Author name code: peter ADS astronomy entries on 2022-09-14 author:"Peter, Hardi" ------------------------------------------------------------------------ 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: Reconfiguration and Eruption of a Solar Filament by Magnetic Reconnection with an Emerging Magnetic Field Authors: Li, Leping; Peter, Hardi; Pradeep Chitta, Lakshmi; Song, Hongqiang; Xu, Zhe; Xiang, Yongyuan Bibcode: 2022ApJ...935...85L Altcode: 2022arXiv220704579L Both observations and simulations suggest that the solar filament eruption is closely related to magnetic flux emergence. It is thought that the eruption is triggered by magnetic reconnection between the filament and the emerging flux. However, the details of such a reconnection are rarely presented. In this study, we report the detailed reconnection between a filament and its nearby emerging fields, which led to the reconfiguration and subsequent partial eruption of the filament located over the polarity inversion line of active region 12816. Before the reconnection, we observed repeated brightenings in the filament at a location that overlies a site of magnetic flux cancellation. Plasmoids form at this brightening region, and propagate bidirectionally along the filament. These indicate the tether-cutting reconnection that results in the formation and eruption of a flux rope. To the northwest of the filament, magnetic fields emerge, and reconnect with the context ones, resulting in repeated jets. Afterwards, other magnetic fields emerge near the northwestern filament endpoints, and reconnect with the filament, forming the newly reconnected filament and loops. A current sheet repeatedly occurs at the interface, with the mean temperature and emission measure of 1.7 MK and 1.1×1028 cm-5. Plasmoids form in the current sheet, and propagate along it and further along the newly reconnected filament and loops. The newly reconnected filament then erupts, while the unreconnected filament remains stable. We propose that besides the orientation of emerging fields, some other parameters, such as the position, distance, strength, and area, are also crucial for triggering the filament eruption. 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: Small-scale coronal brightenings as seen by Solar Orbiter Authors: Peter, Hardi; Berghmans, David; Chitta, Lakshmi Pradeep Bibcode: 2022cosp...44.1323P Altcode: The corona of the Sun shows variability over a wide range of scales, in space, time and energy. The power-law-like distributions small-scale coronal brightenings events have been used to propose self-similarity of the involved processes. Already during the cruise phase, Solar Orbiter was close enough to the Sun so that images provided by the Extreme Ultraviolet Imager (EUI) are among the highest resolution coronal data acquired so far. The small brightenings found in the quiet Sun, now often termed campfires, could be considered as the small end of the distribution of coronal transients. Mostly, these coronal brightenings occur very low in the atmosphere, essentially just above the chromosphere. Still they show a variety of morphology, ranging from dot-like to loop-like with propagating disturbances, small jets, or miniature flux-rope eruptions. This variety of the phenomenology on the smallest resolvable scales points towards a conclusion that there is not one single process that can drive small-scale brightenings. This is supported by studies relating the EUV brightenings to the underlying magnetic field: a part of the cases shows a relation to changes of the surface magnetic flux, while in other cases it is very hard to find any connection to the magnetic field. In the quiet Sun these small brightenings are mostly found at the edges of bright elements of the chromospheric network, which they have in common with another class of brightenings seen at lower temperatures in the transition region, namely explosive events. These transients have been proposed as being due to reconnection, have a similar size and lifetime as the small brightenings, but mostly seem to lack a component at high temperatures. It might well be that these explosive events are related to one particular type of the campfires, e.g. the jet-types, but that remains to be studied. Modelling work on small brightenings is not yet very abundant. One 3D MHD model of the quiet Sun shows coronal brightenings that share properties with the observations. Here the brightening is caused (mostly) by component reconnection, but also one case of a twisted flux rope is found in the simulation data, other (future) models most likely will reveal that also different processes could produce similar brightenings. Because of timing and the mission profile, so far remote sensing observations have been taken mostly in regions of quiet Sun. With the perihelion in March 2022 Solar Orbiter will not only be closer than 0.3 AU from the Sun, providing coronal observations at even higher resolution than before, but also active region observations are planned. These might extend the zoo of the small-scale coronal transients to small features in active regions that might show properties similar or different from the quiet Sun coronal brightenings. Either way, this will provide a challenge for our understanding of the small-scale corona. Title: Transient small-scale brightenings in the quiet solar corona: a model for campfires observed with Solar Orbiter Authors: Chen, Yajie; Peter, Hardi; Berghmans, David; Tian, Hui; Auchère, Frédéric; Przybylski, Damien Bibcode: 2022cosp...44.2557C Altcode: Recent observations by the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed prevalent small-scale transient brightenings in the quiet solar corona termed "campfires". To understand the generation mechanism of these coronal brightenings, we constructed a self-consistent and time-dependent quiet-Sun model extending from the upper convection zone to the lower corona using a realistic three-dimensional radiation magnetohydrodynamic simulation. From the model we have synthesized the coronal emission in the EUI 174 passband. We identified several transient coronal brightenings similar to those in EUI observations. The size and lifetime of these coronal brightenings are mostly 0.5-4 Mm and ~2 min, respectively. These brightenings are generally located at a height of 2-4 Mm above the photosphere, and the local plasma is often heated above 1 MK. By examining the magnetic field structures before and after the occurrence of brightenings, we concluded that these coronal brightenings are generated by component magnetic reconnection between interacting bundles of magnetic field lines or neighboring field lines within highly twisted flux ropes. Occurring in the coronal part of the atmosphere, these events generally reveal no obvious signature of flux emergence or cancellation in photospheric magnetograms. These transient coronal brightenings may play an important role in heating of the local coronal plasma. Title: Parallel Plasma Loops and the Energization of the Solar Corona Authors: Peter, Hardi; Chitta, Lakshmi Pradeep; Chen, Feng; Pontin, David I.; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.; Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.; Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P. Bibcode: 2022ApJ...933..153P Altcode: 2022arXiv220515919P The outer atmosphere of the Sun is composed of plasma heated to temperatures well in excess of the visible surface. We investigate short cool and warm (<1 MK) loops seen in the core of an active region to address the role of field-line braiding in energizing these structures. We report observations from the High-resolution Coronal imager (Hi-C) that have been acquired in a coordinated campaign with the Interface Region Imaging Spectrograph (IRIS). In the core of the active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS show plasma loops at different temperatures that run in parallel. There is a small but detectable spatial offset of less than 1″ between the loops seen in the two bands. Most importantly, we do not see observational signatures that these loops might be twisted around each other. Considering the scenario of magnetic braiding, our observations of parallel loops imply that the stresses put into the magnetic field have to relax while the braiding is applied: the magnetic field never reaches a highly braided state on these length scales comparable to the separation of the loops. This supports recent numerical 3D models of loop braiding in which the effective dissipation is sufficiently large that it keeps the magnetic field from getting highly twisted within a loop. 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: Abundance diagnostics in active regions with Solar Orbiter/SPICE Authors: Giunta, Alessandra; Peter, Hardi; Parenti, Susanna; Buchlin, Eric; Thompson, William; Auchere, Frederic; Kucera, Therese; Carlsson, Mats; Janvier, Miho; Fludra, Andrzej; Hassler, Donald M.; Grundy, Timothy; Sidher, Sunil; Guest, Steve; Leeks, Sarah; Fredvik, Terje; Young, Peter Bibcode: 2022cosp...44.2583G Altcode: With the launch of Solar Orbiter in February 2020, we are now able to fully explore the link between the solar activity on the Sun and the inner heliosphere. Elemental abundance measurements provide a key tracer to probe the source regions of the solar wind and to track it from the solar surface and corona to the heliosphere. Abundances of elements with low first ionisation potential (FIP) are enhanced in the corona relative to high-FIP elements, with respect to the photosphere. This is known as the FIP effect, which is measured as abundance bias (FIP bias) of low and high FIP elements. This effect is vital for understanding the flow of mass and energy through the solar atmosphere. The comparison between in-situ and remote sensing composition data, coupled with modelling, will allow us to trace back the source of heliospheric plasma. Solar Orbiter has a unique combination of in-situ and remote sensing instruments that will help to make such a comparison. In particular, the SPICE (Spectral Imaging of the Coronal Environment) EUV spectrometer records spectra in two wavelength bands, 70.4-79.0 nm and 97.3-104.9 nm. SPICE is designed to provide spectroheliograms using a core set of emission lines arising from ions of both low-FIP and high-FIP elements such as C, N, O, Ne, Mg, S and Fe. These lines are formed over a wide range of temperatures from 20,000 K to over 1 million K, enabling the analysis of the different layers of the solar atmosphere. SPICE spectroheliograms can be processed to produce FIP bias maps, which can be compared to in-situ measurements of the solar wind composition of the same elements. During the Solar Orbiter Cruise Phase, SPICE observed several active regions. We will present some of these observations and discuss the SPICE diagnostic potential to derive relative abundances (e.g., Mg/Ne) and the FIP bias in those regions. Title: The SPICE spectrograph on Solar Orbiter: an introduction and results from the first Orbits Authors: Auchère, Frédéric; Peter, Hardi; Parenti, Susanna; Buchlin, Eric; Thompson, William; Auchere, Frederic; Teriaca, Luca; Kucera, Therese; Carlsson, Mats; Janvier, Miho; Fludra, Andrzej; Giunta, Alessandra; Schuehle, Udo; Hassler, Donald M.; Grundy, Timothy; Sidher, Sunil; Fredvik, Terje; Plowman, Joseph; Aznar Cuadrado, Regina Bibcode: 2022cosp...44.1338A Altcode: The Spectral Imaging of the Coronal Environment (SPICE) instrument is the EUV imaging spectrometer on board the Solar Orbiter mission. With its ability to derive physical properties of the coronal plasma, SPICE is a key component of the payload to establish the connection between the source regions and the in-situ measurements of the solar wind. The spacecraft was successfully launched in February 2020 and completed its cruise phase in December 2021. During this period, the remote sensing instruments were mostly operated during limited periods of time for 'checkout' engineering activities and synoptic observations. Nonetheless, several of these periods provided enough opportunities already to obtain new insights on coronal physics. During the march 2022 perihelion - close to 0.3 AU - SPICE will provide its highest spatial resolution data so far. Coordinated observations between the remote sensing and in-situ instruments will provide the first opportunity to use the full potential of the Solar Orbiter mission. We will review the instrument characteristics and present initial results from the cruise phase and first close encounter. Title: Observation of Magnetic Switchback in the Solar Corona Authors: Telloni, Daniele; Zank, Gary P.; Stangalini, Marco; Downs, Cooper; Liang, Haoming; Nakanotani, Masaru; Andretta, Vincenzo; Antonucci, Ester; Sorriso-Valvo, Luca; Adhikari, Laxman; Zhao, Lingling; Marino, Raffaele; Susino, Roberto; Grimani, Catia; Fabi, Michele; D'Amicis, Raffaella; Perrone, Denise; Bruno, Roberto; Carbone, Francesco; Mancuso, Salvatore; Romoli, Marco; Da Deppo, Vania; Fineschi, Silvano; Heinzel, Petr; Moses, John D.; Naletto, Giampiero; Nicolini, Gianalfredo; Spadaro, Daniele; Teriaca, Luca; Frassati, Federica; Jerse, Giovanna; Landini, Federico; Pancrazzi, Maurizio; Russano, Giuliana; Sasso, Clementina; Berghmans, David; Auchère, Frédéric; Aznar Cuadrado, Regina; Chitta, Lakshmi P.; Harra, Louise; Kraaikamp, Emil; Long, David M.; Mandal, Sudip; Parenti, Susanna; Pelouze, Gabriel; Peter, Hardi; Rodriguez, Luciano; Schühle, Udo; Schwanitz, Conrad; Smith, Phil J.; Verbeeck, Cis; Zhukov, Andrei N. Bibcode: 2022arXiv220603090T Altcode: Switchbacks are sudden, large radial deflections of the solar wind magnetic field, widely revealed in interplanetary space by the Parker Solar Probe. The switchbacks' formation mechanism and sources are still unresolved, although candidate mechanisms include Alfvénic turbulence, shear-driven Kelvin-Helmholtz instabilities, interchange reconnection, and geometrical effects related to the Parker spiral. This Letter presents observations from the Metis coronagraph onboard Solar Orbiter of a single large propagating S-shaped vortex, interpreted as first evidence of a switchback in the solar corona. It originated above an active region with the related loop system bounded by open-field regions to the East and West. Observations, modeling, and theory provide strong arguments in favor of the interchange reconnection origin of switchbacks. Metis measurements suggest that the initiation of the switchback may also be an indicator of the origin of slow solar wind. Title: Doppler shifts of spectral lines formed in the solar transition region and corona Authors: Chen, Yajie; Peter, Hardi; Przybylski, Damien; Tian, Hui; Zhang, Jiale Bibcode: 2022A&A...661A..94C Altcode: 2022arXiv220304691C Context. Emission lines formed in the transition region and corona dominantly show redshifts and blueshifts, respectively.
Aims: We investigate the Doppler shifts in a 3D radiation magnetohydrodynamic (MHD) model of the quiet Sun and compare these to observed properties. We concentrate on Si IV 1394 Å originating in the transition region and examine the Doppler shifts of several other spectral lines at different formation temperatures.
Methods: We constructed a radiation MHD model extending from the upper convection zone to the lower corona using the MURaM code. In this quiet Sun model, the magnetic field is self-consistently maintained by the action of a small-scale dynamo in the convection zone, and it is extrapolated to the corona as an initial condition. We synthesized the profiles of several optically thin emission lines, which formed at temperatures from the transition region into the corona. We investigated the spatial structure and coverage of redshifts and blueshifts and how this changes with the line-formation temperature.
Results: The model successfully reproduces the observed change of average net Doppler shifts from redshifted to blueshifted from the transition region into the corona. In particular, the model shows a clear imbalance of area coverage of redshifts versus blueshifts in the transition region of ca. 80% to 20%, even though it is even a bit larger on the real Sun. We determine that (at least) four processes generate the systematic Doppler shifts in our model, including pressure enhancement in the transition region, transition region brightenings unrelated to coronal emission, boundaries between cold and hot plasma, and siphon-type flows.
Conclusions: We show that there is not a single process that is responsible for the observed net Doppler shifts in the transition region and corona. Because current 3D MHD models do not yet fully capture the evolution of spicules, which is one of the key ingredients of the chromosphere, most probably these have yet to be added to the list of processes responsible for the persistent Doppler shifts. 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: Spectroscopic observation of a transition region network jet Authors: Gorman, J.; Chitta, L. P.; Peter, H. Bibcode: 2022A&A...660A.116G Altcode: 2022arXiv220211375G
Aims: Ubiquitous transition region (TR) network jets are considered to be substantial sources of mass and energy to the corona and solar wind. We conduct a case study of a network jet to better understand the nature of mass flows along its length and the energetics involved in its launch.
Methods: We present an observation of a jet with the Interface Region Imaging Spectrograph (IRIS), while also using data from the Solar Dynamics Observatory (SDO) to provide further context. The jet was located within a coronal hole close to the disk center.
Results: We find that a blueshifted secondary component of TR emission is associated with the jet and is persistent along its spire. This component exhibits upward speeds of approximately 20-70 km s−1 and shows enhanced line broadening. However, plasma associated with the jet in the upper chromosphere shows downflows of 5-10 km s−1. Finally, the jet emanates from a seemingly unipolar magnetic footpoint.
Conclusions: While a definitive magnetic driver is not discernible for this event, we infer that the energy driving the network jet is deposited at the top of the chromosphere, indicating that TR network jets are driven from the mid-atmospheric layers of the Sun. The energy flux associated with the line broadening indicates that the jet could be powered all the way into the solar wind.

Movie associated with Fig. 3 is available at https://www.aanda.org Title: Anisotropic nonthermal motions in the transition region of solar active regions Authors: Mou, Chaozhou; Peter, Hardi; Xia, Lidong; Huang, Zhenghua Bibcode: 2022A&A...660A...3M Altcode: Context. We study the nonthermal motions in the transition region of active regions (ARs) using center-to-limb observations of Si IV 1394 Å and 1403 Å lines.
Aims: The aim of this study is to investigate the characteristics of nonthermal motions in the transition region of ARs using the center-to-limb measurement.
Methods: We used Interface Region Imaging Spectrograph (IRIS) observations of the Si IV doublet lines 1393.78 Å and 1402.77 Å from the transition region of ARs to analyze nonthermal motions. We compared our analyzed data to similar data for the quiet Sun (QS) regions. We derive the nonthermal width by performing a single Gaussian fitting on the average spectral line profiles of Si IV doublet lines. We carried out a statistical analysis of the nonthermal width using the center-to-limb measurement.
Results: We find that the nonthermal motions are clearly decreasing on the disk with increasing distance from disk center. Only close to the limb, clearly beyond μ = 0.4, does the nonthermal broadening increase again towards the limb. The decrease in the nonthermal broadening in ARs away from disk center is markedly different from what is seen in the QS.
Conclusions: We conclude that nonthermal motions are anisotropic in ARs, with the vertical component being greater than the horizontal one. In contrast, we find the situation in the QS to be consistent with the findings of previous studies; only opacity effects are responsible for the increase in the nonthermal widths towards the limb, and nonthermal motions are isotropic in the QS. Title: Coronal condensation as the source of transition-region supersonic downflows above a sunspot Authors: Chen, Hechao; Tian, Hui; Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Hou, Zhenyong Bibcode: 2022A&A...659A.107C Altcode: 2021arXiv211201354C Context. Plasma loops or plumes rooted in sunspot umbrae often harbor downflows with speeds of 100 km s−1. These downflows are supersonic at transition region temperatures of ∼0.1 MK. The source of these flows is not well understood.
Aims: We aim to investigate the source of sunspot supersonic downflows (SSDs) in active region 12740 using simultaneous spectroscopic and imaging observations.
Methods: We identified SSD events from multiple raster scans of a sunspot by the Interface Region Imaging Spectrograph, and we calculated the electron densities, mass fluxes, and velocities of these SSDs. The extreme-ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the EUV Imager onboard the Solar Terrestrial Relations Observatory were employed to investigate the origin of these SSDs and their associated coronal rain.
Results: Almost all the identified SSDs appear at the footpoints of sunspot plumes and are temporally associated with the appearance of chromospheric bright dots inside the sunspot umbra. Dual-perspective EUV imaging observations reveal a large-scale closed magnetic loop system spanning the sunspot region and a remote region. We observed that the SSDs are caused by repeated coronal rain that forms and flows along these closed magnetic loops toward the sunspot. One episode of coronal rain clearly indicates that reconnection near a coronal X-shaped structure first leads to the formation of a magnetic dip. Subsequently, hot coronal plasma catastrophically cools from ∼2 MK in the dip region via thermal instability. This results in the formation of a transient prominence in the dip, from which the cool gas mostly slides into the sunspot along inclined magnetic fields under the gravity. This drainage process manifests as a continuous rain flow, which lasts for ∼2 h and concurrently results in a nearly steady SSD event. The total mass of condensation (1.3 × 1014 g) and condensation rate (1.5 × 1010 g s−1) in the dip region were found to be sufficient to sustain this long-lived SSD event, which has a mass transport rate of 0.7 − 1.2 × 1010 g s−1.
Conclusions: Our results demonstrate that coronal condensation in magnetic dips can result in the quasi-steady sunspot supersonic downflows.

Movies associated to Figs. 1, 3, 6, 7, and 11 are available at https://www.aanda.org Title: peterpeterp/tc_emulator: version 3 Authors: Peter Bibcode: 2022zndo...6223723P Altcode: scripts for the third submission at WCD 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: JuliaData/DataFrames.jl: v1.3.2 Authors: Kamiński, Bogumił; Myles White, John; Powerdistribution; Bouchet-Valat, Milan; Garborg, Sean; Quinn, Jacob; Kornblith, Simon; Cjprybol; Stukalov, Alexey; Bates, Douglas; Short, Tom; DuBois, Chris; Harris, Harlan; Squire, Kevin; Pdeffebach; Arslan, Alex; Anthoff, David; Kleinschmidt, Dave; Noack, Andreas; Shah, Viral B.; Mellnik, Alex; Arakaki, Takafumi; Mohapatra, Tanmay; Peter; Karpinski, Stefan; Lin, Dahua; Timema; ExpandingMan; Oswald, Florian; Arraes Jardim Chagas, Ronan Bibcode: 2022zndo...3376177K Altcode: 2021zndo...3376177M DataFrames v1.3.2 Diff since v1.3.1 Closed issues: Variance in runtime reduction functions (#2956) use of map in ByRow (#2957) Replace and Missing Values (#2976) Subset and Missing Values (#2977) copying of columns in select! and transform! (#2978) Unexpected Behavior of Combined Column Selection (#2980) Merged pull requests: Add a note about df.col .= v broadcasting changes (#2971) (@bkamins) Update working_with_dataframes.md (#2973) (@alfaromartino) Clean up join code (#2975) (@bkamins) Add links to docs, rephrase a bit (#2979) (@nalimilan) fix aliasing detection in sort! (#2981) (@bkamins) make sure ByRow invokes generic map (#2982) (@bkamins) make sure we use source column only once (#2983) (@bkamins) Update subset to handle large number of selectors better (#2989) (@bkamins) Title: Transient small-scale brightenings in the quiet solar corona: A model for campfires observed with Solar Orbiter Authors: Chen, Yajie; Przybylski, Damien; Peter, Hardi; Tian, Hui; Auchère, F.; Berghmans, D. Bibcode: 2021A&A...656L...7C Altcode: 2021arXiv210410940C Context. Recent observations by the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have characterized prevalent small-scale transient brightenings in the corona above the quiet Sun termed campfires.
Aims: In this study we search for comparable brightenings in a numerical model and then investigate their relation to the magnetic field and the processes that drive these events.
Methods: We used the MURaM code to solve the 3D radiation magnetohydrodynamic equations in a box that stretches from the upper convection zone to the corona. The model self-consistently produces a supergranular network of the magnetic field and a hot corona above this quiet Sun. For the comparison with the model, we synthesized the coronal emission as seen by EUI in its 174 Å channel, isolated the seven strongest transient brightenings, and investigated the changes of the magnetic field in and around these in detail.
Results: The transients we isolated have a lifetime of about 2 min and are elongated loop-like features with lengths around 1 Mm to 4 Mm. They tend to occur at heights of about 2 Mm to 5 Mm above the photosphere, a bit offset from magnetic concentrations that mark the bright chromospheric network, and they reach temperatures of above 1 MK. As a result, they very much resemble the larger campfires found in observations. In our model most events are energized by component reconnection between bundles of field lines that interact at coronal heights. In one case, we find that untwisting a highly twisted flux rope initiates the heating.
Conclusions: Based on our study, we propose that the majority of campfire events found by EUI are driven by component reconnection and our model suggests that this process significantly contributes to the heating of the corona above the quiet Sun.

Movies associated to Figs. 2, 4, 5, and A1 are available at https://www.aanda.org Title: Campfires observed by EUI: What have we learned so far? Authors: Berghmans, David; Auchere, F.; Zhukov, Andrei; Mierla, Marilena; Chen, Yajie; Peter, Hardi; Panesar, Navdeep; Chitta, Lakshmi Pradeep; Antolin, Patrick; Aznar Cuadrado, Regina; Tian, Hui; Hou, Zhenyong; Podladchikova, Olena Bibcode: 2021AGUFMSH21A..02B Altcode: Since its very first light images of the corona, the EUI/HRIEUV telescope onboard Solar Orbiter has observed small localised brightenings in the Quiet Sun. These small localised brightenings, have become known as campfires, and are observed with length scales between 400 km and 4000 km and durations between 10 sec and 200 sec. The smallest and weakest of these HRIEUV brightenings have not been previously observed. Simultaneous observations from the EUI High-resolution Lyman- telescope (HRILYA) do not show localised brightening events, but the locations of the HRIEUV events clearly correspond to the chromospheric network. Comparisons with simultaneous AIA images shows that most events can also be identified in the 17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although they appear weaker and blurred. Some of the larger campfires have the appearance of small interacting loops with the brightening expanding from the contact point of the loops. Our differential emission measure (DEM) analysis indicated coronal temperatures. We determined the height for a few of these campfires to be between 1 and 5 Mm above the photosphere. We interpret these events as a new extension to the flare-microflare-nanoflare family. Given their low height, the EUI campfires could stand as a new element of the fine structure of the transition region-low corona, that is, as apexes of small-scale loops that undergo internal heating all the way up to coronal temperatures. 3D MHD simulations with the MURaM code revealed brightenings that are in many ways similar to the campfires by EUI. The brightenings in the simulations suggest that campfires are triggered by component reconnection inside flux bundles rather than flux emergence or cancellation. Nevertheless, some of the observed campfires can be clearly linked to flux cancellation events and, intriguingly, are preceded by an erupting cool plasma structure. Analysis of the dynamics of campfires revealed that some have the appearance of coronal microjets, the smallest coronal jets observed in the quiet Sun. The HRIEUV images also reveal transient jets on a somewhat bigger scale with repeated outflows on the order of 100 km s1. In this paper we will provide an overview of the campfire related phenomena that EUI has observed and discuss the possible relevance for coronal heating. 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: 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: Revisiting the formation mechanism for coronal rain from previous studies Authors: Li, Le-Ping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song, Hong-Qiang Bibcode: 2021RAA....21..255L Altcode: 2021arXiv210701339L Solar coronal rain is classified generally into two categories: flare-driven and quiescent coronal rain. Th latter is observed to form along both closed and open magnetic field structures. Recently, we proposed that some of the quiescent coronal rain events, detected in the transition region and chromospheric diagnostics, along loop-like paths could be explained by the formation mechanism for quiescent coronal rain facilitated by interchange magnetic reconnection between open and closed field lines. In this study, we revisited 38 coronal rain reports from the literature. From theseearlier works, we picked 15 quiescent coronal rain events out of the solar limb, mostly suggested to occur in active region closed loops due to thermal nonequilibrium, to scrutinize their formation mechanism. Employing the extreme ultraviolet images and line-of-sight magnetograms, the evolution of the quiescent coronal rain events and their magnetic fields and context coronal structures is examined. We find that six, comprising 40%, of the 15 quiescent coronal rain events could be totally or partially interpreted by the formation mechanism for quiescent coronal rain along open structures facilitated by interchange reconnection. The results suggest that the quiescent coronal rain facilitated by interchange reconnection between open and closed field lines deserves more attention. Title: Formation of a Solar Filament by Magnetic Reconnection and Coronal Condensation Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song, Hongqiang Bibcode: 2021ApJ...919L..21L Altcode: 2021arXiv210905669L In solar filament formation mechanisms, magnetic reconnection between two sets of sheared arcades forms helical structures of the filament with numerous magnetic dips, and cooling and condensation of plasma trapped inside the helical structures supply mass to the filament. Although each of these processes, namely, magnetic reconnection and coronal condensation have been separately reported, observations that show the whole process of filament formation are rare. In this Letter, we present the formation of a sigmoid via reconnection between two sets of coronal loops, and the subsequent formation of a filament through cooling and condensation of plasma inside the newly formed sigmoid. On 2014 August 27, a set of loops in the active region 12151 reconnected with another set of loops that are located to the east. A longer twisted sigmoidal structure and a set of shorter lower-lying loops then formed. The observations coincide well with the tether-cutting model. The newly formed sigmoid remains stable and does not erupt as a coronal mass ejection. From the eastern endpoint, signatures of injection of material into the sigmoid (as brightenings) are detected, which closely outline the features of increasing emission measure at these locations. This may indicate the chromospheric evaporation caused by reconnection, supplying heated plasma into the sigmoid. In the sigmoid, thermal instability occurs, and rapid cooling and condensation of plasma take place, forming a filament. The condensations then flow bidirectionally to the filament endpoints. Our results provide a clear observational evidence of the filament formation via magnetic reconnection and coronal condensation. Title: Flare-induced decay-less transverse oscillations in solar coronal loops Authors: Mandal, Sudip; Tian, Hui; Peter, Hardi Bibcode: 2021A&A...652L...3M Altcode: 2021arXiv210702247M Evidence of flare-induced, large-amplitude, decay-less transverse oscillations is presented. A system of multithermal coronal loops, as observed with the Atmospheric Imaging Assembly (AIA), exhibit decay-less transverse oscillations after a flare erupts nearby one of the loop footpoints. Measured oscillation periods lie between 4.2 min and 6.9 min wherein the displacement amplitudes range from 0.17 Mm to 1.16 Mm. A motion-magnification technique has been employed to detect the preflare decay-less oscillations. These oscillations have similar periods (between 3.7 min and 5.0 min) similar to the previous ones, but their amplitudes (0.04 Mm to 0.12 Mm) are found to be significantly smaller. No phase difference is found among oscillating threads of a loop when observed through a particular AIA channel or when their multichannel signatures are compared. These features suggest that the occurrence of a flare in this case neither changed the nature of these oscillations (decaying versus decay-less), nor the oscillation periods. The only effect the flare has is to increase the oscillation amplitudes.

Movie is available at https://www.aanda.org Title: Stellar X-rays and magnetic activity in 3D MHD coronal models Authors: Zhuleku, J.; Warnecke, J.; Peter, H. Bibcode: 2021A&A...652A..32Z Altcode: 2021arXiv210200982Z Context. Observations suggest a power-law relation between the coronal emission in X-rays, LX, and the total (unsigned) magnetic flux at the stellar surface, Φ. The physics basis for this relation is poorly understood.
Aims: We use three-dimensional (3D) magnetohydrodynamics (MHD) numerical models of the coronae above active regions, that is, strong concentrations of magnetic field, to investigate the LX versus Φ relation and illustrate this relation with an analytical model based on simple well-established scaling relations.
Methods: In the 3D MHD model horizontal (convective) motions near the surface induce currents in the coronal magnetic field that are dissipated and heat the plasma. This self-consistently creates a corona with a temperature of 1 MK. We run a series of models that differ in terms of the (unsigned) magnetic flux at the surface by changing the (peak) magnetic field strength while keeping all other parameters fixed.
Results: In the 3D MHD models we find that the energy input into the corona, characterized by either the Poynting flux or the total volumetric heating, scales roughly quadratically with the unsigned surface flux Φ. This is expected from heating through field-line braiding. Our central result is the nonlinear scaling of the X-ray emission as LX ∝ Φ3.44. This scaling is slightly steeper than found in recent observations that give power-law indices of up to only 2 or 3. Assuming that on a real star, not only the peak magnetic field strength in the active regions changes but also their number (or surface filling factor), our results are consistent with observations.
Conclusions: Our model provides indications of what causes the steep increase in X-ray luminosity by four orders of magnitude from solar-type activity to fast rotating active stars. 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: 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: 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: On-disk Solar Coronal Condensations Facilitated by Magnetic Reconnection between Open and Closed Magnetic Structures Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song, Hongqiang Bibcode: 2021ApJ...910...82L Altcode: 2021arXiv210204605L Coronal condensation and rain are a crucial part of the mass cycle between the corona and chromosphere. In some cases, condensation and subsequent rain originate in the magnetic dips formed during magnetic reconnection. This provides a new and alternative formation mechanism for coronal rain. Until now, only off-limb, rather than on-disk, condensation events during reconnection have been reported. In this paper, employing extreme-ultraviolet (EUV) images of the Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO), we investigate the condensations facilitated by reconnection from 2011 July 14-15, when STEREO was in quadrature with respect to the Sun-Earth line. Above the limb, in STEREO/EUV Imager (EUVI) 171 Å images, higher-lying open structures move downward, reconnect with the lower-lying closed loops, and form dips. Two sets of newly reconnected structures then form. In the dips, bright condensations occur in the EUVI 304 Å images repeatedly, which then flow downward to the surface. In the on-disk observations by SDO/Atmospheric Imaging Assembly (AIA) in the 171 Å channel, these magnetic structures are difficult to identify. Dark condensations appear in the AIA 304 Å images, and then move to the surface as on-disk coronal rain. The cooling and condensation of coronal plasma is revealed by the EUV light curves. If only the on-disk observations were be available, the relation between the condensations and reconnection, shown clearly by the off-limb observations, could not be identified. Thus, we suggest that some on-disk condensation events seen in transition region and chromospheric lines may be facilitated by reconnection. Title: Transient small-scale brightenings in the quiet Sun corona: a model for "campfires" observed with Solar Orbiter Authors: Chen, Yajie; Przybylski, Damien; Peter, Hardi; Tian, Hui Bibcode: 2021EGUGA..23.5061C Altcode: Recent observations by the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter have revealed prevalent small-scale transient brightenings in the quiet solar corona termed campfires. To understand the generation mechanism of these coronal brightenings, we constructed a self- consistent and time-dependent quiet-Sun model extending from the upper convection zone to the lower corona using a realistic 3D radiation MHD simulation. From the model we have synthesized the coronal emission in the EUI 174 Å passband. We identified several transient coronal brightenings similar to those in EUI observations. The size and lifetime of these coronal brightenings are 2-4 Mm and ∼2 min, respectively. These brightenings are located at a height of 2-4 Mm above the photosphere, and the surrounding plasma is often heated above 1 MK. These findings are consistent with the observational characterisation of the campfires. Through a comparison of the magnetic field structures before and after the occurrence of brightenings, we conclude that these coronal brightenings are generated by component magnetic reconnection between interacting bundles of field lines or the relaxation of highly twisted flux ropes. Occurring in the coronal part of the atmosphere, these events show no measurable signature in the photosphere. These transient coronal brightenings may play an important role in heating of the local coronal plasma. Title: First data for abundance diagnostics with SPICE, the EUV spectrometer on-board Solar Orbiter Authors: Zambrana Prado, Natalia; Buchlin, Éric; Peter, Hardi Bibcode: 2021EGUGA..2315555Z Altcode: Linking solar activity on the surface and in the corona to the heliosphere is one of Solar Orbiter"s main goals. Its EUV spectrometer SPICE (SPectral Imaging of the Coronal Environment) will provide relative abundance measurements which will be key in this quest, as different structures on the Sun have different abundances as a consequence of the FIP (First Ionization Potential) effect. From the 16th to the 22nd of November 2020, the Solar Orbiter remote sensing checkout window STP-122 was carried out. During this period of observations, SPICE was lucky to catch a small AR in its field of view. We carried out abundance specific observations in order to provide relative FIP bias measurements with SPICE. Furthermore, data from other types of observations carried out during that same week allow us to identify the spectral lines that could be used for abundance diagnostics. We take the SPICE instrument characteristics into account to give recommendations regarding the types of studies to carry out to obtain such abundance measurements. 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: Extreme-ultraviolet bursts and nanoflares in the quiet-Sun transition region and corona Authors: Chitta, L. P.; Peter, H.; Young, P. R. Bibcode: 2021A&A...647A.159C Altcode: 2021arXiv210200730C The quiet solar corona consists of myriads of loop-like features, with magnetic fields originating from network and internetwork regions on the solar surface. The continuous interaction between these different magnetic patches leads to transient brightenings or bursts that might contribute to the heating of the solar atmosphere. The literature on a variety of such burst phenomena in the solar atmosphere is rich. However, it remains unclear whether such transients, which are mostly observed in the extreme ultraviolet (EUV), play a significant role in atmospheric heating. We revisit the open question of these bursts as a prelude to the new high-resolution EUV imagery expected from the recently launched Solar Orbiter. We use EUV image sequences recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) to investigate statistical properties of the bursts. We detect the bursts in the 171 Å filter images of AIA in an automated way through a pixel-wise analysis by imposing different intensity thresholds. By exploiting the high cadence (12 s) of the AIA observations, we find that the distribution of lifetimes of these events peaks at about 120 s. However, a significant number of events also have lifetimes shorter than 60 s. The sizes of the detected bursts are limited by the spatial resolution, which indicates that a larger number of events might be hidden in the AIA data. We estimate that about 100 new bursts appear per second on the whole Sun. The detected bursts have nanoflare-like energies of 1024 erg per event. Based on this, we estimate that at least 100 times more events of a similar nature would be required to account for the energy that is required to heat the corona. When AIA observations are considered alone, the EUV bursts discussed here therefore play no significant role in the coronal heating of the quiet Sun. If the coronal heating of the quiet Sun is mainly bursty, then the high-resolution EUV observations from Solar Orbiter may be able to reduce the deficit in the number of EUV bursts seen with SDO/AIA at least partly by detecting more such events. Title: A magnetic reconnection model for hot explosions in the cool atmosphere of the Sun Authors: Ni, Lei; Chen, Yajie; Peter, Hardi; Tian, Hui; Lin, Jun Bibcode: 2021A&A...646A..88N Altcode: 2020arXiv201107692N Context. Ultraviolet (UV) bursts and Ellerman bombs (EBs) are transient brightenings observed in the low solar atmospheres of emerging flux regions. Magnetic reconnection is believed to be the main mechanism leading to formation of the two activities, which are usually formed far apart from each other. However, observations also led to the discovery of co-spatial and co-temporal EBs and UV bursts, and their formation mechanisms are still not clear. The multi-thermal components in these events, which span a large temperature range, challenge our understanding of magnetic reconnection and heating mechanisms in the partially ionized lower solar atmosphere.
Aims: We studied magnetic reconnection between the emerging magnetic flux and back ground magnetic fields in the partially ionized and highly stratificated low solar atmosphere. We aim to explain the multi-thermal characteristics of UV bursts, and to find out whether EBs and UV bursts can be generated in the same reconnection process and how they are related with each other. We also aim to unearth the important small-scale physics in these events.
Methods: We used the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform simulations. The background magnetic fields and emerging fields at the solar surface are reasonably strong. The initial plasma parameters are based on the C7 atmosphere model. We simulated cases with different resolutions, and included the effects of ambipolar diffusion, radiative cooling, and heat conduction. We analyzed the current density, plasma density, temperature, and velocity distributions in the main current sheet region, and synthesized the Si IV emission spectrum.
Results: After the current sheet with dense photosphere plasma emerges and reaches 0.5 Mm above the solar surface, plasmoid instability appears. The plasmoids collide and coalesce with each other, which causes the plasmas with different densities and temperatures to be mixed up in the turbulent reconnection region. Therefore, the hot plasmas corresponding to the UV emissions and colder plasmas corresponding to the emissions from other wavelengths can move together and occur at about the same height. In the meantime, the hot turbulent structures concentrate above 0.4 Mm, whereas the cool plasmas extend to much lower heights to the bottom of the current sheet. These phenomena are consistent with published observations in which UV bursts have a tendency to be located at greater heights close to corresponding EBs and all the EBs have partial overlap with corresponding UV bursts in space. The synthesized Si IV line profiles are similar to that observed in UV bursts; the enhanced wing of the line profiles can extend to about 100 km s-1. The differences are significant among the numerical results with different resolutions, indicating that the realistic magnetic diffusivity is crucial to revealing the fine structures and realistic plasmas heating in these reconnection events. Our results also show that the reconnection heating contributed by ambipolar diffusion in the low chromosphere around the temperature minimum region is not efficient. Title: Magnetic Reconnection between Loops Accelerated By a Nearby Filament Eruption Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song, Hongqiang; Ji, Kaifan; Xiang, YongYuan Bibcode: 2021ApJ...908..213L Altcode: 2020arXiv201208710L Magnetic reconnection modulated by nonlocal disturbances in the solar atmosphere has been investigated theoretically, but rarely observed. In this study, employing Hα and extreme ultraviolet (EUV) images and line-of-sight magnetograms, we report the acceleration of reconnection by an adjacent filament eruption. In Hα images, four groups of chromospheric fibrils are observed to form a saddle-like structure. Among them, two groups of fibrils converge and reconnect. Two newly reconnected fibrils then form and retract away from the reconnection region. In EUV images, similar structures and evolution of coronal loops are identified. The current sheet forms repeatedly at the interface of reconnecting loops, with a width and length of 1-2 and 5.3-7.2 Mm and a reconnection rate of 0.18-0.3. It appears in the EUV low-temperature channels, with an average differential emission measure (DEM) weighed temperature and EM of 2 MK and 2.5 × 1027 cm-5. Plasmoids appear in the current sheet and propagate along it, and then further along the reconnection loops. The filament, located to the southeast of the reconnection region, erupts and pushes away the loops covering the reconnection region. Thereafter, the current sheet has a width and length of 2 and 3.5 Mm and a reconnection rate of 0.57. It becomes much brighter and appears in the EUV high-temperature channels, with an average DEM-weighed temperature and EM of 5.5 MK and 1.7 × 1028 cm-5. In the current sheet, more hotter plasmoids form. More thermal and kinetic energy is hence converted. These results suggest that the reconnection is significantly accelerated by the propagating disturbance caused by the nearby filament eruption. Title: New observational support for the role of magnetic field line braiding in solar coronal heating Authors: Pontin, David; Peter, Hardi; Yeates, Anthony; Pradeep Chitta, L.; Candelaresi, Simon; Hornig, Gunnar; Bushby, Paul Bibcode: 2021cosp...43E1796P Altcode: We present here new work that links models of magnetic field line braiding in coronal loops to observations of the photosphere and corona. We describe analysis of photospheric flows that quantifies the rate at which coronal magnetic field lines are braided. The results suggest that the photospheric motions induce complex tangling of the coronal field on a timescale of minutes to hours. New data from DKIST promises to further improve such estimates. Theoretical models show that this persistent tangling inevitably leads to the onset of reconnection and a turbulent heating of the plasma in the corona. We go on to describe synthetic emissions in a 3D magnetohydrodynamic model of the turbulent decay of an initially-braided magnetic field. We discuss how previously unexplained key features of observed emission line profiles in coronal loops - such as non-thermal widths and non-Gaussian profiles - are reproduced in the synthesised spectra. 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: Relative coronal abundance diagnostics with Solar Orbiter/SPICE Authors: Zambrana Prado, N.; Buchlin, E.; Peter, H.; Young, P. R.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Aznar Cuadrado, R.; Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Sidher, S.; Teriaca, L.; Thompson, W. T.; Williams, D. Bibcode: 2020AGUFMSH038..09Z Altcode: Linking solar activity on the surface and in the corona to the inner heliosphere is one of Solar Orbiter's main goals. Its UV spectrometer SPICE (SPectral Imaging of the Coronal Environment) will provide relative abundance measurements which will be key in this quest as different structures on the Sun have different abundances as a consequence of the FIP (First Ionization Potential) effect. Solar Orbiter's unique combination of remote sensing and in-situ instruments coupled with observation from other missions such as Parker Solar Probe will allow us to compare in-situ and remote sensing composition data. With the addition of modeling, these new results will allow us to trace back the source of heliospheric plasma. As high telemetry will not always be available with SPICE, we have developed a method for measuring relative abundances that is both telemetry efficient and reliable. Unlike methods based on Differential Emission Measure (DEM) inversion, the Linear Combination Ratio (LCR) method does not require a large number of spectral lines. This new method is based on linear combinations of UV spectral lines. The coefficients of the combinations are optimized such that the ratio of two linear combinations of radiances would yield the relative abundance of two elements. We present some abundance diagnostics tested on different combinations of spectral lines observable by SPICE. Title: Dynamics and thermal structure in the quiet Sun seen by SPICE Authors: Peter, H.; Aznar Cuadrado, R.; Schühle, U.; Teriaca, L.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Buchlin, E.; Caminade, S.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Schmutz, W. K.; Sidher, S.; Thompson, W. T.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH038..03P Altcode: We will present some of the early data of the Spectral Imaging of the Coronal Environment (SPICE) instrument on Solar Orbiter. One of the unique features of SPICE is its capability to record a wide range of wavelengths in the extreme UV with the possibility to record spectral lines giving access to a continuous plasma temperature range from 10.000 K to well above 1 MK. The data taken so far were for commissioning purposes and they can be used for a preliminary evaluation of the science performance of the instrument. Here we will concentrate on sample spectra covering the whole wavelength region and on the early raster maps acquired in bright lines in the quiet Sun close to disk center. Looking at different quiet Sun features we investigate the thermal structure of the atmosphere and flow structures. For this we apply fits to the spectral profiles and check the performance in terms of Doppler shifts and line widths to retrieve the structure of the network in terms of dynamics. While the amount of data available so far is limited, we will have a first look on how quiet Sun plasma responds to heating events. For this, we will compare spectral lines forming at different temperatures recorded at strictly the same time. Title: Spectroscopic Observations of the Eruption of an Filament and Associated Magnetic Reconnection Authors: Hu, H.; Liu, Y. D.; Peter, H.; Chitta, L. P.; Wang, R. Bibcode: 2020AGUFMSH0010013H Altcode: We analyze the spectroscopic data from Interface Region Imaging Spectrograph (IRIS) and images from Solar Dynamics Observatory (SDO) to investigate the eruption and associated magnetic reconnection of a filament structure in an active region (AR). Doppler maps derived from Si IV 1394 Å based on single Gaussian fitting indicate that the filament structure ascended rapidly after the birth of the situated AR. The ascent apparently ceased ~2 days after the rapid ascent. The filament structure gradually ascended again ~1 day before the eruption. Blue and red shifts of ~30 km/s are observed in a narrow flare region between two filament threads ~2 hours before the eruption, which are probably the signatures of the outflows of magnetic reconnection between the two filament threads. Eventually the upper filament thread erupted and the lower filament thread remained. Downward motion with a red shift of ~30 km/s and density enhancement are also observed in the regions of flare ribbons during the eruption. This work pictures the evolution of a filament structure before an eruption and the associated magnetic reconnection between two filament threads. Title: First Results From SPICE EUV Spectrometer on Solar Orbiter Authors: Fludra, A.; Caldwell, M.; Giunta, A. S.; Grundy, T.; Guest, S.; Sidher, S.; Auchere, F.; Carlsson, M.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, E.; Caminade, S.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Leeks, S.; Mueller, D.; Parenti, S.; Schmutz, W. K.; Schühle, U.; Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH038..02F Altcode: SPICE (Spectral Imaging of Coronal Environment) is one of the remote sensing instruments onboard Solar Orbiter. It is an EUV imaging spectrometer observing the Sun in two wavelength bands: 69.6-79.4 nm and 96.6-105.1 nm. SPICE is capable of recording full spectra in these bands with exposures as short as 1s. SPICE is the only Solar Orbiter instrument that can measure EUV spectra from the disk and low corona of the Sun and record all spectral lines simultaneously. SPICE uses one of three narrow slits, 2"x11', 4''x11', 6''x11', or a wide slit 30''x14'. The primary mirror can be scanned in a direction perpendicular to the slit, allowing raster images of up to 16' in size.

We present an overview of the first SPICE data taken on several days during the instrument commissioning carried out by the RAL Space team between 2020 April 21 and 2020 June 14. We also include results from SPICE observations at the first Solar Orbiter perihelion at 0.52AU, taken between June 16-21st. We give examples of full spectra from the quiet Sun near disk centre and provide a list of key spectral lines emitted in a range of temperatures between 10,000 K and over 1 million K, from neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur and magnesium. We show examples of first raster images in several strong lines, obtained with different slits and a range of exposure times between 5s and 180s. We describe the temperature coverage and density diagnostics, determination of plasma flows, and discuss possible applications to studies of the elemental abundances in the corona. We also show the first off-limb measurements with SPICE, as obtained when the spacecraft pointed at the limb. Title: Relation of Coronal Rain Originating from Coronal Condensations to Interchange Magnetic Reconnection Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song, Hongqiang Bibcode: 2020ApJ...905...26L Altcode: 2020arXiv201100709L Using extreme-ultraviolet images, we recently proposed a new and alternative formation mechanism for coronal rain along magnetically open field lines due to interchange magnetic reconnection. In this paper we report coronal rain at chromospheric and transition region temperatures originating from the coronal condensations facilitated by reconnection between open and closed coronal loops. For this, we employ the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory. Around 2013 October 19, a coronal rain along curved paths was recorded by IRIS over the southeastern solar limb. Related to this, we found reconnection between a system of higher-lying open features and lower-lying closed loops that occurs repeatedly in AIA images. In this process, the higher-lying features form magnetic dips. In response, two sets of newly reconnected loops appear and retract away from the reconnection region. In the dips, seven events of cooling and condensation of coronal plasma repeatedly occur due to thermal instability over several days, from October 18 to 20. The condensations flow downward to the surface as coronal rain, with a mean interval between condensations of ∼6.6 hr. In the cases where IRIS data were available we found the condensations to cool all the way down to chromospheric temperatures. Based on our observations we suggest that some of the coronal rain events observed at chromospheric temperatures could be explained by the new and alternative scenario for the formation of coronal rain, where the condensation is facilitated by interchange reconnection. Title: Calibrating optical distortions in the Solar Orbiter SPICE spectrograph Authors: Thompson, W. T.; Schühle, U.; Young, P. R.; Auchere, F.; Carlsson, M.; Fludra, A.; Hassler, D.; Peter, H.; Aznar Cuadrado, R.; Buchlin, E.; Caldwell, M.; DeForest, C.; Fredvik, T.; Harra, L. K.; Janvier, M.; Kucera, T. A.; Giunta, A. S.; Grundy, T.; Müller, D.; Parenti, S.; Caminade, S.; Schmutz, W. K.; Teriaca, L.; Williams, D.; Sidher, S. Bibcode: 2020AGUFMSH0360029T Altcode: The Spectral Imaging of the Coronal Environment (SPICE) instrument on Solar Orbiter is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths from 70.4-79.0 nm and 97.3-104.9 nm. A single-mirror off-axis paraboloid focuses the solar image onto the entrance slit of the spectrometer section. A Toroidal Variable Line Space (TVLS) grating images the entrance slit onto a pair of MCP-intensified APS detectors. Ray-tracing analysis prior to launch showed that the instrument was subject to a number of small image distortions which need to be corrected in the final data product. We compare the ray tracing results with measurements made in flight. Co-alignment with other telescopes on Solar Orbiter will also be examined. Title: First results from the EUI and SPICE observations of Alpha Leo near Solar Orbiter first perihelion Authors: Buchlin, E.; Teriaca, L.; Giunta, A. S.; Grundy, T.; Andretta, V.; Auchere, F.; Peter, H.; Berghmans, D.; Carlsson, M.; Fludra, A.; Harra, L.; Hassler, D.; Long, D.; Rochus, P. L.; Schühle, U.; Aznar Cuadrado, R.; Caldwell, M.; Caminade, S.; DeForest, C.; Fredvik, T.; Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp, E.; Kucera, T. A.; Müller, D.; Parenti, S.; Schmutz, W. K.; Sidher, S.; Smith, P.; Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH0360024B Altcode: On June 16th 2020 Solar Orbiter made a dedicated observing campaign where the spacecraft pointed to the solar limb to allow some of the high resolution instruments to observe the ingress (at the east limb) and later the egress (west limb) of the occultation of the star Alpha Leonis by the solar disk. The star was chosen because its luminosity and early spectral type ensure high and stable flux at wavelengths between 100 and 122 nanometers, a range observed by the High Resolution EUI Lyman alpha telescope (HRI-LYA) and by the long wavelength channel of the SPICE spectrograph. Star observations, when feasible, allow to gather a great deal of information on the instrument performances, such as the radiometric performance and the instrument optical point spread function (PSF).

We report here the first results from the above campaign for the two instruments. 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: First results from combined EUI and SPICE observations of Lyman lines of Hydrogen and He II Authors: Teriaca, L.; Aznar Cuadrado, R.; Giunta, A. S.; Grundy, T.; Parenti, S.; Auchere, F.; Vial, J. C.; Fludra, A.; Berghmans, D.; Carlsson, M.; Harra, L.; Hassler, D.; Long, D.; Peter, H.; Rochus, P. L.; Schühle, U.; Buchlin, E.; Caldwell, M.; Caminade, S.; DeForest, C.; Fredvik, T.; Gissot, S.; Heerlein, K.; Janvier, M.; Kraaikamp, E.; Kucera, T. A.; Mueller, D.; Schmutz, W. K.; Sidher, S.; Smith, P.; Stegen, K.; Thompson, W. T.; Verbeeck, C.; Williams, D.; Young, P. R. Bibcode: 2020AGUFMSH0360003T Altcode: The Solar Orbiter spacecraft carries a powerful set of remote sensing instruments that allow studying the solar atmosphere with unprecedented diagnostic capabilities. Many such diagnostics require the simultaneous usage of more than one instrument. One example of that is the capability, for the first time, to obtain (near) simultaneous spatially resolved observations of the emission from the first three lines of the Lyman series of hydrogen and of He II Lyman alpha. In fact, the SPectral Imaging of the Coronal Environment (SPICE) spectrometer can observe the Lyman beta and gamma lines in its long wavelength (SPICE-LW) channel, the High Resolution Lyman Alpha (HRI-LYA) telescope of the Extreme Ultraviolet Imager (EUI) acquires narrow band images in the Lyman alpha line while the Full Disk Imager (FSI) of EUI can take images dominated by the Lyman alpha line of ionized Helium at 30.4 nm (FSI-304). Being hydrogen and helium the main components of our star, these very bright transitions play an important role in the energy budget of the outer atmosphere via radiative losses and the measurement of their profiles and radiance ratios is a fundamental constraint to any comprehensive modelization effort of the upper solar chromosphere and transition region. Additionally, monitoring their average ratios can serve as a check out for the relative radiometric performance of the two instruments throughout the mission. Although the engineering data acquired so far are far from ideal in terms of time simultaneity (often only within about 1 h) and line coverage (often only Lyman beta was acquired by SPICE and not always near simultaneous images from all three telescopes are available) the analysis we present here still offers a great opportunity to have a first look at the potential of this diagnostic from the two instruments. In fact, we have identified a series of datasets obtained at disk center and at various positions at the solar limb that allow studying the Lyman alpha to beta radiance ratio and their relation to He II 30.4 as a function of the position on the Sun (disk center versus limb and quiet Sun versus coronal holes). 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: JuliaData/DataFrames.jl: v0.22.1 Authors: Myles White, John; Kamiński, Bogumił; Powerdistribution; Bouchet-Valat, Milan; Garborg, Sean; Quinn, Jacob; Kornblith, Simon; Cjprybol; Stukalov, Alexey; Bates, Douglas; Short, Tom; DuBois, Chris; Harris, Harlan; Squire, Kevin; Arslan, Alex; Pdeffebach; Anthoff, David; Kleinschmidt, Dave; Noack, Andreas; Shah, Viral B.; Mellnik, Alex; Arakaki, Takafumi; Mohapatra, Tanmay; Peter; Karpinski, Stefan; Lin, Dahua; Timema; ExpandingMan; Oswald, Florian; White, Lyndon Bibcode: 2020zndo...4282946M Altcode: DataFrames v0.22.1 Diff since v0.22.0 Closed issues: eltype width taken into accounet in display even if it is not shown (#2540) Final ellipsis appears on next row (#2544) clarify the interface for crossjoin when makeunique=true (#2545) Two small typos in docs (#2550) Merged pull requests: Fix size of float columns without eltypes (#2542) (@ronisbr) Spaces after commas (#2546) (@kescobo) Optional args style (#2547) (@kescobo) Zero after decimal (#2548) (@kescobo) issue #2550 fix two small typos in docs (#2551) (@roualdes) Switch from travis to GitHub Actions for CI testing (#2552) (@quinnj) error when using one dimension for indexing (#2553) (@bkamins) Add link to CI status badge (#2555) (@nalimilan) 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: Evidence for and Analysis of Multiple Hidden Coronal Strands in Cross-sectional Emission Profiles: Further Results from NASA's High-resolution Solar Coronal Imager Authors: Williams, Thomas; Walsh, Robert W.; Peter, Hardi; Winebarger, Amy R. Bibcode: 2020ApJ...902...90W Altcode: 2020arXiv200902210W Previous work utilizing NASA's High-resolution Coronal Imager (Hi-C 2.1) 172 Å observations revealed that, even at the increased spatial scales available in the dataset, there may be evidence for coronal structures that are still not fully resolved. In this follow-up study, cross-section slices of coronal strands are taken across the Hi-C 2.1 field of view. Following previous loop-width studies, the background emission is removed to isolate the coronal strands. The resulting intensity variations are reproduced by simultaneously fitting multiple Gaussian profiles using a nonlinear least-squares curve-fitting method. In total, 183 Gaussian profiles are examined for possible structures that are hinted at in the data. The full width at half maximum is determined for each Gaussian, which are then collated and analyzed. The most frequent structural widths are ≍450-575 km with 47% of the strand widths beneath NASA's Solar Dynamics Observatory Atmospheric Imaging Assembly (AIA) resolving scale (600-1000 km). Only 17% reside beneath an AIA pixel width (435 km) with just 6% of the strands at the Hi-C 2.1 resolving scale (≍220-340 km). These results suggest that non-Gaussian shaped cross-sectional emission profiles observed by Hi-C 2.1 are the result of multiple strands along the integrated line of sight that can be resolved, rather than being the result of even finer sub-resolution elements. Title: Observations and Modeling of the Onset of Fast Reconnection in the Solar Transition Region Authors: Guo, L. -J.; De Pontieu, Bart; Huang, Y. -M.; Peter, H.; Bhattacharjee, A. Bibcode: 2020ApJ...901..148G Altcode: 2020arXiv200911475G Magnetic reconnection is a fundamental plasma process that plays a critical role not only in energy release in the solar atmosphere, but also in fusion, astrophysics, and other space plasma environments. One of the challenges in explaining solar observations in which reconnection is thought to play a critical role is to account for the transition of the dynamics from a slow quasi-continuous phase to a fast and impulsive energetic burst of much shorter duration. Despite the theoretical progress in identifying mechanisms that might lead to rapid onset, a lack of observations of this transition has left models poorly constrained. High-resolution spectroscopic observations from NASA's Interface Region Imaging Spectrograph now reveal tell-tale signatures of the abrupt transition of reconnection from a slow phase to a fast, impulsive phase during UV bursts or explosive events in the Sun's atmosphere. Our observations are consistent with numerical simulations of the plasmoid instability, and provide evidence for the onset of fast reconnection mediated by plasmoids and new opportunities for remote-sensing diagnostics of reconnection mechanisms on the Sun. 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 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: 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: 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: 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: 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: Stellar coronal X-ray emission and surface magnetic flux Authors: Zhuleku, J.; Warnecke, J.; Peter, H. Bibcode: 2020A&A...640A.119Z Altcode: 2020arXiv200613978Z Context. Observations show that the coronal X-ray emission of the Sun and other stars depends on the surface magnetic field.
Aims: Using power-law scaling relations between different physical parameters, we aim to build an analytical model to connect the observed X-ray emission to the surface magnetic flux.
Methods: The basis for our model are the scaling laws of Rosner, Tucker & Vaiana (RTV) that connect the temperature and pressure of a coronal loop to its length and energy input. To estimate the energy flux into the upper atmosphere, we used scalings derived for different heating mechanisms, such as field-line braiding or Alfvén wave heating. We supplemented this with observed relations between active region size and magnetic flux and derived scalings of how X-ray emissivity depends on temperature.
Results: Based on our analytical model, we find a power-law dependence of the X-ray emission on the magnetic flux, LX ∝ Φm, with a power-law index m being in the range from about one to two. This finding is consistent with a wide range of observations, from individual features on the Sun, such as bright points or active regions, to stars of different types and varying levels of activity. The power-law index m depends on the choice of the heating mechanism, and our results slightly favor the braiding and nanoflare scenarios over Alfvén wave heating. In addition, the choice of instrument will have an impact on the power-law index m because of the sensitivity of the observed wavelength region to the temperature of the coronal plasma.
Conclusions: Overall, our simple analytical model based on the RTV scaling laws gives a good representation of the observed X-ray emission. Therefore we might be able to understand stellar coronal activity though a collection of basic building blocks, like loops, which we can study in spatially resolved detail on the Sun. Title: peterpeterp/atlantic_ace_seasonal_forecast: v2 Authors: Peter Bibcode: 2020zndo...3925816P Altcode: No description provided. Title: Non-thermal line broadening due to braiding-induced turbulence in solar coronal loops Authors: Pontin, D. I.; Peter, H.; Chitta, L. P. Bibcode: 2020A&A...639A..21P Altcode: 2020arXiv200811915P
Aims: Emission line profiles from solar coronal loops exhibit properties that are unexplained by current models. We investigate the non-thermal broadening associated with plasma heating in coronal loops that is induced by magnetic field line braiding.
Methods: We describe the coronal loop by a 3D magnetohydrodynamic model of the turbulent decay of an initially-braided magnetic field. From this, we synthesised the Fe XII line at 193 Å that forms around 1.5 MK.
Results: The key features of current observations of extreme ultraviolet (UV) lines from the corona are reproduced in the synthesised spectra: (i) Typical non-thermal widths range from 15 to 20 km s-1. (ii) The widths are approximately independent of the size of the field of view. (iii) There is a correlation between the line intensity and non-thermal broadening. (iv) Spectra are found to be non-Gaussian, with enhanced power in the wings of the order of 10-20%.
Conclusions: Our model provides an explanation that self-consistently connects the heating process to the observed non-thermal line broadening. The non-Gaussian nature of the spectra is a consequence of the non-Gaussian nature of the underlying velocity fluctuations, which is interpreted as a signature of intermittency in the turbulence. Title: Observation and Modeling of High-temperature Solar Active Region Emission during the High-resolution Coronal Imager Flight of 2018 May 29 Authors: Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.; Ugarte-Urra, Ignacio; Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub, Leon; Kobayashi, Ken; McKenzie, David; Morton, Richard; Rachmeler, Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert Bibcode: 2020ApJ...896...51W Altcode: Excellent coordinated observations of NOAA active region 12712 were obtained during the flight of the High-resolution Coronal Imager (Hi-C) sounding rocket on 2018 May 29. This region displayed a typical active region core structure with relatively short, high-temperature loops crossing the polarity inversion line and bright "moss" located at the footpoints of these loops. The differential emission measure (DEM) in the active region core is very sharply peaked at about 4 MK. Further, there is little evidence for impulsive heating events in the moss, even at the high spatial resolution and cadence of Hi-C. This suggests that active region core heating is occurring at a high frequency and keeping the loops close to equilibrium. To create a time-dependent simulation of the active region core, we combine nonlinear force-free extrapolations of the measured magnetic field with a heating rate that is dependent on the field strength and loop length and has a Poisson waiting time distribution. We use the approximate solutions to the hydrodynamic loop equations to simulate the full ensemble of active region core loops for a range of heating parameters. In all cases, we find that high-frequency heating provides the best match to the observed DEM. For selected field lines, we solve the full hydrodynamic loop equations, including radiative transfer in the chromosphere, to simulate transition region and chromospheric emission. We find that for heating scenarios consistent with the DEM, classical signatures of energy release, such as transition region brightenings and chromospheric evaporation, are weak, suggesting that they would be difficult to detect. Title: The Drivers of Active Region Outflows into the Slow Solar Wind Authors: Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina; Warren, Harry P.; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub, Leon; Kobayashi, Ken; McIntosh, Scott W.; McKenzie, David; Morton, Richard; Rachmeler, Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert Bibcode: 2020ApJ...894..144B Altcode: 2020arXiv200407461B Plasma outflows from the edges of active regions have been suggested as a possible source of the slow solar wind. Spectroscopic measurements show that these outflows have an enhanced elemental composition, which is a distinct signature of the slow wind. Current spectroscopic observations, however, do not have sufficient spatial resolution to distinguish what structures are being measured or determine the driver of the outflows. The High-resolution Coronal Imager (Hi-C) flew on a sounding rocket in 2018 May and observed areas of active region outflow at the highest spatial resolution ever achieved (250 km). Here we use the Hi-C data to disentangle the outflow composition signatures observed with the Hinode satellite during the flight. We show that there are two components to the outflow emission: a substantial contribution from expanded plasma that appears to have been expelled from closed loops in the active region core and a second contribution from dynamic activity in active region plage, with a composition signature that reflects solar photospheric abundances. The two competing drivers of the outflows may explain the variable composition of the slow solar wind. Title: Relative abundance diagnostics with SPICE, the EUV spectrometer on-board Solar Orbiter Authors: Zambrana Prado, Natalia; Buchlin, Eric; Peter, Hardi Bibcode: 2020EGUGA..2220154Z Altcode: With the launches of Parker Solar Probe and Solar Orbiter, we are closer than ever to linking solar activity on the surface and in the corona to the inner heliosphere. In this quest, relative abundance measurements will be key as different structures on the Sun have different abundances as a consequence of the FIP (First Ionization Potential) effect.Comparing in-situ and remote sensing composition data, coupled with modeling, will allow us to trace back the source of heliospheric plasma. Solar Orbiter has a unique combination of in-situ and remote sensing instruments that will hopefully allow us to make such comparisons.High telemetry will not always be available with SPICE (SPectral Imaging of the Coronal Environment), the EUV spectrometer on board Solar Orbiter. We have therefore developed a method for measuring relative abundances that is both telemetry efficient and reliable. Unlike methods based on Differential Emission Measure (DEM) inversion, the Linear Combination Ratio (LCR) method does not require a large number of spectral lines. This new method is based on optimized linear combinations of only a few UV spectral lines. We present some abundance diagnostics applied to synthesized radiances of spectral lines observable by SPICE. Title: Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712 Authors: Williams, Thomas; Walsh, Robert W.; Winebarger, Amy R.; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; Golub, Leon; Kobayashi, Ken; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Rachmeler, Laurel A.; Savage, Sabrina L.; Testa, Paola; Tiwari, Sanjiv K.; Warren, Harry P.; Watkinson, Benjamin J. Bibcode: 2020ApJ...892..134W Altcode: 2020arXiv200111254W Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 2018 May 29 from the White Sands Missile Range, NM, USA. On this occasion, 329 s of 17.2 nm data of target active region AR 12712 were captured with a cadence of ≈4 s, and a plate scale of 0.129 arcsec pixel-1. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm, we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of multi-scale Gaussian normalization, strands from a region of low emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands and those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ≈202 km, though the more typical strand widths seen are ≈513 km. For coronal strands within the region of low emission, the most likely width is significantly narrower than the high-emission strands at ≈388 km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need for a permanent solar observatory with the resolving power of Hi-C. Title: Observations and modeling of the onset of fast reconnection in the solar transition region Authors: Bhattacharjee, Amitava; Guo, Lijia; de Pontieu, Bart; Huang, Yi-Min; Peter, Hardi Bibcode: 2020APS..DPPP10005B Altcode: Magnetic reconnection is a fundamental plasma process that plays a critical role not only in energy release in the solar atmosphere, but also in fusion, astrophysical, and other space plasma environments. One of the challenges in explaining solar observations in which reconnection is thought to play a critical role is to account for the transition of the dynamics from a slow quasi-continuous phase to a fast and impulsive energetic burst of much shorter duration. Despite the theoretical progress in identifying mechanisms that might lead to rapid onset, a lack of observations of this transition has left models poorly constrained. High-resolution spectroscopic observations from NASA's Interface Region Imaging Spectrograph (IRIS) now reveal tell-tale signatures of the abrupt transition of reconnection from a slow phase to a fast, impulsive phase during explosive events in the Sun's atmosphere. Our observations are consistent with numerical simulations of the plasmoid instability, and provide evidence for the onset of fast reconnection mediated by plasmoids and new opportunities for remote-sensing diagnostics of reconnection mechanisms on the Sun. Title: Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lanes Authors: Panesar, Navdeep K.; Sterling, Alphonse C.; Moore, Ronald L.; Winebarger, Amy R.; Tiwari, Sanjiv K.; Savage, Sabrina L.; Golub, Leon E.; Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Testa, Paola; Walsh, Robert W.; Warren, Harry P. Bibcode: 2019ApJ...887L...8P Altcode: 2019arXiv191102331P We present high-resolution, high-cadence observations of six, fine-scale, on-disk jet-like events observed by the High-resolution Coronal Imager 2.1 (Hi-C 2.1) during its sounding-rocket flight. We combine the Hi-C 2.1 images with images from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS), and investigate each event’s magnetic setting with co-aligned line-of-sight magnetograms from the SDO/Helioseismic and Magnetic Imager (HMI). We find that (i) all six events are jetlet-like (having apparent properties of jetlets), (ii) all six are rooted at edges of magnetic network lanes, (iii) four of the jetlet-like events stem from sites of flux cancelation between majority-polarity network flux and merging minority-polarity flux, and (iv) four of the jetlet-like events show brightenings at their bases reminiscent of the base brightenings in coronal jets. The average spire length of the six jetlet-like events (9000 ± 3000 km) is three times shorter than that for IRIS jetlets (27,000 ± 8000 km). While not ruling out other generation mechanisms, the observations suggest that at least four of these events may be miniature versions of both larger-scale coronal jets that are driven by minifilament eruptions and still-larger-scale solar eruptions that are driven by filament eruptions. Therefore, we propose that our Hi-C events are driven by the eruption of a tiny sheared-field flux rope, and that the flux rope field is built and triggered to erupt by flux cancelation. Title: The SPICE (Spectral Imaging of the Coronal Environment) Ultraviolet Imaging Spectrograph Investigation Authors: Hassler, D.; Auchere, F.; Carlsson, M.; Fludra, A.; Giunta, A. S.; Mueller, D.; Peter, H.; Parenti, S.; Teriaca, L.; Fredvik, T. Bibcode: 2019AGUFMSH24A..02H Altcode: One of the primary objectives of the Solar Orbiter mission is to link remote sensing observations of the solar surface structures with in-situ observations of solar wind streams. The SPICE (Spectral Imaging of the Coronal Environment) instrument will characterize the plasma properties of regions near the Sun to directly compare with in-situ measurements from both Solar Orbiter & Parker Solar Probe. Specifically, SPICE will map outflow velocities of surface features to solar wind structures with similar composition (FIP, M/q) measured in-situ by the SWA/HIS instrument on Solar Orbiter. These observations will help discriminate models of solar wind origin by matching composition signatures in solar wind streams to surface feature composition, and discriminate physical processes that inject material from closed structures into solar wind streams.

This presentation will provide an overview of the SPICE investigation, including science & measurement objective, instrument design, capabilities and performance as measured during calibration prior to delivery to the Solar Orbiter spacecraft. The presentation will also provide a description of the operations concept and data processing during the mission. Title: Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO Authors: Tiwari, Sanjiv K.; Panesar, Navdeep K.; Moore, Ronald L.; De Pontieu, Bart; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.; Rachmeler, Laurel A.; Kobayashi, Ken; Testa, Paola; Warren, Harry P.; Brooks, David H.; Cirtain, Jonathan W.; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Walsh, Robert W. Bibcode: 2019ApJ...887...56T Altcode: 2019arXiv191101424T The second Hi-C flight (Hi-C 2.1) provided unprecedentedly high spatial and temporal resolution (∼250 km, 4.4 s) coronal EUV images of Fe IX/X emission at 172 Å of AR 12712 on 2018 May 29, during 18:56:21-19:01:56 UT. Three morphologically different types (I: dot-like; II: loop-like; III: surge/jet-like) of fine-scale sudden-brightening events (tiny microflares) are seen within and at the ends of an arch filament system in the core of the AR. Although type Is (not reported before) resemble IRIS bombs (in size, and brightness with respect to surroundings), our dot-like events are apparently much hotter and shorter in span (70 s). We complement the 5 minute duration Hi-C 2.1 data with SDO/HMI magnetograms, SDO/AIA EUV images, and IRIS UV spectra and slit-jaw images to examine, at the sites of these events, brightenings and flows in the transition region and corona and evolution of magnetic flux in the photosphere. Most, if not all, of the events are seated at sites of opposite-polarity magnetic flux convergence (sometimes driven by adjacent flux emergence), implying likely flux cancellation at the microflare’s polarity inversion line. In the IRIS spectra and images, we find confirming evidence of field-aligned outflow from brightenings at the ends of loops of the arch filament system. In types I and II the explosion is confined, while in type III the explosion is ejective and drives jet-like outflow. The light curves from Hi-C, AIA, and IRIS peak nearly simultaneously for many of these events, and none of the events display a systematic cooling sequence as seen in typical coronal flares, suggesting that these tiny brightening events have chromospheric/transition region origin. Title: The High-Resolution Coronal Imager, Flight 2.1 Authors: Rachmeler, Laurel A.; Winebarger, Amy R.; Savage, Sabrina L.; Golub, Leon; Kobayashi, Ken; Vigil, Genevieve D.; Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.; Peter, Hardi; Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P.; Alexander, Caroline; Ansell, Darren; Beabout, Brent L.; Beabout, Dyana L.; Bethge, Christian W.; Champey, Patrick R.; Cheimets, Peter N.; Cooper, Mark A.; Creel, Helen K.; Gates, Richard; Gomez, Carlos; Guillory, Anthony; Haight, Harlan; Hogue, William D.; Holloway, Todd; Hyde, David W.; Kenyon, Richard; Marshall, Joseph N.; McCracken, Jeff E.; McCracken, Kenneth; Mitchell, Karen O.; Ordway, Mark; Owen, Tim; Ranganathan, Jagan; Robertson, Bryan A.; Payne, M. Janie; Podgorski, William; Pryor, Jonathan; Samra, Jenna; Sloan, Mark D.; Soohoo, Howard A.; Steele, D. Brandon; Thompson, Furman V.; Thornton, Gary S.; Watkinson, Benjamin; Windt, David Bibcode: 2019SoPh..294..174R Altcode: 2019arXiv190905942R The third flight of the High-Resolution Coronal Imager (Hi-C 2.1) occurred on May 29, 2018; the Sounding Rocket was launched from White Sands Missile Range in New Mexico. The instrument has been modified from its original configuration (Hi-C 1) to observe the solar corona in a passband that peaks near 172 Å, and uses a new, custom-built low-noise camera. The instrument targeted Active Region 12712, and captured 78 images at a cadence of 4.4 s (18:56:22 - 19:01:57 UT; 5 min and 35 s observing time). The image spatial resolution varies due to quasi-periodic motion blur from the rocket; sharp images contain resolved features of at least 0.47 arcsec. There are coordinated observations from multiple ground- and space-based telescopes providing an unprecedented opportunity to observe the mass and energy coupling between the chromosphere and the corona. Details of the instrument and the data set are presented in this paper. Title: Generation of solar spicules and subsequent atmospheric heating Authors: Samanta, Tanmoy; Tian, Hui; Yurchyshyn, Vasyl; Peter, Hardi; Cao, Wenda; Sterling, Alphonse; Erdélyi, Robertus; Ahn, Kwangsu; Feng, Song; Utz, Dominik; Banerjee, Dipankar; Chen, Yajie Bibcode: 2019Sci...366..890S Altcode: 2020arXiv200602571S Spicules are rapidly evolving fine-scale jets of magnetized plasma in the solar chromosphere. It remains unclear how these prevalent jets originate from the solar surface and what role they play in heating the solar atmosphere. Using the Goode Solar Telescope at the Big Bear Solar Observatory, we observed spicules emerging within minutes of the appearance of opposite-polarity magnetic flux around dominant-polarity magnetic field concentrations. Data from the Solar Dynamics Observatory showed subsequent heating of the adjacent corona. The dynamic interaction of magnetic fields (likely due to magnetic reconnection) in the partially ionized lower solar atmosphere appears to generate these spicules and heat the upper solar atmosphere. Title: Repeated Coronal Condensations Caused by Magnetic Reconnection between Solar Coronal Loops Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Zhang, Jun; Su, Jiangtao; Song, Hongqiang; Hou, Yijun; Xia, Chun Bibcode: 2019ApJ...884...34L Altcode: We recently presented coronal condensations, caused by magnetic reconnection (MR) between coronal loops from extreme ultraviolet observations, over the course of one day, on 2012 January 19. In this paper, by investigating the loops over an extended period of time from January 16 to 20, we present a case for repeated coronal condensations caused by repeated MR between them. In these five days, MR between higher-lying open loops and lower-lying closed loops occurs repeatedly, forming magnetic dips in the higher-lying open loops. During the MR process, cooling and condensation of coronal plasma occur repeatedly. Early on January 16, cooling, but not condensation, of coronal plasma happens. Later, condensation appears at the edge of the dips and falls down along the loops as coronal rains. On January 17, a similar condensation happens at the edge of the higher-lying dips and falls down along the loops. However, another condensation appears in the lower-lying dips and rains down across them. From January 18 to 19, multiple condensations mostly occur at the edge of the dips and fall down both along the loops and across the dips. On January 20, five condensations sequentially appear and rain down across the dips. Overall, 15 condensation events occur in five days, lasting from 0.5 to 15.6 hr. We suggest that the formation of coronal condensations by MR between loops is common in the solar corona. The repeated MR between loops thus plays an essential role in the mass cycle of coronal plasma by initiating repeated catastrophic cooling and condensation. Title: On the influence of magnetic helicity on X-rays emission of solar and stellar coronae Authors: Warnecke, Jörn; Peter, Hardi Bibcode: 2019arXiv191006896W Altcode: Observation of solar-like stars show a clear relation between X-ray emission and their rotation. Higher stellar rotation can lead to a larger magnetic helicity production in stars. We aim to understand the relation between magnetic helicity on the surface of a star to their coronal X-ray emission. We use 3D MHD simulations to model the corona of the solar-like stars. We take an observed magnetogram as in photospheric activity input, and inject different values of magnetic helicity. We use synthesis emission to calculate the X-ray emission flux of each simulation and investigate how this scales with injected magnetic helicity. We find that for larger injected magnetic helicities an increase in temperature and an increase in X-ray emission. The X-ray emission scaled cubicly with the injected helicity. We can related this to increase of horizontal magnetic field and therefore higher Poynting flux at the coronal base. Using typical scaling of magnetic helicity production with stellar rotation, we can explain the increase of X-ray emission with rotation only by an increase of magnetic helicity at the surface of a star. Title: Plasmoid-mediated reconnection in solar UV bursts Authors: Peter, H.; Huang, Y. -M.; Chitta, L. P.; Young, P. R. Bibcode: 2019A&A...628A...8P Altcode: 2019arXiv190704335P Context. Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection.
Aims: To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere.
Methods: We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma-β at the height of this X point.
Results: The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes.
Conclusions: Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-β, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.

Movie attached to Fig. 2 is available at https://www.aanda.org Title: Hot prominence spicules launched from turbulent cool solar prominences Authors: Chitta, L. P.; Peter, H.; Li, L. Bibcode: 2019A&A...627L...5C Altcode: 2019arXiv190609125C A solar filament is a dense cool condensation that is supported and thermally insulated by magnetic fields in the rarefied hot corona. Its evolution and stability, leading to either an eruption or disappearance, depend on its coupling with the surrounding hot corona through a thin transition region, where the temperature steeply rises. However, the heating and dynamics of this transition region remain elusive. We report extreme-ultraviolet observations of quiescent filaments from the Solar Dynamics Observatory that reveal prominence spicules propagating through the transition region of the filament-corona system. These thin needle-like jet features are generated and heated to at least 0.7 MK by turbulent motions of the material in the filament. We suggest that the prominence spicules continuously channel the heated mass into the corona and aid in the filament evaporation and decay. Our results shed light on the turbulence-driven heating in magnetized condensations that are commonly observed on the Sun and in the interstellar medium.

The movie associated to Fig. 1 is available at https://www.aanda.org. Title: Effects of Coronal Density and Magnetic Field Distributions on a Global Solar EUV Wave Authors: Hu, Huidong; Liu, Ying D.; Zhu, Bei; Peter, Hardi; He, Wen; Wang, Rui; Yang, Zhongwei Bibcode: 2019ApJ...878..106H Altcode: 2019arXiv190501211H We investigate a global extreme-ultraviolet (EUV) wave associated with a coronal mass ejection (CME)-driven shock on 2017 September 10. The EUV wave is transmitted by north- and south-polar coronal holes (CHs), which is observed by the Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory A (STEREO-A) from opposite sides of the Sun. We obtain key findings on how the EUV wave interacts with multiple coronal structures, and its connection with the CME-driven shock: (1) the transmitted EUV wave is still connected with the shock that is incurvated to the Sun, after the shock has reached the opposite side of the eruption; (2) the south CH transmitted EUV wave is accelerated inside an on-disk, low-density region with closed magnetic fields, which implies that an EUV wave can be accelerated in both open and closed magnetic field regions; (3) part of the primary EUV wavefront turns around a bright point (BP) with a bipolar magnetic structure when it approaches a dim, low-density filament channel near the BP; (4) the primary EUV wave is diffused and apparently halted near the boundaries of remote active regions (ARs) that are far from the eruption, and no obvious AR related secondary waves are detected; (5) the EUV wave extends to an unprecedented scale of ∼360° in latitudes, which is attributed to the polar CH transmission. These results provide insights into the effects of coronal density and magnetic field distributions on the evolution of an EUV wave, and into the connection between the EUV wave and the associated CME-driven shock. Title: Plasma injection into a solar coronal loop Authors: Li, L. P.; Peter, H. Bibcode: 2019A&A...626A..98L Altcode: 2019arXiv190507800L Context. The details of the spectral profiles of extreme UV emission lines from solar active regions contain key information for investigating the structure, dynamics, and energetics of the solar upper atmosphere.
Aims: We characterize the line profiles not only through the Doppler shift and intensity of the bulk part of the profile. More importantly, we investigate the excess emission and asymmetries in the line wings to study twisting motions and helicity.
Methods: We used a raster scan of the Interface Region Imaging Spectrograph (IRIS) in an active region. We concentrated on the Si IV line at 1394 Å, which forms just below 0.1 MK, and followed the plasma that moves in a cool loop from one footpoint to the other. We applied single-Gaussian fits to the line core, determined the excess emission in the red and blue wings, and derived the asymmetry of the red and blue wings.
Results: The blue wing excess at one footpoint shows injection of plasma into the loop that then flows to the other side. At the same footpoint, redshifts in the line core indicate that energy is deposited at around 0.1 MK. The enhanced pressure would then push the cool plasma down and inject some plasma into the loop. In the middle part of the loop, the spectral tilts of the line profiles indicate that the magnetic field has a helical structure, and the line wings are symmetrically enhanced. This is an indication that the loop is driven through the injection of helicity at the loop feet.
Conclusions: If the loop is driven to be helical, then the magnetic field can be expected to be in a turbulent state, as has been shown by existing magnetohydrodynamics models. The turbulent motions might explain the (symmetric) line wing enhancements that have also been seen in loops at coronal temperatures, but are not understood so far.

The movie associated to Fig. 7 is available at https://www.aanda.org Title: Data-driven model of the solar corona above an active region Authors: Warnecke, J.; Peter, H. Bibcode: 2019A&A...624L..12W Altcode: 2019arXiv190300455W
Aims: We aim to reproduce the structure of the corona above a solar active region as seen in the extreme ultraviolet (EUV) using a three-dimensional magnetohydrodynamic (3D MHD) model.
Methods: The 3D MHD data-driven model solves the induction equation and the mass, momentum, and energy balance. To drive the system, we feed the observed evolution of the magnetic field in the photosphere of the active region AR 12139 into the bottom boundary. This creates a hot corona above the cool photosphere in a self-consistent way. We synthesize the coronal EUV emission from the densities and temperatures in the model and compare this to the actual coronal observations.
Results: We are able to reproduce the overall appearance and key features of the corona in this active region on a qualitative level. The model shows long loops, fan loops, compact loops, and diffuse emission forming at the same locations and at similar times as in the observation. Furthermore, the low-intensity contrast of the model loops in EUV matches the observations.
Conclusions: In our model the energy input into the corona is similar as in the scenarios of fieldline-braiding or flux-tube tectonics, that is, energy is transported to the corona through the driving of the vertical magnetic field by horizontal photospheric motions. The success of our model shows the central role that this process plays for the structure, dynamics, and heating of the corona. Title: Flame-like Ellerman Bombs and Their Connection to Solar Ultraviolet Bursts Authors: Chen, Yajie; Tian, Hui; Peter, Hardi; Samanta, Tanmoy; Yurchyshyn, Vasyl; Wang, Haimin; Cao, Wenda; Wang, Linghua; He, Jiansen Bibcode: 2019ApJ...875L..30C Altcode: 2019arXiv190301981C Ellerman bombs (EBs) are small-scale intense brightenings in Hα wing images, which are generally believed to be signatures of magnetic reconnection around the temperature minimum region of the solar atmosphere. They have a flame-like morphology when observed near the solar limb. Recent observations from the Interface Region Imaging Spectrograph (IRIS) reveal another type of small-scale reconnection event called an ultraviolet (UV) burst, in the lower solar atmosphere. Though previous observations have shown a clear coincidence between the occurrence of some UV bursts and EBs, the exact relationship between these two phenomena is still debated. We investigate the spatial and temporal relationship between flame-like EBs and UV bursts using joint near-limb observations between the 1.6 m Goode Solar Telescope (GST) and IRIS. In total, 161 EBs have been identified from the GST observations, and ∼20 of them reveal signatures of UV bursts in the IRIS images. Interestingly, we find that these UV bursts have a tendency to appear at the upper parts of their associated flame-like EBs. The intensity variations of most EB-related UV bursts and their corresponding EBs match well. Our results suggest that some of these UV bursts and EBs likely form at different heights during a common reconnection process. Title: Energetics of magnetic transients in a solar active region plage Authors: Chitta, L. P.; Sukarmadji, A. R. C.; Rouppe van der Voort, L.; Peter, H. Bibcode: 2019A&A...623A.176C Altcode: 2019arXiv190201650C Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface.
Aims: We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations.
Methods: We used photospheric Fe I 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code.
Results: The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m-2 through the photosphere.
Conclusions: We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops. Title: Investigating the Transition Region Explosive Events and Their Relationship to Network Jets Authors: Chen, Yajie; Tian, Hui; Huang, Zhenghua; Peter, Hardi; Samanta, Tanmoy Bibcode: 2019ApJ...873...79C Altcode: 2019arXiv190111215C Recent imaging observations with the Interface Region Imaging Spectrograph (IRIS) have revealed prevalent intermittent jets with apparent speeds of 80-250 km s-1 from the network lanes in the solar transition region (TR). Additionally, spectroscopic observations of the TR lines have revealed the frequent presence of highly non-Gaussian line profiles with enhanced emission at the line wings, often referred to as explosive events (EEs). Using simultaneous imaging and spectroscopic observations from IRIS, we investigate the relationship between EEs and network jets. We first identify EEs from the Si IV 1393.755 Å line profiles in our observations, then examine related features in the 1330 Å slit-jaw images. Our analysis suggests that EEs with double peaks or enhancements in both wings appear to be located at either the footpoints of network jets or transient compact brightenings. These EEs are most likely produced by magnetic reconnection. We also find that EEs with enhancements only at the blue wing are mainly located on network jets, away from the footpoints. These EEs clearly result from the superposition of the high-speed network jets on the TR background. In addition, EEs showing enhancement only at the red wing of the line are often located around the jet footpoints, which is possibly caused by the superposition of reconnection downflows on the background emission. Moreover, we find some network jets that are not associated with any detectable EEs. Our analysis suggests that some EEs are related to the birth or propagation of network jets, and that others are not connected to network jets. 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: Quasi-periodic Fast Propagating Magnetoacoustic Waves during the Magnetic Reconnection Between Solar Coronal Loops Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep; Su, Jiangtao; Song, Hongqiang; Xia, Chun; Hou, Yijun Bibcode: 2018ApJ...868L..33L Altcode: 2018arXiv181108553L Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) multi-wavelength images, we have presented coronal condensations caused by magnetic reconnection between a system of open and closed solar coronal loops. In this Letter, we report the quasi-periodic fast magnetoacoustic waves propagating away from the reconnection region upward across the higher-lying open loops during the reconnection process. On 2012 January 19, reconnection between the higher-lying open loops and lower-lying closed loops took place, and two sets of newly reconnected loops formed. Thereafter, cooling and condensations of coronal plasma occurred in the magnetic dip region of higher-lying open loops. During the reconnection process, disturbances originating from the reconnection region propagate upward across the magnetic dip region of higher-lying loops with the mean speed and mean speed amplitude of 200 and 30 km s-1, respectively. The mean speed of the propagating disturbances decreases from ∼230 km s-1 to ∼150 km s-1 during the coronal condensation process, and then increases to ∼220 km s-1. This temporal evolution of the mean speed anti-correlates with the light curves of the AIA 131 and 304 Å channels that show the cooling and condensation process of coronal plasma. Furthermore, the propagating disturbances appear quasi-periodically with a peak period of 4 minutes. Our results suggest that the disturbances represent the quasi-periodic fast propagating magnetoacoustic (QFPM) waves originating from the magnetic reconnection between coronal loops. Title: Solar Ultraviolet Bursts Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.; Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr Bibcode: 2018SSRv..214..120Y Altcode: 2018arXiv180505850Y The term "ultraviolet (UV) burst" is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017. 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: Coronal Condensations Caused by Magnetic Reconnection between Solar Coronal Loops Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep; Su, Jiangtao; Xia, Chun; Song, Hongqiang; Hou, Yijun Bibcode: 2018ApJ...864L...4L Altcode: 2018arXiv180809626L Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) multi-wavelength images, we report the coronal condensation during the magnetic reconnection (MR) between a system of open and closed coronal loops. Higher-lying magnetically open structures, observed in AIA 171 Å images above the solar limb, move downward and interact with the lower-lying closed loops, resulting in the formation of dips in the former. An X-type structure forms at the interface. The interacting loops reconnect and disappear. Two sets of newly reconnected loops then form and recede from the MR region. During the MR process, bright emission appears sequentially in the AIA 131 and 304 Å channels repeatedly in the dips of higher-lying open structures. This indicates the cooling and condensation process of hotter plasma from ∼0.9 MK down to ∼0.6 MK, and then to ∼0.05 MK, also supported by the light curves of the AIA 171, 131, and 304 Å channels. The part of higher-lying open structures supporting the condensation participate in the successive MR. Without support from underlying loops, the condensation then rains back to the solar surface along the newly reconnected loops. Our results suggest that the MR between coronal loops leads to the condensation of hotter coronal plasma and its downflows. MR thus plays an active role in the mass cycle of coronal plasma because it can initiate the catastrophic cooling and condensation. This underlines that the magnetic and thermal evolution has to be treated together and cannot be separated, even in the case of catastrophic cooling. 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: Amueller/Word_Cloud: Wordcloud 1.5.0 Authors: Mueller, Andreas; Fillion-Robin, Jean-Christophe; Boidol, Raphael; Tian, Font; Nechifor, Paul; yoonsubKim; Peter; Rampin, Remi; Corvellec, Marianne; Medina, Juan; Dai, Yuchao; Petrushev, Baze; Langner, Karol M.; Hong; Alessio; Ozsvald, Ian; vkolmakov; Jones, Terry; Bailey, Eric; Rho, Valentina; IgorAPM; Roy, Divakar; May, Chandler; foobuzz; Piyush; Kian Seong, Low; Van Goey, Jeroen; Seden Smith, James; Gus; Mai, Feng Bibcode: 2018zndo....594952M Altcode: A little word cloud generator in Python 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: Kinetic Simulation of Slow Magnetosonic Waves and Quasi-Periodic Upflows in the Solar Corona Authors: He, Jiansen; Peter, Hardi; Vocks, Christian; Marsch, Eckart; Wang, Linghua; Zhang, Lei; Ruan, Wenzhi Bibcode: 2018cosp...42E1405H Altcode: Quasi-periodic disturbances of emission-line parameters are frequently observed in the corona. These disturbances propagate upward along the magnetic field with speeds of _100 km/s. This phenomenon has been interpreted as evidence of the propagation of slow magnetosonic waves or has been argued to be a signature of intermittent outflows superposed on the background plasmas. Here we aim to present a new "wave + flow" model to interpret these observations. In our scenario, the oscillatory motion is a slow-mode wave, and the flow is associated with a beam created by the wave-particle interaction owing to Landau resonance. With the help of a kinetic model, we simulate the propagation of slow-mode waves and the generation of beam flows. We find that weak periodic beam flows can be generated by to Landau resonance in the solar corona, and the phase with the strongest blueward asymmetry is ahead of that with the strongest blueshift by about 1/4 period. We also find that the slow wave dampsto the level of 1/e after the transit time of two wave periods, owing to Landau damping and Coulomb collisions in our simulation. This damping timescale is similar to that resulting from thermal conduction in the MHD regime.The beam flow is weakened/attenuated with increasing wave period and decreasing wave amplitude since Coulomb collisions become more and more dominant over the wave action. We suggest that this "wave + flow"kinetic model provides an alternative explanation for the observed quasi-periodic propagating perturbations in various parameters in the solar corona. Title: Two Solar Tornadoes Observed with the Interface Region Imaging Spectrograph Authors: Yang, Zihao; Peter, Hardi; Su, Yang; Samanta, Tanmoy; Zhang, Jingwen; Tian, Hui; Chen, Yajie Bibcode: 2018cosp...42E3746Y Altcode: The barbs or legs of some prominences show an apparent motion of rotation, which are often termed solar tornadoes. It is under debate whether the apparent motion is a real rotating motion, or caused by oscillations or counter-streaming flows. We present analysis results from spectroscopic observations of two tornadoes by the Interface Region Imaging Spectrograph. Each tornado was observed for more than 2.5 hours. Doppler velocities are derived through a single Gaussian fit to the Mg II k 2796Å and Si IV 1393Å line profiles. We find coherent and stable red and blue shifts adjacent to each other across the tornado axes, which appears to favor the interpretation of these tornadoes as rotating cool plasmas with temperatures of 10^4 K-10^5 K. This interpretation is further supported by simultaneous observations of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, which reveal periodic motions of dark structures in the tornadoes. Our results demonstrate that spectroscopic observations can provide key information to disentangle different physical processes in solar prominences. Title: Concurrence of Cool and Warm Jets by Magnetic Flux Emerging from below the Solar Chromosphere to the Transition Region Authors: He, Jiansen; Tu, Chuanyi; Peter, Hardi; Wang, Linghua; Yang, Liping; Zhang, Lei; Yan, Limei Bibcode: 2018cosp...42E1402H Altcode: In the solar atmosphere, jets are ubiquitous at various spatial-temporal scales. They are important for understanding the energy and mass transports in the solar atmosphere. According to recent observational studies, the high-speed network jets are likely to be intermittent but continual sources of mass and energy for the solar wind. Here, weconduct a magnetohydrodynamics simulation to investigate the mechanism of these network jets. A combination of magnetic flux emergence and horizontal advection is used to drive the magnetic reconnection in the transition region between a strong magnetic loop and a background open flux. The simulation results show that not only a fast warm jet, much similar to the network jets, is found, but also an adjacent slow cool jet, mostly like classical spicules, is launched. Differing from the fast warm jet as driven by magnetic reconnection, the slow cool jet is mainly accelerated by gradients of both thermal pressure and magnetic pressure near the outer border of the mass-concentrated region compressed by the emerging loop. These results provide a new perspective on our understanding of the formation of both the slow cool jets from the solar chromosphere and the fast warm jets from the solar transition region. Title: A Si IV/O IV Electron Density Diagnostic for the Analysis of IRIS Solar Spectra Authors: Young, P. R.; Keenan, F. P.; Milligan, R. O.; Peter, H. Bibcode: 2018ApJ...857....5Y Altcode: 2018arXiv180301721Y Solar spectra of ultraviolet bursts and flare ribbons from the Interface Region Imaging Spectrograph (IRIS) have suggested high electron densities of > {10}12 cm-3 at transition region temperatures of 0.1 MK, based on large intensity ratios of Si IV λ1402.77 to O IV λ1401.16. In this work, a rare observation of the weak O IV λ1343.51 line is reported from an X-class flare that peaked at 21:41 UT on 2014 October 24. This line is used to develop a theoretical prediction of the Si IV λ1402.77 to O IV λ1401.16 ratio as a function of density that is recommended to be used in the high-density regime. The method makes use of new pressure-dependent ionization fractions that take account of the suppression of dielectronic recombination at high densities. It is applied to two sequences of flare kernel observations from the October 24 flare. The first shows densities that vary between 3× {10}12 and 3× {10}13 cm-3 over a seven-minute period, while the second location shows stable density values of around 2× {10}12 cm-3 over a three-minute period. Title: Mass and energy supply of a cool coronal loop near its apex Authors: Yan, Limei; Peter, Hardi; He, Jiansen; Xia, Lidong; Wang, Linghua Bibcode: 2018A&A...611A..49Y Altcode: Context. Different models for the heating of solar corona assume or predict different locations of the energy input: concentrated at the footpoints, at the apex, or uniformly distributed. The brightening of a loop could be due to the increase in electron density ne, the temperature T, or a mixture of both. Aim. We investigate possible reasons for the brightening of a cool loop at transition region temperatures through imaging and spectral observation.
Methods: We observed a loop with the Interface Region Imaging Spectrograph (IRIS) and used the slit-jaw images together with spectra taken at a fixed slit position to study the evolution of plasma properties in and below the loop. We used spectra of Si IV, which forms at around 80 000 K in equilibrium, to identify plasma motions and derive electron densities from the ratio of inter-combination lines of O IV. Additional observations from the Solar Dynamics Observatory (SDO) were employed to study the response at coronal temperatures (Atmospheric Imaging Assembly, AIA) and to investigate the surface magnetic field below the loop (Helioseismic and Magnetic Imager, HMI).
Results: The loop first appears at transition region temperatures and later also at coronal temperatures, indicating a heating of the plasma in the loop. The appearance of hot plasma in the loop coincides with a possible accelerating upflow seen in Si IV, with the Doppler velocity shifting continuously from -70 km s-1 to -265 km s-1. The 3D magnetic field lines extrapolated from the HMI magnetogram indicate possible magnetic reconnection between small-scale magnetic flux tubes below or near the loop apex. At the same time, an additional intensity enhancement near the loop apex is visible in the IRIS slit-jaw images at 1400 Å. These observations suggest that the loop is probably heated by the interaction between the loop and the upflows, which are accelerated by the magnetic reconnection between small-scale magnetic flux tubes at lower altitudes. Before and after the possible heating phase, the intensity changes in the optically thin (Si IV) and optical thick line (C II) are mainly contributed by the density variation without significant heating.
Conclusions: We therefore provide evidence for the heating of an envelope loop that is affected by accelerating upflows, which are probably launched by magnetic reconnection between small-scale magnetic flux tubes underneath the envelope loop. This study emphasizes that in the complex upper atmosphere of the Sun, the dynamics of the 3D coupled magnetic field and flow field plays a key role in thermalizing 1D structures such as coronal loops.

An animation associated to Fig. 1 is available at https://www.aanda.org 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: Magnetic Reconnection at the Earliest Stage of Solar Flux Emergence Authors: Tian, Hui; Zhu, Xiaoshuai; Peter, Hardi; Zhao, Jie; Samanta, Tanmoy; Chen, Yajie Bibcode: 2018ApJ...854..174T Altcode: 2018arXiv180106785T On 2016 September 20, the Interface Region Imaging Spectrograph observed an active region during its earliest emerging phase for almost 7 hr. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory observed continuous emergence of small-scale magnetic bipoles with a rate of ∼1016 Mx s-1. The emergence of magnetic fluxes and interactions between different polarities lead to the frequent occurrence of ultraviolet (UV) bursts, which exhibit as intense transient brightenings in the 1400 Å images. In the meantime, discrete small patches with the same magnetic polarity tend to move together and merge, leading to the enhancement of the magnetic fields and thus the formation of pores (small sunspots) at some locations. The spectra of these UV bursts are characterized by the superposition of several chromospheric absorption lines on the greatly broadened profiles of some emission lines formed at typical transition region temperatures, suggesting heating of the local materials to a few tens of thousands of kelvin in the lower atmosphere by magnetic reconnection. Some bursts reveal blue- and redshifts of ∼100 km s-1 at neighboring pixels, indicating the spatially resolved bidirectional reconnection outflows. Many such bursts appear to be associated with the cancellation of magnetic fluxes with a rate of the order of ∼1015 Mx s-1. We also investigate the three-dimensional magnetic field topology through a magnetohydrostatic model and find that a small fraction of the bursts are associated with bald patches (magnetic dips). Finally, we find that almost all bursts are located in regions of large squashing factor at the height of ∼1 Mm, reinforcing our conclusion that these bursts are produced through reconnection in the lower atmosphere. Title: Two Solar Tornadoes Observed with the Interface Region Imaging Spectrograph Authors: Yang, Zihao; Tian, Hui; Peter, Hardi; Su, Yang; Samanta, Tanmoy; Zhang, Jingwen; Chen, Yajie Bibcode: 2018ApJ...852...79Y Altcode: 2017arXiv171108968Y The barbs or legs of some prominences show an apparent motion of rotation, which are often termed solar tornadoes. It is under debate whether the apparent motion is a real rotating motion, or caused by oscillations or counter-streaming flows. We present analysis results from spectroscopic observations of two tornadoes by the Interface Region Imaging Spectrograph. Each tornado was observed for more than 2.5 hr. Doppler velocities are derived through a single Gaussian fit to the Mg II k 2796 Å and Si IV 1393 Å line profiles. We find coherent and stable redshifts and blueshifts adjacent to each other across the tornado axes, which appears to favor the interpretation of these tornadoes as rotating cool plasmas with temperatures of 104 K-105 K. This interpretation is further supported by simultaneous observations of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, which reveal periodic motions of dark structures in the tornadoes. Our results demonstrate that spectroscopic observations can provide key information to disentangle different physical processes in solar prominences. Title: Formation of Cool and Warm Jets by Magnetic Flux Emerging from the Solar Chromosphere to Transition Region Authors: Yang, Liping; Peter, Hardi; He, Jiansen; Tu, Chuanyi; Wang, Linghua; Zhang, Lei; Yan, Limei Bibcode: 2018ApJ...852...16Y Altcode: In the solar atmosphere, jets are ubiquitous at various spatial-temporal scales. They are important for understanding the energy and mass transports in the solar atmosphere. According to recent observational studies, the high-speed network jets are likely to be intermittent but continual sources of mass and energy for the solar wind. Here, we conduct a 2D magnetohydrodynamics simulation to investigate the mechanism of these network jets. A combination of magnetic flux emergence and horizontal advection is used to drive the magnetic reconnection in the transition region between a strong magnetic loop and a background open flux. The simulation results show that not only a fast warm jet, much similar to the network jets, is found, but also an adjacent slow cool jet, mostly like classical spicules, is launched. Differing from the fast warm jet driven by magnetic reconnection, the slow cool jet is mainly accelerated by gradients of both thermal pressure and magnetic pressure near the outer border of the mass-concentrated region compressed by the emerging loop. These results provide a different perspective on our understanding of the formation of both the slow cool jets from the solar chromosphere and the fast warm jets from the solar transition region. Title: The temporal evolution of explosive events and its implication on reconnection dynamics Authors: Guo, L.; Liu, W.; De Pontieu, B.; Huang, Y. M.; Peter, H.; Bhattacharjee, A. Bibcode: 2017AGUFMSH43A2803G Altcode: Transition-region explosive events and other bursts seen in extreme UV light are characterized by broad spectral line profiles, and the more violent ones show a strong enhancement of emission. They are thought to be driven by magnetic reconnection, because of their characteristic spectral profiles often indicating strong Alfvénic flows, and because of the fact that they typically occur where magnetic flux concentrations of opposite polarity intersect. In this presentation, we will focus on the temporal evolution of transition-region explosive events. In particular, we will investigate fast onsets of these events and the rapid oscillations of intensity during these event. The fast onset refers to the beginning of an explosive event, where the intensities and the widths of its line profiles increase dramatically (often within less than 10 seconds) and the rapid oscillations of intensity refer to blinks of emission that usually last less than 10 seconds during the event. In order to interpret and understand underlying mechanisms of these observations, we conduct numerical simulation of an explosive event and calculate its spectra. We observe a similar temporal evolution in the synthetic Si IV spectra when the explosive event is driven by time-dependent reconnection—plasmoid instability. The qualitative agreement between observations and simulations suggests that the temporal evolution of Si IV spectra of explosive events are closely related to reconnection dynamics. Title: Current systems of coronal loops in 3D MHD simulations Authors: Warnecke, J.; Chen, F.; Bingert, S.; Peter, H. Bibcode: 2017A&A...607A..53W Altcode: 2016arXiv161106170W
Aims: We study the magnetic field and current structure associated with a coronal loop. Through this we investigate to what extent the assumptions of a force-free magnetic field break down and where they might be justified.
Methods: We analyze a three-dimensional (3D) magnetohydrodynamic (MHD) model of the solar corona in an emerging active region with the focus on the structure of the forming coronal loops. The lower boundary of this simulation is taken from a model of an emerging active region. As a consequence of the emerging magnetic flux and the horizontal motions at the surface a coronal loop forms self-consistently. We investigate the current density along magnetic field lines inside (and outside) this loop and study the magnetic and plasma properties in and around this loop. The loop is defined as the bundle of field lines that coincides with enhanced emission in extreme UV.
Results: We find that the total current along the emerging loop changes its sign from being antiparallel to parallel to the magnetic field. This is caused by the inclination of the loop together with the footpoint motion. Around the loop, the currents form a complex non-force-free helical structure. This is directly related to a bipolar current structure at the loop footpoints at the base of the corona and a local reduction of the background magnetic field (I.e., outside the loop) caused by the plasma flow into and along the loop. Furthermore, the locally reduced magnetic pressure in the loop allows the loop to sustain a higher density, which is crucial for the emission in extreme UV. The action of the flow on the magnetic field hosting the loop turns out to also be responsible for the observed squashing of the loop.
Conclusions: The complex magnetic field and current system surrounding it can only be modeled in 3D MHD models where the magnetic field has to balance the plasma pressure. A one-dimensional coronal loop model or a force-free extrapolation cannot capture the current system and the complex interaction of the plasma and the magnetic field in the coronal loop, despite the fact that the loop is under low-β conditions. Title: Compact solar UV burst triggered in a magnetic field with a fan-spine topology Authors: Chitta, L. P.; Peter, H.; Young, P. R.; Huang, Y. -M. Bibcode: 2017A&A...605A..49C Altcode: 2017arXiv170608059C Context. Solar ultraviolet (UV) bursts are small-scale features that exhibit intermittent brightenings that are thought to be due to magnetic reconnection. They are observed abundantly in the chromosphere and transition region, in particular in active regions.
Aims: We investigate in detail a UV burst related to a magnetic feature that is advected by the moat flow from a sunspot towards a pore. The moving feature is parasitic in that its magnetic polarity is opposite to that of the spot and the pore. This comparably simple photospheric magnetic field distribution allows for an unambiguous interpretation of the magnetic geometry leading to the onset of the observed UV burst.
Methods: We used UV spectroscopic and slit-jaw observations from the Interface Region Imaging Spectrograph (IRIS) to identify and study chromospheric and transition region spectral signatures of said UV burst. To investigate the magnetic topology surrounding the UV burst, we used a two-hour-long time sequence of simultaneous line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) and performed data-driven 3D magnetic field extrapolations by means of a magnetofrictional relaxation technique. We can connect UV burst signatures to the overlying extreme UV (EUV) coronal loops observed by the Atmospheric Imaging Assembly (AIA).
Results: The UV burst shows a variety of extremely broad line profiles indicating plasma flows in excess of ±200 km s-1 at times. The whole structure is divided into two spatially distinct zones of predominantly up- and downflows. The magnetic field extrapolations show a persistent fan-spine magnetic topology at the UV burst. The associated 3D magnetic null point exists at a height of about 500 km above the photosphere and evolves co-spatially with the observed UV burst. The EUV emission at the footpoints of coronal loops is correlated with the evolution of the underlying UV burst.
Conclusions: The magnetic field around the null point is sheared by photospheric motions, triggering magnetic reconnection that ultimately powers the observed UV burst and energises the overlying coronal loops. The location of the null point suggests that the burst is triggered low in the solar chromosphere.

Movies associated to Figs. 2 and 4 are available at http://www.aanda.org Title: Sentinel-1A - First precise orbit determination results Authors: Peter, H.; Jäggi, A.; Fernández, J.; Escobar, D.; Ayuga, F.; Arnold, D.; Wermuth, M.; Hackel, S.; Otten, M.; Simons, W.; Visser, P.; Hugentobler, U.; Féménias, P. Bibcode: 2017AdSpR..60..879P Altcode: Sentinel-1A is the first satellite of the European Copernicus programme. Equipped with a Synthetic Aperture Radar (SAR) instrument the satellite was launched on April 3, 2014. Operational since October 2014 the satellite delivers valuable data for more than two years. The orbit accuracy requirements are given as 5 cm in 3D. In order to fulfill this stringent requirement the precise orbit determination (POD) is based on the dual-frequency GPS observations delivered by an eight-channel GPS receiver.

The Copernicus POD (CPOD) Service is in charge of providing the orbital and auxiliary products required by the PDGS (Payload Data Ground Segment). External orbit validation is regularly performed by comparing the CPOD Service orbits to orbit solutions provided by POD expert members of the Copernicus POD Quality Working Group (QWG). The orbit comparisons revealed systematic orbit offsets mainly in radial direction (approx. 3 cm). Although no independent observation technique (e.g. DORIS, SLR) is available to validate the GPS-derived orbit solutions, comparisons between the different antenna phase center variations and different reduced-dynamic orbit determination approaches used in the various software packages helped to detect the cause of the systematic offset. An error in the given geometry information about the satellite has been found. After correction of the geometry the orbit validation shows a significant reduction of the radial offset to below 5 mm. The 5 cm orbit accuracy requirement in 3D is fulfilled according to the results of the orbit comparisons between the different orbit solutions from the QWG. 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 Width Distribution of Loops and Strands in the Solar Corona—Are We Hitting Rock Bottom? Authors: Aschwanden, Markus J.; Peter, Hardi Bibcode: 2017ApJ...840....4A Altcode: 2017arXiv170101177A In this study, we analyze Atmospheric Imaging Assembly (AIA) and Hi-C images in order to investigate absolute limits for the finest loop strands. We develop a model of the occurrence-size distribution function of coronal loop widths, characterized by the lower limit of widths w min, the peak (or most frequent) width w p , the peak occurrence number n p , and a power-law slope a. Our data analysis includes automated tracing of curvilinear features with the OCCULT-2 code, automated sampling of the cross-sectional widths of coronal loops, and fitting of the theoretical size distribution to the observed distribution. With Monte Carlo simulations and variable pixel sizes {{Δ }}x, we derive a first diagnostic criterion to discriminate whether the loop widths are unresolved ({w}p/{{Δ }}x≈ 2.5+/- 0.2) or fully resolved (if {w}p/{{Δ }}x≳ 2.7). For images with resolved loop widths, we can apply a second diagnostic criterion that predicts the lower limit of loop widths as a function of the spatial resolution. We find that the loop widths are marginally resolved in AIA images but are fully resolved in Hi-C images, where our model predicts a most frequent (peak) value at {w}p≈ 550 {km}, in agreement with recent results of Brooks et al. This result agrees with the statistics of photospheric granulation sizes and thus supports coronal heating mechanisms operating on the macroscopic scale of photospheric magneto-convection, rather than nanoflare braiding models on unresolved microscopic scales. 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: Miniature loops in the solar corona Authors: Barczynski, K.; Peter, H.; Savage, S. L. Bibcode: 2017A&A...599A.137B Altcode: 2016arXiv161108513B Context. Magnetic loops filled with hot plasma are the main building blocks of the solar corona. Usually they have lengths of the order of the barometric scale height in the corona that is 50 Mm.
Aims: Previously it has been suggested that miniature versions of hot loops exist. These would have lengths of only 1 Mm barely protruding from the chromosphere and spanning across just one granule in the photosphere. Such short loops are well established at transition region temperatures (0.1 MK), and we investigate if such miniature loops also exist at coronal temperatures (>1 MK).
Methods: We used extreme UV (EUV) imaging observations from the High-resolution Coronal Imager (Hi-C) at an unprecedented spatial resolution of 0.3'' to 0.4''. Together with EUV imaging and magnetogram data from the Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) data from Hinode we investigated the spatial, temporal and thermal evolution of small loop-like structures in the solar corona above a plage region close to an active region and compared this to a moss area within the active region.
Results: We find that the size, motion and temporal evolution of the loop-like features are consistent with photospheric motions, suggesting a close connection to the photospheric magnetic field. Aligned magnetograms show that one of their endpoints is rooted at a magnetic concentration. Their thermal structure, as revealed together with the X-ray observations, shows significant differences to moss-like features.
Conclusions: Considering different scenarios, these features are most probably miniature versions of hot loops rooted at magnetic concentrations at opposite sides of a granule in small emerging magnetic loops (or flux tubes). Title: A Form of Nascent Solar Wind Outflow: Beam flow Generated by Slow-Mode Waves Through Landau Resonance in the Weakly Collisional Solar Atmosphere Authors: He, J.; Ruan, W.; Zhang, L.; Vocks, C.; Marsch, E.; Tu, C.; Peter, H.; Wang, L. Bibcode: 2016AGUFMSH51B2589H Altcode: Quasi-periodic disturbances of emission-line parameters are frequently observed in the corona. These disturbances propagate upward along the magnetic field with speeds 100 km s-1. This phenomenon has been interpreted as an evidence of the propagation of slow magnetosonic waves or argued to be a signature of the intermittent outflows superposed on the background plasmas. Here we aim to present a new "wave + flow" model to interpret these observations. In our scenario, the oscillatory motion is a slow mode wave, and the flow is associated with a beam created by the wave-particle interaction owing to Landau resonance. With the help of a kinetic model, we simulate the propagation of slow mode waves and the generation of beam flows. We find that weak periodic beam flows can be generated owing to Landau resonance in the solar corona, and the phase with strongest blueward asymmetry is ahead of that with strongest blueshift by about 1/4 period. We also find that the slow wave damps to the level of 1/e after the transit time of two wave periods, owing to Landau damping and Coulomb collisions in our simulation. This damping time scale is similar to that resulting from thermal-conduction in the MHD regime. The beam flow is weakened/attenuated with increasing wave period and decreasing wave amplitude since Coulomb collision becomes more and more dominant over the wave action. We suggest that this "wave + flow" kinetic model provides an alternative explanation for the observed quasi-periodic propagating perturbations in various parameters in the solar corona. Therefore, the compressible slow-mode waves, which can be driven and launched by magnetic reconnection, vertical piston oscillation, or periodic horizontal squeezing, is believed to play an important role in accelerating the plasmas into a form of the solar wind nascent outflows. Title: The importance of high-resolution observations of the solar corona Authors: Winebarger, A. R.; Cirtain, J. W.; Golub, L.; Walsh, R. W.; De Pontieu, B.; Savage, S. L.; Rachmeler, L.; Kobayashi, K.; Testa, P.; Brooks, D.; Warren, H.; Mcintosh, S. W.; Peter, H.; Morton, R. J.; Alexander, C. E.; Tiwari, S. K. Bibcode: 2016AGUFMSH31B2577W Altcode: The spatial and temporal resolutions of the available coronal observatories are inadequate to resolve the signatures of coronal heating. High-resolution and high-cadence observations available with the Interface Region Imaging Spectrograph (IRIS) and the High-resolution Coronal Imager (Hi-C) instrument hint that 0.3 arcsec resolution images and < 10 s cadence provide the necessary resolution to detect heating events. Hi-C was launched from White Sands Missile Range on July 11, 2012 (before the launch with IRIS) and obtained images of a solar active region in the 19.3 nm passband. In this presentation, I will discuss the potential of combining a flight in Hi-C with a 17.1 nm passband, in conjunction with IRIS. This combination will provide, for the first time, a definitive method of tracing the energy flow between the chromosphere and corona and vice versa. Title: Fine flow structures in the transition region small-scale loops Authors: Yan, L.; Peter, H.; He, J.; Wei, Y. Bibcode: 2016AGUFMSH31B2569Y Altcode: The observation and model have suggested that the transition region EUV emission from the quiet sun region is contributed by very small scale loops which have not been resolved. Recently, the observation from IRIS has revealed that this kind of small scale loops. Based on the high resolution spectral and imaging observation from IRIS, much more detail work needs to be done to reveal the fine flow features in this kind of loop to help us understand the loop heating. Here, we present a detail statistical study of the spatial and temporal evolution of Si IV line profiles of small scale loops and report the spectral features: there is a transition from blue (red) wing enhancement dominant to red (blue) wing enhancement dominant along the cross-section of the loop, which is independent of time. This feature appears as the loop appear and disappear as the loop un-visible. This is probably the signature of helical flow along the loop. The result suggests that the brightening of this kind of loop is probably due to the current dissipation heating in the twisted magnetic field flux tube. Title: GREGOR first results Authors: Peter, Hardi; Forveille, Thierry; Alves, Joao Bibcode: 2016A&A...596E...1P Altcode: No abstract at ADS Title: Magnetic reconnection between a solar filament and nearby coronal loops Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Priest, Eric; Chen, Huadong; Guo, Lijia; Chen, Feng; Mackay, Duncan Bibcode: 2016NatPh..12..847L Altcode: 2016arXiv160503320L Magnetic reconnection is difficult to observe directly but coronal structures on the Sun often betray the magnetic field geometry and its evolution. Here we report the observation of magnetic reconnection between an erupting filament and its nearby coronal loops, resulting in changes in the filament connection. X-type structures form when the erupting filament encounters the loops. The filament becomes straight, and bright current sheets form at the interfaces. Plasmoids appear in these current sheets and propagate bi-directionally. The filament disconnects from the current sheets, which gradually disperse and disappear, then reconnects to the loops. This evolution suggests successive magnetic reconnection events predicted by theory but rarely detected with such clarity in observations. Our results confirm the three-dimensional magnetic reconnection theory and have implications for the evolution of dissipation regions and the release of magnetic energy for reconnection in many magnetized plasma systems. Title: The SPICE Spectral Imager on Solar Orbiter: Linking the Sun to the Heliosphere Authors: Fludra, Andrzej; Haberreiter, Margit; Peter, Hardi; Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina; Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Davila, Joseph; Giunta, Alessandra; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo Bibcode: 2016cosp...41E.607F Altcode: The SPICE (Spectral Imaging of the Coronal Environment) instrument is one of the key remote sensing instruments onboard the upcoming Solar Orbiter Mission. SPICE has been designed to contribute to the science goals of the mission by investigating the source regions of outflows and ejection processes which link the solar surface and corona to the heliosphere. In particular, SPICE will provide quantitative information on the physical state and composition of the solar atmosphere plasma. For example, SPICE will access relative abundances of ions to study the origin and the spatial/temporal variations of the 'First Ionization Potential effect', which are key signatures to trace the solar wind and plasma ejections paths within the heliosphere. Here we will present the instrument and its performance capability to attain the scientific requirements. We will also discuss how different observation modes can be chosen to obtain the best science results during the different orbits of the mission. To maximize the scientific return of the instrument, the SPICE team is working to optimize the instrument operations, and to facilitate the data access and their exploitation. Title: Solar abundances with the SPICE spectral imager on Solar Orbiter Authors: Giunta, Alessandra; Haberreiter, Margit; Peter, Hardi; Vial, Jean-Claude; Harrison, Richard; Parenti, Susanna; Innes, Davina; Schmutz, Werner; Buchlin, Eric; Chamberlin, Phillip; Thompson, William; Bocchialini, Karine; Gabriel, Alan; Morris, Nigel; Caldwell, Martin; Auchere, Frederic; Curdt, Werner; Teriaca, Luca; Hassler, Donald M.; DeForest, Craig; Hansteen, Viggo; Carlsson, Mats; Philippon, Anne; Janvier, Miho; Wimmer-Schweingruber, Robert; Griffin, Douglas; Baudin, Frederic; Davila, Joseph; Fludra, Andrzej; Waltham, Nick; Eccleston, Paul; Gottwald, Alexander; Klein, Roman; Hanley, John; Walls, Buddy; Howe, Chris; Schuehle, Udo; Gyo, Manfred; Pfiffner, Dany Bibcode: 2016cosp...41E.681G Altcode: Elemental composition of the solar atmosphere and in particular abundance bias of low and high First Ionization Potential (FIP) elements are a key tracer of the source regions of the solar wind. These abundances and their spatio-temporal variations, as well as the other plasma parameters , will be derived by the SPICE (Spectral Imaging of the Coronal Environment) EUV spectral imager on the upcoming Solar Orbiter mission. SPICE is designed to provide spectroheliograms (spectral images) using a core set of emission lines arising from ions of both low-FIP and high-FIP elements. These lines are formed over a wide range of temperatures, enabling the analysis of the different layers of the solar atmosphere. SPICE will use these spectroheliograms to produce dynamic composition maps of the solar atmosphere to be compared to in-situ measurements of the solar wind composition of the same elements (i.e. O, Ne, Mg, Fe). This will provide a tool to study the connectivity between the spacecraft (the Heliosphere) and the Sun. We will discuss the SPICE capabilities for such composition measurements. Title: Kinetic Simulation of Slow Magnetosonic Waves and Quasi-Periodic Upflows in the Solar Corona Authors: Ruan, Wenzhi; He, Jiansen; Zhang, Lei; Vocks, Christian; Marsch, Eckart; Tu, Chuanyi; Peter, Hardi; Wang, Linghua Bibcode: 2016ApJ...825...58R Altcode: 2016arXiv160101823R Quasi-periodic disturbances of emission-line parameters are frequently observed in the corona. These disturbances propagate upward along the magnetic field with speeds of ∼100 km s-1. This phenomenon has been interpreted as evidence of the propagation of slow magnetosonic waves or has been argued to be a signature of intermittent outflows superposed on the background plasmas. Here we aim to present a new “wave + flow” model to interpret these observations. In our scenario, the oscillatory motion is a slow-mode wave, and the flow is associated with a beam created by the wave-particle interaction owing to Landau resonance. With the help of a kinetic model, we simulate the propagation of slow-mode waves and the generation of beam flows. We find that weak periodic beam flows can be generated by to Landau resonance in the solar corona, and the phase with the strongest blueward asymmetry is ahead of that with the strongest blueshift by about 1/4 period. We also find that the slow wave damps to the level of 1/e after the transit time of two wave periods, owing to Landau damping and Coulomb collisions in our simulation. This damping timescale is similar to that resulting from thermal conduction in the MHD regime. The beam flow is weakened/attenuated with increasing wave period and decreasing wave amplitude since Coulomb collisions become more and more dominant over the wave action. We suggest that this “wave + flow” kinetic model provides an alternative explanation for the observed quasi-periodic propagating perturbations in various parameters in the solar corona. Title: Scaling laws of coronal loops compared to a 3D MHD model of an active region Authors: Bourdin, Ph. -A.; Bingert, S.; Peter, H. Bibcode: 2016A&A...589A..86B Altcode: 2016arXiv160305276B Context. The structure and heating of coronal loops have been investigated for decades. Established scaling laws relate fundamental quantities like the loop apex temperature, pressure, length, and coronal heating.
Aims: We test these scaling laws against a large-scale 3D magneto-hydrodynamics (MHD) model of the solar corona, which became feasible with current high-performance computing.
Methods: We drove an active region simulation with photospheric observations and find strong similarities to the observed coronal loops in X-rays and extreme-ultraviolet (EUV) wavelength. A 3D reconstruction of stereoscopic observations shows that our model loops have a realistic spatial structure. We compared scaling laws to our model data extracted along an ensemble of field lines. Finally, we fit a new scaling law that represents hot loops and also cooler structures, which was not possible before based only on observations.
Results: Our model data gives some support for scaling laws that were established for hot and EUV-emissive coronal loops. For the Rosner-Tucker-Vaiana (RTV) scaling law we find an offset to our model data, which can be explained by 1D considerations of a static loop with a constant heat input and conduction. With a fit to our model data we set up a new scaling law for the coronal heat input along magnetic field lines.
Conclusions: RTV-like scaling laws were fitted to hot loops and therefore do not predict well the coronal heat input for cooler structures that are barely observable. The basic differences between 1D and self-consistent 3D modeling account for deviations between earlier scaling laws and ours. We also conclude that a heating mechanism by MHD-turbulent dissipation within a braided flux tube would heat the corona stronger than is consistent with our model corona. Title: A closer look at a coronal loop rooted in a sunspot umbra Authors: Chitta, L. P.; Peter, H.; Young, P. R. Bibcode: 2016A&A...587A..20C Altcode: 2015arXiv151203831C Context. Extreme UV (EUV) and X-ray loops in the solar corona connect regions of enhanced magnetic activity, but they are not usually rooted in the dark umbrae of sunspots because the strong magnetic field found there suppresses convection. This means that the Poynting flux of magnetic energy into the upper atmosphere is not significant within the umbra as long as there are no light bridges or umbral dots.
Aims: Here we report a rare observation of a coronal loop rooted in the dark umbra of a sunspot without any traces of light bridges or umbral dots. This allows us to investigate the loop without much confusion from background or line-of-sight integration effects.
Methods: We used the slit-jaw images and spectroscopic data from the Interface Region Imaging Spectrograph (IRIS) and concentrate on the line profiles of O iv and Si iv that show persistent strong redshifted components in the loop rooted in the umbra. Using the ratios of O iv, we can estimate the density and thus investigate the mass flux. The coronal context and temperature diagnostics of these observations is provided through the EUV channels of the Atmospheric Imaging Assembly (AIA).
Results: The coronal loop, embedded within cooler downflows, hosts supersonic downflows. The speed of more than 100 km s-1 is on the same order of magnitude in the transition region lines of O iv and Si iv, and is even seen at comparable speed in the chromospheric Mg II lines. At a projected distance of within 1'' of the footpoint, we see a shock transition to smaller downflow speeds of about 15 km s-1 being consistent with mass conservation across a stationary isothermal shock.
Conclusions: We see no direct evidence for energy input into the loop because the loop is rooted in the dark uniform part of the umbra with no light bridges or umbral dots near by. Thus one might conclude that we are seeing a siphon flow driven from the footpoint at the other end of the loop. However, for a final result data of similar quality at the other footpoint are needed, but this is too far away to be covered by the IRIS field of view. Title: A Numerical Investigation of the Recurrent High-speed Jets as a Possibility of Solar Wind Origin Authors: Yang, Liping; He, Jiansen; Peter, Hardi; Tu, Chuanyi; Zhang, Lei; Marsch, Eckart; Wang, Linghua; Feng, Xueshang Bibcode: 2015arXiv151201868Y Altcode: In the solar atmosphere, jets are prevalent and they are significant for the mass and energy transport. Here we conduct numerical simulations to investigate the mass and energy contributions of the recently observed high-speed jets to the solar wind. With a one-dimensional hydrodynamic solar wind model, the time-dependent pulses are imposed at the bottom to simulate the jets. The simulation results show that without other energy source, the injected plasmas are accelerated effectively to be a transonic wind with a substantial mass flux. The rapid acceleration occurs close to the Sun, and the resulting asymptotic speed, number density at 0.3 AU, as well as mass flux normalized to 1 AU are compatible with in situ observations. As a result of the high speed, the imposed pulses generate a train of shocks traveling upward. By tracing the motions of the injected plasma, it is found that these shocks heat and accelerate the injected plasmas successively step by step to push them upward and eventually allow them to escape. The parametric studies show that increasing the speed of the imposed pulses or their temperature gives a considerably faster, and hotter solar wind, while increasing their number density or decreasing their recurring period only bring a denser solar wind. These studies provide a possibility that the ubiquitous high-speed jets are a substantial mass and energy contributions to the solar wind. Title: Limitations of force-free magnetic field extrapolations: Revisiting basic assumptions Authors: Peter, H.; Warnecke, J.; Chitta, L. P.; Cameron, R. H. Bibcode: 2015A&A...584A..68P Altcode: 2015arXiv151004642P Context. Force-free extrapolations are widely used to study the magnetic field in the solar corona based on surface measurements.
Aims: The extrapolations assume that the ratio of internal energy of the plasma to magnetic energy, the plasma β, is negligible. Despite the widespread use of this assumption observations, models, and theoretical considerations show that β is of the order of a few percent to more than 10%, and thus not small. We investigate what consequences this has for the reliability of extrapolation results.
Methods: We use basic concepts starting with force and energy balance to infer relations between plasma β and free magnetic energy to study the direction of currents in the corona with respect to the magnetic field, and to estimate the errors in the free magnetic energy by neglecting effects of the plasma (β ≪ 1). A comparison with a 3D magneto-hydrodynamics (MHD) model supports our basic considerations.
Results: If plasma β is of the order of the relative free energy (the ratio of the free magnetic energy to the total magnetic energy) then the pressure gradient can balance the Lorentz force. This is the case in solar corona, and therefore the currents are not properly described. In particular, the error in terms of magnetic energy by neglecting the plasma is of the order of the free magnetic energy, so that the latter cannot be reliably determined by an extrapolation.
Conclusions: While a force-free extrapolation might capture the magnetic structure and connectivity of the coronal magnetic field, the derived currents and free magnetic energy are not reliable. Thus quantitative results of extrapolations on the location and amount of heating in the corona (through current dissipation) and on the energy storage of the magnetic field (e.g. for eruptive events) are limited. Title: Mass and Energy Transfer Between the Solar Photosphere and Corona Authors: Peter, H. Bibcode: 2015AGUFMSH23D..03P Altcode: The problem of chromospheric and coronal heating is also a problem of mass supply to the corona. On average we see redshifts at transition region temperatures of the order of 10 km/s. If interpreted as downflows, this would quickly empty the corona, and fresh material has to be transported into the corona. Several models have been proposed to understand this mass cycle between the different atmospheric layers. However, as of yet all these proposals have serious shortcomings. On the observational side open questions remain, too. With the new IRIS mission we can observe the transition region at unprecedented spatial and spectral resolution, but the observational results are still puzzling. In particular the finding that the spatial distribution of line widths and Doppler shifts do not change with increasing resolution is against physical intuition. This shows that even with IRIS we still have significant velocity gradients along the line-of-sight, indicating that shocks might play a significant role. Likewise the temporal evolution might be a key for our understanding of the mass cycle. It might well be that the filling and draining of hot plasma occurs on significantly different time scales, which might be part of the difficulty to arrive at a conclusive observational picture. Considering the progress made for the quiet Sun, it seems clear that the processes responsible for the mass exchange are not resolved (yet). Therefore one might wonder to what extent one could use larger and resolved individual events in more active parts of the Sun to understand the details of the mass transport. In particular a common understanding of reconnection events such as Ellerman bombs in the photosphere, explosive events in the transition region and the recently discovered IRIS bombs in-between might provide the key to better understand the mass cycle throughout the atmospheric layers from the photosphere to the corona. Title: Origin of the High-speed Jets Fom Magnetic Flux Emergence in the Solar Transition Region as well as Their Mass and Energy Contribuctions to the Solar Wind Authors: Liping, Y.; He, J.; Peter, H.; Tu, C. Y.; Feng, X. S. Bibcode: 2015AGUFMSH31B2406L Altcode: In the solar atmosphere, the jets are ubiquitous and found to be at various spatia-temporal scales. They are significant to understand energy and mass transport in the solar atmosphere. Recently, the high-speed transition region jets are reported from the observation. Here we conduct a numerical simulation to investigate the mechanism in their formation, as well as their mass and energy contributions to the solar wind. Driven by the supergranular convection motion, the magnetic reconnection between the magnetic loop and the background open flux occurring in the transition region is simulated with a two-dimensional MHD model. The simulation results show that not only a fast hot jet, much resemble the found transition region jets, but also a adjacent slow cool jet, mostly like classical spicules, is launched. The force analysis shows that the fast hot jet is continually driven by the Lorentz force around the reconnection region, while the slow cool jet is induced by an initial kick through the Lorentz force associated with the emerging magnetic flux. Also, the features of the driven jets change with the amount of the emerging magnetic flux, giving the varieties of both jets.With the developed one-dimensional hydrodynamic solar wind model, the time-dependent pulses are imposed at the bottom to simulate the jet behaviors. The simulation results show that without other energy source, the injected plasmas are accelerated effectively to be a transonic wind with a substantial mass flux. The rapid acceleration occurs close to the Sun, and the resulting asymptotic speeds, number density at 0.3 AU, as well as mass flux normalized to 1 AU are compatible with in site observations. As a result of the high speed, the imposed pulses lead to a train of shocks traveling upward. By tracing the motions of the injected plasma, it is found that these shocks heat and accelerate the injected plasma to make part of them propagate upward and eventually escape. The parametric study shows that as the speed and temperature of the imposed pulses increase, we get an increase of the speed and temperature of the driven solar wind, which do not be influenced by the increase of the number density of the imposed pulses. When the recurring period of the imposed pulses decreases, the obtained solar wind becomes slower and cooler. Title: Origin of Both the Fast Hot Jet and the Slow Cool Jet from Magnetic Flux Emergence and Advection in the Solar Transition Region Authors: Yang, Liping; Peter, Hardi; He, Jiansen; Tu, Chuanyi; Wang, Linghua; Zhang, Lei; Feng, Xueshang Bibcode: 2015arXiv151201869Y Altcode: In the solar atmosphere, the jets are ubiquitous and found to be at various spatia-temporal scales. They are significant to understand energy and mass transport in the solar atmosphere. Recently, the high-speed transition region jets are reported from the observation. Here we conduct a numerical simulation to investigate the mechanism in their formation. Driven by the supergranular convection motion, the magnetic reconnection between the magnetic loop and the background open flux occurring in the transition region is simulated with a two-dimensional magnetohydrodynamics model. The simulation results show that not only a fast hot jet, much resemble the found transition region jets, but also a adjacent slow cool jet, mostly like classical spicules, is launched. The force analysis shows that the fast hot jet is continually driven by the Lorentz force around the reconnection region, while the slow cool jet is induced by an initial kick through the Lorentz force associated with the emerging magnetic flux. Also, the features of the driven jets change with the amount of the emerging magnetic flux, giving the varieties of both jets. These results will inspire our understanding of the formation of the prevalence of both the fast hot jet and slow cool jet from the solar transition region and chromosphere. Title: Heating and cooling of coronal loops observed by SDO Authors: Li, L. P.; Peter, H.; Chen, F.; Zhang, J. Bibcode: 2015A&A...583A.109L Altcode: 2015arXiv150904510L Context. One of the most prominent processes to have been suggested as heating the corona to well above 106 K builds on nanoflares, which are short bursts of energy dissipation.
Aims: We compare observations to model predictions to test the validity of the nanoflare process.
Methods: Using extreme UV data from AIA/SDO and HMI/SDO line-of-sight magnetograms, we study the spatial and temporal evolution of a set of loops in active region AR 11850.
Results: We find a transient brightening of loops in emission from Fe xviii forming at about 7.2 MK, while at the same time these loops dim in emission from lower temperatures. This points to a fast heating of the loop that goes along with evaporation of material that we observe as apparent upward motions in the image sequence. After this initial phase lasting some 10 min, the loops brighten in a sequence of AIA channels that show progressively cooler plasma, indicating that this cooling of the loops lasts about one hour. A comparison to the predictions from a 1D loop model shows that this observation supports the nanoflare process in (almost) all aspects. In addition, our observations show that the loops get broader while getting brighter, which cannot be understood in a 1D model.

Movie associated to Fig. 1 is available in electronic form at http://www.aanda.org Title: Self-Absorption in the Solar Transition Region Authors: Yan, Limei; Peter, Hardi; He, Jiansen; Tian, Hui; Xia, Lidong; Wang, Linghua; Tu, Chuanyi; Zhang, Lei; Chen, Feng; Barczynski, Krzysztof Bibcode: 2015ApJ...811...48Y Altcode: 2015arXiv150105706Y Transient brightenings in the transition region of the Sun have been studied for decades and are usually related to magnetic reconnection. Recently, absorption features due to chromospheric lines have been identified in transition region emission lines raising the question of the thermal stratification during such reconnection events. We analyze data from the Interface Region Imaging Spectrograph in an emerging active region. Here the spectral profiles show clear self-absorption features in the transition region lines of Si iv. While some indications existed that opacity effects might play some role in strong transition region lines, self-absorption has not been observed before. We show why previous instruments could not observe such self-absorption features, and discuss some implications of this observation for the corresponding structure of reconnection events in the atmosphere. Based on this we speculate that a range of phenomena, such as explosive events, blinkers or Ellerman bombs, are just different aspects of the same reconnection event occurring at different heights in the atmosphere. Title: Using coronal seismology to estimate the magnetic field strength in a realistic coronal model Authors: Chen, F.; Peter, H. Bibcode: 2015A&A...581A.137C Altcode: 2015arXiv150800593C
Aims: Coronal seismology is used extensively to estimate properties of the corona, e.g. the coronal magnetic field strength is derived from oscillations observed in coronal loops. We present a three-dimensional coronal simulation, including a realistic energy balance in which we observe oscillations of a loop in synthesised coronal emission. We use these results to test the inversions based on coronal seismology.
Methods: From the simulation of the corona above an active region, we synthesise extreme ultraviolet emission from the model corona. From this, we derive maps of line intensity and Doppler shift providing synthetic data in the same format as obtained from observations. We fit the (Doppler) oscillation of the loop in the same fashion as done for observations to derive the oscillation period and damping time.
Results: The loop oscillation seen in our model is similar to imaging and spectroscopic observations of the Sun. The velocity disturbance of the kink oscillation shows an oscillation period of 52.5 s and a damping time of 125 s, which are both consistent with the ranges of periods and damping times found in observations. Using standard coronal seismology techniques, we find an average magnetic field strength of Bkink = 79 G for our loop in the simulation, while in the loop the field strength drops from roughly 300 G at the coronal base to 50 G at the apex. Using the data from our simulation, we can infer what the average magnetic field derived from coronal seismology actually means. It is close to the magnetic field strength in a constant cross-section flux tube, which would give the same wave travel time through the loop.
Conclusions: Our model produced a realistic looking loop-dominated corona, and provides realistic information on the oscillation properties that can be used to calibrate and better understand the result from coronal seismology.

A movie associated with Fig. 1 is available in electronic form at http://www.aanda.org Title: Coronal energy input and dissipation in a solar active region 3D MHD model Authors: Bourdin, Ph. -A.; Bingert, S.; Peter, H. Bibcode: 2015A&A...580A..72B Altcode: 2015arXiv150703573B Context. We have conducted a 3D MHD simulation of the solar corona above an active region (AR) in full scale and high resolution, which shows coronal loops, and plasma flows within them, similar to observations.
Aims: We want to find the connection between the photospheric energy input by field-line braiding with the coronal energy conversion by Ohmic dissipation of induced currents.
Methods: To this end we compare the coronal energy input and dissipation within our simulation domain above different fields of view, e.g. for a small loops system in the AR core. We also choose an ensemble of field lines to compare, e.g., the magnetic energy input to the heating per particle along these field lines.
Results: We find an enhanced Ohmic dissipation of currents in the corona above areas that also have enhanced upwards-directed Poynting flux. These regions coincide with the regions where hot coronal loops within the AR core are observed. The coronal density plays a role in estimating the coronal temperature due to the generated heat input. A minimum flux density of about 200 Gauss is needed in the photosphere to heat a field line to coronal temperatures of about 1 MK.
Conclusions: This suggests that the field-line braiding mechanism provides the coronal energy input and that the Ohmic dissipation of induced currents dominates the coronal heating mechanism. Title: Division II: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2015IAUTB..28..106V Altcode: The Business Meeting of Commission 10 was held as part of the Business Meeting of Division II (Sun and Heliosphere), chaired by Valentin Martínez-Pillet, the President of the Division. The President of Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took the chair for the business meeting of C10. She summarised the activities of C10 over the triennium and the election of the incoming OC. Title: Thermal Diagnostics with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions Authors: Cheung, Mark C. M.; Boerner, P.; Schrijver, C. J.; Testa, P.; Chen, F.; Peter, H.; Malanushenko, A. Bibcode: 2015ApJ...807..143C Altcode: 2015arXiv150403258C We present a new method for performing differential emission measure (DEM) inversions on narrow-band EUV images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. The method yields positive definite DEM solutions by solving a linear program. This method has been validated against a diverse set of thermal models of varying complexity and realism. These include (1) idealized Gaussian DEM distributions, (2) 3D models of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a nonlinear force-free field, and (3) thermodynamic models from a fully compressible, 3D MHD simulation of active region (AR) corona formation following magnetic flux emergence. We then present results from the application of the method to AIA observations of Active Region 11158, comparing the region's thermal structure on two successive solar rotations. Additionally, we show how the DEM inversion method can be adapted to simultaneously invert AIA and Hinode X-ray Telescope data, and how supplementing AIA data with the latter improves the inversion result. The speed of the method allows for routine production of DEM maps, thus facilitating science studies that require tracking of the thermal structure of the solar corona in time and space. Title: Magnetic jam in the corona of the Sun Authors: Chen, F.; Peter, H.; Bingert, S.; Cheung, M. C. M. Bibcode: 2015NatPh..11..492C Altcode: 2015arXiv150501174C The outer solar atmosphere, the corona, contains plasma at temperatures of more than a million kelvin--more than 100 times hotter than the solar surface. How this gas is heated is a fundamental question tightly interwoven with the structure of the magnetic field. Together this governs the evolution of coronal loops, the basic building block prominently seen in X-rays and extreme ultraviolet (EUV) images. Here we present numerical experiments accounting for both the evolving three-dimensional structure of the magnetic field and its complex interaction with the plasma. Although the magnetic field continuously expands as new magnetic flux emerges through the solar surface, plasma on successive field lines is heated in succession, giving the illusion that an EUV loop remains roughly at the same place. For each snapshot the EUV images outline the magnetic field. However, in contrast to the traditional view, the temporal evolution of the magnetic field and the EUV loops can be quite different. This indicates that the thermal and the magnetic evolution in the outer atmosphere of a cool star should be treated together, and should not be simply separated as predominantly done so far. Title: What can large-scale magnetohydrodynamic numerical experiments tell us about coronal heating? Authors: Peter, H. Bibcode: 2015RSPTA.37350055P Altcode: The upper atmosphere of the Sun is governed by the complex structure of the magnetic field. This controls the heating of the coronal plasma to over a million kelvin. Numerical experiments in the form of three-dimensional magnetohydrodynamic simulations are used to investigate the intimate interaction between magnetic field and plasma. These models allow one to synthesize the coronal emission just as it would be observed by real solar instrumentation. Large-scale models encompassing a whole active region form evolving coronal loops with properties similar to those seen in extreme ultraviolet light from the Sun, and reproduce a number of average observed quantities. This suggests that the spatial and temporal distributions of the heating as well as the energy distribution of individual heat deposition events in the model are a good representation of the real Sun. This provides evidence that the braiding of fieldlines through magneto-convective motions in the photosphere is a good concept to heat the upper atmosphere of the Sun. Title: Coronal Heating By the Interaction between Emerging Active Regions and the Quiet Sun Observed By the Solar Dynamics Observatory Authors: Zhang, Jun; Zhang, Bin; Li, Ting; Yang, Shuhong; Zhang, Yuzong; Li, Leping; Chen, Feng; Peter, Hardi Bibcode: 2015ApJ...799L..27Z Altcode: The question of what heats the solar corona remains one of the most important puzzles in solar physics and astrophysics. Here we report Solar Dynamics Observatory Atmospheric Imaging Assembly observations of coronal heating by the interaction between emerging active regions (EARs) and the surrounding quiet Sun (QS). The EARs continuously interact with the surrounding QS, resulting in dark ribbons which appear at the boundary of the EARs and the QS. The dark ribbons visible in extreme-ultraviolet wavelengths propagate away from the EARs with speeds of a few km s-1. The regions swept by the dark ribbons are brightening afterward, with the mean temperature increasing by one quarter. The observational findings demonstrate that uninterrupted magnetic reconnection between EARs and the QS occurs. When the EARs develop, the reconnection continues. The dark ribbons may be the track of the interface between the reconnected magnetic fields and the undisturbed QS’s fields. The propagating speed of the dark ribbons reflects the reconnection rate and is consistent with our numerical simulation. A long-term coronal heating which occurs in turn from nearby the EARs to far away from the EARs is proposed. Title: Numerical Simulation of Fast-mode Magnetosonic Waves Excited by Plasmoid Ejections in the Solar Corona Authors: Yang, Liping; Zhang, Lei; He, Jiansen; Peter, Hardi; Tu, Chuanyi; Wang, Linghua; Zhang, Shaohua; Feng, Xueshang Bibcode: 2015ApJ...800..111Y Altcode: The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to ~10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfvén speed of ~1000 km s-1. In the k - ω diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15 Mm from the wave source region, we estimate the energy flux of FMWs to be E ~ 7.0 × 106 erg cm-2 s-1, which is ~50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves. Title: Why is Non-Thermal Line Broadening of Spectral Lines in the Lower Transition Region of the Sun Independent of Spatial Resolution? Authors: De Pontieu, B.; McIntosh, S.; Martinez-Sykora, J.; Peter, H.; Pereira, T. M. D. Bibcode: 2015ApJ...799L..12D Altcode: 2017arXiv171006807D Spectral observations of the solar transition region (TR) and corona show broadening of spectral lines beyond what is expected from thermal and instrumental broadening. The remaining non-thermal broadening is significant (5-30 km s-1) and correlated with intensity. Here we study spectra of the TR Si iv 1403 Å line obtained at high resolution with the Interface Region Imaging Spectrograph (IRIS). We find that the large improvement in spatial resolution (0.″33) of IRIS compared to previous spectrographs (2″) does not resolve the non-thermal line broadening which, in most regions, remains at pre-IRIS levels of about 20 km s-1. This invariance to spatial resolution indicates that the processes behind the broadening occur along the line-of-sight (LOS) and/or on spatial scales (perpendicular to the LOS) smaller than 250 km. Both effects appear to play a role. Comparison with IRIS chromospheric observations shows that, in regions where the LOS is more parallel to the field, magneto-acoustic shocks driven from below impact the TR and can lead to significant non-thermal line broadening. This scenario is supported by MHD simulations. While these do not show enough non-thermal line broadening, they do reproduce the long-known puzzling correlation between non-thermal line broadening and intensity. This correlation is caused by the shocks, but only if non-equilibrium ionization is taken into account. In regions where the LOS is more perpendicular to the field, the prevalence of small-scale twist is likely to play a significant role in explaining the invariance and correlation with intensity. Title: Coronal loops above an active region: Observation versus model Authors: Bourdin, Philippe-A.; Bingert, Sven; Peter, Hardi Bibcode: 2014PASJ...66S...7B Altcode: 2014PASJ..tmp..113B; 2014arXiv1410.1216B We conducted a high-resolution numerical simulation of the solar corona above a stable active region. The aim is to test the field line braiding mechanism for a sufficient coronal energy input. We also check the applicability of scaling laws for coronal loop properties like the temperature and density. Our 3D MHD model is driven from below by Hinode observations of the photosphere, in particular a high-cadence time series of line-of-sight magnetograms and horizontal velocities derived from the magnetograms. This driving applies stress to the magnetic field and thereby delivers magnetic energy into the corona, where currents are induced that heat the coronal plasma by Ohmic dissipation. We compute synthetic coronal emission that we directly compare to coronal observations of the same active region taken by Hinode. In the model, coronal loops form at the same places as they are found in coronal observations. Even the shapes of the synthetic loops in 3D space match those found from a stereoscopic reconstruction based on STEREO spacecraft data. Some loops turn out to be slightly over-dense in the model, as expected from observations. This shows that the spatial and temporal distribution of the Ohmic heating produces the structure and dynamics of a coronal loops system close to what is found in observations. Title: Why Is Non-thermal Line Broadening of Lower Transition Region Lines Independent of Spatial Resolution? Authors: De Pontieu, B.; Mcintosh, S. W.; Martínez-Sykora, J.; Peter, H.; Pereira, T. M. D. Bibcode: 2014AGUFMSH51C4175D Altcode: Spectral observations of the solar transition region (TR) and corona typically show broadening of the spectral lines beyond what is expected from thermal and instrumental broadening. The remaining non-thermal broadening is significant (10-30 km/s), correlated with the intensity, and has been attributed to waves, macro and micro turbulence, nanoflares, etc... Here we study spectra of the low TR Si IV 1403 Angstrom line obtained at high spatial and spectral resolution with the Interface Region Imaging Spectrograph (IRIS). We find that the large improvement in spatial resolution (0.33 arcsec) of IRIS compared to previous spectrographs (2 arcsec) does not resolve the non-thermal line broadening which remains at pre-IRIS levels of 20 km/s. This surprising invariance to spatial resolution indicates that the physical processes behind the non-thermal line broadening either occur along the line-of-sight (LOS) and/or on spatial scales (perpendicular to the LOS) smaller than 250 km. Both effects appear to play a role. Comparison with IRIS chromospheric observations shows that, in regions where the LOS is more parallel to the field, magneto-acoustic shocks driven from below impact the low TR leading to strong non-thermal line broadening from line-of-sight integration across the shock at the time of impact. This scenario is confirmed by advanced MHD simulations. In regions where the LOS is perpendicular to the field, the prevalence of small-scale twist is likely to play a significant role in explaining the invariance and the correlation with intensity. Title: Planck 2013 results Authors: Alves, Joao; Bertout, Claude; Combes, Françoise; Ferrara, Andrea; Forveille, Thierry; Guillot, Tristan; Napiwotzki, Ralf; Peter, Hardi; Shore, Steve; Tolstoy, Eline; Walmsley, Malcolm Bibcode: 2014A&A...571E...1A Altcode: No abstract at ADS Title: Hot explosions in the cool atmosphere of the Sun Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.; De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint, L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V. Bibcode: 2014Sci...346C.315P Altcode: 2014arXiv1410.5842P The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by the Interface Region Imaging Spectrograph (IRIS) reveal that it is difficult to determine what is up and down, even in the cool 6000-kelvin photosphere just above the solar surface: This region hosts pockets of hot plasma transiently heated to almost 100,000 kelvin. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection. Title: Conversion from mutual helicity to self-helicity observed with IRIS Authors: Li, L. P.; Peter, H.; Chen, F.; Zhang, J. Bibcode: 2014A&A...570A..93L Altcode: 2014arXiv1410.5597L Context. In the upper atmosphere of the Sun observations show convincing evidence for crossing and twisted structures, which are interpreted as mutual helicity and self-helicity.
Aims: We use observations with the new Interface Region Imaging Spectrograph (IRIS) to show the conversion of mutual helicity into self-helicity in coronal structures on the Sun.
Methods: Using far UV spectra and slit-jaw images from IRIS and coronal images and magnetograms from SDO, we investigated the evolution of two crossing loops in an active region, in particular, the properties of the Si IV line profile in cool loops.
Results: In the early stage two cool loops cross each other and accordingly have mutual helicity. The Doppler shifts in the loops indicate that they wind around each other. As a consequence, near the crossing point of the loops (interchange) reconnection sets in, which heats the plasma. This is consistent with the observed increase of the line width and of the appearance of the loops at higher temperatures. After this interaction, the two new loops run in parallel, and in one of them shows a clear spectral tilt of the Si IV line profile. This is indicative of a helical (twisting) motion, which is the same as to say that the loop has self-helicity.
Conclusions: The high spatial and spectral resolution of IRIS allowed us to see the conversion of mutual helicity to self-helicity in the (interchange) reconnection of two loops. This is observational evidence for earlier theoretical speculations.

Movie associated with Fig. 1 and Appendix A are available in electronic form at http://www.aanda.org Title: Prevalence of small-scale jets from the networks of the solar transition region and chromosphere Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.; Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves, K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber, M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W. Bibcode: 2014Sci...346A.315T Altcode: 2014arXiv1410.6143T As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80 to 250 kilometers per second from the narrow bright network lanes of this interface region. These jets have lifetimes of 20 to 80 seconds and widths of ≤300 kilometers. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with amplitudes of ~20 kilometers per second. Many jets reach temperatures of at least ~105 kelvin and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind. Title: Molecular absorption in transition region spectral lines Authors: Schmit, D. J.; Innes, D.; Ayres, T.; Peter, H.; Curdt, W.; Jaeggli, S. Bibcode: 2014A&A...569L...7S Altcode: 2014arXiv1409.1702S
Aims: We present observations from the Interface Region Imaging Spectrograph (IRIS) of absorption features from a multitude of cool atomic and molecular lines within the profiles of Si IV transition region lines. Many of these spectral lines have not previously been detected in solar spectra.
Methods: We examined spectra taken from deep exposures of plage on 12 October 2013. We observed unique absorption spectra over a magnetic element which is bright in transition region line emission and the ultraviolet continuum. We compared the absorption spectra with emission spectra that is likely related to fluorescence.
Results: The absorption features require a population of sub-5000 K plasma to exist above the transition region. This peculiar stratification is an extreme deviation from the canonical structure of the chromosphere-corona boundary. The cool material is not associated with a filament or discernible coronal rain. This suggests that molecules may form in the upper solar atmosphere on small spatial scales and introduces a new complexity into our understanding of solar thermal structure. It lends credence to previous numerical studies that found evidence for elevated pockets of cool gas in the chromosphere.

Movies associated to Figs. 1 and 2 are available in electronic form at http://www.aanda.org Title: Observations of Subarcsecond Bright Dots in the Transition Region above Sunspots with the Interface Region Imaging Spectrograph Authors: Tian, H.; Kleint, L.; Peter, H.; Weber, M.; Testa, P.; DeLuca, E.; Golub, L.; Schanche, N. Bibcode: 2014ApJ...790L..29T Altcode: 2014arXiv1407.1060T Observations with the Interface Region Imaging Spectrograph (IRIS) have revealed numerous sub-arcsecond bright dots in the transition region above sunspots. These bright dots are seen in the 1400 Å and 1330 Å slit-jaw images. They are clearly present in all sunspots we investigated, mostly in the penumbrae, but also occasionally in some umbrae and light bridges. The bright dots in the penumbrae typically appear slightly elongated, with the two dimensions being 300-600 km and 250-450 km, respectively. The long sides of these dots are often nearly parallel to the bright filamentary structures in the penumbrae but sometimes clearly deviate from the radial direction. Their lifetimes are mostly less than one minute, although some dots last for a few minutes or even longer. Their intensities are often a few times stronger than the intensities of the surrounding environment in the slit-jaw images. About half of the bright dots show apparent movement with speeds of ~10-40 km s-1 in the radial direction. Spectra of a few bright dots were obtained and the Si IV 1402.77 Å line profiles in these dots are significantly broadened. The line intensity can be enhanced by one to two orders of magnitude. Some relatively bright and long-lasting dots are also observed in several passbands of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, and they appear to be located at the bases of loop-like structures. Many of these bright dots are likely associated with small-scale energy release events at the transition region footpoints of magnetic loops. Title: Discovery of the Sun's million-degree hot corona Authors: Peter, Hardi; Dwivedi, Bhola N. Bibcode: 2014FrASS...1....2P Altcode: 2014FrASS...1....2H As time goes by, discoveries become common knowledge, and often the person who first changed a paradigm gets forgotten. One such case is the discovery that the Sun's corona is a million degrees hot - much hotter than its surface. While we still work on solving how the Sun heats the corona, the name of the discoverer seems to be forgotten. Instead, other people get the credit who contributed important pieces to the puzzle, but the person who solved this puzzle was someone else. In this historical note we show that this credit should go to Hannes Alfvén. Title: Exploring the Components of IRIS Spectra: More Shift, Twist, and Sway Than Shake, Rattle, and Roll Authors: McIntosh, Scott W.; De Pontieu, Bart; Peter, Hardi Bibcode: 2014AAS...22431304M Altcode: The beautifully rich spectra of the IRIS spacecraft offer an unparalleled avenue to explore the mass and energy transport processes which sustain the Sun's outer atmosphere. In this presentation we will look in detail at the various components of the spectrographic data and place them in context with Slit-Jaw imaging and EUV imaging from SDO/AIA. We will show that the line profiles display many intriguing features including the clear signatures of strong line-of-sight flows (in all magnetized regions) that are almost always accompanied by transverse and torsional motions at the finest resolvable scales. We will demonstrate that many interesting relationships develop when studying the spectra statistically. These relationships indicate IRIS's ability to spectrally and temporally resolve the energetic processes affecting the outer solar atmosphere. Title: A model for the formation of the active region corona driven by magnetic flux emergence Authors: Chen, F.; Peter, H.; Bingert, S.; Cheung, M. C. M. Bibcode: 2014A&A...564A..12C Altcode: 2014arXiv1402.5343C
Aims: We present the first model that couples the formation of the corona of a solar active region to a model of the emergence of a sunspot pair. This allows us to study when, where, and why active region loops form, and how they evolve.
Methods: We use a 3D radiation magnetohydrodynamics (MHD) simulation of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a 3D MHD coronal model. The coronal model accounts for the braiding of the magnetic fieldlines, which induces currents in the corona to heat up the plasma. We synthesize the coronal emission for a direct comparison to observations. Starting with a basically field-free atmosphere we follow the filling of the corona with magnetic field and plasma.
Results: Numerous individually identifiable hot coronal loops form, and reach temperatures well above 1 MK with densities comparable to observations. The footpoints of these loops are found where small patches of magnetic flux concentrations move into the sunspots. The loop formation is triggered by an increase in upward-directed Poynting flux at their footpoints in the photosphere. In the synthesized extreme ultraviolet (EUV) emission these loops develop within a few minutes. The first EUV loop appears as a thin tube, then rises and expands significantly in the horizontal direction. Later, the spatially inhomogeneous heat input leads to a fragmented system of multiple loops or strands in a growing envelope.

Animation associated with Fig. 2 is available in electronic form at http://www.aanda.org Title: MASC: Magnetic Activity of the Solar Corona Authors: Auchere, Frederic; Fineschi, Silvano; Gan, Weiqun; Peter, Hardi; Vial, Jean-Claude; Zhukov, Andrei; Parenti, Susanna; Li, Hui; Romoli, Marco Bibcode: 2014cosp...40E.149A Altcode: We present MASC, an innovative payload designed to explore the magnetic activity of the solar corona. It is composed of three complementary instruments: a Hard-X-ray spectrometer, a UV / EUV imager, and a Visible Light / UV polarimetric coronagraph able to measure the coronal magnetic field. The solar corona is structured in magnetically closed and open structures from which slow and fast solar winds are respectively released. In spite of much progress brought by two decades of almost uninterrupted observations from several space missions, the sources and acceleration mechanisms of both types are still not understood. This continuous expansion of the solar atmosphere is disturbed by sporadic but frequent and violent events. Coronal mass ejections (CMEs) are large-scale massive eruptions of magnetic structures out of the corona, while solar flares trace the sudden heating of coronal plasma and the acceleration of electrons and ions to high, sometimes relativistic, energies. Both phenomena are most probably driven by instabilities of the magnetic field in the corona. The relations between flares and CMEs are still not understood in terms of initiation and energy partition between large-scale motions, small-scale heating and particle acceleration. The initiation is probably related to magnetic reconnection which itself results magnetic topological changes due to e.g. flux emergence, footpoints motions, etc. Acceleration and heating are also strongly coupled since the atmospheric heating is thought to result from the impact of accelerated particles. The measurement of both physical processes and their outputs is consequently of major importance. However, despite its fundamental importance as a driver for the physics of the Sun and of the heliosphere, the magnetic field of our star’s outer atmosphere remains poorly understood. This is due in large part to the fact that the magnetic field is a very difficult quantity to measure. Our knowledge of its strength and orientation is primarily based on extrapolations from photospheric observations, not from direct measurements. These extrapolations require strong assumptions on critical but unobserved quantities and thus fail to accurately reproduce the complex topologies inferred from remote-sensing observations of coronal structures in white light, EUV, and X-rays. Direct measurements of the coronal magnetic field are also clearly identified by the international heliophysics community as a key element susceptible to lead to major breakthroughs in the understanding of our star. MASC is thus designed to answer the following top-level scientific questions: 1. What is the global magnetic field configuration in the corona? 2. What is the role of the magnetic field in the triggering of flares and CMEs? 3. What is the role of the magnetic field in the acceleration mechanisms of the solar winds? 4. What is the energy spectrum and in particular what are the highest energies to which charged particles can be accelerated in the solar corona? MASC will address these fundamental questions with a suite of instruments composed of an X-ray spectrometer, a UV / EUV imager, and a coronagraph working in the visible and at Lyman alpha. The spectrometer will provide information on the energetics of solar flares, in particular at very high energies of accelerated particles. The UV / EUV imager will provide constraints on the temperature of the flaring and non-flaring corona. The coronagraph will provide the number density of free electrons in the corona, maps of the outflow velocity of neutral hydrogen, and measurements of the coronal magnetic field, via the Hanle effect. These measurements will be performed at all steps of the flare-CME processes, thus providing a detailed picture of the solar coronal dynamics in the quiet and eruptive periods. Title: Magnetic Field Diagnostics and Spatio-Temporal Variability of the Solar Transition Region Authors: Peter, H. Bibcode: 2013SoPh..288..531P Altcode: 2013arXiv1303.1825P Magnetic field diagnostics of the transition region from the chromosphere to the corona faces us with the problem that one has to apply extreme-ultraviolet (EUV) spectro-polarimetry. While for the coronal diagnostics techniques already exist in the form of infrared coronagraphy above the limb and radio observations on the disk, one has to investigate EUV observations for the transition region. However, so far the success of such observations has been limited, but various current projects aim to obtain spectro-polarimetric data in the extreme UV in the near future. Therefore it is timely to study the polarimetric signals we can expect from these observations through realistic forward modeling. Title: Numerical Simulations of Chromospheric Anemone Jets Associated with Moving Magnetic Features Authors: Yang, Liping; He, Jiansen; Peter, Hardi; Tu, Chuanyi; Zhang, Lei; Feng, Xueshang; Zhang, Shaohua Bibcode: 2013ApJ...777...16Y Altcode: Observations with the space-based solar observatory Hinode show that small-scale magnetic structures in the photosphere are found to be associated with a particular class of jets of plasma in the chromosphere called anemone jets. The goal of our study is to conduct a numerical experiment of such chromospheric anemone jets related to the moving magnetic features (MMFs). We construct a 2.5 dimensional numerical MHD model to describe the process of magnetic reconnection between the MMFs and the pre-existing ambient magnetic field, which is driven by the horizontal motion of the magnetic structure in the photosphere. We include thermal conduction parallel to the magnetic field and optically thin radiative losses in the corona to account for a self-consistent description of the evaporation process during the heating of the plasma due to the reconnection process. The motion of the MMFs leads to the expected jet and our numerical results can reproduce many observed characteristics of chromospheric anemone jets, topologically and quantitatively. As a result of the tearing instability, plasmoids are generated in the reconnection process that are consistent with the observed bright moving blobs in the anemone jets. An increase in the thermal pressure at the base of the jet is also driven by the reconnection, which induces a train of slow-mode shocks propagating upward. These shocks are a secondary effect, and only modulate the outflow of the anemone jet. The jet itself is driven by the energy input due to the reconnection of the MMFs and the ambient magnetic field. Title: Temperature dependence of ultraviolet line parameters in network and internetwork regions of the quiet Sun and coronal holes Authors: Wang, X.; McIntosh, S. W.; Curdt, W.; Tian, H.; Peter, H.; Xia, L. -D. Bibcode: 2013A&A...557A.126W Altcode:
Aims: We study the temperature dependence of the average Doppler shift and the non-thermal line width in network and internetwork regions for both the quiet Sun (QS) and the coronal hole (CH), by using observations of the Solar Ultraviolet Measurements of Emitted Radiation instrument onboard the Solar and Heliospheric Observatory spacecraft.
Methods: We obtain the average Doppler shift and non-thermal line width in the network regions of QS, internetwork regions of QS, network regions of CH, and internetwork regions of CH by applying a single-Gaussian fit to the line profiles averaged in each of the four regions. The formation temperatures of the lines we use cover the range from 104 to 1.2 × 106 K. Two simple scenarios are proposed to explain the temperature dependence of the line parameters in the network regions. In one of the scenarios, the spectral line consists of three components: a rapid, weak upflow generated in the lower atmosphere, a nearly static background, and a slow cooling downflow. In the other scenario, there are just two components, which include a bright core component and a faint wide tail one.
Results: An enhancement of the Doppler shift magnitude and the non-thermal line width in network regions compared to the internetwork regions is reported. We also report that most transition region lines are less redshifted (by 0-8 km s-1) and broader (by 0-5 km s-1) in CH compared to the counterparts of QS. In internetwork regions, the difference in the Doppler shifts between the coronal hole and the QS is slightly smaller, especially for the lines with formation temperatures lower than 2 × 105 K. And the two simple scenarios can reproduce the variation in the line parameters with the temperature very well.
Conclusions: Our results suggest that the physical processes in network and internetwork regions are different and that one needs to separate network and internetwork when discussing dynamics and physical properties of the solar atmosphere. The agreement between the results of the observation and our scenarios suggests that the temperature dependence of Doppler shifts and line widths might be caused by the different relative contributions of the three components at different temperatures. The results may shed new light on our understanding of the complex chromosphere-corona mass cycle. However, the existing observational results do not allow us to distinguish between the two scenarios. At this stage, a high-resolution instrument Interface Region Imaging Spectrograph is highly desirable. Title: Structure of solar coronal loops: from miniature to large-scale Authors: Peter, H.; Bingert, S.; Klimchuk, J. A.; de Forest, C.; Cirtain, J. W.; Golub, L.; Winebarger, A. R.; Kobayashi, K.; Korreck, K. E. Bibcode: 2013A&A...556A.104P Altcode: 2013arXiv1306.4685P
Aims: We use new data from the High-resolution Coronal Imager (Hi-C) with its unprecedented spatial resolution of the solar corona to investigate the structure of coronal loops down to 0.2''.
Methods: During a rocket flight, Hi-C provided images of the solar corona in a wavelength band around 193 Å that is dominated by emission from Fe xii showing plasma at temperatures around 1.5 MK. We analyze part of the Hi-C field-of-view to study the smallest coronal loops observed so far and search for the possible substructuring of larger loops.
Results: We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal loops. Their coronal segments above the chromosphere have a length of only about 1 Mm and a thickness of less than 200 km. They could be interpreted as the coronal signature of small flux tubes breaking through the photosphere with a footpoint distance corresponding to the diameter of a cell of granulation. We find that loops that are longer than 50 Mm have diameters of about 2'' or 1.5 Mm, which is consistent with previous observations. However, Hi-C really resolves these loops with some 20 pixels across the loop. Even at this greatly improved spatial resolution, the large loops seem to have no visible substructure. Instead they show a smooth variation in cross-section.
Conclusions: That the large coronal loops do not show a substructure on the spatial scale of 0.1'' per pixel implies that either the densities and temperatures are smoothly varying across these loops or it places an upper limit on the diameter of the strands the loops might be composed of. We estimate that strands that compose the 2'' thick loop would have to be thinner than 15 km. The miniature loops we find for the first time pose a challenge to be properly understood through modeling.

Appendices are available in electronic form at http://www.aanda.org Title: VizieR Online Data Catalog: 3D-MHD model of a solar active region corona (Bourdin+, 2013) Authors: Bourdin, P. -A.; Bingert, S.; Peter, H. Bibcode: 2013yCat..35550123B Altcode: 2013yCat..35559123B Parameter and setup files used for a 3D-MHD simulation with the Pencil Code. The parameters are needed to reproduce the simulation, while the setup files show which modules of the Pencil Code were used to conduct the simulation.

The parameters file are in the state as used at the end of the simulation, when the analysis was performed. With the logfile, one can reconstruct the state at any time during the simulation run (this applies to "run.in").

The code revision logfile indicates which code revision was used when, where only changes in the configuration are listed together with the full initial and final configuration.

All *.in files are in Fortran Namelist format. The *.in and *.local files are all ready to be used with Pencil Code. The Pencil Code can be obtained at: http://pencil-code.nordita.org/

(7 data files). Title: Observationally driven 3D magnetohydrodynamics model of the solar corona above an active region Authors: Bourdin, Ph. -A.; Bingert, S.; Peter, H. Bibcode: 2013A&A...555A.123B Altcode: 2013arXiv1305.5693B Context.
Aims: The goal is to employ a 3D magnetohydrodynamics (MHD) model including spectral synthesis to model the corona in an observed solar active region. This will allow us to judge the merits of the coronal heating mechanism built into the 3D model.
Methods: Photospheric observations of the magnetic field and horizontal velocities in an active region are used to drive our coronal simulation from the bottom. The currents induced by this heat the corona through Ohmic dissipation. Heat conduction redistributes the energy that is lost in the end through optically thin radiation. Based on the MHD model, we synthesized profiles of coronal emission lines which can be directly compared to actual coronal observations of the very same active region.
Results: In the synthesized model data we find hot coronal loops which host siphon flows or which expand and lose mass through draining. These synthesized loops are at the same location as and show similar dynamics in terms of Doppler shifts to the observed structures. This match is shown through a comparison with Hinode data as well as with 3D stereoscopic reconstructions of data from STEREO.
Conclusions: The considerable match to the actual observations shows that the field-line braiding mechanism leading to the energy input in our corona provides the proper distribution of heat input in space and time. From this we conclude that in an active region the field-line braiding is the dominant heating process, at least at the spatial scales available to current observations.

Parameters and simulation log-files 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/555/A123 Title: Injection of Plasma into the Nascent Solar Wind via Reconnection Driven by Supergranular Advection Authors: Yang, Liping; He, Jiansen; Peter, Hardi; Tu, Chuanyi; Chen, Wenlei; Zhang, Lei; Marsch, Eckart; Wang, Linghua; Feng, Xueshang; Yan, Limei Bibcode: 2013ApJ...770....6Y Altcode: To understand the origin of the solar wind is one of the key research topics in modern solar and heliospheric physics. Previous solar wind models assumed that plasma flows outward along a steady magnetic flux tube that reaches continuously from the photosphere through the chromosphere into the corona. Inspired by more recent comprehensive observations, Tu et al. suggested a new scenario for the origin of the solar wind, in which it flows out in a magnetically open coronal funnel and mass is provided to the funnel by small-scale side loops. Thus mass is supplied by means of magnetic reconnection that is driven by supergranular convection. To validate this scenario and simulate the processes involved, a 2.5 dimensional (2.5D) numerical MHD model is established in the present paper. In our simulation a closed loop moves toward an open funnel, which has opposite polarity and is located at the edge of a supergranulation cell, and magnetic reconnection is triggered and continues while gradually opening up one half of the closed loop. Its other half connects with the root of the open funnel and forms a new closed loop which is submerged by a reconnection plasma stream flowing downward. Thus we find that the outflowing plasma in the newly reconnected funnel originates not only from the upward reconnection flow but also from the high-pressure leg of the originally closed loop. This implies an efficient supply of mass from the dense loop to the dilute funnel. The mass flux of the outflow released from the funnel considered in our study is calculated to be appropriate for providing the mass flux at the coronal base of the solar wind, though additional heating and acceleration mechanisms are necessary to keep the velocity at the higher location. Our numerical model demonstrates that in the funnel the mass for the solar wind may be supplied from adjacent closed loops via magnetic reconnection as well as directly from the footpoints of open funnels. Title: Parameterisation of coronal heating: spatial distribution and observable consequences Authors: van Wettum, T.; Bingert, S.; Peter, H. Bibcode: 2013A&A...554A..39V Altcode: 2013arXiv1303.1371V
Aims: We investigate the difference in the spatial distribution of the energy input for parameterisations of different mechanisms to heat the corona of the Sun and possible impacts on the coronal emission.
Methods: We use a 3D magneto-hydrodynamic (MHD) model of a solar active region as a reference and compare the Ohmic-type heating in this model to parameterisations for alternating current (AC) and direct current (DC) heating models; in particular, we use Alfvén wave and MHD turbulence heating. We extract the quantities needed for these two parameterisations from the reference model and investigate the spatial distribution of the heat input in all three cases, globally and along individual field lines. To study differences in the resulting coronal emission, we employ 1D loop models with a prescribed heat input based on the heating rate we extracted along a bundle of field lines.
Results: On average, all heating implementations show a rough drop of the heating rate with height. This also holds for individual field lines. While all mechanisms show a concentration of the energy input towards the low parts of the atmosphere, for individual field lines the concentration towards the foot points is much stronger for the DC mechanisms than for the Alfvén wave AC case. In contrast, the AC model gives a stronger concentration of the emission towards the foot points. This is because the more homogeneous distribution of the energy input leads to higher coronal temperatures and a more extended transition region.
Conclusions: The significant difference in the concentration of the heat input towards the foot points for the AC and DC mechanisms and the pointed difference in the spatial distribution of the coronal emission for these cases show that the two mechanisms should be discriminable by observations. Before drawing final conclusions, these parameterisations should be implemented in new 3D models in a more self-consistent way. 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: 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: Coronal structure and dynamics above an active region - MHD model versus observation Authors: Bourdin, Philippe-A.; Bingert, Sven; Peter, Hardi Bibcode: 2013enss.confE..56B Altcode: We present a one-to-one comparison between an observed active region and a 3D MHD model including spectral synthesis. We set up the 3D MHD model from the photosphere to the corona and use the actually observed photospheric magnetograms and horizontal motions as a lower boundary condition to drive the 3D coronal model. Following Parker's model for field-line braiding this induces currents that are dissipated and heat the corona. From the 3D MHD model we synthesize emission spectra in EUV and X-rays that can be compared directly to the Hinode/EIS and XRT observation of the active region we model. We find that the hot coronal loops that form in the model occur at just the same places as they are found in the actual observations. Moreover, their spatial structure and the flows along the loops as seen in the synthesized intensity and Doppler maps compare well to the actual observations. By this we present the first coronal model driven by photospheric observations that provides a one-to-one match to the coronal structure and dynamics observed for that same active region. This shows that the distribution of the energy input in time and space through the field-line braiding is close to the real solar coronal energy deposition. Title: "Nanoflare heating in the solar corona - Parker was right" Authors: Bingert, Sven; Peter, Hardi Bibcode: 2013enss.confE..48B Altcode: We investigate the energy distribution of the heat input in a 3D MHD model of a solar active region. We find that the energy is deposited mostly in quantities around 10^17 J, which is very close to the nanoflare energy derived by Parker. For our study we employ a 3D MHD model of the solar corona to investigate the spatial and temporal evolution of the heat input based on Ohmic dissipation. The model is driven by horizontal motions shuffling around the magnetic field in the photosphere. This induces currents and their dissipation heats the coronal plasma, very similar to the field-line braiding mechanism proposed by Parker. We find a power-law behavior of the energy input due to Ohmic dissipation indicating that the system is scale invariant. However considering the coronal part of the computational box alone, we find a knee in the energy distribution and a preferred energy for the heat deposition. This energy coincides with the nanoflare energy proposed by Parker. While Parker considered a quite idealized setup, we model a realistic corona, accounting for gravity, heat conduction and radiative losses, and account for the full interaction of plasma and magnetic field. Our results are remarkable, because even in the complex and dynamic setup we find a result that basically supports Parker's original proposal. By synthesizing coronal emission from our model we can go one step further, though, and show that the resulting model corona really resembles the observed corona in structure and dynamics. Title: Studies of the dynamics and energetics of cool plasma ejections into the corona Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi Bibcode: 2013enss.confE.142Z Altcode: The corona is highly dynamic and shows transient events on various scales in space and time. Most of these features are related to changes in the magnetic field structure or impulsive heating caused by the conversion of magnetic to thermal energy. We are applying three-dimensional magnetohydrodynamic models in order to investigate the structure and dynamics in the upper solar atmosphere above a small active region. Emission line spectra are synthesized from the model and compared to spectra and images observed by current space-based instruments, such as the EUV Imaging Spectrometer (EIS) onboard the Hinode satellite and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. We investigate mass and energy flows between the solar chromosphere and corona and discuss possible scenarios for a mass cycle between the lower and upper solar atmosphere. In particular, we have studied the processes that lead to the formation and ejection of a confined plasma ejection into the solar corona. A description of the nature of this particular feature will be presented which is found to be a hydrodynamic phenomenon triggered by a heating event above the chromosphere. A detailed analysis of 1D coronal loop models has been performed to understand how the plasma responds to a heating pulse. The results confirm the formation mechanism of the blob observed in the 3D model. This raises the question if other small-scale ejection features seen on the Sun could also be based on hydrodynamic processes instead of being plasmoid-type phenomena as it is usually assumed. Title: Coupled model for the formation of an active region corona Authors: Chen, Feng; Bingert, Sven; Peter, Hardi; Cameron, Robert; Schüssler; , Manfred; Cheung, Mark C. M. Bibcode: 2013enss.confE..21C Altcode: We will present the first model that couples the formation of an active region corona to a model of the emergence. This allows us to study when, where, and why active region loops form, and how they evolve. For this we use an existing 3D radiation MHD model of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a coronal model. Our 3D MHD coronal model accounts for the braiding of the magnetic field lines that induces currents in the corona that is getting filled with the emerging magnetic field. Starting with a basically field-free atmosphere we follow the flux emergence until numerous individually identifiable hot coronal loops have been formed. The temperatures in the coronal loops of well above 1 MK are reached at densities corresponding to actually observed active region loops. The loops develop over a very short time period of the order of several minutes through the evaporation of material from the chromosphere. Because we have full access to the heating rate as a function of time and space in our computational domain we can determine the conditions under which these loops form. Title: Nanoflare statistics in an active region 3D MHD coronal model Authors: Bingert, S.; Peter, H. Bibcode: 2013A&A...550A..30B Altcode: 2012arXiv1211.6417B
Aims: We investigate the statistics for the spatial and temporal distribution of the energy input into the corona in a three-dimensional magneto-hydrodynamical (3D MHD) model. The model describes the temporal evolution of the corona above an observed active region. The model is driven by photospheric granular motions that braid the magnetic field lines. This induces currents that are dissipated, thereby leading to transient heating of the coronal plasma. We evaluate the transient heating as subsequent heating events and analyze their statistics. The results are then interpreted in the context of observed flare statistics and coronal heating mechanisms. Observed solar flares and other smaller transients cover a wide range of energies. The frequency distribution of energies follow a power law, the lower end of the distribution given by the detection limit of current instrumentation. One particular heating mechanism is based on the occurrence of so-called nanoflares, i.e. very low-energy deposition events.
Methods: To conduct the numerical experiment we use a high-order finite-difference code that solves the partial differential equations for the conservation of mass, the momentum and energy balance, and the induction equation. The energy balance includes Spitzer heat conduction and optically thin radiative losses in the corona.
Results: The temporal and spatial distribution of the Ohmic heating in the 3D MHD model follows a power law and can therefore be understood as a system in a self-organized critical state. The slopes of the power law are similar to the results based on observations of flares and smaller transients. We find that the coronal heating is dominated by events similar to the so-called nanoflares with energies on the order of 1017 J or 1024 erg. Title: Constant cross section of loops in the solar corona Authors: Peter, H.; Bingert, S. Bibcode: 2012A&A...548A...1P Altcode: 2012arXiv1209.0789P Context. The corona of the Sun is dominated by emission from loop-like structures. When observed in X-ray or extreme ultraviolet emission, these million K hot coronal loops show a more or less constant cross section.
Aims: In this study we show how the interplay of heating, radiative cooling, and heat conduction in an expanding magnetic structure can explain the observed constant cross section.
Methods: We employ a three-dimensional magnetohydrodynamics (3D MHD) model of the corona. The heating of the coronal plasma is the result of braiding of the magnetic field lines through footpoint motions and subsequent dissipation of the induced currents. From the model we synthesize the coronal emission, which is directly comparable to observations from, e.g., the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (AIA/SDO).
Results: We find that the synthesized observation of a coronal loop seen in the 3D data cube does match actually observed loops in count rate and that the cross section is roughly constant, as observed. The magnetic field in the loop is expanding and the plasma density is concentrated in this expanding loop; however, the temperature is not constant perpendicular to the plasma loop. The higher temperature in the upper outer parts of the loop is so high that this part of the loop is outside the contribution function of the respective emission line(s). In effect, the upper part of the plasma loop is not bright and thus the loop actually seen in coronal emission appears to have a constant width.
Conclusions: From this we can conclude that the underlying field-line-braiding heating mechanism provides the proper spatial and temporal distribution of the energy input into the corona - at least on the observable scales.

Movies associated to Figs. 1 and 2 are available in electronic form at http://www.aanda.org Title: Mass flows between the chromosphere and corona - comparison of 1D and 3D coronal loop models Authors: Zacharias, P.; Bingert, S.; Peter, H. Bibcode: 2012AGUFMSH33B2239Z Altcode: The ejection and return of cool transition region plasma into the corona will be discussed in the context of our three-dimensional magnetohydrodynamic (3D MHD) model of the solar corona. The dynamics of the ejection are investigated in more detail in a 1D loop model and are compared to observations from Hinode/EIS and SDO/AIA. Results from the 3D model analysis serve as input for the 1D loop model. In the 3D case, a heating pulse is the main driver of the ejection. To mimic the situation in 1D, a heating pulse is injected at different heights along the loop with varying amplitude and width. As a consequence, the heating rate is strongly increased in a localized area and leads to enhanced evaporation that causes the material to rise. We present results that show the successful reproduction of the ejection in a 1D loop model following the injection of a heating pulse. In contrast to earlier studies, where similar heating events lead to both redshifts in transition region emission lines and blueshifts in coronal emission lines, preliminary results of our parameter study show exclusively upflows along the loop and almost no downflows during the heating phase. We will discuss these findings in terms of the mass cycle between the chromosphere and corona. Title: Active region coronal loops in a large scale self consistent 3D MHD model Authors: Bingert, S.; Peter, H. Bibcode: 2012AGUFMSH33B2238B Altcode: Observations of the solar corona in EUV show a dominance of loops above active regions. These loops show an almost constant cross-section which is in contradiction to the expanding magnetic field. To understand their appearance the detailed knowledge of the distribution of the coronal heating and heat transport is needed. We investigate the building process of these structures in a large-scale three dimensional magneto-hydrodynamic (MHD) model of the solar corona above an active region. The model expands from the photosphere up to a height of 80 Mm and spans over 100x100 Mm^2 in the horizontal directions. The dynamics of the model is driven by photopsheric granular motions driving the magnetic field in an active region as observed by HMI/SDO. The model solves the set of MHD equations, i.e. the induction equation along with the conservation of mass, momentum and energy. The energy equation includes radiative losses for the optically thin corona and anisotropic Spitzer heat conduction. It is essential to treat this term properly to get the correct coronal pressure, and for the synthesis of coronal emission lines. Heat conduction is the dominant process in the hot corona and therefore limits the progress of of the numerical experiment. To improve the performance of the model we extend the standard set of the MHD equations to include the time evolution of the heat flux vector.; Synthetic MgX emission aligned with the magnetic field lines connecting the main polarities of the active region. Isocontur show a roughly constant cross-section of the emission structure. Title: Persistent Doppler Shift Oscillations Observed with Hinode/EIS in the Solar Corona: Spectroscopic Signatures of Alfvénic Waves and Recurring Upflows Authors: Tian, Hui; McIntosh, Scott W.; Wang, Tongjiang; Ofman, Leon; De Pontieu, Bart; Innes, Davina E.; Peter, Hardi Bibcode: 2012ApJ...759..144T Altcode: 2012arXiv1209.5286T Using data obtained by the EUV Imaging Spectrometer on board Hinode, we have performed a survey of obvious and persistent (without significant damping) Doppler shift oscillations in the corona. We have found mainly two types of oscillations from February to April in 2007. One type is found at loop footpoint regions, with a dominant period around 10 minutes. They are characterized by coherent behavior of all line parameters (line intensity, Doppler shift, line width, and profile asymmetry), and apparent blueshift and blueward asymmetry throughout almost the entire duration. Such oscillations are likely to be signatures of quasi-periodic upflows (small-scale jets, or coronal counterpart of type-II spicules), which may play an important role in the supply of mass and energy to the hot corona. The other type of oscillation is usually associated with the upper part of loops. They are most clearly seen in the Doppler shift of coronal lines with formation temperatures between one and two million degrees. The global wavelets of these oscillations usually peak sharply around a period in the range of three to six minutes. No obvious profile asymmetry is found and the variation of the line width is typically very small. The intensity variation is often less than 2%. These oscillations are more likely to be signatures of kink/Alfvén waves rather than flows. In a few cases, there seems to be a π/2 phase shift between the intensity and Doppler shift oscillations, which may suggest the presence of slow-mode standing waves according to wave theories. However, we demonstrate that such a phase shift could also be produced by loops moving into and out of a spatial pixel as a result of Alfvénic oscillations. In this scenario, the intensity oscillations associated with Alfvénic waves are caused by loop displacement rather than density change. These coronal waves may be used to investigate properties of the coronal plasma and magnetic field. 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: Commission 10: Solar Activity Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk, James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul; Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi; Vršnak, Bojan; Yan, Yihua Bibcode: 2012IAUTA..28...69V Altcode: Commission 10 of the International Astronomical Union has more than 650 members who study a wide range of activity phenomena produced by our nearest star, the Sun. Solar activity is intrinsically related to solar magnetic fields and encompasses events from the smallest energy releases (nano- or even picoflares) to the largest eruptions in the Solar System, coronal mass ejections (CMEs), which propagate into the Heliosphere reaching the Earth and beyond. Solar activity is manifested in the appearance of sunspot groups or active regions, which are the principal sources of activity phenomena from the emergence of their magnetic flux through their dispersion and decay. The period 2008-2009 saw an unanticipated extended solar cycle minimum and unprecedentedly weak polar-cap and heliospheric field. Associated with that was the 2009 historical maximum in galactic cosmic rays flux since measurements begun in the middle of the 20th Century. Since then Cycle 24 has re-started solar activity producing some spectacular eruptions observed with a fleet of spacecraft and ground-based facilities. In the last triennium major advances in our knowledge and understanding of solar activity were due to continuing success of space missions as SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched by the breathtaking images of the solar atmosphere produced by the Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the framework of NASA's Living with a Star program. In August 2012, at the time of the IAU General Assembly in Beijing when the mandate of this Commission ends, we will be in the unique position to have for the first time a full 3-D view of the Sun and solar activity phenomena provided by the twin STEREO missions about 120 degrees behind and ahead of Earth and other spacecraft around the Earth and ground-based observatories. These new observational insights are continuously posing new questions, inspiring and advancing theoretical analysis and modelling, improving our understanding of the physics underlying magnetic activity phenomena. Commission 10 reports on a vigorously evolving field of research produced by a large community. The number of refereed publications containing `Sun', `heliosphere', or a synonym in their abstracts continued the steady growth seen over the preceding decades, reaching about 2000 in the years 2008-2010, with a total of close to 4000 unique authors. This report, however, has its limitations and it is inherently incomplete, as it was prepared jointly by the members of the Organising Committee of Commission 10 (see the names of the primary contributors to the sections indicated in parentheses) reflecting their fields of expertise and interest. Nevertheless, we believe that it is a representative sample of significant new results obtained during the last triennium in the field of solar activity. 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: Large scale MHD model of the solar corona above time dependent HMI/SDO magnetograms Authors: Bingert, S.; Peter, H. Bibcode: 2012decs.confE...3B Altcode: The SDO spacecraft provides a unique tool to observe the solar atmosphere simultaneously in the photosphere and the corona. The magnetic field and the energy transport couples the whole system, which requires a model that describes the atmosphere all the way from the photosphere into the corona. We present the results of a large scale three dimensional magneto-hydrodynamic model of the solar corona, that is driven by the (time variable) magnetic field in the photosphere as observed by HMI/SDO. The results of the 3D MHD model are then used to synthesize the coronal emission and is directly compared to AIA/SDO observations. The domain of the numerical model spans over 100x100 Mm^2 in horizontal directions and reaches a height of 80 Mm, thus containing the full (small) active region. The spatial resolution is sufficient to resolve thin loops and fine structure in the transition region and corona. This large scale model includes all needed physics, such as anisotropic heat conduction and radiative loss to account for a proper coronal pressure. Based on the data we also derive basic parameters, e.g. the energy flux through the domain or the structure and energy content of the coronal magnetic field. Title: Coronal loops with constant cross-section reproduced in 3D MHD models Authors: Peter, Hardi; Bingert, Sven Bibcode: 2012decs.confE..59P Altcode: EUV and X-ray images of the solar corona show loops with a more or less constant cross-section. Because the magnetic field is expanding with height, one would expect the coronal loops to expand with height. Suggestions on special magnetic structures have been made to understand the constant cross section of the loops, e.g. introducing helicity. However no convincing picture could be presented yet. We present results from a 3D MHD box model of a solar active region, which is heated through braiding of magnetic field lines and subsequent Ohmic dissipation. From the MHD model we synthesize emission as it would be observed with AIA/SDO. These synthetic images clearly show EUV loops with constant cross-section and thus can reproduce the observed structures. The analysis of the densities and temperatures in relation to the magnetic structure in the 3D model box shows that the constant cross section is a result of the temperature and density variation in the loop structure perpendicular to the magnetic field. These results underline that one has to account for the three-dimensional nature of the corona even when investigating a seemingly one-dimensional structure such as a coronal loop. Title: Ejection of cool plasma into the corona - comparison of results from a 3D MHD model with results from AIA/SDO, EIS/Hinode and a 1D loop model Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi Bibcode: 2012decs.confE..48Z Altcode: The formation and subsequent ejection of cool plasma into the corona will be discussed, as observed in our three-dimensional magnetohydrodynamic (3D MHD) model of the solar atmosphere extending from the photosphere into the corona. The model accounts properly for the energy balance, especially for heat conduction and radiative losses, allowing us to reliably synthesize the profiles of optically thin extreme ultraviolet emission lines and compare them to existing observations. A detailed description of the nature of this particular phenomenon will be provided. The analysis of the various forces acting upon the plasma in the 3D model shows that the pressure gradient which is driving the ejection is due to Ohmic dissipation of currents resulting from the braiding of the magnetic field lines by photospheric plasma motions. Preliminary results of a parameter study on the reproduction of the phenomenon in a one-dimensional loop model support the scenario of a heating event that leads to the ejection of cool plasma into the corona in both, the 1D loop model and the 3D model. In addition, results of the numerical model will be compared to observations from the Extreme Ultraviolet Imaging Spectrometer (EIS) onboard Hinode and the Atmospheric Imaging Assembly (AIA) onboard SDO, where we have also found evidence of cool plasma ejecta that are moving along magnetic field lines. Title: Siphon flow in a cool magnetic loop Authors: Bethge, C.; Beck, C.; Peter, H.; Lagg, A. Bibcode: 2012A&A...537A.130B Altcode: 2011arXiv1111.5564B Context. Siphon flows that are driven by a gas pressure difference between two photospheric footpoints of different magnetic field strength connected by magnetic field lines are a well-studied phenomenon in theory, but observational evidence is scarce. Aims. We investigate the properties of a structure in the solar chromosphere in an active region to find out whether the feature is consistent with a siphon flow in a magnetic loop filled with chromospheric material.

Methods. We derived the line-of-sight (LOS) velocity of several photospheric spectral lines and two chromospheric spectral lines, Ca II H 3968.5 *Aring; and He I 10830 Å, in spectropolarimetric observations of NOAA 10978 done with the Tenerife Infrared Polarimeter (TIP-II) and the POlarimetric LIttrow Spectrograph (POLIS). The structure can be clearly traced in the LOS velocity maps and the absorption depth of He I. The magnetic field configuration in the photosphere is inferred directly from the observed Stokes parameters and from inversions with the HELIX+ code. Data from the full-disk Chromospheric Telescope (ChroTel) in He I in intensity and LOS velocity are used for tracking the temporal evolution of the flow, along with TRACE Fe IX/X 171 Å data for additional information about coronal regions related to the structure under investigation.

Results. The inner end of the structure is located in the penumbra of a sunspot. It shows downflows whose strength decreases with decreasing height in the atmosphere. The flow velocity in He I falls abruptly from above 40 km s-1 to about zero further into the penumbra. A slight increase of emission is seen in the Ca II H spectra at the endpoint. At the outer end of the structure, the photospheric lines that form higher up in the atmosphere show upflows that accelerate with height. The polarization signal near the outer end shows a polarity opposite to that of the sunspot, the magnetic field strength of 580 G is roughly half as large as at the inner end. The structure exists for about 90 min. Its appearance is preceeded by a brightening in its middle in the coronal TRACE data.

Conclusions. The observed flows match theoretical predictions of chromospheric and coronal siphon flows, with accelerating upflowing plasma at one footpoint with low field strength and decelerating downflowing plasma at the other end. A tube shock at the inner end is probable, but the evidence is not conclusive. The TRACE data suggest that the structure forms because of a reorganization of field lines after a reconnection event. Title: Catastrophic cooling and cessation of heating in the solar corona Authors: Peter, H.; Bingert, S.; Kamio, S. Bibcode: 2012A&A...537A.152P Altcode: 2011arXiv1112.3667P Context. Condensations in the more than 106 K hot corona of the Sun are commonly observed in the extreme ultraviolet (EUV). While their contribution to the total solar EUV radiation is still a matter of debate, these condensations certainly provide a valuable tool for studying the dynamic response of the corona to the heating processes.
Aims: We investigate different distributions of energy input in time and space to investigate which process is most relevant for understanding these coronal condensations.
Methods: For a comparison to observations we synthesize EUV emission from a time-dependent, one-dimensional model for coronal loops, where we employ two heating scenarios: simply shutting down the heating and a model where the heating is very concentrated at the loop footpoints, while keeping the total heat input constant.
Results: The heating off/on model does not lead to significant EUV count rates that one observes with SDO/AIA. In contrast, the concentration of the heating near the footpoints leads to thermal non-equilibrium near the loop top resulting in the well-known catastrophic cooling. This process gives a good match to observations of coronal condensations.
Conclusions: This shows that the corona needs a steady supply of energy to support the coronal plasma, even during coronal condensations. Otherwise the corona would drain very fast, too fast to even form a condensation.

Movies are available in electronic form at http://www.aanda.org Title: The Chromospheric Telescope Authors: Bethge, C.; Peter, H.; Kentischer, T. J.; Halbgewachs, C.; Elmore, D. F.; Beck, C. Bibcode: 2011A&A...534A.105B Altcode: 2011arXiv1108.4880B
Aims: We introduce the Chromospheric Telescope (ChroTel) at the Observatorio del Teide in Izaña on Tenerife as a new multi-wavelength imaging telescope for full-disk synoptic observations of the solar chromosphere. We describe the design of the instrument and summarize its performance during the first one and a half years of operation. We present a method to derive line-of-sight velocity maps of the full solar disk from filtergrams taken in and near the He i infrared line at 10 830 Å.
Methods: ChroTel observations are conducted using Lyot-type filters for the chromospheric lines of Ca ii K, Hα, and He i 10 830 Å. The instrument operates autonomically and gathers imaging data in all three channels with a cadence of down to one minute. The use of a tunable filter for the He i line allows us to determine line-shifts by calibrating the line-of-sight velocity maps derived from the filtergram intensities with spectrographic data from the Tenerife Infrared Polarimeter at high spatial and spectral resolution.
Results: The robotic operation and automated data reduction have proven to operate reliably in the first one and and half years. The achieved spatial resolution of the data is close to the theoretical limit of 2 arcsec in Hα and Ca ii K and 3 arcsec in He i. Line-of-sight velocities in He i can be determined with a precision of better than 3-4 km s-1 when co-temporal spectrographic maps are available for calibration.
Conclusions: ChroTel offers a unique combination of imaging in the most important chromospheric lines, along with the possibility to determine line-of-sight velocities in one of the lines. This is of interest for scientific investigations of large-scale structures in the solar chromosphere, as well as for context imaging of high-resolution solar observations. 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: Ejection of cool plasma into the hot corona Authors: Zacharias, P.; Peter, H.; Bingert, S. Bibcode: 2011A&A...532A.112Z Altcode: 2011arXiv1106.5972Z Context. The corona is highly dynamic and shows transient events on various scales in space and time. Most of these features are related to changes in the magnetic field structure or impulsive heating caused by the conversion of magnetic to thermal energy.
Aims: We investigate the processes that lead to the formation, ejection and fall of a confined plasma ejection that was observed in a numerical experiment of the solar corona. By quantifying physical parameters such as mass, velocity, and orientation of the plasma ejection relative to the magnetic field, we provide a description of the nature of this particular plasma ejection.
Methods: The time-dependent three-dimensional magnetohydrodynamic (3D MHD) equations are solved in a box extending from the chromosphere, which serves as a reservoir for mass and energy, to the lower corona. The plasma is heated by currents that are induced through field line braiding as a consequence of photospheric motions included in the model. Spectra of optically thin emission lines in the extreme ultraviolet range are synthesized, and magnetic field lines are traced over time. We determine the trajectory of the plasma ejection and identify anomalies in the profiles of the plasma parameters.
Results: Following strong heating just above the chromosphere, the pressure rapidly increases, leading to a hydrodynamic explosion above the upper chromosphere in the low transition region. The explosion drives the plasma, which needs to follow the magnetic field lines. The ejection is then moving more or less ballistically along the loop-like field lines and eventually drops down onto the surface of the Sun. The speed of the ejection is in the range of the sound speed, well below the Alfvén velocity.
Conclusions: The plasma ejection observed in a numerical experiment of the solar corona is basically a hydrodynamic phenomenon, whereas the rise of the heating rate is of magnetic nature. The granular motions in the photosphere lead (by chance) to a strong braiding of the magnetic field lines at the location of the explosion that in turn is causing strong currents which are dissipated. Future studies need to determine if this process is a ubiquitous phenomenon on the Sun on small scales. Data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (AIA/SDO) might provide the relevant information.

Appendix and movie are available in electronic form at http://www.aanda.org Title: Investigation of mass flows in the transition region and corona in a three-dimensional numerical model approach Authors: Zacharias, P.; Peter, H.; Bingert, S. Bibcode: 2011A&A...531A..97Z Altcode: 2011arXiv1105.5491Z Context. The origin of solar transition region redshifts is not completely understood. Current research is addressing this issue by investigating three-dimensional magneto-hydrodynamic models that extend from the photosphere to the corona.
Aims: By studying the average properties of emission line profiles synthesized from the simulation runs and comparing them to observations with present-day instrumentation, we investigate the origin of mass flows in the solar transition region and corona.
Methods: Doppler shifts were determined from the emission line profiles of various extreme-ultraviolet emission lines formed in the range of T = 104-106 K. Plasma velocities and mass flows were investigated for their contribution to the observed Doppler shifts in the model. In particular, the temporal evolution of plasma flows along the magnetic field lines was analyzed.
Results: Comparing observed vs. modeled Doppler shifts shows a good correlation in the temperature range log (T/[K]) = 4.5-5.7, which is the basis of our search for the origin of the line shifts. The vertical velocity obtained when weighting the velocity by the density squared is shown to be almost identical to the corresponding Doppler shift. Therefore, a direct comparison between Doppler shifts and the model parameters is allowed. A simple interpretation of Doppler shifts in terms of mass flux leads to overestimating the mass flux. Upflows in the model appear in the form of cool pockets of gas that heat up slowly as they rise. Their low temperature means that these pockets are not observed as blueshifts in the transition region and coronal lines. For a set of magnetic field lines, two different flow phases could be identified. The coronal part of the field line is intermittently connected to subjacent layers of either strong or weak heating, leading either to mass flows into the loop (observed as a blueshift) or to the draining of the loop (observed as a redshift). Title: Intermittent heating in the solar corona employing a 3D MHD model Authors: Bingert, S.; Peter, H. Bibcode: 2011A&A...530A.112B Altcode: 2011arXiv1103.6042B
Aims: We investigate the spatial and temporal evolution of the heating of the corona of a cool star such as our Sun in a three-dimensional magneto-hydrodynamic (3D MHD) model.
Methods: We solve the 3D MHD problem numerically in a box representing part of the (solar) corona. The energy balance includes Spitzer heat conduction along the magnetic field and optically thin radiative losses. The self-consistent heating mechanism is based on the braiding of magnetic field lines rooted in the convective photosphere. Magnetic stress induced by photospheric motions leads to currents in the atmosphere that heat the corona through Ohmic dissipation.
Results: While the horizontally averaged quantities, such as heating rate, temperature, or density, are relatively constant in time, the simulated corona is highly variable and dynamic, on average reaching the temperatures and densities found in observations. The strongest heating per particle is found in the transition region from the chromosphere to the corona. The heating is concentrated in current sheets roughly aligned with the magnetic field and is transient in time and space. This supports the idea that numerous small heating events heat the corona, often referred to as nanoflares. Title: Asymmetries of solar coronal extreme ultraviolet emission lines Authors: Peter, H. Bibcode: 2010A&A...521A..51P Altcode: 2010arXiv1004.5403P Context. The profiles of emission lines formed in the corona contain information on the dynamics and the heating of the hot plasma. Only recently has data with sufficiently high spectral resolution become available for investigating the details of the profiles of emission lines formed well above 106 K. These show enhanced emission in the line wings, which has not been understood yet.
Aims: We study the underlying processes leading to asymmetric line profiles, in particular the responsible plasma flows and line broadening mechanisms in a highly filamentary and dynamic atmosphere.
Methods: Line profiles of Fe XV formed at 2.5 MK acquired by the Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode solar space observatory are studied using multi Gaussian fits, with emphasis on the resulting line widths and Doppler shifts.
Results: In the major part of the active region, the spectra are best fit by a narrow line core and a broad minor component. The latter contributes some 10% to 20% to the total emission, is about a factor of 2 broader than the core, and shows strong blueshifts of up to 50 km s-1, especially in the footpoint regions of the loops. On average, the line width increases from the footpoints to the loop top for both components. A component with high upflow speeds can be found also in small restricted areas.
Conclusions: The coronal structures consist of at least two classes that are not resolved spatially but only spectroscopically and that are associated with the line core and the minor component. Because of their huge line width and strong upflows, it is proposed that the major part of the heating and the mass supply to the corona is actually located in source regions of the minor component. It might be that these are identical to type II spicules. The siphon flows and draining loops seen in the line core component are consistent with structures found in a three-dimensional magneto-hydrodynamic (3D MHD) coronal model. Despite the quite different appearance of the large active region corona and small network elements seen in transition region lines, both show similar line profile characteristics. This indicates that the same processes govern the heating and dynamics of the transition region and the corona. Title: Dynamics of Active Regions: Observations and 3D MHD Modeling Authors: Peter, Hardi; Bingert, S. Bibcode: 2010AAS...21630005P Altcode: We present data from Hinode/EIS, compare these to 3D MHD coronal models and outline forward models using SDO data. EUV lines originating in an active region show asymmetric line profiles that are interpreted using multi-Gaussian line fitting. Depending of the structure, a minor component can show high upflow speeds of above 100 km/S or it can be subject to strong heating resulting in a very strong broadening in combination with upflows of some 50 km/s. The latter ones are found near loop footpoints and might be associated with type II spicules feeding mass into coronal structures.

The bulk part of the active region emission shows loops with either siphon-type loops, associated with asymmetric heating, or with downflows on both legs, associated with a lack of heating, i.e. cooling. We find these typical structures also in 3D MHD models where we can directly investigate the dynamics in the computational box. The properties of the observed coronal spectral profiles is surprisingly similar to those in the transition region suggesting common mechanisms driving the heating and dynamics from the cool transition region to the hot corona. This is also supported by the 3D MHD models which treat the chromosphere - transition region - corona system as one single system.

Based on this work we will present the concept to use HMI/SDO magnetic field data as input at the lower boundary of our 3D coronal models. The coronal model will then be used to synthesize the emission that would be observed by AIA/SDO, which then can be directly compared to the actual AIA observations. As the numerical simulations are computationally time-consuming, we will not be able to show final results based on HMI data, but will present first results and results from older data sets. Title: On the nature of coronal loops above the quiet sun network Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B. V. Bibcode: 2010AdSpR..45..310B Altcode: The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network. Title: Spectral analysis of 3D MHD models of coronal structures Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi Bibcode: 2009AdSpR..43.1451Z Altcode: 2009arXiv0904.2312Z We study extreme-ultraviolet emission line spectra derived from three-dimensional magnetohydrodynamic models of structures in the corona. In order to investigate the effects of increased magnetic activity at photospheric levels in a numerical experiment, a much higher magnetic flux density is applied at the photosphere as compared to the Sun. Thus, we can expect our results to highlight the differences between the Sun and more active, but still solar-like stars. We discuss signatures seen in extreme-ultraviolet emission lines synthesized from these models and compare them to observed signatures in the spatial distribution and temporal evolution of Doppler shifts in lines formed in the transition region and corona. This is of major interest to test the quality of the underlying magnetohydrodynamic model to heat the corona, i.e. currents in the corona driven by photospheric motions (flux braiding). Title: Commission 10: Solar Activity Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy, Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter, Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu Bibcode: 2009IAUTA..27...79K Altcode: 2008arXiv0809.1444K Commission 10 deals with solar activity in all of its forms, ranging from the smallest nanoflares to the largest coronal mass ejections. This report reviews scientific progress over the roughly two-year period ending in the middle of 2008. This has been an exciting time in solar physics, highlighted by the launches of the Hinode and STEREO missions late in 2006. The report is reasonably comprehensive, though it is far from exhaustive. Limited space prevents the inclusion of many significant results. The report is divided into the following sections: Photosphere and chromosphere; Transition region; Corona and coronal heating; Coronal jets; flares; Coronal mass ejection initiation; Global coronal waves and shocks; Coronal dimming; The link between low coronal CME signatures and magnetic clouds; Coronal mass ejections in the heliosphere; and Coronal mass ejections and space weather. Primary authorship is indicated at the beginning of each section. Title: Doppler shifts in the transition region and corona. Mass cycle between the chromosphere and the corona Authors: Zacharias, P.; Bingert, S.; Peter, H. Bibcode: 2009MmSAI..80..654Z Altcode: Emission lines in the transition region and corona show persistent line shifts. It is a major challenge to understand the dynamics in the upper atmosphere and thus these line shifts, which are a signature of the mass cycle between the chromosphere and the corona. We examine EUV emission line profiles synthesized from a 3D MHD coronal model of a solar-like corona, in particular of an active region surrounded by strong chromospheric network. This allows us to investigate the physical processes leading to the line Doppler shifts, since we have access to both, the synthetic spectra and the physical parameters, i.e. magnetic field, temperature and density in the simulation box. By analyzing the evolution of the flows along field lines together with the changing magnetic structure we can investigate the mass cycle. We find evidence that loops are loaded with mass during a reconnection process, leading to upflows. After the loops disconnect from the reconnection site, they cool and drain which leads to the observed redshifts. Previous 1D loop models (neglecting the 3D nature) assumed that heating leads to evaporation and upflows followed by a cooling phase after the heating stops. The scenario modeled here is quite different, as it shows that the continuously changing three-dimensional magnetic structure is of pivotal importance to understand the mass balance between the chromosphere and the corona. Title: Spectral Analysis of 3D MHD Models of Quiet Sun and Active Region Structures Authors: Zacharias, P.; Bingert, S.; Peter, H. Bibcode: 2008ESPM...12.3.39Z Altcode: We study EUV emission line spectra derived from 3D MHD models of structures in the corona, in particular of an active region surrounded by a strong chromospheric network. The 3D MHD models account properly for the energy balance, especially for heat conduction and radiative losses. This allows us to reliably synthesize the profiles of EUV emission lines observable with current EUV spectrometers, e.g. SUMER/SOHO and EIS/Hinode. We investigate the temporal evolution and spatial distribution of the Doppler shifts of the EUV emission lines synthesized from these models. This is of major interest for the underlying mechanism of the heating of the solar corona, i.e. dissipation of currents in the corona driven by photospheric motions (flux braiding). Based on the 3D MHD models we can also derive intensity maps as they will be observed through the coronal channels of the AIA-instrument onboard SDO and we can analyze how to process the AIA maps in order to derive physical quantities such as temperatures and densities. Since we have access not only to the synthetic spectra, but also to the magnetic field in the box we can explore how the magnetic field relates to the emission in various coronal lines. In the present paper we will show results on the evolution of spectral properties such as line shifts or widths in different modeled coronal structures and compare these to results from SUMER/SOHO as well as EIS/Hinode data. Title: On the Nature of Coronal Loops Authors: Bingert, S.; Zacharias, P.; Peter, H.; Gudiksen, B. Bibcode: 2008ESPM...12.3.29B Altcode: The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity.

Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different.

We will compare two different models of an active region, one for two basically isolated sunspots, and another one for two sunspots surrounded by magnetic field patches similar to the chromospheric network.

The current paradigm is that these loops follow magnetic field lines as pearls on a string, and thus the majority of present corona models describe structures following the field lines.

Our study challenges this paradigm by showing through a three-dimensional model that coronal structures in complex magnetic field geometries might appear loop-like while they are not aligned with the magnetic field.

Using a forward model approach, both models result in emission from the corona being concentrated in loop structures.

In the first case the loops seen in EUV and X-ray emission are following the magnetic field.

However, in the second case, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field, but are more related to the current sheets formed in response to the footpoint motions of the magnetic field.

This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network or in the complex magnetic structures within chromospheric network patches. Title: 12th European Solar Physics Meeting Authors: Peter, H. Bibcode: 2008ESPM...12.....P Altcode: The meeting will cover all aspects of solar physics and is the ideal place for European and international solar researchers to come together. It begins with a session, where we ask experienced researchers to give a broad view on what the real key questions in our field are. This will give a stimulating start for the discussions during the meeting. The main part of the meeting will be session 2 and 3, where the problems and results in solar physics will be discussed from the observationally as well as from the theory side. This does not mean that observations and theory will be split. In the contrary, twice we will look at the same problems from two sides: what new observations drive us to run new models and how do new models motivate new concepts of observational approaches. In a way, we do the journey from the core of the Sun to the outer edge of the heliosphere twice. Following these major sessions of the meeting we will have room to discuss how solar work relates to other stars, and vice versa, and how to understand the Sun-Earth system. The final session will deal with plans for future research in our field, including the European Solar Telescope (EST), the status of the Solar Orbiter or activities in theory and modeling. Title: The control and data concept for the robotic solar telescope ChroTel Authors: Halbgewachs, C.; Bethge, Ch.; Caligari, P.; Elmore, D.; Kentischer, T. J.; Peter, H.; Sigwarth, M.; Schmidt, W. Bibcode: 2008SPIE.7019E..2TH Altcode: 2008SPIE.7019E..93H The solar telescope ChroTel is designed as a robotic telescope so that no user interaction is necessary for observation. The telescope will start tracking in the morning as soon as weather conditions are appropriate and will process a user defined observation routine until sunset. Weather conditions and system status are continuously monitored to close the telescope shutter in case of bad weather or to drive to the stow position in case of an error. The ChroTel control software was programmed in LabVIEW. Title: ChroTel: a robotic telescope to observe the chromosphere of the Sun Authors: Kentischer, T. J.; Bethge, Ch.; Elmore, D. F.; Friedlein, R.; Halbgewachs, C.; Knölker, M.; Peter, H.; Schmidt, W.; Sigwarth, M.; Streander, K. Bibcode: 2008SPIE.7014E..13K Altcode: 2008SPIE.7014E..36K The Chromospheric Telescope (ChroTel) is a 10 cm robotic telescope to observe the full solar disk with a 2k × 2k CCD at high temporal cadence. It is located at the Observatorio del Teide, Tenerife, Spain, next to the 70 cm German Vacuum Tower Telescope (VTT). ChroTel contains a turret system that relays a stabilized image of the solar disk into a laboratory within the VTT building. The control design allows a fully robotic operation. Observations are carried out in three chromospheric wavelengths (CaK: 393 nm, Ha: 652 nm, HeI 1083 nm). Title: On the nature of coronal loops Authors: Peter, H.; Bingert, S.; Gudiksen, B. V. Bibcode: 2008AGUSMSP41C..05P Altcode: No abstract at ADS Title: Editorial Authors: Walmsley, M.; Peter, H. Bibcode: 2008A&A...481E..17W Altcode: In September 2006 the solar space observatory Hinode (Japanese for sunrise) was launched. The project is led by the Japan Aerospace Exploration Agency (JAXA) together with the National Astronomical Observatory of Japan (NAOJ) and contributions from the United States and the United Kingdom. The European Space Agency (ESA) is providing ground station coverage through the Svalbard Satellite Station in Norway. The Hinode satellite carries a solar optical telescope (SOT), a X-ray telescope (XRT), and an EUV imaging spectrometer (EIS). Together, these permit an investigation of the interior of the Sun, and all atmospheric regions, from the photosphere and chromosphere to the corona, addressing the origin of the Sun's magnetic field, the driving force behind solar eruptive events, and the nature of the hot corona. As a recognition of the impact provided by the new solar observations using HINODE, we publish a “special feature” in this issue. It consists of 18 letters which document these new results. Title: On the nature of coronal loops Authors: Bingert, Sven; Zacharias, Pia; Peter, Hardi Bibcode: 2008cosp...37..302B Altcode: 2008cosp.meet..302B The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. In these models we account for the mass, momentum and energy balance including heat conduction and radiative losses. The heating is through current dissipation in the corona driven by photospheric motions (flux braiding). Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We will compare two different models of an active region, one for two basically isolated sunspots, and another one for two sunspots surrounded by strong magnetic field patches mimicking a strong chromospheric network. Both models result in emission from the model corona being concentrated in loop structures. In the first case the loops seen in EUV and X-ray emission are aligned with the magnetic field, following the general paradigm. However, in the second case, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field, but are related to the current sheets formed in response to the footpoint motions of the magnetic field. For the Sun this result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network. These models are now ready to face detailed comparisons with EUV spectroscopic observations from Hinode and imaging from SDO/AIA. Title: 3D MHD models compared to EUV observations of quiet Sun and active region structures Authors: Zacharias, Pia; Bingert, Sven; Peter, Hardi Bibcode: 2008cosp...37.3579Z Altcode: 2008cosp.meet.3579Z We study EUV emission line spectra derived from 3D MHD models of structures in the corona, in particular of an active region surrounded by a strong chromospheric network. The 3D MHD models account properly for the energy balance, especially for heat conduction and radiative losses, which allows us to reliably synthesize the profiles of EUV emission lines observable with current EUV spectrometers, i.e. SUMER/SOHO and EIS/Hinode. Thus we can directly compare these synthesized spectra to real observations with these instruments. We will discuss differences of models with different levels of magnetic activity in terms of signatures seen in EUV lines synthesized from these models and compare them to signatures found, e.g., in the spatial distribution and temporal evolution of Doppler shifts of lines formed in the transition region and corona. This is of major interest to test the quality of the underlying model to heat the corona, i.e. currents in the corona driven by photospheric motions (flux braiding). Based on the 3D MHD model we can also derive intensity maps as they will be observed through the coronal channels of the AIA-instrument on SDO. Through this we can provide some guidance on how to process the AIA maps in order to derive physical quantities such as temperature and density through a combination of filters. Title: The correlation between coronal Doppler shifts and the supergranular network Authors: Aiouaz, T.; Peter, H.; Lemaire, P. Bibcode: 2007A&A...466..689A Altcode: No abstract at ADS Title: The Coronal Heating Paradox Authors: Aschwanden, Markus J.; Winebarger, Amy; Tsiklauri, David; Peter, Hardi Bibcode: 2007ApJ...659.1673A Altcode: The ``coronal heating problem'' has been with us over 60 years, and hundreds of theoretical models have been proposed without an obvious solution in sight. In this paper we point out that observations show no evidence for local heating in the solar corona, but rather for heating below the corona in the transition region and upper chromosphere, with subsequent chromospheric evaporation as known in flares. New observational evidence for this scenario comes from (1) the temperature evolution of coronal loops, (2) the overdensity of hot coronal loops, (3) upflows in coronal loops, (4) the Doppler shift in coronal loops, (5) upward propagating waves, (6) the energy balance in coronal loops, (7) the magnetic complexity in the transition region, (8) the altitude of nanoflares and microflares, (9) the cross section of elementary loops, as well as (10) 3D MHD simulations of coronal heating. The phrase ``coronal heating problem'' is therefore a paradoxical misnomer for what should rather be addressed as the ``chromospheric heating problem'' and ``coronal loop filling process.'' This paradigm shift substantially reduces the number of relevant theoretical models for coronal heating in active regions and the quiet Sun, but our arguments do not apply to coronal holes and the extended heliospheric corona. Title: Das Heliophysikalische Jahr 2007 in Deutschland Authors: Heber, Bernd; Fichtner, Horst; Scherer, Klaus; Büchner, Jörg; Peter, Hardi Bibcode: 2007S&W....46d..18H Altcode: No abstract at ADS Title: Modeling the (upper) solar atmosphere including the magnetic field Authors: Peter, H. Bibcode: 2007AdSpR..39.1814P Altcode: 2007astro.ph..3575P The atmosphere of the Sun is highly structured and dynamic in nature. From the photosphere and chromosphere into the transition region and the corona plasma- β changes from above to below one, i.e., while in the lower atmosphere the energy density of the plasma dominates, in the upper atmosphere the magnetic field plays the governing role - one might speak of a "magnetic transition". Therefore the dynamics of the overshooting convection in the photosphere, the granulation, is shuffling the magnetic field around in the photosphere. This leads not only to a (re-)structuring of the magnetic field in the upper atmosphere, but induces also the dynamic reaction of the coronal plasma, e.g., due to reconnection events. Therefore the (complex) structure and the interaction of various magnetic patches is crucial to understand the structure, dynamics and heating of coronal plasma as well as its acceleration into the solar wind. The present article will emphasize the need for three-dimensional modeling accounting for the complexity of the solar atmosphere to understand these processes. Some advances on 3D modeling of the upper solar atmosphere in magnetically closed as well as open regions will be presented together with diagnostic tools to compare these models to observations. This highlights the recent success of these models which in many respects closely match the observations. Title: S ynergies With Other Missions Concerning Ultraviolet Imaging And Spectroscopy Authors: Peter, H. Bibcode: 2007ESASP.641E..18P Altcode: The Solar Orbiter will provide a number of unique opportunities to observe the Sun from close-by, at co-rotation and from high latitudes not accessible for observatories in Earth orbit or at L1. Nevertheless other missions providing information in the VUV using imaging and spectroscopic instruments can provide valuable complementary information. In phases when the Orbiter observes parts of the Sun also accessible from other space observatories, the latter ones can provide data with a higher rate than the orbiter and through this allow a larger field of view and/or a faster time cadence. Also imagers could cover more wavelength bands and spectrometers might provide a larger spectral coverage and higher spectral resolution than possible on the Orbiter. The different profiles of the instrumentation on-board the Orbiter and other space observatories will answer different aspects on the physical problems tackled by future models and by this supply complementary information to better understand our Sun. Instrumentation complementing Solar Orbiter, especially concerning fast raster maps with full spectral coverage and VUV spectro-polarimetry to investigate coronal magnetic fields, are highly desirable. In order to answer questions to be posed by elaborate future coronal models the solar community should be really ambitious and aim at a combination of such instruments in near Earth orbit for coordinated observations with the Solar Orbiter Title: Link between the chromospheric network and magnetic structures of the corona Authors: Jendersie, S.; Peter, H. Bibcode: 2006A&A...460..901J Altcode: 2006astro.ph..9280J Context: . Recent work suggested that the traditional picture of the corona above the quiet Sun being rooted in the magnetic concentrations of the chromospheric network alone is strongly questionable.
Aims: . Building on that previous study we explore the impact of magnetic configurations in the photosphere and the low corona on the magnetic connectivity from the network to the corona. Observational studies of this connectivity are often utilizing magnetic field extrapolations. However, it is open to which extent such extrapolations really represent the connectivity found on the Sun, as observations are not able to resolve all fine scale magnetic structures. The present numerical experiments aim at contributing to this question.
Methods: . We investigated random salt-and-pepper-type distributions of kilo-Gauss internetwork flux elements carrying some 1015 to 1017 Mx, which are hardly distinguishable by current observational techniques. These photospheric distributions are then extrapolated into the corona using different sets of boundary conditions at the bottom and the top. This allows us to investigate the fraction of network flux which is connected to the corona, as well as the locations of those coronal regions which are connected to the network patches.
Results: . We find that with current instrumentation one cannot really determine from observations, which regions on the quiet Sun surface, i.e. in the network and internetwork, are connected to which parts of the corona through extrapolation techniques. Future spectro-polarimetric instruments, such as with Solar B or Gregor, will provide a higher sensitivity, and studies like the present one could help to estimate to which extent one can then pinpoint the connection from the chromosphere to the corona.
Conclusions: . Title: First VUV Sun-As-A-Star Spectrum Compared to Other Cool Stars Authors: Peter, H. Bibcode: 2006ESASP.617E..10P Altcode: 2006soho...17E..10P No abstract at ADS Title: First high spectral resolution VUV full-Sun spectrum compared to cool stars Authors: Peter, Hardi Bibcode: 2006A&A...449..759P Altcode: This paper reports the first full-Sun vacuum ultraviolet (VUV) emission line profile originating from the transition region from the chromosphere to the corona. It is based on a raster scan of the whole solar disk using SUMER/SOHO. The full-Sun spectrum has a spectral resolution which allows an investigation of details in the line profile as well as a thorough comparison to stellar spectra as obtained, e.g. with FUSE or STIS/HST. The full-Sun spectrum shows enhanced emission in the wings, and is well described by a double Gaussian fit with a narrow and a broad component. It is shown that the broad component is due to structures on the solar surface, especially those related to the magnetic chromospheric network. Thus it is proposed that the broad components of other solar-like stars are also a consequence of the mixture of surface structures, and not necessarily a signature of small-scale heating processes like explosive events, as it is commonly argued. A comparison to spectra of luminous cool stars shows that the line asymmetries of these stars might also be a surface structure effect and not or only partly due to opacity effects in their cool dense winds. These comparisons show the potential of high quality full-Sun VUV spectra and their value for the study of solar-stellar connections. As an example, this study proposes that αCen A has a considerably higher amount of magnetic flux concentrated in the chromospheric magnetic network than the Sun. Title: Forward Modeling of the Corona of the Sun and Solar-like Stars: From a Three-dimensional Magnetohydrodynamic Model to Synthetic Extreme-Ultraviolet Spectra Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke Bibcode: 2006ApJ...638.1086P Altcode: 2005astro.ph..3342P A forward model is described in which we synthesize spectra from an ab initio three-dimensional MHD simulation of an outer stellar atmosphere, where the coronal heating is based on braiding of magnetic flux due to photospheric footpoint motions. We discuss the validity of assumptions such as ionization equilibrium and investigate the applicability of diagnostics like the differential emission measure inversion. We find that the general appearance of the synthesized corona is similar to the solar corona and that, on a statistical basis, integral quantities such as average Doppler shifts or differential emission measures are reproduced remarkably well. The persistent redshifts in the transition region, which have puzzled theorists since their discovery, are explained by this model as caused by the flows induced by the heating through braiding of magnetic flux. While the model corona is only slowly evolving in intensity, as is observed, the amount of structure and variability in Doppler shift is very large. This emphasizes the need for fast coronal spectroscopic observations, as the dynamical response of the corona to the heating process manifests itself in a comparably slow evolving coronal intensity but rapid changes in Doppler shift. Title: High resolution solar telescope GREGOR Authors: Peter, H.; von der Luehe, O. Bibcode: 2006cosp...36.3628P Altcode: 2006cosp.meet.3628P GREGOR is a new 1 5 m solar telescope assembled on Tenerife Spain by a German consortium together with international partners It is designed for high-precision measurements of the magnetic field and the gas motion in the solar photosphere and chromosphere with a resolution of 70 km on the Sun and for high resolution stellar spectroscopy The telescope has a Gregorian configuration mounted in an open structure with a retractable dome for thermal control and features a multi-conjugate adaptive optics system GREGOR will be equipped with several post-focus instruments including polarimetric longslit spectrographs for the visible and infrared and a high resolution Fabry-Perot filter spectrometer for 3D spectroscopy Apart from a report on the present status of the instrument the talk will focus on a selection of science goals of GREGOR Among these are the emergence evolution and disappearance of small-scale magnetic flux the energy budget of sunspots and the structure of their penumbrae as well as the dynamics of the chromosphere In this context an attempt will be made to put future observations with GREGOR in the context of modeling of the photosphere chromosphere and corona Title: Evidence for coronal plasma oscillations over supergranular cells Authors: Gontikakis, C.; Peter, H.; Dara, H. C. Bibcode: 2006IAUS..233..189G Altcode: Evidence of coronal oscillations over the interior of supergranular cells was found through SUMER observations. The observations are rasters of quiet Sun regions and the oscillations were detected, in the Ne VIII 770 Å Doppler maps, as a characteristic pattern. It should be noted that the Ne VIII ion has coronal formation temperature (650 000 K) and that reports of oscillations in the quiet Sun corona are scarce. Magnetic extrapolation from MDI magnetogram showed that at the location where the oscillation was detected, the gas and magnetic pressures get equalized (β=1) higher in the atmosphere, compared to the surrounding, non oscillating quiet Sun. This could indicate a non-compressible wave propagating inside the gas dominated medium of the cell causing the detected oscillation. Title: Modelling the solar atmosphere including the magnetic field Authors: Peter, H. Bibcode: 2006cosp...36.2989P Altcode: 2006cosp.meet.2989P The lower corona and transition region of the Sun are highly structured through the magnetic field and are very dynamic To account for both the structure and the dynamics of the solar atmosphere a 3D MHD model has to be employed The paper will present recent 3D MHD coronal models and will discuss them with special emphasis on a possible comparison to observations Results will be presented for spectra derived from the complex coronal models which can be treated as synthetic observations The agreement of these to actual average observations of the solar outer atmosphere is very good especially with respect to Doppler shifts and the emission measure The results from the 3D MHD models will also be compared to modeling stellar atmospheres through a superposition of a large number of individual loops as derived from potential field extrapolations This will show the limitations as well as the potential of such multi-loop models Finally the results of modern models of the solar corona will be put into the context of stellar observations and possible future directions in coronal modeling will be discussed Title: Forward modeling of coronal funnels Authors: Aiouaz, T.; Peter, H.; Keppens, R. Bibcode: 2005A&A...442L..35A Altcode: We propose a forward modeling approach of coronal funnels to investigate the outer layers of the solar atmosphere with respect to their thermodynamical properties and resulting emission line spectra. We investigate the plasma flow out of funnels with a new 2D MHD time dependent model including the solar atmosphere all the way from the chromosphere to the corona. The plasma in the funnel is treated in the single-fluid MHD approximation including radiative losses, anisotropic thermal conduction, and two different parameterized heating functions. We obtain plasma properties (e.g. density, temperature and flow speed) within the funnel for each heating function. From the results of the MHD calculation we derive spectral profiles of a low corona emission line (Ne VIII, 770 Å). This allows us e.g. to study the Doppler shifts across the funnel. These results indicate a systematic variation of the Doppler shifts in lines formed in the low corona depending on the heating function used. The line shift above the magnetic field concentration in the network is stronger than in the inter-network in both cases. However, for one of the heating functions, the maximum blue-shift (outflow) is not to be found in the very center of the funnel but in the vicinity of the center. This is not the case of the second heating function where the maximum is well aligned with the centre of the funnel. This model directly relates for the first time the form of the heating function to the thermodynamic and spectral properties of the plasma in a funnel. Title: Coronal Heating Through Braiding of Magnetic Field Lines Synthesized Coronal EUV Emission and Magnetic Structure Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A. Bibcode: 2005ESASP.596E..14P Altcode: 2005ccmf.confE..14P No abstract at ADS Title: Coronal oscillation above a supergranular cell of the quiet Sun chromospheric network? Authors: Gontikakis, C.; Peter, H.; Dara, H. C. Bibcode: 2005A&A...441.1191G Altcode: We have detected an oscillation in the low corona, using a raster of the SUMER EUV spectrograph in the Ne viii, 770 Å line formed at about 700 000 K. The oscillation was found in the Ne viii Doppler map above the interior of a supergranular cell of the chromospheric network in the quiet Sun, while it is absent in line radiance. The photospheric magnetic field, extrapolated to coronal levels, was used to relate this phenomenon to the magnetic structure. This oscillation phenomenon, reported here for the first time, seems to occur only above the darkest cells of the chromospheric network. We interpret our findings as a collective non-compressible oscillation of the corona above the whole cell interior. This oscillation may originate at the chromosphere and the driving wave may undergo a mode conversion at the top chromosphere, where the magnetic pressure equals the gas pressure. Our interpretation cannot be definitive and should be verified with more data. Title: First VUV full-Sun spectrum of the transition region with high spectral resolution compared to cool stars Authors: Peter, Hardi Bibcode: 2005astro.ph.10319P Altcode: This paper reports the first full-Sun vacuum ultraviolet (VUV) emission line profile originating from the transition region from the chromosphere to the corona. It is based on a raster scan of the whole solar disk using SUMER/SOHO. The full-Sun spectrum has a spectral resolution which allows an investigation of details in the line profile as well as a thorough comparison to stellar spectra as obtained, e.g. with FUSE or STIS/HST. The full-Sun spectrum shows enhanced emission in the wings, and is well described by a double Gaussian fit with a narrow and a broad component. It is shown that the broad component is due to structures on the solar surface, especially those related to the magnetic chromospheric network. Thus it is proposed that the broad components of other solar-like stars are also a consequence of the mixture of surface structures, and not necessarily a signature of small-scale heating processes like explosive events, as it is commonly argued. A comparison to spectra of luminous cool stars shows that the line asymmetries of these stars might also be a surface structure effect and not or only partly due to opacity effects in their cool dense winds. These comparisons show the potential of high quality full-Sun VUV spectra and their value for the study of solar-stellar connections. As an example, this study proposes that alpha Cen A has a considerably higher amount of magnetic flux concentrated in the chromospheric magnetic network than the Sun. Title: EUV Emission from a 3D MHD Coronal Model: Temporal Variability in a Synthesized Corona Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å. Bibcode: 2005ESASP.592..527P Altcode: 2005soho...16E..98P; 2005ESASP.592E..98P No abstract at ADS Title: The Structure of the Base of the Corona Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, Ake Bibcode: 2005ESASP.592..471B Altcode: 2005ESASP.592E..84B; 2005soho...16E..84B No abstract at ADS Title: Relation of the Chromospheric Network to Coronal Funnels and the Solar Wind Authors: Aiouaz, T.; Peter, H.; Keppens, R. Bibcode: 2005ESASP.592..135A Altcode: 2005ESASP.592E..20A; 2005soho...16E..20A No abstract at ADS Title: High-speed coronal rain Authors: Müller, D. A. N.; De Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2005A&A...436.1067M Altcode: At high spatial and temporal resolution, coronal loops are observed to have a highly dynamic nature. Recent observations with SOHO and TRACE frequently show localized brightenings "raining" down towards the solar surface. What is the origin of these features? Here we present for the first time a comparison of observed intensity enhancements from an EIT shutterless campaign with non-equilibrium ionization simulations of coronal loops in order to reveal the physical processes governing fast flows and localized brightenings. We show that catastrophic cooling around the loop apex as a consequence of footpoint-concentrated heating offers a simple explanation for these observations. An advantage of this model is that no external driving mechanism is necessary as the dynamics result entirely from the non-linear character of the problem. Title: The correlation between coronal Doppler shifts and the supergranular network Authors: Aiouaz, T.; Peter, H.; Lemaire, P. Bibcode: 2005A&A...435..713A Altcode: We examine properties of line profiles as found with large raster scans of the solar corona acquired by the UV spectrometer SUMER on board SOHO. The observed regions include an equatorial coronal hole, a polar coronal hole, as well as surrounding quiet Sun areas. In order to reveal the network and remove strong local brightenings, a filter is applied to a continuum image. The filtered continuum image, the intensity image and the dopplergram are used to produce “scatter diagrams” (dispersion plots). We find correlations between the chromospheric network, the Ne VIII (770 Å) intensity and the Ne VIII (770 Å) Doppler shift in quiet Sun areas and in coronal holes. We establish that the maximum outflow (blue-shift) at low corona temperatures does not appear in the centre of the network but rather near network boundaries. Furthermore the maximum blue-shift seems to appear in the dark regions in Ne VIII line intensity, which is in agreement with Wilhelm (2000). The opposite correlation appears for very low intensities (less than half of the average intensity), revealing in these regions a lack of energy to either accelerate the solar wind or produce any detectable radiation. The absence of magnetic field concentration in these regions in a reconstructed magnetogram from a MDI/SOHO series seems to confirm the lack of energy. Title: Tackling the coronal heating problem using 3D MHD coronal simulations with spectral synthesis Authors: Peter, H.; Gudiksen, B. V.; Nordlund, A. Bibcode: 2005ESASP.560...59P Altcode: 2005csss...13...59P No abstract at ADS Title: On the Outflow at Solar Corona Heights Authors: Aiouaz, T.; Peter, H.; Lemaire, P. Bibcode: 2004ESASP.575..331A Altcode: 2004soho...15..331A No abstract at ADS Title: Forward Modelling of Coronal Funnels Authors: Aiouaz, T.; Peter, H.; Keppens, R. Bibcode: 2004ESASP.575..337A Altcode: 2004soho...15..337A No abstract at ADS Title: Oscillations Over a Supergranular Cell Observed with SUMER Authors: Gontikakis, C.; Peter, H.; Dara, H. C. Bibcode: 2004ESASP.575..131G Altcode: 2004soho...15..131G No abstract at ADS Title: Statistical Comparison of Blinkers and Explosive Events Authors: Brkovic, A.; Peter, H. Bibcode: 2004ESASP.575..471B Altcode: 2004soho...15..471B No abstract at ADS Title: Analysis of Synthetic EUV Spectra from 3d Models of the Corona Authors: Bingert, S.; Peter, H.; Gudiksen, B.; Nordlund, A.; Dobler, W. Bibcode: 2004ESASP.575..348B Altcode: 2004soho...15..348B No abstract at ADS Title: Synthetic EUV Spectra from 3D MHD Coronal Simulations: Coronal Heating Through Magnetic Braiding Authors: Peter, H.; Gudiksen, B. V.; Nordlund, Å. Bibcode: 2004ESASP.575...50P Altcode: 2004soho...15...50P No abstract at ADS Title: Coronal Heating through Braiding of Magnetic Field Lines Authors: Peter, Hardi; Gudiksen, Boris V.; Nordlund, Åke Bibcode: 2004ApJ...617L..85P Altcode: 2004astro.ph..9504P Cool stars such as our Sun are surrounded by a million degree hot outer atmosphere, the corona. For more than 60 years, the physical nature of the processes heating the corona to temperatures well in excess of those on the stellar surface have remained puzzling. Recent progress in observational techniques and numerical modeling now opens a new window to approach this problem. We present the first coronal emission-line spectra synthesized from three-dimensional numerical models describing the evolution of the dynamics and energetics as well as of the magnetic field in the corona. In these models the corona is heated through motions on the stellar surface that lead to a braiding of magnetic field lines inducing currents that are finally dissipated. These forward models enable us to synthesize observed properties such as (average) emission-line Doppler shifts or emission measures in the outer atmosphere, which until now have not been understood theoretically, even though many suggestions have been made in the past. As our model passes these observational tests, we conclude that the flux braiding mechanism is a prime candidate for being the dominant heating process of the magnetically closed corona of the Sun and solar-like stars. Title: Thermal Instability as the Origin of High Speed Coronal Rain Authors: Müller, D. A. N.; de Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2004ESASP.575..291M Altcode: 2004soho...15..291M No abstract at ADS Title: Dynamics of solar coronal loops. II. Catastrophic cooling and high-speed downflows Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H. Bibcode: 2004A&A...424..289M Altcode: 2004astro.ph..5538M This work addresses the problem of plasma condensation and ``catastrophic cooling'' in solar coronal loops. We have carried out numerical calculations of coronal loops and find several classes of time-dependent solutions (static, periodic, irregular), depending on the spatial distribution of a temporally constant energy deposition in the loop. Dynamic loops exhibit recurrent plasma condensations, accompanied by high-speed downflows and transient brightenings of transition region lines, in good agreement with features observed with TRACE. Furthermore, these results also offer an explanation for the recent EIT observations of \cite{DeGroof+al2004AA} of moving bright blobs in large coronal loops. In contrast to earlier models, we suggest that the process of catastrophic cooling is not initiated by a drastic decrease of the total loop heating but rather results from a loss of equilibrium at the loop apex as a natural consequence of heating concentrated at the footpoints of the loop, but constant in time. Title: Statistical comparison of transition region blinkers and explosive events Authors: Brković, A.; Peter, H. Bibcode: 2004A&A...422..709B Altcode: Explosive events and blinkers are two observational classes of transients seen on the quiet Sun and an investigation of the significance of and relationship between such events may be critical for understanding basic processes at work in the solar atmosphere. We analysed five time-series spectra of the quiet Sun of transition region O XV 629 Å, O XVI 1032 Å and O XVI 1038 Å lines. We investigated how often explosive events occurred during the course of a blinker at the same location and found that slightly more than a half of all explosive events happened during about one third of all blinkers. In some cases during a blinker more than one explosive event was registered. The largest average maximum relative intensity enhancement was for blinkers with explosive events, followed by the blinkers without explosive events, with the least being for the explosive events which did not happen during the course of a blinker. Due to these differences among the maximum enhancements between these events we suppose that blinkers and explosive events are two independent phenomena. Intensity light curves of blinkers show that events can be separated into two classes, specifically: 1) simple blinkers with smooth increase in intensity having only one significant peak, and 2) complex blinkers characterised by multiple (2-4) significant peaks. These two classes were equally represented when frequencies of their occurrences were averaged over five analysed data-sets. The analysis of the line profile parameters and their correlations for these two classes did not give any result which could further distinguish between them. During blinkers the intensity peaks mostly at the middle of their durations, while the line width peaks somewhat earlier than the intensity. This was a general character for both simple and complex blinkers, as well as for all explosive events. Title: Transition region blinkers versus explosive events Authors: Brkovic, A.; Peter, H. Bibcode: 2004IAUS..223..449B Altcode: 2005IAUS..223..449B Explosive events and blinkers are two observational classes of transients seen on the quiet Sun and an investigation of the significance of and relationship between such events may be critical for understanding basic processes at work in the solar atmosphere. Our analysis showed that blinkers and explosive events are independent phenomena which have to be explained separately. Title: Analysis of Intensities, Line Widths and Line Shifts during Blinkers Authors: Brkovic, A.; Peter, H. Bibcode: 2004ESASP.547..251B Altcode: 2004soho...13..251B Explosive events and blinkers are two major observational classes of transients seen on the quiet Sun. We analysed five time-series spectra of transition region lines O V 629 Å, O VI 1032 Å and O VI 1038 Å and detected 209 blinkers and 55 explosive events. In 44 cases both events were present at the same pixel simultaneously. This means that about 21% of blinkers coincided with explosive events and that 80% of explosive events had intensity enhancements typical for blinkers on the time scale longer than an hour. In more than a half of the eleven remaining explosive events we found that intensity enhancements relative to the pre-event level were on average on the 100% level on the time scale somewhat longer than duration of explosive event, i.e., shorter than ten minutes. The blinkers without an explosive event signature could be separated into two observational classes: 1) Simple blinkers with smooth increase in intensity having only one significant peak and 2) Complex or oscillating blinkers characterised by multiple (2-4) significant peaks. By percentage the class 1) was represented with about 45% and the class 2) with about 34%. Title: Thermal non-equilibrium in coronal loops: A road to complex evolution Authors: Müller, Daniel; de Groof, A.; Hansteen, V. H.; Peter, H. Bibcode: 2004IAUS..223..289M Altcode: 2005IAUS..223..289M At high spatial and temporal resolution, coronal loops are observed to have a highly dynamic nature. Recent observations with SOHO and TRACE frequently show localized brightening "raining" down towards the solar surface. What is the origin of these features? Here we present for the first time a comparison of observed intensity enhancements from an EIT shutterless campaign with non-equilibrium ionization simulations of coronal loops in order to reveal the physical processes governing fast flows and localized brightening. We show that catastrophic cooling around the loop apex as a consequence of footpoint-concentrated heating offers a simple explanation for these observations. An advantage of this model is that no external driving mechanism is necessary as the dynamics result entirely from the non-linear character of the system. Title: Catastrophic Cooling and High-Speed Downflows in Solar Coronal Loops Authors: Müller, D.; Peter, H.; Hansteen, V. Bibcode: 2004IAUS..219..765M Altcode: 2003IAUS..219E..48M We report numerical simulations of the condensation of plasma in short solar coronal loops which has several interesting physical consequences. Firstly we propose a connection between small cool loops which presumably constitute the solar transition region and prominences in the sense that the same physical mechanism governs their dynamics namely the onset of instability and runaway cooling due to strong radiative losses. Secondly we show that the temporal evolution of these loop models exhibit a cyclic pattern of chromospheric evaporation condensation formation motion of the condensation region to either side of the loop and finally loop reheating with a period of 4000 - 6000 s for a loop of 10 Mm length. Thirdly we have synthesized transition region lines from these simulations which show strong periodic intensity variations making condensation processes in loops a candidate to account for the observed temporal variability of these lines. Title: Plasma Condensation in Solar Coronal Loops: II. "Catastrophic Cooling" and High-Speed Downflows Authors: Müller, D. A. N.; Peter, H.; Hansteen, V. H. Bibcode: 2004ESASP.547..199M Altcode: 2004soho...13..199M The second part of this work focuses on the application of the concept of plasma condensation to large coronal loops. In contrast to the short loops analyzed in Müller et al. (2003a), these models can more easily be compared to SOHO and TRACE observations. From our numerical calculations of coronal loops we find several classes of time-dependent solutions (static, periodic, irregular), depending on the spatial dependence of a temporally constant energy deposition in the loop. One of these classes is in remarkably close agreement with the features observed with TRACE, described by Schrijver (2001): Emission in C IV (154.8 nm), developing initially near the loop tops, cool plasma sliding down on both sides of the loop, downflow velocities of up to 100 km/s, and a downward acceleration which is substantially reduced with respect to the solar surface gravity. Furthermore, these results also offer an explanation for the observations of De Groof et al. (2003a,b). In contrast to earlier models, we suggest that the process of catastrophic cooling does not have to be initiated by a drastic decrease of the loop heating. It can also result from a loss of equilibrium at the loop apex which is a natural consequence if the loop is heated predominantly at the footpoints, but constant in time. Title: SOHO/SUMER Results: Mass Flows Authors: Peter, H. Bibcode: 2004IAUS..219..575P Altcode: 2003IAUS..219E.195P Hardly any part of the chromosphere and the low corona of the Sun is in a static state. Spectrographs reveal line shifts and non-thermal broadening indicating mass supply to the corona draining of coronal material siphon flows wave propagation and more. Imaging instruments show apparent motions of bright or dark structures indicating the presence of flows and waves too.

This paper will review recent observations of mass and wave motions in the low corona focusing on spectroscopic investigations and will discuss their relevance to the understanding of the dynamics and energetics of the corona.

Special emphasis will be devoted to the question what these solar observations might teach us with respect to stellar coronae and how we could use observations and models of stellar coronae to improve our knowledge of the Sun. Title: Structure and Dynamics of the Low Corona of the Sun (With 13 Figures) Authors: Peter, Hardi Bibcode: 2004RvMA...17...87P Altcode: No abstract at ADS Title: Dynamics and Properties of Coronal Funnels Authors: Aiouaz, T.; Peter, H.; Lemaire, P.; Keppens, R. Bibcode: 2004ESASP.547..375A Altcode: 2004soho...13..375A Coronal funnels are open magnetic structures connecting the chromosphere with the solar corona [5, 3]. We investigate the stationary plasma flow out of funnels with a 2D- MHD model. The funnel area function is derived from a magnetic field model and the funnel is approximately 10 Mm high and 20 Mm wide. The energy balance includes radiative losses, thermal conduction, and a parametrized heating function. We adjust the parameters to the quantities measured in the lower solar corona. We obtained 2D plasma properties (e.g. density, temperature, flow speed, etc.) within the funnel. From the results of the MHD calculation we synthesize emision profiles of various lines formed in the transition region from the chromosphere to the corona. This allows us to study e.g. the Doppler shifts at various temperatures across the funnel and thus enables a detailed comparison of the model results with observations. For this we investigate SUMER data and study Doppler shifts perpendicular to the chromospheric network for different emission lines, where a tessalation technique is used to derive the outlines of the chromospheric network. In this paper typical results are presented for the Ne VIII(770.4 Å) line. Preliminary results show that these model caclulations compare well to the observations. Title: Plasma Condensation in Solar Coronal Loops -- I. Basic Processes Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2004ESASP.547..285M Altcode: 2004soho...13..285M In the first part of this work, we report numerical calculations of the condensation of plasma in short coronal loops, which has several interesting physical consequences. We propose a connection between small, cool loops, which presumably constitute the solar transition region, and prominences in the sense that the same physical mechanism governs their dynamics, namely the onset of instability and runaway cooling due to strong radiative losses. We show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000 - 8000 s for a loop of 10 Mm length. Furthermore, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence. In the second part of this work (Müller et al., 2003b), we apply this concept to large coronal loops. Title: Heating the magnetically open ambient background corona of the Sun by Alfvén waves Authors: Peter, H.; Vocks, C. Bibcode: 2003A&A...411L.481P Altcode: Observations of UVCS/SOHO of very high emission line widths in the outer corona suggest that the open field regions there are heated by ion-cyclotron resonance absorption of Alfvén waves resulting in the ions being much hotter than the electrons. In the lower corona it is usually assumed that the effective Coulomb-interactions ensure equal temperatures of ions and electrons. However, recent models have shown that in regions of strong magnetic field gradients the ion-cyclotron absorption can be so efficient that the ion temperature overcomes the electron temperature. In this paper we will present new observational results from SUMER/SOHO showing that the lines of O 5 and S 6 have very large line widths just above the limb. The peak line width occurs at about 10 arcsec above the limb and corresponds to ion temperatures of more than 3x 106 K. We compare these observational results to new models in which plasma in coronal funnels is heated and accelerated by means of ion-cyclotron absorption of high-frequency waves. As our model is in good qualitative agreement with the observations we come to the conclusion that the open corona in coronal funnels could well be heated by an ion-cyclotron absorption mechanism, even close to the Sun in the low corona. Title: Dynamics of solar coronal loops. I. Condensation in cool loops and its effect on transition region lines Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003A&A...411..605M Altcode: We report numerical calculations of the condensation of plasma in short coronal loops, which have several interesting physical consequences. Firstly, we propose a connection between small, cool loops (T < 106 K), which constitute one of the basic components of the solar transition region, and prominences, in the sense that the same physical mechanism governs their dynamics: Namely the onset of instability and runaway cooling due to strong radiative losses. Secondly, we show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000-8000 s for a loop of 10 Mm length. Thirdly, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence. Title: Sizes of quiet Sun transition region structures Authors: Gontikakis, C.; Peter, H.; Dara, H. C. Bibcode: 2003A&A...408..743G Altcode: We studied the morphology of the transition region of the quiet Sun, with data obtained by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer (SUMER) and the Extreme-Ultraviolet Imaging Telescope (EIT), in September 1996. We analyzed lines emitted in the chromosphere, the low transition region and the low corona. We computed the mean autocorrelation function for the radiance images in order to estimate the characteristic size of the features present in the transition region. Moreover different estimate McCly-mont. Moreover, we calculated autocorrelation functions for the dopplergrams and line width images deduced from the SUMER data. In addition to the line core of the C IV line, we investigated a broader tail component, whose origin is still unclear. We found that the size of the bright radiance features is always larger than the size of the structures of the dopplergrams and Doppler widths, at any altitude. The network features seem to diminish at a temperature around 105 K, due to the thermodynamic properties of the transition region. The mean size of the structures of the tail component radiance is smaller than the one of the core radiance. Title: Relation of transition region blinkers to the low chromosphere Authors: Brković, A.; Peter, H. Bibcode: 2003A&A...406..363B Altcode: We investigated the coupling between the low chromosphere, lower and upper transition region through the study of blinkers. Blinkers are extreme ultraviolet (EUV) small-scale transient brightenings best detected in transition region lines showing an intensity enhancement on the 60-100% level on average. We analysed time-series spectra of the \cii 1037 Å (2-4x 104 K, lower transition region) and \oxvi 1038 Å (3x 105 K, upper transition region) lines, as well as the \ci continuum at 1042 Å formed about 1 Mm above the solar surface, obtained with the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) spectrometer on SOHO. Blinkers detected in \cii and \oxvi have similar enhancement levels. We found a modest space-time overlap between blinkers in the two lines and a small correlation between \cii or \oxvi blinkers and the corresponding light curve in the chromospheric continuum. This suggests a relation of EUV blinkers to the chromosphere, which could support the idea that blinkers are the consequence of granular collisions. The average duration of \oxvi blinkers amounts to 10.0 min and of \cii blinkers to 6.7 min. Title: Dynamics of Coronal loops: "Catastrophic Cooling" and High-speed Downflows Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003ANS...324...13M Altcode: 2003ANS...324..B09M No abstract at ADS Title: Dynamics and Properties of Coronal Funnels Authors: Aiouaz, T.; Peter, H.; Lemaire, Philippe; Keppens, Rony Bibcode: 2003ANS...324....7A Altcode: 2003ANS...324..B01A No abstract at ADS Title: Condensation in Cool Coronal Loops and its Effect on Transition Region Lines Authors: Müller, D. A. N.; Hansteen, V. H.; Peter, H. Bibcode: 2003ANS...324..108M Altcode: 2003ANS...324..P14M No abstract at ADS Title: Ion-cyclotron Heating in the Low Corona Authors: Peter, Hardi; Vocks, Christian Bibcode: 2003ANS...324R..16P Altcode: 2003ANS...324..B13P No abstract at ADS Title: Relation of Transition Region Blinkers to the Low Chromosphere Authors: Brković, Alen; Peter, Hardi Bibcode: 2003ANS...324..107B Altcode: 2003ANS...324..P13B No abstract at ADS 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: Explosive events and transition region blinkers: Time variability of non-Gaussian quiet Sun EUV spectra Authors: Peter, H.; Brković, A. Bibcode: 2003A&A...403..287P Altcode: The transition region (TR) from the chromosphere to the corona of the Sun and solar type stars is a very dynamic regime. On the Sun at least two major observational classes of transients can be seen: explosive events and blinkers. Besides these transients there is also a more steady spectral component in transition region lines that accounts for the deviation of the line profile from a (single) Gaussian shape, i.e. for enhanced wings that are well described by a second broad Gaussian, a tail component. The present paper discusses the spectral properties of these features in order to learn more about a possible connection between blinkers, explosive events and tail components. This paper will show that explosive events are most probably not related to the tail components or to transition region blinkers. During a blinker not only the intensity rises, as described by many CDS studies, but also the line width and shift are changing, as we show with SUMER spectra. The variation in line shift and width is closely related to the intensity variation. The line width drops to very small values at the midst of a blinker, almost reaching the thermal line width, while at the same time the Doppler shifts reach a maximum value. From this we may conclude that blinkers are driven by events in the chromosphere heating a loop asymmetrically and powering a strong laminar flow through the loop. Title: Two-component structure of the solar transition region Authors: Gontikakis, C.; Peter, H.; Dara, H. C. Bibcode: 2002ESASP.506..625G Altcode: 2002svco.conf..625G; 2002ESPM...10..625G We studied the morphology of the quiet Sun in the Transition Region, using observations of the SUMER spectrograph in three emission lines (Si II, 1533 Å, C IV, 1548 Å and Ne VIII, 770 Å), emitted respectively in the chromosphere, the low transition region and the low corona, as well as EIT images in the four passbands of the instrument. We computed the mean autocorrelation function for the intensity images in order to estimate the characteristic size of the features present in the Transition Region. Moreover, we deduced autocorrelation functions for the Dopplergrams and line width images. Specifically, using images of the C IV emission lines tail component, as this is estimated by a double Gaussian fit, we studied the morphology of the coronal funnels, as they appear at 105K (formation temperature of the C IV line). We found that the size of the intensity bright features is always larger than the size of the structures of the dopplergrams and dopplerwidths, at all altitudes. The mean size of the structures of the tail component intensity is smaller than the one of the core intensity component. Title: Multi-component transition region structure of the Sun and stars Authors: Peter, H. Bibcode: 2002ASPC..277..291P Altcode: 2002sccx.conf..291P No abstract at ADS Title: Open and closed magnetic structures in the transition region Authors: Peter, Hardi Bibcode: 2002ESASP.505..525P Altcode: 2002solm.conf..525P; 2002IAUCo.188..525P To trace back the origin of the solar wind to the transition region and chromosphere it is of importance to distinguish between magnetically open and closed regions. As it is not yet possible to measure coronal fields with the required accuracy, one has to fall back on interpreting the observed spectra. If heated by the same energy flux a magnetically closed region should be significantly denser than an open region. This is because the solar wind carries away up to 90% of the energy input. Therefore the emission is dominated by closed structures almost everywhere on the Sun. The noticeable difference in density of open and closed regions allows to distinguish open and closed regions by means of spectroscopy. As the non-thermal broadening, e.g. due to waves, is larger in a less dense plasma, one expects two-component spectral profiles in areas where open and closed regions are not resolved spatially: a narrow intense line core from the dense low coronal loops and a broad weaker tail component from coronal funnels. The latter ones might be either feet of large less dense loops or the base of the solar wind. This paper will present results on the spectroscopic deconvolution of magnetically open and closed regions, e.g., wave transport of energy in the coronal funnels. The results will also be related to stellar transition region observations. Title: Hierarchy of chromospheric structures and their relation to the magnetic field Authors: Peter, Hardi Bibcode: 2002ESASP.505..155P Altcode: 2002IAUCo.188..155P; 2002solm.conf..155P While the plasma is dominating the magnetic field in the photosphere, the roles are exchanged in the corona. In a "traditional" one-dimensional picture the change from plasma to magnetically dominated (at plasma β = 1) takes place somewhere in the chromosphere. Therefore the magnetic structure of the chromosphere is one of the keys to understand the relevant processes in the solar atmosphere. The most prominent structure of the chromosphere is the network with strong magnetic fields. The interior of the super-granulation cells are non-magnetic and dominated by shocks producing the well known grains. The oscillations can be heavily influenced by the magnetic structure: e.g. in case of a low canopy the usually prominent 3 minute intensity fluctuations might vanish. The structure of the network, however, is far more complicated. A large number of phenomena, spicules being the most prominent one, are found in the network. While the chromospheric structures in the visible and near UV are now studied for more than 100 years, it was not until recently that also the EUV emission lines and continua from the chromosphere can be studied. Furthermore EUV images in lines formed at coronal temperatures revealed new chromospheric phenomena, like the "moss", and their relation to the "classical" chromosphere is still unclear. This paper will try to review (some of) the important chromospheric structures and discuss their relation to photospheric and coronal processes, especially with respect to the magnetic field. Title: Time variability of coronal funnels Authors: Brković, A.; Peter, H. Bibcode: 2002ESASP.505..215B Altcode: 2002IAUCo.188..215B; 2002solm.conf..215B Transition region spectra from the network are commonly composed by two components: a narrow one from smaller loops and a broad tail component that can be interpreted as originating from coronal funnels. We present a time series analysis of tail components for one network region. A comparison of fitted spectra obtained by applying running means with full time resolution spectra shows that typical explosive events are not responsible for the observed tail components. The correlation between core and tail intensities on time-scales of minutes is not significant. Title: Composition of the solar chromosphere and transition region Authors: Peter, Hardi Bibcode: 2002AdSpR..30...13P Altcode: The composition of the solar plasma is an important tool to study several physical processes, e.g. to trace back the source region of the solar wind. The most prominent effect is the change of elemental abundances from the photosphere to the corona according to the first ionisation potential, the FIP-effect. It is widely accepted that this takes place in the chromosphere, where the particles turn from neutral to ionised. Many models for the element fractionation have been presented so far. Basically they all make use of the separation of ions and neutrals, but still, there is no conclusion as to which of them is the most important physical process leading to this separation. The situation is a bit like with coronal heating: many suggestions, but very limited ability to to prove them right or wrong. Of course, the ideal way to test the models for the composition in the chromosphere and the transition region would be to perform direct observations of that region. But unlike in the corona a reliable inversion of the spectral observations of chromospheric and transition region emission lines is nearly impossible. But probably new thoughts on the structure of the chromosphere and the transition region combined with advanced forward modeling will finally lead to some new insights to this puzzling question. 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: The nature of the solar transition region Authors: Peter, Hardi Bibcode: 2002ESASP.508..237P Altcode: 2002soho...11..237P One of the keys to the understanding of coronal heating is the structure of the thin transition region from the chromosphere to the corona. All the energy that is ultimately heating the corona has to be transported somehow from the photosphere through this thin layer. As the "natural" time-scales are very small in the transition region the analysis of the EUV emission formed between 104 to 106K provides a suitable tool to study effects of the energy transport. As we are far from spatially resolving the transition region, spectroscopy can be used to deconvolve the emission from different structures. If either the line width or the line shift or both signatures differ, one can still study two regions even though they are not spatially resolved. This is especially true if the different structures are on top of each other ("vertically"). One example are large coronal funnels arching over smaller transition region loops. These spectroscopic techniques can also be used to interpret observations of stellar transition regions. This paper will try to give an overview of the various transition region structures and their physical nature, as well as on the required observational techniques. Title: New views of the solar transition region Authors: Peter, Hardi Bibcode: 2002AdSpR..30..501P Altcode: Two decades ago it was discovered that emission lines from the solar transition region from the chromosphere to the corona are composed by two Gaussian components: a narrow core and a broad second component contributing up to 25% to the total intensity. New observations with SUMER/SOHO allow for the first time statistically significant results on the widths and Doppler shifts of the broad second components. Evidence will be presented that the two components of the line profile correspond to two radically different physical regimes. Of course, the results for the dominant line core fit into the well known properties of transition region lines, e.g. the redshifts. In contrast, the second broad components, which occur only in the bright network, show different properties. Most striking is the fact that large parts of the network show blueshifts in the second components, while the core is redshifted at the same location. The results suggest that the second component is formed in Gabriel-type coronal funnels and that these are heated in the same manner as open field regions, i.e. the solar wind. However, the analysis of the Doppler shifts shows that these funnels should not be magnetically open, but rather the footpoint regions of large coronal loops spanning across several network elements. Title: Multi-component structure of the solar transition region Authors: Peter, Hardi Bibcode: 2001ESASP.493..327P Altcode: 2001sefs.work..327P No abstract at ADS Title: On the nature of the transition region from the chromosphere to the corona of the Sun Authors: Peter, H. Bibcode: 2001A&A...374.1108P Altcode: One of the keys to understand coronal heating is to understand the (magnetic) structure of the atmosphere below the respective coronal features. The EUV emission lines formed in the thin transition region from the chromosphere to the corona present a sensitive tool to study the structure and dynamics of the lowermost corona and its connection to the chromosphere. Data from the SUMER spectrograph on SOHO show for the first time that broad components are a common feature of emission line profiles formed from some 40 000 K to 106 K. The contribution of that tail component to the total intensity of the line exhibits a trend with line formation temperature that peaks in the middle transition region with smaller contribution at high and low temperatures. The line width of the tail component shows a monotonic increase with temperature that is consistent with a passing Alfvén wave, which is in contrast to the trend in width of the line core. Together with previous observations this presents evidence that the line core and the tail component are formed in radically different physically regimes. It is proposed that these are small closed loops and coronal funnels, respectively, with the latter being the footpoints of large coronal loops. The new results on the structuring of the transition region will improve understanding on heating the corona and accelerating the solar wind. Title: An Observational Manifestation of Magnetoatmospheric Waves in Internetwork Regions of the Chromosphere and Transition Region Authors: McIntosh, S. W.; Bogdan, T. J.; Cally, P. S.; Carlsson, M.; Hansteen, V. H.; Judge, P. G.; Lites, B. W.; Peter, H.; Rosenthal, C. S.; Tarbell, T. D. Bibcode: 2001ApJ...548L.237M Altcode: We discuss an observational signature of magnetoatmospheric waves in the chromosphere and transition region away from network magnetic fields. We demonstrate that when the observed quantity, line or continuum emission, is formed under high-β conditions, where β is the ratio of the plasma and magnetic pressures, we see fluctuations in intensity and line-of-sight (LOS) Doppler velocity consistent with the passage of the magnetoatmospheric waves. Conversely, if the observations form under low-β conditions, the intensity fluctuation is suppressed, but we retain the LOS Doppler velocity fluctuations. We speculate that mode conversion in the β~1 region is responsible for this change in the observational manifestation of the magnetoatmospheric waves. Title: Coronal Heating and Solar Wind Acceleration: Future Work on Observations Authors: Peter, Hardi Bibcode: 2001SSRv...95..107P Altcode: The space-based observatories SOHO and TRACE have shown some very interesting results on the structure and dynamics of the Sun and its atmosphere, e.g., the extremely high ion temperatures or the enormous variability in the corona. But one question is still open to debate: how to use these data to distinguish between different types of physical heating processes, as, e.g., absorption of high-frequency Alfvén-waves or reconnection events? This paper will discuss some possibilities on how to tackle this type of question. These include observations of ion temperature anisotropies and electron temperatures in the corona as well as measurements of coronal magnetic fields. Emphasis will be put on simultaneous observations of the whole solar atmosphere from the photosphere into the solar wind and on solar-stellar connections. In the light of these ideas new proposed space missions as well as ground based efforts will be discussed. Title: Erratum: Multi-component structure of solar and stellar transition regions Authors: Peter, H. Bibcode: 2000A&A...364..933P Altcode: No abstract at ADS Title: Multi-component structure of solar and stellar transition regions Authors: Peter, H. Bibcode: 2000A&A...360..761P Altcode: Emission lines from the solar transition region between the chromosphere and the corona often show a two Gaussian component profile with a core and a broad second component contributing up to 25% to the total intensity. For the first time a systematic study of the broadening and Doppler shift of the second weaker components is performed using SUMER on SOHO to explore the spatial structures of the second components. It is found that the two component structure is basically restricted to the bright chromospheric network. The narrow core component shows the familiar transition region redshifts, with hardly any blueshifts in the network. The broad second components are blueshifted compared to the core, but are still predominantly redshifted. However, quite large areas in the network (up to 20''× 20'') show concentrations of blueshifts in the second component. In the inter-network the line profile has a single Gaussian shape and shows small red- and also some blueshifts. It is suggested that the two components in the network correspond with two spatially unresolved physical regimes in quiet Sun network: small scale loops and larger scale coronal loop structures anchored in the network. The footpoint regions of the latter are of a funnel-type and form a "canopy" above inter-network regions of the chromosphere. Shocks propagating upward from the non- magnetic chromosphere interact with this canopy, which leads to the transition region inter-network emission. A further analysis, especially of emission lines originating from higher temperatures, is required to confirm this scenario. The distribution and correlations of the line intensities, shifts and widths show that these physical regimes are heated by different mechanisms. This sheds new light on the interpretation of stellar observations in terms of coronal heating. A comparison to existing studies of stellar transition regions shows the need for more thorough theoretical investigations on the formation of stellar transition region lines. Title: On the Doppler Shifts of Solar Ultraviolet Emission Lines Authors: Peter, H.; Judge, P. G. Bibcode: 1999ApJ...522.1148P Altcode: We examine emission-line profiles observed with the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument during the roll of the SOHO spacecraft on 1997 March 20. SUMER data were acquired in selected wavelength bands including lines from the low chromosphere to the corona. Our main aim is to determine the center-to-limb behavior of emission lines formed in the chromosphere, transition region, and corona, especially of the observed Doppler shifts, to try to form a consistent picture of the basic kinematic properties of the emitting plasmas. To achieve this we combine the roll data with data from the full disk discussed elsewhere and fitted Gaussian profiles to the cores of the line profiles. The Doppler-shift data at large spatial scales (>50") clearly reveal center-to-limb redshift behavior consistent with a cosϑ variation in all transition region lines. The three ``coronal'' lines in the data set (of Ne VIII and Mg X) reveal center-to-limb behavior consistent with disk-center blueshifts, in contradiction to some previous work. The redshift to blueshift transition occurs at electron temperatures of about 5×105 K. Furthermore, we present evidence for an outflow of the fast solar wind from the coronal holes throughout the whole transition region. These results confirm and extend earlier work and point toward a (re-) measurement of rest wavelengths of lines formed at coronal temperatures in the laboratory. Together these results provide a firmer observational foundation for the development of classes of models to account for the well-known redshifts and point to the need to develop models that can also account for the coronal-line blueshifts. Title: The Chromosphere in Coronal Holes and the Quiet-Sun Network: an HE I (584 Å) Full-Disk Scan by SUMER/SOHO Authors: Peter, H. Bibcode: 1999ApJ...522L..77P Altcode: A raster scan by Solar Ultraviolet Measurements of Emitted Radiation/Solar and Heliospheric Observatory (SOHO) of the full solar disk containing over 2 million individual spectra of the He I line at 584 Å provides the possibility to study small- and large-scale variations of intensity, line shift, and line width. New striking features are the enhanced line width and the blueshifts in the coronal holes as well as the long-known reduced intensity there. The relation of intensity versus line shift and the variation of the line shift in the coronal holes indicate that the blueshifts in the holes are not consistent with a uniform outflow. The observations suggest that optical depth effects might be responsible for the observed blueshifts and line widths. These results motivate new observations by SOHO and ground-based facilities. Title: Doppler shifts of solar UV emission lines and the source region of the (fast) solar wind Authors: Peter, Hardi Bibcode: 1999AIPC..471..281P Altcode: 1999sowi.conf..281P Full-disk observations with the UV spectrograph SUMER on board SOHO offer the possibility to study emission line profiles on the whole solar disk, beyond what was possible in previous UV studies, e.g. with HRTS or on board Skylab. SUMER data acquired during a SOHO spacecraft roll are complementary to the full-disk data as they cover a wider range of emission lines. In both data sets the center-to-limb variation of the Doppler shifts can be studied, allowing a precise determination of the Doppler shift at disk center. The most interesting result is the opposite center-to-limb variation of the lines formed below and above 5.105 K and the blueshift of the hotter lines found at disk center. This is of importance for the understanding of physical models for the transition region from the chromosphere to the corona. By comparing the quiet Sun and coronal hole regions the outflow velocity of the fast solar wind and the variation of the cross-section of the flow channels of the fast solar wind throughout the transition region can be estimated. Finally by comparing different methods of wavelength calibration one can re-evaluate the rest (or ``laboratory'') wavelengths, which are but poorly known for the hotter lines. Title: Analysis of Transition-Region Emission-Line Profiles from Full-Disk Scans of the Sun Using the SUMER Instrument on SOHO Authors: Peter, H. Bibcode: 1999ApJ...516..490P Altcode: We examine statistical properties of line profiles seen in full-disk observations with the UV spectrograph SUMER on board SOHO. In the SUMER data archive, full-disk data with complete spectral information are available only for wavelength regions including the He I (584 Å), Ne VIII (770 Å), C III (977 Å) and C IV (1548 Å) emission lines. In this paper we will concentrate on C IV and Ne VIII. Collectively these data provide us with the unique opportunity to study the properties of line profiles in the lower and upper solar transition-region beyond what could be achieved with earlier instruments. In particular, these data reveal the center-to-limb behavior of line shifts and line widths for the first time in a statistically meaningful way.

For C IV these data show the well-known redshift of the transition-region lines in the quiet Sun and a clear correlation of the Doppler shift and the nonthermal broadening to the intensity as a characteristic of the network structure. This correlation is not found in the coronal holes. No indications for the network can be found in Ne VIII. For Ne VIII we find a center-to-limb variation of the line shift opposite to C IV, which leads to the conclusion that Ne VIII must be blueshifted at disk center. This also shows the need for a remeasurement of the wavelength of Ne VIII in the laboratory. The center-to-limb variation of the line width leads to the conclusion that the nonresolved motions are not isotropic with a preference for the vertical component. Both lines show a clear signal for an outflow in the polar coronal holes. We discuss the implications of these and other observations for models of the transition-region and corona. The line shift behavior of Ne VIII and other lines from the upper transition-region deserves deeper investigation and probably has pivotal importance in our understanding of the solar transition-region. We are pursuing such work. Title: The chromospheric network and the solar wind outflow Authors: Peter, H. Bibcode: 1999AGAb...15...12P Altcode: 1999AGM....15..A15P It is widely accepted that the quiet solar wind originates from the magnetically dominated chromospheric network, but there is still no definite proof of this concept of co-existing closed magnetic structures (loops) and open funnels. The emission of typical transition region EUV lines, like C IV (1548 Å), is redshifted in the network elements by up to 10 km/s or more. Most models trying to explain these redshifts assume that the emission is concentrated in closed magnetic structures. Two recent studies of SUMER data show that the emission of coronal lines, like Ne VIII (770 Å), is blueshifted in the network. Simple considerations of the energy budget in open and closed regions suggest that the inward heat flux, and by this the density and the emissivity in closed field regions should be at least a factor of 10 higher compared to open field regions. In conclusion the emission in both, transition region and coronal lines, should be dominated by closed field regions. Following these considerations, the line profiles obtained from spectra with good signal-to-noise ratio should show a core, originating from the closed loops, and a second (10%) component that is due to emission from open field regions in the network. This paper will present evidence for this scenario. Analyzing the differences of network and inter-network regions with respect to line shifts and widths of the core and the second component of the emission lines will have impact on the understanding of the structure of the network and the formation of the solar wind. A comparison to spectra of stellar transition regions will shed new light on their use to better understand the structure of stellar coronae. Title: Element fractionation in the solar chromosphere driven by ionization-diffusion processes Authors: Peter, Hardi Bibcode: 1998A&A...335..691P Altcode: An ionization-diffusion mechanism is proposed to understand the first ionization potential (FIP) fractionation as observed in the solar corona and the solar wind. The enrichment of the low-FIP elements (<10 eV) compared to the high-FIP elements in a large variety of phenomena, as e. slow and fast wind or polar plumes, is explained. Also the special behaviour of the heavy noble gases becomes perceptive and the absolute fractionation, i. in relation to hydrogen, can be calculated and fits well to the measurements. Additionally the theoretical velocity-dependence of the fractionation will be used to determine the velocities of the solar wind in the chromosphere. The main achievement of this paper is the explanation of a great variety of fractionation-related phenomena on the basis of one single model. Title: Element Separation in the Chromosphere Ionization-Diffusion Models for the FIP-Effect Authors: Peter, Hardi Bibcode: 1998SSRv...85..253P Altcode: Ionization-diffusion mechanisms to understand the first ionization potential (FIP) fractionation as observed in the solar corona and the solar wind are reviewed. The enrichment of the low-FIP elements (<10 eV) compared to the high-FIP elements, seen in e.g. slow and fast wind or polar plumes, is explained. The behaviour of the heavy noble gases becomes understandable. The absolute fractionation, i.e. in relation to hydrogen, can be calculated and fits well to the measurements. The theoretical velocity-dependence of the fractionation will with used to determine the velocities of the solar wind in the chromosphere. Title: Hydrogen and helium in the solar chromosphere: a background model for fractionation Authors: Peter, Hardi; Marsch, Eckart Bibcode: 1998A&A...333.1069P Altcode: A multi-fluid model for a hydrogen-helium mixture in an ionization-diffusion layer in the (solar) chromosphere is presented. The purpose of this model is to serve as a background for fractionation models calculating the abundance variations of minor species from the photosphere to the solar wind. The emphasis will be on the determination of the (mean) flow velocity in that ionization layer. The equations of continuity and momentum of every component, neutral and (singly) ionized for both elements, will be solved together with an energy equation including heating and radiative losses. Special attention will be paid to the ionization and the elastic collisions as well as to resonant charge exchange. One of the main results is the connection of the particle flux through the chromosphere with the ionization rate, i. with the photon flux in the UV. Furthermore the abundance variations of helium are discussed with the result, that the ion-neutral separation processes leading to the fractionation of the minor elements cannot explain the fractionation of helium as measured in the solar wind. Title: The Structure of the Chromosphere Properties Pertaining to Element Fractionation Authors: Judge, P. G.; Peter, H. Bibcode: 1998SSRv...85..187J Altcode: We review the structure and dynamics of the solar chromosphere with emphasis on the quiet Sun and properties that are relevant to element fractionation mechanisms. Attention is given to the chromospheric magnetic field, its connections to the photosphere, and to the dynamical evolution of the chromosphere. While some profound advances have been made in the “unmagnetized” chromosphere, our knowledge of the magnetically controlled chromosphere, more relevant for the discussion of element fractionation, is limited. Given the dynamic nature of the chromosphere and the poorly understood magnetic linkage to the corona, it is unlikely that we will soon know the detailed processes leading to FIP fractionation. Title: Element Separation in the Chromosphere Authors: Peter, H. Bibcode: 1998sce..conf..253P Altcode: No abstract at ADS Title: The Structure of the Chromosphere Authors: Judge, P. G.; Peter, H. Bibcode: 1998sce..conf..187J Altcode: No abstract at ADS Title: Mehrflüssigkeitsmodelle der unteren Sonnenatmosphäre und Schlußfolgerungen für den Sonnenwind Title: Mehrflüssigkeitsmodelle der unteren Sonnenatmosphäre und Schlußfolgerungen für den Sonnenwind Title: Multi-fluid models of the lower solar atmosphere and conclusions for the solar wind; Authors: Peter, Hardi Bibcode: 1997PhDT.......272P Altcode: No abstract at ADS Title: Ionization Layer of Hydrogen in the Solar Chromosphere and the Solar Wind Mass Flux Authors: Peter, H.; Marsch, E. Bibcode: 1997ESASP.404..591P Altcode: 1997cswn.conf..591P No abstract at ADS Title: Superpenumbral vortex structures. Authors: Peter, H. Bibcode: 1996NAWG.1996..197P Altcode: Spectroheliograms in Hα often show vortex structures in the superpenumbral area of sunspots which covers a region from the outer boundary of the spot to about five spot radii. It is possible to find an analytical solution of the magnetohydrodynamic (MHD) basic equations for a special (one dimensional) case of an axisymmetric horizontal layer around a sunspot. Despite its simplicity, this solution reproduces the main facts of the statistical observations and quantitative measurements. Title: Velocity-dependent fractionation in the solar chromosphere. Authors: Peter, H. Bibcode: 1996A&A...312L..37P Altcode: An explanation of the observed patterns of the first ionisation potential (FIP) fractionation of element abundances in the slow as well as in the fast speed solar wind is proposed. In the former the low-FIP (<10eV) elements are enriched in relation to the high-FIP elements by a factor of 4 compared to the photosphere. In the latter this separation is weaker, or vanishes at all. This velocity dependence can be understood by a simple analysis of atomic and plasma parameters in the solar chromosphere. For this purpose an appropriate layer of constant (hydrogen) density (10^16^m-3) and temperature (10^4^K) is assumed. It is permeated by ionising photons and embedded in a homogeneous vertical magnetic field. Only trace gases in a hydrogen background are investigated. Instead of solving the equations only the asymptotic behaviour is analysed. At the lower boundary a neutral gas mixture and a constant velocity of the neutrals is assumed. Thus there the differential equations for a trace gas are reduced to an algebraic system. This results in the constant total particle flux of the respective minor species. The obtained fractionation depends not only on atomic parameters, i.e. ionisation times and collision frequencies, but also on the plasma parameters, i.e. density, temperature and velocity. Using typical values for the chromosphere a mass flow velocity of about 200m/s leads to the observed fractionation in the slow wind, while 1000m/s correspond to the patterns in the high speed solar wind. Title: Superpenumbral vortices Authors: Peter, Hardi Bibcode: 1996MNRAS.278..821P Altcode: Spectroheliograms, especially in Hα, often show vortex structures in the superpenumbral area of a sunspot, which covers a region from the outer boundary of the spot to about five spot radii. Attempts were made at the beginning of this century to understand the physics of these solar vortices, but pure hydrodynamical or electrodynamical theories failed. The model presented in this paper is based on simplified magnetohydrodynamic equations. The assumptions chosen render an analytical solution practicable, and an obvious explanation for this long-standing problem is obtained. Although this magnetohydrodynamic theory is accomplished in a simple way, it is able to reproduce the main facts of the statistical observations and quantitative measurements. Title: First Steps to a Multi-fluid Model of the Solar Transition Region Authors: Peter, H.; Marsch, E. Bibcode: 1996ApL&C..34...83P Altcode: No abstract at ADS Title: Superpenumbral Vortex Structures Authors: Peter, H. Bibcode: 1996ApL&C..34...77P Altcode: No abstract at ADS Title: Superpenumbral vortex structures and Coriolis force Authors: Peter, H. Bibcode: 1994smf..conf..222P Altcode: No abstract at ADS Title: The yellow amplitude of RS Leporis. Authors: Germann, R.; Kohl, M.; Locher, K.; Peter, H. Bibcode: 1986BBSAG..81....5G Altcode: No abstract at ADS Title: 99th list of minima of eclipsing binaries. Authors: Andrakakou, M.; Boninsegna, R.; Dequinze, R.; Elias, D. P.; Germann, R.; Locher, K.; Louis, P.; Pampaloni, C.; Peter, H.; Stoikidis, N.; Wils, P. Bibcode: 1983BBSAG..66....1A Altcode: No abstract at ADS Title: 97th list of minima of eclipsing binaries. Authors: Diethelm, R.; Elias, D. P.; Germann, R.; Kohl, M.; Locher, K.; Mavrofridis, G.; Mourikis, D.; Nezry, E.; Nikolaou, I.; Pampaloni, C.; Peter, H.; Schildknecht, T.; Stefanopoulos, G.; Stoikidis, N. Bibcode: 1983BBSAG..64....1D Altcode: No abstract at ADS Title: 98th list of minima of eclipsing binaries. Authors: Diethelm, R.; Elias, D. P.; Germann, R.; Keller, T.; Locher, K.; Nezry, E.; Pampaloni, C.; Parris, A.; Peter, H.; Schildknecht, T.; Stefanopoulos, G.; Stoikidis, N.; Wils, P. Bibcode: 1983BBSAG..65....1D Altcode: No abstract at ADS Title: 94th - 96th list of minima of eclipsing binaries. Authors: Boistel, G.; Diethelm, R.; Elias, D. P.; Germann, R.; Locher, K.; Mavrofridis, G.; Mourikis, D.; Ralincourt, P.; Peter, H.; Schildknecht, T.; Stefanopoulos, G.; Stoikidis, N.; Boninsegna, R.; Kohl, M.; Le Borgne, J. -F.; Leyman, R.; Louis, P.; Andrakakou, M. Bibcode: 1982BBSAG..61....1B Altcode: No abstract at ADS Title: 91st - 93rd list of minima of eclipsing binaries. Authors: Amsler, S.; Andrakakou, M.; Boninsegna, R.; Cadalbert, R.; Delhaye, D.; Dokic, P.; Elias, D. P.; Germann, R.; Grüebler, T.; Häfliger, M.; Horowitz, L.; Kukan, J.; Le Borgne, J. -F.; Leyman, D.; Leyman, R.; Locher, K.; Mammoliti, S.; Mavrofridis, G.; Mouillard, C.; Mouillard, G.; Mourikis, D.; Nikolaou, I.; Peter, H.; Staub, B.; Stucky, A.; Stefanopoulos, G.; Stoikidis, N.; Zwing, W.; Capol, L.; Diethelm, R.; Häring, R.; Hunn, D.; Kägi, J.; Kocian, K.; Kohl, M.; Maranta, C.; Biedermann, B.; Ferrand, S.; Hasler, N.; Kaiser, A.; Liégeois, J. -P.; Louis, P.; Schildknecht, T. Bibcode: 1982BBSAG..58....1A Altcode: No abstract at ADS Title: Jost Bürgi und seine Himmelsgloben. Authors: Peter, H. Bibcode: 1981Orion..39...40P Altcode: No abstract at ADS Title: 89th - 90th list of minima of eclipsing binaries. Authors: Andrakakou, M.; Boistel, G.; Boninsegna, R.; Diethelm, R.; Elias, D. P.; Germann, R.; Leyman, R.; Locher, K.; Matagne, P.; Mourikis, D.; Nezry, E.; Nikolaou, I.; Parris, A.; Peter, H.; Poretti, E.; Ralincourt, P.; Vialle, J.; Capol, L.; Contopoulos, N.; Maranta, C.; Mavrofridis, G.; Pampaloni, C.; Stoikidis, N.; Zwing, W. Bibcode: 1981BBSAG..56....1A Altcode: No abstract at ADS Title: 85th - 88th list of minima of eclipsing binaries. Authors: Boistel, G.; Boninsegna, R.; Diethelm, R.; Germann, R.; Locher, K.; Mallama, A. D.; Mavrofridis, G.; Peter, H.; Stefanopoulos, G.; Stoikidis, N.; Andrakakou, M.; Andreolo, L.; Elias, D. P.; Ficola, L.; Franchini, M.; Mugnaini, P.; Pampaloni, C.; Parris, A.; Poretti, E.; Mourikis, D.; Nikolaou, I. Bibcode: 1981BBSAG..52....1B Altcode: No abstract at ADS Title: 79th - 81st list of minima of eclipsing binaries. Authors: Agnesoni, C.; Andrakakou, M.; Boistel, G.; Buzzoni, A.; Diethelm, R.; Germann, R.; Locher, K.; Mourikis, D.; Pampaloni, C.; Peter, H.; Poretti, E.; Ralincourt, P.; Troispoux, G.; Chiantini, L.; Ficola, L.; Mugnaini, P.; Antoniadou, T.; Boninsegna, R.; Elias, D. Bibcode: 1980BBSAG..46....1A Altcode: No abstract at ADS Title: 82nd - 84th list of minima of eclipsing binaries. Authors: Andrakakou, M.; Diethelm, R.; Elias, D. P.; Germann, R.; Leyman, R.; Locher, K.; Mavrofridis, G.; Mourikis, D.; Parris, A.; Peter, H.; Stoikidis, N.; Franchini, M.; Nikolaou, I.; Pampaloni, C.; Boistel, G.; Buzzoni, A.; Leyman, D.; Nezry, E.; Ralincourt, P.; Troispoux, G. Bibcode: 1980BBSAG..49....1A Altcode: No abstract at ADS Title: 77th - 78th list of minima of eclipsing binaries. Authors: Andrakakou, M.; Buzzoni, A.; Diethelm, R.; Germann, R.; Locher, K.; Marot, A.; Mavrofridis, G.; Mourikis, D.; Del Parigi, A.; Parris, A.; Peter, H.; Poretti, E.; Ralincourt, P.; Stoikidis, N.; Troispoux, G.; Franchini, M.; Ferraro, F.; Le Borgne, J. -F.; Leyman, R.; Mengoli, G.; Pampaloni, C.; Plasmati, C.; Remis, J.; Stefanopoulos, G. Bibcode: 1979BBSAG..44....1A Altcode: No abstract at ADS Title: 74th - 76th list of minima of eclipsing binaries. Authors: Agnesoni, C.; Berquet, R.; Boninsegna, R.; Buzzoni, A.; Clovin, J. -P.; Cutispoto, G.; Danthine, P.; Diethelm, R.; Germann, R.; Leyman, R.; Locher, K.; van Loo, E.; Pampaloni, C.; Peter, H.; Poretti, E.; Ralincourt, P.; Stefanopoulos, G.; Troispoux, G.; Zaccaria, N.; Andrakakou, M.; Chiantini, L.; Dimou, G.; Mourikis, D.; Parris, A.; Stoikidis, N.; Franchini, M.; Nezry, E. Bibcode: 1979BBSAG..41....1A Altcode: No abstract at ADS Title: 71st - 73rd of minima of eclipsing binaries. Authors: Boninsegna, R.; Buzzoni, A.; Carradori, T.; Clovin, J. -P.; Diethelm, R.; Dore, R.; Germann, R.; Le Borgne, J. -F.; Locher, K.; Lucentini, E.; Nezry, E.; Pampaloni, C.; Del Parigi, A.; Penna, M.; Peter, H.; Poretti, E.; Ralincourt, P.; Troispoux, G.; Andrakakou, M.; Ferraro, F.; Franchini, M.; Mourikis, D.; Plasmati, C.; Royer, A.; Agnesoni, C.; Mavrofridis, G.; Stefanopoulos, G.; Zaccaria, N. Bibcode: 1978BBSAG..38....1B Altcode: No abstract at ADS Title: V342 Aquilae: probable recent shallowing of the minimum I. Authors: Diethelm, R.; Locher, K.; Peter, H. Bibcode: 1978BBSAG..38....7D Altcode: No abstract at ADS Title: 69th and 70th list of minima of eclipsing binaries. Authors: Agnesoni, C.; Albert, P.; Benucci, M.; Boninsegna, R.; Bourgeois, J.; Buzzoni, A.; Carradori, T.; Clovin, J. -P.; Danthine, P.; Diethelm, R.; Duquesne, J. -L.; Ferraro, F.; Franchini, M.; Frangeul, M.; Frère, A.; Germann, R.; Hevesi, Z.; Leyman, R.; Locher, K.; Lucentini, E.; Nezry, E.; Pampaloni, C.; Del Parigi, A.; Peter, H.; Piazza, R.; Plasmati, C.; Poretti, E.; Ralincourt, P.; Remis, J.; Royer, A.; Troispoux, G.; Vespe, F.; Zaccaria, N.; Livi, A.; Penna, M. Bibcode: 1978BBSAG..36....1A Altcode: No abstract at ADS Title: 67th, 68th list of minima of eclipsing binaries. Authors: Albert, P.; Boninsegna, R.; Bourgeois, J.; Clovin, J. -P.; Danthine, P.; Diethelm, R.; Frangeul, M.; Germann, R.; Lardinois, V.; Le Borgne, J. -F.; Le Strat, P.; Leydon, R.; Locher, K.; Marot, A.; Mons, P.; Penna, M.; Royer, A.; Remis, J.; Troispoux, G.; Wabniz, S.; Benucci, M.; Bouzin, B.; Buzzoni, A.; Duquesne, J. -L.; Ferrara, F.; de Francesco, M.; Hevesi, Z.; Livi, A.; Le Jehan, S.; Leyman, R.; Maniet, T.; Nezry, E.; Del Parigi, A.; Pampaloni, C.; Plasmati, C.; Peter, H.; Poretti, E.; Ralincourt, P.; Travaglino, F.; Tuboly, V.; Vespe, F.; Zaccaria, N. Bibcode: 1977BBSAG..34....1A Altcode: No abstract at ADS Title: Deceptive observations of UU Canis Maioris. Authors: Locher, K.; Peter, H. Bibcode: 1977BBSAG..32....5L Altcode: No abstract at ADS Title: 62nd - 66th list of minima of eclipsing binaries. Authors: Boninsegna, R.; Clovin, J. -P.; Diethelm, R.; Doby, P.; Frangeul, M.; Germann, R.; Hevesi, Z.; Locher, K.; Marot, A.; Peter, H.; Poretti, E.; Remis, J.; Troispoux, G.; Behagle, M.; Le Borgne, J. -F.; Ralincourt, P.; Royer, A.; Seretti, A.; Squelard, J.; Zaccaria, N.; Travaglino, F.; Tuboly, V.; Berrilli, F.; Boninsegna, V.; Bouzin, B.; Cayla, P.; Le Saout, M.; Leydon, R.; Wabniz, S.; Buzzoni, A.; Kratochwill, R.; Lucentini, E.; Del Parigi, A.; Plasmati, C. Bibcode: 1976BBSAG..29....1B Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Diethelm, R.; Figer, A.; Germann, R.; Locher, K.; Peter, H.; Royer, A.; Tuboly, V.; Zajàcz, G.; Behagle, M.; Boninsegna, R.; Bourgeois, J.; Carnevali, P.; Clovin, J. -P.; Doby, P.; Fenyvesi, A.; Frangeul, M.; Le Borgne, J. -F.; Leydon, R.; Marot, A.; Poretti, E.; Ralincourt, P.; Remis, J.; Seretti, A.; Wilmet, M.; Troispoux, G.; Aresi, P.; Le Saout, M. Bibcode: 1976BBSAG..25....1D Altcode: No abstract at ADS Title: The totality duration of TY Lib. Authors: Peter, H.; Locher, K. Bibcode: 1975BBSAG..22....5P Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Carnevali, P.; Diethelm, R.; Figer, A.; Germann, R.; Le Borgne, J. -F.; Locher, K.; Mauron, N.; Marot, A.; Peter, H.; Ralincourt, P.; Remis, J.; Sanchez, C.; Domec, C.; Hevesi, Z.; Rolland, R.; Roudier, T.; Behagle, M.; Royer, A.; Dumarchi, G. Bibcode: 1975BBSAG..19....1C Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Bonneville, T.; Chetanneau, A.; Desprez, F.; Diethelm, R.; Dumarchi, G.; Fries, A.; Germann, R.; Le Borgne, J. -F.; Leydon, R.; Locher, K.; Marot, A.; Mauron, N.; Peter, H.; Ralincourt, P.; Remis, J.; Rolland, R.; Romoli, C.; Royer, A.; Sieber, W.; Steimer, P.; Baumann, M.; Boninsegna, R.; Bourgeois, J.; Carnevali, P.; Clovin, J. -P.; Kissling, M.; Maurin, L.; Troispoux, G.; Wüthrich, T.; Zajàcz, G. Bibcode: 1975BBSAG..23....1B Altcode: No abstract at ADS Title: 1. Südwestdeutsche Regionaltagung der VdS in Karlsruhe. Authors: Peter, H. Bibcode: 1974Orion..32..232P Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Diethelm, R.; Figer, A.; Germann, R.; Locher, K.; Peter, H.; Remis, J.; Rolland, R.; Zajacz, G.; Carnevali, P.; Mauron, N.; Ralincourt, P.; Romoli, C.; Hevesi, Z. Bibcode: 1974BBSAG..16....1D Altcode: No abstract at ADS Title: List of minima of eclipsing binaries. Authors: Diethelm, R.; Germann, R.; Locher, K.; Peter, H.; Wittwer, H.; Forster, S.; Morger, P. Bibcode: 1974BBSAG..13....1D Altcode: No abstract at ADS Title: The totality duration of TU Her. Authors: Peter, H.; Locher, K. Bibcode: 1973BBSAG...9....3P Altcode: No abstract at ADS Title: List of minima of eclipsing binaries. Authors: Diethelm, R.; Germann, R.; Gliba, G.; Locher, K.; Mallama, A.; Morger, P.; Peter, H.; Aeberli, R.; Boss, U.; Forbes, S.; Heer, A.; Jucker, A.; Jucker, B.; Kobelt, V.; Senn, M.; Staub, J. Bibcode: 1973BBSAG..10....1D Altcode: No abstract at ADS Title: The totality duration duration of TZ Eri. Authors: Peter, H.; Locher, K. Bibcode: 1973BBSAG..11....6P Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Diethelm, R.; Germann, R.; Locher, K.; Mallama, T.; Meier, R.; Morger, P.; Peter, H. Bibcode: 1973BBSAG...7....1D Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Diethelm, R.; Germann, R.; Giger, M.; Locher, K.; Peter, H.; Schäpper, F. Bibcode: 1972BBSAG...5....1D Altcode: No abstract at ADS Title: Lists of minima of eclipsing binaries. Authors: Diethelm, R.; Germann, R.; Locher, K.; Peter, H. Bibcode: 1972BBSAG...1....1D Altcode: No abstract at ADS