) in two infrared FeI lines at 15 648.5 Å and 15 652.8 Å are inverted employing a technique based on response functions to retrieve the atmospheric stratification at every point in the sunspot. In order to improve the results for the umbra, profiles of Zeeman split OH lines blending the FeI 15 652.8 Å are also consistently fit. Thus we obtain maps of temperature, line-of-sight velocity, magnetic field strength, inclination, and azimuth, as a function of both location within the sunspot and height in the atmosphere. We present these maps for an optical depth range between log tau5 = 0 and log tau5 = -1.5, where these lines provide accurate results. We find decreasing magnetic field strength with increasing height all over the sunspot, with a particularly large vertical field gradient of ~ -4 G km-1 in the umbra. We also observe the so called ``spine'' structures in the penumbra, i.e. extended radial features with a stronger and more vertical magnetic field than the surroundings. Also we found that the magnetic field zenith angle increases with height. From the velocity map it is clear that the Evershed flow avoids the spines and mostly concentrates in the more inclined intervening field. The field inclination at a few locations in the outer penumbra in lower layers goes beyond 90o. These locations coincide with the strongest flows in the velocity map. Title: Three-dimensional magnetic field topology in a region of solar coronal heating Authors: Solanki, S. K.; Lagg, A.; Woch, J.; Krupp, N.; Collados, M. Bibcode: 2003Natur.425..692S Altcode: Flares and X-ray jets on the Sun arise in active regions where magnetic flux emerges from the solar interior amd interacts with the ambient magnetic field. The interactions are believed to occur in electric current sheets separating regions of opposite magnetic polarity. The current sheets located in the corona or upper chromosphere have long been thought to act as an important source of coronal heating, requiring their location in the corona or upper chromosphere. The dynamics and energetics of these sheets are governed by a complex magnetic field structure that, until now, has been difficult to measure. Here we report the determination of the full magnetic vector in an interaction region near the base of the solar corona. The observations reveal two magnetic features that characterize young active regions on the Sun: a set of rising magnetic loops and a tangential discontinuity of the magnetic field direction, the latter being the observational signature of an electric current sheet. This provides strong support for coronal heating models based on the dissipation of magnetic energy at current sheets. Title: On the origin of solar variability, with an application to the search for extrasolar planets Authors: Seleznyov, A. D.; Solanki, S. K.; Krivova, N. A. Bibcode: 2003ESASP.539..589S Altcode: 2003toed.conf..589S Detection of planetary transits holds great promise for the search of terrestrial planets. However, most stars are variable at the level of the signal produced by the transit of an Earth-like planet. Hence, intrinsic stellar variability can lead to "false" planet transit detections. An understanding of the origin of the stellar variability is needed to ensure reliable transit detections. We consider the Sun as the closest and best studied star and analyze its variability on timescales relevant to the transit effect, namely from an hour to several days. Total and spectral solar irradiance measurements obtained by the VIRGO instrument on board the SOHO spacecraft have been analyzed by applying Fourier and wavelet techniques. Preliminary results suggest that at the time scales of interest solar variability is driven partly by solar magnetic activity, which dominates at longer time scales, and convection, in particular solar granulation, which dominates at shorter time scales. As part of a more quantitative analysis a simple numerical model of the irradiance variations due to granulation has been constructed. Irradiance variability of stars with different surface gravity was calculated in the frequency band of relevance to transits. Title: Molecular Lines as Diagnostics of Solar and Stellar Magnetic Fields Authors: Berdyugina, S. V.; Solanki, S. K.; Lagg, A. Bibcode: 2003csss...12..210B Altcode: Thanks to recent advances in theory we can now calculate molecular line profiles in the presence of magnetic fields with high accuracy, both in the Zeeman and Paschen-Back regimes (Berdyugina et al. 2000; Berdyugina & Solanki 2001a). The synthetic Stokes profiles of various molecular species (e.g. TiO, OH, MgH, CN, FeH) have been compared with profiles observed in sunspots. The agreement between the theory and observations is remarkable. For example, the mutually opposite polarities of different OH lines are reproduced without invoking any free parameters, except the magnetic field strength and sunspot temperature. Introducing molecular lines into the inversion of sunspot spectra leads to significant improvements in the deduced magnetic field vector. Here we investigate how molecular lines can be used to deduce magnetic parameters of cool stars. We find that such lines are of great interest for measuring magnetic fields on cooler stars and in starspots. Title: Solar total and spectral irradiance: modelling and a possible impact on climate Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2003ESASP.535..275K Altcode: 2003iscs.symp..275K There is growing evidence that solar variability influences the Earth's climate, although the underlying mechanism is not yet understood. Variations in the solar total and spectral irradiance often play a central role within various processes that have been suggested. Whereas changes in the total irradiance can affect the overall energy balance of the Earth's atmosphere, variations in its spectral distribution, in particular in the UV, have a pronounced effect on the chemistry of the Earth's upper atmosphere. Measurements of the solar total irradiance are only available since 1978 and the spectral irradiance record is even shorter. This calls for a reconstruction of irradiance variations at earlier times with the help of models. We first outline our current understanding of the main mechanism responsible for irradiance variations and describe the efforts to reconstruct them. The reconstructed total and UV irradiance is then employed to estimate the solar contribution to global warming, with particular emphasis to the period since 1970. Title: Solar irradiance fluctuations on short timescales Authors: Solanki, S. K.; Seleznyov, A. D.; Krivova, N. A. Bibcode: 2003ESASP.535..285S Altcode: 2003iscs.symp..285S Although solar irradiance variability at time-scales of days to the solar cycle has been well studied, comparatively little is known about the causes of such variations on shorter time-scales. We present an analysis that aims to distinguish between magnetic and convective causes. It suggests that on time-scales longer than 1-2 days magnetic structures are the dominant source while for time-scales shorter than a few hours convection appears to dominate. We also present a simple granulation model that includes the various paths of granule birth and death and compare its output with VIRGO data. Title: Quiet-Sun inter-network magnetic fields observed in the infrared Authors: Khomenko, E. V.; Collados, M.; Solanki, S. K.; Lagg, A.; Trujillo Bueno, J. Bibcode: 2003A&A...408.1115K Altcode: This paper presents the results of an investigation of the quiet Sun's magnetic field based on high-resolution infrared spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter (TIP) at the German VTT of the Observatorio del Teide. We observed two very quiet regions at disc centre. The seeing was exceptionally good during both observing runs, being excellent during one of them. In both cases the network was intentionally avoided to the extent possible, to focus the analysis on the characteristics of the weak polarization signals of the inter-network regions. We find that the Stokes V profile of Fe I 15648 Å line in almost 50% of the pixels and Stokes Q and/or U in 20% of the pixels have a signal above 10-3 (in units of continuum intensity Ic), which is significantly above the noise level of 2-3 x 10-4. This implies that we detect fluxes as low as 2 x 1015 Mx/px. We find evidence that we have detected most of the net flux that is in principle detectable at 1'' resolution with the Zeeman effect. The observed linear polarization resulting from the transverse Zeeman effect indicates that the magnetic fields have a broad range of inclinations, although most of the pixels show polarization signatures which imply an inclination of about 20o. Nearly 30% of the selected V-profiles have irregular shapes with 3 or more lobes, suggesting mixed polarities with different LOS velocity within the resolution element. The profiles are classified using a single value decomposition approach. The spatial distribution of the magnetic signal shows that profiles of different classes (having different velocities, splitting, asymmetries) are clustered together and form patches, close to the spatial resolution in size. Most of the field is found to be located in intergranular lanes. The statistical properties of the mainly inter-network field sampled by these observations are presented, showing that most of the observed fields are weak with relatively few kG features. The field strength distribution peaks at 350 G and has a FWHM of 300 G. Other parameters, such as profile asymmetries, filling factors and line-of-sight velocities are also determined and discussed.

Based on observations with the German Vacuum Tower Telescope (VTT) operated by the Kiepenheuer-Institut für Sonnenphysik at the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias (IAC). Title: Dependence of UV radiance of the quiet Sun on the solar cycle: Surface magnetic fields as the cause Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2003A&A...407..359P Altcode: The quiet-Sun UV radiance depends on the solar cycle, as shown by data collected by the SUMER spectrograph on the Solar and Heliospheric Observatory (SOHO). The cause of this dependence is still unclear. Here the hypothesis is tested for the He I 584 Å line that these variations are due to changes in the magnetic network. The quiet-Sun variability is investigated with the two EUV instruments CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) and the MDI (Michelson Doppler Imager) magnetograph on SOHO. Using a monthly data set of co-spatial and co-temporal observations of quiet Sun areas near disk centre we follow the evolution of the quiet Sun over four years from solar cycle minimum to maximum conditions and find that the magnetic flux of the quiet network increases during this period. Furthermore, its variation is well correlated with the radiance change in the He I 584 Å line. Also, we find that the largest fractional change is in the flux of the strong network elements (largest average field strengths), while the weaker elements do not exhibit a significant change. Title: Hot coronal loop oscillations observed with SUMER: Examples and statistics Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E.; Kliem, B. Bibcode: 2003A&A...406.1105W Altcode: We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe Xix and Fe Xxi, with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s-1. The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2-3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8-102 km s-1, together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

Table \ref{osctab} and Appendices A and B are only available in electronic form at http://www.edpsciences.org Title: Long-Term Cosmic Ray Intensities: Physical Reconstruction Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schuessler, M.; Kovaltsov, G. A. Bibcode: 2003ICRC....7.4041U Altcode: 2003ICRC...28.4041U Solanki et al. (2000) have recently calculated the open solar magnetic flux for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a simple spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected differential spectra and integral intensity of galactic cosmic rays at the Earth's orbit since 1610. The calculated cosmic ray integral intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, using the specific yield function of cosmogenic 10 Be radionuclide production in the atmosphere, we also calculate the expected 10 Be production rate which exhibits an excellent agreement with the actual 10 Be abundance in polar ice over the last 400 years. Here we present a physical model for the long-term reconstruction of cosmic ray intensity at 1 AU. The reconstruction is based on a combination of the solar magnetic flux model and a heliospheric model. This model allows us to calculate the expected intensity of galactic cosmic rays (GCR) at the Earth's orbit for the last 400 years. Details can be found in [25]. Using the numerical recip e of Solanki et al. [21] and the group sunspot number series (Fig. 1.a) [11] we have calculated the open solar magnetic flux Fo since 1610 as shown in Fig. 1.b. In order to calculate galactic cosmic ray (GCR) spectra we use a spherically symmetric quasi-steady sto chastic simulation model described in detail elsewhere [24], which reliably describes the long-term GCR modulation during the last 50 years. In this model, the most important parameter of the heliospheric modulation of GCR is the modulation strength [10]: Φ = (D - rE )V /(3κo), where D = 100 AU is the heliospheric boundary and rE = 1 AU, V = 400 km/s is the constant solar wind velocity and κo is the rigidity indep endent part of the diffusion coefficient. Thus, all changes in the modulation strength Φ in our model are related to the changing diffusion Title: Overview of Recent Results on Coronal Loop Oscillations Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E. Bibcode: 2003ANS...324...19W Altcode: 2003ANS...324..B18W No abstract at ADS Title: Infrared Polarimetry at the MPAe: The Solar Atmosphere from the Photosphere to the Upper Chromosphere Authors: Lagg, Andreas; Woch, Joachim; Solanki, Sami K.; Mathew, Shibu; Borrero, Juan M.; Krupp, N.; Raouafi, N. E. Bibcode: 2003ANS...324...29L Altcode: 2003ANS...324..D04L No abstract at ADS Title: SUNRISE: Balloon-borne High-Resolution Observation of the Sun Authors: Solanki, S. K.; Curdt, W.; Gandorfer, A.; Schüssler, M.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.; Sunrise Team Bibcode: 2003ANS...324..113S Altcode: 2003ANS...324..P20S No abstract at ADS Title: Determining the Inclination of the Rotation Axis of a Sun-like Star Authors: Gizon, L.; Solanki, S. K. Bibcode: 2003ApJ...589.1009G Altcode: Asteroseismology provides us with the possibility of determining the angle, i, between the direction of the rotation axis of a pulsating Sun-like star and the line of sight. A knowledge of i is important not just for obtaining improved stellar parameters, but also in order to determine the true masses of extrasolar planets detected from the radial velocity shifts of their central stars. By means of Monte Carlo simulations, we estimate the precision of the measurement of i and other stellar parameters. We find that the inclination angle can be retrieved accurately when i>~30deg for stars that rotate at least twice as fast as the Sun. Title: Slow-mode standing waves observed by SUMER in hot coronal loops Authors: Wang, T. J.; Solanki, S. K.; Innes, D. E.; Curdt, W.; Marsch, E. Bibcode: 2003A&A...402L..17W Altcode: We report the first detection of postflare loop oscillations seen in both Doppler shift and intensity. The observations were recorded in an Fe Xix line by the SUMER spectrometer on SOHO in the corona about 70 min after an M-class flare on the solar limb. The oscillation has a period of about 17 min in both the Doppler velocity and the intensity, but their decay times are different (i.e., 37 min for the velocity and 21 min for the intensity). The fact that the velocity and the intensity oscillations have exactly a 1/4-period phase difference points to the existence of slow-mode standing waves in the oscillating loop. This interpretation is also supported by two other pieces of evidence: (1) the wave period and (2) the amplitude relationship between the intensity and velocity are as expected for a slow-mode standing wave. Title: Variability of EUV-spectra from the quiet upper solar atmosphere: Intensity and Doppler shift Authors: Brković, A.; Peter, H.; Solanki, S. K. Bibcode: 2003A&A...403..725B Altcode: We have studied SUMER and CDS time series of spectra and images of quiet-Sun regions at the solar disc centre. The data contain ultraviolet emission lines sampling temperatures of the chromosphere, transition region and corona. We find a high correlation between average net Doppler shifts and relative brightness variabilities of the studied lines (correlation coefficient of 0.92), suggesting a connection between the two quantities. The anti-correlation between differential emission measures and relative brightness variabilities is weaker (correlation coefficient of -0.78). We discuss the observed relationships on the basis of differential emission measures and linear wave calculations. Title: Can solar variability explain global warming since 1970? Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2003JGRA..108.1200S Altcode: The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations and climate modeling. This approach suffers from uncertainties that are difficult to estimate. Such uncertainties are introduced because the employed models may not include important but complex processes or mechanisms or may treat these in too simplified a manner. Here we take a more empirical approach. We employ time series of the most relevant solar quantities, the total and UV irradiance between 1856 and 1999 and the cosmic rays flux between 1868 and 1999. The time series are constructed using direct measurements wherever possible and reconstructions based on models and proxies at earlier times. These time series are compared with the climate record for the period 1856 to 1970. The solar records are scaled such that statistically the solar contribution to climate is as large as possible in this period. Under this assumption we repeat the comparison but now including the period 1970-1999. This comparison shows without requiring any recourse to modeling that since roughly 1970 the solar influence on climate (through the channels considered here) cannot have been dominant. In particular, the Sun cannot have contributed more than 30% to the steep temperature increase that has taken place since then, irrespective of which of the three considered channels is the dominant one determining Sun-climate interactions: tropospheric heating caused by changes in total solar irradiance, stratospheric chemistry influenced by changes in the solar UV spectrum, or cloud coverage affected by the cosmic ray flux. Title: A stream of particles from the begin {equation}ta Pictoris disc: A possible ejection mechanism Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2003A&A...402L...5K Altcode: Recently, a stream of particles originating from the direction of begin {equation}ta Pictoris, a young main sequence star surrounded by a dust disc, has been reported (Baggaley \cite{baggaley-2000}). Standard mechanisms of particle ejection from a disc fail to reproduce the properties of this stream. We find that scattering by a giant proto-planet with properties taken from the literature is consistent with the observations. The fact that a straightforward ejection mechanism reproduces the data supports the identification of the particle stream's source with beta Pic. Our work also indicates that protoplanetary dust discs form a potentially rich source of large interstellar grains, as widely detected in the Solar System. Title: Interplanetary and solar surface properties of coronal holes observed during solar maximum Authors: Zhang, J.; Woch, J.; Solanki, S. K.; von Steiger, R.; Forsyth, R. Bibcode: 2003JGRA..108.1144Z Altcode: Data from the Solar Wind Ion Composition Spectrometer (SWICS) on board the Ulysses spacecraft and synoptic maps from Kitt Peak are used to analyze the relatively short-lived coronal holes which exist during the maximum phase of the solar activity cycle 23. They are compared with the persistent polar coronal holes which prevail around solar minimum. A solar wind velocity increase coinciding with a shift of the ionic charge composition toward lower charge states serves as a robust criterion for identifying solar wind streams emanating from solar maximum holes. This allows an unambiguous association of every stream identified in interplanetary space with a coronal hole on the solar surface with consistent magnetic polarity. Solar wind streams emanating from the solar maximum holes generally show lower velocities of 400 to 600 km/s compared to the polar hole stream velocities of 700 to 800 km/s. However, the SWICS O7+/O6+ charge-state ratios, which are a proxy for coronal temperatures, do not reveal a consistent difference. Though a number of solar maximum holes have a significantly, up to three times, higher temperature compared to the polar coronal holes, the majority of the investigated holes and specifically those with new cycle polarity have a coronal temperature within the range of polar hole temperatures. Likewise, the magnetic flux density in the solar maximum holes and in the polar coronal holes, as derived from the synoptic maps, is not strikingly different. Therefore any intrinsic difference between solar maximum holes and polar coronal holes is small. The striking discrepancy in their kinetic properties, namely the slower velocity of the solar wind streams emanating from solar maximum holes, may partly be attributed to deceleration of the solar wind during propagation to the spacecraft. The discrepancy may also be influenced by active regions in close proximity to the coronal holes, which presumably is more likely for smaller holes. There may, however, be a tendency for the faster wind streams to emanate from cooler holes. Title: Ejection of Dust from Planetary Systems into Interstellar Space Authors: Krivov, A.; Krivova, N.; Solanki, S.; Titov, V. Bibcode: 2003EAEJA.....4264K Altcode: Extensive observational and theoretical research of the last two decades have made it clear that young exoplanetary systems comprise not only planets themselves, but also disks of small bodies and of dust-sized material these bodies replenish. In much the same way as in the young Solar System, interaction of the solids in a circumstellar disk with planets at late stages of planet formation should hang the material on the outskirts of the system, creating exosolar Kuiper belts and Oort clouds. A portion of the material is ejected by the exoplanets from the disks into interstellar space, which may represent an important source of large interstellar dust grains in the Galaxy. Here we study how the efficiency of the ejection mechanism and the involved timescales depend on the mass and orbital parameters of the planet, as well as on the `unperturbed' spatial distribution of disk particles. We also derive the distribution of the final velocities of the ejected material. The study is done semianalytically with the aid of the Oepik-Weidenschilling statistical theory of planetary encounters and numerically with direct Monte-Carlo orbital simulations. The results may have particular implications to the beta Pictoris system: they lend further support to the idea (Krivova and Solanki 2002) that a collimated stream of interstellar radiometeors detected by Baggaley (2000) originates at beta Pic and consists of grains ejected by a presumed jovian planet orbiting this star. Title: Cycles and cyclicities of the Sun Authors: Solanki, S. K.; Krivova, N. A. Bibcode: 2003ASPC..292..423S Altcode: 2003ipc..conf..423S The solar activity cycle is discussed in the broader context of solar variability. It is pointed out that the Sun exhibits periodic, cyclic, chaotic and stochastic phenomena. The origins of solar variability are mentioned and the importance of secular variations of the solar cycle for a putative influence of the Sun on the Earth's climate is briefly discussed. Title: SUNRISE: a balloon-borne telescope for high resolution solar observations in the visible and UV Authors: Solanki, Sami K.; Gandorfer, Achim M.; Schuessler, Manfred; Curdt, W.; Lites, Bruce W.; Martinez-Pillet, Valentin; Schmidt, Wolfgang; Title, Alan M. Bibcode: 2003SPIE.4853..129S Altcode: Sunrise is a light-weight solar telescope with a 1 m aperture for spectro-polarimetric observations of the solar atmosphere. The telescope is planned to be operated during a series of long-duration balloon flights in order to obtain time series of spectra and images at the diffraction-limit and to study the UV spectral region down to ~200 nm, which is not accessible from the ground. The central aim of Sunrise is to understand the structure and dynamics of the magnetic field in the solar atmosphere. Through its interaction with the convective flow field, the magnetic field in the solar photosphere develops intense field concentrations on scales below 100 km, which are crucial for the dynamics and energetics of the whole solar atmosphere. In addition, Sunrise aims to provide information on the structure and dynamics of the solar chromosphere and on the physics of solar irradiance changes. Sunrise is a joint project of the Max-Planck-Institut fuer Aeronomie (MPAe), Katlenburg-Lindau, with the Kiepenheuer-Institut fuer Sonnenphysik (KIS), Freiburg, the High-Altitude Observatory (HAO), Boulder, the Lockheed-Martin Solar and Astrophysics Lab. (LMSAL), Palo Alto, and the Instituto de Astrofi sica de Canarias, La Laguna, Tenerife. In addition, there are close contacts with associated scientists from a variety of institutes. Title: On the nature of moving magnetic feature pairs around sunspots Authors: Zhang, Jun; Solanki, S. K.; Wang, Jingxiu Bibcode: 2003A&A...399..755Z Altcode: Employing data recorded by the Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric Observatory (SOHO), we have identified 144 pairs of opposite magnetic polarity moving magnetic features (MMFs) in two active regions (NOAA ARs 8375 and 9236). The following results are obtained: (1) The majority of MMF pairs first appears at a distance of 1000 to 5000 km from the outer boundary of the sunspot, although MMF pairs appearing closer to the sunspot may be missed. (2) MMF bipoles are not randomly oriented. The member of an MMF pair further from the sunspot has the polarity of the parent sunspot in 85% of the cases. Furthermore, the orientations of MMF pairs are associated with the twist of the sunspot superpenumbra deduced from Hα images. (3) The mean lifetime of the studied MMFs is around 4 hours. (4) The separation between the two polarities of the MMFs falls in the range of 1100-1700 km. This separation remains almost unchanged, even decreases slightly as the MMF pairs move outwards. (5) MMFs are observed to cluster at particular azimuths around the parent sunspot, in particular in AR 8375. (6) MMF pairs move approximately radially outward from sunspots at an average speed of around 0.5 km s-1. Their motion is deflected towards large concentrations of magnetic flux of opposite polarity to that of the parent sunspot. A qualitative model based on these and other observations is presented. MMF pairs are proposed to be part of a U-loop emanating from the sunspot's magnetic canopy. Possible mechanisms leading to the formation of such a loop are discussed. Title: Reconstruction of solar irradiance variations in cycle 23: Is solar surface magnetism the cause? Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M.; Unruh, Y. C. Bibcode: 2003A&A...399L...1K Altcode: A model of solar irradiance variations is presented which is based on the assumption that solar surface magnetism is responsible for all total irradiance changes on time scales of days to years. A time series of daily magnetograms and empirical models of the thermal structure of magnetic features (sunspots, faculae) are combined to reconstruct total (and spectral) irradiance from 1996 to 2002. Comparisons with observational data reveal an excellent correspondence, although the model only contains a single free parameter. This provides strong support for the hypothesis that solar irradiance variations are caused by changes in the amount and distribution of magnetic flux at the solar surface. Title: Intercalibration of SUMER and CDS on SOHO. III. SUMER and CDS-GIS Authors: Pauluhn, Anuschka; Lang, Jim; Breeveld, Eddie R.; Solanki, Sami K.; Schühle, Udo Bibcode: 2003ApOpt..42..657P Altcode: Simultaneous observation of the same solar sources with different instruments is one way to test prelaunch radiometric calibrations and to detect changes in responsivity with time of extreme-ultraviolet instruments in space. Here we present the results of intercalibration of the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrometer (detectors A and B) and the GIS (Grazing Incidence Spectrometer), one of two spectrometers that compose the CDS (Coronal Diagnostic Spectrometer) on the Solar and Heliospheric Observatory (SOHO). The two instruments observed simultaneously radiances of emission lines at or near the center of the solar disk. The emission line chosen for intercomparison was Ne viii at 770 Å. However, such an intercomparison of the SUMER and CDS-GIS measurements means comparing two data sets with large differences in resolution and field of view. The latter difference, especially, introduces differences in the measured intensities caused by the solar variability that is relatively strong in the 770- Å line. Using a statistical approach to overcome this problem, we found that the ratio of the GIS to the SUMER average radiances amounted to 2.6 +/- 0.9 before the SOHO ’s loss of attitude and to 2.1 +/- 0.7 afterward. These findings confirm earlier estimates of the GIS ’s responsivity being too low, and an update of the GIS calibration is recommended. Despite the large differences in resolution and field of view of the two instruments, the shapes of their normalized and rescaled histograms of the radiances agree well and therefore represent characteristic features of the Ne viii line. Title: The Magnetic Field from the Sun to the Interstellar Medium Authors: Solanki, Sami K. Bibcode: 2003IAUJD...7E..11S Altcode: The magnetic field of the Sun is responsible for a vast variety of phenomena on the Sun and in the heliosphere. The field extends from the bottom of the convection zone into the interstellar medium and is the main agent that couples the various layers of the atmosphere together. The strength structure spatial scales complexity and evolution of the magnetic field changes very significantly with distance from the solar surface. In particular the energy density of the field compared to that of the gas (thermal kinetic) is a strong function of location. Furthermore although the global magnetic structure becomes simpler with increasing height complex fine-scale features are seen at all heights. Title: New Molecular Indicators of Sunspot Magnetic Fields: Infrared OH Lines Authors: Berdyugina, S. V.; Solanki, S. K.; Lagg, A. Bibcode: 2003ASPC..286..299B Altcode: 2003ctmf.conf..299B No abstract at ADS Title: Molecular lines observations with THEMIS Authors: Arnaud, J.; Faurobert, M.; Raouafi, N. -E.; Solanki, S. K. Bibcode: 2003sf2a.conf..111A Altcode: 2003sf2a.confE..39A Lines of molecules like MgH, C2, TiO, ... represent powerful tools for the study of the solar atmosphere. Their observations on the Sun can also help to constrain the polarizability and Lande factors calculations which are much more complex than for atomic lines. We will present THEMIS polarimetric observations of these lines to illustrate some aspects of their use for solar physics. Title: Structure of a simple sunspot from the inversion of IR spectral data Authors: Mathew, S. K.; Solanki, S. K.; Lagg, A.; Collados, M.; Berdyugina, S. V.; Frutiger, C.; Krupp, N.; Woch, J. Bibcode: 2003AN....324..388M Altcode: Analysis of spectral data of two neighboring infrared lines, Fe i 15648.5 Å (g = 3) and Fe i 15652.9 Å (g_eff = 1.53) are carried out for a simple sunspot when it was near the solar disk center (mu = 0.92), to understand the basic structure of sunspot magnetic field. Inversions of Stokes profiles are carried out to derive different atmospheric parameters both as a function of location within the sunspot and height in the atmosphere. As a result of the inversion we have obtained maps of magnetic field strength, temperature, line-of-sight velocity, field inclination and azimuth for different optical depth layers between log (tau_ {5}) = 0 and log (tau_ {5}) = -2.0 . In this paper we present few results from our inversion for a layer averaged between log (tau_ {5}) from 0.0 to -0.5. Title: Doppler Oscillations of Active Region Loops: Steps towards Coronal Seismology Authors: Curdt, W.; Wang, T. J.; Dammasch, I. E.; Solanki, S. K. Bibcode: 2003HvaOB..27...83C Altcode: Oscillations of coronal loops -- subject of theoretical work for a long time -- can help to determine coronal plasma parameters not otherwise accessible. Therefore, the Doppler oscillations recently observed by the SUMER spectrometer on SOHO are of extreme interest and constitute a significant contribution to the old, but rejuvenated field of coronal seismology. High-velocity oscillation events in hot EUV flare lines are seen almost every time these lines brighten. Such events seem to be a common feature of active region loops. The oscillations always have an impulsive trigger and are strongly damped while they cool down. Lines formed at normal coronal temperatures do not show any signature of these oscillations. Title: Commission 12: Solar radiation and structure (Rayonnement et structure solaires) Authors: Solanki, Sami K. Bibcode: 2003IAUTA..25...90S Altcode: 2003IAUTr..25A..90S No abstract at ADS Title: Sunspots: An overview Authors: Solanki, Sami K. Bibcode: 2003A&ARv..11..153S Altcode: Sunspots are the most readily visible manifestations of solar magnetic field concentrations and of their interaction with the Sun's plasma. Although sunspots have been extensively studied for almost 400 years and their magnetic nature has been known since 1908, our understanding of a number of their basic properties is still evolving, with the last decades producing considerable advances. In the present review I outline our current empirical knowledge and physical understanding of these fascinating structures. I concentrate on the internal structure of sunspots, in particular their magnetic and thermal properties and on some of their dynamical aspects. Title: Hot loop oscillations seen by SUMER Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E. Bibcode: 2003AN....324..340W Altcode: No abstract at ADS Title: Magnetic Elements Near the Solar Limb: Inversions Based on a Flux-tube Model Authors: Frutiger, C.; Solanki, S. K.; Gandorfer, A. Bibcode: 2003ASPC..307..344F Altcode: No abstract at ADS Title: Molecules as Diagnostics of Solar and Stellar Magnetic Fields Authors: Berdyugina, S. V.; Solanki, S. K.; Stenflo, J. O. Bibcode: 2003ASPC..307..181B Altcode: No abstract at ADS Title: Modeling the Fine Structure of a Sunspot Penumbra through the Inversion of Stokes Profiles Authors: Borrero, J. M.; Lagg, A.; Solanki, S. K.; Frutiger, C.; Collados, M.; Bellot Rubio, L. R. Bibcode: 2003ASPC..286..235B Altcode: 2003ctmf.conf..235B No abstract at ADS Title: Asteroseismic Determination of a Star Authors: Gizon, Laurent; Solanki, Sami K. Bibcode: 2003IAUJD..12E..19G Altcode: Asteroseismology provides us with the possibility of determining the angle i between the direction of the rotation axis of a pulsating Sun-like star and the line of sight. A knowledge of i is important not just for obtaining improved stellar parameters but also in order to determine the true masses of extra-solar planets detected from the radial velocity shifts of their central stars. By means of Monte-Carlo simulations we estimate the precision of the measurement of i and other stellar parameters. We find that the inclination angle can be retrieved accurately for sufficiently large i for stars that rotate at least twice faster than the Sun. Title: Doppler oscillations in hot coronal loops Authors: Curdt, W.; Wang, T. J.; Innes, D. E.; Solanki, S. K.; Dammasch, I. E.; Kliem, B.; Ofman, L. Bibcode: 2002ESASP.506..581C Altcode: 2002ESPM...10..581C; 2002svco.conf..581C Recently, a new kind of damped oscillations of hot coronal loops was revealed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO. Such events seem to be a common feature observed in active region loops, seen very often when these lines brighten. The oscillations always have an impulsive trigger and are strongly damped while they cool down. However, in lines formed at coronal temperatures of ≍2 MK never any signature of these oscillations has been observed. In this study, we present the main properties of Doppler oscillations derived from a statistical study of 17 flare-like events, and a comparison with TRACE transverse loop oscillations. We also discuss the oscillation modes and their damping mechanism. Title: Was one sunspot cycle in the 18th century really lost? Authors: Krivova, N. A.; Solanki, S. K.; Beer, J. Bibcode: 2002A&A...396..235K Altcode: The unusually long 4th solar cycle has recently been proposed by Usoskin et al. (2001) to be composed of two cycles. They argue that a weak and short cycle might have been lost in sparse sunspot data at the end of the 18th century. Here we check this hypothesis in different ways. First, we consider the sunspot number record in greater detail and compare in a statistical sense the sunspot observations of the period in question with those at other times. In a statistical sense the sunspot numbers recorded at the time of the proposed new cycle minimum are extremely untypical for other minima in the solar cycle record, but quite usual for the declining phase of the solar cycle. We also analyse other available proxies of solar activity, such as variations of the cosmogenic nuclides 10Be and 14C as well as auroral activity. These historical records are sufficiently long and provide an independent testimony of the cyclic behaviour of solar activity at the end of the 18th century. We found no evidence for a lost cycle in any of these data sets. Finally, we compare the proposed new cycle with the other cycles in the sunspot record. This reveals that the proposed ``missing'' cycle has very unusual properties, much more so than the original, standard cycle 4. Taken together, the evidence from these various tests strongly suggests that no cycle was missed and that the official sunspot cycle numbering and parameters are correct. Title: Search for a relationship between solar cycle amplitude and length Authors: Solanki, S. K.; Krivova, N. A.; Schüssler, M.; Fligge, M. Bibcode: 2002A&A...396.1029S Altcode: The cross-correlation between time series of solar cycle length and amplitude suggests that the length precedes the amplitude. The relationship between the two is found to be more complex than a simple lag or phase shift, however. A simple empirical model is constructed which allows the amplitude of a given cycle to be predicted with relatively high accuracy from the lengths of earlier cycles. This result not only adds to the means at our disposal for predicting the amplitudes of future cycles, but also implies that the solar dynamo carries a memory of the length of one cycle over into the next. It may also have a bearing on why solar cycle length correlates better with the Earth's temperature record than cycle amplitude (Friis-Christensen & Lassen \cite{Friis-Christensen:Lassen:1991}). Thoughts on possible physical causes are presented. Title: A physical reconstruction of cosmic ray intensity since 1610 Authors: Usoskin, Ilya G.; Mursula, Kalevi; Solanki, Sami K.; Schüssler, Manfred; Kovaltsov, Gennady A. Bibcode: 2002JGRA..107.1374U Altcode: The open solar magnetic flux has been recently reconstructed by [2000, 2002] for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected intensity of galactic cosmic rays at the Earth's orbit since 1610. This new, physical reconstruction of the long-term cosmic ray intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, it resolves the problems related to previous reconstruction for the last 140 years based on linear correlations. We also calculate the flux of 2 GeV galactic protons and compare it to the cosmogenic 10Be level in polar ice in Greenland and Antarctica. An excellent agreement between the calculated and measured levels is found over the last 400 years. Title: The 1.3-year and 156-day periodicities in sunspot data: Wavelet analysis suggests a common origin Authors: Krivova, N. A.; Solanki, S. K. Bibcode: 2002A&A...394..701K Altcode: Helioseismic data have revealed a 1.3-year periodicity in the solar rotation rate near the bottom of the solar convection zone. In order to test whether these rotation rate variations have a significant impact on the solar dynamo, we search for such a periodicity in tracers of relatively freshly emerged flux at the solar surface, namely sunspots. Sunspot areas and sunspot number time series are studied with the help of the wavelet transform. Significant power at this period (1.28 years) is indeed found and is observed to vary strongly with time. This provides independent support for the presence of a 1.3 year periodicity in solar data. The power at the 154-158-day Rieger period of solar flares is seen to vary approximately in phase with the 1.28-year period. Based on this we propose that the Rieger period is the third harmonic (3x 156 days= 1.28 years) of the 1.3-year period. If the rotation rate of the Sun does vary with 1.3 years then the enhanced flaring with the Rieger period may finally be driven by the 1.3 year periodicity. However, the power in both periods is also found to approximately follow the total number of sunspots. Therefore we cannot rule out that the 1.3-year and 156-day periods are harmonics of the solar activity cycle. Finally, our analysis of a calibrated sunspot area record reveals that the 156-day period continues into the most recent cycles, in contrast to earlier results. Title: Sunrise: a 1-m balloon borne solar telescope Authors: Solanki, S. K.; Schüssler, M.; Curdt, W.; Lites, B. W.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.; Sunrise Team Bibcode: 2002ESASP.505...27S Altcode: 2002solm.conf...27S; 2002IAUCo.188...27S Sunrise is a light-weight solar telescope with a 1 m aperture for spectro-polarimetric observations of the solar atmosphere. The telescope is planned to be operated during a series of long-duration balloon flights in order to obtain time series of spectra and images at the diffraction-limit and to study the UV spectral region down to ≅200 nm, which is not accessible from the ground. The central aim of Sunrise is to understand the structure and dynamics of the magnetic field in the solar atmosphere. Interacting with the convective flow field, the magnetic field in the solar photosphere develops intense field concentrations on scales below 100 km, which are crucial for the dynamics and energetics of the whole solar atmosphere. In addition, Sunrise aims to provide information on the structure and dynamics of the solar chromosphere and on the physics of solar irradiance changes. Title: Thermal-magnetic relation of a sunspot as inferred from the inversion of 1.5 μm spectral data Authors: Mathew, S. K.; Solanki, S. K.; Lagg, A.; Krupp, N.; Woch, J.; Collados, M.; Berdyugina, S.; Frutiger, C. Bibcode: 2002ESASP.505..501M Altcode: 2002IAUCo.188..501M; 2002solm.conf..501M We present the thermal-magnetic relation in a simple, isolated sunspot deduced from the inversion of 1.56 μm spectropolarimetric data. Due to the high Zeeman sensitivity of the g = 3, Fe I 1.5648 μm line, we can study this relationship in the entire sunspot. An inversion technique based on response functions is used to derive various parameters, both as a function of location within the sunspot and of height in the atmosphere. In this paper we attempt to relate field strength, vertical and radial field components and the field inclination with temperature. Title: Initial features of an X-class flare observed with SUMER and TRACE Authors: Wang, T. J.; Solanki, S. K.; Innes, D. E.; Curdt, W. Bibcode: 2002ESASP.505..607W Altcode: 2002IAUCo.188..607W; 2002solm.conf..607W A class X1.5 flare started on the solar limb at 00:43 UT on 21 April 2002, which was associated with a CME observed at 01:27 UT by LASCO C2. The coordinated analyses of this flare include TRACE 195 Å images and SUMER spectra in lines of Fe XXI, Fe XII, and C II. We find that: 1) The flare began with a jet seen by TRACE, which was detected by SUMER in the C II line as a strong brightening with blue shifts up to 170 km s-1. At that time only weak emission was detected in Fe XII and Fe XXI. 2) Subsequently, a weak looplike brightening started south of the jet, moving outwards with an average speed of about 150 km s-1. The SUMER spectra responded this moving loop as separatingly brightenings, visible only in the Fe XXI line. The southwards moving component contains red- and blue-shifted emission features and has an apparent speed of ~120 km s-1. The absence of signatures in Fe XII and C II lines indicates that the moving weak loop seen by TRACE corresponds to the emission from very hot plasma, in a blend line in the 195 Å bandpass due to Fe XXIV formed at T > 10 MK. 3) The trigger mechanism of the flare and associated CME can be interpreted in the same way as that proposed by Wang et al. (2002) for an event with similar initial features. Title: Harold Jeffreys Lecture: Solar variability and climate change: is there a link? Authors: Solanki, Sami K. Bibcode: 2002A&G....43e...9S Altcode: Radiation from the Sun makes Earth a habitable planet. Fluctuations in the solar output are therefore likely to affect the climate on Earth, but establishing both how the output of he Sun varies and how such variations influence Earth's climate have proved tricky. But increased amounts of data from the Sun and about the climate on Earth over recent years mean that rapid progress is being made. In this paper, I review the current debate on the influence of the Sun and summarize the state of play in this area of solar physics. Title: Statistical properties of magnetic fields in intranetwork Authors: Khomenko, E. V.; Collados, M.; Lagg, A.; Solanki, S. K.; Trujillo Bueno, J. Bibcode: 2002ESASP.505..445K Altcode: 2002IAUCo.188..445K; 2002solm.conf..445K We report a study of the quiet sun's magnetic field based on high-resolution infrared spectropolarimetric observations (TIP/VTT). We find that in almost 50% of the pixels Stokes V and in 15% the Stokes Q and/or U profiles have a signal above 10-3. The statistical properties of the mainly intranetwork field sampled by these observations are presented, showing that most of the observed fields are weak (the field strength distribution peaks at 350 G and has a FWHM of 300 G) with very few kG features. The magnetized regions occupy a very small fill fractions (about 2%). The field changes properties on granular spatial scales and the size of the patches formed by similar profiles is close to 1". Most of the parameters of the observed polarization profiles show correlations with granulation parameters. Title: How large is the Sun's total magnetic flux? Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M. Bibcode: 2002ESASP.505..461K Altcode: 2002solm.conf..461K; 2002IAUCo.188..461K Variations in the radiative output of the Sun are directly allied to changes in the amount and distribution of solar surface magnetic field. The variability of irradiance on time-scales significantly shorter than a solar cycle can be produced by the spatial and temporal evolution of active regions, whereas its secular variations ensue from changes of the quiet-Sun magnetic flux on longer time-scales. The relative amounts of the magnetic flux in active regions and in the quiet Sun as well as their cyclic evolution have been studied by Harvey (1994) on the basis of the NSO/Kitt Peak synoptic maps. Since a single pixel of such a map is much bigger than individual small-scale magnetic elements and opposite polarities may be present within the same pixel, some magnetic flux went uncounted, so that the flux obtained by Harvey (1994) represents a lower limit to the true amount of magnetic flux present on the Sun. We use MDI full-disc and high-resolution magnetograms to estimate the total magnetic flux of the Sun. Title: Surface magnetic fields and UV radiance of the quiet Sun Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2002ESASP.505..521P Altcode: 2002solm.conf..521P; 2002IAUCo.188..521P In order to investigate the correspondence between the quiet-Sun UV radiance and changes in the magnetic network during the solar cycle, we study the quiet-Sun variability with the two EUV instruments CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) and the MDI (Michelson Doppler Imager) magnetograph on SOHO. Using a monthly data set of co-spatial and co-temporal observations of quiet-Sun areas near disk centre we follow the evolution of the quiet Sun over four years from solar minimum to solar maximum conditions. Although the magnetic flux of the quiet network increases by a rather low percentage over the solar cycle, its variation is well correlated with the radiance change in the He I 584 Å line. We test our data set of quiet-Sun measurements for a pixelwise relation between magnetic flux density and radiance. Title: Hot loop oscillations seen by SUMER: examples and statistics Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E. Bibcode: 2002ESASP.505..199W Altcode: 2002solm.conf..199W; 2002IAUCo.188..199W We measure physical parameters of Doppler-shift oscillations in 17 flare-like events. These events have been recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO, along a slit fixed above limb active regions. The selected spectral windows contain emission lines with formation temperatures from ~104 to 107K. The events were only detected in hot flare lines, without any signature in lines formed around 2×106K. Similarly, the Doppler shift oscillations occur in regions coincident with hot soft X-ray loops, but not with EUV loops. The oscillations have periods of 11 - 31 min, with an exponential decay time of 5.5 - 29 min, and show an initial large shift pulse with peak velocities up to 200 km s-1. Several indications suggest that the Doppler oscillations are incompressible coronal loop oscillations, that are usually excited impulsively by weak flare (or microflare) events that also produced a strong emission increase at 5 - 8×106K. Title: Doppler Shift Oscillations of Hot Solar Coronal Plasma Seen by SUMER: A Signature of Loop Oscillations? Authors: Wang, Tongjiang; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E. Bibcode: 2002ApJ...574L.101W Altcode: We report observations of strongly damped Doppler shift oscillations detected in a flare line, Fe XIX, with the Solar Ultraviolet Measurement of Emitted Radiation spectrometer. Spectra were recorded above an active region at the western limb of the Sun, from lines with formation temperatures ranging from 0.01 to 10 MK. However, the oscillations were seen only in the hot plasma (>6 MK) lines. The Doppler oscillations have periods of 14-18 minutes, with an exponential decay time of 12-19 minutes, and show an initial large blueshift pulse with peak velocities up to 77 km s-1. Several indications suggest that the Doppler oscillations are incompressible coronal loop oscillations that are excited impulsively by a flarelike event that also produced a strong increase in Fe XIX emission. Title: The magnetic structure of sunspots and starspots Authors: Solanki, S. K. Bibcode: 2002AN....323..165S Altcode: The single most important quantity determining the properties of sunspots and presumably starspots is their magnetic field. First an overview of the magnetic structure of sunspots is given, some of the progress made in recent years is described and some of the unsolved questions are pointed out. Both observational and theoretical aspects are dealt with. Finally, the magnetic structure of starspots is discussed. After presenting the evidence for (and against) their magnetic nature the signature of starspots in Zeeman Doppler images is described. It is pointed out that if the properties of sunspots are extrapolated to giant spots on rapidly rotating stars then the observed signature can be at least qualitatively explained. Title: Modelling solar irradiance variations: separate models for the network and active region faculae Authors: Wenzler, T.; Solanki, S. K.; Fluri, D. M.; Frutiger, C.; Fligge, M.; Ortiz, A. Bibcode: 2002ESASP.508..231W Altcode: 2002soho...11..231W In order to determine to what extent solar surface magnetism affects solar irradiance we need to reconstruct the irradiance from magnetograms. This process requires the use of model atmospheres. Here we present two model atmospheres describing faculae in active regions and the network. The models have been constructed such that they reproduce various data sets simultaneously. Title: Intercalibration of CDS and SUMER Authors: Pauluhn, A.; Lang, J.; Schühle, U.; Solanki, S. K.; Wilhelm, K.; Pike, C. D.; Thompson, W. T.; Rüedi, I.; Hollandt, J.; Huber, M. C. E. Bibcode: 2002ESASP.508..223P Altcode: 2002soho...11..223P The outcome of the Joint Observing Programme (JOP) Intercal-01, which is the intercalibration of the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument (detectors A and B) and the two CDS (Coronal Diagnostic Spectrometer) instruments, the Normal Incidence Spectrometer (NIS) and the Grazing Incidence Spectrometer (GIS), is presented. Recent calibration updates of both instruments have been employed, and the results indicate a very good correlation and agreement of the measured radiances within the individual uncertainties. Title: Relative brightness variability vs. averaged Doppler shift in the quiet Sun Authors: Brković, A.; Peter, H.; Solanki, S. K. Bibcode: 2002ESASP.508..281B Altcode: 2002soho...11..281B We studied SUMER and CDS time series of spectra and images of quiet-Sun regions at disc centre. Ultraviolet emission lines sampling temperatures of the chromosphere, transition region and corona were recorded. We found a high correlation between average net Doppler shifts and relative brightness variabilities of the studied lines. We point to some basic ideas which could eventually model the variability-Doppler shift relationship. Title: How does the magnetic cycle change radiance and irradiance of the Sun? Authors: Solanki, Sami K. Bibcode: 2002ESASP.508..173S Altcode: 2002soho...11..173S The Sun's magnetic field, which is concentrated into flux tubes in the photosphere, affects the radiance. Depending on the size of a flux tube it radiates more (for slender tubes) or less (for broad tubes) strongly than the quiet Sun. The Sum of all magnetic features on the whole solar disc affects the Sun's irradiance. As the number and the size distribution of the flux tubes changes over the solar cycle it produces a variation of total irradiance, which is very similar to the observed variation. There is also evidence that the Sun's magnetic field exhibits a secular variation, which is expected to produce a slow irradiance change, whose amplitude is expected to be larger than the cyclic variation. In this review some of the relevant processes taking place in flux tubes are pointed out and models describing the irradiance variations over the solar cycle based on the Sun's magnetic field are discussed. Finally, the results of the modelling of the secular variation of the Sun's magnetic field are presented. Title: Total solar magnetic flux: dependence on spatial resolution of magnetograms Authors: Krivova, N. A.; Solanki, S. K.; Fligge, M. Bibcode: 2002ESASP.508..155K Altcode: 2002soho...11..155K The variability of the solar radiative output is tied to the evolution of the surface magnetic field. Irradiance changes on time-scales of the solar rotation are governed by the varying distribution of the magnetic features and the evolution of sunspots and individual active regions, whereas the total amount of magnetic flux in particular small-scale magnetic elements seems to dominate the irradiance variations on the time scale of the solar cycle. Possibly of even greater relevance for climate on Earth are secular variations of solar irradiance, which are at least partly caused by changes in the quiet-Sun magnetic flux on long time scales. Using NSO/Kitt Peak synoptic charts Harvey (1994) found that the total magnetic flux in active regions at activity maximum is about 3 times higher than the flux at activity minimum. The size of small-scale magnetic elements is, however, far below the resolution of currently obtainable magnetograms. At the same time, their distribution on the solar surface is highly non-uniform, with magnetic elements of opposite polarities often being grouped close together. This leads to an apparent cancellation of the flux within a relatively large pixel of a Kitt Peak synoptic chart and underestimates the total magnetic flux, mainly in the quiet Sun. Using MDI full-disc and high-resolution magnetograms and artificially reducing their spatial resolution by binning several pixels together we study the influence of the resolution on the measured total magnetic flux. Title: On the intensity contrast of solar photospheric faculae and network elements Authors: Ortiz, A.; Solanki, S. K.; Domingo, V.; Fligge, M.; Sanahuja, B. Bibcode: 2002A&A...388.1036O Altcode: 2002astro.ph..7008O Sunspots, faculae and the magnetic network contribute to solar irradiance variations. The contribution due to faculae and the network is of basic importance, but suffers from considerable uncertainty. We determine the contrasts of active region faculae and the network, both as a function of heliocentric angle and magnetogram signal. To achieve this, we analyze near-simultaneous full disk images of photospheric continuum intensity and line-of-sight magnetic field provided by the Michelson Doppler Interferometer (MDI) on board the SOHO spacecraft. Starting from the surface distribution of the solar magnetic field we first construct a mask, which is then used to determine the brightness of magnetic features, and the relatively field-free part of the photosphere separately. By sorting the magnetogram signal into different bins we are able to distinguish between the contrasts of different concentrations of magnetic field. We find that the center-to-limb variation (CLV) of the contrast changes strongly with magnetogram signal. Thus, the contrasts of active region faculae (large magnetogram signal) and the network (small signal) exhibit a very different CLV, showing that the populations of magnetic flux tubes that underly the two kinds of features are different. The results are compatible with, on average, larger flux tubes in faculae than in the network. This implies that these elements need to be treated separately when reconstructing variations of the total solar irradiance with high precision. We have obtained an analytical expression for the contrast of photospheric magnetic features as a function of both position on the disk and spatially averaged magnetic field strength, by performing a 2-dimensional fit to the observations. We also provide a linear relationship between magnetogram signal and the mu =cos (theta ), where theta is the heliocentric angle, at which the contrast is maximal. Finally, we show that the maximum contrast per unit magnetic flux decreases rapidly with increasing magnetogram signal, supporting earlier evidence that the intrinsic contrast of magnetic flux tubes in the network is higher. Title: Oscillating hot loops observed by SUMER Authors: Wang, T. J.; Solanki, S. K.; Curdt, W.; Innes, D. E.; Dammasch, I. E. Bibcode: 2002ESASP.508..465W Altcode: 2002soho...11..465W We report observations of Doppler shift oscillations in hot flare lines emitted from active region loops. The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO recorded spectra of limb active regions loops in several emission lines with formation temperatures from ~104 to 107K. The events were only detected in the hot flare lines, without any signature in lines formed around 2×106K. There is a large shift pulse of up to 190 km s-1 during the rising phase of the flux which is followed by two or three periods of strongly damped alternating red and blue shift oscillations with periods in the range 12-31 min. Slow mode standing waves match the observed period. However, the initial large Doppler shift pulse suggests that the waves are impulsively generated. Unlike the oscillating loops seen in the TRACE images, these Doppler shift oscillations are sometimes seen without an associated flare. Title: Variation of the facular and network contrast during the rising phase of cycle 23 Authors: Ortiz, A.; Domingo, V.; Sanahuja, B.; Solanki, S. K. Bibcode: 2002ESASP.508..185O Altcode: 2002soho...11..185O Magnetic activity contributes to solar irradiance variations, both on short and long time-scales. While sunspots and active region faculae are the dominant contributors to irradiance changes on time-scales of days to weeks, the origin of the long-term increase of the irradiance between activity minimum and maximum (~0.1%) is still debated. It has been proposed that the small-scale magnetic elements composing the enhanced and quiet network contribute substantially to this increase. To contribute to this debate, we attempt to see if there is a change in the radiative properties of these elements along the solar cycle, and to evaluate such a change. We use near-simultaneous full disk magnetograms and images of the photospheric continuum intensity provided by MDI/SOHO. We have studied the center-to-limb variations (CLV) of the contrast as a function of magnetic strength and we are now analyzing how the noise level of the images changes throughout time, as a preliminary step towards an analysis of the temporal irradiance variations. Title: Dependence of UV radiance of the quiet Sun on the solar cycle Authors: Pauluhn, A.; Solanki, S. K. Bibcode: 2002ESASP.508..227P Altcode: 2002soho...11..227P Recent SOHO measurements of UV radiance of the quiet Sun show a dependence of the radiance on the solar cycle. We study the hypothesis that changes in the magnetic network are causing these variations. The quiet-Sun variability is investigated with the two EUV instruments CDS and SUMER and the MDI magnetograph on SOHO. We find that although the magnetic flux of the quiet network increases by a rather low percentage over the rising part of the solar cycle, its variation is well correlated with the radiance change in the He I 584 Å line. The main change occurs in the flux of the strong network elements while the weaker elements do not exhibit a significant change. Title: The Sun at solar minimum: North - south asymmetry of the polar coronal holes Authors: Zhang, J.; Woch, J.; Solanki, S. K.; von Steiger, R. Bibcode: 2002GeoRL..29.1236Z Altcode: 2002GeoRL..29h..77Z Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic charts derived from Kitt Peak magnetograms are used to compare the south and north polar coronal holes which existed during the declining/minimum phase of the solar activity cycle from 1992 to 1997. The kinetic properties of the solar wind emanating from the two polar coronal holes, as represented by solar wind speed, do not differ significantly. However, the electron temperature in the two coronal holes inferred from ionic charge composition data, namely the O7+/O6+ ratio, show consistent differences, with the south polar hole being 10 to 15% hotter. The ground-based magnetograms show that the north polar coronal hole covers a larger part of the solar surface than the southern one. The total magnetic flux and, specifically, the flux density of the north polar coronal hole is considerably lower for the whole interval of time between 1992 and 1997. This strongly indicates that the difference in coronal hole temperature between the southern and northern coronal hole is intrinsic and is not due to the fact that the Ulysses observations in the south and north coronal hole streams were made at different phases of the solar cycle. Thus the differences found represent a real north-south asymmetry during this time period. Title: Quiet-Sun variability observed with SUMER and CDS Authors: Brković, A.; Solanki, S. K.; Rüedi, I. Bibcode: 2002A&A...385..257B Altcode: Brightness variations of solar features are investigated using time series of images and spectra of quiet-Sun regions at disc centre obtained with the Coronal Diagnostic Spectrometer (CDS) and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument onboard the SOHO spacecraft. Ultraviolet emission lines sampling temperatures of the chromosphere, transition region and corona were recorded, with the \hei 584.3 Å and \oxv 629.7 Å lines being recorded simultaneously by both instruments. A comparison shows that both instruments give similar results except that SUMER reveals a factor of three higher absolute and relative variability than CDS. Simple tests suggest that the higher spatial resolution of SUMER compared to CDS, and the broad slit used for the CDS observations, are responsible for this difference. This points to the need for higher spatial resolution for future variability studies. The SUMER results confirm and extend to lower temperatures the trends deduced in an earlier paper from CDS data. Title: The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields. I. Theoretical spectral patterns in the Zeeman regime Authors: Berdyugina, S. V.; Solanki, S. K. Bibcode: 2002A&A...385..701B Altcode: An overview of the theory of the Zeeman effect in diatomic molecules for the limiting Hund's cases (a) and (b) is given and a numerical approach for the intermediate coupling case (a-b) is developed. In contrast to earlier derivations, which were limited to doublets, this approach is valid for terms of any multiplicity. General properties of the Zeeman effect for the various cases are deduced. Finally, calculated Landé factors for prominent molecular bands in sunspot and cool-star spectra are employed to predict the general behaviour of these bands in the presence of a magnetic field below the Paschen-Back limit. The limiting field strength is calculated and listed. Title: Solar irradiance variations and climate Authors: Solanki, S. K.; Fligge, M. Bibcode: 2002JASTP..64..677S Altcode: 2002JATP...64..677S The irradiance of the Sun is observed to vary in phase with the solar cycle at an amplitude of /~0.1% and a period of roughly 11 years. There is indirect evidence that the irradiance also exhibits a larger secular variation. Direct measurements only cover the last 21/2 solar cycles and a longer record is needed to study the possible coupling with climate. Therefore, it is necessary to develop successful models of solar irradiance variations and to reconstruct it back into the past. In the present paper, the current observational knowledge and the state-of-the-art of the modelling are introduced and reviewed. Title: Models for solar magnetic loops. II. Comparison with SOHO-CDS observations on the solar disk Authors: Brković, A.; Landi, E.; Landini, M.; Rüedi, I.; Solanki, S. K. Bibcode: 2002A&A...383..661B Altcode: The present work describes a detailed comparison between SOHO-CDS observations of active region loops with a static, isobaric loop model developed assuming a temperature-independent heating function in the energy balance equation and a variable loop cross-section. The loop model is described in Landini & Landi (2002). Observations of an active region recorded by CDS have been analyzed. Additional data from the EIT and MDI instruments on board the SOHO satellite, and broad band soft X-rays images from the Yohkoh satellite, have been used to complement the CDS dataset. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT, MDI and Yohkoh images and a loop structure has been identified. Two other loop structures are visible but their footpoints are not clearly identified, and have not been analyzed. Electron density, temperature and pressure along the selected loop structure have been measured by means of line ratio techniques. These quantities have been used to test the assumption of constant pressure adopted in the theoretical model, and to compare their values with its predictions. The loop filling factor has also been estimated from the CDS data after assumptions on the loop geometry have been made. Comparison with CDS data has shown that a classical model is not able to reproduce the observations; despite the large uncertainties, mainly given by the limited CDS spatial resolution, indications suggest that agreement occurs only if an ``ad hoc'' isothermal region is added on top of the loop and a large conductive flux at the base is assumed. Suggestions for improvements of theoretical loop models and further studies with the EIS instrument on Solar-B, due for launch in 2005, are given. Title: Secular variation of the Sun's magnetic flux Authors: Solanki, S. K.; Schüssler, M.; Fligge, M. Bibcode: 2002A&A...383..706S Altcode: We present an extension of the model of \citet{Solanki:etal:2000} that allows us to reconstruct the time evolution of both the total and the open magnetic flux at the solar surface since 1700. The flux emerging in large active regions is determined using the sunspot number as a proxy, while the flux emergence in small ephemeral regions is described by an extended cycle whose amplitude and length are related to the corresponding sunspot cycle. Both types of regions contribute to the open flux, which is the source of the heliospheric field. The overlap of the activity cycles of ephemeral regions leads to a secular variation of the total cycle-related magnetic flux (active region flux + ephemeral region flux + open flux). The model results indicate that the total surface flux has doubled in the first half of the last century. The evolution of the open flux is in good agreement with the reconstruction by \citet{Lockwood:etal:1999}. Title: Solar variability and global warming Authors: Krivova, N.; Solanki, S. Bibcode: 2002cosp...34E.274K Altcode: 2002cosp.meetE.274K The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations entering climate modelling. Given the complexity of the climate system, however, such modelling is perforce based on simplifying assumptions, which leaves it open to criticism. We take a complementary approach. We assume that the Sun has been responsible for climate change prior to 1970. Then, using reconstructions and measured records of relevant solar quantities as well as of the cosmic-ray flux, we estimate which fraction of the dramatic temperature rise after that date could be due to the influence of the Sun. We show that at least in the most recent past (since 1970) the solar influence on climate cannot have been significant. Title: The solar atmosphere Authors: Solanki, S. K.; Hammer, R. Bibcode: 2002css1.book.1065S Altcode: No abstract at ADS Title: Irradiance Medels based on Magnetic Activity Authors: Solanki, S.; Krivova, N.; Unruh, Y. Bibcode: 2002cosp...34E1357S Altcode: 2002cosp.meetE1357S An introduction is given to the modelling of irradiance variations based on solar surface magnetism. We describe how a time series of daily magnetograms and empirical models of the thermal structure of magnetic features (sunspots, magnetic elements) are combined to reconstruct total and spectral irradiance on a time scale of weeks to years. Comparisons with observational data reveal an excellent correspondence. On a longer time scale of decades to centuries we first need to know how the Sun's magnetic field evolved. Hence reconstructions of the Sun's total magnetic flux are briefly introduced, before we present longer term reconstructions of the total and spectral irradiance since the Maunder minimum. Title: Physical reconstruction of the long-term heliospheric modulation of cosmic rays Authors: Usoskin, I.; Mursula, K.; Solanki, S.; Shuessler, M.; Kovaltsov, G. Bibcode: 2002cosp...34E.902U Altcode: 2002cosp.meetE.902U Open solar magnetic flux has been recently reconstructed by Solanki et al. (2000, 2002) for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected intensity of galactic cosmic rays at the Earth's orbit since 1610. This calculated cosmic ray intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, we calculate the flux of 2 GeV galactic protons and compare it to the cosmogenic 10 B e level in polar ice in Greenland and Antarctica. An excellent agreement between the calculated and actual levels is found over the last 400 years. Title: Solar Variability Authors: Solanki, S. K. Bibcode: 2002ISSIR...2....1S Altcode: 2002rcs..conf....1S; 2002ESASR...2....1S With increasing sensitivity, wavelength coverage and photometric accuracy (including reliability in calibrations) it is becoming ever more clear that the Sun is a variable star at practically all wavelengths and timescales. A short and incomplete overview is given of the forms which this variability can take. Some of the underlying physics is also very briefly touched upon. Title: The Sun At Solar Minimum: North - South Asymmetry of The Polar Coronal Holes Authors: Woch, J.; Zhang, J.; Solanki, S. K.; von Steiger, R. Bibcode: 2002EGSGA..27.4007W Altcode: Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic charts derived from Kitt Peak magnetograms are used to compare the south and north polar coronal holes which existed during the declining/minimum phase of the solar activity cycle from 1992 to 1997. The kinetic properties of the solar wind emanating from the two polar coronal holes, as represented by solar wind speed, do not differ significantly. However, the electron temperature in the two coronal holes inferred from ionic charge composition data, namely the O7+/O6+ ratio, show consis- tent differences, with the south polar hole being 10 to 15% hotter. The ground-based magnetograms show that the north polar coronal hole covers a larger part of the so- lar surface than the southern one. The total magnetic flux and, specifically, the flux density of the north polar coronal hole is considerably lower for the whole interval of time between 1992 and 1997. This strongly indicates that the difference in coronal hole temperature between the southern and northern coronal hole is intrinsic and is not due to the fact that the Ulysses observations in the south and north coronal hole streams were made at different phases of the solar cycle Thus the differences found represents a real north-south asymmetry. Title: Interplanetary and solar surface properties of coronal holes Authors: Woch, J.; Zhang, J.; Solanki, S.; von Steiger, R. Bibcode: 2002cosp...34E.967W Altcode: 2002cosp.meetE.967W Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyse coronal holes throughout the solar activity cycle. The large polar coronal holes existing during the declining/minimum phase of solar cycle 22 show a persistent north-south asymmetry. The coronal temperature, inferred from ionic charge-state distributions, and the magnetic flux density are significantly lower in the north polar coronal hole. The temperature of the emerging north polar hole of solar cycle 23 is remarkably similar to that of solar cycle 22, confirming that the coronal temperatures of the polar coronal holes do not evolve in the course of the activity cycle. Solar wind streams emanating from the small-scale coronal holes observed around solar maximum generally show lower velocities compared to the polar coronal hole streams. However, the coronal temperatures do not reveal a consistent difference. Though a large number of solar maximum holes have a significantly higher temperature compared to the polar coronal holes the majority has a coronal temperature within the range of polar hole temperatures. Above all, the latter holds for solar maximum coronal holes having a magnetic polarity consistent with the polarity of the new solar cycle. Likewise, the magnetic flux density in the solar maximum holes and in the polar coronal holes, as derived from the synoptic maps, is not strikingly different. Therefore, it can be concluded that there is no indication for an intrinsic difference of solar maximum and polar coronal holes. Title: How much of the solar irradiance variations is caused by the magnetic field at the solar surface? Authors: Solanki, S. K.; Fligge, M. Bibcode: 2002AdSpR..29.1933S Altcode: The contribution to total solar irradiance variations by the magnetic field at the solar surface is estimated. Detailed models of the irradiance changes on the basis of magnetograms show that magnetic features at the solar surface account for over 90% of the irradiance variations on a solar rotation time scale and at least 70% on a solar cycle time scale. If the correction to the VIRGO record proposed by Fröhlich & Finsterle (2001) is accepted, then magnetic features at the solar surface are responsible for over 90% of the solar cycle irradiance variations as well. Title: Intercalibration of CDS and SUMER Authors: Pauluhn, A.; Lang, J.; Schuhle, U.; Solanki, S. K.; Wilhelm, K.; Thompson, W. T.; Pike, C. D.; Ruedi, I.; Hollandt, J.; Huber, M. C. E. Bibcode: 2002ISSIR...2..235P Altcode: 2002rcs..conf..235P; 2002ESASR...2..235P Simultaneous observations of the same solar features with different instruments provide a way to compare radiometric calibrations and detect changes in responsivity with time of EUV instruments in space within the combined uncertainties of the individual instruments. Here we present the intercalibration of the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument (detectors A and B) and the two CDS (Coronal Diagnostic Spectrometer) instruments, the Normal Incidence Spectrometer (NIS) and the Grazing Incidence Spectrometer (GIS) on the Solar and Heliospheric Observatory (SOHO). This work describes the results of the Joint Observing Programme Intercal 01 and presents quiet-Sun comparisons from March 1996 up to February 2001, which represents the complete set of all available Intercal 01 measurements. Recent calibration updates of both instruments are employed, and the results indicate a very good correlation and agreement of the measured radiances within the combined uncertainties. Title: Properties of ultraviolet lines observed with the Coronal Diagnostic Spectrometer (CDS/SOHO) in coronal holes and the quiet Sun Authors: Stucki, K.; Solanki, S. K.; Pike, C. D.; Schühle, U.; Rüedi, I.; Pauluhn, A.; Brković, A. Bibcode: 2002A&A...381..653S Altcode: We present an analysis of 14 ultraviolet emission lines belonging to different atoms and ions observed inside polar coronal holes and in the normal quiet Sun. The observations were made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO). This study extends previous investigations made with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer to higher temperatures. We compare line intensities, shifts and widths in coronal holes with the corresponding values obtained in the quiet Sun. While all lines formed at temperatures above 7 x 105 K show clearly the presence of the hole in their intensities, differences in line width are more subtle, with cooler lines being broader in coronal holes, while hotter lines tend to be narrower. According to the present data all lines are blueshifted inside the coronal hole compared to the normal quiet Sun. Almost all the lines formed between 80 000 K and 600 000 K (i.e. transition-region lines) show a correlation between blueshifts and brightness within coronal holes. This is in agreement with the conclusion reached by Hassler et al. (\cite{Hassler1999}) that the fast solar wind emanates from the network and supports our previous study (Stucki et al. 2000b). For coronal lines, this trend seems to be reversed. Title: Reconstruction of Cosmic Ray Intensity Since 1610 Authors: Usoskin, I. G.; Mursula, K.; Solanki, S. K.; Schüssler, M.; Kovaltsov, G. A. Bibcode: 2002EGSGA..27.5173U Altcode: Open solar magnetic flux has been recently reconstructed by Solanki et al. (2000, 2002) for the last 400 years from sunspot data. Using this reconstructed magnetic flux as an input to a spherically symmetric quasi-steady state model of the heliosphere, we calculate the expected intensity of galactic cosmic rays at the Earth's orbit since 1610. This calculated cosmic ray intensity is in good agreement with the neutron monitor measurements during the last 50 years. Moreover, we calculate the flux of 2 GeV galactic protons and compare it to the cosmogenic 10Be level in polar ice in Greenland and Antarctica. An excellent agreement between the calculated and actual levels is found over the last 400 years. Title: Physical reconstruction of long-term solar activity Authors: Usoskin, I.; Solanki, S.; Schuessler, M.; Mursula, K.; Kovaltsov, G. Bibcode: 2002cosp...34E.901U Altcode: 2002cosp.meetE.901U For many applications in dynamo theory and solar-terrestrial research it is important to know the evolution of solar activity on long time scales (centuries to millennia). Previous reconstructions were based upon either multi-harmonic backward extrapolation of the known sunspot records or on the assumption of a linear relation between terrestrial proxies (e.g., cosmogenic isotope abundance) and solar activity. Here we present, for the first time, a physical reconstruction of sunspot activity on long time scales from the cosmogenic 10 Be records. We use a numerical inversion of a combined physical solar-heliospheric model (Usoskin et al., 2002), which is essentially non-linear. Using physical rather than empirical relations on all steps, we present a reconstruction of sunspot activity since the 15th century. Uncertainties of the reconstruction are discussed in details. It is important that the current high level of sunspot activity is unique on the millennium time scale. Title: Coronal Holes During Solar Maximum: Swics/ulysses and Kitt Peak Observations Authors: Zhang, J.; Woch, J.; Solanki, S. K.; von Steiger, R. Bibcode: 2002EGSGA..27.4034Z Altcode: Synoptic maps of the solar coronal magnetic field from Kitt Peak show the existence of small-scale coronal holes at low and mid latitudes during the maximum phase of the present solar cycle. The solar wind originating from the most prominent holes can be unambiguously identified in interplanetary space at distances of 2 to 4 AU with the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses. The coronal hole plasma is characterized by an enhanced velocity and a decreased O7+/O6+ ratio com- pared to the ambient solar wind. The decreased ratio is indicative for a lower coronal temperature in the source region on the Sun. Though clearly distinguishable from the ambient solar wind plasma, the solar wind originating from small scale coronal holes has a lower velocity and less reduced O7+/O6+ ratio than the solar wind emanating from the large-scale polar coronal holes during solar minimum. This difference is re- flected in synoptic coronal hole maps and magnetograms. The magnetic flux density of the small scale coronal holes is considerably lower compared to the flux density of the polar coronal holes. We suggest that the two types of coronal holes are intrinsically different. Title: Noise reduction in helioseismic power spectra\ by non-orthogonal wavelets Authors: Solanki, S. K.; Régulo, C.; Fligge, M.; Kosovichev, A. G. Bibcode: 2001A&A...379.1039S Altcode: We present a method to reduce noise in helioseismic power spectra using a non-orthogonal wavelet transform based on quadratic spline functions. The quality of our method is tested by applying it to artificially generated time-series approximating solar p-modes. The mode frequencies and line widths obtained from least-squares fits to the smoothed spectra are compared with the corresponding parameters deduced from maximum likelihood fits to the original spectra. The results from both approaches are very similar and suggest that there is no major bias in either of these rather independent approaches. As a practical example we denoise parts of the power spectrum obtained from the two first years of operation of the GOLF instrument onboard SOHO. Title: Zeeman-split opposite-polarity OH lines in sunspot spectra: Resolution of a puzzle Authors: Berdyugina, S. V.; Solanki, S. K. Bibcode: 2001A&A...380L...5B Altcode: We present the first synthetic Zeeman-split Stokes I and V profiles of OH lines. They explain the puzzling observations reported by Harvey (\cite{har85}) of two pairs of lines from the infrared (2, 0) band with Stokes V profiles exhibiting opposite polarities. Our new perturbation calculations of the Zeeman effect in diatomic molecules, which allow states of any multiplicity to be treated, resolve the puzzle. They reveal that the unusual behaviour of these lines stems from the fact that the two pairs have effective Landé factors of similar magnitude, but of opposite signs. Title: Intercalibration of SUMER and CDS on SOHO. II. SUMER detectors A and B and CDS NIS Authors: Pauluhn, Anuschka; Rüedi, Isabelle; Solanki, Sami K.; Schühle, Udo; Wilhelm, Klaus; Lang, Jim; Thompson, William T.; Hollandt, Jörg Bibcode: 2001ApOpt..40.6292P Altcode: Results of an intercalibration between the extreme-ultraviolet spectrometers Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on board the Solar and Heliospheric Observatory (SOHO) are reported. The results of the joint observing program Intercal_01 are described, and intercalibration results up to July 2000 of both SUMER detectors A and B and of the CDS Normal Incidence Spectrometer (NIS) are presented. The instruments simultaneously observed radiance of emission lines at the center of the Sun, and three lines have been chosen for intercomparison: He I 584 Å, Mg X 609 Å, and Mg X 624 Å. Initially the same area was observed by both instruments, but, after restrictions were imposed by the scanning mechanism of SUMER in November 1996, the instruments viewed areas of different sizes. Nevertheless, the temporal correlation between the two instruments remained good through June 1998, when contact with the SOHO spacecraft was lost. Until then the CDS instrument measured (33+/-5)% and (38+/-7)% (+/-1σ) higher intensity than SUMER in the Hz I 584-Å line on average for detectors A and B, respectively. Data from SUMER detector B agreed well for Mg X 609 Å and Mg X 624 Å with the CDS intensities, showing offsets of (2+/-10)% and (9+/-15)%, much less than the data of detector A with offsets of (7+/-8)% and (16+/-7)% for the two lines, respectively, relative to CDS. Finally, the intercalibration measurements after the loss and recovery of the SOHO spacecraft are analyzed. The data for observations from November 1998 to July 2000 are compared, and it is shown that, although the responses of the instruments have changed, the CDS and the SUMER still perform well, and their temporal correlation is good. Title: High-resolution solar polarimetry with Sunrise Authors: Schmidt, W.; Solanki, S. K.; Lites, B. W.; Title, A. M.; Martínez Pillet, V. Bibcode: 2001AN....322..363S Altcode: Sunrise is a solar telescope with an aperture of 1 m, and is dedicated for spectropolarimetric measurements in the visible and the near UV. The total wavelength range is 200 to 1000 nm for narrowband imaging and diagnostic spectroscopy. Sunrise is planned as a stratospheric long-duration balloon mission with a first flight in 2006 Title: A formation mechanism of magnetic elements in regions of mixed polarity Authors: Gadun, A. S.; Solanki, S. K.; Sheminova, V. A.; Ploner, S. R. O. Bibcode: 2001SoPh..203....1G Altcode: We present 2-D, fully compressible radiation-MHD simulations of the solar photospheric and subphotospheric layers that run for 2 hours of solar time starting from a magnetic configuration with mixed polarities. In the atmospheric layers the simulation reveals a correlation between field strength and inclination, with a nearly vertical strong-field magnetic component and a more horizontal weak-field component, in agreement with the observations. Our simulation also shows that magnetic flux is converted from one of these states to the other. In particular, magnetic flux sheets can also be formed when a new downflow lane starts due to granule fragmentation. The dynamics of the granulation and field-line reconnection are found to play a role in the initial stages of a magnetic element's formation. The simulation predicts that during or shortly after their formation magnetic elements could be associated with oppositely polarized flux at a small spatial scale. Title: The sun's variable spectrum and its terrestrial effects Authors: Fligge, M.; Solanki, S. K. Bibcode: 2001A&AT...20..467F Altcode: The solar radiation exhibits strong temporal variations not only in intensity but also in its spectrum which are linked to the evolution of the Sun through its magnetic activity cycle. The irradiance variability increases from longer to shorter wavelengths, with variations in the UV being orders of magnitude larger than those in the visible. While 99% of the total solar radiative output occurs at wavelengths longer than 300 nm, solar UV radiation accounts for up to 30% of the total variation. The continuous rise of the Earth's surface temperature over the last decades urgently raises a question about the physical origin of so-called global warming. A reliable estimate of the influence of human activity on the Earth's climate requires detailed knowledge about the natural sources of variability. One of the unknown boundary conditions for models of the evolution of the Earth's climate is the variation of the solar irradiance. Title: The influence of an inclined rotation axis on solar irradiance variations Authors: Knaack, R.; Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2001A&A...376.1080K Altcode: Compared with Sun-like stars, the irradiance variations of the Sun over the solar cycle appear to be relatively small for its average activity level (Lockwood et al. \cite{Lockwood:etal:1992}; Radick et al. \cite{Radick:etal:1998}). It has been proposed that the special position of Earth-based observers in the ecliptic plane may give the impression of a subdued solar photometric variability (Schatten \cite{Schatten:1993}). The aim of the present paper is to examine the influence on irradiance variations of a solar rotation axis inclined towards the observer. A three-component model is used to calculate relative flux variations of a given active-region distribution on the surface of the Sun as a function of inclination and wavelength. Wavelength-dependent intensity spectra are used to describe the contributions of the undisturbed photosphere, sunspots and faculae. The spectra result from models that have successfully been used to reproduce a host of solar data and thus represent realistic estimates of the radiative output from these solar features. We find that an inclined rotation axis increases the total solar irradiance variations maximally by 40%. The most probable value is approximately 6%. This is much less than that suggested by former studies, which were based on simple contrast functions. In the averaged Strömgren filters we estimate a most probable increase of the solar variability of 30%. In addition, we estimate the dependence of the flux in the chromospheric Ca II H&K lines on inclination. We find that the average chromospheric activity level depends only slightly on the inclination angle. The chromospheric variability of Sun-like stars, however, is significantly affected. Nonetheless, our results indicate that a different average inclination of stellar rotation axes relative to the observer cannot explain the discrepancy between the brightness variations of the Sun and Sun-like stars. Title: Variations of solar spectral irradiance from near UV to the infrared-measurements and results Authors: Fligge, M.; Solanki, S. K.; Pap, J. M.; Fröhlich, C.; Wehrli, C. Bibcode: 2001JASTP..63.1479F Altcode: 2001JATP...63.1479F Solar spectral irradiance variations are known to exhibit a strong wavelength dependence with the amount of variability increasing towards shorter wavelengths. The bulk of solar radiation is emitted at visible and infrared wavelengths. Thus, the spectral radiation length of 300nm accounts for 99% of the total solar radiative output. Deposited in the Earth's troposphere and biosphere, this part of the solar irradiance spectrum determines direct solar radiative forcing and is therefore of particular interest for climate studies. First, measurements of solar irradiance and irradiance variability from near UV to the IR are reviewed with particular emphasis on the results obtained from the Variability of Irradiance and Gravity Oscillations (VIRGO) on SOHO and Solar Spectrum Measurement (SOLSPEC) instruments. In the second part a model is presented which describes solar spectral irradiance variations in terms of the changing distribution of solar surface magnetic features. Title: The lower polar atmosphere and the solar dynamo: perspectives for the Solar Orbiter Authors: Solanki, Sami K. Bibcode: 2001ESASP.493...35S Altcode: 2001sefs.work...35S The Solar Orbiter will, by flying out of the ecliptic, allow its battery of remote-sensing instruments to focus on the Sun's polar regions, providing solar physicists with a unique opportunity to study this enigmatic part of the Sun for the first time. Some of the scientific questions which may be addressed by the Solar Orbiter in its out of the ecliptic phase are presented here, with emphasis being placed on those related to the lower solar atmosphere and the solar interior. Title: Analysis of blinkers and EUV brightenings in the quiet Sun observed with CDS Authors: Brković, A.; Solanki, S. K.; Rüedi, I. Bibcode: 2001A&A...373.1056B Altcode: Movies of quiet Sun regions at disc centre obtained with the Coronal Diagnostic Spectrometer (CDS) onboard the SOHO spacecraft are used to study the properties of transient brightenings seen in the extreme ultraviolet (EUV), so-called blinkers, at three different temperatures sampled simultaneously in the chromospheric He I 584.3 Å (2 x 104 K), the transition region O V 629.7 Å (2.5 x 105 K) and coronal Mg IX 368.1 Å (106 K) lines. Blinkers, here defined somewhat differently than in previous studies, were clearly detected in the O V and He I lines. Brightenings of the Mg IX line were also seen. A thorough analysis of blinker properties is carried out and their detailed properties are determined. Blinkers are found to be present in both bright (network) and dark (intranetwork) regions, but their number density is larger in the brighter areas (in O V) although the rest of their properties appear to be unaffected. The average sizes of brightenings range from 2.8 Mm2 in Mg IX, 12.4 Mm2 in He I to 23.5 Mm2 in O V. The durations of blinkers are in the range 3-110 min, with the average durations being 23 min in He I, about 16 min in O V and 12 min in Mg IX. The frequency distributions of ratio of peak to background intensity, excess energy and size follow power laws with exponents <-5 for the intensity ratio, and between -1 and -3 for the other two parameters. The correlation coefficients between pairs of ratio, energy and size are at least 0.5, while other pairs of parameters describing the blinkers appear to be uncorrelated. The best correlation is between size and energy. The blinker durations exhibit a distribution whose form is compatible with a log-normal function. Finally, blinkers in the 3 lines (i.e. 3 temperature regimes) are poorly correlated; with the correlation coefficient being always less than 0.4. This suggests that to a large extent the transition region reacts independently of the corona and chromosphere to energy deposition, so that these parts of the atmosphere are at least partly decoupled from each other. This agrees with the expectations from models having separate transition-region loops, but contradicts the classical picture of the transition region, as being heated dominantly by energy conduction from the corona. Title: Small-Scale Magnetic Elements in 2-D Nonstationary Magnetogranulation Authors: Gadun, A. S.; Solanki, S. K. Bibcode: 2001ASSL..259..295G Altcode: 2001dysu.conf..295G 2-D simulations of magnetogranulation provide evidence of a close connection between the magnetic field and nonstationary thermal convection. Fragmentation of large granules can lead to the formation of compact nearly vertical magnetic tubes from a weaker horizontal field. Conversely, the dissolution of granules can lead to a merging of magnetic elements and either to field cancellation (leading to the transformation of strong vertical field to its weaker horizontal state) or to the formation of broader and stronger magnetic structures. Title: Diagnostics from Spectral Irradiance Measurements Authors: Unruh, Y. C.; Solanki, S. K. Bibcode: 2001AGUSM..SP32B01U Altcode: Solar irradiance varies in tune with solar activity. The irradiance changes show a strong wavelength dependence, being an order of magnitude larger in the UV than in the visible. The spectral dependence of the irradiance variations can be used to differentiate between mechanisms that may be responsible for the irradiance changes. I review some of the mechanisms that have been invoked in the past to explain the solar irradiance variations and compare their predicted spectral irradiance changes with solar irradiance measurements. The comparisons imply that surface magnetic features alone are sufficient to explain solar irradiance data. Title: Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO Authors: Pauluhn, A.; Solanki, S. K.; Schühle, U.; Wilhelm, K.; Lang, J.; Thompson, W. T.; Rüedi, I.; Hollandt, J.; Huber, M. C. E. Bibcode: 2001SSRv...97...63P Altcode: Since the beginning of the SOHO (Solar and Heliospheric Observatory) mission an intercalibration programme was carried out which included simultaneous observations of the EUV instruments CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) of common targets on the quiet Sun. The observations in the chromospheric line of He i (584 Å) and the two coronal lines of Mg x (609 Å and 624 Å) thus cover the long period of 4 years and provide a data set highly suitable not only for instrumental comparison but also for studies of the quiet Sun's long term variability. Up to the SOHO accident, both instruments show a very good temporal correlation and stability. Even after the loss and recovery of the spacecraft, when the instruments had been exposed to extreme temperature conditions, the performance of the CDS and SUMER instruments is still good, as is the temporal correlation. However, the ratio between the efficiencies of the two instruments, which remained constant with time until the SOHO accident seems to have changed afterwards. In the coronal lines both instruments show an increase of average radiances towards the solar maximum. Title: Empirical models of solar magnetic flux-tubes and their non-magnetic surroundings Authors: Frutiger, C.; Solanki, S. K. Bibcode: 2001A&A...369..646F Altcode: A powerful method for the analysis of the structure of small scale magnetic elements in the solar photosphere is the inversion of Stokes spectra. In previous papers based on such inversions \cite{BellotRubio:etal:1997, BellotRubio:etal:1999} and \cite{Frutiger:etal:1999} have argued in favor of models with rather different dynamic properties. In this paper we return to this debate and compare results returned by inversions based on new multi-component models applied to several Fe i, Fe ii and C i spectral line profiles obtained in active region plage with a Fourier Transform Spectrometer. These inversions differ from earlier ones by the fact that mass conservation is strictly imposed both inside the magnetic elements and on the surrounding external flow field. These flux-tube models are not only able to reproduce the characteristic Stokes V asymmetries and line-shifts observed in active regions plages or network elements, but also the Stokes I line profiles, including line bisectors. It is confirmed that from the quality of the fits alone it is not possible to distinguish between the steady flow proposed by \cite{BellotRubio:etal:1997} and the oscillatory model of \cite{Frutiger:Solanki:1998}. If, however, physical constraints are imposed (e.g. mass conservation or that the flow retains the same direction over height in the flux tube) then the oscillatory model is found to be superior. In addition, the current investigation also provides the first inversion-based model of abnormal granulation. Title: Large Doppler Shifts in X-Ray Plasma: An Explosive Start to Coronal Mass Ejection Authors: Innes, D. E.; Curdt, W.; Schwenn, R.; Solanki, S.; Stenborg, G.; McKenzie, D. E. Bibcode: 2001ApJ...549L.249I Altcode: We report observations, taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer, of spatially resolved high red and blue Doppler shifts (up to 650 km s-1) from X-ray-emitting plasma in the corona above a flare. The high Doppler shifts are seen minutes after a fast, faint optical front is seen racing through the same part of the corona in images taken with the Mirror Coronagraph for Argentina. The association of the large-scale fast optical emission front with soft X-ray emission and high Doppler shifts suggests plasma heating and acceleration in the wake of a shock. Title: Division II: Sun and Heliosphere Authors: Benz, Arnold O.; Bogdan, T.; Foukal, P. V.; Melrose, D. B.; Solanki, S.; Vandas, M.; Webb, D. F. Bibcode: 2001IAUTB..24..110B Altcode: No abstract at ADS Title: The Formation of One-Lobed Stokes V Profiles in an Inhomogeneous Atmosphere Authors: Ploner, S. R. O.; Schussler, M.; Solanki, S. K.; Sheminova, V. A.; Gadun, A. S.; Frutiger, C. Bibcode: 2001ASPC..236..371P Altcode: 2001aspt.conf..371P We assess the diagnostic potential of the observed pathological Stokes V profiles that differ strongly from the customary, nearly antisymmetric two-lobed shape. In particular, we consider the formation of one-lobed Stokes V profiles using the results of an MHD simulation. We find that the majority of one-lobed profiles is produced in regions of weak horizontal field with significant cancellation caused by mixed polarity along the line of sight. A minority of one-lobed profiles originates close to strong magnetic field concentrations with strong gradients of velocity and magnetic field strength. Title: Stellar Irradiance Variations Caused by Magnetic Activity: The Influence of an Inclined Rotation Axis Authors: Knaack, R.; Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2001ASPC..248..227K Altcode: 2001mfah.conf..227K No abstract at ADS Title: A Model for the Decline of Coronal X-ray Emission of Cool Giant Stars Authors: Holzwarth, V.; Schüssler, M.; Solanki, S. K. Bibcode: 2001ASPC..248..259H Altcode: 2001mfah.conf..259H No abstract at ADS Title: Are the Sun's Brightness Variations Really Tamer than Those of Other Comparable Solar-type Stars? (CD-ROM Directory: contribs/unruh1) Authors: Unruh, Y. C.; Knaack, R.; Fligge, M.; Solanki, S. K. Bibcode: 2001ASPC..223..748U Altcode: 2001csss...11..748U No abstract at ADS Title: Consistent Empirical Models of Solar Magnetic Flux Tubes and the Surrounding Convection (CD-ROM Directory: contribs/frutige2) Authors: Frutiger, C.; Solanki, S. K. Bibcode: 2001ASPC..223..632F Altcode: 2001csss...11..632F No abstract at ADS Title: The Quiet-Sun Variability as seen by CDS and by SUMER Authors: Brkovic, A.; Solanki, S. K.; Rüedi, I. Bibcode: 2001IAUS..203..381B Altcode: Blinkers are transient brightenings seen in the extreme ultraviolet. These brightenings are candidates for microflare activity. Here we determine their properties, using co-aligned observations with SUMER and CDS, in the latter case with an open slit. We use CDS to find evidence for blinkers in the quiet Sun, and SUMER to measure the velocities, and line width changes associated with them. Title: Magnetic Splitting of Molecular Lines in Sunspot Authors: Berdyugina, S. V.; Frutiger, C.; Solanki, S. K. Bibcode: 2001IAUS..203..254B Altcode: A study of molecular lines in sunspots is of particular interest because of their high temperature and pressure sensitivity. Many of them are also magnetically sensitive, but this was not yet widely investigated. With high-resolution, high signal-to-noise Fourier spectroscopy in four Stokes parameters now available, the use of molecular lines for studying the structure of sunspots brings real gains. One is the extension of spot models, including magnetic field, up to layers, where atomic lines suffer from NLTE effects but molecules can still be treated in the LTE approximation. Equally important is the fact that since molecular lines are extremely temperature sensitive they can be used to probe the thermal and magnetic structure of the coolest parts of sunspots. We present calculations of splitting and the Stokes parameters for a number of molecular lines in the visible and near-infrared regions. Our first selections are the green system of MgH A2Π-X2σ and the TiO triplet α, γ' and γ systems as the most studied band systems in the sunspot spectrum. The calculations involve different regimes of the molecular Zeeman effect, up to the complete Paschen-Back effect for individual lines. We look for molecular lines which can be used along with atomic lines to derive magnetic, thermal and dynamic properties of the umbra. Title: An Example of Reconnection and Magnetic Flux Recycling near the Solar Surface Authors: Ploner, S. R. O.; Schüssler, M.; Solanki, S. K.; Gadun, A. S. Bibcode: 2001ASPC..236..363P Altcode: 2001aspt.conf..363P No abstract at ADS Title: Statistical Features of the Quiet Sun in EUV Authors: Pauluhn, A.; Solanki, S. K.; Rüedi, I.; Landi, E.; Schühle, U. Bibcode: 2001IAUS..203..416P Altcode: The frequency distribution of the extreme ultraviolet (EUV) intensities in the quiet Sun has in the past usually been modelled using two Gaussians. Here we test this and other distribution functions against observed distributions with exceptional statistics. The data were obtained in a number of spectral lines observed with two extreme ultraviolet spectrometers, CDS (Coronal Diagnostic Spectrometer) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on board the Solar and Heliospheric Observatory (SOHO). We show that the frequency distribution of the radiance is best modelled by a lognormal distribution or by a sum of a lognormal and a Gaussian. The fact that the radiance distribution of the quiet Sun including the network and the intranetwork is better reproduced by a single lognormal distribution function than by two Gaussians suggests that the same heating processes are acting in both types of features. The shape of the distribution function shows a clear temperature dependence. Title: Small-scale Photospheric Structure of the Solar Magnetic Fields outside Sunspots Authors: Solanki, S. K. Bibcode: 2001ASPC..248...45S Altcode: 2001mfah.conf...45S No abstract at ADS Title: The Molecular Zeeman Effect and Solar Magnetic Fields Authors: Berdyugina, S. V.; Frutiger, C.; Solanki, S. K.; Livingston, W. Bibcode: 2001ASPC..236..551B Altcode: 2001aspt.conf..551B No abstract at ADS Title: Solar and Stellar Magnetic Fields: the Molecular Zeeman Effect as a Probe Authors: Berdyugina, S. V.; Solanki, S. K.; Frutiger, C. Bibcode: 2001ASPC..248...99B Altcode: 2001mfah.conf...99B No abstract at ADS Title: Magnetic Fields in Cool Stars (CD-ROM Directory: contribs/moreno) Authors: Moreno Insertis, F.; Saar, S. H.; Solanki, S. K. Bibcode: 2001ASPC..223..435M Altcode: 2001csss...11..435M No abstract at ADS Title: Empirical Models of Stellar Convection (CD-ROM Directory: contribs/frutige1) Authors: Frutiger, C.; Solanki, S. K. Bibcode: 2001ASPC..223..626F Altcode: 2001csss...11..626F No abstract at ADS Title: Buried Flux Tubes in the Coronal Graveyard (CD-ROM Directory: contribs/schussle) Authors: Schüssler, M.; Holzwarth, V.; Solanki, S. K.; Charbonnel, C. Bibcode: 2001ASPC..223.1114S Altcode: 2001csss...11.1114S No abstract at ADS Title: Magnetic Fields Across the Hertzsprung-Russell Diagram Authors: Mathys, G.; Solanki, S. K.; Wickramasinghe, D. T. Bibcode: 2001ASPC..248.....M Altcode: 2001mfah.conf.....M No abstract at ADS Title: Variations of the Solar Spectral Irradiance Authors: Solanki, S. K.; Fligge, M.; Unruh, Y. C. Bibcode: 2001IAUS..203...66S Altcode: Not just the total solar irradiance is known to vary, but also the solar spectrum. Observations and models of solar spectral irradiance variations are presented. Title: Radiance of Solar Spectral Lines observed with CDS and SUMER on SOHO (CD-ROM Directory: contribs/pauluhn) Authors: Pauluhn, A.; Schühle, U.; Solanki, S. K.; Rüedi, I.; Lang, J.; Pike, C. D.; Thompson, W. T.; Huber, M. C. E. Bibcode: 2001ASPC..223..721P Altcode: 2001csss...11..721P No abstract at ADS Title: Noise Reduction in GOLF Spectra Using Wavelets (CD-ROM Directory: contribs/regulo) Authors: Régulo, C.; Roca Cortés, T.; Solanki, S. K.; Fligge, M.; GOLF Team Bibcode: 2001ASPC..223..734R Altcode: 2001csss...11..734R No abstract at ADS Title: High-resolution Solar Polarimetry with Sunrise Authors: Schmidt, W.; Solanki, S. K.; Schüssler, M.; Curdt, W.; Lites, B. W.; Title, A. M.; Martinez Pillet, V. Bibcode: 2001AGM....18S1001S Altcode: Sunrise is a 1m balloon-borne solar telescope. It is equipped with a spectrograph polarimeter which combines vector-polarimetry in the visible with diagnostic spectroscopy in the visible and the UV, down to 200 nm. The instrumentation includes a filter-magnetograph and a medium-band filtergraph. The wavelength bands of the latter include the CH-band (430.6 nm) and a UV continuum at 205 nm. Diffraction limited resolution in the UV will be achieved by employing a phase diversity technique. The main telescope is based on a lightweight silicon-carbide mirror, developed within the Solar Lite program. During the long-duration flight at Antarctica, foreseen for late 2005, Sunrise will continuously observe the sun for a period of about ten days, with constant image quality across the full field of view. In-flight alignment of the telescope optics will be controlled by a wavefront sensor. The main goal of Sunrise is to understand the structure and dynamics of the magnetic field in the atmosphere of the sun. To this end, Sunrise will observe small magnetic flux concentrations with dimensions of less than 70 km with high polarimetric accuracy. At the same time, Sunrise will provide diffraction-limited filtergrams of the photosphere and chromosphere with a resolution down to 35 km at a wavelength of 200 nm. Title: Successful spectral synthesis of Zeeman-split molecular bands in sunspot spectra Authors: Berdyugina, S. V.; Frutiger, C.; Solanki, S. K.; Livingstone, W. Bibcode: 2000A&A...364L.101B Altcode: We present the first spectral synthesis of Zeeman-split Stokes profiles of the MgH A2Pi -X2BLAigma green system and TiO gamma -system. The calculations involve different regimes of the molecular Zeeman effect, up to the complete Paschen-Back effect for individual lines. The synthetic spectra are compared with observations of Stokes I and V in sunspot umbrae. We find that although the Stokes I spectra are reasonably reproduced, some lines are obviously still missing from the employed line lists. The Stokes V spectra turn out to be much cleaner since the missing lines do not appear to be Zeeman-split. We thus provide the first good fit to Zeeman-split molecular lines, including profiles with unconventional Stokes V shapes, determined by the Paschen-Back effect. Based on observations from the Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii Title: Comparison of far-ultraviolet emission lines formed in coronal holes and the quiet Sun Authors: Stucki, K.; Solanki, S. K.; Schühle, U.; Rüedi, I.; Wilhelm, K.; Stenflo, J. O.; Brković, A.; Huber, M. C. E. Bibcode: 2000A&A...363.1145S Altcode: We present an analysis of 26 far-ultraviolet emission lines belonging to 19 atoms and ions observed on both sides of the boundary of polar coronal holes as well as other quiet Sun areas along the limb. The observations were made with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO). We compare line intensities, shifts and widths in coronal holes with the corresponding values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show on average an increasingly stronger blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, with the coolest lines in our sample (formation temperature ~ 104 K) indicating a small relative redshift. With respect to the rest wavelength, however, only lines formed above 5 * 105 K show blueshifts in coronal holes, which is not very different from the quiet Sun. The width of the lines is generally larger (by a few kilometers per second) inside the coronal hole. Intensity measurements clearly show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, and provide evidence for a slightly enhanced emission in polar coronal holes for lines formed below 105 K. This last result is, however, less certain than the rest due to relatively poor statistics. Intensity histograms also exhibit distinct differences between coronal hole and quiet-Sun data. For cooler chromospheric lines, such as Ni II, the coronal holes display a greater spread in intensities than the quiet Sun. Transition-region lines, e.g. O IV, do not reveal such differences, while Ne VIII shows characteristics of a coronal line with lower average intensity and lower intensity spread inside holes. Title: Sunspot Magnetic Fields Authors: Solanki, S. Bibcode: 2000eaa..bookE2298S Altcode: The magnetic field is the central quantity determining the property of a SUNSPOT. It permeates every part of sunspots and by greatly reducing the convective transport of heat from below is also responsible for sunspot darkness. Conversely, sunspots were the first astronomical objects recognized (by G E Hale in 1908) to harbor a magnetic field.... Title: Evolution of the Sun's large-scale magnetic field since the Maunder minimum Authors: Solanki, S. K.; Schüssler, M.; Fligge, M. Bibcode: 2000Natur.408..445S Altcode: The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations. Title: Modelling solar irradiance variations: Comparison with observations, including line-ratio variations Authors: Unruh, Y. C.; Solanki, S. K.; Fligge, M. Bibcode: 2000SSRv...94..145U Altcode: Solar irradiance variations show a strong temporal and spectral dependence. The progression of the Sun through its activity cycle as well as solar rotation are mirrored in the irradiance variations. The spectral dependence is such that the variations are several magnitudes larger in the EUV than in the visible or infrared. We present a simple 3-component model that is based on the assumption that changes in the solar flux are exclusively due to changes in spot and facular coverage. We compare our model to observations of the spectral solar irradiance variations. Despite its simplicity, we find that the agreement between our model and the observations is surprisingly good. We also explore the reliability and the limitations of our approach by comparing observations of the solar facular contrast and of the changes in spectral line depths with our calculations. Title: Modelling Short-Term Spectral Irradiance Variations Authors: Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2000SSRv...94..139F Altcode: On time-scales of the solar rotation most of the solar irradiance variations are caused by the changing distribution of solar surface magnetic features. We model these short-term irradiance variations using calculations of sunspot and facular contrasts as a function of wavelength and limb angle on the Sun. The position of active regions on the solar disc is derived from the MDI magnetograms. The reconstructed irradiance variations are compared with total and spectral irradiance measurements obtained by the VIRGO experiment on SOHO. Title: Sunspot Models Authors: Solanki, S. Bibcode: 2000eaa..bookE2300S Altcode: Models of SUNSPOTS aim either to reproduce observed properties of sunspots, or to understand the physical processes occurring in them. Sunspot models are of very diverse types, ranging from empirically derived models of their thermal stratification to MHD models of their magnetic configuration and evolution. The final aim of all modelling is to obtain a consistent and detailed description and und... Title: Reconstruction of Past Solar Irradiance Authors: Solanki, S. K.; Fligge, M. Bibcode: 2000SSRv...94..127S Altcode: Accurate measurements of solar irradiance started in 1978, but a much longer time series is needed in order to uncover a possible influence on the Earth's climate. In order to reconstruct the irradiance prior to 1978 we require both an understanding of the underlying causes of solar irradiance variability as well as data describing the state of the Sun (in particular its magnetic field) at the relevant epochs. Evidence is accumulating that on the time-scale of the solar cycle or less, variations in solar irradiance are produced mainly by changes in the amount and distribution of magnetic flux on the solar surface. The main solar features contributing to a darkening of the Sun are sunspots, while active-region faculae and the network lead to a brightening. There is also increasing evidence for secular changes of the solar magnetic field and the associated of solar brightness variability. In part the behavior of sun-like stars is used as a guide of such secular changes. Under the assumption that solar irradiance variations are due to solar surface magnetism on all relevant time scales it is possible to reconstruct the irradiance with some reliability from today to around 1874, and with lower accuracy back to the Maunder minimum. One major problem is the decreasing amount and accuracy of the relevant data with age. In this review the various reconstructions of past solar irradiance are presented and the assumptions underlying them are scrutinized. Title: Solar Photospheric Magnetic Flux Tubes: Observations Authors: Solanki, S. Bibcode: 2000eaa..bookE2013S Altcode: The magnetic field on the solar surface (SOLAR MAGNETIC FIELD) is highly filamentary, with a considerable fraction being in the form of flux tubes (roughly speaking bundles of concentrated field lines). Small-scale flux tubes have diameters below approximately 400 km. They appear bright and are the basic magnetic features underlying the magnetic network and active region plages (SOLAR CHROMOSPHER... Title: Statistics of quiet Sun extreme ultraviolet intensities Authors: Pauluhn, A.; Solanki, S. K.; Rüedi, I.; Landi, E.; Schühle, U. Bibcode: 2000A&A...362..737P Altcode: The frequency distribution of the extreme ultraviolet (EUV) emission line intensities in the quiet Sun has in the past often been modelled using two Gaussians. This gives adequate fits to observed distributions of average statistical significance. In this paper we test this and other distribution functions against observed distributions with exceptional statistical significance. The data were obtained in a number of spectral lines observed with two extreme ultraviolet spectrometers on board the Solar and Heliospheric Observatory (SOHO). In this way, the influence of spatial resolution and other instrument-specific parameters can be identified. The observations span a period of more than two years and provide a very large data set of radiance measurements of the quiet Sun at or near solar disk centre. We show that the frequency distribution of the radiance is best modelled by a lognormal distribution. The fact that the radiance distribution of the quiet Sun including the network and the intranetwork is better reproduced by a single lognormal distribution function than by two Gaussians suggests that the same heating processes are acting in both types of features. The parameters of the lognormal fit show a clear temperature dependence, with the transition region lines exhibiting the largest skewness of the distribution and the chromospheric intensity distributions being the most symmetric. Title: On the relationship between shift and intensity of ultraviolet lines in coronal holes and the quiet Sun Authors: Stucki, K.; Solanki, S. K.; Schühle, U.; Rüedi, I. Bibcode: 2000A&A...362L..49S Altcode: We study the relationship between wavelength shifts and intensities of chromospheric, transition-region and coronal ultraviolet emission lines in polar coronal holes and in the normal quiet Sun using SUMER data. Within coronal holes almost all the lines showing the network and formed above 30 000 K show a correlation between blueshifts and brightness. This extends and supports the conclusion reached by Hassler et al. (1999) that the fast solar wind emanates from the network. In the normal quiet Sun, however, we find that only lines formed above 2-3 * 105 K show such a trend, the cooler lines being more redshifted in the network. This suggests that either there is a fundamental difference in the initial acceleration of the solar wind in coronal holes and the normal quiet Sun, or that the wavelength-shift versus brightness relationship in the quiet Sun stems from other processes or structures (loops) than in coronal holes (open field lines). Title: Size-dependent properties of simulated 2-D solar granulation Authors: Gadun, A. S.; Hanslmeier, A.; Pikalov, K. N.; Ploner, S. R. O.; Puschmann, K. G.; Solanki, S. K. Bibcode: 2000A&AS..146..267G Altcode: Two time-dependent sets of two-dimensional hydrodynamic models of solar granulation have been analyzed to obtain dependence of simulated thermal convection on the horizontal size of the convection cells. The two sets of models treat thermal convection either as fully non-stationary, multiscale convection (granular convection is a surface phenomenon) or as quasi-steady-state convection cells (they treat granular convection as a collection of deep-formed cells). The following results were obtained: 1) quasi-steady convection cells can be divided into 3 groups according to their properties and evolution, namely small-scale (up to L ~ 900 km), intermediate-scale (1000-1500 km) and large-scale (larger 1500 km) convection cells. For the first group thermal damping due to radiative exchange of energy, mostly in the horizontal direction, is very important. Large-scale cells build up a pressure excess, which can lead to their total fragmentation. Similar processes also acts on the fully non-stationary convection. 2) The largest horizontal size of convection cells for which steady-state solutions can be obtained is about 1500 km. This corresponds to granules, i.e. the bright parts of the convection cells, with a diameter of about 1000 km. 3) In addition to the zone of high convective instability associated with the partial ionization of hydrogen, we identify another layer harboring important dynamic processes in steady-state models. Just below the hydrogen-ionization layer pressure fluctuations and the acoustic flux are reduced. Steady-state models with reflecting lateral boundaries even exhibit an inversion of pressure fluctuations there. 4) From observational point of view the surface convection differs from steady-state deep treatment of thermal convection in the dependence of vertical granular velocities on their sizes for small-scale inhomogeneous. However, they cannot be distinguished by the dependence of temperature or emergent intensity of brightness structures. 5) Both kinds of models demonstrate the inversion of density in subphotospheric layers. It is more pronounced in small-scale cells and inside hot upflows. 6) The brightness of simulated granules linearly increases with their size for small granules and is approximately constant or even decreases slightly for larger granules. For intergranular lanes the simulations predict a decrease of their brightness with increasing size. It falls very rapidly for narrow lanes and remains unchanged for broader lanes. 7) A quantitative comparison of the brightness properties of simulated granulation with real observations shows that the strong size-dependence of the properties of the smallest simulated granules is not accessible to current observations due to their limited spatial resolution. The observed size dependences result rather from spatial smoothing and the granule-finding algorithm. We do not exclude, however, an influence of the limitations of the 2-D treatment of thermal convection on the present results. Title: The solar spectral irradiance since 1700 Authors: Fligge, M.; Solanki, S. K. Bibcode: 2000GeoRL..27.2157F Altcode: The change in the irradiance spectrum of the Sun from 1700 to the last solar minimum is determined and compared to the change in the spectrum between activity minimum and maximum. For this purpose we have used detailed model flux spectra of solar magnetic features. Also, time-series of the solar spectral irradiance since 1700 in different wavelength bands are reconstructed. We expect that these reconstructions are more accurate than previously published ones, although they suffer (like all reconstructions of solar irradiance on such time-scales) from uncertainties in our knowledge of the evolution of the solar network with time. Title: Spin and orbital angular momentum exchange in binary star systems. II. Ascending the giant branch: a new path to FK Comae stars Authors: Keppens, R.; Solanki, S. K.; Charbonnel, C. Bibcode: 2000A&A...359..552K Altcode: Using the model by Keppens (1997), we investigate the angular momentum (AM) evolution in asymmetric binary star systems from Zero-Age Main Sequence times until at least one component has ascended the giant branch. We concentrate on stars ranging in mass from 0.9 Msun - 1.7 Msun, in almost synchronous, short period systems (P_orb<9 days). We address synchronization and circularization by tidal interaction, allowing for structural evolution and stellar winds. A Weber-Davis prescription is used to quantify the wind influence, thereby accounting for changes in its acceleration mechanism from the interplay of the evolving thermal-magneto-centrifugal effects. We identify a scenario for fast in-spiraling components with d ln P_orb/dt =~ -{cal O}(10-8) which is primarily driven by fast structural evolution as the heaviest component ascends the giant branch. This leads to the formation of contact systems, which ultimately coalesce and form FK Comae-like objects on relatively short timescales due to the continuing expansion of the primary. The obtained mass loss rates and orbital period variations d ln P_orb/dt are confronted with their observed ranges. The predicted mass loss rates agree with the solar value on the main sequence and with the Reimers relation in the giant phase. Observations of period evolution in close, active binaries suggest, however, that other influences than those considered here must play an important role. Finally, we point out how the mass asymmetry of the binary system can be a crucial ingredient in the angular momentum evolution: while the primary dictates the spin-orbital AM exchange in the system, the slowly evolving lighter component can develop an efficient magneto-centrifugally driven wind and thereby drain the AM from the system. Title: Cyclic Evolution of Sunspots: Gleaning New Results from Old Data Authors: Solanki, S. K.; Fligge, M.; Pulkkinen, P.; Hoyng, P. Bibcode: 2000JApA...21..163S Altcode: No abstract at ADS Title: Properties of the solar granulation obtained from the inversion of low spatial resolution spectra Authors: Frutiger, C.; Solanki, S. K.; Fligge, M.; Bruls, J. H. M. J. Bibcode: 2000A&A...358.1109F Altcode: The spectra of cool stars are rich in information on elemental abundances, convection and non-thermal heating. Extracting this information is by no means straightforward, however. Here we demonstrate that an inversion technique may not only provide the stratification of the classical parameters describing a model atmosphere, but can also determine the properties of convection at the stellar surface. The inversion technique is applied to spectra of photospheric lines, one recorded at the quiet solar disk center, the other integrated over the whole disk. We find that a model based on a single plane-parallel atmosphere gives unsatisfactory fits to the spectral lines and suffers from considerable uncertainties in the derived temperature stratification. Also, the elemental abundances returned by the inversion are not particularly reliable. These problems are greatly reduced if two atmospheric components, corresponding to granular up- and downflows are allowed for. The best results are obtained if the line profiles and bisectors of a neutral and ionized species are fit and the results are constrained using a simple mass conservation scheme. We find that inversions based on two- and three-component models of disk-integrated spectra give similar results to inversions of disk-center observations, although with somewhat lower accuracy. This similarity is promising for future applications of line profile inversions to the study of late-type stars and in particular their convection. Title: Properties of Flux Tubes and the Relation with Solar Irradiance Variability Authors: Fligge, M.; Solanki, S. K. Bibcode: 2000JApA...21..275F Altcode: No abstract at ADS Title: Is solar mesogranulation a surface phenomenon? Authors: Ploner, S. R. O.; Solanki, S. K.; Gadun, A. S. Bibcode: 2000A&A...356.1050P Altcode: Convection is the main form of energy transport in the subsurface layers of the sun and other cool stars. The imprint of cellular convection can be directly observed on the solar surface, with a hierarchy of four size scales. The smallest observed convection cells, called granules, have typical horizontal sizes of 1,000-2,000 km and have been successfully reproduced by numerical simulations \citep{spruit:97,stein:nordlund:98}. Cells at three larger scales are also detected \citep{leighton:etal:62,november:etal:81,beck:etal:98}, but these have so far not been amenable to numerical modelling, so that their formation scenarios remain untested. Here we present a numerical simulation which resolves both the granular and the next larger, mesogranular, scale. The mesogranules have horizontal extents of 5,000-10,000 km. Our 2D simulation reproduces key properties of both granules and mesogranules. In addition, our simulation demonstrates that the observed mesogranulation is driven close to the solar surface and therefore rules out the text-book explanation of mesogranulation as cellular convection driven by superadiabaticity in the deeper layer where neutral helium ionizes. By proxy, this result also casts doubt on the traditional explanation of supergranulation, even larger convection cells with diameters of 20,000-30,000 km, as being driven by the yet deeper second ionization of helium. Title: Distribution of magnetic flux on the solar surface and low-degree p-modes Authors: Moreno-Insertis, F.; Solanki, S. K. Bibcode: 2000MNRAS.313..411M Altcode: The frequencies of solar p-modes are known to change over the solar cycle. There is also recent evidence that the relation between frequency shift of low-degree modes and magnetic flux or other activity indicators differs between the rising and falling phases of the solar cycle, leading to a hysteresis in such diagrams. We consider the influence of the changing large-scale surface distribution of the magnetic flux on low-degree (l<=3) p-mode frequencies. To that end, we use time-dependent models of the magnetic flux distribution and study the ensuing frequency shifts of modes with different order and degree as a function of time. The resulting curves are periodic functions (in simple cases just sine curves) shifted in time by different amounts for the different modes. We show how this may easily lead to hysteresis cycles comparable to those observed. Our models suggest that high-latitude fields are necessary to produce a significant difference in hysteresis between odd- and even-degree modes. Only magnetic field distributions within a small parameter range are consistent with the observations by Jiménez-Reyes et al. Observations of p-mode frequency shifts are therefore capable of providing an additional diagnostic of the magnetic field near the solar poles. The magnetic distribution that is consistent with the p-mode observations also appears reasonable compared with direct measurements of the magnetic field. Title: Reconstruction of Past Solar Irradiance Authors: Solanki, S. K.; Fligge, M. Bibcode: 2000svc..book..127S Altcode: No abstract at ADS Title: Long-Term Changes in Solar Irradiance Authors: Solanki, S.; Fligge, M. Bibcode: 2000ESASP.463...51S Altcode: 2000sctc.proc...51S No abstract at ADS Title: Modelling irradiance variations from the surface distribution of the solar magnetic field Authors: Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2000A&A...353..380F Altcode: An important question in solar physics is to what extent solar surface magnetism affects the solar irradiance. Previous attempts to answer this question have employed proxies of the magnetic field to reconstruct the irradiance and compare it with observations. Here we present the first model calculations of solar irradiance variations based on variations of the surface distribution of the solar magnetic field. The irradiance reconstruction makes use of sunspot and facular contrasts calculated as a function of wavelength and limb angle on the Sun. The position and size of magnetic features on the solar disk are extracted from full-disk magnetograms obtained by the Michelson Doppler Interferometer (MDI) onboard the SOHO spacecraft. The reconstructed spectral irradiance variations are compared with total and spectral contrast measurements obtained by the VIRGO instrument onboard SOHO. Our reconstructions are able to reproduce variations on the time-scale of the solar rotation with much greater accuracy than previous models based on disk-integrated magnetic proxies. Title: The influence of an inclined rotation axis on solar irradiance variations Authors: Knaack, R.; Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2000ssls.work...45K Altcode: The irradiance variations of the Sun over the solar cycle are relatively small compared with sun-like stars (Radick et al., 1998). It has been proposed that a random distribution of stellar rotation axes relative to the ecliptic plane could explain their larger variability (Schatten, 1993). We examine the influence on total irradiance variations of the inclination of the solar rotation axis relative to an observer. A three-component model is used to calculate the relative flux variations of a given distribution of active regions on the surface of the Sun as a function of the inclination. One component is the quiet Sun, another is a sunspot component and the third describes the faculae. We find that the inclination effect increases the total irradiance variations of the Sun maximally by 40%. The most probable value is approximately 5%. This is much less than former studies (Schatten, 1993, Radick et al., 1998) suggested. Title: Commission 12: Solar Radiation and Structure (Radiation et Structure Solaires) Authors: Foukal, Peter; Solanki, Sami; Mariska, J.; Baliunas, S.; Dravins, D.; Duvall, T.; Fang, C.; Gaizauskas, V.; Heinzel, P.; Kononovich, E.; Koutchmy, S.; Melrose, D.; Stix, M.; Suematsu, Y.; Deubner, F. Bibcode: 2000IAUTA..24...73F Altcode: No abstract at ADS Title: Solar Surface Magnetism and the Increase of Solar Irradiance between Activity Minimum and Maximum Authors: Fligge, M.; Solanki, Sami K.; Meunier, Nadege; Unruh, Yvonne C. Bibcode: 2000ESASP.463..117F Altcode: 2000sctc.proc..117F No abstract at ADS Title: On the Contrast of Faculae and Small Magnetic Features Authors: Ortiz, A.; Solanki, S. K.; Fligge, M.; Domingo, V.; Sanahuja, B. Bibcode: 2000ESASP.463..399O Altcode: 2000sctc.proc..399O Sunspots, faculae and the magnetic network contribute to solar irradiance variations. The contribution due to faculae and the network is important for understanding solar irradiance variations, but suffers from considerable uncertainty. We focus our study on the faculae and the network which produce an increase in the irradiance. Data from the Michelson Doppler Interferometer (MDI) are employed. Starting from the surface distribution of the solar magnetic field we build a mask to detect bright features and study their contrast dependence on limb angle and magnetic field. By sorting the magnetic field strength into different bins we can distinguish between different associated bright features. We find that the contrast of active region faculae and the network exhibits different centre to limb variations, implying that they need to be treated separately when reconstructing variations of the total solar irradiance. Title: Modelling Solar Irradiance Variations: Comparison with Observations, Including Line-Ratio Variations Authors: Unruh, Y. C.; Solanki, S. K.; Fligge, M. Bibcode: 2000svc..book..145U Altcode: No abstract at ADS Title: Modelling Short-Term Spectral Irradiance Variations Authors: Fligge, M.; Solanki, S. K.; Unruh, Y. C. Bibcode: 2000svc..book..139F Altcode: No abstract at ADS Title: EUV brightness variations in the quiet Sun Authors: Brković, A.; Rüedi, I.; Solanki, S. K.; Fludra, A.; Harrison, R. A.; Huber, M. C. E.; Stenflo, J. O.; Stucki, K. Bibcode: 2000A&A...353.1083B Altcode: The Coronal Diagnostic Spectrometer (CDS) onboard the SOHO satellite has been used to obtain movies of quiet Sun regions at disc centre. These movies were used to study brightness variations of solar features at three different temperatures sampled simultaneously in the chromospheric He I 584.3 Ä (2 * 104 K), the transition region O V 629.7 Ä (2.5 * 105 K) and coronal Mg IX 368.1 Ä (106 K) lines. In all parts of the quiet Sun, from darkest intranetwork to brightest network, we find significant variability in the He I and O V line, while the variability in the Mg IX line is more marginal. The relative variability, defined by rms of intensity normalised to the local intensity, is independent of brightness and strongest in the transition region line. Thus the relative variability is the same in the network and the intranetwork. More than half of the points on the solar surface show a relative variability, determined over a period of 4 hours, greater than 15.5% for the O V line, but only 5% of the points exhibit a variability above 25%. Most of the variability appears to take place on time-scales between 5 and 80 minutes for the He I and O V lines. Clear signs of ``high variability'' events are found. For these events the variability as a function of time seen in the different lines shows a good correlation. The correlation is higher for more variable events. These events coincide with the (time averaged) brightest points on the solar surface, i.e. they occur in the network. The spatial positions of the most variable points are identical in all the lines. Title: Division II: The Sun and Heliosphere: (Le Soleil et Heliosphere) Authors: Foukal, Peter; Ai, Guoxiang; Benz, Arnold; Engvold, Oddbjorn; Solanki, Sami; Vandas, Marek; Verheest, Frank Bibcode: 2000IAUTA..24...65F Altcode: No abstract at ADS Title: The evolution of solar granules deduced from 2-D simulations Authors: Ploner, S. R. O.; Solanki, S. K.; Gadun, A. S. Bibcode: 1999A&A...352..679P Altcode: The evolution of solar granules is investigated on the basis of two dimensional numerical solutions of the hydrodynamic equations describing a compressible, radiatively coupled and gravitationally stratified medium representative of the solar surface layers. The simulation covers 17 Mm on the solar surface and was run for over 5 h of solar time, hence allowing the evolution of over 400 granules to be followed. A statistical investigation of the temporal evolution of granules therefore becomes feasible. Two types of granules can be distinguished by their means of death: fragmenting and dissolving granules. Properties and average evolutionary histories of these two types of granules are considered. It is found that fragmenting granules are in general large at birth and expand further with time. It is confirmed that fragmentation into two (or more) parts is produced by buoyancy braking, which in turn is initiated by the stronger horizontal flows in larger granules. This last property, finally, is due to mass conservation. The expansion, however, is due to a pressure excess relative to neighbouring granules. The pressure excess is particularly marked if the neighbours are dissolving granules. In contrast, dissolving granules are born small and shrink before finally disappearing. The shrinkage is caused by their neighbours which generally posses excess gas pressure and larger horizontal flows. In summary, according our findings the fate of a granule is decided by its properties at birth and the company it keeps. Evidence is presented suggesting that the evolution of both types of granules is driven by events near the solar surface. Title: Is the FIP effect present inside solar photospheric magnetic flux tubes? Authors: Sheminova, V. A.; Solanki, S. K. Bibcode: 1999A&A...351..701S Altcode: The first determination of the elemental composition in the photospheric layers of solar magnetic flux tubes is described. Stokes I and V profiles of 13 elements observed in solar active region plage and in the network are analysed. The abundances are obtained for elements with high (C, O) and low (Al, Ca, Cr, Na, Ni, Sc, Si, Ti, Y, Zn) first ionization potential (FIP) in order to investigate to what extent the abundance anomalies observed in the upper solar atmosphere (FIP-effect) are already present in the photospheric layers of flux tubes, which are the source of much of the gas in the upper atmosphere. Various sources of error are considered and the uncertainties introduced by them are estimated. There are hints of a weak FIP-effect in the flux tubes, corresponding to an overabundance of a factor of 1.1-1.2 of the low-FIP elements relative to high-FIP elements, as compared to the quiet photosphere. However, our data set a firm upper limit of 1.3-1.6 on this factor, which is well below the enhancement seen in many parts of the upper solar atmosphere. Title: Relationship between Line Shift and Intensity Inside Coronal Holes Authors: Stucki, K.; Solanki, S. K.; Rüedi, I.; Schüehle, U. Bibcode: 1999ESASP.446..633S Altcode: 1999soho....8..633S We analyse SUMER spectra of a group of lines belonging to chromospheric, transition region and coronal ions obtained on both sides of the boundary of polar coronal holes as well as at other locations along the limb. We study the relationship between line shifts and intensities in coronal holes and compare to values obtained in the quiet Sun. We find that within coronal holes, a trend can be detected in some transition region lines, like O V and N V, in the sense that the lines are more strongly blueshifted in brighter regions. This is in agreement with the conclusion reached by Hassler et al. (1999, Science 283, 810-813) that the fast solar wind emanates from the network. Furthermore, a correlation of line shifts of ions formed at higher temperatures (Ne VIII, Fe XII) with intensities of lines showing network structures (Si I, N III) is presented. Title: Hanle effect observations with the CA BT I 4227 Å line Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1999A&A...350.1060B Altcode: The Hanle effect in the Ca i 4227 Ä line has been explored through the analysis of a large number of Stokes profile recordings obtained on the quiet Sun with the beam-splitter polarimeter system at IRSOL (Istituto Ricerche Solari Locarno). In contrast to previous Hanle observations with this line, which were limited to the Stokes I and Q parameters, we are now in a position to study the combined effects of Hanle depolarization (via Stokes Q) and rotation of the plane of linear polarization (via Stokes U) with the same methods that we recently applied to the Sr ii 4078 Ä line. The Hanle histograms for the distributions of the depolarization and rotation parameters are very similar for the two lines and show that there must be mixed contributions to the Hanle signals from spatially unresolved magnetic fields with random orientations (which do not contribute to Stokes U) and partially resolved magnetic fields with a net orientation of the field vectors. Field strengths in the range 5-10 G are preferred. We also determine the ``Hanle efficiency profile'', which shows how the Hanle effect is confined to the Doppler core but vanishes in the line wings. It is wider than the corresponding profile for the Sr line, as expected from the difference in atomic weight and wavelength between the two lines. The Q/I profiles of the Ca i 4227 Ä line have minima around the Doppler core which turn negative (polarization perpendicular to the limb) for limb distances mu =cos theta >~ 0.2, a likely signature of partial redistribution effects. Title: Loop Models from SOHO Observations Authors: Landini, M.; Brkovic, A.; Landi, E.; Rüedi, I.; Solanki, S. Bibcode: 1999ESASP.446..423L Altcode: 1999soho....8..423L SOHO CDS, GIS and NIS, observations are used to evaluate the electron temperature, density and pressure of active region loops. The measurements are used to constrain empirical models of loops, which are compared with an improved version of theoretical models from Landini and Monsignori Fossi 1975. Energy balance is investigated to evaluate temperature, pressure and heating release along the loop. Title: Magnetic Field Structuring Authors: Solanki, S. K. Bibcode: 1999ESASP.446...25S Altcode: 1999soho....8...25S No abstract at ADS Title: Two-dimensional simulation of solar granulation: description of technique and comparison with observations Authors: Gadun, A. S.; Solanki, S. K.; Johannesson, A. Bibcode: 1999A&A...350.1018G Altcode: The physical properties of the solar granulation are analyzed on the basis of 2-D fully compressible, radiation-hydrodynamic simulations and the synthetic spectra they produce. The basic physical and numerical treatment of the problem as well as tests of this treatment are described. The simulations are compared with spatially averaged spectral observations made near disk centre and high resolution spectra recorded near the solar limb. The present simulations reproduce a significant number of observed features, both at the centre of the solar disc and near the solar limb. Reproduced observables include the magnitude of continuum and line-core intensity fluctuations, line bisectors and correlations between different line parameters. Spatially averaged line shifts near disc centre, however, are not so well reproduced, as are individual correlations between line parameters near the solar limb. Possible causes of these discrepancies are discussed. The present models predict the existence of two photospheric layers at which the temperature fluctuations change sign. We point out a diagnostic of the hitherto undetected upper sign reversal based on high spatial resolution spectral observations of a sample of lines formed over a wide range of heights in the photosphere. Title: Characteristics Of Blinkers Observed With CDS Authors: Brkovic, A.; Ruedi, I.; Solanki, S. K. Bibcode: 1999ESASP.446..191B Altcode: 1999soho....8..191B Blinkers are transient brightenings seen in the extreme ultraviolet. They probably can not heat the corona, but are candidates for microflare activity. Here we determine their properties on the basis of a larger sample than previously studied. We used the Normal Incidence Spectrometer of the Coronal Diagnostic Spectrometer (CDS) onboard the SOHO satellite in its movie mode, i.e. 90"x240" slit, to find evidence for blinkers in the quiet Sun. The He I 584.3 A (20'000 K), the O V 629.7 (250'000 K) and the Mg IX 368.1 (1'000'000 K) lines are recorded simultaneously at a cadence of 31 seconds for a duration of 4 hours each on December 3 1996. Harrison (1997) defined the blinker as a phenomenon showing an enhancement of a factor 2-3 in the flux of transition region lines at network junctions. The criterion we used was similar to his. We applied the threshold to the O V line, and identified 74 distinct blinkers. The average properties of these blinkers and the scatter around these mean values were then determined. These properties include the duration of the blinker, the energy content of the event, ratio of maximum to minimum brightness and the sizes of these brightenings. We find that the overall brightening is often composed of a number of shorter events. Finally, we checked if these blinkers were also present in the images obtained in the two other lines, of He I and Mg IX. In the He I line we found 65 events and in the Mg IX line 16 events. On average the brightening was smaller in these lines than in O V. The work is still in progress. We plan to investigate how the locations of these events are related to the distribution of the magnetic field and if they are also related to other phenomena. References: Harrison, R.A.: 1997, Solar Physics 162, 467. Title: Oscillations of Sunspot Magnetic Fields: MDI Observations of a Symmetrical Sunspot Authors: Rüedi, I.; Solanki, S. K. Bibcode: 1999ASPC..184..131R Altcode: We report on sunspot magnetic field oscillations observed using the MDI instrument on the SOHO spacecraft. Clear oscillations are seen in both the 3- and 5-min bands, but appear to be intermittent. Title: A reconstruction of total solar irradiance since 1700 Authors: Solanki, S. K.; Fligge, M. Bibcode: 1999GeoRL..26.2465S Altcode: The irradiance of the Sun is reconstructed from 1700 to the present, whereby the contributions of active regions and the quiet Sun are modelled separately. A method is proposed which allows the contribution of active-region faculae and sunspots to irradiance changes to be isolated even when only a single proxy of solar activity, such as sunspot relative number, Rz, is available. The resulting reconstruction explicitly takes the non-linear relationship between Rz and irradiance variations into account. Nevertheless, due to the decreasing accuracy of the solar proxy data the accuracy of the reconstruction decreases at earlier epochs. The main uncertainty, however, lies in the reconstruction of the quiet-sun irradiance variations. Title: Infrared lines as probes of solar magnetic features. XV. Evershed flow in cool, weak penumbral fields Authors: Rüedi, I.; Solanki, S. K.; Keller, C. U. Bibcode: 1999A&A...348L..37R Altcode: Observations of Ti I lines at 2.2 mu m show that the Evershed flow takes place in cool, almost horizontal channels with a low magnetic field strength (~ 500-900 G) that does not appear to change significantly across the penumbra. This property might allow an outward directed siphon flow to exist along such cool flux tubes. Title: Expansion of solar magnetic flux tubes large and small Authors: Solanki, S. K.; Finsterle, W.; Rüedi, I.; Livingston, W. Bibcode: 1999A&A...347L..27S Altcode: In the solar photosphere the magnetic field of magnetic elements and sunspots is known to expand with height. In the case of sunspots this expansion is known to be very rapid, with the field forming an almost horizontal canopy. In this contribution we present new results on the superpenumbral canopy of sunspots based on fits to Stokes I and V profiles of infrared spectral lines. The new models take pressure balance across the boundary of the canopy field into account, which leads to significantly lower canopy base heights than previously determined from similar data. Due to the lower canopy base height, the density above the canopy base is larger, so that estimates of the mass transported by the Evershed effect in the canopy need to be revised upwards: approximately 15-50% of the mass flowing through the penumbra travels beyond the sunspot boundary above the canopy base. A comparison with small flux tubes leads to the surprising result that although the two types of features have magnetic fluxes that differ by 5-6 orders of magnitude, their relative rate of expansion with height is very similar, suggesting that at least in this respect sunspots can be described by the thin-tube approximation. The remaining small differences between the relative expansion of the two types of flux tubes is qualitatively compatible with the presence of magnetic flux that returns into the solar interior at the spot boundary, as has been proposed by Westendorp Plaza et al. (1997). Title: Determination of solar cycle length variations using the continuous wavelet transform Authors: Fligge, M.; Solanki, S. K.; Beer, J. Bibcode: 1999A&A...346..313F Altcode: The length of the sunspot cycle determined by Friis-Christensen & Lassen (1991) correlates well with indicators of terrestrial climate, but has been criticized as being subjective. In the present paper we present a more objective and general cycle-length determination. Objectivity is achieved by using the continuous wavelet transform based on Morlet wavelets and carrying out a careful error analysis. Greater generality comes from the application of this technique to different records of solar activity, e.g. sunspot number, sunspot area, plage area or (10) Be records. The use of different indicators allows us to track cycle length variations back to the 15th century. All activity indicators give cycle length records which agree with each other within the error bars, whereby the signal due to the solar cycle is weaker within (10) Be than in the other indicators. In addition, all records exhibit cycle length variations which are, within the error bars, in accordance with the record originally proposed by Friis-Christensen & Lassen (1991). In the 16th century, however, the (10) Be record suggests a much longer cycle than the auroral record used by Friis-Christensen & Lassen. Also, the presence of a distinct 11-year cycle in the (10) Be record during the Maunder Minimum is confirmed. By combining the results from all the indicators a composite of the solar cycle length is constructed, which we expect to be more reliable than the length derived from individual records. Title: The spectral dependence of facular contrast and solar irradiance variations Authors: Unruh, Y. C.; Solanki, S. K.; Fligge, M. Bibcode: 1999A&A...345..635U Altcode: We present model calculations of facular and sunspot contrasts as a function of wavelength and limb angle on the Sun. These are the first such calculations; they assume LTE and are based on opacity distribution functions (ODFs). The calculated facular contrasts as a function of limb angle fit into the general picture of contrast measurements, and the behaviour of the contrast with wavelength at a given limb angle is in excellent agreement with the measurements. The calculated intensity spectra are used to construct the solar flux spectrum for different levels of solar activity. It is assumed that the irradiance or flux variations are due to changes in the sunspot and facular filling factors. The model atmosphere used to calculate the facular intensities has been tuned so that the calculated irradiance variations match the observed total and spectral irradiance variations during the last solar cycles. The model calculations have also been used to estimate the relative importance of continuum and spectral-line variations in producing irradiance variations. The results suggest that the continuum variations only contribute negligibly to the total irradiance variations on solar-cycle time scales. Title: Influence of torsional waves in solar magnetic flux tubes on spectral lines Authors: Ploner, S. R. O.; Solanki, S. K. Bibcode: 1999A&A...345..986P Altcode: The influence of torsional waves propagating along a thin, vertical, photospheric flux tube on Zeeman-split polarized line profiles (Stokes profiles) is investigated using a simple MHD model. In the presence of such a wave spatially resolved Stokes profiles are found to oscillate strongly in wavelength, amplitude and blue-red asymmetry. Qualitatively, torsional waves induce similar changes into the line profiles as kink waves (Ploner & Solanki 1997). The magnitude of the line parameter variation depends strongly on the observed location with respect to the flux-tube axis. The spatially averaged Stokes V and Q profiles are found to follow the torsional wave with double the wave frequency, some parameters of Stokes U fluctuate directly at the wave frequency, however. The other main feature of the spatially averaged profiles is their comparatively small reaction to the wave. The reason for the latter is that most polarized light is produced near the centre of the flux tube where, however, the torsional wave produces only weak perturbations. Temporally and spatially averaged Stokes profiles are found to be only negligibly shifted, but strongly broadened. The sign of the small remaining asymmetry is opposite in Stokes Q to that in V and U. The amplitude of the wave and the location of the flux tube on the solar disk have a strong influence on the magnitude of the perturbation of the Stokes profiles. Title: Formation of small-scale magnetic elements: surface mechanism Authors: Gadun, A. S.; Sheminova, V. A.; Solanki, S. K. Bibcode: 1999KFNT...15..387G Altcode: 2012arXiv1210.3499G; 1999KFNT...15e.387G The first results of a two-dimensional MHD simulation of solar magnetogranulation are given. The medium was treated as compressible, gravitationally stratified, radiatively coupled, partially ionized, and turbulent. The evolution of magnetogranulation was simulated in course of 2 hours of hydrodynamic (solar) time. A surface (magnetic plume-like) mechanism which forms thin magnetic elements was found to exist. This sort of field formation occurs due to fragmentation of large-scale granules. Active role of such mechanism shows that the magnetogranulation not only concentrates and intensifies the global magnetic flux at the boundaries of convective cells but also forms nearly vertical compact magnetic tubes by involving the weak horizontal field of the photosphere, which in general may be of local nature. Title: Limits on gravity-induced depolarization of light from the white dwarf Grw +70°8247 Authors: Solanki, Sami K.; Haugan, Mark P.; Mann, R. B. Bibcode: 1999PhRvD..59d7101S Altcode: We use measurements of the polarization of light from a magnetic white dwarf to impose sharp constraints on the gravity-induced birefringence of space predicted by a broad class of nonmetric gravitation theories. Since gravity-induced birefringence violates the Einstein equivalence principle, our measurements test this foundation of general relativity and other metric gravitation theories in a new setting. Title: Can Chromospheric Activity mimic a Polar Spot? Authors: Bruls, J. H. M. J.; Schüssler, M.; Solanki, S. K. Bibcode: 1999ASPC..158..182B Altcode: 1999ssa..conf..182B No abstract at ADS Title: Solar Line Bisectors in the Infrared Authors: Puschmann, K.; Hanslmeier, A.; Solanki, S. K. Bibcode: 1999ASSL..239..227P Altcode: 1999msa..proc..227P In the present work we made some analysis with respect to shifts and asymmetries of infrared solar spectral lines to get information about vertical velocity fields in the deep layers of the Solar Photosphere. For the analysis of shifts and asymmetries of bisectors were used. We averaged bisectors belonging to a group of similar line parameter. Therefore it was possible, to analyse correlations between asymmetries and line shifts and corresponding vertical velocities with line parameters and to discuss the results with other literature. Title: Inversion of Stokes profiles Authors: Frutiger, C.; Solanki, S. K.; Fligge, M.; Bruls, J. H. M. J. Bibcode: 1999ASSL..243..281F Altcode: 1999sopo.conf..281F No abstract at ADS Title: Observations of the Hanle effect in the Ca I 4227 and Sr II 4078 Å lines Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1999ASSL..243...31B Altcode: 1999sopo.conf...31B No abstract at ADS Title: Formation of small-scale magnetic elements: surface mechanism. Authors: Gadun, A. S.; Sheminova, V. A.; Solanki, S. K. Bibcode: 1999KPCB...15..291G Altcode: The authors present results of a two-dimensional MHD simulation of the solar magnetogranulation. The medium was assumed to be compressible, gravitationally stratified, radiatively coupled, partially ionized, and turbulent. The simulated magnetogranulation evolved over the course of two hours of hydrodynamic (solar) time. A surface (magnetic plume-like) mechanism which forms thin magnetic elements was found to operate during the process of granule fragmentation. The activity of such a mechanism suggests that the magnetogranulation can concentrate and intensify the global magnetic flux at the boundaries of convective cells and can also form nearly vertical compact magnetic flux tubes by involving the weak horizontal photospheric field, which may be, in general, of local (turbulent) nature. Title: Coronal Hole Properties Observed with SUMER Authors: Stucki, K.; Solanki, S. K.; Rüedi, I.; Stenflo, J. O.; Brković , A.; Schühle, U.; Wilhelm, K.; Huber, M. C. E. Bibcode: 1999SSRv...87..315S Altcode: We analyze SUMER spectra of 14 lines belonging to 12 ions, obtained on both sides of the boundary of polar coronal holes as well as at other locations along the limb. We compare line intensities, shifts and widths in coronal holes with values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show an increasingly stronger blueshift in coronal holes relative to the quiet Sun at an equal heliospheric angle. The width of the lines is generally larger (by a few km/s) inside the coronal hole. Intensity measurements show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, with evidence for a slightly enhanced emission in polar coronal holes for lines formed below 105 K. Title: Coronal Holes Versus Normal Quiet Sun Observed with SUMER Authors: Stucki, K.; Solanki, S. K.; Rüedi, I.; Stenflo, J. O.; Brković, A.; Schühle, U.; Wilhelm, K.; Huber, M. C. E. Bibcode: 1999Ap&SS.264...53S Altcode: 1998Ap&SS.264...53S We present a preliminary analysis of spectral lines obtained with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO), as observed during three observing campaigns. From the 70 observed spectral lines, we selected 12, representing 9 ions or atoms, in order to analyse line intensities, shifts and widths in polar coronal holes as well as in the normal quiet Sun. We find that coronal lines show a distinct blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, while there is no evidence for such a shift for lines formed at temperatures below 10^5 K. The widths of lines formed at temperatures above 3 - 10^4 K are slightly increased inside the coronal hole, but unaffected for lower temperatures. Intensity measurements clearly show the center-to-limb variation, as well as an intensity diminution inside the coronal hole for lines formed above approximately 10^5 K. Title: Spots and Plages: the Solar Perspective Authors: Solanki, S. K. Bibcode: 1999ASPC..158..109S Altcode: 1999ssa..conf..109S No abstract at ADS Title: Loop Models from SOHO Observations Authors: Landini, M.; Brković , A.; Landi, E.; Rüedi, I.; Solanki, S. Bibcode: 1999SSRv...87..245L Altcode: The Coronal Diagnostic Spectrometer (CDS) on SOHO is a grazing/normal incidence spectrograph, aimed to produce stigmatic spectra of selected regions of the solar surface in six spectral windows of the extreme ultraviolet from 150 Å to 785 Å (Harrison et al. 1995). In the present work, CDS, EIT, MDI and Yohkoh observations of active region lops have been analyzed. These observations are part of JOP 54. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT and MDI images, and loop structures have been clearly identified using Fe XVI emission lines. Density sensitive lines and lines from adjacent stages of ionization of Fe ions have been used to measure electron density and temperature along the loop length; these measurements have been used to determine the electron pressure along the loop and test the constant pressure assumption commonly used in loop modeling. The observations have been compared with a static, isobaric loop model (Landini and Monsignori Fossi 1975) assuming a temperature-constant heating function in the energy balance equation. Good agreement is found for the temperature distribution along the loop at the coronal level. The model pressure is somewhat higher than obtained from density sensitive line ratios. Title: Granulation Near the Solar Limb: Observations and 2-D Modeling Authors: Gadun, A. S.; Solanki, S. K.; Johannesson, A. Bibcode: 1999ASSL..239..201G Altcode: 1999msa..proc..201G Based on high spatial resolution spectra and using 2-D fully compressible, HD simulations of granules we analyze the correlations between spectral line parameters: their CLV, height-dependence and sensitivity to spatial resolution. We also point out some discrepancies between the model results and observations. Title: Signatures of Coronal Hole Spectra Between 660 Å and 1460 Å Measured with SUMER on SOHO Authors: Schühle, U.; Curdt, W.; Wilhelm, K.; Solanki, S. K.; Stucki, K. Bibcode: 1999SSRv...87..299S Altcode: Spectra of the northern polar coronal hole measured with the SUMER spectrometer on SOHO on 25 October 1996 are analyzed. We present spectra taken at locations on the solar disk where part of the spectrometer slit intersects a polar coronal hole region and an area of brighter emission from outside of the coronal hole area. By comparing the line intensities between the parts of the spectrum taken inside the "dark" area of the coronal holes and the brighter regions, we work out the signatures of the specific coronal hole in the chromosphere, transition region and lower corona. We find that emissions of neutral atom lines, of which there are many in the spectrum of SUMER, show no difference between the coronal hole and the bright boundary areas, whereas all ionized species show strong intensity enhancements, including the continuum emissions of carbon and hydrogen. These enhancements are larger than in normal quiet Sun areas. Title: On the expansion of large and small flux tubes with height Authors: Solanki, S. K.; Rüedi, I.; Finsterle, W.; Livingston, W. Bibcode: 1999ASSL..243..397S Altcode: 1999sopo.conf..397S No abstract at ADS Title: Sunspot magnetic oscillations: Comparison between observations and models Authors: Rüedi, I.; Solanki, S. K.; Bogdan, T.; Cally, P. Bibcode: 1999ASSL..243..337R Altcode: 1999sopo.conf..337R No abstract at ADS Title: Brightness Variations in the Solar Atmosphere as Seen by SOHO Authors: Brkovic, A.; Rüedi, I.; Solanki, S. K.; Huber, M. C. E.; Stenflo, J. O.; Stucki, K.; Harrison, R.; Fludra, A. Bibcode: 1999ASSL..239..231B Altcode: 1999msa..proc..231B We present preliminary results of a statistical analysis of the brightness variations of solar features at different levels in the solar atmosphere. We observed quiet Sun regions at disc centre using the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO). We find significant variability at all time scales in all parts of the quiet Sun, from darkest intranetwork to brightest network. Such variations are observed simultaneously in the chromospheric He I 584.33 Angstroms (2 \cdot 10^4 K) line, the transition region O V 629.74 Angstroms (2.5 \cdot 10^5 K) and coronal Mg IX 368.06 Angstroms (10^6 K) line. The relative variability is independent of brightness and most of the variability appears to take place on time scales longer than 5 minutes for all 3 spectral lines. No significant differences are observed between the different data sets. Title: Infrared lines as probes of solar magnetic features. XIV. TI i and the cool components of sunspots Authors: Rueedi, I.; Solanki, S. K.; Keller, C. U.; Frutiger, C. Bibcode: 1998A&A...338.1089R Altcode: The first systematic observations of sunspot umbrae using the lines of the Ti Ii multiplet at 2.2 mu m are presented. Their diagnostic capabilities are investigated, developed and used to investigate the magnetic and velocity structure of a sunspot. These lines are most sensitive to cool plasma. In addition, they are extremely Zeeman sensitive. We find that a sunspot is composed of two distinct cool magnetic components. One of them is fairly vertical, has a large magnetic field strength and is associated with the central (umbral) part of the sunspot. The other component is strongest near the outer boundary of the spot (penumbra), is much more inclined, has a very low magnetic field strength and shows the signature of the Evershed effect. In contrast to the smooth transition of field strength from the darkest part of the umbra to the outer penumbral boundary usually visible in observations carried out in other spectral lines, the Ti Ii lines exhibit a sharp transition between the two magnetic components. Title: Hanle diagnostics of solar magnetic fields: the SR II 4078 Angstrom line Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1998A&A...337..565B Altcode: The Hanle depolarization and rotation effects in the Sr ii 4078\tsAngstroms line have been explored with the instrumentation at IRSOL (Istituto Ricerche Solari Locarno) by recording the Stokes I, Q, and U line profiles with high spectral resolution and polarimetric accuracy in a large number of regions across the solar disk. >From the extracted line parameters we have constructed ``Hanle histograms'' showing the statistical distributions of the Hanle rotation and depolarization effects. Comparison with theoretical calculations allow these histograms to be understood in terms of magnetic fields with a strength of about 5-10\ts G, which is similar to the field strengths previously found through analysis of Q/I Hanle depolarization in the Ca i 4227\tsAngstroms line. While small-scale magnetic fields with spatially unresolved angular distributions contribute to the observed Hanle depolarization effects, the observed Hanle rotation effects in Stokes U are due to spatially resolved fields with net large-scale orientations (e.g. global or canopy-type fields). We have also for the first time determined empirical ``Hanle efficiency profiles'', derived independently for the Hanle rotation and depolarization effects. They show how the Hanle efficiency has its maximum in the Doppler core of the line and then rapidly decreases to become zero in the line wings. Title: Doppler imaging: the polar SPOT controversy Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schuessler, M. Bibcode: 1998A&A...336..231B Altcode: Doppler imaging studies have revealed that most rapidly rotating cool stars have high-latitude spots, which in many cases cover the stellar poles. The spectroscopic signature of polar spots is a filling in of the cores of spectral lines, which become flat-bottomed and may show bumps. Although the existence of polar spots is corroborated by spectroscopic and photometric measurements, and although theoretical models predict polar spots, they remain controversial. Most notably, it has been proposed that the line core filling in might also be caused by chromospheric activity. We present a NLTE radiative transfer analysis of 14 of the most-used Doppler-imaging lines which demonstrates that chromospheric activity can produce filling in of the observed line profiles only in a few of these lines. Moreover, such filling in is in general not of the type observed in the spectra of active stars. We are able to produce a flat-bottomed line core by concentrating the chromospheric activity near the poles, but only for two of the strongest lines, Fe i 5497 Angstroms and Fe i 6430 Angstroms. In the observations, however, also the weaker lines have flat-bottomed cores. Therefore, it is unlikely that polar spots are an artifact due to misinterpretation of the spectral signature of chromospheric activity. Nevertheless, we cannot exclude that chromospheric activity provides part of the filling in of the cores of some stronger lines; we present a diagnostic that may help to separate the contributions of chromospheric activity and spots. Title: Do solar magnetic elements harbor downflows? Authors: Frutiger, C.; Solanki, S. K. Bibcode: 1998A&A...336L..65F Altcode: In a recent paper Bellot Rubio et al. (1997) inverted Zeeman split Stokes profiles to infer the stratification of the temperature, velocity and magnetic field in the photospheric layers of solar magnetic elements (modeled as thin flux tubes). One controversial result of their inversions is the presence of a strong downflow within the flux tubes. In the model underlying their inversion such a downflow is necessary to reproduce the asymmetric shape of the observed V profiles. We present inversions based on two different flux-tube models, both of which reproduce the Stokes I and V profiles obtained in plages and the network with high accuracy, including the V profile asymmetry. One model is almost identical to that employed by Bellot Rubio et al. (1997), and results in a significant downflow within the flux tube. The other, although similar in most respects, has mass conservation enforced inside the flux tubes, i.e. they contain both an upflow and a downflow which could arise from oscillations or siphon flows. Hence, current data may not be sufficiently sensitive to distinguish between the two velocity structures, so that there is no compelling evidence for a net downflow of matter inside magnetic elements. From a physical point of view the model incorporating mass conservation is to be preferred. Title: Oscillations of sunspot magnetic fields Authors: Rueedi, I.; Solanki, S. K.; Stenflo, J. O.; Tarbell, T.; Scherrer, P. H. Bibcode: 1998A&A...335L..97R Altcode: We report on velocity and magnetic field oscillations observed in sunspots using the MDI instrument onboard SOHO. In addition to the well-known velocity oscillations, the data clearly show highly localised oscillations of the magnetogram signal in different parts of the sunspots. We show that only oscillations of the magnetic field vector can produce the observed magnetogram oscillations, and that the observed phase relations suggest an origin in terms of magnetoacoustic gravity waves. Title: A model of solar total and spectral irradiance variations Authors: Fligge, M.; Solanki, S. K.; Unruh, Y. C.; Froehlich, C.; Wehrli, Ch. Bibcode: 1998A&A...335..709F Altcode: We model solar spectral irradiance variations under the assumption that they are produced by sunspots and faculae alone. The model is based on three components, i.e. quiet Sun, assumed to be temporally invariant, sunspots and faculae whose temporal variations are mimicked using time series of sunspot areas and Mg II core-to-wing ratios. The detailed flux spectrum for each component is employed. The results are compared to spectral irradiance measurements at 402 nm, 500 nm and 862 nm obtained between February 22 and December 31, 1996 by the three-channel sunphotometers (SPM) which are part of the VIRGO package onboard SOHO. Our model shows a good correlation with the measured variations of the three colour channels and of the total irradiance. Since it also successfully reproduces changes in the UV spectral irradiance, irradiance variations and observed variations of the faculae-to-spots filling factor ratio since 1978, our model supports earlier suggestions that a large part of the solar irradiance variations is caused by magnetic fields at the solar surface. Finally, we use the model to test whether the difference between the magnitude of solar and stellar brightness variations reported by Lockwood et al. (1992) is due to the particular filters they use for their stellar observations. Our results suggest that this effect can explain only a small part of the discrepancy. Title: On the reliability of Stokes diagnostics of magnetic elements away from solar disc centre Authors: Solanki, S. K.; Steiner, O.; Buente, M.; Murphy, G.; Ploner, S. R. O. Bibcode: 1998A&A...333..721S Altcode: {The spectropolarimetric signature of models of small-scale magnetic features is well understood at the centre of the solar disc, but has been little studied near the solar limb, mainly because the detailed geometry of the flux tubes must then be taken into account in a realistic analysis. We present multi-ray calculations of Stokes profiles through arrays of 2-D magnetohydrostatic models of small flux tubes. We compare the Stokes profile shapes and Stokes based diagnostics (Q to V ratio, V amplitude, magnetic line ratio, centre of gravity wavelengths, etc.) resulting from plane-parallel and 2-D flux-tube models at different limb distances for two lines in the visible and an infrared H-band line. For the visible lines around 5250 Angstroms all the diagnostics we have studied, with the exception of the Q and U to V ratio, are significantly affected by the finite size of the flux tubes and the passage of the rays through non-magnetic material near the limb. We show that magnetic filling factors and the global magnetic flux may be underestimated using the usual calibration techniques. In addition, near the solar limb the magnetic line ratio can move into a regime that is forbidden according to simpler models. The spatially averaged longitudinal field derived from the centre-of-gravity method also becomes strongly model dependent. The thermal structure of the non-magnetic atmosphere appears to play a particularly important role in determining the values of these diagnostic parameters. The infrared line at 1.5648 mu m, on the one hand, reacts little to the external atmosphere and provides superior diagnostics of both the magnetic field strength and filling factor. On the other hand, for a flux tube with a sharp boundary this line shows little dependence on the flux-tube diameter, in contrast to the earlier findings of Zayer et al. (1989). } Title: Temporal Evolution of Artificial Solar Granules Authors: Ploner, S. R. O.; Solanki, S. K.; Gadun, A. S.; Hanslmeier, A. Bibcode: 1998SSRv...85..261P Altcode: We study the evolution of artificial granulation on the basis of 2-D hydrodynamical simulations. These clearly show that granules die in two different ways. One route to death is the well known bifurcation or fragmentation of a large granule into 2 smaller ones (exploding granules). The other pathway to death is characterized by merging intergranular lanes and the accompanying dissolution of the granule located between them. It is found that the lifetime and maximum brightness is independent of the way in which granules evolve and die. They clearly differ in size, however, with exploding granules being in general significantly larger. Title: Structure of the Solar Photosphere Authors: Solanki, Sami K. Bibcode: 1998SSRv...85..175S Altcode: The majority of measured solar abundances refer to the solar photosphere. In general, when determining photospheric abundances a plane-parallel atmosphere and LTE are assumed. However, the photosphere is structured by granulation, magnetic fields and p-modes. They change line profiles by the thermal inhomogeneities and wavelength shifts they introduce. A brief description of the first two of these phenomena is given and some of the ways in which they influence abundances are pointed out. Departures from LTE also occur. The magnitude of the errors introduced into elemental abundances by neglecting such departures is also briefly discussed. Title: Foreword Authors: Fröhlich, C.; Huber, M. C. E.; Solanki, S. K.; von Steiger, R. Bibcode: 1998SSRv...85...11F Altcode: No abstract at ADS Title: Long-term behavior of emission from solar faculae: steps towards a robust index Authors: Fligge, M.; Solanki, S. K. Bibcode: 1998A&A...332.1082F Altcode: Facular emission is one of the major contributors to long-term solar irradiance variations. Reconstructions of past facular variations, however, are strongly hampered by the lack of reliable proxies, particularly on time-scales longer than a solar cycle. We consider the five potential facular proxies with records covering more than 40 years. By suitably weighting and combining them we create a new proxy. In comparison with sunspot relative number the combined proxy exhibits enhanced values during solar cycles 16 and 17. This suggests that the Sun may have been brighter during those cycles than earlier models, which used sunspot numbers as the facular proxy, indicate. Title: Hanle depolarisation in the solar chromosphere Authors: Bianda, M.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1998A&A...331..760B Altcode: A new polarimeter and an extension of the polarimetric technique of Donati et al. (1990) to higher polarisation values are presented. With this system, which allows low-noise polarimetric observations in the blue and near-UV part of the solar spectrum, we have recorded more than 200 Q/I profiles of Ca I 4227 Angstroms with an accuracy of 2-3x 10(-4) . We find that while the Q/I line shape outside the line core is very stable from one part of the solar surface to the next, the line core exhibits large variations relative to the rest of the profile. These variations are best interpreted in terms of partial depolarisation produced by the Hanle effect in the presence of weak fields. We obtain a quantitative and model-independent estimate of the depolarisation, from which we deduce field strengths of 5-15 G at the level of line-core formation, i.e. in the low to mid chromosphere. We also find evidence that the field strength is distributed around these average values with a FWHM that is approximately the same as the average value. Title: Infrared lines as probes of solar magnetic features. XIII. The relative flux in weak and strong quiet-sun magnetic fields Authors: Meunier, N.; Solanki, S. K.; Livingston, W. C. Bibcode: 1998A&A...331..771M Altcode: An estimate of the fraction of magnetic flux in intrinsically weak-field form, i.e. fields with less than 1 kG intrinsic strength, in the quiet Sun is presented. We find that on average approximately 2/3 of the flux is in weak-field form, although our data allow a range of values between 25%\ and 85%. These estimates have been derived with the help of radiative transfer model calculations from low-noise scans through the quiet Sun at fixed wavelengths within a Zeeman sensitive spectral line at 1.56 mu m. They represent the first rapidly modulated polarimetric observations of solar 1.56 mu m radiation. These scans show that the polarimetric signature near the core of the line exhibits a very different spatial structure than in the outer line wings. Since the outer part of the line profile is only sensitive to intrinsically strong magnetic fields, while the central part reacts also to intrinsically weak fields, this implies that intrinsically weak fields are distributed differently on the surface than strong fields are (weak-field features lie closer together). The spatial distribution suggests that the strong fields we are observing are network elements, whereas the weak fields are intranetwork features. Further results, such as the average distance between weak and strong field features, flux distribution, spatial power spectra and the Stokes asymmetry of intrinsically weak fields are also presented. In particular, we find that the flux per magnetic feature is distributed lognormally, in agreement with a similar finding for sunspot umbral areas by Bogdan et al. (1988). Title: Solar irradiance since 1874 revisited Authors: Solanki, S. K.; Fligge, M. Bibcode: 1998GeoRL..25..341S Altcode: We reconstruct the solar irradiance since 1874 employing an evolved version of previously published models and improved sunspot and facular data. A good correlation between reconstructed irradiance and Earth's global air temperature on time scales longer than the solar cycle is obtained and, in contrast to many earlier models, solar irradiance does not on average lag behind global temperature prior to 1975, although the exact time lag between the two quantities depends somewhat on details of the reconstruction. Since that epoch, however, air temperatures have increased by 0.2 K, whereas solar irradiance has risen a disproportionately smaller amount. Therefore, unless the influence of solar variability on Earth is very strongly non-linear, at least this most recent temperature increase reflects the influence of man-made greenhouse gases or non-solar sources of natural variability. Title: Velocity fields below the magnetic canopy of solar flux tubes: evidence for high-speed downflows? Authors: Briand, C.; Solanki, S. K. Bibcode: 1998A&A...330.1160B Altcode: It is well-established that velocities in the immediate surroundings of solar magnetic elements produce an asymmetry in the Stokes V profiles emerging from the magnetic feature. Conversely, the observed Stokes V asymmetry can be used to infer the velocity field. Taking as constraints the area asymmetries of the Stokes V profiles of two lines of neutral magnesium (lambda 457nm and lambda 517nm) observed near the center of the solar disk, the (vertical) component of the velocity field below the magnetic canopy of flux tubes is investigated. We find that the strong Mg I b_2 line at 517nm qualitatively extends the diagnostic capabilities of Stokes V asymmetry, mainly due to the fact that it is sensitive to velocities over a large range of heights and hence also at relatively large distances from the flux tube axis. In order to retrieve the observed area asymmetry of both lines, up- as well as downflows have to be introduced in the models. If the temperature differences between the two flows are neglected then a downflow of 1.5 - 2km s(-1) close to the edge of the flux tube and an almost equally strong upflow at greater distances (corresponding to the central part of a granule) reproduces the observed area asymmetries. If we take into account that the temperature in the downflow is lower than in the upflow, we can only reproduce the observations if the downflow is fast (>= 5 km s(-1) ) and concentrated into narrow lanes. Title: The photospheric sources of jets Authors: Solanki, S. K. Bibcode: 1998ESASP.421..115S Altcode: 1998sjcp.conf..115S No abstract at ADS Title: Complex magnetic fields in an active region Authors: Bernasconi, P. N.; Keller, C. U.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1998A&A...329..704B Altcode: High-resolution observations of the full Stokes vector in Fe\sc i spectral lines around 5250 Angstroms obtained at the Swedish Vacuum Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a complex active region reveal the presence of anomalously shaped Stokes profiles indicating the coexistence of at least two magnetic components within the same spatial resolution element. These Stokes profiles have been analyzed with an inversion code based on a 3-component atmospheric model with two magnetic and one field-free component. The fits to the observations in a magnetic region that resembles a small penumbra reveal the presence of a horizontal magnetic field component with an average field strength of /line{B}=840 G, a mean filling factor of /lineα=0.12, and an average temperature /line{T}=5400 K at log {tau_ {5000}}=-1.5 embedded in the main ``penumbral'' magnetic field that has /line{B}=1500 G, /lineα=0.56, and /line{T}=4900 K. The horizontal component exhibits a mean outflow of 2.7 km s(-1) which is mainly due to the Evershed flow. In a region where there are strong downflows up to 7 km s(-1) , we infer the possible presence of a shock front whose height changes along the slit. The height variation can be explained by a change of the gas pressure at the base of the photosphere below the shock front as proposed by Thomas & Montesinos (1991). Small plages with field strengths below 900 G have been observed in the vicinity of some pores. Finally, we present a puzzling field structure at the boundary between two adjacent pores. Ambiguous results suggest that although the inversion code is able to successfully invert even very complex Stokes profiles, we are far from a complete description of the field structure in complex magnetic regions. We warn that magnetograms and fits to data involving only a single magnetic component may hide the true complexity of the magnetic structure in at least some parts of active regions. Title: Solar Composition and Its Evolution -- From Core to Corona Authors: Fröhlich, C.; Huber, M. C. E.; Solanki, S. K.; von Steiger, R. Bibcode: 1998sce..conf.....F Altcode: No abstract at ADS Title: Search for a Robust Index of Long-Term Facular Variations Authors: Fligge, M.; Solanki, S. K. Bibcode: 1998ASPC..140..317F Altcode: 1998ssp..conf..317F No abstract at ADS Title: Kink Waves in Solar Flux Tubes and Polarized Line Profiles Authors: Ploner, S. R. O.; Solanki, S. K. Bibcode: 1998ASPC..154..679P Altcode: 1998csss...10..679P Waves are thought to play a fundamental role in the heating processes of the chromosphere and corona of the Sun and other cool stars. Of particular interest are periodic motions in so called flux tubes, elements of concentrated magnetic field. We investigate the influence of kink waves in flux tubes on polarized line profiles and demonstrate that they have a large effect. This should help us to set limits on the energy flux in such waves. Title: Temporal Evolution of Artificial Solar Granules Authors: Ploner, S. R. O.; Solanki, S. K.; Gadun, A. S.; Hanslmeier, A. Bibcode: 1998sce..conf..261P Altcode: No abstract at ADS Title: A Non-LTE Analysis of Doppler Imaging Lines Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schussler, M. Bibcode: 1998ASPC..154.1959B Altcode: 1998csss...10.1959B Doppler imaging studies have revealed that most stars with high activity levels have polar spots. Although their existence is corroborated by spectroscopic and photometric measurements, and although theoretical models have been produced that include polar spots, their existence remains controversial. Based on a NLTE radiative transfer analysis of the most-used Doppler-imaging lines we reject the claim that chromospheric activity might be responsible for the features in the spectral lines that are commonly interpreted as polar spots. Title: Torsional waves in solar flux tubes and polarized line profiles. Authors: Ploner, S. R. O.; Solanki, S. K. Bibcode: 1998joso.proc..113P Altcode: Waves are thought to play a fundamental role in the heating of the chromosphere and corona of the Sun and other cool stars. Of particular interest are periodic motions in so called flux tubes, elements of concentrated magnetic field. The authors investigate the influence of torsional waves from different locations in flux tubes on polarized line profiles. They find that the magnitude of the influence of those waves strongly depends on the location within the flux tube. Title: A model of the wavelength dependence of solar irradiance variations Authors: Solanki, S. K.; Unruh, Y. C. Bibcode: 1998A&A...329..747S Altcode: The variation of the solar irradiance over the solar cycle has a strong wavelength dependence, being larger at shorter wavelengths. Here we present simple models of the spectral dependence of irradiance variations between solar activity maximum and minimum. We find that the observations (which concentrate on the UV) cannot be reproduced by a change in effective temperature of the Sun (or of parts of its surface) alone. We can, however, reproduce the data with either a 2-component or a 3-component model, of which one component is the quiet Sun, another is a facular component, and the third (in the case of the 3-component model) represents the temperature stratification of sunspots. The facular component is found to be very close to the facular models F or P of Fontenla et al. (1993). The success of these models supports the assumption underlying many studies of total solar irradiance variations that these are caused mainly by magnetic fields at the solar surface. Our investigation also allows an improved estimate of the relative contribution of the various layers in the solar photosphere and of the different wavelength regions to the total irradiance variations. Title: The Wavelength Dependence of Solar Irradiance Variations Authors: Unruh, Yvonne C.; Solanki, Sami K. Bibcode: 1998ASPC..154..700U Altcode: 1998csss...10..700U The observed irradiance variations over a solar cycle show a strong wavelength dependence; the variations are strongest in the UV and very weak in the visible. Here we present a simple model of the spectral dependence of the solar irradiance. We can exclude models that postulate a change in the effective temperature of the Sun or of parts of the Sun. However, we find that the observed irradiance variations can be matched with a model that includes the different temperature stratification of the quiet photosphere, the faculae and the sunspots. Title: Structure of the Solar Photosphere Authors: Solanki, S. K. Bibcode: 1998sce..conf..175S Altcode: No abstract at ADS Title: Manifestations of Solar Magnetic Fields Authors: Solanki, S. K. Bibcode: 1998HiA....11..857S Altcode: The magnetism of the Sun manifests itself in innumerable ways, many of which constitute what is referred to as solar magnetic activity, while others are counted among the phenomena of the quiet Sun. After a brief overview of the structure of the solar magnetic field, a few examples of its manifestations are pointed out. Title: Modelling Spectral Irradiance Variations obtained by VIRGO Authors: Fligge, M.; Solanki, S. K.; Unruh, Y.; Frohlich, C.; Wehrli, Ch. Bibcode: 1998ASPC..140..311F Altcode: 1998ssp..conf..311F No abstract at ADS Title: The flare of November 29, 1996 observed by SOHO/CDS Authors: Czaykowska, A.; Rank, G.; Ruedi, I.; Solanki, S. K.; de Pontieu, B. Bibcode: 1998cee..workE..32C Altcode: We present flare and post-flare observations obtained with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO) on November 29, 1996. On this day at around 20:40 UT, an M 1.0/1F flare occurred in the solar active region NOAA 7999 and was accidentally observed by the Normal Incidence Spectrometer (NIS), one of the two spectrometers of CDS (Harrison et al., 1995). The data consist of two rasters lasting for 135 minutes each and both cover an area of 4 times 4 arcminutes. The first raster is pointed at the northern part of the active region during the flare whereas the second one covers the southern part of the active region after the main phase of the flare. The observations were part of the Joint Observing Program (JOP) 54 which is aimed at the investigation of scaling laws in coronal loops. Consequently the details of observations such as line list and exposure time weren't convenient for flare observations. In addition a flare can lead to over-exposures, i.e., saturation of the CCD detector pixels, and a burn-in degeneration of the detector in bright lines. Therefore observations of flares with CDS are avoided and the flare from November 29, 1996, is so far the only noteworthy flare observed by CDS. In our case we have remarkable saturation in the chromospheric He I line at 584 AA and the coronal Fe XVI lines at 335 AA and 361 AA, which are formed at an equilibrium temperature of about 2.5 cdot 10^6 K. Another effect of illumination on the CCD detector being too high is that the electron well of each pixel may fill and hence bleed to adjacent pixels. This effect is clearly seen in our data. As the flare occurred, the 2 times 240 arcseconds slit was being rastered across the active region from west to east. We thus have a convolution of spatial and temporal effects which are not easy to separate. However, we have spectral information of each pixel in all lines and exposures which are not saturated. Hence, line parameters such as intensity and relative Doppler shifts can be calculated. Moreover, the line list contains two density sensitive line pairs, Fe XII 338 AA/364 AA and Fe XIII 348 AA / 360 AA (see, e.g., Mason et al., 1997) which can be used to determine the electron density. Using this data we intend to study the temporal evolution of characteristics and geometry of the loop during the flare. Title: Solar Magnetic Fields: an Introduction Authors: Solanki, Sami K. Bibcode: 1998LNP...507...41S Altcode: 1998sspt.conf...41S The magnetic field of the Sun is thought to be produced by a dynamo in the solar interior and exhibits its greatest influence on the solar plasma in the tenuous outer layers of the solar atmosphere, where it lies at the heart of almost every major phenomenon. Most direct observations of the magnetic field are restricted to the solar surface, however. Both observational and theoretical methods have been employed heavily to obtain information on and an understanding of solar magnetism. It is the aim of these lecture notes to impart some of this knowledge to the reader: knowledge both of the magnetic field and of some of the methods used to investigate it. Title: Sunspot Oscillations Observed with MDI Authors: Rüedi, I.; Solanki, S. K.; Stenflo, J. O.; Scherrer, P. H. Bibcode: 1998ESASP.417..281R Altcode: 1998cesh.conf..281R No abstract at ADS Title: Solar composition and its evolution - from core to corona. Proceedings. ISSI (International Space Science Institute) Workshop, Bern (Switzerland), 26 - 30 Jan 1998. Authors: Fröhlich, C.; Huber, M. C. E.; Solanki, S. K.; von Steiger, R. Bibcode: 1998SSRv...85.....F Altcode: The following topics were dealt with: solar composition, solar evolution, the solar interior, helioseismology, photosphere, chromosphere, solar corona, solar wind, the Sun as a Sun-like star. Title: Evolved Stars: What Happens to Activity Off the Main Sequence Authors: Strassmeier, K. G.; Fekel, F. C.; Gray, D. F.; Hatzes, A. P.; Schmitt, J. H. M. M.; Solanki, S. K. Bibcode: 1998ASPC..154..257S Altcode: 1998csss...10..257S Magnetic activity on the main sequence has been well studied, in contrast to researches on sub-giants, giants, and supergiant stars. In this discussion we will address three main topics associated with activity in evolved stars: (1) rotation regimes for evolved stars; (2) rotation-activity relations in the H-R diagram; (3) polar spots. Title: Noise Reduction in Astronomical Spectra: A New Wavelet-Based Method Authors: Fligge, M.; Solanki, S. K. Bibcode: 1998ASPC..154..833F Altcode: 1998csss...10..833F We present a wavelet technique to de-noise astronomical spectra based on non-orthogonal wavelet packets and compare it to Fourier-smoothing and other wavelet-based de-noising algorithms. It is found to give better results than any other tested de-noising algorithm and is particularly successful in recovering weak signals that are practically drowned by the noise. Title: Solar Oscillations in CO at 4.6 Microns Authors: Livingston, W.; Solanki, S.; Muglach, K.; Wallace, L. Bibcode: 1998ASPC..135..186L Altcode: 1998hcsp.conf..186L The fundamental band lines of CO originate in the solar temperature minimum and lower cool parts of the chromosphere. We have observed velocity and intensity oscillations in the quiet sun disk center and extreme limb, sunspots, and plage. We have also looked for CO oscillations in the Sun-as-a-star (but detect nothing). We compare the power spectra of these various features, noting well separated 3 min and 5 min peaks. We interpret our observations in terms of the models of Carlsson and Stein (1995), and Solanki, et al., (1994). Active Region Seismology: Results from TON Project Bala Balachandran (Dept. Physics, National Tsing Hua University, Hsinchu, Taiwan 30043, R.O.C.) Taiwan Oscillation Network (TON) is a ground based observation facility for the study of solar oscillations. The data consists of full-disc K-line images taken at every minute. Using the 512 images time series, we have tried to infer the location of the magnetic field below the photosphere using the property of the active regions to absorb the p-mode power. Two consecutive days, one just when the magnetic flux appears on the surface and the other the day before the emergence, were analysed. For this, an annular region of 15 deg. around the region of emerging flux was chosen for the analysis. The modes were decomposed into incoming and outgoing waves and the absorption coefficient was calculated as the fractional difference of the two types. We found that the absorption is stronger after the emergence of the flux and the flux was at a depth of about 4000 km below the surface just before its appearance on the surface; that is about 15 hrs before. It was also reported earlier by other groups that the quiet sun exhibits acoustic emission. It is believed to be an observational effect and the cause is yet to find out. We have carried out a systematic analysis of the quiet sun emission using 512 images. The analysis is similar to the active region study, but the region of interest is the solar disc centre. We found that there exists negative absoption or emission for most modes and the negative absorption increases with the angular distance from the centre of the sun. Title: Wavelength Dependence of Solar Irradiance Variability from VIRGO Onboard SOHO Authors: Wehrli, C.; Fröhlich, C.; Anklin, M.; Fligge, M.; SOlanki, S. K.; Unruh, Y. Bibcode: 1997ESASP.415..497W Altcode: 1997cpsh.conf..497W No abstract at ADS Title: Polar spots and stellar spindown: is dynamo saturation needed? Authors: Solanki, S. K.; Motamen, S.; Keppens, R. Bibcode: 1997A&A...325.1039S Altcode: Dynamo saturation is often invoked when calculating the rotational evolution of cool stars. At rapid rotation rates a saturated dynamo reduces the angular momentum carried away by the stellar wind. This, in turn, may explain the high rotation rates present in the distribution of rotation periods in young clusters. Here we point out that concentration of magnetic flux near the poles of rapidly rotating cool stars provides an alternative to dynamo saturation. A high-latitude concentration of field on rapid rotators saturates the angular momentum loss induced by the stellar wind, due to the reduced torque arm. We show that the inclusion of this effect in model calculations is able to reproduce the observed high rotation rates without the need for dynamo saturation. Taken together with the results of O'Dell et al. (1995A&A...294..715O) this argues against dynamo saturation at low rotation rates. Title: Noise reduction in astronomical spectra using wavelet packets Authors: Fligge, M.; Solanki, S. K. Bibcode: 1997A&AS..124..579F Altcode: The wavelet representation of a signal offers greater flexibility in de-noising astronomical spectra than classical Fourier smoothing due to the additional wavelength resolution of the decomposed signal. We present here a new wavelet-based approach to noise reduction. It is similar to an application of the splitting algorithm of a wavelet packets analysis using non-orthogonal wavelets. It clearly separates the signal from the noise, in particular also at the noise-dominated highest frequencies. This allows a better suppression of the noise, so that the spectrum de-noised in this manner provides a closer approximation of the uncorrupted signal than in the case of a single wavelet transformation or a Fourier transform. We test this method on intensity and circularly polarized spectra of the sun and compare with Fourier and other wavelet-based de-noising algorithms. Our technique is found to give better results than any other tested de-noising algorithm. It is shown to be particularly successful in recovering weak signals that are practically drowned by the noise. Title: Influence of kink waves in solar magnetic flux tubes on spectral lines. Authors: Ploner, S. R. O.; Solanki, S. K. Bibcode: 1997A&A...325.1199P Altcode: The kink-mode waves in solar magnetic flux tubes have been proposed as the carriers of significant amounts of energy into the upper atmosphere. Their observational signature, i.e. their influence on polarized spectral lines, is investigated theoretically using a simple MHD model and radiative transfer calculations. The results of a large model grid are presented. The wave is found to periodically shift, broaden, strengthen and change the asymmetry of circularly and linearly polarized profiles (Stokes V and Q, respectively). For most, but not all line parameters it exhibits its largest influence close to the limb. The time fluctuations of all quantities are in phase for Stokes Q and V, except for their amplitudes, which are in antiphase. The signal of the wave in time-averaged profiles depends on the wave frequency. Stokes Q and V exhibit opposite senses of the blue-red asymmetry and wavelength shift. This behaviour is found to be a result of the dependence on frequency of the phase relation between velocity and flux tube inclination. As the wave frequency approaches the cutoff frequency the phase relation changes and the time-averaged line asymmetry and shift decrease rapidly. High frequency kink waves are found to be an extremely efficient source of Stokes Q and V asymmetry, while maintaining a small line shift. Title: Simulations of Zeeman-split CA II K-line Stokes profiles with angle-dependent partial redistribution. Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 1997A&A...325.1179B Altcode: The formulation of the polarized radiative transfer equations for Zeeman-split spectral lines is still incomplete for cases with frequency-dependent line source function, e.g. when partial frequency distribution (PRD) of line photons occurs (strong resonance lines). Under the well-founded assumption that the field-free approximation works equally well for lines with partial redistribution as for lines with complete redistribution (CRD), we decouple the computation of the level populations (accounting for PRD effects) from the formal Stokes vector solution. The level populations are obtained with the Hubeny & Lites (1995ApJ...455..376H) version of Carlsson's (1986UppOR..33....1C) MULTI code, and the formal solution follows by means of a modified version of the Murphy & Rees (1990, NCAR Technical Note NCAR/TN-348+IA) SPSR code. Due to lack of the appropriate equations concerning the combination of partial redistribution and Zeeman splitting of spectral lines, an ad hoc partial redistribution approach (basically allowing for wavelength dependence of the line source function) is implemented in the SPSR code. This combination of codes is used to study the relevance of partial redistribution to the Caii K-line diagnostics of solar plage regions using grids of flux-tube models. In addition to magnetic fields, velocity fields play an important role in the formation of the K-line profiles, but these invalidate the commonly-used angle-averaged PRD formalism. We therefore extended the Hubeny & Lites (1995ApJ...455..376H) angle-averaged PRD version of MULTI code to the angle-dependent case, which allows line profiles to be computed under PRD conditions for arbitrary (but not too large) velocity fields. This code is subsequently used to perform the non-magnetic non-LTE statistical equilibrium computations for a few schematic velocity structures in plage flux tubes and in the surrounding non-magnetic atmosphere. The SPSR code is again used to obtain the K-line Stokes profiles. We investigate in particular the asymmetry of the K line intensity and circularly polarized profiles produced by the velocity fields. Title: Polar spots and stellar spindown: is dynamo saturation needed? Authors: Solanki, S. K.; Motamen, S.; Keppens, R. Bibcode: 1997A&A...324..943S Altcode: Dynamo saturation is often invoked when calculating the rotational evolution of cool stars. At rapid rotation rates a saturated dynamo reduces the angular momentum carried away by the stellar wind. This, in turn, may explain the high rotation rates present in the distribution of rotation periods in young clusters. Here we point out that concentration of magnetic flux near the poles of rapidly rotating cool stars provides an alternative to dynamo saturation. A high-latitude concentration of field on rapid rotators saturates the angular momentum loss induced by the stellar wind, due to the reduced torque arm. We show that the inclusion of this effect in model calculations is able to reproduce the observed high rotation rates without the need for dynamo saturation. Taken together with the results of O'Dell et al. (1995A&A...294..715O) this argues against dynamo saturation at low rotation rates. Title: Inter-Cycle Variations of Solar Irradiance: Sunspot Areas as a Pointer Authors: Fligge, M.; Solanki, S. K. Bibcode: 1997SoPh..173..427F Altcode: Most of the present models and reconstructions of solar irradiance use the concept of Photometric Sunspot Index (PSI) to account for the influence of sunspots on solar brightness. Since PSI is based on measured sunspot areas a firm database of such areas is essential. We show, however, that a significant disagreement exists between the data provided by the Royal Greenwich Observatory (from 1874 to 1976) and newer measurements provided by the observatories of Rome, Yunnan, Catania, and the US Air Force. The overlap of the time intervals over which sunspot areas were measured at Greenwich and Rome allows us to quantify the difference between the Greenwich and other data sets. We find that the various data sets differ, at least in a statistical sense, mainly by a correction factor of between 1.15 and 1.25. Title: Center-to-limb variation of the second solar spectrum. Authors: Stenflo, J. O.; Bianda, M.; Keller, C. U.; Solanki, S. K. Bibcode: 1997A&A...322..985S Altcode: The linear polarization that is caused by scattering processes in the solar atmosphere has been refered to as the "second solar spectrum", since it is structurally as rich as the ordinary intensity spectrum but quite different in appearance and information contents. One of the most used and theoretically best understood lines in the second solar spectrum is the SrI 4607Å line, which has served as a diagnostic tool for determinations of spatially unresolved, turbulent magnetic fields via the Hanle effect. Here we present the detailed center-to-limb variation of the scattering polarization in this line for a number of new data sets obtained both with an electrooptical modulation system (ZIMPOL) and a non-modulating beam splitter system (at IRSOL, Locarno), to provide improved observational constraints for theoretical modelling. The amplitude and width of the polarization profile, the amount of continuum polarization, as well as the depth and width of the intensity profile have been evaluated and carefully corrected for spectral broadening and stray light. While there is generally good agreement between the five data sets, some systematic differences are shown to be of solar rather than instrumental origin, most likely due to spatially varying Hanle depolarization across the solar disk. A number of other spectral lines have been observed with the ZIMPOL system at two different limb distances (μ=0.1 and 0.2) to allow us to compare the steepness of the center-to-limb variation of their polarization amplitudes. The steepest variation is exhibited by the continuum polarization, which declines by approximately a factor of 6 when going the 15 arcsec distance from μ=0.1 to μ=0.2. The spectral lines with the steepest center-to-limb variation are molecular lines, the CaII infrared triplet, and Hα. In contrast the SrI 4607 and BaII 4554Å lines have only moderately steeper center-to-limb variations than that of an ideal, purely dipole-scattering atmosphere, for which the polarization ratio between μ=0.1 and μ=0.2 is 1.38. These center-to-limb variations may be used to constrain temperature-density models of the upper photosphere and chromosphere. Title: Translimb Spectroscopy with SOHO/SUMER Authors: Ayres, T. R.; Lemaire, P.; Schuhle, U.; Wilhelm, K.; Ruedi, I.; Solanki, S. Bibcode: 1997SPD....28.0104A Altcode: 1997BAAS...29..879A We have used SUMER to obtain deep exposures of the 1300--1400 Angstroms spectrum, at the extreme limb and off-limb. Previous ``translimb'' studies in the thermal infrared had revealed remarkable extensions of cold material (T ~ 3000 K)---traced by carbon monoxide emission lines---into the heart of the hot chromosphere. A main objective of our program was to search for corresponding far-UV signatures of the ``thermally-bifurcated'' low chromosphere; for example, radiatively fluoresced emissions of the CO A--X 4th-positive system (collisional excitation would be negligible in cold gas). We conducted two separate observing programs with SUMER. Both made use of the 1('') -diameter circular aperture, translated across the limb in the minimum motor step increments of 0.('') 375, along the central meridian in the Northern polar coronal hole. The first program executed for nine hours beginning 19UT 25 Oct 1996. The full wavelength range was 1340--1400 Angstroms. It was recorded in two overlapping segments, placing key regions of the spectrum alternately on the KBr and bare parts of the detector, to help isolate 2nd-order features. Each segment was integrated for 500 s, and 32 pairs were obtained to span a 12('') swath centered on the optical limb. The second program was conducted 00--09UT 01 Dec 1996. It consisted of a single wavelength setting (1300--1340 Angstroms) with exposure time 500 s, but twice the spatial coverage of the earlier series: 64 steps, for a total displacement of 24('') . The strong chromospheric resonance lines of atomic oxygen (1302--1305 Angstroms) and ionized carbon (1334--1335 Angstroms) were observed on the bare part of the MCP camera. We report our progress in cataloging the rich, diverse translimb emission spectrum; and our efforts to deduce fundamental properties of the thermally heterogeneous chromosphere. [-3mm] SOHO is a project of international cooperation; the participation of TRA was supported by NASA grant NAG5-3226. Title: Magnetic field measurements on moderately active cool dwarfs. Authors: Rueedi, I.; Solanki, S. K.; Mathys, G.; Saar, S. H. Bibcode: 1997A&A...318..429R Altcode: We present a careful analysis of 13 high-quality optical spectra of low to moderately active late-type dwarfs (G1-K5) aimed at determining their magnetic parameters. Among our sample only one star, ɛ Eri (spatially averaged field strength=~165+/-30G), exhibits the unambiguous signature of a magnetic field, a few are candidates and the remaining show no sign of a magnetic field in the observed spectra. Our analysis is based on an inversion of the spectra using detailed numerical solutions of the Unno-Rachkovsky equations, for multiple spectral lines at different positions on the stellar disk, and including magneto-optical effects. It gives results for ɛ Eri which are in good agreement with the detailed analysis of infrared spectra by Valenti et al. (1995ApJ...439..939V). However, the low value of the spatially averaged field strength of these recent analyses imply that most values of the magnetic flux determined previously for moderately active stars are probably too large, often by considerable amounts. We find that the magnetic flux can be reliably determined if considerable care is taken in the analysis, but the magnetic field strength and filling factor cannot be determined separately for moderately active stars with optical spectra of spectral resolution <=10^5^ and S/N<=250. In the case of ɛ Eri we are able to constrain the temperature of the detected magnetic features, which we find to be similar to or hotter than the non-magnetic surroundings, providing the first direct evidence that the detected field is in the form of plages. We also find that if an inversion approach is used, which determines various line broadening parameters simultaneously in a self-consistent manner, the presence of a magnetic field is not as obvious as some previous analyses have suggested. In addition, we determine fundamental parameters of the stellar sample. Title: Properties of Brightenings Seen in CDS Movies Authors: Rüedi, I.; Brkovic, A.; Solanki, S. K.; Harrison, R.; Fludra, A.; Huber, M. C. E.; Stenflo, J. O.; Stucki, K. Bibcode: 1997ESASP.404..641R Altcode: 1997cswn.conf..641R No abstract at ADS Title: Dynamics of Flux Tubes in the Solar Atmosphere: Observations Authors: Solanki, S. K. Bibcode: 1997LNP...489...49S Altcode: 1997shpp.conf...49S Ground-based observations of the dynamics of solar magnetic flux tubes are reviewed. First a brief overview of the range of dynamic phenomena observed in both the largest and smallest flux tubes, i.e. in sunspots and magnetic elements, is given. Then three such phenomena — the Evershed effect, steady flows in small-scale magnetic features and oscillations in such features — are selected and discussed in detail. Title: Empirical Modelling and Thermal Structure of Sunspots Authors: Solanki, Sami K. Bibcode: 1997ASPC..118..178S Altcode: 1997fasp.conf..178S An overview of empirical models of sunspot thermal structure is presented. First a few remarks are made on empirical modelling in general. After that the literature on the various types of models, 1- and 2-component models of the umbra and penumbra, is reviewed. The evidence for or against the dependence of umbral temperature on various parameters like sunspot size, magnetic field strength and phase of the solar cycle is critically reviewed. Title: Umbral Polarimetric Measurements Using the TI I Multiplet at 2.2 MU M %The temperature gradient of a sunspot umbra Authors: Ruedi, I.; Solanki, S. K.; Livingston, W. Bibcode: 1997ASPC..118..237R Altcode: 1997fasp.conf..237R We present the first measurements of sunspot magnetic fields using the lines of the Ti I multiplet at 2.2 mu m. These lines are most sensitive to the plasma in the umbra. The observed line profiles suggest that the temperature gradient in mid-photospheric layers of a sunspot umbra is steeper than that of the standard sunspot models of Maltby et al. (1986). Title: Velocity fields around magnetic flux tubes. Authors: Briand, C.; Solanki, S. K. Bibcode: 1997joso.proc...55B Altcode: No abstract at ADS Title: The beat of the solar chromosphere's cold heart. Authors: Solanki, S. K.; Livingston, W.; Muglach, K.; Wallace, L. Bibcode: 1996A&A...315..303S Altcode: The cold heart of the solar chromosphere is best investigated using fundamental band lines of carbon monoxide, CO, at 4.7μm. We have obtained time series of CO spectra in the quiet sun and in active-region plage at the solar limb and at disk centre. In addition, we have recorded time series in sunspot umbrae located near disk centre. The power spectra and RMS amplitudes of the quiet-sun oscillations at disk centre and at the limb are not compatible with a generally hot chromosphere which is periodically cooled, but support recent suggestions that the low chromosphere is pervasively cool, interspersed with hot, possibly shocked material. In the plage the CO oscillations provide indirect evidence for the expansion of hot material (probably inside magnetic elements) with height. In umbrae the CO lines exhibit well-separated 3min and 5min peaks. We also present spectra of the phase shift between velocity and intensity oscillations of CO lines. At disk centre in the quiet sun the phase shift is on the whole similar to that seen in atomic lines formed near the classical temperature minimum, although with some properties peculiar to CO. In plages the quality of the phase shift is marginal, but suggests either large radiative damping or propagating waves in the 4 mHz frequency range. Finally, in sunspots the phase shift resembles that of atomic chromospheric lines in some umbrae. Title: Distribution of starspots on cool stars. I. Young and main sequence stars of 1Msun_. Authors: Schuessler, M.; Caligari, P.; Ferriz-Mas, A.; Solanki, S. K.; Stix, M. Bibcode: 1996A&A...314..503S Altcode: Sunspots are restricted to a latitude band within 30degof the solar equator. In contrast, the latitudes of spots on the surfaces of rapidly rotating cool stars can range from their polar regions, for RS CVn systems and for T Tauri stars leaving the Hayashi track, to mid latitudes for stars close to or on the main sequence. In order to find an explanation for these observed spot latitudes we have applied the criteria for the undulatory instability (Parker instability) of a toroidal magnetic flux tube embedded in the convective overshoot layer below the outer convection zone and calculated the non-linear evolution of the rising magnetic loops formed by this instability. We describe the results for a star of one solar mass in different phases of its evolution before and on the main sequence. We find that there usually is a range of latitudes at which magnetic flux can emerge on the stellar surface. The mean latitude of emergence shifts towards the poles for increasingly rapid rotation. The internal structure of the star, however, plays an almost equally important role in determining the latitude of magnetic emergence. For stars of solar mass only the youngest objects, with extremely deep convection zones, should show spots emerging at the stellar poles. Pre-main sequence stars at an age of 10^7^ y (convection zone reaching down half-way to the centre) exhibit high latitude, but not truly polar spots, while a main sequence star of one solar mass, even at high rotation rates, only shows intermediate latitude spots. These results are found to be in good agreement with Doppler images of young rapid rotators. Title: Infrared lines as probes of solar magnetic features. XII. Magnetic flux tubes: evidence of convective collapse? Authors: Solanki, S. K.; Zufferey, D.; Lin, H.; Rueedi, I.; Kuhn, J. R. Bibcode: 1996A&A...310L..33S Altcode: The magnetic field in the solar photosphere is mainly composed of magnetic flux tubes. Their formation is not well understood, largely due to an absence of observational tests of theoretical predictions. Here we use infrared polarimetric data to test and confirm the prediction that whereas the field strength of large flux tubes is almost independent of their magnetic flux, small flux tubes show a strong dependence. Our work thus strengthens the case for convective collapse as the source of concentrated solar magnetic fields. We also present the first direct measurement of the intrinsic field strength of typical intranetwork elements. A significant fraction of them is in equipartition with the kinetic energy of convection. Nevertheless, our results suggest that as far as their internal structure is concerned intranetwork magnetic features are better described by flux tubes than by turbulent fields. Title: Book Review: The Sun as a variable star ; solar and stellar irradiance variations (IAU colloquium 143) / Cambridge U Press, 1994 Authors: Pap, J. M.; Fröhlich, C.; Hudson, H. D.; Solanki, S. K. Bibcode: 1996SSRv...76..354P Altcode: No abstract at ADS Title: On the detection of shocks in the solar granulation. Authors: Solanki, S. K.; Rueedi, I.; Bianda, M.; Steffen, M. Bibcode: 1996A&A...308..623S Altcode: We investigate the spectral signature of a hydrodynamic simulation of solar granulation and compare it with high resolution observations. The model gives the correct qualitative trend of increasing line width with decreasing continuum intensity seen by Nesis et al. (1992) and interpreted by them as a sign of post-shock turbulence. We find, however, that the profiles in the dark downflow lanes are broader even when there is no horizontal transonic flow or shock in the vicinity. We conclude that the observations of Nesis et al. do not provide any firm evidence for the presence of granular transonic flows. Nevertheless, the simulation predicts a promising diagnostic of shocked horizontal flows. We find that at the locations of the shocks the line profiles are particularly broad, especially near the solar limb. We present observations of quiet solar regions that show this specific signature of shocks and thus support the theoretical prediction of transonic granular flows. Title: Measurement of the full Stokes vector of He I 10830 Å Authors: Rüedi, I.; Keller, C. U.; Solanki, S. K. Bibcode: 1996SoPh..164..265R Altcode: First observations of the full Stokes vector in the upper chromosphere are presented. The He I 10830 Å line, which has been shown to give reliable measurements of the line-of-sight component of the magnetic field vector, has been used for this purpose. It is shown that the difference between the appearance of chromospheric and photospheric magnetic structures observed close to the solar limb is largely due to the difference in height to which they refer and projection effects. The observations do suggest, however, that the magnetic field above sunspot penumbrae is somewhat more vertical in the chromosphere than in the photosphere. Title: The Influence of Sunspot Canopies on Magnetic Inclination Measurements in Solar Plages Authors: Solanki, Sami K.; Finsterle, Wolfgang; Rüedi, Isabelle Bibcode: 1996SoPh..164..253S Altcode: Sunspots are known to have large, low-lying magnetic canopies, i.e. horizontal magnetic fields overlying a field-free medium, that cover substantial fractions of active region plage. In this paper we consider the influence of such canopies on the inclination of plage magnetic fields. We find that for observations in spectral lines like 5250.2Å the neglect of a sunspot canopy when determining magnetic inclination angles of plage fields can introduce errors exceeding 5-10°. This is particularly true if the observations do not have high spatial resolution. Thus this effect may explain some of the measurements of substantially inclined fields in solar plages. Furthermore we find that the Fe I 15648 Å line is far superior in giving correct flux-tube inclinations in the presence of a sunspot magnetic canopy. Finally, the inversion of full Stokes profiles is shown to produce more reliable results than results obtained by considering only ratios of individual Stokes profile parameters. Title: Inversion of Stokes Vector Profiles in Terms of a 3-Component Model Authors: Bernasconi, P. N.; Solanki, S. K. Bibcode: 1996SoPh..164..277B Altcode: Various spectropolarimetric observations show peculiar Stokes profiles that reveal the coexistence of at least two magnetic components in the same resolution element. An example is given by observations of the full Stokes vector in a complex active region performed with the ZIMPOL I Stokes polarimeter. In order to deduce the physical parameters of the observed regions from such measured profiles, we have extended an existing inversion code, so that it can now fit the data with models composed of up to three different atmospheric components. Two of these components are magnetic and may possess different field strengths, field geometries, temperature stratifications, and velocity fields. The third component describes the field free atmosphere surrounding the magnetic features. Title: Flux Tube Dynamics in Active Stars Authors: Caligari, P.; Schüssler, M.; Solanki, S. K.; Schaerer, D.; Stix, M. Bibcode: 1996ApL&C..34...17C Altcode: No abstract at ADS Title: New constraints on gravity-induced birefringence Authors: Solanki, Sami K.; Haugan, Mark P. Bibcode: 1996PhRvD..53..997S Altcode: A wide class of gravitation theories predicts gravity-induced birefringence. For Moffat's NGT, the prototypical theory of this type, Gabriel, Haugan, Mann, and Palmer used the predicted gravitational birefringence and observations of solar polarization to constrain the Sun's nonsymmetric charge lsolar. We improve on this constraint by making use of improved knowledge of the solar source of polarization and of a refined analysis procedure. We obtain l2solar< (305 km)2. Title: PRD vs. CRD CaII K Stokes profiles from solar plage Authors: Bruls, J. H. M. J.; Solanki, S. K.; Uitenbroek, H. Bibcode: 1996ASPC..109..113B Altcode: 1996csss....9..113B No abstract at ADS Title: Solar and stellar magnetic flux tubes (review) Authors: Solanki, S. K. Bibcode: 1996IAUS..176..201S Altcode: No abstract at ADS Title: Observing stellar surface structure with the ESO-VLT interferometer Authors: von der Luhe, O.; Solanki, S.; Reinheimer, Th. Bibcode: 1996IAUS..176..147V Altcode: No abstract at ADS Title: Interesting lines in the infrared solar spectrum. III. A polarimetric survey between 1.05 and 2.50 μm. Authors: Rueedi, I.; Solanki, S. K.; Livingston, W.; Harvey, J. Bibcode: 1995A&AS..113...91R Altcode: We present a survey of Stokes I, V and Q in the near infrared spectral range between 1.05 and 2.50μm, based on Fourier transform spectrometer data obtained in a sunspot umbra and in an active region plage. We discuss the diagnostic potential of a number of lines for studying solar magnetic features. These include the Ti I multiplet near 22000A, the Fe I g=3 line at 15648.5A, He I 10830A, high excitation C I lines, as well as CO and CN lines. As an example, we provide the first estimate of the field strength gradient averaged over the height range covered by the photosphere and chromosphere. We also identify lines that show large Stokes V or Q amplitudes and analyze their profile asymmetries. In our J-band plage spectrum the Stokes V and Q profiles show an extremely large blue-red asymmetry, whose origin is not yet clear. As an incidental result we obtain the first measured value of the vertical gradient of the magnetic field strength in a plage. Averaged over approximately 1500-2000km it corresponds to roughly 0.6G/km. Title: Infrared lines as probes of solar magnetic features. XI. Structure of a sunspot umbra with a light bridge. Authors: Rueedi, I.; Solanki, S. K.; Livingston, W. Bibcode: 1995A&A...302..543R Altcode: We investigate the magnetic structure of an umbra and the light bridge it contains using spectra at 1.56μm. The magnetic field strength in the umbra reaches over 3500G on the first day of observation and 3200G on the second and third days, while the field strength in the light bridge is reduced by 1000-1400G relative to the nearby umbra. The magnetic vector is also found to be much more horizontal in the light bridge. There is evidence that the boundary between the strong and weak magnetic field is less than 1" wide, implying the presence of large currents there. At some locations a downflow of up to 1.5km/s is seen in the light bridge relative to the umbral material. Finally, we discuss the blending of the 1.5648μm line in sunspots and develop the depth of the OH 15650.7A line as a diagnostic of umbral temperatures. Title: Interesting lines in the infrared solar spectrum. II. Unblended lines between λ1.0 and λ1.8 μm. Authors: Ramsauer, J.; Solanki, S. K.; Biemont, E. Bibcode: 1995A&AS..113...71R Altcode: We list 603 spectral lines between 1.0 and 1.8μm that are judged to be relatively unblended from a visual inspection of spectra of the quiet sun. Much of the atomic data of relevance to studies of solar and stellar magnetism, convection and atmospheric structure are also provided. Particular attention is paid to blending by telluric lines. We determine the level of blending both in the presence and the absence of telluric lines. We also describe how telluric blends may be removed from spectra with high spectral resolution. Title: Book Review: Planetary systems : formation, evolution and detection / Kluwer, 1994 Authors: Solanki, S. K.; Švestka, Zdeněk; Svestka, Z. Bibcode: 1995SoPh..159..403S Altcode: 1995SoPh..159..403B No abstract at ADS Title: Empirical models of solar magnetic elements: constraints imposed by MgI Stokes profiles. Authors: Briand, C.; Solanki, S. K. Bibcode: 1995A&A...299..596B Altcode: Although the temperature structure of small-scale magnetic features in the lower and middle photosphere has been constrained quite well, there are still considerable uncertainties in the upper photospheric and lower chromospheric thermal structure. As a step towards an improvement of this situation we investigate, using a non-LTE analysis, the diagnostic capabilities of the Stokes I and V profiles of the Mg I b_2_ 517.3nm and the Mg I 457.1nm lines. We find that the V profile of the former line can constrain the magnetic element thermal and velocity structure near the temperature minimum, which goes beyond the capabilities of the commonly used Fe I and II lines. The λ457.1nm line, on the other hand, does not provide any additional information on its own. A comparison of synthetic profiles with plage and network Stokes I and V spectra confirms the findings of Bruls & Solanki (???) that the chromospheric temperature rise starts at a substantially lower height in magnetic elements than in the quiet Sun. Some of the ambiguities in previous empirical models of magnetic elements are also removed. We confirm that small-scale magnetic features are associated with larger line broadening velocities than the quiet Sun, particularly in the higher layers. Finally, the Mg I b_2_ line is revealed to be a direct diagnostic of the merging height of magnetic elements. Title: VizieR Online Data Catalog: Lines in the infrared solar spectrum (Ramsauer+, 1995) Authors: Ramsauer, J.; Solanki, S. K.; Biemont, E. Bibcode: 1995yCat..41130071R Altcode: We list 603 spectral lines between 1.0, 1.8um that are judged to be relatively unblended from a visual inspection of spectra of the quiet sun. Much of the atomic data of relevance to studies of solar and stellar magnetism, convection and atmospheric structure are also provided. Particular attention is paid to blending by telluric lines. We determine the level of blending both in the presence and the absence of telluric lines. We also describe how telluric blends may be removed from spectra with high spectral resolution. (4 data files). Title: The Evershed effect: rise and fall of the wave model. Authors: Buente, M.; Solanki, S. K. Bibcode: 1995A&A...297..861B Altcode: Adiabatic surface and trapped body waves have recently been proposed as the source of the Evershed effect. We show that such waves fail to simultaneously reproduce the main features of the Evershed effect in a set of spectral lines with different temperature sensitivities. This result sets constraints on all models that attempt to explain the Evershed effect on the basis of a wave-like correlation between temperature and velocity. In the photospheric layers in which the waves are expected to propagate, dissipative effects due to radiative diffusion are likely to damp the wave-induced temperature perturbations. We show, however, that in the line forming penumbral layers radiatively damped ("isothermal") waves shift spectral lines of neutral and ionized species in opposite directions. We conclude that, in spite of a number of highly attractive features, linear wave models have great difficulty in explaining the Evershed effect. Title: Infrared lines as probes of solar magnetic features. IX. MgI 12μm diagnostics of solar plage. Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 1995A&A...293..240B Altcode: The MgI 12.32μm line is a prime diagnostic for studying upper photospheric magnetic fields on the Sun. We study for the first time its behavior using flux-tube models with magnetic field strengths and filling factors characteristic of solar plage in order to establish the line's capabilities for measuring plage magnetic fields. We show that this line is only moderately sensitive to the range of temperatures expected for solar plages, including the chromospheric temperature rise. This low sensitivity considerably enhances this line as a magnetic field diagnostic for the upper photosphere of plages. We find that besides being highly sensitive to the field strength in flux tubes, the shape and Zeeman splitting of this line are almost equally sensitive to the magnetic filling factor at the flux-tube base. This combined sensitivity leads to the large range of profile shapes observed in plages. Not only can the 12μm emission lines be used to determine the sub-spatial-resolution distribution of flux-tube field strengths, but also the sub-spatial-resolution distribution of filling factors (i.e. they can provide an estimate of the "clumpiness" of the flux-tube distribution on a small scale). We also provide evidence that simple two-component modeling of magnetic flux tubes, without taking into account the height-dependence of the flux-tube size, may lead to erroneous conclusions in the case of the MgI 12μm lines, and that at least 1.5-D computations are required to adequately model these lines in magnetic flux tubes. Computations also show that standard flux-tube models simultaneously reproduce the observed splitting of the g=3 FeI lines at 1.5648μm (formed in the low photosphere) and at 525.02nm (middle photosphere), as well as of the 12.32μm emission line (upper photosphere). Our computations thus support the currently standard view that solar plages are composed of flux tubes with kGauss fields in the lower and mid photosphere, and that the thin-tube approximation is an adequate representation of the magnetic stratification in these flux tubes. Title: FTS Polarimetric Survey of the Infrared Solar Spectrum Between 1.0 and 2.5 MU M Authors: Ruedi, I.; Solanki, S. K.; Livingston, W.; Harvey, J. Bibcode: 1995ASPC...81..107R Altcode: 1995lahr.conf..107R No abstract at ADS Title: Oscillations in Active Plage Regions as Observed in 1.56 Micron Lines Authors: Muglach, K.; Solanki, S. K.; Livingston, W. C. Bibcode: 1995itsa.conf..387M Altcode: No abstract at ADS Title: Infrared lines as probes of solar magnetic features. VIII. MgI 12μm diagnostics of sunspots. Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson, M. Bibcode: 1995A&A...293..225B Altcode: Due to their large Zeeman sensitivity, the MgI lines at 12μm are important diagnostics of solar magnetism. The formation of their central emission features is now understood, enabling quantitative modeling and diagnostic application of these lines. We supply the first systematic analysis of solar MgI 12μm Stokes profiles employing detailed line-profile synthesis. We compute Stokes profiles of MgI 12.32μm for the quiet Sun, for sunspot penumbrae and for the extended ("superpenumbral") magnetic canopies surrounding sunspots. We use these computations to analyze recent MgI 12μm observations by Hewagama et al. (1993). Our results are the following: (1) -Saha-Boltzmann temperature sensitivity explains that the emission peaks are stronger in penumbrae than in the quiet Sun, and that they disappear in umbrae. (2) -The formation heights of the emission features are approximately the same in penumbrae and in the quiet Sun, namely τ_500_=~10^-3^. (3) -The simple Seares formula allows relatively accurate determinations of field strength and magnetic inclination. (4) -The observed excess broadening of the σ-component peaks compared with the π component in penumbrae is well explained by primarily horizontal, smooth radial variation of the magnetic field strength. Additional small-scale variations are less than {DELTA}B =~200G. (5) -The vertical field gradients dB/dz in penumbrae range from 0.7G/km to 3G/km; the larger gradients occur near the umbra, the smaller ones near the outer edge of the penumbra. (6) -The MgI 12μm lines are well-suited to measure the base heights of superpenumbral magnetic canopies. These heights range between 300km and 500km above τ_500_=1 out to twice the sunspot radius, in excellent agreement with determinations from other infrared lines. Title: Solar line bisectors in the infrared Authors: Puschmann, K.; Hanslmeier, A.; Solanki, S. K. Bibcode: 1995IAUS..176P.117P Altcode: No abstract at ADS Title: Observation of Surface Activity on Cool Giants with the VLT Interferometer Authors: von der Lühe, O.; Schüssler, M.; Solanki, S. K.; Caligari, P. Bibcode: 1995svlt.conf...94V Altcode: No abstract at ADS Title: Magnetic Field Measurements in the Infrared Authors: Solanki, S. K. Bibcode: 1995itsa.conf..341S Altcode: No abstract at ADS Title: Infrared lines as probes of solar magnetic features. X. HeI 10830A as a diagnostic of chromospheric magnetic fields. Authors: Rueedi, I.; Solanki, S. K.; Livingston, W. C. Bibcode: 1995A&A...293..252R Altcode: The need for a simple but quantitative diagnostic of upper chromospheric magnetic fields is keenly felt. We develop the HeI 10830A line as such a diagnostic. An application to observations of an active region allows us to compare the magnetic field in the upper chromosphere with the field in the underlying photosphere. In general, the magnetic field in the chromosphere is found to be significantly more homogeneous. We find that dB/dz in the umbra of a large sunspot (0.4-0.6G/km) is similar to other determinations of this quantity over an equivalent height range. Also, dB/dz decreases outwards in the spot. Thus, in the outer penumbra it has dropped to 0.1-0.3 G/km. A comparison of these values with the results of dB/dz measurements in the photosphere suggests that dB/dz decreases with height. We also find evidence for magnetic canopies near sunspots and for the conservation of magnetic flux with height in solar plages when averaged horizontally over a few arc s. Observations of complex Stokes V profiles at the neutral line in a sunspot penumbra (crossover effect) suggest that the upper chromospheric penumbral magnetic field is not fluted to the same extent as the photospheric field. The large line broadening of He I (up to 10km/s) is found to be due to motions which are largely field aligned. Title: Magnetic flux observed over entire solar disk Authors: Livingston, Willam; Solanki, Sami Bibcode: 1994AAS...18512306L Altcode: 1994BAAS...26.1523L A single channel Babock-type magnetograph, using the favorable Fe 1.5648 microns (g=3) line, reveals fields everywhere from equator to the polar limb. Good seeing is essential. Experimental setup includes a 2.5 arcsec image slicer as the input, the NIM modulator driven at 20Hz, and an exit slit of 0.20 Angstroms. Noise is 5 (10(-3) ) percent polarization with a time constant of 3 sec and is negligible. Presumably we mainly sense the intranetwork fields in agreement with S. Martin (1988). New results are that the fields (flux) may strengthen near the limb and that the best signal is obtained at line center. The latter could indicate unresolved doppler shifts or the transverse Zeeman effect. (Both circular and linear polarized light is detected by the modulator). Field strength will be deduced by a statistical analysis of signal vs. position on the line profile. Title: New contribution functions for Zeeman split spectral lines Authors: Solanki, S. K.; Bruls, J. H. M. J. Bibcode: 1994A&A...286..269S Altcode: The recently introduced emergent line radiation contribution function (Achmad + de Jager + Nieuwenhuijzen 1991; Gurtovenko + Sheminova + Sarychev 1991) is extended to the Stokes vector of Zeeman split spectral lines. This contribution function is compared to the line depresssion contribution function on the basis of an analytical relation and of numerical NLTE computations of a set of spectral lines. In general the two contribution functions give very similar results, although in some cases the one or the other of them is more suitable. Title: The deep layers of solar magnetic elements Authors: Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M.; Solanki, S. K. Bibcode: 1994A&A...285..648G Altcode: We compare self-consistent theoretical models of solar magnetic flux sheets with spectropolarimetric observations of a solar plage and a network region. Our observational diagnostics mainly provide information on temperature and magnetic field of the deep photospheric layers. They are used to constrain the two free parameters of the models, viz. width and initial evacuation of the flux sheets. We find that the width of flux sheets in the network is approximately 200 km, while it is 300-350 km in an active plage. The flux sheets turn out to be less evacuated than previously thought, so they have continuum intensities close to unity. Since these are average values, however, our results do not exclude the presence of either smaller and brighter or larger and darker magnetic structures. Title: Infrared lines as probes of solar magnetic features. 7: On the nature of the Evershed effect in sunspots Authors: Solanki, S. K.; Montavon, C. A. P.; Livingston, W. Bibcode: 1994A&A...283..221S Altcode: Stokes I and V profiles of 1.56 micron lines are observed in sunspots and their surroundings near the solar limb. An analysis of these observations confirms that the magnetic field of the sunspots continues beyond their visible boundaries in the form of an extensive, low-lying (supernumbral) canopy. We also find that the V profiles, which are only formed above the canopy base, exhibit the Evershed effect with line shifts of 0.5-2 km/s, while the non-magnetic material below the canopy is at rest. The deduced geometry of the flow outside the sunspot suggests a resolution of the long standing dispute about the existence or not of an Evershed flow outside the visible sunspot. In addition, the low density of the gas above the canopy base implies that the mass flux in the magnetic canopy is much smaller (approximately equals 10%) than the mass flux in the penumbra, where the 1.5 micron lines exhibit wavelength shifts of approximately 4 km/s. The present observations cast doubt on the standard interpretation of the photospheric Evershed effect in terms of a steady flow, in particular if it is driven by the siphon-flow mechanism. Title: 1.5 MU M Observations the Depth of Sunspot Penumbrae Authors: Solanki, S. K.; Ruedi, I.; Livingston, W.; Schmidt, H. U. Bibcode: 1994IAUS..154..471S Altcode: No abstract at ADS Title: Magnetic Field Strength and Filling Factor Sensitivity of the MgI 12 MU M Infrared Lines in Solar Plage Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 1994ASPC...68..294B Altcode: 1994sare.conf..294B No abstract at ADS Title: Flux Tube Shredding Its Infrared Signature Authors: Bunte, M.; Steiner, O.; Solanki, S. K.; Pizzo, V. J. Bibcode: 1994IAUS..154..459B Altcode: No abstract at ADS Title: Are Magnetic Features on epsilon ERI Cold or Hot? Authors: Rueedi, I.; Solanki, S. K.; Mathys, G.; Saar, S. H. Bibcode: 1994ASPC...64..465R Altcode: 1994csss....8..465R We analyse high-quality spectra of epsilon Eri with the aim of determining its magnetic and thermal parameters. From a careful analysis we conclude that it is not possible to separate the field strength and filling factor of this star based on data obtained in the visible spectral range. The data do suggest, however, that the magnetic field of the star is concentrated in cool, starspot-like structures. Title: How Accurate Are Stellar Magnetic Field Measurements? II. Analysis of Disk-Integrated Flux Tube Models Authors: Saar, S. H.; Buente, M.; Solanki, S. K. Bibcode: 1994ASPC...64..474S Altcode: 1994csss....8..474S We analyze disk-integrated line profiles generated in atmospheres embedded with fluxtubes to test the accuracy of simple radiative transfer (RT) analyses in determining stellar magnetic parameters. Title: Distribution of Magnetic Flux on the Surface of Rapidly Rotating Stars Authors: Caligari, P.; Schussler, M.; Stix, M.; Solanki, S. K. Bibcode: 1994ASPC...64..387C Altcode: 1994csss....8..387C No abstract at ADS Title: A Search for Sunspot Canopies Using a Vector Magnetograph Authors: Adams, M.; Solanki, S. K.; Hagyard, M. J.; Moore, R. L. Bibcode: 1994ASPC...64..342A Altcode: 1994csss....8..342A No abstract at ADS Title: New Light on the Heart of Darkness of the Solar Chromosphere Authors: Solanki, S. K.; Livingston, W.; Ayres, T. Bibcode: 1994Sci...263...64S Altcode: Solar carbon monoxide spectra indicate the existence of a cool (less than 4000 kelvin) component to the solar chromosphere coexisting with the hot, bright gas at 6000 to 7000 kelvin. However, both the existence and the location of the cool component have been controversial. New high-resolution spectra show that carbon monoxide goes into emission just beyond the limb, allowing it to be probed without photospheric contamination. The cool component has temperatures as low as 3000 to 3500 kelvin and appears to cover 50 to 85 percent of the quiet solar surface. There is a steep temperature rise to normal chromospheric temperatures at a height of 900 to 1100 kilometers. Large horizontal velocities are seen, suggesting that the cool component is maintained by the supersonic adiabatic expansion of upwelling gas in overshooting granules. Title: Some consequences of an uncombed and inhomogeneous penumbra Authors: Solanki, S. K.; Montavon, C. A. P. Bibcode: 1994ASIC..433..239S Altcode: No abstract at ADS Title: Preliminary properties of pores derived from 1.56 micron lines Authors: Muglach, K.; Solanki, S. K.; Livingston, W. C. Bibcode: 1994ASIC..433..127M Altcode: No abstract at ADS Title: Poster Proceedings from IAU Colloquium 143: The Sun as a Variable Star: Solar and Stellar Irradiance Variations Authors: Pap, J. M.; Frohlich, C.; Hudson, H. S.; Solanki, S. K. Bibcode: 1994svsp.coll.....P Altcode: 1994IAUCo.143P....P No abstract at ADS Title: The upper photosphere and lower chromosphere of small-scale magnetic features Authors: Solanki, S. K.; Bruls, J. H. M. J.; Steiner, O.; Ayres, T.; Livingston, W.; Uitenbroek, H. Bibcode: 1994ASIC..433...91S Altcode: No abstract at ADS Title: Properties of Magnetic Features from the Analysis of Near-Infrared Spectral Lines Authors: Solanki, S. K. Bibcode: 1994IAUS..154..393S Altcode: No abstract at ADS Title: Must the Magnetic Field Saturate on Rapidly Rotating Stars? Authors: Solanki, S. K. Bibcode: 1994ASPC...64..477S Altcode: 1994csss....8..477S No abstract at ADS Title: Solar Magnetic Elements: Models Compared with Observations Authors: Grossmann-Doerth, U.; Knolker, M.; Schussler, M.; Solanki, S. K. Bibcode: 1994ASPC...68...96G Altcode: 1994sare.conf...96G No abstract at ADS Title: Solar line bisectors in the infrared. Authors: Puschmann, K.; Anslmeier, A.; Solanki, S. Bibcode: 1994AGAb...10..117P Altcode: No abstract at ADS Title: On the nature of the Evershed effect Authors: Solanki, S. K.; Montavon, C. A. P.; Livingston, W. Bibcode: 1994smf..conf..173S Altcode: No abstract at ADS Title: Magnetic fields in the upper chromosphere: He I 10830 Å as an almost ideal diagnostic Authors: Rüedi, I.; Solanki, S. K.; Livingston, W. C. Bibcode: 1994smf..conf..363R Altcode: No abstract at ADS Title: Flux tube dynamics in pre-main-sequence and giant stars. Authors: Caligari, P.; Schaerer, D.; Schüssler, M.; Solanki, S. Bibcode: 1994AGAb...10...92C Altcode: No abstract at ADS Title: MgI 12 μm diagnostics of sunspot penumbrae Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson, M. Bibcode: 1994smf..conf..191B Altcode: No abstract at ADS Title: Irradiance Effects of Small-Scale Magnetic Fields on the Sun Authors: Solanki, S. K. Bibcode: 1994svsp.coll..226S Altcode: 1994IAUCo.143P.226S No abstract at ADS Title: Surface waves: Origin of the Evershed phenomenon? Authors: Bünte, M.; Solanki, S. K. Bibcode: 1994smf..conf..179B Altcode: No abstract at ADS Title: A Search for Sunspot Canopies Using a Vector Magnetograph Authors: Adams, M.; Solanki, S. K.; Hagyard, M.; Moore, R. L. Bibcode: 1993SoPh..148..201A Altcode: Using a magnetograph, we examine four sunspots for evidence of a magnetic canopy at the penumbra/photosphere boundary. The penumbral edge is determined from the photometric intensity and is defined to correspond to the value of the average intensity minus twice the standard deviation from the average. From a comparison of the location of this boundary with the location of contours of the vertical and horizontal components of the magnetic field, we conclude that the data are best represented by canopy-type fields close to all four sunspots. There is some evidence that the magnetic inclination in the canopies is 5°-15° with respect to the horizontal and that the canopy base height lies in the middle/upper photosphere. The observations further suggest that the magnetic canopy of a sunspot begins at its outer penumbral boundary. Title: Evidence for siphon flows with shocks in solar magnetic flux tubes Authors: Degenhardt, D.; Solanki, S. K.; Montesinos, B.; Thomas, J. H. Bibcode: 1993A&A...279L..29D Altcode: We synthesize profiles of the infrared line Fe I 15648.5 A (g = 3) for a recently developed theoretical model of siphon flows along photospheric magnetic loops. The synthesized line profiles are compared with the observations from which Rueedi et al. (1992) deduced the presence of such flows across the neutral line of an active region plage. This comparison supports the interpretation of Rueedi et al. (1992). It also suggests that the average footpoint separation of the observed loops carrying the siphon flow is 8-15 sec and that the siphon flow experiences a standing tube shock in the downstream leg near the top of the arch. Title: Infrared lines as probes of solar magnetic features. VI. The thermal-magnetic relation and Wilson depression of a simple sunspot Authors: Solanki, S. K.; Walther, U.; Livingston, W. Bibcode: 1993A&A...277..639S Altcode: We analyse the relationship between the temperature and the magnetic vector, as derived from 1.56 μm spectra, in a simple sunspot. Due to the high Zeeman sensitivity of the = 3, 1.56 μm line, we can study this relationship throughout the sunspot. We confirm the field-strength, B, vs. temperature, T, relationship found by Kopp & Rabin (1992). In addition, we also find a linear relation between the magnetic inclination angle, γ‧, and T. An analysis based on the assumption of magnetohydrostatic force balance gives an estimate of the Wilson depression, ZW, throughout the sunspot. Our analysis supports a jump in ZW of 200-500 km at the umbral boundary and agrees with a relatively constant ZW in the penumbra. In addition, we constrain various gradients of magnetic parameters and judge the strength of magnetic curvature forces. For example, we set tight limits on the vertical gradient of γ‧, the magnetic inclination angle to the vertical, in the penumbra. Finally, we discuss the consequences for, e.g., the model proposed by Sanchez Almeida & Lites (1992) to explain the broad-band circular polarization measured in sunspots. Title: Uncombed fields as the source of the broad-band circular polarization of sunspots. Authors: Solanki, S. K.; Montavon, C. A. P. Bibcode: 1993A&A...275..283S Altcode: We investigate the production of broad-band circular polarization (BBC) of sunspots using a model inspired by the discovery of small-scale variations of magnetic inclination in sunspot penumbrae. The proposed model is a simple representation of a horizontal magnetic flux tube embedded in an inclined magnetic field, with a flow in either one or both components (Evershed flow). This model is shown to produce a sizable BBC without requiring substantial net vertical gradients of the magnetic vector. It reproduces the BBC observations in a qualitatively correct and quantitatively acceptable manner for reasonable values of its free parameters. In addition, our model also explains the crossover effect observed in sunspot penumbrae. The fits to the observations support the presence of a larger velocity in the horizontal magnetic filaments than in the inclined field. We point out that the BBC is complementary to high resolution images in that it diagnoses the height variation of the small-scale structure of sunspot magnetic fields. By combining the results from both techniques we outline the 3-D fine-scale structure of the magnetic field in the photospheric layers of sunspot penumbrae. The most important result is that the two differently inclined components of the field do not simply form vertical sheets, but rather must have a horizontal interface near the level of line formation. Title: Surface waves as the origin of the Evershed phenomenon Authors: Bunte, M.; Darconza, G.; Solanki, S. K. Bibcode: 1993A&A...274..478B Altcode: We investigate the spectral signature of magnetoacoustic-gravity surface waves (MAGS-waves), which we expect to exist at the interface between the lower boundary of the magnetic field of the sunspot penumbra and the non-magnetic gas below. MAGS-waves have a number of properties that make them attractive candidates for explaining the photo spheric Evershed effect: 1. Since they transport only energy but no mass, they resolve the problem of mass conservation at the outer penumbral boundary. 2. Since they are restricted to magnetic interfaces, they are only (or dominantly) present in the penumbra and absent in the umbra. 3. The phase relation between the temperature, the horizontal, and vertical velocity perturbations leads to line shifts and asymmetries in observations at the limb, but none at disc centre. 4. The amplitude of the velocity perturbation drops approximately exponentially with height, in good agreement with observations. 5. Waves travelling in opposite directions with respect to the observer produce oppositely directed line shifts and asymmetries, making it straightforward to explain the opposite shifts and asymmetries observed in the limb-side and centre-side penumbra. Title: The chromospheric temperature rise in solar magnetic flux tubes Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 1993A&A...273..293B Altcode: We set constraints on the location and steepness of the chromospheric temperature rise in solar magnetic flux tubes by synthesizing Stokes V profiles in non-local thermodynamic equilibrium (NLTE) of a set of "clean" Fe I and Fe II lines of different strengths. We compute profiles for several flux-tube models of the magnetic features within solar network and plage. Comparison with observed V profiles indicates that both in network and plage the chromospheric temperature rise starts at larger optical depth within the flux tubes than in the surrounding non-magnetic atmosphere. The exact onset cannot be determined with the present set of lines because they cannot easily distinguish between the location and initial steepness of the temperature rise. Assuming a similar T(τ) gradient as in quiet-Sun models, the chromosphere sets in 200 - 300 km deeper in flux tubes than in the quiet Sun. The similarity between the plage and network results suggests that in the low chromosphere the heating per flux tube is almost independent of magnetic filling factor. Title: Smallscale Solar Magnetic Fields - an Overview Authors: Solanki, Sami K. Bibcode: 1993SSRv...63....1S Altcode: An overview is given of the observational and the theoretical methods used to investigate solar magnetic fields. It includes an introduction to the Stokes parameters, their radiative transfer in the presence of a magnetic field, and empirical techniques used to measure various properties of solar magnetic features, such as the strength and direction of the magnetic field, magnetic flux, temperature, velocity, size and lifetime. The MHD equations are introduced and some of the most common simplifications used to describe solar magnetic features are outlined. The application of these techniques to small-scale magnetic features is surveyed. The results of empirical and theoretical investigations of small-scale solar magnetic features are reviewed. Current views on their magnetic structure, thermal stratification, velocity field, size, distribution and evolution are presented. Finally, some open questions concerning small-scale solar magnetic fields are listed. Title: Centre-to-limb variation of the Stokes V asymmetry in solar magnetic flux tubes Authors: Buente, M.; Solanki, S. K.; Steiner, O. Bibcode: 1993A&A...268..736B Altcode: The center-to-limb variation (CLV) of synthetic Stokes V line profiles of the spectral lines Fe 15250.22 A and Fe I 5083.35 A is computed and compared with observations. The basic elements of the hydromagnetic model used to calculate the Stokes V profiles consist of a vertical cylindrical flux tube surrounded by a field-free plasma in stationary motion with a downflow along the tube boundary and an upflow further away from it. It is shown that an array of such magnetic flux tubes reproduces the peculiar observed CLV of the Stokes V asymmetry, in particular the sign reversal of the asymmetry near the limb. We determine a minimum number of model components needed for the reproduction of the Stokes V area asymmetry and study the influence of various free model parameters on it. Satisfactory agreement between synthetic and observed Stokes V profiles can only be achieved by including a considerable amount of realism into the model such as a granular temperature-velocity correlation. This suggests that the basic picture of magnetic structures in the quiet solar network and active region plages is correct. Title: Field Strength vs. Temperature Relation and the Structure of Sunspots Authors: Solanki, S. K.; Walther, U.; Livingston, W. Bibcode: 1993ASPC...46...48S Altcode: 1993IAUCo.141...48S; 1993mvfs.conf...48S No abstract at ADS Title: Siphon Flow Across the Magnetic Neutral-Line of an Active Region Authors: Solanki, S. K.; Ruedi, I.; Rabin, D. Bibcode: 1993ASPC...46..534S Altcode: 1993IAUCo.141..534S; 1993mvfs.conf..534S No abstract at ADS Title: Are sunspot penumbrae deep or shallow? Authors: Solanki, S. K.; Schmidt, H. U. Bibcode: 1993A&A...267..287S Altcode: From the strength and inclination of the magnetic field measured across large symmetric sunspots, we estimate the fraction of the total magnetic flux of the sunspot passing through the solar surface in the penumbra. It is found that on average approximately 1/2-2/3 of the total magnetic flux of the spot emerges in its penumbra. Sunspot penumbrae are therefore deep, i.e. the tau = 1 level does not correspond to the lower magnetic boundary of the spot in its penumbra (except perhaps near its outer edge). Furthermore, the analyzed data do not support the passage of any significant amount of magnetic flux through the solar surface (in either direction) at or beyond the edge of the sunspot. The observations support models of the sunspot magnetic field which are bounded by a relatively sharp current sheet. Evidence for a substantial deviation from a potential field in the penumbra is found in the analyzed symmetric sunspots. Finally, at the height of line formation the field strength averaged over the whole umbra of all the analyzed sunspots is approximately 2250 G, while the field strength averaged over the whole sunspot is roughly 1350 G. The latter value is similar to the field strength measured in small-scale magnetic features. Title: Evershed Effect in Sunspots and Their Canopies Authors: Solanki, S. K.; Montavon, C.; Livingston, W. Bibcode: 1993ASPC...46...52S Altcode: 1993IAUCo.141...52S; 1993mvfs.conf...52S No abstract at ADS Title: Why rapid rotators have polar spots. Authors: Schuessler, M.; Solanki, S. K. Bibcode: 1992A&A...264L..13S Altcode: Starspots on magnetically active, cool stars preferentially appear near the poles. We suggest that this preference of high latitudes is due to the rapid rotation to these stars which leads to a dominance of the Coriolis force over the buoyancy force in the dynamics of magnetic flux tubes. As a consequence, flux tubes erupting from the deep parts of the stellar convection zone follow a path nearly parallel to the axis of rotation and thus necessarily surface at high latitudes, unless their initial field strength exceeds a critical value for which buoyancy becomes dominant again. It is shown that for stars with rotation periods below about 10 days flux tubes with such large field strength (of the order of 10 exp 6 G) cannot be formed and stored since they are unstable with respect to non-axisymmetric disturbances. Consequently, magnetically active stars with rapid rotation exhibit magnetic flux eruption at high latitudes and polar starspots. Title: Infrared lines as probes of solar magnetic features. V - The magnetic structure of a simple sunspot and its canopy Authors: Solanki, S. K.; Rueedi, I.; Livingston, W. Bibcode: 1992A&A...263..339S Altcode: The Zeeman Lande g = 3 Fe I line at 15648 A is used here to determine the strength and inclination of the magnetic field of a simple, relatively symmetric sunspot. The largest field strength observed in the sunspot is approximately 3100 G. The field strength at different points along the outer penumbral edge varies beween 800 and 1000 G. The angle of inclination at the outer penumbral boundary is close to 80 deg. A low-lying, almost horizontal superpenumbral magnetic canopy best reproduces the observations. The magnetic field strength in the canopy is consistent with a magnetic monopole. Evidence of small magnetic elements underlying the superpenumbral canopy is presented. Title: Infrared lines as probes of solar magnetic features. I - A many-line analysis of a network region Authors: Muglach, K.; Solanki, S. K. Bibcode: 1992A&A...263..301M Altcode: The results are presented of an analysis of a Fourier transform spectrometer Stokes I and V spectrum in the H band of a solar network region. From a many-line statistical analysis of the measured Stokes F profiles, it is concluded that there is no evidence for s systematic up or downflow greater than 200-300 m/s within the magnetic element of the observed region. The Stokes V profiles in the H band show a blue-red asymmetry having the same sign as in the visible, but a smaller magnitude. The lines with g(eff) greater than about 1.5 are more or less completely Zeeman split in the observed region, and only lines with g(eff) values well below unity can be considered to be in the weak-field regime. An analysis based on radiative transfer calculations of 16 selected spectral lines in a flux tube model shows that a thin tube with a field strength of 2050 G at tau (1.6) = 1 , corresponding to 1500 G at z = 0, fits the profiles best. Title: Infrared lines as probes of solar magnetic features. II - Diagnostic capabilities of Fe I 15648.5 A and 15652.9 A Authors: Solanki, S. K.; Rueedi, I. K.; Livingston, W. Bibcode: 1992A&A...263..312S Altcode: The properties of two Fe I lines at 15648.5 A and 15652.9 A in the infrared H-band are investigated using numerical radiative transfer calculations. It is shown that using this combination of a Lande g = 3 and a g(eff) = 1.53 line it is possible to measure the field strength in solar magnetic features simply and with great accuracy. The g = 3 line alone allows field strengths as low as 300-600 G to be measured. By forming the line ratio between the V profiles of these lines it is, in principle, possible to measure field strengths as low as 100 G, thus achieving a sensitivity otherwise only possible with the emission lines near 12 microns. Other diagnostic capabilities of these lines are also analyzed. Title: Infrared lines as probes of solar magnetic features. III - Strong and weak magnetic fields in plages Authors: Rueedi, I.; Solanki, S. K.; Livingston, W.; Stenflo, J. O. Bibcode: 1992A&A...263..323R Altcode: The diagnostics developed by Zayer et al. 1989 are applied to 27 Stokes V spectra of solar plates. The 1.5 micron lines at 15648 A and 15652 A are used to measure magnetic field strengths between 400 G and 1700 G at z = 0 in active region plages with an accuracy of up to 2-3 percent. The results confirm the accuracy of the line-ratio technique of Stenflo (1973). It is estimated that the fraction of net magnetic flux in strong-field form is close to 90 percent, supporting the view that the small-scale magnetic fields are concentrated very efficiently. The data clearly contradict the claims of Zirin and Popp (1989) based on 12 micron lines that there is at most indirect evidence for kG fields in solar plages. They also contradict the large fraction of weak-field flux reported by Del Toro et al. (1990). Title: Continuum brightness of solar magnetic elements Authors: Solanki, S. K.; Brigljevic, V. Bibcode: 1992A&A...262L..29S Altcode: Ratios between the Stokes V profiles of C I lines and Fe II lines are used to determine the temperature of the continuum-forming layers of solar magnetic features. From the derived temperature stratification the continuum intensity of the spatially unresolved magnetic features is obtained independently of spatial resolution. First results suggest that whereas magnetic features have, on average, a brighter continuum than the quiet sun in a region of small magnetic filling, they are darker in a region with large filling factor. Consequences for flux-tube sizes and the interpretation of the global solar luminosity variation are discussed. Title: Infrared lines as probes of solar magnetic features. IV - Discovery of a siphon flow Authors: Rueedi, I.; Solanki, S. K.; Rabin, D. Bibcode: 1992A&A...261L..21R Altcode: Spectra of two neighboring IR lines, Fe I 15648.5 A and Fe I 15652.9 A, are analyzed. The spectra were obtained with an IR array detector in active region plages with the entrance slit of the spectrograph placed across the polarity-inversion (neutral) line. Near the neutral line the positive polarity field is weaker (about 1200 G) and shows an upflow of up to 2 km/s, while the negative polarity field is stronger (about 1500 G) and exhibits a downflow of up to 1 km/s. This configuration corresponds to the expected signature of a siphon flow along a loop connecting flux tubes across the neutral line. Title: Waves in solar magnetic flux tubes - The observational signature of undamped longitudinal tube waves Authors: Solanki, S. K.; Roberts, B. Bibcode: 1992MNRAS.256...13S Altcode: Linear calculations of undamped magnetoacoustic waves utilizing the thin-tube approximation are presented and their influence on the Stokes I and V line parameters of a set of photospheric spectral lines is analyzed. To improve the observational diagnostics of flux tube waves, the dependence of various line parameters, such as the Stokes V zero-crossing wavelength, Stokes V amplitude, width area and asymmetry, on wave parameters is studied. It is determined that it is possible to set a lower limit on the energy flux transported by longitudinal tube waves by measuring time series of the Stokes V zero-crossing wavelength of a selected group of lines. Title: Polarimetry with an imaging FTS. Authors: Stenflo, J. O.; Solanki, S. K. Bibcode: 1992ESASP.344..197S Altcode: 1992spai.rept..197S The concept of an imaging FTS polarimeter based on piezoelastic modulation and direct demodulation in partially masked CCDs is outlined. To illustrate its principle the currently operational FTS polarimeter at the McMath telescope as well as the detection scheme of the ETH polarimeter, based on partially masked CCD cameras, are briefly described. Title: Small-scale physics of convection and magnetic fields. Authors: Solanki, S. K. Bibcode: 1992ESASP.344...27S Altcode: 1992spai.rept...27S An overview is given of the current knowledge of the properties and the physics of small-scale magnetic features, magnetoconvection and convection. This review concentrates on the theoretical aspects. Some examples are given of features or processes which require very high spatial resolution to be observed. Title: CA II K Line Diagnostics of Two Dimensional Models of the Solar Chromosphere Authors: Solanki, S. K.; Buente, M.; Steiner, O.; Uitenbroek, H. Bibcode: 1992ASPC...26..294S Altcode: 1992csss....7..294S No abstract at ADS Title: Magnetic Fields in Stellar Photospheres (Invited Review) Authors: Solanki, S. Bibcode: 1992ASPC...26..211S Altcode: 1992csss....7..211S No abstract at ADS Title: How Accurate are Stellar Magnetic Field Measurements? Authors: Saar, S. H.; Solanki, S. K. . Bibcode: 1992ASPC...26..259S Altcode: 1992csss....7..259S We study the accuracy of two magnetic analysis methods by applying them to a large grid of lines computed using detailed 1.5-D radiative transfer (RT) calculations in a variety of stellar atmospheres imbedded with fluxtubes. We find errors of < +/- 20% when using the more realistic analysis method if the fluxtube and external quiet atmospheres are similar; errors can increase substantially if this condition is relaxed. Title: The Chromospheric Temperature Rise in Fluxtubes Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 1992ASPC...26..512B Altcode: 1992csss....7..512B No abstract at ADS Title: Strong; Weak Solar Magnetic Fields Authors: Solanki, S. K.; Ruedi, I.; Livingston, W. C.; Stenflo, J. O. Bibcode: 1992ASPC...26..262S Altcode: 1992csss....7..262S No abstract at ADS Title: Two-dimensional models of the solar chromosphere. I - The CA II K line as a diagnostic: 1.5-D radiative transfer Authors: Solanki, S. K.; Steiner, O.; Uitenbroeck, H. Bibcode: 1991A&A...250..220S Altcode: Ca II K line calculations for a 5-level atom and partial frequency redistribution along multiple rays passing through 2D MHD models of magnetic flux tubes in the solar atmosphere are presented. Temperature stratifications corresponding to various empirical chromospheric models are incorporated into the models. The influence of a number of model parameters on individual and spatially averaged line profiles is considered in detail. The profiles are also compared with observations. It is found that the present models can, at least qualitatively, reproduce a wide variety of observations if the temperature within the magnetic component lies between that of model F of Vernazza et al. (1981) and of model VALP of Ayres et al. (1986) and if the temperature in the nonmagnetic part of the atmosphere corresponds to that of the COOLC model of Ayres et al. Observations that are consistent with this model are presented. Title: The effect of non-linear oscillations in magnetic flux tubes on Stokes V asymmetry Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1991A&A...249..239G Altcode: The present knowledge on, and interpretation of, the asymmetry of Stokes V of spectral lines thought to be formed in and around magnetic flux tubes is reviewed. Crude models of nonlinear oscillations in the flux tube were investigated in order to explain the observed values of Stokes V amplitude and area asymmetry. It was found that flux tube models with quasi-oscillatory motions, consisting of a slow upflow and a rapid downflow in the magnetic region, as well as a downflow in the nonmagnetic surroundings, may reproduce the observations, both asymmetries and zero-crossing shift of Stokes V. Title: Flux tube diagnostics based on the Stokes V profiles of infrared H-band lines. Authors: Muglach, K.; Solanki, S. K. Bibcode: 1991sopo.work..489M Altcode: Stokes V profiles of the unblended Fe I lines in the wavelength range 1.5 - 1.8 μ (H-band) observed in the solar network are analysed. The data are consistent with an absence of stationary flows larger than 0.3 km s-1. The infrared Stokes V profiles are asymmetric in the same sense as lines in the visible, but tend to be of smaller magnitude, in particular the area asymmetry. The widths of the Stokes V profiles are consistent with kG field strengths. Finally, model calculations allow an upper limit of 2 to be set on the continuum intensity of small magnetic features relative to the quiet Sun. Title: Stokes profile formation heights in solar magnetic flux tubes. Authors: Larson, B.; Solanki, S. K.; Grossmann-Doerth, U. Bibcode: 1991sopo.work..479L Altcode: The formation heights of magnetically split lines in small solar magnetic flux tubes are investigated. In particular, the authors are interested in how the heights of formation depend on different flux tube parameters, like magnetic field strength, temperature and temperature gradient, as well as on line parameters, such as line strength, excitation potential and ionization stage. Title: Filigree and flux tube physics. Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K. Bibcode: 1991sia..book..890S Altcode: This review covers the properties of the small-scale (outside sunspots) magnetic field from observational and theoretical points of view. Special emphasis is put on the physics of small isolated magnetic concentrations (flux tubes). Topics discussed include the basic observational properties, the origin and disappearance of the small-scale field, the properties of individual magnetic elements, their influence on the solar irradiance and their interaction with solar oscillations. Title: Inversion of Stokes V profiles: The structure of solar magnetic fluxtubes and its dependence on the filling factor. Authors: Keller, C. U.; Solanki, S. K.; Stenflo, J. O.; Zayer, I. Bibcode: 1991sopo.work..387K Altcode: The authors present results from an inversion procedure that derive the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of magnetic fluxtubes from observed Stokes V spectra near disk center. In a first step the inversion has been applied to 10 Fe I and Fe II Stokes V profiles of a plage and a network region to obtain reliable models of the fluxtubes. In a second step the dependence of the fluxtube structure on the filling factor has been studied with spectra of 3 Fe I lines from 23 different regions based on the models derived in the first step. Title: Center-to-limb variation of the Stokes V asymmetry in solar magnetic flux tubes. Authors: Bünte, M.; Steiner, O.; Solanki, S. K. Bibcode: 1991sopo.work..468B Altcode: The center-to-limb-variation of synthetic Stokes V line profiles of the spectral line Fe I 5250.22 Å is presented and compared with observations. These synthetic profiles are calculated using models that contain the main features of the current basic pictures of small scale magnetic fields on the Sun. Title: Magnetic Field Measurements on Cool Stars. Authors: Solanki, S. K. Bibcode: 1991RvMA....4..208S Altcode: No abstract at ADS Title: Dependence of the properties of solar magnetic flux tubes on filling factor. II - Results of an inversion approach Authors: Zayer, I.; Stenflo, J. O.; Keller, C. U.; Solanki, S. K. Bibcode: 1990A&A...239..356Z Altcode: The dependence of the properties of solar magnetic elements on the magnetic filling factor is studied using Stokes V spectra of three lines observed near the center of the solar disk. The inversion technique developed by Keller et al. (1990) is applied to three neighboring spectral lines, and the average temperature difference, the magnetic field strength, and the nonstationary velocity in the relevant line-forming layers are quantitatively determined. Quantitative evidence is provided for the dependence of the temperature within flux tubes on the amount of magnetic flux. The flux tubes are found to become cooler and their field strengths, at a given optical depth, to become larger as the filling factor increases. The presence of kilogauss field strengths within flux tubes is reconfirmed. Title: The influence of vertical magnetic field gradients on the measured field strength and filling factor in late-type stars. Authors: Grossmann-Doerth, U.; Solanki, S. K. Bibcode: 1990A&A...238..279G Altcode: The influence of a vertical gradient of the magnetic field in late type stars on the measurement of magnetic field strengths and filling factors is studied. Line profiles and contribution functions of spectral lines with large Lande factors are calculated in model stellar atmospheres in the presence of a magnetic field with a vertical gradient. It is found that the four lines, which have often been used to measure solar and stellar magnetic fields in the past, are formed at heights sufficiently different to account for differences in apparent field strength of up to 1000 G if the fields are assumed to be similarly structured as in the sun. Thus it is concluded that the vertical gradient of the field may contribute to a good part of the discrepancy between different published measurements of field strength and filling factor in the K2 dwarf Epsilon-Eri. Title: Solar magnetic field strength determinations from high spatial resolution filtergrams Authors: Keller, C. U.; Stenflo, J. O.; Solanki, S. K.; Tarbell, T. D.; Title, A. M. Bibcode: 1990A&A...236..250K Altcode: Circularly polarized images with high spatial resolution (better than 1 arcsec) of a solar active region, obtained with a tunable filter in the wings of Fe I 5247.1 A and Fe I 5250.2 A, have been analyzed in terms of the magnetic line ratio technique introduced by Stenflo (1973). Whenever a measurable amount of polarization is present, the distribution of the observed magnetic-line ratio is compatible with a unique value, which is randomly blurred by noise due to the photon statistics, the CCD camera, and atmospheric distortions. There is no need for a distribution of field strengths to explain the observed distribution of the magnetic line ratio. Consequently, the observations are compatible with a unique magnetic field strength in solar small-scale magnetic elements of about 1000 G at the level of line formation. For a thin flux tube, this corresponds to a field strength of approximately 2000 G at the level of continuum formation, which is in excellent agreement with previous field strength determinations from low spatial resolution spectra (4-10 arcsec). Title: How magnetic is the solar chromosphere? Authors: Solanki, S. K.; Steiner, O. Bibcode: 1990A&A...234..519S Altcode: The lower solar chromosphere is thought to have a very inhomogeneous temperature structure, with hot magnetic flux tubes surrounded by cool (T less than 4000 K) nonmagnetic gas (Ayres et al., 1986). The effect of such a thermally bifurcated atmosphere on the structure of the magnetic field in the chromosphere is considered. It is shown that magnetic flux tubes expand much more rapidly if the atmosphere is thermally bifurcated than if it is homogeneous. They merge and form a magnetic canopy with an almost horizontal base which does not exceed approximately 800-1000 km above tau = 1, irrespective of the magnetic filling factor. Hence the middle and upper chromosphere is filled with a magnetic field almost everywhere on the sun. The consequences of this result both for the sun and for other late type stars are discussed. Title: Structure of solar magnetic fluxtubes from the inversion of Stokes spectra at disk center Authors: Keller, C. U.; Steiner, O.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1990A&A...233..583K Altcode: The paper presents an inversion procedure that derives the temperature stratification, the turbulent velocity, and the magnetic field strength of the photospheric layers of small-scale magnetic fields from observed Stokes V spectra and the continuum intensity. The inversion is based on the determination of a small number of model flux parameters by a nonlinear least squares fitting algorithm. The minimization of the sum of the squared differences between observed and synthetic observables makes it possible to determine the temperature stratification and the magnetic field strength. Title: Solar line asymmetries and the magnetic filling factor Authors: Brandt, P. N.; Solanki, S. K. Bibcode: 1990A&A...231..221B Altcode: Results are presented of an analysis of high-quality Fourier transform spectrometer spectra obtained in facular regions near solar disk center having different amounts of magnetic flux or, equivalently, different filling factors, alpha. The line parameters (width, equivalent width, depth, bisector shape, and wavelength) of 32 spectral lines and their variation with alpha are studied. Line depth is seen to decrease with increasing filling factor for all the observed lines, while the line width increases, even after compensation has been made for the Zeeman broadening. For strong or temperature-insensitive lines the equivalent width stays practically unchanged, but decreases for weak lines. The findings are qualitatively explained by a decrease in the contrast between the upflow and downflow components of the nonmagnetic part of the atmosphere in active regions (abnormal granulation) coupled to either a decrease in the average temperature of the nonmagnetic part or a significant increase in small-scale velocity. Title: Interesting lines in the infrared solar spectrum between 1.49 and 1.8 microns Authors: Solanki, S. K.; Biemont, E.; Muerset, U. Bibcode: 1990A&AS...83..307S Altcode: Lists are presented of two groups of spectral lines interesting for the study of solar and stellar magnetic fields, convection and atmospheric structure and appearing in the IR solar spectrum between 1.49 and 1.8 microns. The first group contains 130 spectral lines judged to be relatively unblended from a visual inspection of the spectra, while the second is composed of 30 lines exhibiting large Stokes V amplitudes in a quiet network, region. Some interesting aspects of these lines are discussed. In particular, it is pointed out that blending due to magnetically unsplit spectral lines, such as telluric lines, can seriously affect all four Stokes parameters. Title: Flux tube diagnostics based on infrared H-band lines. Authors: Muglach, K.; Solanki, S. K. Bibcode: 1990AGAb....5...45M Altcode: No abstract at ADS Title: Empirical Models of Photospheric Flux Tubes Authors: Solanki, S. K. Bibcode: 1990IAUS..138..103S Altcode: No abstract at ADS Title: The Observational Signature of Flux Tube Waves and an Upper Limit on the Energy Flux Transported by Them Authors: Solanki, S. K.; Roberts, B. Bibcode: 1990IAUS..138..259S Altcode: No abstract at ADS Title: Waves in solar photospheric flux tubes and their influence on the observable spectrum Authors: Solanki, S. K.; Roberts, B. Bibcode: 1990GMS....58..181S Altcode: Linear calculations of undamped magnetoacoustic waves in thin solar magnetic flux tubes are presented and their influence on the Stokes V profiles of various iron lines is studied. This is a necessary first step for the diagnostics of the properties of flux tube waves, in particular the amount of energy transported by them into the upper atmosphere. It is shown that, with sufficiently high spatial resolution, observations can distinguish between standing and propagating waves on the basis of line parameters of photospheric spectral lines alone. Particular attention is given to exploring quantitative diagnostics for the wave amplitude, since it is currently the most important unknown parameter determining the energy flux carried by the waves. It is found that although this parameter can be derived relatively simply if the thermal fluctuations produced by the wave are ignored (i.e., for an isothermal wave), the task becomes much more complex for the more realistic case of a coupled variation of temperature and velocity. Title: Quantitative explanation of Stokes V asymmetry in solar magnetic flux tubes Authors: Solanki, S. K. Bibcode: 1990GMS....58..185S Altcode: A 2D flux-tube model of a solar magnetic element is presently used to calculate Stokes profiles of four spectral lines with various properties. This model, which uses empirically-derived temperature and magnetic field strength values, is able to reproduce the asymmetry between the blue and red Stokes V wings in addition to other line parameters observed near the disk center in solar active-region plages. The Stokes V symmetry is a straightforward consequence of the current view of magnetic elements that are embedded in cool, downflowing intergranular lanes, in the presence of large-amplitude nonstationary mass motions within magnetic elements. Title: At what heights are spectral lines formed in solar magnetic flux tubes? Authors: Larsson, B.; Solanki, S.; Grossmann-Doerth, U. Bibcode: 1990nba..meet..169L Altcode: 1990taco.conf..169L Illustrative examples, taken from a study of the formation of magnetically split lines in solar magnetic flux tubes, are presented. A particular interest is: in how the heights of formation depend on different flux tube parameters, like the magnetic field strength, the temperature and temperature gradient, as well as on line parameters, such as line strength, excitation potential and ionization stage. The result should help to improve the construction of empirical models of magnetic features and provide a new understanding of the spectral diagnostics used in the study of the solar magnetic field. Title: The origin and the diagnostic capabilities of the Stokes V asymmetry observed in solar faculae and the network Authors: Solanki, S. K. Bibcode: 1989A&A...224..225S Altcode: The origin of the Stokes V asymmetry observed near disk center in solar active regions and in the quiet network is studied with the help of a two-dimensional flux tube model of solar magnetic elements. It is shown that the observed relative area and amplitude asymmetry, the zero-crossing wavelengths, and the width of four Fe I and II Stokes V profiles belonging to lines with widely different properties can be reproduced relatively well within the framework of the model incorporating certain features. The sensitivity of the Stokes V asymmetry to the thermal and velocity structure in and around solar magnetic elements is also demonstrated. It is the Stokes V line parameter most sensitive to the atmosphere in the vicinity of magnetic elements, and can also become a powerful diagnostic of the mass motions inside the magnetic elements. Values of the temperature and velocity in the immediate surroundings of these elements at the height of line formation are derived for the first time. Title: Stokes V asymmetry and shift of spectral lines Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1989A&A...221..338G Altcode: Further evidence is given for the interpretation of the observed unshifted and asymmetric Stokes V profiles in the solar atmosphere in terms of a magnetic flux concentration expanding with height and surrounded by a downflow. A general proof is given that a V profile originating in an atmosphere in which magnetic field and flow are spatially separated along the line of sight has a zero-crossing wavelength which is unshifted with respect to the rest wavelength of the line center. Heuristic considerations for the dependence of the asymmetry of Stokes V on Zeeman shift, Doppler shift, line width, and strength are described and confirmed by exploratory calculations. Title: The internal magnetic field structure of solar magnetic elements Authors: Solanki, S.; Zayer, I.; Stenflo, J. O. Bibcode: 1989hsrs.conf..409S Altcode: No abstract at ADS Title: The internal magnetic distribution and the diameters of solar magnetic elements. Authors: Zayer, I.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1989A&A...211..463Z Altcode: A diagnostic is proposed for the horizontal distribution and vertical gradient of the magnetic field with spatially unresolved solar magnetic elements. Radiative transfer calculations are conducted to model the magnetic field structure, which is fitted to observational data from a Fourier transform spectrometer. It is found that a vertical gradient of the magnetic field strength must be present in solar magnetic fluxtubes and that, in the lower photosphere, the field strength inside the fluxtube must be horizontally nearly constant. The magnetic field strengths and gradients derived at the disk center satisfy observations near 0.6 microns, if the appropriate model fluxtube size is chosen. Title: On the Analysis of Stokes J and V Profiles in the Infrared Authors: Solanki, S. K. Bibcode: 1989AGAb....3...20S Altcode: No abstract at ADS Title: Magnetic fields in late-type dwarfs : preliminary results from a multiline approach. Authors: Mathys, G.; Solanki, S. K. Bibcode: 1989A&A...208..189M Altcode: Preliminary results of a multiline approach to the diagnosis of magnetic fields in late-type dwarfs are presented. The spectra of fours stars observed with the Coude Echelle Spectrometer of the European Southern Observatory are analyzed using the Stenflo-Lindegren technique, which permits empirical relations to be obtained between various parameters characterizing a statistical sample of observed line profiles and the atomic quantities pertaining to the corresponding transitions. These empirical relations are first tested on solar and stellar data and then interpreted with the help of a simple model of the line formation, and constraints on the magnetic field properties (mean strength, filling factor, thermodynamic properties of the magnetic regions compared to the nonmagnetic parts of the stellar surface) are derived. Title: Magnetic fields in late-type dwarfs: Preliminary results from a multiline approach Authors: Mathys, G.; Solanki, S. K. Bibcode: 1988STIN...9019928M Altcode: Results of a multiline approach to the diagnosis of magnetic fields in late-type dwarfs are presented. The spectra of four stars observed with the Coude echelle spectrometer of the European Southern Observatory are analyzed using the Stenflo-Lindegren technique, which permits empirical relations to be obtained between various parameters characterizing a statistical sample of observed line profiles and the atomic quantities pertaining to the corresponding transitions. These empirical relations are tested on solar and stellar data and interpreted with a simple model of the line formation. Constraints on the magnetic field properties (mean strength, filling factor, thermodynamic properties of the magnetic regions compared to the nonmagnetic parts of the stellar surface) are derived. In spite of the crudeness of approximations, the results are compatible with those obtained with other methods. The advantages of the Stenflo-Lindegren technique over other approaches are sketched. Title: Unshifted, asymmetric Stokes V-profiles - Possible solution of a riddle Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1988A&A...206L..37G Altcode: We show that Stokes V-profiles originating in an atmosphere in which a magnetic field and a systematic velocity field are spatially separated along the line of sight are asymmetric with unshifted zero-crossings. Such a configuration is probably realized in the peripheral parts of magnetic flux concentrations in the solar photosphere: Since the magnetic field flares out with height, the line of sight traverses both a static magnetic region and a downdraft in the non-magnetic surroundings. V-profiles formed in these parts of the flux concentation will be strongly asymmetric but unshifted while the V-profiles from the central parts are symmetric and unshifted. The resulting mean V-profiles show the characteristics of the observed profiles, i.e. they are unshifted and asymmetric. Title: Interpretation of broad band circular polarization measurements using Stokes V spectra Authors: Muerset, U.; Stenflo, J. O.; Solanki, S. K. Bibcode: 1988A&A...204..279M Altcode: The wavelength dependence and the center to limb variation of the broad band circular polarization of solar active regions are determined by integrating over spectra with a large wavelength range obtained with a Fourier transform spectrometer (FTS). It is shown that the broad band circular polarization (BBC) is due mainly to the asymmetry of the Stokes V profiles. The approximate contributions of lines of different depths to the total broad band signal are analyzed. The diagnostic contents of BBC observations with low spatial resolution, such as those of Kemp et al. (1987), are discussed. These observations are reproduced with the help of simulated broad band polarization data obtained by integrating FTS spectra. It is shown that the spatial distribution of the net field on the solar surface (within a single large spatial resolution element) can affect the measured BBC signal considerably, and may even change its sign. Title: Contribution and response functions for Stokes line profiles formed in a magnetic field Authors: Grossmann-Doerth, U.; Larsson, B.; Solanki, S. K. Bibcode: 1988A&A...204..266G Altcode: Expressions defining the contribution functions of the 'line depression' Stokes profiles formed in a general magnetic field are derived. Such contribution functions are better suited to determining the heights at which the bound-bound transitions responsible for spectral lines are important than the contribution functions to the 'intensity' Stokes parameters defined by Van Ballegooijen (1985). Expressions defining response functions for both 'intensity' and 'line depression' Stokes parameters are also derived for an arbitrary atmosphere and magnetic field. A code for calculating the various Stokes contribution functions is described and some example calculations are presented. These clearly demonstrate the superior diagnostic value of the contribution functions to the 'line depression' Stokes profiles. Title: Can stationary velocity fields explain the Stokes V asymmetry observed in solar magnetic elements? Authors: Solanki, S. K.; Pahlke, K. D. Bibcode: 1988A&A...201..143S Altcode: A mechanism for producing the observed asymmetry between the amplitudes and areas of the blue and red wings of Stokes V profiles, which is based on velocity gradients, is explored. It is found that the velocity gradients do not produce a Stokes V area asymmetry in very weak lines. A stationary velocity gradient is neither able to reproduce the observed asymmetry of lines of different strengths and excitation potentials nor simultaneously reproduce the widths and asymmetries of strong lines. If the asymmetries of Fe I 5250.2 A, Fe I 6301.5 A, and Fe I 6302.5 A are reproduced, it is shown that their zero crossing wavelengths are too strongly redshifted as compared to observations. Title: Magnetic field strength in solar flux tubes - A model atmosphere independent determination Authors: Sanchez Almeida, J.; Collados, M.; del Toro Iniesta, J. C.; Solanki, S. K. Bibcode: 1988A&A...196..266S Altcode: The "line ratio method" (Stenflo, 1973) has been extensively used in the past to carry out measurements of the magnetic field strength in spatially unresolved magnetic flux concentrations. The authors present here a new variant of this technique, which is particularly simple as it does not depend on any radiative transfer calculations and thus the assumption of a model atmosphere is not required. General properties of the transfer equation lead to a relationship between the circular polarization generated by two lines which are identical except for their Landé factors. This can be used to directly determine the field strength from the measured line profiles. In order to test the method the authors have applied it to experimental data. A comparison with the traditional line ratio method is shown. Title: Continuum intensity of magnetic flux concentrations - Are magnetic elements bright points? Authors: Schuessler, M.; Solanki, S. K. Bibcode: 1988A&A...192..338S Altcode: The authors use the weakening of temperature sensitive spectral lines (Fe I λ5250.2 and λ5247.1) in a strong plage region to derive a lower limit (≈1.4 of the quiet photospheric value) for the continuum intensity in magnetic flux concentrations. This suggests an identification of magnetic elements with the observed photospheric bright points. The authors discuss the implications of this result for the quantitative determination of magnetic flux and propose a procedure for obtaining information on the physical structure of the non-magnetic surroundings of flux concentrations. Title: Magnetic Field of Late-Type Stars : a New Approach Authors: Mathys, G.; Solanki, S. K. Bibcode: 1988IAUS..132..325M Altcode: Magnetic field diagnosis in three late-type stars is performed using the multiline technique originally developed by Stenflo and Lindegren (1977) to study spatially unresolved magnetic features on the sun. Title: A new technique for the measurement of stellar magnetic fields: First Results Authors: Solanki, S. K.; Mathys, G. Bibcode: 1988ASSL..143...39S Altcode: 1988acse.conf...39S A technique for determining strengths and filling factors of stellar magnetic fields is presented. It is based on a statistical analysis of a large number of unblended Fe I lines and was first introduced by Stenflo and Lindegren (1977). It has been tested in solar active regions and is now applied to late type stars. First results of an analysis of high S/N spectra of three stars are presented and compared to previous measurements. Title: Velocity and temperature in solar magnetic fluxtubes from a statistical centre-to-limb analysis Authors: Pantellini, F. G. E.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1988A&A...189..263P Altcode: Stokes I and V profiles of a large number of unblended Fe I lines observed at different positions on the solar disk have been analyzed. Asymmetries in the Stokes V profile are noted. The center-to-limb variation of the zero-crossing wavelength of the weak and medium strong lines is found to be consistent with the absence of stationary flows in fluxtubes. The stronger lines are shown to be more blueshifted than the weak ones at all limb distances. The fluxtube temperature structure is confirmed to be a function of the filling factor. The rms velocity amplitude is found to increase somewhat when approaching the limb, in contrast to what would be expected if the mass motions were mostly vertical. Title: NLTE effects in solar magnetic fluxtubes Authors: Solanki, S. K.; Steenbock, W. Bibcode: 1988A&A...189..243S Altcode: NLTE calculations of approximately 200 Fe I and Fe II spectral lines with a comprehensive model atom are presented for a quiet sun model and a grid of fluxtube models. These calculations are compared with LTE results and the departures from LTE in fluxtubes are estimated. For all the fluxtube models tested, the departures from LTE ionization for the Fe I lines are found to be larger than for the quiet sun. Fe II lines show almost no difference between LTE and NLTE. The influence of temperature gradients on the departures is also studied and discussed. The profiles of five Fe I and one Fe II lines are analyzed in detail and the errors in empirically determined velocities and temperatures in fluxtubes, introduced by the assumption of LTE, are estimated. It is found that assuming LTE has practically no effect on the determined velocities, but that it may lead to an overestimation of the temperature in fluxtubes at the heights at which the lines are formed. Title: Properties of solar magnetic fluxtubes from only two spectral lines Authors: Solanki, S. K.; Keller, C.; Stenflo, J. O. Bibcode: 1987A&A...188..183S Altcode: A method for the determination of the magnetic field strength, velocity, and temperatures inside solar flux tubes, in addition to their inclinations and filling factors, is presented which requires only the Stokes V and Q profiles of the Fe I 5250.2 A and Fe I 5247.1 A spectral lines. Application of the procedure to spectra of the two lines obtained at various distances from the solar limb shows that considerable velocity broadening is required at all positions on the disk in order to reproduce the polarimeter data. The center to limb variation of the 5250/5247 Stokes V and Q line ratios is found to contain little information on the height variation of the magnetic field in the context of one-dimensional models. Title: The Photospheric Layers of Solar Magnetic Flux Tubes Authors: Solanki, S. K. Bibcode: 1987PhDT.......251S Altcode: This thesis is concerned with the structure and properties of magnetic fluxtubes in the solar photosphere. After a brief introduction outlining the importance of magnetic fields for the Sun and the stars, with particular emphasis on the role of fluxtubes, a brief description of polarised light, its properties, production, radiative transfer and measurement is given. There follows an overview of spectroscopy and polarimetry with a Fourier transform spectrometer (FTS). The data forming the basis of the thesis, FTS spectra of Stokes I, V, and partly also Q in the visible and the infrared (1.5-1.7μm), are described. The analysis procedure is partly based on the weak-field approximation, which is used to determine an approximation of the magnetically unsplit Stokes I inside the fluxtubes. The lines to be analysed (a set of 450 unblended Fe lines) are listed and their effective Landé factors are determined from laboratory measurements. The line profiles are parameterised and a statistical analysis of these parameters is carried out. Line of sight bulk and turbulent velocities, temperatures and the magnetic field in magnetic fluxtubes are determined, partly directly from the measured profiles, but partly on the basis of radiative transfer calculations in model fluxtubes. It is shown that fluxtubes do not harbour significant net flows and that fluxtubes in the network are on average hotter than those in plage regions. Finally, MHD models of cylindrically symmetric fluxtubes are presented. The effects of magnetic tension are included. A self-consistent formalism is developed to take the influence of neighbouring fluxtubes into account. The dependence of the merging height on various parameters of the fluxtubes is discussed. It is shown that the thin fluxtube approximation is quite reliable for fluxtubes with radii less than approximately 100 km near the solar surface. Title: Diagnostics of solar magnetic fluxtubes with the infrared line Fe I lambda 15648.54 A Authors: Stenflo, J. O.; Solanki, S. K.; Harvey, J. W. Bibcode: 1987A&A...173..167S Altcode: Fourier Transform Spectrometer recordings of May 1984 have been used to explore the center-to-limb variation of the Stokes I, V, and Q profiles of the solar infrared Fe I line at 15,648.54 A. The aim is to examine the new possibilities offerred for the diagnostics of the spatially unresolved magnetic flux tubes on the sun when lines with complete Zeeman splitting are used. Comparison is made with the line-ratio method, which must be used at visible wavelengths, where the splitting is incomplete. The Stokes V asymmetries observed in the infrared line are small or even of opposite sign as compared with the corresponding asymmetries observed at visible wavelengths. This suggests that the time-averaged height gradient of the Doppler velocities inside the fluxtubes becomes small and may change its sign when moving down to the bottom of the fluxtube photosphere. Title: The photospheric layers of solar magnetic fluxtubes Authors: Solanki, Sami Khan Bibcode: 1987PhDT.......295S Altcode: No abstract at ADS Title: Center-to-limb variation of Stokes profiles and the diagnostics of solar magnetic fluxtubes Authors: Stenflo, J. O.; Solanki, S. K.; Harvey, J. W. Bibcode: 1987A&A...171..305S Altcode: Simultaneous recordings of the Stokes I, Q, and V spectra have been performed with a Fourier transform spectrometer in 10 solar regions distributed over various center-to-limb distances, from disk center to the extreme limb. The observational material and the recording technique used are presented. The authors then evaluate the Stokes profile parameters for a small selected set of spectral lines to explore the potential of this qualitatively new data set for the diagnostics of spatially unresolved magnetic fluxtubes. Title: Magnetic fields: observations and theory. Authors: Solanki, S. K. Bibcode: 1987PAICz..66...95S Altcode: 1987eram....1...95S Theoretical model calculations of magnetic elements in the photospheric layers of solar active regions are compared with the results of observations. Title: Structure of Magnetic Flux tubes as derived from Observations with Moderate Spatial Resolution Authors: Solanki, S. K. Bibcode: 1987rfsm.conf...67S Altcode: Motions with large amplitudes and probably a vertical velocity gradient are present in the fluxtubes or their immediate surroundings. The exact nature of such motions is unknown. Candidates are: stationary up- and downflows in different fluxtubes, so that on the average little vertical mass transfer takes place. Oscillations or waves in fluxtubes are another possibility, whereby these motions are not in phase in different fluxtubes. Finally, due to the geometry of the fluxtubes (expansion with height), it is possible that motions in their immediate surroundings may also affect Stokes V. Title: Intensity profiles in fluxtubes. Authors: Sanches Almeida, J.; Collados, M.; del Toro Iniesta, J. C.; Solanki, S. K. Bibcode: 1987PAICz..66..261S Altcode: 1987eram....1..261S Spectroscopic analysis of the light coming from an atmosphere is a powerful tool for revealing its properties. The problem when using conventional spectroscopy for solar fluxtubes is their unresolved character: magnetic and non-magnetic regions of a plage have to be observed as a whole. With the aim of obtaining the true intensity spectrum of an unresolved tube, the authors have developed a simple method which can reconstruct the intensity generated in the magnetic component. Only observed parameters are used: intensity and circular polarization in the plage and intensity in the quiet photosphere. Title: FTS Measurements of Solar Line Asymmetries in Quiet and Active Regions Authors: Brandt, P. N.; Solanki, S. K. Bibcode: 1987rfsm.conf...82B Altcode: Spatially averaged Fourier transform spectra (FTS) with their well known high spectral resolution, their highly symmetric apparatus profile, low scattered light and high S/N ratio can be used as a complementing tool for the diagnosis of atmospheric parameters. The possibility to use many lines measured strictly simultaneously and covering a wide range of excitation potentials, heights of formation etc. represents another advantage of the FTS and at the same time reduces possible errors due to line blending. The main motivation for the present investigation of plage versus quiet sun line profiles came from the exciting results by Livingston & Holweger (1982) and by Livingston (1983) on the possible cycle dependence of line equivalent widths and asymmetries, as measured in integrated sunlight. Preliminary results are given. Title: Some effects of finite spectral resolution on the Stokes V profile Authors: Solanki, S. K.; Stenflo, J. O. Bibcode: 1986A&A...170..120S Altcode: The authors consider some general effects of spectral smearing on the Stokes V profile of the Fe I 5250.2 Å line, as observed on the sun with a Fourier transform spectrometer at a very high spectral resolution. The effects of low spectral resolution are numerically simulated by convoluting the observed line profiles with model instrumental profiles. The authors also try to find some Stokes V parameters which are insensitive to the spectral resolution. Finally, they show that the large Stokes V zero-crossing wavelength shifts observed by Wiehr (1985) and by Scholier and Wiehr (1985) from complete profiles, and by Giovanelli and Slaughter (1978) with the line-centre-magnetogram technique of Giovanelli and Ramsey (1971) are compatible with the results of Stenflo and Harvey (1985) and Solanki (1985, 1986). Title: Velocities in solar magnetic fluxtubes. Authors: Solanki, S. K. Bibcode: 1986A&A...168..311S Altcode: Stokes V zero-crossing wavelengths and I(V) profiles of a large number of unblended Fe I and II lines are used to study the velocities in solar magnetic flux tubes. No net flows are seen in the photospheric layers of flux tubes at disk center in both active region plages and the quiet network. Stokes V zero-crossing wavelength shifts relative to the wavelengths of the Stokes I profiles do not support the presence of downflows inside flux tubes. One dimensional LTE model calculations are used to show that the observed line profiles formed inside a flux tube can only be reproduced if the model profiles are broadened by velocity. The dependence of the total turbulent velocity on line strength in the flux tube is found to differ strongly from its dependence in the quiet photosphere. Title: Lines in the wavelength range λλ 4300 6700 Å with large stokes V amplitudes outside sunspots Authors: Solanki, S. K.; Pantellini, F. G. E.; Stenflo, J. O. Bibcode: 1986SoPh..107...57S Altcode: 1987SoPh..107...57S A list of solar spectral lines in the wavelength λλ 4300-6700 å exhibiting large Stokes V amplitudes in observed spectra of active region plages and the quiet network is presented. Title: Can velocity gradients explain the observed Stokes V asymmetry in the absence of large zero-crossing shifts? Authors: Pahlke, K. D.; Solanki, S. K. Bibcode: 1986MitAG..65..162P Altcode: No abstract at ADS Title: Structure and merging of solar magnetic fluxtubes Authors: Pneuman, G. W.; Solanki, S. K.; Stenflo, J. O. Bibcode: 1986A&A...154..231P Altcode: In the present, expansion technique-based model of the solar atmosphere's magnetic flux tubes, account is taken of the effects of field line curvature, internal structural variations, and the merging of the tube with its contiguous neighbors as it expands (through the use of a small 'seed' magnetic field between the tubes that has no influence on the solution in the limit of vanishing strength). For the solutions obtained, the internal magnetic structure of the tube evolves in height in a nonself-similar manner, although the gas pressure can vary self-similarly for the special case in which internal temperature is both uniform and equal to the external temperature. In the vicinity of the merging height, the field approaches uniformity consistent with a vertical tube with constant cross section. Title: High Spectral Resolution and Properties of Small Magnetic Flux Tubes Authors: Solanki, S. K. Bibcode: 1985tphr.conf..172S Altcode: No abstract at ADS Title: Models of solar magnetic fluxtubes - Constraints imposed by Fe I and II lines Authors: Solanki, S. K.; Stenflo, J. O. Bibcode: 1985A&A...148..123S Altcode: The diagnostic contents of the Stokes I and V profiles of about 50 unblended Fe II lines have been explored and used to set new constraints on the temperature structure of magnetic fluxtubes. The simultaneous use of Fe I and II lines makes it possible to determine the temperature in both the upper and lower fluxtube photosphere. The Fe II lines further make it possible to obtain model-insensitive values of the magnetic filling factors. Empirically determined effective Lande factors of most of the unblended iron lines in the visible part of the solar spectrum are presented and compared with the corresponding LS coupling values. Title: High spectral resolution and properties of small magnetic fluxtubes. Authors: Solanki, S. K. Bibcode: 1985MPARp.212..172S Altcode: Some empirical results pertaining to properties of small magnetic fluxtubes are described. Using data obtained with a Fourier transform spectrometer it has been possible to set constraints on the temperature structure of fluxtubes, on the types of mass motions and velocities inside them, and on the magnetic field strength near their τ = 1 level. The Stokes V asymmetries observed in fluxtube spectra are also discussed and evidence is presented favouring the hypothesis that they are produced by velocities. Title: Can Velocity Gradients Explain the Observed Stokes V Asymmetry in the Absence of Large Zero-Crossing Shifts? Authors: Pahlke, K. D.; Solanki, S. K. Bibcode: 1985MitAG..65..162P Altcode: No abstract at ADS Title: The photospheric temperature structure of magnetic fluxtubes. Authors: Solanki, S. K. Bibcode: 1984ESASP.220...63S Altcode: 1984ESPM....4...63S The temperature stratifications of plage and network fluxtubes are determined by comparing a large number of observed Fe I and Fe II lines with LTE model calculations. The data are also compared with profiles published in the literature. It is shown that in order to reproduce the data correctly, a sharp dip in the fluxtube temperature must occur in the photosphere, where the temperature of fluxtube and surroundings become similar (at equal optical depth). The effects of different pressure stratifications on the depths of the iron lines are briefly discussed and it is shown that the pressure stratification giving the best agreement with the data corresponds to a rapid expansion of the fluxtube near the height at which the temperature dip occurs. Title: Properties of solar magnetic fluxtubes as revealed by Fe I lines Authors: Solanki, S. K.; Stenflo, J. O. Bibcode: 1984A&A...140..185S Altcode: The information contents in the solar spectrum for modelling of magnetic fluxtubes is explored by analysing the statistical properties of the Stokes I and V line profiles of 400 unblended Fe I lines. Methods of determining the temperature structure, photospheric magnetic field strength, magnetic filling factor, and microturbulence velocity are presented and used to provide estimates of these quantities in enhanced network and plage regions. Analysis of the magnetic line broadening shows that the magnetic field strength is approximately equal in network and plage regions, consistent with previous results. On the other hand, the temperature structures of the plage fluxtubes and the network elements are observed to be substantially different. The variation of the Stokes V line asymmetries with line strength are found to be similar in the different solar regions, indicating similar velocity structures in plages and network elements. Title: Diagnostics of solar magnetic fluxtubes using a Fourier transform spectrometer Authors: Stenflo, J. O.; Solanki, S.; Harvey, J. W.; Brault, J. W. Bibcode: 1984A&A...131..333S Altcode: An overview is presented of the diagnostic contents for fluxtube modelling of Fourier transform spectrometer recordings of the longitudinal Zeeman effect near the solar disk center made in April 1979. The observations and data reductions are summarized and the application of the weak-field model to the Stokes profiles is examined. The significance of telluric lines and blends and the validity of LS coupling are considered. The magnetic fluxes, intrinsic field strengths, and area factors are discussed and the thermodynamic properties of fluxtubes are addressed. Mass motions inside the fluxtubes and the height variation of fluxtube parameters are considered.