Author name code: socas-navarro ADS astronomy entries on 2022-09-14 author:"Socas Navarro, Hector" ------------------------------------------------------------------------ Title: Searching for technosignatures in exoplanetary systems with current and future missions Authors: Haqq-Misra, Jacob; Schwieterman, Edward W.; Socas-Navarro, Hector; Kopparapu, Ravi; Angerhausen, Daniel; Beatty, Thomas G.; Berdyugina, Svetlana; Felton, Ryan; Sharma, Siddhant; De la Torre, Gabriel G.; Apai, Dániel; TechnoClimes 2020 workshop participants Bibcode: 2022AcAau.198..194H Altcode: 2022arXiv220600030H Technosignatures refer to observational manifestations of technology that could be detected through astronomical means. Most previous searches for technosignatures have focused on searches for radio signals, but many current and future observing facilities could also constrain the prevalence of some non-radio technosignatures. This search could thus benefit from broader participation by the astronomical community, as contributions to technosignature science can also take the form of negative results that provide statistically meaningful quantitative upper limits on the presence of a signal. This paper provides a synthesis of the recommendations of the 2020 TechnoClimes workshop, which was an online event intended to develop a research agenda to prioritize and guide future theoretical and observational studies technosignatures. The paper provides a high-level overview of the use of current and future missions to detect exoplanetary technosignatures at ultraviolet, optical, or infrared wavelengths, which specifically focuses on the detectability of atmospheric technosignatures, artificial surface modifications, optical beacons, space engineering and megastructures, and interstellar flight. This overview does not derive any new quantitative detection limits but is intended to provide additional science justification for the use of current and planned observing facilities as well as to inspire astronomers conducting such observations to consider the relevance of their ongoing observations to technosignature science. This synthesis also identifies possible technology gaps with the ability of current and planned missions to search for technosignatures, which suggests the need to consider technosignature science cases in the design of future mission concepts. Title: The search for transit technosignatures Authors: Socas-Navarro, Hector Bibcode: 2022cosp...44.2814S Altcode: As a byproduct of the exoplanet revolution, we can now search for technosignatures that would potentially leave an imprint on a planet's transit light-curve data. This is an exciting new window that has opened in recent years. We can now examine the space environment around a planet in search for artificial megastructures or large collections artifacts, such as satellite belts. This talk summarizes some possible technosignatures that might be within the reach of current or upcoming instrumentation, with emphasis on photometric observations. The axes of merit or the ichnoscale diagram may be used to provide a quantitative framework in which one can arrange technosignatures. A more detailed discussion will be presented on the search for geosynchronous artifacts, such as Clarke exobelts. These have the appealing peculiarity of being "Occam-friendly", as well as a natural extrapolation of current human technology and having typical lifetime scales of at least millions of years. Title: Further support and a candidate location for Planet 9 Authors: Socas-Navarro, Hector Bibcode: 2022arXiv220507675S Altcode: The existence of a hypothetical Planet 9 lurking in the outer solar system has been invoked as a plausible explanation for the the anomalous clustering in the orbits of some trans-Neptunian objects. Here we propose to use meteors arriving at Earth as messengers with the potential of revealing the presence of a hitherto undiscovered massive object. The peculiar meteor CNEOS 2014-01-08, recently put forward as the first interstellar meteor, might be one such messenger. Its origin in the sky overlaps with the predicted band of possible Planet 9 orbits and it is actually compatible with the highest probability region. The odds of this coincidence being due to chance are of ~0.5\%. Furthermore, some statistical anomalies about CNEOS 2014-01-08 are resolved under the hypothesis that it was flung at Earth by Planet 9. Based on the available data, we propose the region at coordinates RA:50.0{\pm}4°, dec:11.8{\pm}1.8° in the constellation of Aries, as the first candidate location for Planet 9. Title: Limitations of the Ca II 8542 Å Line for the Determination of Magnetic Field Oscillations Authors: Felipe, Tobias; Socas Navarro, Hector; Sangeetha, C. R.; Milic, Ivan Bibcode: 2021ApJ...918...47F Altcode: 2021arXiv210702160F Chromospheric umbral oscillations produce periodic brightenings in the core of some spectral lines, known as umbral flashes. They are also accompanied by fluctuations in velocity, temperature, and, according to several recent works, magnetic field. In this study, we aim to ascertain the accuracy of the magnetic field determined from inversions of the Ca II 8542 Å line. We have developed numerical simulations of wave propagation in a sunspot umbra. Synthetic Stokes profiles emerging from the simulated atmosphere were computed and then inverted using the NICOLE code. The atmospheres inferred from the inversions have been compared with the original parameters from the simulations. Our results show that the inferred chromospheric fluctuations in velocity and temperature match the known oscillations from the numerical simulation. In contrast, the vertical magnetic field obtained from the inversions exhibits an oscillatory pattern with a ~300 G peak-to-peak amplitude, which is absent in the simulation. We have assessed the error in the inferred parameters by performing numerous inversions with slightly different configurations of the same Stokes profiles. We find that when the atmosphere is approximately at rest, the inversion tends to favor solutions that underestimate the vertical magnetic field strength. On the contrary, during umbral flashes, the values inferred from most of the inversions are concentrated at stronger fields than those from the simulation. Our analysis provides a quantification of the errors associated with the inversions of the Ca II 8542 Å line and suggests caution with the interpretation of the inferred magnetic field fluctuations. Title: Exploring the Sun's upper atmosphere with neural networks: Reversed patterns and the hot wall effect Authors: Socas-Navarro, H.; Asensio Ramos, A. Bibcode: 2021A&A...652A..78S Altcode: We have developed an inversion procedure designed for high-resolution solar spectro-polarimeters, such as those of Hinode and the DKIST. The procedure is based on artificial neural networks trained with profiles generated from random atmospheric stratifications for a high generalization capability. When applied to Hinode data, we find a hot fine-scale network structure whose morphology changes with height. In the middle layers, this network resembles what is observed in G-band filtergrams, but it is not identical. Surprisingly, the temperature enhancements in the middle and upper photosphere have a reversed pattern. Hot pixels in the middle photosphere, possibly associated with small-scale magnetic elements, appear cool at the log τ500 = −3 and −4 level, and vice versa. Finally, we find hot arcs on the limb side of magnetic pores. We interpret them as the first piece of direct observational evidence of the "hot wall" effect, which is a prediction of theoretical models from the 1970's. Title: Concepts for future missions to search for technosignatures Authors: Socas-Navarro, Hector; Haqq-Misra, Jacob; Wright, Jason T.; Kopparapu, Ravi; Benford, James; Davis, Ross; TechnoClimes 2020 workshop participants Bibcode: 2021AcAau.182..446S Altcode: 2021arXiv210301536S New and unique opportunities now exist to look for technosignatures (TS) beyond traditional SETI radio searches, motivated by tremendous advances in exoplanet science and observing capabilities in recent years. Space agencies, both public and private, may be particularly interested in learning about the community's views as to the optimal methods for future TS searches with current or forthcoming technology. This report is an effort in that direction. We put forward a set of possible mission concepts designed to search for TS, although the data supplied by such missions would also benefit other areas of astrophysics. We introduce a novel framework to analyze a broad diversity of TS in a quantitative manner. This framework is based on the concept of ichnoscale, which is a new parameter related to the scale of a TS cosmic footprint, together with the number of potential targets where such TS can be searched for, and whether or not it is continuous in time. Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST) Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio, Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart; Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa, Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler, Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun, Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres, Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.; Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini, Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena; Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor; Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael; Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli, Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys, Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.; Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis, Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson, Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.; Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.; Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava, Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas, Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST Instrument Scientists; DKIST Science Working Group; DKIST Critical Science Plan Community Bibcode: 2021SoPh..296...70R Altcode: 2020arXiv200808203R The National Science Foundation's Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute. Title: Long-term evolution of three light bridges developed on the same sunspot Authors: Griñón-Marín, A. B.; Pastor Yabar, A.; Centeno, R.; Socas-Navarro, H. Bibcode: 2021A&A...647A.148G Altcode: 2021arXiv210204392B; 2021arXiv210204392G One important feature of sunspots is the presence of light bridges. These structures are elongated and bright (as compared to the umbra) features that seem to be related to the formation and evolution of sunspots. In this work, we studied the long-term evolution and the stratification of different atmospheric parameters of three light bridges formed in the same host sunspot by different mechanisms. To accomplish this, we used data taken with the GREGOR Infrared Spectrograph installed at the GREGOR telescope. These data were inverted to infer the physical parameters of the atmosphere where the observed spectral profiles were formed of the three light bridges. We find that, in general, the behaviour of the three light bridges is typical of this kind of structure with the magnetic field strength, inclination, and temperature values between the values at the umbra and the penumbra. We also find that they are of a significantly non-magnetic character (particularly at the axis of the light bridges) as it is deduced from the filling factor. In addition, within the common behaviour of the physical properties of light bridges, we observe that each one exhibits a particular behaviour. Another interesting result is that the light bridge cools down, the magnetic field decreases, and the magnetic field lines get more inclined higher in the atmosphere. Finally, we studied the magnetic and non-magnetic line-of-sight velocities of the light bridges. The former shows that the magnetic component is at rest and, interestingly, its variation with optical depth shows a bi-modal behaviour. For the line-of-sight velocity of the non-magnetic component, we see that the core of the light bridge is at rest or with shallow upflows and clear downflows sinking through the edges. Title: Semi-empirical Models of Spicule from Inversion of Ca II 8542 Å Line Authors: Kuridze, David; Socas-Navarro, Hector; Koza, Július; Oliver, Ramon Bibcode: 2021ApJ...908..168K Altcode: 2020arXiv201203702K We study a solar spicule observed off-limb using high-resolution imaging spectroscopy in the Ca II 8542 Å line obtained with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope. Using a new version of the non-LTE code NICOLE specifically developed for this problem we invert the spicule single- and double-peak line profiles. This new version considers off-limb geometry and computes atomic populations by solving the 1D radiative transfer assuming a vertical stratification. The inversion proceeds by fitting the observed spectral profiles at 14 different heights with synthetic profiles computed in the model by solving the radiative transfer problem along its length. Motivated by the appearance of double-peak Ca II 8542 Å spicule profiles, which exhibit two distinct emission features well separated in wavelength, we adopt a double-component scenario. We start from the ansatz that the spicule parameters are practically constant along the spicule axis for each component, except for a density drop. Our results support this ansatz by attaining very good fits to the entire set of 14 × 4 profiles (14 heights and 4 times). We show that the double-component model with uniform temperature of 9560 K, exponential decrease of density with a height scale of 1000-2000 km, and the counter-oriented line-of-sight velocities of components reproduce the double-peak line profiles at all spicule segments well. Analyses of the numerical response function reveals the necessity of the inversions of spectra at multiple height positions to obtain height-dependent, degeneracy-free reliable models with a limited number of free parameters. Title: Mapping the Sun's upper photosphere with artificial neural networks Authors: Socas-Navarro, Hector; Asensio Ramos, Andres Bibcode: 2021arXiv210111445S Altcode: We have developed an inversion procedure designed for high-resolution solar spectro-polarimeters, such as Hinode/SP or DKIST/ViSP. The procedure is based on artificial neural networks trained with profiles generated from random atmospheric stratifications for a high generalization capability. When applied to Hinode data we find a hot fine-scale network structure whose morphology changes with height. In the middle layers this network resembles what is observed in G-band filtergrams but it is not identical. Surprisingly, the temperature enhancements in the middle and upper photosphere have a reversed pattern. Hot pixels in the middle photosphere, possibly associated to small-scale magnetic elements, appear cool at the log(tau_500)=-3 and -4 level, and viceversa. Finally, we find hot arcs on the limb side of magnetic pores, which we interpret as the first direct observational evidence of the "hot wall" effect in temperature. Title: Downflowing umbral flashes as evidence of standing waves in sunspot umbrae Authors: Felipe, T.; Henriques, V. M. J.; de la Cruz Rodríguez, J.; Socas-Navarro, H. Bibcode: 2021A&A...645L..12F Altcode: 2021arXiv210104188F Context. Umbral flashes are sudden brightenings commonly visible in the core of some chromospheric lines. Theoretical and numerical modeling suggests that they are produced by the propagation of shock waves. According to these models and early observations, umbral flashes are associated with upflows. However, recent studies have reported umbral flashes in downflowing atmospheres.
Aims: We aim to understand the origin of downflowing umbral flashes. We explore how the existence of standing waves in the umbral chromosphere impacts the generation of flashed profiles.
Methods: We performed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized with the NICOLE code.
Results: For freely propagating waves, the chromospheric temperature enhancements of the oscillations are in phase with velocity upflows. In this case, the intensity core of the Ca II 8542 Å atmosphere is heated during the upflowing stage of the oscillation. However, a different scenario with a resonant cavity produced by the sharp temperature gradient of the transition region leads to chromospheric standing oscillations. In this situation, temperature fluctuations are shifted backward and temperature enhancements partially coincide with the downflowing stage of the oscillation. In umbral flash events produced by standing oscillations, the reversal of the emission feature is produced when the oscillation is downflowing. The chromospheric temperature keeps increasing while the atmosphere is changing from a downflow to an upflow. During the appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing most of the time, and only 38% of the flashed profiles are associated with downflows.
Conclusions: We find a scenario that remarkably explains the recent empirical findings of downflowing umbral flashes as a natural consequence of the presence of standing oscillations above sunspot umbrae. Title: Spatially resolved measurements of the solar photospheric oxygen abundance Authors: Cubas Armas, M.; Asensio Ramos, A.; Socas-Navarro, H. Bibcode: 2020A&A...643A.142C Altcode: 2020arXiv201002151C
Aims: We report the results of a novel determination of the solar oxygen abundance using spatially resolved observations and inversions. We seek to derive the photospheric solar oxygen abundance with a method that is robust against uncertainties in the model atmosphere.
Methods: We use observations with spatial resolution obtained at the Vacuum Tower Telescope to derive the oxygen abundance at 40 different spatial positions in granules and intergranular lanes. We first obtain a model for each location by inverting the Fe I lines with the NICOLE inversion code. These models are then integrated into a hierarchical Bayesian model that is used to infer the most probable value for the oxygen abundance that is compatible with all the observations. The abundance is derived from the [O I] forbidden line at 6300 Å taking into consideration all possible nuisance parameters that can affect the abundance.
Results: Our results show good agreement in the inferred oxygen abundance for all the pixels analyzed, demonstrating the robustness of the analysis against possible systematic errors in the model. We find a slightly higher oxygen abundance in granules than in intergranular lanes when treated separately (log(ɛO) = 8.83 ± 0.02 vs. log(ɛO) = 8.76 ± 0.02), which is a difference of approximately 2-σ. This tension suggests that some systematic errors in the model or the radiative transfer still exist but are small. When taking all pixels together, we obtain an oxygen abundance of log(ɛO) = 8.80 ± 0.03, which is compatible with both granules and lanes within 1-σ. The spread of results is due to both systematic and random errors. Title: The penumbral solar filaments from the photosphere to the chromosphere Authors: Murabito, M.; Ermolli, I.; Giorgi, F.; Stangalini, M.; Guglielmino, S. L.; Jafarzadeh, S.; Socas-Navarro, H.; Romano, P.; Zuccarello, F. Bibcode: 2020JPhCS1548a2017M Altcode: The magnetic field structure of sunspots above the photosphere remain poorly understood due to limitations in observations and the complexity of these atmospheric layers. In this regard, we studied the large isolated sunspot (70”× 80”) located in the active region NOAA 12546 with spectro-polarimetric measurements acquired along the Fe I 617.3 nm and Ca II 854.2 nm lines with the IBIS/DST instrument, under excellent seeing conditions lasting more than three hours. Using the Non Local Thermodynamic Equilibrium inversion code we inverted both line measurements simultaneously to retrieve the three-dimensional magnetic and thermal structure of the penumbral region from the bottom of the photosphere to the middle chromosphere. The analysis of data acquired at spectral ranges unexplored allow us to show clear evidence of the spine and intra-spine structure of the magnetic field at chromospheric heights. In particular, we found a peak-to-peak variations of the magnetic field strength and inclination of about 200 G and 10° chromospheric heights, respectively, and of about 300 G and 20° in the photosphere. We also investigated the structure of the magnetic field gradient in the penumbra along the vertical and azimuthal directions, confirming previous results reported in the literature from data taken at the spectral region of the He I 1083 nm triplet. Title: Discovery of long-period magnetic field oscillations and motions in isolated sunspots Authors: Griñón-Marín, A. B.; Pastor Yabar, A.; Socas-Navarro, H.; Centeno, R. Bibcode: 2020A&A...635A..64G Altcode: 2020arXiv200106030G We analyse the temporal evolution of the inclination component of the magnetic field vector for the penumbral area of 25 isolated sunspots. Compared to previous works, the use of data from the HMI instrument aboard the SDO observatory facilitates the study of a very long time series (≈1 week) with a good spatial and temporal resolution. We used the wavelet technique and we found some filamentary-shaped events with large wavelet power. Their distribution of periods is broad, ranging from the lower limit for this study of 48 min up to 63 h. An interesting property of these events is that they do not appear homogeneously all around the penumbra but they seem to concentrate at particular locations. The cross-comparison of these wavelet maps with AIA data shows that the regions where these events appear are visually related to the coronal loops that connect the outer penumbra to one or more neighbouring opposite polarity flux patches. Title: The 3D structure of the penumbra at high resolution from the bottom of the photosphere to the middle chromosphere Authors: Murabito, Mariarita; Ermolli, Ilaria; Giorgi, Fabrizio; Stangalini, Marco; Guglielmino, Salvo L.; Jafarzadeh, Shahin; Socas-Navarro, Hector; Romano, Paolo; Zuccarello, Francesca Bibcode: 2020IAUS..354..448M Altcode: Sunspots are the most prominent feature of the solar magnetism in the photosphere. Although they have been widely investigated in the past, their structure remains poorly understood. Indeed, due to limitations in observations and the complexity of the magnetic field estimation at chromospheric heights, the magnetic field structure of sunspot above the photosphere is still uncertain. Improving the present knowledge of sunspot is important in solar and stellar physics, since spot generation is seen not only on the Sun, but also on other solar-type stars. In this regard, we studied a large, isolated sunspot with spectro-polarimeteric measurements that were acquired at the Fe I 6173 nm and Ca II 8542 nm lines by the spectropolarimeter IBIS/DST under excellent seeing conditions lasting more than three hours. Using the Non-LTE inversion code NICOLE, we inverted both line measurements simultaneously, to retrieve the three-dimensional magnetic and thermal structure of the penumbral region from the bottom of the photosphere to the middle chromosphere. Our analysis of data acquired at spectral ranges unexplored in previous studies shows clear spine and intra-spine structure of the penumbral magnetic field at chromopheric heights. Our investigation of the magnetic field gradient in the penumbra along the vertical and azimuthal directions confirms results reported in the literature from analysis of data taken at the spectral region of the He I 1083 nm triplet. Title: High-resolution spectroscopy of Boyajian's star during optical dimming events Authors: Martínez González, M. J.; González-Fernández, C.; Asensio Ramos, A.; Socas-Navarro, H.; Westendorp Plaza, C.; Boyajian, T. S.; Wright, J. T.; Collier Cameron, A.; González Hernández, J. I.; Holgado, G.; Kennedy, G. M.; Masseron, T.; Molinari, E.; Saario, J.; Simón-Díaz, S.; Toledo-Padrón, B. Bibcode: 2019MNRAS.486..236M Altcode: 2019MNRAS.tmp..828M; 2018arXiv181206837M Boyajian's star is an apparently normal main-sequence F-type star with a very unusual light curve. The dipping activity of the star, discovered during the Kepler mission, presents deep, asymmetric, and aperiodic events. Here we present high-resolution spectroscopic follow-up during some dimming events recorded post-Kepler observations, from ground-based telescopes. We analyse data from the HERMES, HARPS-N, and FIES spectrographs to characterize the stellar atmosphere and to put some constraints on the hypotheses that have appeared in the literature concerning the occulting elements. The star's magnetism, if existing, is not extreme. The spots on the surface, if present, would occupy 0.02 per cent of the area, at most. The chromosphere, irrespective of the epoch of observation, is hotter than the values expected from radiative equilibrium, meaning that the star has some degree of activity. We find no clear evidence of the interstellar medium or exocoments being responsible for the dimmings of the light curve. However, we detect at 1-2σ level, a decrease of the radial velocity of the star during the first dip recorded after the Kepler observations. We claim the presence of an optically thick object with likely inclined and high impact parameter orbits that produces the observed Rossiter-McLaughlin effect. Title: Can a negative-mass cosmology explain dark matter and dark energy? Authors: Socas-Navarro, H. Bibcode: 2019A&A...626A...5S Altcode: 2019arXiv190208287S A recent study by Farnes (2018, A&A, 620, A92) proposed an alternative cosmological model in which both dark matter and dark energy are replaced with a single fluid of negative mass. This paper presents a critical review of that model. A number of problems and discrepancies with observations are identified. For instance, the predicted shape and density of galactic dark matter halos are incorrect. Also, halos would need to be less massive than the baryonic component, otherwise they would become gravitationally unstable. Perhaps the most challenging problem in this theory is the presence of a large-scale version of the "runaway effect", which would result in all galaxies moving in random directions at nearly the speed of light. Other more general issues regarding negative mass in general relativity are discussed, such as the possibility of time-travel paradoxes.

The simulation video is available at https://www.aanda.org Title: Height Dependence of the Penumbral Fine-scale Structure in the Inner Solar Atmosphere Authors: Murabito, Mariarita; Ermolli, I.; Giorgi, F.; Stangalini, M.; Guglielmino, S. L.; Jafarzadeh, S.; Socas-Navarro, H.; Romano, P.; Zuccarello, F. Bibcode: 2019ApJ...873..126M Altcode: 2018arXiv181209029M We studied the physical parameters of the penumbra in a large and fully developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the Non-LTE inversion COde (NICOLE) code, we obtained the three-dimensional structure of the magnetic field in the penumbra from the bottom of the photosphere up to the middle chromosphere. We analyzed the azimuthal and vertical gradient of the magnetic field strength and inclination. Our results provide new insights on the properties of the penumbral magnetic fields in the chromosphere at atmospheric heights unexplored in previous studies. We found signatures of the small-scale spine and intraspine structure of both the magnetic field strength and inclination at all investigated atmospheric heights. In particular, we report typical peak-to-peak variations of the field strength and inclination of ≈300 G and ≈20°, respectively, in the photosphere, and of ≈200 G and ≈10° in the chromosphere. In addition, we estimated the vertical gradient of the magnetic field strength in the studied penumbra: we find a value of ≈0.3 G km-1 between the photosphere and the middle chromosphere. Interestingly, the photospheric magnetic field gradient changes sign from negative in the inner to positive in the outer penumbra. Title: Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions Authors: Bai, Xianyong; Socas-Navarro, Hector; Nóbrega-Siverio, Daniel; Su, Jiangtao; Deng, Yuanyong; Li, Dong; Cao, Wenda; Ji, Kaifan Bibcode: 2019ApJ...870...90B Altcode: 2018arXiv181103723B Dynamical jets are generally found on light bridges (LBs), which are key to studying sunspot decay. So far, their formation mechanism is not fully understood. In this paper, we used state-of-the-art observations from the Goode Solar Telescope, the Interface Region Imaging Spectrograph, the Spectro-polarimeter on board Hinode, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to analyze the fan-shaped jets on LBs in detail. A continuous upward motion of the jets in the ascending phase is found from the Hα velocity that lasts for 12 minutes and is associated with the Hα line wing enhancements. Two mini jets appear on the bright fronts of the fan-shaped jets visible in the AIA 171 and 193 Å channels, with a time interval as short as 1 minute. Two kinds of small-scale convective motions are identified in the photospheric images, along with the Hα line wing enhancements. One seems to be associated with the formation of a new convection cell, and the other manifests as the motion of a dark lane passing through the convection cell. The finding of three-lobe Stokes V profiles and their inversion with the NICOLE code indicate that there are magnetic field lines with opposite polarities in LBs. From the Hα -0.8 Å images, we found ribbon-like brightenings propagating along the LBs, possibly indicating slipping reconnection. Our observation supports the idea that the fan-shaped jets under study are caused by magnetic reconnection, and photospheric convective motions play an important role in triggering the magnetic reconnection. Title: On the Connection Between Planets, Dark Matter and Cancer Authors: Socas-Navarro, Hector Bibcode: 2018arXiv181202482S Altcode: In a recent paper, Zioutas and Valachovic (2018) claim that dark matter is responsible for a significant fraction of the melanoma skin cancer. This conclusion is drawn from their observation of a significant correlation between skin melanoma incidence in the US and the inner planets positions (especially those of Mercury and Earth). Here I present a number of objections to their interpretation. Some (but not all) of the counterarguments are based on the analysis of a larger dataset from the same source, considering more cancer types and separating by patient attributes, such as race. One of the counterarguments is that, if the melanoma fluctuations with periods similar to planetary orbits were produced by dark matter density enhancements on Earth, then we would have to conclude that the black population is somehow immune to dark matter, a conclusion that seems incompatible with the current WIMP paradigm. Interestingly, some periodicities are present in the data, including the ones reported by Zioutas and Valachovic, but I argue that they must have a societal rather than astronomical origin. Title: Inversions of synthetic umbral flashes: Effects of scanning time on the inferred atmospheres Authors: Felipe, T.; Socas-Navarro, H.; Przybylski, D. Bibcode: 2018A&A...614A..73F Altcode: 2018arXiv180205028F Context. The use of instruments that record narrowband images at selected wavelengths is a common approach in solar observations. They allow scanning of a spectral line by sampling the Stokes profiles with two-dimensional images at each line position, but require a compromise between spectral resolution and temporal cadence. The interpretation and inversion of spectropolarimetric data generally neglect changes in the solar atmosphere during the scanning of line profiles.
Aims: We evaluate the impact of the time-dependent acquisition of various wavelengths on the inversion of spectropolarimetric profiles from chromospheric lines during umbral flashes.
Methods: Numerical simulations of nonlinear wave propagation in a sunspot model were performed with the code MANCHA. Synthetic Stokes parameters in the Ca II 8542 Å line in NLTE were computed for an umbral flash event using the code NICOLE. Artificial profiles with the same wavelength coverage and temporal cadence from reported observations were constructed and inverted. The inferred atmospheric stratifications were compared with the original simulated models.
Results: The inferred atmospheres provide a reasonable characterization of the thermodynamic properties of the atmosphere during most of the phases of the umbral flash. The Stokes profiles present apparent wavelength shifts and other spurious deformations at the early stages of the flash, when the shock wave reaches the formation height of the Ca II 8542 Å line. These features are misinterpreted by the inversion code, which can return unrealistic atmospheric models from a good fit of the Stokes profiles. The misguided results include flashed atmospheres with strong downflows, even though the simulation exhibits upflows during the umbral flash, and large variations in the magnetic field strength.
Conclusions: Our analyses validate the inversion of Stokes profiles acquired by sequentially scanning certain selected wavelengths of a line profile, even in the case of rapidly changing chromospheric events such as umbral flashes. However, the inversion results are unreliable during a short period at the development phase of the flash. Title: Possible Photometric Signatures of Moderately Advanced Civilizations: The Clarke Exobelt Authors: Socas-Navarro, Hector Bibcode: 2018ApJ...855..110S Altcode: 2018arXiv180207723S This paper puts forward a possible new indicator of the presence of moderately advanced civilizations on transiting exoplanets. The idea is to examine the region of space around a planet where potential geostationary or geosynchronous satellites would orbit (hereafter, the Clarke exobelt). Civilizations with a high density of devices and/or space junk in that region, but otherwise similar to ours in terms of space technology (our working definition of “moderately advanced”), may leave a noticeable imprint on the light curve of the parent star. The main contribution to such a signature comes from the exobelt edge, where its opacity is maximum due to geometrical projection. Numerical simulations have been conducted for a variety of possible scenarios. In some cases, a Clarke exobelt with a fractional face-on opacity of ∼10-4 would be easily observable with existing instrumentation. Simulations of Clarke exobelts and natural rings are used to quantify how they can be distinguished by their light curves. Title: Alfvén wave dissipation in the solar chromosphere Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.; Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt, Rebecca L. Bibcode: 2018NatPh..14..480G Altcode: 2018arXiv181007712G Magnetohydrodynamic Alfvén waves1 have been a focus of laboratory plasma physics2 and astrophysics3 for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence4, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes5. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond. Title: The First Post-Kepler Brightness Dips of KIC 8462852 Authors: Boyajian, Tabetha. S.; Alonso, Roi; Ammerman, Alex; Armstrong, David; Asensio Ramos, A.; Barkaoui, K.; Beatty, Thomas G.; Benkhaldoun, Z.; Benni, Paul; Bentley, Rory O.; Berdyugin, Andrei; Berdyugina, Svetlana; Bergeron, Serge; Bieryla, Allyson; Blain, Michaela G.; Capetillo Blanco, Alicia; Bodman, Eva H. L.; Boucher, Anne; Bradley, Mark; Brincat, Stephen M.; Brink, Thomas G.; Briol, John; Brown, David J. A.; Budaj, J.; Burdanov, A.; Cale, B.; Aznar Carbo, Miguel; Castillo García, R.; Clark, Wendy J.; Clayton, Geoffrey C.; Clem, James L.; Coker, Phillip H.; Cook, Evan M.; Copperwheat, Chris M.; Curtis, J. L.; Cutri, R. M.; Cseh, B.; Cynamon, C. H.; Daniels, Alex J.; Davenport, James R. A.; Deeg, Hans J.; De Lorenzo, Roberto; de Jaeger, Thomas; Desrosiers, Jean-Bruno; Dolan, John; Dowhos, D. J.; Dubois, Franky; Durkee, R.; Dvorak, Shawn; Easley, Lynn; Edwards, N.; Ellis, Tyler G.; Erdelyi, Emery; Ertel, Steve; Farfán, Rafael. G.; Farihi, J.; Filippenko, Alexei V.; Foxell, Emma; Gandolfi, Davide; Garcia, Faustino; Giddens, F.; Gillon, M.; González-Carballo, Juan-Luis; González-Fernández, C.; González Hernández, J. I.; Graham, Keith A.; Greene, Kenton A.; Gregorio, J.; Hallakoun, Na'ama; Hanyecz, Ottó; Harp, G. R.; Henry, Gregory W.; Herrero, E.; Hildbold, Caleb F.; Hinzel, D.; Holgado, G.; Ignácz, Bernadett; Ilyin, Ilya; Ivanov, Valentin D.; Jehin, E.; Jermak, Helen E.; Johnston, Steve; Kafka, S.; Kalup, Csilla; Kardasis, Emmanuel; Kaspi, Shai; Kennedy, Grant M.; Kiefer, F.; Kielty, C. L.; Kessler, Dennis; Kiiskinen, H.; Killestein, T. L.; King, Ronald A.; Kollar, V.; Korhonen, H.; Kotnik, C.; Könyves-Tóth, Réka; Kriskovics, Levente; Krumm, Nathan; Krushinsky, Vadim; Kundra, E.; Lachapelle, Francois-Rene; LaCourse, D.; Lake, P.; Lam, Kristine; Lamb, Gavin P.; Lane, Dave; Lau, Marie Wingyee; Lewin, Pablo; Lintott, Chris; Lisse, Carey; Logie, Ludwig; Longeard, Nicolas; Lopez Villanueva, M.; Whit Ludington, E.; Mainzer, A.; Malo, Lison; Maloney, Chris; Mann, A.; Mantero, A.; Marengo, Massimo; Marchant, Jon; Martínez González, M. J.; Masiero, Joseph R.; Mauerhan, Jon C.; McCormac, James; McNeely, Aaron; Meng, Huan Y. A.; Miller, Mike; Molnar, Lawrence A.; Morales, J. C.; Morris, Brett M.; Muterspaugh, Matthew W.; Nespral, David; Nugent, C. R.; Nugent, Katherine M.; Odasso, A.; O'Keeffe, Derek; Oksanen, A.; O'Meara, John M.; Ordasi, András; Osborn, Hugh; Ott, John J.; Parks, J. R.; Rodriguez Perez, Diego; Petriew, Vance; Pickard, R.; Pál, András; Plavchan, P.; Pollacco, Don; Pozo Nuñez, F.; Pozuelos, F. J.; Rau, Steve; Redfield, Seth; Relles, Howard; Ribas, Ignasi; Richards, Jon; Saario, Joonas L. O.; Safron, Emily J.; Sallai, J. Martin; Sárneczky, Krisztián; Schaefer, Bradley E.; Schumer, Clea F.; Schwartzendruber, Madison; Siegel, Michael H.; Siemion, Andrew P. V.; Simmons, Brooke D.; Simon, Joshua D.; Simón-Díaz, S.; Sitko, Michael L.; Socas-Navarro, Hector; Sódor, Á.; Starkey, Donn; Steele, Iain A.; Stone, Geoff; Strassmeier, Klaus G.; Street, R. A.; Sullivan, Tricia; Suomela, J.; Swift, J. J.; Szabó, Gyula M.; Szabó, Róbert; Szakáts, Róbert; Szalai, Tamás; Tanner, Angelle M.; Toledo-Padrón, B.; Tordai, Tamás; Triaud, Amaury H. M. J.; Turner, Jake D.; Ulowetz, Joseph H.; Urbanik, Marian; Vanaverbeke, Siegfried; Vanderburg, Andrew; Vida, Krisztián; Vietje, Brad P.; Vinkó, József; von Braun, K.; Waagen, Elizabeth O.; Walsh, Dan; Watson, Christopher A.; Weir, R. C.; Wenzel, Klaus; Westendorp Plaza, C.; Williamson, Michael W.; Wright, Jason T.; Wyatt, M. C.; Zheng, WeiKang; Zsidi, Gabriella Bibcode: 2018ApJ...853L...8B Altcode: 2018arXiv180100732B We present a photometric detection of the first brightness dips of the unique variable star KIC 8462852 since the end of the Kepler space mission in 2013 May. Our regular photometric surveillance started in 2015 October, and a sequence of dipping began in 2017 May continuing on through the end of 2017, when the star was no longer visible from Earth. We distinguish four main 1%-2.5% dips, named “Elsie,” “Celeste,” “Skara Brae,” and “Angkor,” which persist on timescales from several days to weeks. Our main results so far are as follows: (i) there are no apparent changes of the stellar spectrum or polarization during the dips and (ii) the multiband photometry of the dips shows differential reddening favoring non-gray extinction. Therefore, our data are inconsistent with dip models that invoke optically thick material, but rather they are in-line with predictions for an occulter consisting primarily of ordinary dust, where much of the material must be optically thin with a size scale ≪1 μm, and may also be consistent with models invoking variations intrinsic to the stellar photosphere. Notably, our data do not place constraints on the color of the longer-term “secular” dimming, which may be caused by independent processes, or probe different regimes of a single process. Title: Ca II 8542 Å brightenings induced by a solar microflare Authors: Kuckein, C.; Diercke, A.; González Manrique, S. J.; Verma, M.; Löhner-Böttcher, J.; Socas-Navarro, H.; Balthasar, H.; Sobotka, M.; Denker, C. Bibcode: 2017A&A...608A.117K Altcode: 2017arXiv170906861K
Aims: We study small-scale brightenings in Ca II 8542 Å line-core images to determine their nature and effect on localized heating and mass transfer in active regions.
Methods: High-resolution two-dimensional spectroscopic observations of a solar active region in the near-infrared Ca II 8542 Å line were acquired with the GREGOR Fabry-Pérot Interferometer attached to the 1.5-m GREGOR telescope. Inversions of the spectra were carried out using the NICOLE code to infer temperatures and line-of-sight (LOS) velocities. Response functions of the Ca II line were computed for temperature and LOS velocity variations. Filtergrams of the Atmospheric Imaging Assembly (AIA) and magnetograms of the Helioseismic and Magnetic Imager (HMI) were coaligned to match the ground-based observations and to follow the Ca II brightenings along all available layers of the atmosphere.
Results: We identified three brightenings of sizes up to 2'' × 2'' that appeared in the Ca II 8542 Å line-core images. Their lifetimes were at least 1.5 min. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or Interface Region Imaging Spectrograph (IRIS) bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels; and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of 20'' next to the brightenings. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to - 2.2 km s-1. The structure did not disappear during the observations. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF.
Conclusions: We detected Ca II brightenings that coincided with the footpoint location of an MF. The MF event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the MF.

The movie associated to Fig. 2 is available at http://www.aanda.org Title: Design of an adaptable Stokes polarimeter for exploring chromospheric magnetism Authors: Louis, Rohan E.; Bayanna, A. Raja; Socas Navarro, Héctor Bibcode: 2017AdSpR..60.1547L Altcode: 2017arXiv170607186L The chromosphere is a highly complex and dynamic layer of the Sun, that serves as a conduit for mass and energy supply between two, very distinct regions of the solar atmosphere, namely, the photosphere and corona. Inferring magnetic fields in the chromosphere, has thus become an important topic, that can be addressed with large-aperture solar telescopes to carry out highly sensitive polarimetric measurements. In this article, we present a design of a polarimeter for investigating the chromospheric magnetic field. The instrument consists of a number of lenses, two ferro-electric liquid crystals, a Wollaston prism, and a CCD camera. The optical design is similar to that of a commercial zoom lens which allows a variable f# while maintaining focus and aberrations well within the Airy disc. The optical design of the Adaptable ChRomOspheric POLarimeter (ACROPOL) makes use of off-the-shelf components and is described for the 70 cm Vacuum Tower Telescope and the 1.5 m GREGOR telescope at Observatorio del Teide, Tenerife, Spain. Our design shows that the optical train can be separated into two units where the first unit, consisting of a single lens, has to be changed while going from the VTT to the GREGOR configuration. We also discuss the tolerances within which, diffraction limited performance can be achieved with our design. Title: A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils Authors: Henriques, V. M. J.; Mathioudakis, M.; Socas-Navarro, H.; de la Cruz Rodríguez, J. Bibcode: 2017ApJ...845..102H Altcode: 2017arXiv170605311H We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II λ8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier “sunspot plumes,” which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers. Title: Search for torsional oscillations in isolated sunspots Authors: Griñón-Marín, A. B.; Socas-Navarro, H.; Centeno, R. Bibcode: 2017A&A...604A..36G Altcode: 2017arXiv170601952G; 2017arXiv170601952B In this work we seek evidence for global torsional oscillations in alpha sunspots. We have used long time series of continuum intensity and magnetic field vector maps from the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO) spacecraft. The time series analysed here span the total disk passage of 25 isolated sunspots. We found no evidence of global long-term periodic oscillations in the azimuthal angle of the sunspot magnetic field within 1 degree. This study could help us to understand the sunspot dynamics and its internal structure. Title: Uncertainties in the solar photospheric oxygen abundance Authors: Cubas Armas, M.; Asensio Ramos, A.; Socas-Navarro, H. Bibcode: 2017A&A...600A..45C Altcode: 2017arXiv170106809C
Aims: The purpose of this work is to better understand the confidence limits of the photospheric solar oxygen abundance derived from three-dimensional models using the forbidden [OI] line at 6300 Å, including correlations with other parameters involved.
Methods: We worked with a three-dimensional empirical model and two solar intensity atlases. Bayesian inference was employed as a tool to determine the most probable value for the solar oxygen abundance given the model chosen. We considered a number of error sources, such as uncertainties in the continuum derivation, in the wavelength calibration and in the abundance/strength of Ni.
Results: Our results show correlations between the effects of several parameters employed in the derivation. The Bayesian analysis provides robust confidence limits taking into account all of these factors in a rigorous manner. We obtain that, given the empirical three-dimensional model and the atlas observations employed here, the most probable value for the solar oxygen abundance is log (ɛO) = 8.86 ± 0.04. However, we note that this uncertainty does not consider possible sources of systematic errors due to the model choice. Title: Inference of the chromospheric magnetic field orientation in the Ca II 8542 Å line fibrils Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez González, M. J.; Socas-Navarro, H. Bibcode: 2017A&A...599A.133A Altcode: 2016arXiv161206088A Context. Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes.
Aims: Use statistical analysis to test whether the association between fibrils and magnetic field lines is justified.
Methods: We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro-polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image.
Results: We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of 16°, while this dispersion goes up to 34° in less magnetized regions. Title: First Detection of Sign-reversed Linear Polarization from the Forbidden [O I] 630.03 nm Line Authors: de Wijn, A. G.; Socas-Navarro, H.; Vitas, N. Bibcode: 2017ApJ...836...29D Altcode: 2017arXiv170108793D We report on the detection of linear polarization of the forbidden [O I] 630.03 nm spectral line. The observations were carried out in the broader context of the determination of the solar oxygen abundance, an important problem in astrophysics that still remains unresolved. We obtained spectro-polarimetric data of the forbidden [O I] line at 630.03 nm as well as other neighboring permitted lines with the Solar Optical Telescope of the Hinode satellite. A novel averaging technique was used, yielding very high signal-to-noise ratios in excess of 105. We confirm that the linear polarization is sign-reversed compared to permitted lines as a result of the line being dominated by a magnetic dipole transition. Our observations open a new window for solar oxygen abundance studies, offering an alternative method to disentangle the Ni I blend from the [O I] line at 630.03 nm that has the advantage of simple LTE formation physics. Title: Filamentary Oscillations in the Penumbra of Sunspots Authors: Griñon, Ana Bélen; Pastor Yabar, A.; Socas-Navarro, H.; Centeno, R. Bibcode: 2017psio.confE.111G Altcode: 2017psio.confE.111A No abstract at ADS Title: Filamentary Oscillations in the Penumbra of Sunspots Authors: Grión-Marín, A. B.; Socas-Navarro, H.; Centeno, R. Bibcode: 2016usc..confE..66G Altcode: 2016usc..confE..66B The issue of long-term (on scales of several hours to days) morphological changes in sunspots, and particularly the possible existence of apparent rotational motions and oscillations, has drawn attention since the early 20th century. This kind of study requires data with high spatial resolution and good temporal sampling and coverage. The HMI instrument on board the Solar Dynamics Observatory routinely measures the full magnetic field vector in sunspots and allows us to track them with consistent image quality and high cadence during their entire disk passage. It is the ideal instrument to analyze the evolution of sunspots, and in particular the azimuthal component of the penumbral magnetic field. We carried out an analysis (Griñón-Marín et al. 2016 -Submitted-) looking for torsional oscillations in the penumbra of sunspots that led to no evidence of this kind of oscillation in the 25 sunspots analyzed. However, we detected filamentary-like oscillations in some areas of the penumbra with periods of several hours. In this contribution I will show their morphological analysis and discuss the possible sources for such oscillations. Title: Small-scale magnetic flux emergence in a sunspot light bridge Authors: Louis, Rohan E.; Bellot Rubio, Luis R.; de la Cruz Rodríguez, Jaime; Socas-Navarro, Héctor; Ortiz, Ada Bibcode: 2015A&A...584A...1L Altcode: 2015arXiv150900741L Context. Light bridges are convective intrusions in sunspots that often show enhanced chromospheric activity.
Aims: We seek to determine the nature of flux emergence in a light bridge and the processes related to its evolution in the solar atmosphere.
Methods: We analyse a sequence of high-resolution spectropolarimetric observations of a sunspot taken at the Swedish 1-m Solar Telescope. The data consist of spectral scans of the photospheric Fe i line pair at 630 nm and the chromospheric Ca ii 854.2 nm line. Bisectors were used to construct Dopplergrams from the Fe i 630.15 nm measurements. We employed LTE and non-LTE inversions to derive maps of physical parameters in the photosphere and chromosphere, respectively.
Results: We observe the onset of blueshifts of about 2 km s-1 near the entrance of a granular light bridge on the limbward side of the spot. The blueshifts lie immediately next to a strongly redshifted patch that appeared six minutes earlier. Both patches can be seen for 25 min until the end of the sequence. The blueshifts coincide with an elongated emerging granule, while the redshifts appear at the end of the granule. In the photosphere, the development of the blueshifts is accompanied by a simultaneous increase in field strength of about 400 G. The field inclination increases by some 25°, becoming nearly horizontal. At the position of the redshifts, the magnetic field is equally horizontal but of opposite polarity. An intense brightening is seen in the Ca ii filtergrams over the blueshifts and redshifts, about 17 min after their detection in the photosphere. The brightening is due to emission in the blue wing of the Ca ii 854.2 nm line, close to its knee. Non-LTE inversions reveal that this kind of asymmetric emission is caused by a temperature enhancement of ~700 K between -5.0 ≤ log τ ≤ -3.0 and a blueshift of 3 km s-1 at log τ = -2.3 that decreases to zero at log τ = -6.0
Conclusions: The photospheric blueshifts and redshifts observed in a granular light bridge seem to be caused by the emergence of a small-scale, flat Ω-loop with highly inclined footpoints of opposite polarity that brings new magnetic field to the surface. The gas motions detected in the two footpoints are reminiscent of a siphon flow. The rising loop is probably confined to the lower atmosphere by the overlying sunspot magnetic field and the interaction between the two flux systems may be responsible for temperature enhancements in the upper photosphere/lower chromosphere. This is the first time that magnetic flux is observed to emerge in the strongly magnetised environment of sunspots, pushed upwards by the convective flows of a granular light bridge.

The movie associated to Fig. 2 is available in electronic form at http://www.aanda.org Title: NICOLE: NLTE Stokes Synthesis/Inversion Code Authors: Socas-Navarro, H. Bibcode: 2015ascl.soft08002S Altcode: NICOLE, written in Fortran 90, seeks the model atmosphere that provides the best fit to the Stokes profiles (in a least-squares sense) of an arbitrary number of simultaneously-observes spectral lines from solar/stellar atmospheres. The inversion core used for the development of NICOLE is the LORIEN engine (the Lovely Reusable Inversion ENgine), which combines the SVD technique with the Levenberg-Marquardt minimization method to solve the inverse problem. Title: An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles Authors: Socas-Navarro, H.; de la Cruz Rodríguez, J.; Asensio Ramos, A.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2015A&A...577A...7S Altcode: 2014arXiv1408.6101S With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

The source code is currently hosted at the following repository: https://github.com/hsocasnavarro/NICOLE Title: The solar oxygen abundance from an empirical three-dimensional model Authors: Socas-Navarro, H. Bibcode: 2015A&A...577A..25S Altcode: 2014arXiv1409.6910S The oxygen abundance in the solar photosphere, and consequently the solar metallicity itself, is still a controversial question with far-reaching implications in many areas of astrophysics. This paper presents a new determination obtained by fitting the forbidden O i line at 6300 Å with an observational 3D model. The approach presented here is novel because previous determinations were based either on 1D empirical stratifications or on 3D theoretical models. The resulting best-fit abundances are log ɛ(O) = 8.90 and log ɛ(Ni) = 6.15. Nevertheless, by introducing minor tweaks in the model and the procedure, it is possible to retrieve very different values, even down to log ɛ(O) = 8.70. This extreme sensitivity of the abundance to possible systematic effects is not specific to this particular work, but probably reflects the real uncertainty inherent to all abundance determinations based on a prescribed model atmosphere.

As with the earlier version, the new 3D model is publicly available and may be downloaded from 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/577/A25 Title: VizieR Online Data Catalog: 3D model for fitting forbidden OI 6300 line (Socas-Navarro, 2015) Authors: Socas-Navarro, H. Bibcode: 2015yCat..35770025S Altcode: 2015yCat..35779025S modelin.mod and modelout.mod contain three-dimensional cubes each column represents the physical stratification of each pixel, with the in and the out suffix referring to the two atmospheric components coexisting with the filling factor. The code manual has details on the file format and the variables stored.

(3 data files). Title: Synthetic Observations of Wave Propagation in a Sunspot Umbra Authors: Felipe, T.; Socas-Navarro, H.; Khomenko, E. Bibcode: 2014ApJ...795....9F Altcode: 2014arXiv1408.6565F Spectropolarimetric temporal series from Fe I λ6301.5 Å and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I λ6301.5 Å line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca II infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca II infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere. Title: Physical properties of a sunspot chromosphere with umbral flashes Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.; Socas-Navarro, H.; van Noort, M. Bibcode: 2013A&A...556A.115D Altcode: 2013arXiv1304.0752D We present new high-resolution spectro-polarimetric Ca IIλ8542 observations of umbral flashes in sunspots. At nearly 0.18 arcsec, and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). From a fast Fourier transform analysis, we find a trend of decreasing power at high temporal frequencies at those locations with more horizontal magnetic fields, for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.

Movie and Appendices A and B are available in electronic form at http://www.aanda.org Title: The European Solar Telescope Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R.; EST Team Bibcode: 2013MmSAI..84..379C Altcode: The European Solar Telescope (EST) is a project to design, build and operate an European Solar 4-meter class telescope to be located in the Canary Islands, with the participation of institutions from fifteen European countries gathered around the consortium EAST (European Association for Solar Telescopes). The project main objective up to the present has been the development of the conceptual design study (DS) of a large aperture Solar Telescope. The study has demonstrated the scientific, technical and financial feasibility of EST. The DS has been possible thanks to the co-financing allocated specifically by the EU and the combined efforts of all the participant institutions. Different existing alternatives have been analysed for all telescope systems and subsystems, and decisions have been taken on the ones that are most compatible with the scientific goals and the technical strategies. The present status of some subsystems is reviewed in this paper. Title: Chromospheric Magnetic Fields: Observations, Simulations and their Interpretation Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012ASPC..463...15D Altcode: 2012arXiv1203.4577D The magnetic field of the quiet-Sun chromosphere remains a mystery for solar physicists. The reduced number of chromospheric lines are intrinsically hard to model and only a few of them are magnetically sensitive. In this work, we use a 3D numerical simulation of the outer layers of the solar atmosphere, to asses the reliability of non-LTE inversions, in this case applied to the Ca II λ8542 Å line. We show that NLTE inversions provide realistic estimates of physical quantities from synthetic observations. Title: The European Solar Telescope Authors: Socas-Navarro, H. Bibcode: 2012IAUSS...6E.212S Altcode: In this presentation I will describe the current status of the European Solar Telescope (EST) project. The EST design has a 4-m aperture to achieve both a large photon collection and very high spatial resolution. It includes a multi-conjugate adaptive system integrated in the light path for diffraction-limited imaging. The optical train is optimized to minimize instrumental polarization and to keep it nearly constant as the telescope tracks the sky. A suite of visible and infrared instruments are planned with a light distribution system that accomodates full interoperability and simultaneous usage. The science drivers emphasize combined observations at multiple heights in the atmosphere to build a connected view of solar magnetism from the photosphere to the corona. Title: 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from the Photosphere to the Corona Authors: Rimmele, T. R.; Tritschler, A.; Wöger, F.; Collados Vera, M.; Socas-Navarro, H.; Schlichenmaier, R.; Carlsson, M.; Berger, T.; Cadavid, A.; Gilbert, P. R.; Goode, P. R.; Knölker, M. Bibcode: 2012ASPC..463.....R Altcode: No abstract at ADS Title: Unusual Stokes V profiles during flaring activity of a delta sunspot Authors: Fischer, C. E.; Keller, C. U.; Snik, F.; Fletcher, L.; Socas-Navarro, H. Bibcode: 2012A&A...547A..34F Altcode: 2012arXiv1209.0983F
Aims: We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808. The region was recorded with the Vector-Spectromagnetograph of the Synoptic Optical Long-term Investigations of the Sun facility. The active region produced several successive X-class flares between 19:00 UT and 24:00 UT on September 13, 2005 and we aim to quantify transient and permanent changes in the magnetic field and velocity field during one of the flares, which has been fully captured.
Methods: The Stokes profiles were inverted using the height-dependent inversion code LILIA to analyze magnetic field vector changes at the flaring site. We report multilobed asymmetric Stokes V profiles found in the δ-sunspot umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere consisting of two components (SIR inversions) to interpret the profile shape. The results are put in context with Michelson Doppler Imager (MDI) magnetograms and reconstructed X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager.
Results: We obtain the magnetic field vector and find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line at the flaring site. At two locations in the umbra we encounter strong fields (~3 kG), as inferred from the Stokes I profiles, which, however, exhibit a low polarization signal. During the flare we observe in addition asymmetric Stokes V profiles at one of these sites. The asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong apparent polarity reversal observed in MDI-magnetograms and a chromospheric hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes profiles result in line-of-sight velocity differences in the range of ~12 km s-1 to 14 km s-1 between the two components in the photosphere. Another possibility is that local atmospheric heating is causing the observed asymmetric Stokes V profile shape. In either case our analysis shows that a very localized patch of ~5″ in the photospheric umbra, co-spatial with a flare footpoint, exhibits a subresolution fine structure. 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: Non-local thermodynamic equilibrium inversions from a 3D magnetohydrodynamic chromospheric model Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012A&A...543A..34D Altcode: 2012arXiv1205.3171D Context. The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet-Sun regions that have a relatively weak photospheric field. During the past decade inversion methods have emerged as powerful tools for analyzing the chromosphere of active regions. The applicability of inversions to infer the stratification of the physical conditions in a dynamic 3D solar chromosphere has not yet been studied in detail.
Aims: This study aims to establish the diagnostic capabilities of non-local thermodynamical equilibrium (NLTE) inversion techniques of Stokes profiles induced by the Zeeman effect in the Ca ii λ8542 Å line.
Methods: We computed the Ca ii atomic level populations in a snapshot from a 3D radiation-MHD simulation of the quiet solar atmosphere in non-LTE using the 3D radiative transfer code Multi3d. These populations were used to compute synthetic full-Stokes profiles in the Ca ii λ8542 Å line using 1.5D radiative transfer and the inversion code Nicole. The profiles were then spectrally degraded to account for finite filter width, and Gaussian noise was added to account for finite photon flux. These profiles were inverted using Nicole and the results were compared with the original model atmosphere.
Results: Our NLTE inversions applied to quiet-Sun synthetic observations provide reasonably good estimates of the chromospheric magnetic field, line-of-sight velocities and somewhat less accurate, but still very useful, estimates of the temperature. Three-dimensional scattering of photons cause cool pockets in the chromosphere to be invisible in the line profile and consequently they are also not recovered by the inversions. To successfully detect Stokes linear polarization in this quiet snapshot, a noise level below 10-3.5 is necessary. Title: Model Selection for Spectropolarimetric Inversions Authors: Asensio Ramos, A.; Manso Sainz, R.; Martínez González, M. J.; Viticchié, B.; Orozco Suárez, D.; Socas-Navarro, H. Bibcode: 2012ApJ...748...83A Altcode: 2012arXiv1201.5063A Inferring magnetic and thermodynamic information from spectropolarimetric observations relies on the assumption of a parameterized model atmosphere whose parameters are tuned by comparison with observations. Often, the choice of the underlying atmospheric model is based on subjective reasons. In other cases, complex models are chosen based on objective reasons (for instance, the necessity to explain asymmetries in the Stokes profiles) but it is not clear what degree of complexity is needed. The lack of an objective way of comparing models has, sometimes, led to opposing views of the solar magnetism because the inferred physical scenarios are essentially different. We present the first quantitative model comparison based on the computation of the Bayesian evidence ratios for spectropolarimetric observations. Our results show that there is not a single model appropriate for all profiles simultaneously. Data with moderate signal-to-noise ratios (S/Ns) favor models without gradients along the line of sight. If the observations show clear circular and linear polarization signals above the noise level, models with gradients along the line are preferred. As a general rule, observations with large S/Ns favor more complex models. We demonstrate that the evidence ratios correlate well with simple proxies. Therefore, we propose to calculate these proxies when carrying out standard least-squares inversions to allow for model comparison in the future. Title: NLTE inversions from a 3D MHD Chromospheric simulation Authors: de la Cruz Rodriguez, J.; Socas-Navarro, H.; Carlsson, M.; Leenaarts, J. Bibcode: 2012decs.confE..80D Altcode: The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet Sun regions with a relatively weak field. Measuring the magnetic field of the solar chromosphere is an outstanding challenge for observers. Inversion codes allow for detailed interpretation of full-Stokes data from spectral lines formed in the chromosphere. However, the applicability of non-LTE inversions to infer physical conditions in the dynamic 3D solar chromosphere, has not yet been studied in detail. In this study, we use a snapshot from a 3D MHD simulation of quiet-sun, extending from the photosphere to the corona, to asses the reliability of non-LTE inversions to infer chromospheric quantities, especially the magnetic field. Title: VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager Authors: Borrero, J. M.; Tomczyk, S.; Kubo, M.; Socas-Navarro, H.; Schou, J.; Couvidat, S.; Bogart, R. Bibcode: 2011SoPh..273..267B Altcode: 2009arXiv0901.2702B In this paper we describe in detail the implementation and main properties of a new inversion code for the polarized radiative transfer equation (VFISV: Very Fast Inversion of the Stokes Vector). VFISV will routinely analyze pipeline data from the Helioseismic and Magnetic Imager (HMI) on-board of the Solar Dynamics Observatory (SDO). It will provide full-disk maps (4096×4096 pixels) of the magnetic field vector on the Solar Photosphere every ten minutes. For this reason VFISV is optimized to achieve an inversion speed that will allow it to invert sixteen million pixels every ten minutes with a modest number (approx. 50) of CPUs. Here we focus on describing a number of important details, simplifications and tweaks that have allowed us to significantly speed up the inversion process. We also give details on tests performed with data from the spectropolarimeter on-board of the Hinode spacecraft. Title: Characterization of telescope polarization properties across the visible and near-infrared spectrum. Case study: the Dunn Solar Telescope Authors: Socas-Navarro, H.; Elmore, D.; Asensio Ramos, A.; Harrington, D. M. Bibcode: 2011A&A...531A...2S Altcode: 2010arXiv1009.2866S Accurate astrophysical polarimetry requires a proper characterization of the polarization properties of the telescope and instrumentation employed to obtain the observations. Determining the telescope and instrument Muller matrix is becoming increasingly difficult with the increase in aperture size, precision requirements and instrument complexity of new and upcoming projects. We have carried out a detailed multi-wavelength characterization of the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak as a case study and explore various possibilites for the determination of its polarimetric properties. We show that the telescope model proposed in this paper is more suitable than that in previous work in that it describes better the wavelength dependence of aluminum-coated mirrors. We explore the adequacy of the degrees of freedom allowed by the model using a novel mathematical formalism. Finally, we investigate the use of polarimeter calibration data taken at different times of the day to characterize the telescope and find that very valuable information on the telescope properties can be obtained in this manner. The results are also consistent with the entrance window polarizer measurements. This general method opens interesting possibilities for the calibration of future large-aperture telescopes and precision polarimetric instrumentation. Title: A high-resolution three-dimensional model of the solar photosphere derived from Hinode observations Authors: Socas-Navarro, H. Bibcode: 2011A&A...529A..37S Altcode: 2011arXiv1101.2100S A new three-dimensional model of the solar photosphere is presented in this paper and made publicly available to the community. This model has the peculiarity of having been obtained by inverting spectro-polarimetric observations, rather than by using numerical radiation hydrodynamical simulations. The data used here are from the spectro-polarimeter onboard the Hinode satellite, which routinely delivers Stokes I, Q, U, and V profiles in the 6302 Å spectral region with excellent quality, stability, and spatial resolution (approximately 0.3''). With this spatial resolution the major granular components are resolved, which implies that the derived model needs no micro- or macroturbulence to properly fit the widths of the observed spectral lines. Not only does this model fit the observed data used for its construction, but it can also fit previous solar atlas observations satisfactorily. Title: The Polarization Optics for the European Solar Telescope Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.; Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.; Snik, F.; Socas-Navarro, H. Bibcode: 2011ASPC..437..329B Altcode: EST, the European Solar Telescope, is a 4-m class solar telescope, which will be located at the Canary Islands. It is currently in the conceptual design phase as a European funded project. In order to fulfill the stringent requirements for polarimetric sensitivity and accuracy, the polarimetry has been included in the design work from the very beginning. The overall philosophy has been to use a combination of techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters, differential measurement schemes, fast modulation and demodulation, and accurate calibration, and at the same time not giving up flexibility. The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and non-varying in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system in which eigenvectors can travel undisturbed through the system. Title: Are solar chromospheric fibrils tracing the magnetic field? Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H. Bibcode: 2011A&A...527L...8D Altcode: 2011arXiv1101.3551D Fibrils are thin elongated features visible in the solar chromosphere in and around magnetized regions. Because of their visual appearance, they have been traditionally considered a tracer of the magnetic field lines. For the first time, we challenge that notion by comparing their orientation to that of the magnetic field, obtained via high-resolution spectropolarimetric observations of Ca ii lines. The short answer to the question posed in the title is that mostly yes, but not always. Title: (1) The effect of magnetic fields on solar abundance determinations (2) The solar photosphere in 3D. This time from observations Authors: Fabbian, D.; Socas-Navarro, H. Bibcode: 2010iac..talk..232F Altcode: 2010iac..talk..186F No abstract at ADS Title: Are chromospheric fibrils tracing the magnetic field? Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H. Bibcode: 2010iac..talk..221D Altcode: 2010iac..talk..178D No abstract at ADS Title: Utilization of redundant polarized solar spectra to infer the polarization properties of the new generation of large aperture solar telescopes Authors: Elmore, David F.; Lin, Haosheng; Socas Navarro, Héctor; Jaeggli, Sarah A. Bibcode: 2010SPIE.7735E..4EE Altcode: 2010SPIE.7735E.147E Spectro-polarimetry plays an important role in the study of solar magnetism and strongly influences the design of the new generation of solar telescopes. Calibration of the polarization properties of the telescope is a critical requirement needed to use these observations to infer solar magnetic fields. However, the large apertures of these new telescopes make direct calibration with polarization calibration optics placed before all the telescope optical elements impractical. It is therefore desirable to be able to infer the polarization properties of the telescope optical elements utilizing solar observations themselves. Taking advantage of the fact that the un-polarized, linearly, and circularly polarized spectra originating from the Sun are uncorrelated, we have developed techniques to utilize observations of solar spectra with redundant combination of the polarization states measured at several different telescope configurations to infer the polarization properties of the telescope as a whole and of its optical elements. We show results of these techniques applied to spectro-plarimetric data obtained at the Dunn Solar Telescope. Title: European Solar Telescope: project status Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Grivel-Gelly, C.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R. Bibcode: 2010SPIE.7733E..0HC Altcode: 2010SPIE.7733E..15C The European Solar Telescope is a project for a 4-meter class telescope to be located in the Canary Islands. EST is promoted by the European Association for Solar Telescopes (EAST). This is a consortium formed by a number of research organizations from fifteen European countries (Austria, Croatia, Czech Republic, France, Germany, Hungary, Italy, the Netherlands, Norway, Poland, Slovak Republic, Spain, Sweden, Switzerland, and United Kingdom). EST will specialize in high spatial and temporal resolution using diverse instruments that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over many scale heights in the solar atmosphere. In this contribution, the status of the development of the Design Study of EST is presented, emphasizing the most important aspects of the optical design, mechanical structure, AO and MCAO systems for wavefront correction, instruments and polarization analysis. Title: The polarization optics for the European Solar Telescope (EST) Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.; Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.; Snik, F.; Socas-Navarro, H. Bibcode: 2010SPIE.7735E..6IB Altcode: 2010SPIE.7735E.214B EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST will be located at the Canary Islands and aims at observations with the best possible spectral, spatial and temporal resolution and best polarimetric performance, of the solar photosphere and chromosphere, using a suite of instruments that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic properties of the plasma over many scale heights, and ranging from λ=350 until 2300 nm. In order to be able to fulfill the stringent requirements for polarimetric sensitivity and accuracy, from the very beginning the polarimetry has been included in the design work. The overall philosophy has been to use a combination of techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters, differential measurement schemes, fast modulation and demodulation, and accurate calibration. The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and nonvarying in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system in which eigenvectors can travel undisturbed through the system. Title: European Solar Telescope: Progress status Authors: Collados, M.; Bettonvil, F.; Cavaller, L.; Ermolli, I.; Gelly, B.; Pérez, A.; Socas-Navarro, H.; Soltau, D.; Volkmer, R.; EST Team Bibcode: 2010AN....331..615C Altcode: In this paper, the present status of the development of the design of the European Solar Telescope is described. The telescope is devised to have the best possible angular resolution and polarimetric performance, maximizing the throughput of the whole system. To that aim, adaptive optics and multi-conjugate adaptive optics are integrated in the optical path. The system will have the possibility to correct for the diurnal variation of the distance to the turbulence layers, by using several deformable mirrors, conjugated at different heights. The present optical design of the telescope distributes the optical elements along the optical path in such a way that the instrumental polarization induced by the telescope is minimized and independent of the solar elevation and azimuth. This property represents a large advantage for polarimetric measurements. The ensemble of instruments that are planned is also presented. Title: Spectro-polarimetry in the era of large solar telescopes Authors: Socas-Navarro, H. Bibcode: 2010AN....331..581S Altcode: 2010arXiv1005.4620S This paper discusses some of the challenges of spectro-polarimetric observations with a large aperture solar telescope such as the ATST or the EST. The observer needs to reach a compromise between spatial and spectral resolution, time cadence, and signal-to-noise ratio, as only three of those four parameters can be pushed to the limit. Tunable filters and grating spectrographs provide a natural compromise as the former are more suitable for high-spatial resolution observations while the latter are a better choice when one needs to work with many wavelengths at full spectral resolution. Given the requirements for the new science targeted by these facilities, it is important that 1) tunable filters have some multi-wavelength capability; and 2) grating spectrographs have some 2D field of view. Title: Observation and analysis of chromospheric magnetic fields . Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort, M.; Rouppe van der Voort, L. Bibcode: 2010MmSAI..81..716D Altcode: 2010arXiv1004.0698D The solar chromosphere is a vigorously dynamic region of the sun, where waves and magnetic fields play an important role. To improve chromospheric diagnostics, we present new observations in Ca II 8542 carried out with the SST/CRISP on La Palma, working in full-Stokes mode. We measured Stokes line profiles in active regions. The line profiles observed close to the solar limb show signals in all four Stokes parameters, while profiles observed close to disk center only show signals above the noise level in Stokes I and V. We used the NLTE inversion code 'NICOLE' to derive atmospheric parameters in umbral flashes present in a small round sunspot without penumbra. Title: Has Hinode Revealed the Missing Turbulent Flux of the Quiet Sun? Authors: Lites, B. W.; Kubo, M.; Socas-Navarro, H.; Berger, T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A. M.; Ichimoto, K.; Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2009ASPC..405..173L Altcode: The Hinode Spectro-Polarimeter has revealed the presence of surprisingly strong horizontal magnetic fields nearly everywhere in the quiet solar atmosphere. These horizontal fields, along with measures of the vertical fields, may be the signature of the ``hidden turbulent flux'' of the quiet Sun. The measured horizontal fields average at least to 55 Gauss: nearly 5 times that of the measured longitudinal apparent flux density. The nature of these fields are reviewed, and discussed in the light of recent magneto-convection numerical simulations of the quiet Sun. Title: Direct Imaging of Fine Structure in the Chromosphere of a Sunspot Umbra Authors: Socas-Navarro, H.; McIntosh, S. W.; Centeno, R.; de Wijn, A. G.; Lites, B. W. Bibcode: 2009ApJ...696.1683S Altcode: 2008arXiv0810.0597S High-resolution imaging observations from the Hinode spacecraft in the Ca II H line are employed to study the dynamics of the chromosphere above a sunspot. We find that umbral flashes and other brightenings produced by the oscillation are extremely rich in fine structure, even beyond the resolving limit of our observations (0farcs22). The umbra is tremendously dynamic to the point that our time cadence of 20 s does not suffice to resolve the fast lateral (probably apparent) motion of the emission source. Some bright elements in our data set move with horizontal propagation speeds of 30 km s-1. We have detected filamentary structures inside the umbra (some of which have a horizontal extension of ~1500 km) which, to our best knowledge, had not been reported before. The power spectra of the intensity fluctuations reveal a few distinct areas with different properties within the umbra that seem to correspond with the umbral cores that form it. Inside each one of these areas the dominant frequencies of the oscillation are coherent, but they vary considerably from one core to another. Title: VizieR Online Data Catalog: Models of solar magnetic structures (Socas-Navarro, 2007) Authors: Socas-Navarro, H. Bibcode: 2009yCat..21690439S Altcode: The observations used in this work were acquired with the new Spectro-Polarimeter for Infrared and Optical Regions (SPINOR; Socas-Navarro et al., 2006SoPh..235...55S) on 2004 June 16. The data set includes two chromospheric lines of the CaII infrared triplet at 849.8 and 854.2nm, recorded by two different cameras, as well as two nearby FeI lines at 849.7 and 853.8nm.

(7 data files). Title: Magnetic Fields of the Quiet Sun: A New Quantitative Perspective From Hinode Authors: Lites, B. W.; Kubo, M.; Socas-Navarro, H.; Berger, T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2008ASPC..397...17L Altcode: This article summarizes results of studies presented in two papers already published: Lites et al. (2007a); Lites et al. (2007b). Please see these for further details. Title: The participation of the Instituto de Astrofísica de Canarias in the European Solar Telescope Authors: Collados, M.; Calcines, A.; Diaz, J. J.; Gracia, F.; Grivel-Gelly, C.; López, R.; Mangharam, H.; Páez, E.; Perez, A.; Rasilla, J. L.; Rodríguez, L. F.; Sánchez-Capuchino, J.; Socas-Navarro, H. Bibcode: 2008SPIE.7012E..32C Altcode: 2008SPIE.7012E.105C This communication reviews the participation of the Instituto de Astrofísica de Canarias (IAC) in the design of the European Solar Telescope. Apart of being the coordinator institution of the whole project, and, as such, responsible for the project managing, the IAC leads several tasks like overall instrument definition or characterization of the atmospheric turbulence profile with height or the definition of adequate detectors. More in particular, the IAC will design and build two long-base SHABAR (SHAdow BAnd Ranger), instruments to measure medium-altitude seeing. The IAC is also responsible for the design, together with other institutions, of the design of grating spectropolarimeters suitable for multiwavelength high spatial and spectral resolution. Title: A New Approach to the Solar Oxygen Abundance Problem Authors: Centeno, R.; Socas-Navarro, H. Bibcode: 2008ApJ...682L..61C Altcode: 2008arXiv0803.0990S; 2008arXiv0803.0990C In this work we present new data that sets strong constraints on the solar oxygen abundance. Our approach, based on the analysis of spectropolarimetric observations, is almost model-independent and therefore extremely robust. The asymmetry of the Stokes V profile of the 6300 Å [O I] and Ni I blend is used as an indicator of the relative abundances of these two elements. The peculiar shape of the profile requires a value of epsilonO = 730 ± 100 ppm (parts per million), or log epsilonO = 8.86 ± 0.07 in the logarithmic scale commonly used in astrophysics. The uncertainty range includes the model dependence as well as uncertainties in the oscillator strengths of the lines. We emphasize that the very low degree of model dependence in our analysis makes it very reliable compared to traditional determinations. Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2008ApJ...674..596S Altcode: The reliability of quiet-Sun magnetic field diagnostics based on the Fe I lines at 6302 Å has been questioned by recent work. Here we present the results of a thorough study of high-resolution multiline observations taken with the new spectropolarimeter SPINOR, comprising the 5250 and 6302 Å spectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Å lines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magnetohydrodynamic constrains. When this is done, the 5250 Å lines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point toward a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet-Sun magnetic properties. Finally, we propose the use of two other Fe I lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: The Horizontal Magnetic Flux of the Quiet-Sun Internetwork as Observed with the Hinode Spectro-Polarimeter Authors: Lites, B. W.; Kubo, M.; Socas-Navarro, H.; Berger, T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2008ApJ...672.1237L Altcode: Observations of very quiet Sun using the Solar Optical Telescope/Spectro-Polarimeter (SOT/SP) aboard the Hinode spacecraft reveal that the quiet internetwork regions are pervaded by horizontal magnetic flux. The spatial average horizontal apparent flux density derived from wavelength-integrated measures of Zeeman-induced linear polarization is BTapp = 55 Mx cm -2, as compared to the corresponding average vertical apparent flux density of | BLapp| = 11 Mx cm -2. Distributions of apparent flux density are presented. Magnetic fields are organized on mesogranular scales, with both horizontal and vertical fields showing "voids" of reduced flux density of a few granules spatial extent. The vertical fields are concentrated in the intergranular lanes, whereas the stronger horizontal fields are somewhat separated spatially from the vertical fields and occur most commonly at the edges of the bright granules. High-S/N observations from disk center to the limb help to constrain possible causes of the apparent imbalance between | BLapp| and BTapp, with unresolved structures of linear dimension on the surface smaller by at least a factor of 2 relative to the SOT/SP angular resolution being one likely cause of this discrepancy. Other scenarios for explaining this imbalance are discussed. The horizontal fields are likely the source of the "seething" fields of the quiet Sun discovered by Harvey et al. The horizontal fields may also contribute to the "hidden" turbulent flux suggested by studies involving Hanle effect depolarization of scattered radiation. Title: Hinode Observations of Flux Emergence in Quiet and Active Regions Authors: Lites, B. W.; Centeno, R.; Kubo, M.; Socas-Navarro, H. Berger, T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2008ASPC..383...71L Altcode: We review briefly the observational understanding of emergence of flux in both the quiet Sun and active regions in the light of first results from the joint Japan/US/UK Hinode mission. That spacecraft is now providing us with our first continuous, high resolution measurements of the photospheric vector magnetic field, along with high resolution observations of the thermal and dynamic properties of the chromosphere and corona. This review is intended to present a few very early results and to highlight the potential for discovery offered by this extraordinary new mission. The discovery of ubiquitous horizontal magnetic flux in the quiet internetwork regions is presented. Title: Hinode Observations of Horizontal Quiet Sun Magnetic Flux and the ``Hidden Turbulent Magnetic Flux'' Authors: Lites, Bruce; Socas-Navarro, Hector; Kubo, Masahito; Berger, Thomas; Frank, Zoe; Shine, Richard A.; Tarbell, Theodore D.; Title, Alan M.; Ichimoto, Kiyoshi; Katsukawa, Yukio; Tsuneta, Saku; Suematsu, Yoshinori; Shimizu, Toshifumi Bibcode: 2007PASJ...59S.571L Altcode: We present observations of magnetic fields of the very quiet Sun near disk center using the Spectro-Polarimeter of the Solar Optical Telescope aboard the Hinode satellite. These observations reveal for the first time the ubiquitous presence of horizontal magnetic fields in the internetwork regions. The horizontal fields are spatially distinct from the vertical fields, demonstrating that they are not arising mainly from buffeting of vertical flux tubes by the granular convection. The horizontal component has an average ``apparent flux density'' of 55Mxcm-2 (assuming the horizontal field structures are spatially resolved), in contrast to the average apparent vertical flux density of 11Mxcm-2. The vertical fields reside mainly in the intergranular lanes, whereas the horizontal fields occur mainly over the bright granules, with a preference to be near the outside edge of the bright granules. The large apparent imbalance of vertical and horizontal flux densities is discussed, and several scenarios are presented to explain this imbalance. Title: Spectropolarimetric Inversions of the Ca II 8498 and 8542 Å Lines in the Quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ApJ...670..885P Altcode: We study non-LTE inversions of the Ca II infrared triplet lines as a tool for inferring physical properties of the quiet Sun. The inversion code is successful in recovering the temperature, velocity, and longitudinal magnetic flux density in the photosphere and chromosphere, but the height range where the inversions are sensitive is limited, especially in the chromosphere. We present results of inverting spectropolarimetric observations of the lines in a quiet-Sun region. We find three distinct ranges in chromospheric temperature: low temperatures in the internetwork, high temperatures in the enhanced magnetic network, and intermediate temperatures associated with low magnetic flux regions in the network. The differences between these regions become more pronounced with height as the plasma-β decreases. These inversions support the picture of the chromosphere, especially close to the magnetic network, being highly inhomogeneous in both the vertical and horizontal directions. Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J. Bibcode: 2007arXiv0710.1099S Altcode: The reliability of quiet Sun magnetic field diagnostics based on the \ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We present here the results of a thorough study of high-resolution multi-line observations taken with the new spectro-polarimeter SPINOR, comprising the 5250 and 6302 Åspectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Ålines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magneto-hydrodynamical constrains. When this is done, the 5250 Ålines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point towards a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet Sun magnetic properties. Finally, we propose the use of two other \ion{Fe}{1} lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others. Title: Emergence of Small-Scale Magnetic Loops in the Quiet-Sun Internetwork Authors: Centeno, R.; Socas-Navarro, H.; Lites, B.; Kubo, M.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Tsuneta, S.; Katsukawa, Y.; Suematsu, Y.; Shimizu, T.; Nagata, S. Bibcode: 2007ApJ...666L.137C Altcode: 2007arXiv0708.0844C We study the emergence of magnetic flux at very small spatial scales (less than 2") in the quiet-Sun internetwork. To this aim, a time series of spectropolarimetric maps was taken at disk center using the instrument SP/SOT on board Hinode. The LTE inversion of the full Stokes vector measured in the Fe I 6301 and 6302 Å lines allows us to retrieve the magnetic flux and topology in the region of study. In the example presented here, the magnetic flux emerges within a granular structure. The horizontal magnetic field appears prior to any significant amount of vertical field. As time goes on, the traces of the horizontal field disappear, while the vertical dipoles drift-carried by the plasma motions-toward the surrounding intergranular lanes. These events take place within typical granulation timescales. Title: Spectropolarimetric Observations of the Ca II λ8498 and λ8542 in the Quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ApJ...663.1386P Altcode: 2007arXiv0707.1310P The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 and 8542 Å lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-β regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-β simulation, are seen and large self-reversals in the intensity profiles are common. Title: The Solar Oxygen Abundance Determined from Polarimetric Observations Authors: Norton, Aimee A.; Socas-Navarro, H. Bibcode: 2007AAS...210.2502N Altcode: 2007BAAS...39..131N In this work we present support for recent claims that advocate a downward revision of the solar oxygen abundance. Our analysis employs spatially-resolved spectro-polarimetric observations including the Fe I lines at 6302 Angstroms, and the O I infrared triplet around 7774 Angstroms, in the quiet Sun. We used the Fe I lines to reconstruct the three-dimensional thermal and magnetic structure of the atmosphere. The simultaneous O I observations were then employed to determine the abundance of oxygen at each pixel, using both LTE and non-LTE (NLTE) approaches to the radiative transfer. In this manner, we obtain values of log eo=8.64 (NLTE) and 8.94 (LTE) dex. We find an unsettling fluctuation of the oxygen abundance over the field of view. This is likely an artifact indicating that, even with this relatively refined strategy, important physical ingredients are still missing in the picture. By examining the spatial distribution of the abundance, we estimate realistic confidence limits of approximately 0.1 dex. Title: The Ca II Infrared Triplet Lines as Diagnostics of Chromospheric Magnetism Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007ASPC..368..139P Altcode: The Ca II infrared (IR) triplet lines are a promising candidate for studying chromospheric magnetism and dynamics. To study how good of a diagnostic the lines are for chromospheric magnetism in the quiet Sun we have constructed a MHD simulation in the high plasma-β regime, analyzed quiet Sun spectropolarimetric data of the lines and used a non-local thermodynamic equilibrium (nLTE) inversion code on the observations. In the simulation, where shocking acoustic waves dominate the dynamics, the Ca lines show a time-varying pattern of disappearing and reappearing Stokes V lobes. Waves are seen in the observations as well, but the dynamics are more complex. Unlike in the simulation, the observed Ca lines do not have similar shapes and the Stokes V asymmetries are determined by the local magnetic topology, not the phase of the wave. The fundamental differences between the observations and the simulation lead one to conclude that a 1D plane parallel atmosphere is not a valid approximation for the chromosphere. Nor can the effects of magnetic fields on the dynamics be neglected. This is further supported by the inversions failure to reproduce line profile asymmetries caused by gradients in the velocity and/or magnetic field. To explain the asymmetries, 3D structures and strongly localized gradients need to be included. The work presented here will be published in more detail elsewhere. Title: The Intrinsic Dimensionality of Spectropolarimetric Data Authors: Asensio Ramos, A.; Socas-Navarro, H.; López Ariste, A.; Martínez González, M. J. Bibcode: 2007ApJ...660.1690A Altcode: 2007astro.ph..1604A The amount of information available in spectropolarimetric data is estimated. To this end, the intrinsic dimensionality of the data is inferred with the aid of a recently derived estimator based on nearest neighbor considerations and obtained applying the principle of maximum likelihood. We show in detail that the estimator correctly captures the intrinsic dimension of artificial data sets with known dimension. The effect of noise in the estimated dimension is analyzed thoroughly, and we conclude that it introduces a positive bias that needs to be accounted for. Real simultaneous spectropolarimetric observations in the visible 630 nm and the near-infrared 1.5 μm spectral regions are also investigated in detail, showing that the near-infrared data set provides more information of the physical conditions in the solar atmosphere than the visible data set. Finally, we demonstrate that the amount of information present in an observed data set is a monotonically increasing function of the number of available spectral lines. Title: Magnetic Flux Emergence In The Quiet Sun Photosphere Authors: Centeno, Rebecca; Lites, B.; Socas-Navarro, H.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Tsuneta, S.; Katsukawa, Y.; Suematsu, Y.; Kubo, M.; Shimizu, T. Bibcode: 2007AAS...210.9406C Altcode: 2007BAAS...39..218C We study the emergence of magnetic flux at very small spacial scales (less than 1 arcsec) in the quiet Sun internetwork. To this aim, several time series of spectropolarimetric maps were taken at disk center using the instrument SP/SOT on board Hinode. The LTE inversion of the full Stokes vector measured in the Fe I 6301 and 6302 lines will allow us to retrieve the magnetic flux and topology in the region of study. We find that the magnetic flux emerges typically within the granular structures. In many cases, the horizontal magnetic field appears prior to any significant amount of vertical field. As time goes on, the traces of the horizontal field dissapear while the the vertical dipoles drift -carried by the plasma motions- towards the surrounding intergranular lanes. Sometimes they stay trapped there for a while but they eventually either disappear by disgregation/cancelation or agregate to other magnetic field concentrations giving rise to larger flux elements. The time scale of these events is of the order of 10-20 minutes. Title: Ubiquitous Horizontal Magnetic Fields in the Quiet Solar Photosphere as Revealed by HINODE Meaurements Authors: Lites, Bruce W.; Socas Navarro, H.; Berger, T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Kubo, M.; Shimizu, T.; Nagata, S.; Hinode Team Bibcode: 2007AAS...210.6303L Altcode: 2007BAAS...39..171L Measurements with the HINODE Spectro-Polarimeter (SP) of the quiet Sun allow characterization of the weak, mixed-polarity magnetic flux at the highest angular resolution to date (0.3"), and with good polarimetric sensitivity(0.025% relative to the continuum). The image stabilization of the HINODE spacecraft allows long integrations with degradation of the image quality only by the evolution of the solar granulation. From the Stokes V profile measurements we find an average solar "Apparent Flux Density" of 14 Mx cm-2, with significant Stokes V signals at every position on the disk at all times. However, there are patches of meso-granular size (5-15") where the flux is very weak. At this high sensitivity, transverse fields produce measurable Stokes Q,U linear polarization signals over a majority of the area, with apparent transverse flux densities in the internetwork significantly larger than the corresponding longitudinal flux densities. When viewed at the center of the solar disk, the Stokes V signals (longitudinal fields) show a preference for occurrence in the intergranular lanes, and the Q,U signals occur preferably over the granule interiors, but neither association is exclusive.

Hinode is an international project supported by JAXA, NASA, PPARC and ESA. We are grateful to the Hinode team for all their efforts in the design, build and operation of the mission. Title: The Solar Oxygen Crisis: Probably Not the Last Word Authors: Socas-Navarro, H.; Norton, A. A. Bibcode: 2007ApJ...660L.153S Altcode: In this work we present support for recent claims that advocate a downward revision of the solar oxygen abundance. Our analysis employs spatially resolved spectropolarimetric observations including the Fe I lines at 6302 Å and the O I infrared triplet around 7774 Å in the quiet Sun. We used the Fe I lines to reconstruct the three-dimensional thermal and magnetic structure of the atmosphere. The simultaneous O I observations were then employed to determine the abundance of oxygen at each pixel, using both LTE and non-LTE (NLTE) approaches to the radiative transfer. In this manner, we obtain values of logɛO=8.63 (NLTE) and 8.93 (LTE) dex. We find an unsettling fluctuation of the oxygen abundance over the field of view. This is likely an artifact indicating that, even with this relatively refined strategy, important physical ingredients are still missing in the picture. By examining the spatial distribution of the abundance, we estimate realistic confidence limits of approximately 0.1 dex. Title: Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T. Bibcode: 2007arXiv0704.0617P Altcode: The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-beta simulation, are seen and large self-reversals in the intensity profiles are common. Title: Semiempirical Models of Solar Magnetic Structures Authors: Socas-Navarro, H. Bibcode: 2007ApJS..169..439S Altcode: This paper presents semiempirical models of various solar magnetic structures, extending from the photosphere to the chromosphere. The models have been derived from non-LTE inversions of high-resolution spectropolarimetric observations of four Ca II and Fe I lines. The observed targets are dark and bright components of a sunspot umbra; dark and bright components of a sunspot penumbra; a canopy between two sunspots; a facula; and a network element. These models may be employed, e.g., to compute realistic synthetic Stokes spectra of photospheric and chromospheric lines. Title: A synthetic stellar polarization atlas from 400 to 1000 nm Authors: Socas-Navarro, H.; Asensio Ramos, A.; Manso Sainz, R. Bibcode: 2007A&A...465..339S Altcode: 2006astro.ph.12388S Context: With the development of new polarimeters for large telescopes, the spectro-polarimetric study of astrophysical bodies is becoming feasible and, indeed, more frequent. In particular, this is permitting the observational study of stellar magnetic fields.
Aims: With the aim to optimize and interpret this kind of observations, we have produced a spectral atlas of circular polarization in a grid of stellar atmospheric models with effective temperatures between 3500 and 10 000 K, surface gravities log(g)=3.5-5, metallicities between 10-2 and 1, and magnetic field strengths of 100, 1000 and 5000 G.
Methods: We have computed the emergent Stokes I and V flux spectra in LTE of more than 105 spectral lines.
Results: The atlas and several numerical tools are available in electronic format and may be downloaded from http://download.hao.ucar.edu/pub/PSA/. In this paper we review and discuss some of its most relevant features, such as which spectral regions and individual lines harbor the strongest signals, what are interesting lines to observe, how to disentangle field strength from filling factor, etc.

Full line lists and spectra are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/ qcat?J/A+A/465/339 Title: VizieR Online Data Catalog: Synthetic stellar polarization 400-1000nm (Socas-Navarro+ 2007) Authors: Socas-Navarro, H.; Asensio Ramos, A.; Manso Sainz, R. Bibcode: 2007yCat..34650339S Altcode: No abstract at ADS Title: A suite of community tools for spectro-polarimetric analysis . Authors: Lites, B.; Casini, R.; Garcia, J.; Socas-Navarro, H. Bibcode: 2007MmSAI..78..148L Altcode: The National Center for Atmospheric Research (NCAR) has undertaken a 3-year initiative to develop the Community Spectro-polarimetric Analysis Center (CSAC). The goal of this effort is to provide the community with standardized tools for extracting the solar magnetic field vector and related atmospheric parameters from spectro-polarimetric observations. The emphasis will be to develop portable, efficient, and well-documented procedures for analysis of data from the many new and upcoming observational facilities, both ground- and space-based. The initial focus of CSAC will be the development of robust methods for inversion of Stokes spectral data, starting with a standard Milne-Eddington inversion that has been the workhorse for analysis of data from e.g. the Advanced Stokes Polarimeter. Upon completion of that code, the program will move to more sophisticated methods that embrace more realistic and detailed models of the solar atmosphere. Very fast methods for inversion (neural networks or pattern recognition techniques, for example) are also candidates. Finally, the CSAC is intended to eventually provide standardized methods for resolution of the 180-degree field azimuth ambiguity, and for visualization of the resulting magnetic field vector maps. CSAC is formulated as a community effort, and as such will receive guidance and input from the community. Title: On the Variation of the Peak Asymmetry of Low-l Solar p Modes Authors: Jiménez-Reyes, S. J.; Chaplin, W. J.; Elsworth, Y.; García, R. A.; Howe, R.; Socas-Navarro, H.; Toutain, T. Bibcode: 2007ApJ...654.1135J Altcode: The resonant peaks of solar p modes show small amounts of asymmetry in frequency. Here, we use five independent sets of contemporaneous data, collected over a ~=8 yr period, to investigate whether peak asymmetry in low angular degree p modes changes over the solar activity cycle. Three of the data sets are from instruments on board the ESA/NASA SOHO spacecraft (GOLF, MDI, and VIRGO/SPM); and two are from ground-based networks (BiSON and GONG). Evidence for variation in asymmetry, well correlated with the activity cycle, is uncovered in the GOLF and BiSON Doppler velocity data. Suggestions of a similar trend are present in the GONG Doppler velocity data. Apparent changes in the MDI Doppler velocity data are somewhat less significant. Meanwhile, analysis of the SPM intensity data failed to uncover any evidence for significant change of the asymmetry parameter. Title: Spectral Line Selection for HMI Authors: Norton, A. A.; Pietarila Graham, J. D.; Ulrich, R. K.; Schou, J.; Tomczyk, S.; Liu, Y.; Lites, B. W.; López Ariste, A.; Bush, R. I.; Socas-Navarro, H.; Scherrer, P. H. Bibcode: 2006ASPC..358..193N Altcode: We present information on two spectral lines, Fe I 6173 Å and Ni I 6768 Å, that were candidates for use in the Helioseismic and Magnetic Imager (HMI) instrument. Both Fe I and Ni I profiles have clean continuum and no blends that threaten performance. The higher Landé factor of Fe I means its operational velocity range in regions of strong magnetic field is smaller than for Ne I. Fe I performs better than Ni I for vector magnetic field retrieval. Inversion results show that Fe I consistently determines field strength and flux more accurately than the Ni I line. Inversions show inclination and azimuthal errors are recovered to ≈2° above 600 Mx/cm2 for Fe I and above 1000 Mx/cm2 for Ni I. The Fe I line was recommended, and ultimately chosen, for use in HMI. Title: Spectral Line Selection for HMI: A Comparison of Fe I 6173 Å and Ni I 6768 Å Authors: Norton, A. A.; Graham, J. Pietarila; Ulrich, R. K.; Schou, J.; Tomczyk, S.; Liu, Y.; Lites, B. W.; Ariste, A. López; Bush, R. I.; Socas-Navarro, H.; Scherrer, P. H. Bibcode: 2006SoPh..239...69N Altcode: 2006SoPh..tmp...88N; 2006astro.ph..8124N We present a study of two spectral lines, Fe I 6173 Å and Ni I 6768 Å, that were candidates to be used in the Helioseismic and Magnetic Imager (HMI) for observing Doppler velocity and the vector magnetic field. The line profiles were studied using the Mt. Wilson Observatory, the Advanced Stokes Polarimeter and the Kitt Peak-McMath Pierce telescope and one-meter Fourier transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua and no blends that threaten instrument performance. The Fe I line is 2% deeper, 15% narrower, and has a 6% smaller equivalent width than the Ni I line. The potential of each spectral line to recover pre-assigned solar conditions is tested using a least-squares minimization technique to fit Milne-Eddington models to tens of thousands of line profiles that have been sampled at five spectral positions across the line. Overall, the Fe I line has a better performance than the Ni I line for vector-magnetic-field retrieval. Specifically, the Fe I line is able to determine field strength, longitudinal and transverse flux four times more accurately than the Ni I line in active regions. Inclination and azimuthal angles can be recovered to ≈2° above 600 Mx cm−2 for Fe I and above 1000 Mx cm−2 for Ni I. Therefore, the Fe I line better determines the magnetic-field orientation in plage, whereas both lines provide good orientation determination in penumbrae and umbrae. We selected the Fe I spectral line for use in HMI due to its better performance for magnetic diagnostics while not sacrificing velocity information. The one exception to the better performance of the Fe I line arises when high field strengths combine with high velocities to move the spectral line beyond the effective sampling range. The higher geff of Fe I means that its useful range of velocity values in regions of strong magnetic field is smaller than Ni I. Title: Erratum: ``Polynomial Approximants for the Calculation of Polarization Profiles in the He I 10830 Å Multiplet'' (ApJS, 160, 312 [2005]) Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2006ApJS..166..441S Altcode: In our published paper there is an error in the order of the rows in Tables 1 through 6. We stated in the paper that the components are sorted in the order of increasing wavelengths in the linear Zeeman splitting (LZS) regime. However, in the published tables the values for the σ+ and σ- components appear reversed. As a result, researchers trying to implement our polynomial approximants might apply the incomplete Paschen-Back splitting (IPBS) corrections to the wrong components. To avoid any possible confusion, here we reproduce the tables with the correct sorting (i.e., in the order of increasing wavelengths in the LZS regime). For clarity, we have also replaced the σ+/- notation with σred/blue.

We thank Rebecca Centeno for bringing this error to our attention. Title: The Ca Ir Triplet As A Diagnostic For Chromospheric Magnetism Authors: Pietarila, Anna M.; Socas-Navarro, H.; Bogdan, T. J. Bibcode: 2006SPD....37.1902P Altcode: 2006BAAS...38..247P The Ca IR triplet lines are a good candidate for observing chromospheric magnetic fields. Simulations of the lines in the high-beta regime, spectropolarimetric observations and inversions of observations give insight to their usability\suitability in observing QS magnetic fields and dynamics. Propagating waves are clearly visible in the simulations, both in Stokes V and I. Observed line profiles are found to be more complicated, though they do indicate the presence of waves. We present both simulated and observed profiles and analyze the ability of the NLTE inversion algorithm to retrieve the underlying physics in the solar atmosphere. Title: High Resolution Spectropolarimetry of Penumbral Formation with IBIS Authors: Reardon, Kevin; Casini, R.; Cavallini, F.; Tomczyk, S.; Rouppe van der Voort, L.; Van Noort, M.; Woeger, F.; Socas Navarro, H.; IBIS Team Bibcode: 2006SPD....37.3503R Altcode: 2006BAAS...38..260R We present the results of first spectropolarimetric observations made with the Interferometric Bidimensional Spectrometer (IBIS) at the NSO/Dunn Solar Telescope. The use of narrowband imaging and post-facto reconstruction techniques allows for observations close to the diffraction limit of the vector magnetic field. We will show observations of the the formation of an individual penumbral filament around a small pore. We measure the magnetic field and velocity field of the forming penumbral filament. The spectropolarimetric mode of IBIS will be available to the community in the fall of 2006. Title: Site testing for the Advanced Technology Solar Telescope Authors: Hill, F.; Beckers, J.; Brandt, P.; Briggs, J.; Brown, T.; Brown, W.; Collados, M.; Denker, C.; Fletcher, S.; Hegwer, S.; Horst, T.; Komsa, M.; Kuhn, J.; Lecinski, A.; Lin, H.; Oncley, S.; Penn, M.; Radick, R.; Rimmele, T.; Socas-Navarro, H.; Streander, K. Bibcode: 2006SPIE.6267E..1TH Altcode: 2006SPIE.6267E..59H The Advanced Solar Technology Telescope (ATST) is a 4-m solar telescope being designed for high spatial, spectral and temporal resolution, as well as IR and low-scattered light observations. The overall limit of performance of the telescope is strongly influenced by the qualities of the site at which it is located. Six sites were tested with a seeing monitor and a sky brightness instrument for 1.5 to 2 years. The sites were Big Bear (California), Haleakala (Hawaii), La Palma (Canary Islands, Spain), Panguitch Lake (Utah), Sacramento Peak (New Mexico), and San Pedro Martir (Baja California, Mexico). In this paper we will describe the methods and results of the site survey, which chose Haleakala as the location of the ATST. Title: Spectro-Polarimetric Observations and Non-Lte Modeling of Ellerman Bombs Authors: Socas-Navarro, Hector; Pillet, Valentín Martínez; Elmore, David; Pietarila, Anna; Lites, Bruce W.; Sainz, Rafael Manso Bibcode: 2006SoPh..235...75S Altcode: 2005astro.ph..8667S Ellerman bombs are bright emission features observed in the wings of Hα, usually in the vicinity of magnetic concentrations. Here we show that they can also be detected in the Ca II infrared triplet lines, which are easier to interpret and therefore allow for more detailed diagnostics. We present full Stokes observations of the 849.8 and 854.2 nm lines acquired with the new spectro-polarimeter SPINOR. The data show no significant linear polarization at the level of 3 × 10−4. The circular polarization profiles exhibit measureable signals with a very intricate pattern of peaks. A non-LTE analysis of the spectral profiles emerging from these features reveals the presence of strong downflows (∼10 {km s−1}) in a hot layer between the upper photosphere and the lower chromosphere. Title: Spinor: Visible and Infrared Spectro-Polarimetry at the National Solar Observatory Authors: Socas-Navarro, Hector; Elmore, David; Pietarila, Anna; Darnell, Anthony; Lites, Bruce W.; Tomczyk, Steven; Hegwer, Steven Bibcode: 2006SoPh..235...55S Altcode: 2005astro.ph..8685S The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.). Title: Simulation of Quiet-Sun Waves in the Ca II Infrared Triplet Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.; Carlsson, M.; Stein, R. F. Bibcode: 2006ApJ...640.1142P Altcode: 2005astro.ph.10744P The Ca II infrared triplet lines around 8540 Å are good candidates for observing chromospheric magnetism. Model spectra of these lines are obtained by combining a radiation hydrodynamic simulation with a Stokes synthesis code. The simulation shows interesting time-varying behavior of the Stokes V profiles as waves propagate through the formation region of the lines. Disappearing and reappearing lobes in the Stokes V profiles as well as profile asymmetries are closely related to the atmospheric velocity gradients. Title: On the fine structure of sunspot penumbrae. III. The vertical extension of penumbral filaments Authors: Borrero, J. M.; Solanki, S. K.; Lagg, A.; Socas-Navarro, H.; Lites, B. Bibcode: 2006A&A...450..383B Altcode: 2005astro.ph.10586B In this paper we study the fine structure of the penumbra as inferred from the uncombed model (flux tube embedded in a magnetic surrounding) when applied to penumbral spectropolarimetric data from the neutral iron lines at 6300 Å. The inversion infers very similar radial dependences in the physical quantities (LOS velocity, magnetic field strength etc.) as those obtained from the inversion of the Fe I 1.56 μm lines. In addition, the large Stokes V area asymmetry exhibited by the visible lines helps to constrain the size of the penumbral flux tubes. As we demonstrate here, the uncombed model is able to reproduce the area asymmetry with striking accuracy, returning flux tubes as thick as 100-300 kilometers in the vertical direction, in good agreement with previous investigations. Title: Influence of the Paschen-Back effect on the results of polarimetric inversions of the He I 10830 Å triplet Authors: Sasso, C.; Lagg, A.; Solanki, S. K.; Socas-Navarro, H. Bibcode: 2006MSAIS...9..126S Altcode: The He I triplet at 10830 Å has a great potential for determining the magnetic field vector in the upper chromosphere. The triplet is Zeeman sensitive (Landé factors 2.0, 1.75 and 1.25) and shows the signature of the Hanle effect under appropriate conditions. Additionally, the Zeeman sublevels are influenced by the Paschen-Back effect leading to changes in strength and in position of the Zeeman components of the transitions forming the triplet. In this work we calculate the influence of the Paschen-Back effect on the Stokes profiles and investigate its relevance to inversions on spectro-polarimetric data obtained with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). Title: Evidence for Fine Structure in the Chromospheric Umbral Oscillation Authors: Centeno, R.; Socas-Navarro, H.; Collados, M.; Trujillo Bueno, J. Bibcode: 2005ApJ...635..670C Altcode: 2005astro.ph.10740C Novel spectropolarimetric observations of the He I multiplet are used to explore the dynamics of the chromospheric oscillation above sunspot umbrae. The results presented here provide strong evidence in support of the two-component model proposed by Socas-Navarro and coauthors. According to this model, the waves propagate only inside channels of subarcsecond width (the ``active'' component), whereas the rest of the umbra remains nearly at rest (the ``quiet'' component). Although the observations support the fundamental elements of that model, there is one particular aspect that is not compatible with our data. We find that, contrary to the scenario as originally proposed, the active component remains through the entire oscillation cycle and harbors both the upflowing and the downflowing phase of the oscillation. Title: Solar Site Survey for the Advanced Technology Solar Telescope. I. Analysis of the Seeing Data Authors: Socas-Navarro, H.; Beckers, J.; Brandt, P.; Briggs, J.; Brown, T.; Brown, W.; Collados, M.; Denker, C.; Fletcher, S.; Hegwer, S.; Hill, F.; Horst, T.; Komsa, M.; Kuhn, J.; Lecinski, A.; Lin, H.; Oncley, S.; Penn, M.; Rimmele, T.; Streander, K. Bibcode: 2005PASP..117.1296S Altcode: 2005astro.ph..8690S The site survey for the Advanced Technology Solar Telescope concluded recently after more than 2 years of data gathering and analysis. Six locations, including lake, island, and continental sites, were thoroughly probed for image quality and sky brightness. The present paper describes the analysis methodology employed to determine the height stratification of the atmospheric turbulence. This information is crucial, because daytime seeing is often very different between the actual telescope aperture (~30 m) and the ground. Two independent inversion codes have been developed to simultaneously analyze data from a scintillometer array and a solar differential image monitor. We show here the results of applying them to a sample subset of data from 2003 May that was used for testing. Both codes retrieve a similar seeing stratification through the height range of interest. A quantitative comparison between our analysis procedure and actual in situ measurements confirms the validity of the inversions. The sample data presented in this paper reveal a qualitatively different behavior for the lake sites (dominated by high-altitude seeing) and the rest (dominated by near-ground turbulence). Title: Are Electric Currents Heating the Magnetic Chromosphere? Authors: Socas-Navarro, H. Bibcode: 2005ApJ...633L..57S Altcode: 2005astro.ph..8689S This Letter presents an analysis of three-dimensional vector currents and temperatures observed in a sunspot from the photosphere to the chromosphere, spanning a range of heights of approximately 1500 km. With this unique data set, based on novel spectropolarimetric observations of the 850 nm spectral region, it is possible to conduct for the first time an empirical study of the relation between currents and chromospheric heating. It is shown that while resistive current dissipation contributes to heat the sunspot chromosphere, it is not the dominant factor. The heating effect of current dissipation is more important in the penumbra of the sunspot, but even there it is still a relatively modest contribution. Title: The Three-dimensional Structure of a Sunspot Magnetic Field Authors: Socas-Navarro, H. Bibcode: 2005ApJ...631L.167S Altcode: 2005astro.ph..8688S We report on observations of the three-dimensional structure of a sunspot magnetic field from the photosphere to the chromosphere, obtained with the new visible/infrared spectropolarimeter SPINOR. The observations, interpreted with a non-LTE modeling technique, reveal a surprisingly complex topology with areas of opposite-sign torsion, suggesting that flux ropes of opposite helicity may coexist together in the same spot. Title: Polynomial Approximants for the Calculation of Polarization Profiles in the He I 10830 Å Multiplet Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2005ApJS..160..312S Altcode: 2005astro.ph..5321S The He I multiplet at 10830 Å is formed in the incomplete Paschen-Back regime for typical conditions found in solar and stellar atmospheres. The positions and strengths of the various components that form the Zeeman structure of this multiplet in the Paschen-Back regime are approximated here by polynomials. The fitting errors are smaller than ~10-2 mÅ in the component positions and ~10-3 in the relative strengths. The approximant polynomials allow for a very fast implementation of the incomplete Paschen-Back regime in numerical codes for the synthesis and inversion of polarization profiles in this important multiplet. Title: High precision polarimetry with the Advanced Technology Solar Telescope Authors: Socas-Navarro, Hector; Elmore, David F.; Keller, Christoph U.; Seagraves, Paul H.; Streander, Kim V.; Card, Gregory L.; Warner, Mark; Kuhn, Jeffrey R.; Mickey, Donald L. Bibcode: 2005SPIE.5901...52S Altcode: This paper addresses the issue of calibrating the Advanced Technology Solar Telescope for high-precision polarimetry, in particular of the optical train above the Gregorian station (where suitable calibration optics will be placed). Conventional techniques would not be adequate for this telescope given its large aperture. Here we explore two different methods that are currently being considered by the design team. The first one is the "sub-aperture" method, which uses small calibration optics above the primary mirror to calibrate a small sub-aperture of the system. This calibration is then extended to the full aperture by means of actual observations. The second method is based on analyzing the polarization observed in a spectral line with a peculiar Zeeman pattern, such as the FeII 614.9 nm line, which does not produce any intrinsic linear polarization. Numerical simulations are presented that show the robustness of both techniques and their respective advantages and disadvantages are discussed. Title: The visible spectro-polarimeter for the advanced technology solar telescope Authors: Elmore, D. F.; Socas-Navarro, H.; Card, G. L.; Streander, K. V. Bibcode: 2005SPIE.5901...60E Altcode: The mission of the ATST visible spectro-polarimeter (ViSP) is to provide precision measurements of the full state of polarization (Stokes parameters) simultaneously at diverse wavelengths in the visible spectrum and fully resolve (or nearly so) the profiles of spectrum lines originating in the solar atmosphere. We present the instrument science requirements, their flow down to instrument specifications, and a preliminary ViSP design. The ViSP spectrograph allows for reconfiguration while maintaining an immediately selectable configuration. We describe how the ViSP will utilize the ATST polarimetry facility. Title: An artificial neural network approach to the solution of molecular chemical equilibrium Authors: Asensio Ramos, A.; Socas-Navarro, H. Bibcode: 2005A&A...438.1021A Altcode: 2005astro.ph..5322A A novel approach is presented for the solution of instantaneous chemical equilibrium problems. The chemical equilibrium can be considered, due to its intrinsically local character, as a mapping of the three-dimensional parameter space spanned by the temperature, hydrogen density and electron density into many one-dimensional spaces representing the number density of each species. We take advantage of the ability of artificial neural networks to approximate non-linear functions and construct neural networks for the fast and efficient solution of the chemical equilibrium problem in typical stellar atmosphere physical conditions. The neural network approach has the advantage of providing an analytic function, which can be rapidly evaluated. The networks are trained with a learning set (that covers the entire parameter space) until a relative error below 1% is reached. It has been verified that the networks are not overtrained by using an additional verification set. The networks are then applied to a snapshot of realistic three-dimensional convection simulations of the solar atmosphere showing good generalization properties. Title: The ATST Site Survey Authors: Hill, F.; Beckers, J.; Brandt, P.; Briggs, J. W.; Brown, T.; Brown, W.; Collados, M.; Denker, C.; Fletcher, S.; Hegwer, S.; Horst, T.; Komsa, M.; Kuhn, J.; Lecinski, A.; Lin, H.; Oncley, S.; Penn, M.; Radick, R.; Rimmele, T.; Socas-Navarro, H.; Soltau, D.; Streander, K. Bibcode: 2005AGUSMSP34A..04H Altcode: The Advanced Technology Solar Telescope (ATST) will be the world's largest aperture solar telescope, and is being designed for high resolution, IR, and coronal research. It must be located at a site that maximizes the scientific return of this substantial investment. We present the instrumentation, analysis and results of the ATST site survey. Two instrumentation sets were deployed at each of six sites to measure seeing as a function of height, and sky brightness as a function of wavelength and off-limb position. Analysis software was developed to estimate the structure function Cn2 as a function of height near the ground, and the results were verified by comparison with in-situ measurements. Additional software was developed to estimate the sky brightness. The statistics of the conditions at the sites were corrected for observing habits and the annualized hours of specific observing conditions were estimated. These results were used to identify three excellent sites suitable to host the ATST: Haleakala, Big Bear and La Palma. Among them, Haleakala is proposed as the optimal location of the ATST, La Palma and Big Bear being viable alternative sites. Title: Polarimetric calibration of large-aperture telescopes. II. Subaperture method Authors: Socas-Navarro, Hector Bibcode: 2005JOSAA..22..907S Altcode: 2004astro.ph.10563S; 2005OSAJ...22..907S A new method for absolute polarimetric calibration of large telescopes is presented. The proposed method is highly accurate and is based on the calibration of a small subaperture, which is then extended to the full system by means of actual observations of an astronomical source. The calibration procedure is described in detail along with numerical simulations that explore its robustness and accuracy. The advantages and disadvantages of this technique with respect to other possible alternatives are discussed. Title: Strategies for Spectral Profile Inversion Using Artificial Neural Networks Authors: Socas-Navarro, H. Bibcode: 2005ApJ...621..545S Altcode: 2004astro.ph.10567S This paper explores three different strategies for the inversion of spectral lines (and their Stokes profiles) using artificial neural networks. It is shown that a straightforward approach in which the network is trained with synthetic spectra from a simplified model leads to considerable errors in the inversion of real observations. This problem can be overcome in at least two different ways that are studied here in detail. The first method makes use of an additional preprocessing autoassociative neural network to project the observed profile into the theoretical model subspace. The second method considers a suitable regularization of the neural network used for the inversion. These new techniques are shown to be robust and reliable when applied to the inversion of both synthetic and observed data. Title: Polarimetric calibration of large-aperture telescopes. I. Beam-expansion method Authors: Socas-Navarro, Hector Bibcode: 2005JOSAA..22..539S Altcode: 2005OSAJ...22..539S; 2004astro.ph.10564S This paper describes a concept for the high-accuracy absolute calibration of the instrumental polarization introduced by the primary mirror of a large-aperture telescope. This procedure requires a small aperture with polarization calibration optics (e.g., mounted on the dome) followed by a lens that opens the beam to illuminate the entire surface of the mirror. The Jones matrix corresponding to this calibration setup (with a diverging incident beam) is related to that of the normal observing setup (with a collimated incident beam) by an approximate correction term. Numerical models of parabolic on-axis and off-axis mirrors with surface imperfections are used to explore its accuracy. Title: Physical Properties of Spicules from Simultaneous Spectropolarimetric Observations of He I and Ca II Lines Authors: Socas-Navarro, H.; Elmore, D. Bibcode: 2005ApJ...619L.195S Altcode: 2004astro.ph.10566S We present full Stokes observations from SPINOR (Spectro-Polarimeter for INfrared and Optical Regions, at the Dunn Solar Telescope) in the Ca II infrared triplet and the He I multiplet at 1083 nm from which some properties of spicules have been derived. There are important advantages in multiline observations, particularly from different elements. We find that the orientation of the plane of polarization is very different for the Ca and He lines, which provides new model-independent direct evidence of magnetic fields in spicules. Our data show that the Ca and He lines have almost identical widths. Since the Ca atom is 10 times heavier than He, we are able to conclude that most of the broadening is nonthermal (~=16 km s-1) and to set an upper limit of 13 kK to the spicular temperatures. The bisectors of the lines span a velocity range of over 15 km s-1 for the He line and 30 km s-1 for the Ca ones. The vertical gradient of line-of-sight velocities is also very different for both elements. We obtain 2.8 km s-1 Mm-1 from He versus 6.4 km s-1 Mm-1 from Ca. These properties, and others from similar observations, should be taken into account in future physical models of spicules. Title: Feature Extraction Techniques for the Analysis of Spectral Polarization Profiles Authors: Socas-Navarro, H. Bibcode: 2005ApJ...620..517S Altcode: 2004astro.ph.10565S This paper introduces a novel feature extraction technique for the analysis of spectral line Stokes profiles. The procedure is based on the use of an autoassociative artificial neural network containing nonlinear hidden layers. The neural network extracts a small subset of parameters from the profiles (features), from which it is then able to reconstruct the original profile. This new approach is compared to two other procedures that have been proposed in previous works, namely principal component analysis and Hermitian function expansions. Depending on the target application, each of these three techniques has some advantages and disadvantages, which are discussed here. Title: Shocks in the Quiet Solar Photosphere: A Rather Common Occurrence Authors: Socas-Navarro, H.; Manso Sainz, R. Bibcode: 2005ApJ...620L..71S Altcode: 2004astro.ph.10568S We present observations of the quiet solar photosphere in the Fe I lines at 6302 Å where at least four different spatial locations exhibit upward-directed supersonic flows. These upflows can only be detected in the circular polarization profiles as a double-peaked structure in the blue lobe of both Fe I lines. We have detected cases of either magnetic polarity in the data. The polarization signals associated with the upflows are very weak, which is probably why they had not been seen before in this type of observation. We propose that the observed flows are the signature of aborted convective collapse, similar to the case reported by Bellot Rubio et al. Our data indicate that this phenomenon occurs frequently in the quiet Sun, which means that many magnetic elements (although the fraction is still unknown) are destroyed even before they are formed completely. The spectral signatures of supersonic upflows reported here are probably present in most spectropolarimetric observations of sufficient signal-to-noise ratio and spatial resolution. Title: Observational Evidence for Small-Scale Mixture of Weak and Strong Fields in the Quiet Sun Authors: Socas-Navarro, H.; Lites, B. W. Bibcode: 2004ApJ...616..587S Altcode: 2004astro.ph.10562S Three different maps of the quiet Sun, observed with the Advanced Stokes Polarimeter (ASP) and the Diffraction-Limited Stokes Polarimeter (DLSP), show evidence of strong (~=1700 G) and weak (<500 G) fields coexisting within the resolution element at both network and internetwork locations. The angular resolution of the observations is of 1" (ASP) and 0.6" (DLSP). Even at the higher DLSP resolution, a significant fraction of the network magnetic patches harbor a mixture of strong and weak fields. Internetwork elements that exhibit kG fields when analyzed with a single-component atmosphere are also shown to harbor considerable amounts of weak fields. Only those patches for which a single-component analysis yields weak fields do not show this mixture of field strengths. Finally, there is a larger fractional area of weak fields in the convective upflows than in the downflows. Title: A Simple Procedure for Optimizing the Height Resolution in Spectral Line Inversions Authors: Socas-Navarro, H. Bibcode: 2004ApJ...614..457S Altcode: This paper presents a method to determine how much detail of the height stratification of atmospheric parameters can be retrieved from the inversion of spectral lines (and their polarization profiles, when available) via χ2-minimization algorithms. An a priori analysis of the response functions and their correlations permits an evaluation of the optimal number of inversion nodes and their locations. A number of problems of practical interest are studied by applying this method to different photospheric and chromospheric models of quiet Sun and sunspots. As a result of this analysis I obtain the optimal discretization for retrieving the temperature and magnetic field stratifications in such atmospheres. I show that the Fe I lines at 6302 Å provide a height resolution for temperature inversions close to Δτ500~=0.5 in the middle photosphere. In comparison, the chromospheric Hα line allows for a resolution of Δτ500~=1.5 through the chromosphere. This kind of analysis may also be useful in other contexts in which χ2-minimization algorithms are employed. Title: Motivation and Initial Results from SPINOR in the Near Infrared Authors: Socas-Navarro, H.; Elmore, D.; Lites, W. Bibcode: 2004astro.ph.10576S Altcode: SPINOR is a new spectro-polarimeter that will serve as a facility instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic operation over a very broad range of wavelengths, from $\sim$400 up to 1600 nm, allowing for the simultaneous observation of several visible and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc). SPINOR is scheduled for commissioning by the end of 2005. In this paper we report on the current status of the project and present actual observations of active regions in the Ca II infrared triplet and the He I multiplet at 1083 nm. Title: The Thermal and Magnetic Structure of Umbral Dots from the Inversion of High-Resolution Full Stokes Observations Authors: Socas-Navarro, H.; Martínez Pillet, V.; Sobotka, M.; Vázquez, M. Bibcode: 2004ApJ...614..448S Altcode: This paper presents the analysis of high-resolution Stokes observations of eight different umbral dots in a sunspot. The spectra were recorded with the La Palma Stokes Polarimeter, attached to the Swedish Vacuum Solar Telescope. The observed line profiles have been inverted to yield the height stratifications of temperature, magnetic field, and line-of-sight velocity, as well as their respective Wilson depressions. We report on systematic differences in the properties of umbral dots with respect to the nearby umbra, including small upflows (~100 m s-1), higher temperatures (~1 kK), and weaker fields (~500 G) with more horizontal orientations (~10°). The field weakening is strongly correlated with the Wilson depression, suggesting that it may be due to an opacity effect (as one is looking at higher layers). The inclination excess, on the other hand, is real and cannot be ascribed to formation height issues. The results obtained from our semiempirical modeling are discussed within the context of the currently existing scenarios for the subsurface structure of sunspots. The observational signatures revealed by our analysis fit well within both the ``spaghetti'' and the monolithic models. Title: Solar site testing for the Advanced Technology Solar Telescope Authors: Hill, Frank; Beckers, Jacques; Brandt, Peter; Briggs, John; Brown, Timothy; Brown, W.; Collados, Manuel; Denker, Carsten; Fletcher, Steven; Hegwer, Steven; Horst, T.; Komsa, Mark; Kuhn, Jeff; Lecinski, Alice; Lin, Haosheng; Oncley, Steve; Penn, Matthew; Rimmele, Thomas R.; Socas-Navarro, Hector; Streander, Kim Bibcode: 2004SPIE.5489..122H Altcode: The location of the Advanced Technology Solar Telescope (ATST) is a critical factor in the overall performance of the telescope. We have developed a set of instrumentation to measure daytime seeing, sky brightness, cloud cover, water vapor, dust levels, and weather. The instruments have been located at six sites for periods of one to two years. Here we describe the sites and instrumentation, discuss the data reduction, and present some preliminary results. We demonstrate that it is possible to estimate seeing as a function of height near the ground with an array of scintillometers, and that there is a distinct qualitative difference in daytime seeing between sites with or without a nearby lake. Title: Multiline Stokes Analysis for the Study of Small-Scale Solar Magnetic Fields Authors: Socas-Navarro, H. Bibcode: 2004ApJ...613..610S Altcode: Combining observations of spectral lines with different Zeeman sensitivities, it is possible to infer some properties of the small-scale solar magnetic fields. This paper presents a theoretical analysis of the diagnostics potential of 14 different spectral lines when used for this purpose. These lines span a broad range of wavelengths from the visible to the near-infrared and have been employed in recent studies of the quiet-Sun magnetism. It is shown here that the visible lines analyzed provide only a rough picture of the unresolved field distribution, but their results are more robust. Infrared lines, on the other hand, have the potential of carrying much richer information on the details of the distribution. However, this depends critically on the sensitivity attained and the analysis performed. The best results are obtained when a suitable combination of visible and infrared lines is employed. The analysis of sample visible and infrared quiet-Sun profiles confirms this assessment and suggests the coexistence of weak and strong fields within the resolution element, which would explain the observational discrepancy that surrounds this issue. Finally, I present a table with optimum combinations of spectral lines for three present or upcoming multiline solar spectropolarimeters (THEMIS, SPINOR, and the TIP+POLIS combination). Title: Instrumentation for the Advanced Technology Solar Telescope Authors: Rimmele, Thomas R.; Hubbard, Robert P.; Balasubramaniam, K. S.; Berger, Tom; Elmore, David; Gary, G. Allen; Jennings, Don; Keller, Christoph; Kuhn, Jeff; Lin, Haosheng; Mickey, Don; Moretto, Gilberto; Socas-Navarro, Hector; Stenflo, Jan O.; Wang, Haimin Bibcode: 2004SPIE.5492..944R Altcode: The 4-m aperture Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at Nasmyth or a small Gregorian area. For example, instruments for observing the faint corona preferably will be mounted at Nasmyth focus where maximum throughput is achieved. In addition, the Nasmyth focus has minimum telescope polarization and minimum stray light. We describe the set of first generation instruments, which include a Visible-Light Broadband Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters, Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter & Spectrometer and a UV-Polarimeter. We also discuss unique and efficient approaches to the ATST instrumentation, which builds on the use of common components such as detector systems, polarimetry packages and various opto-mechanical components. Title: Signatures of Incomplete Paschen-Back Splitting in the Polarization Profiles of the He I λ10830 Multiplet Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Landi Degl'Innocenti, E. Bibcode: 2004ApJ...612.1175S Altcode: 2004astro.ph..9005S We investigate the formation of polarization profiles induced by a magnetic field in the He I multiplet at 10830 Å. Our analysis considers the Zeeman splitting in the incomplete Paschen-Back regime. The effects turn out to be important and produce measurable signatures on the profiles, even for fields significantly weaker than the level-crossing field (~400 G). When compared to profiles calculated with the usual linear Zeeman effect, the incomplete Paschen-Back profiles exhibit the following conspicuous differences: (1) a non-Doppler blueshift of the Stokes V zero-crossing wavelength of the blue component; (2) area and peak asymmetries, even in the absence of velocity and magnetic gradients; and (3) a ~25% reduction in the amplitude of the red component. These features do not vanish in the weak-field limit. The spectral signatures that we analyze in this paper may be found in previous observations published in the literature. Title: Characterization of Magnetic Flux in the Quiet Sun. II. The Internetwork Fields at High Angular Resolution Authors: Lites, B. W.; Socas-Navarro, H. Bibcode: 2004ApJ...613..600L Altcode: This paper presents high-resolution (0.6") spectropolarimetric observations of very quiet solar regions. We make determinations of the magnetic flux density, intrinsic field strength, and magnetic filling factor in these regions. Our results confirm other previous works in that most of the internetwork flux is in the form of intrinsically strong fields. However, in contrast to other recent observations, we do not find a significant increase in the amount of unsigned magnetic flux relative to that measured at an angular resolution of 1", nor do we find that most internetwork lanes are populated by strong flux. We discuss possible causes for this disparity. The lack of additional flux with increased resolution may be used to set constrains on the typical spatial scales at which mixed polarities occur (smaller than ~=0.6"). Title: Magnetic Properties of the Solar Internetwork Authors: Socas-Navarro, H.; Martínez Pillet, V.; Lites, B. W. Bibcode: 2004ApJ...611.1139S Altcode: Advanced Stokes Polarimeter observations are used to study the weakest polarization signals observed in the quiet photosphere with flux densities in the range of 1.5-50 Mx cm-2, which are found in internetwork regions. Our analysis allows us to reach an unprecedented spectropolarimetric sensitivity at the cost of sacrificing spatial resolution. We find evidence for intrinsically different fields in granules and lanes and characterize the average properties of the weakest observable flux concentrations. The magnetic signals observed suggest a strong coupling between magnetic fields and convective flows. Upflows bring up weak fields (equipartition or weaker) to the surface, with stronger upflows carrying larger amounts of flux. The circular polarization profiles observed in the granular regions display a very strongly asymmetric shape, which contrasts with the less asymmetric profiles observed in the downflowing regions. At downflowing locations with speeds of 0.5 km s-1, both weak and strong fields can be found. However, when the downflow speed increases (up to about 1 km s-1) both the mean flux and the intrinsic field strength show a tendency to increase. The asymmetry of the circular polarization profiles also shows a clear trend as a function of magnetic flux density. Low-flux regions display the negative area asymmetry one naturally expects for field strengths decreasing with height embedded in a downflowing environment. As we move to stronger flux density locations, the well-known positive area asymmetry develops and reaches even higher values than those typically found in network regions. These results may have important implications for our understanding of the coupling between magnetic fields and convective processes that pervade the solar photosphere. Title: On the Diagnostic Potential of Hα for Chromospheric Magnetism Authors: Socas-Navarro, H.; Uitenbroek, H. Bibcode: 2004ApJ...603L.129S Altcode: We investigate the feasibility of measuring line-of-sight magnetic field strength in the solar chromosphere through Stokes polarimetry in the hydrogen Hα line. Because of the large intrinsic Doppler width of the hydrogen line, the weak-field approximation applies even for the strongest magnetic fields in the solar atmosphere. We calculate the Hα response functions of Stokes I and V to perturbations in the magnetic field and temperature in two different one-dimensional solar models representing the average quiet Sun and a sunspot umbra. These response functions show that the Hα line exhibits large photospheric sensitivities in these solar models in addition to its chromospheric sensitivity. One particularly striking example is the Stokes V response in the quiet Sun, which is mostly photospheric when the field decreases with height. Title: Magnetic Fields in the Quiet Sun: Observational Discrepancies and Unresolved Structure Authors: Socas-Navarro, H.; Sánchez Almeida, J. Bibcode: 2003ApJ...593..581S Altcode: Observations of magnetically sensitive lines in the visible and the infrared yield apparently contradictory values for the intrinsic field strength in the internetwork quiet Sun. It is shown that this discrepancy can be understood if one assumes that the magnetic field is not homogeneous over the resolution element. The difference between visible and infrared measurements may be used to set constrains on the subpixel distribution of field strengths. We suggest a specific probability density function that seems to satisfy the existing observational constraints. Title: Measuring Solar Magnetic Fields with Artificial Neural Networks Authors: Socas-Navarro, H. Bibcode: 2003NN.....16..355S Altcode: The quantification of the solar magnetic field is a crucial step in modern solar physics to understand the dynamics, activity and variability of our star. Presently, a reliable inference of these fields is only possible by means of a computer-intensive process that has so far limited scientists to the analysis of observations from small regions of the solar disk, and/or very crude spatial and temporal resolution. This work presents a different approach to the problem, in which a multilayer perceptron, trained with known synthetic profiles, is able to recognize the profiles and return the magnetic field used to synthesize them. The network is then confronted with real observations of a sunspot which had been previously inverted using traditional inversion techniques. A quantitative comparison between these two procedures shows the reliability of the network when applied to points having magnetic filling factors larger than approximately 70%. The dramatic decrease in the re!

quired computing time presents an opportunity for the routine analysis of large-scale, high-resolution solar observations. Title: Small-scale Magnetic Fields in the Quiet Sun Authors: Socas-Navarro, H. Bibcode: 2003ASPC..307..330S Altcode: No abstract at ADS Title: The Helioseismic and Magnetic Imager (HMI) on SDO: Full Vector Magnetography with a Filtergraph Polarimeter Authors: Graham, J. D.; Norton, A.; López Ariste, A.; Lites, B.; Socas-Navarro, H.; Tomczyk, S. Bibcode: 2003ASPC..307..131G Altcode: No abstract at ADS Title: The Fine Structure of Sunspot Umbrae (Invited review) Authors: Socas-Navarro, H. Bibcode: 2003ASPC..286..267S Altcode: 2003ctmf.conf..267S No abstract at ADS Title: Zeeman diagnostics of solar magnetic fields Authors: Socas-Navarro, Hector Bibcode: 2002ESASP.505...45S Altcode: 2002solm.conf...45S; 2002IAUCo.188...45S The next generation of space- and ground-based spectropolarimeters demands new diagnostic tools capable of fast (ideally real-time) and reliable interpretation of the Stokes data. This paper presents a critical review of the various techniques that can be used for Solar magnetometry, from traditional approaches, such as the magnetograph formula, to very recent developments using artificial neural networks. Title: Inference of Solar Magnetic Field Parameters from Data with Limited Wavelength Sampling Authors: Graham, Jonathan D.; López Ariste, Arturo; Socas-Navarro, Hector; Tomczyk, Steven Bibcode: 2002SoPh..208..211G Altcode: We investigate the diagnostic potential of polarimetric measurements with filtergraph instruments. Numerical simulations are used to explore the possibility of inferring the magnetic field vector, its filling factor, and the thermodynamics of model atmospheres when only a few wavelength measurements are available. These simulations assume the magnetic Sun to be represented by Milne-Eddington atmospheres. The results indicate that two wavelength measurements are insufficient to reliably determine the magnetic parameters, regardless of whether magnetograph techniques or least-squares fitting inversions are used. However, as few as four measurements analyzed with the inversion technique provide enough information to retrieve the intrinsic magnetic field with an accuracy better than 10% in most cases. Title: Converging Flows in the Penumbra of a δ Sunspot Authors: Lites, B. W.; Socas-Navarro, H.; Skumanich, A.; Shimizu, T. Bibcode: 2002ApJ...575.1131L Altcode: Doppler velocities in the penumbra of a δ-configuration sunspot observed near the limb indicate flows that converge upon the line separating locally positive and negative polarity magnetic field (the polarity inversion line). These flows persist for many hours. Observations of this region with the Advanced Stokes Polarimeter (ASP) reveal a convex vector field geometry with magnetic lines of force arching upward from positive polarity, then downward to negative polarity. The straightforward interpretation of the combined Doppler velocity and vector field information leads to an untenable physical situation: were flows directed from both footpoints toward the tops of arched magnetic lines of force, mass would rapidly load the tops of the arches. However, there is no observational evidence of the dynamics that such a loading would require. To better understand this apparent contradiction, we perform two-component analyses of the observed Stokes spectral profiles in the vicinity of the polarity inversion line, in order to extract information about unresolved structure of the magnetic field and its associated flows. Fits to the observed profiles, obtained by use of two different inversion techniques, suggest strongly that, as in penumbrae of simple sunspots, the field geometry in the convergence zone is ``fluted.'' However, unlike in simple sunspots, which have only an outward-directed Evershed flow in the more horizontal of the field components, at each spatial point our analysis reveals flows in the two components that are oppositely directed. We interpret these observations as indicative of an interleaved system of field lines in the vicinity of the polarity reversal, whereby the convergent streams are able to slip past one another and return downward into the solar interior. Title: The Solar Internetowrk Magnetic Fields Authors: Socas-Navarro, H.; Lites, B. W. Bibcode: 2002AAS...200.5516S Altcode: 2002BAAS...34R.950S The properties of solar photospheric inter-network fields are studied using observations from the Advanced Stokes Polarimeter. By averaging over large areas of the observed map we are able to bring down the noise level to approximately 10-5 and determine typical magnetic properties of granules and intergranular lanes outside the magnetic network. Title: Inference of Solar Vector Magnetic Fields with Filtergraph Instruments Authors: Graham, J. D.; Lites, B. W.; López Ariste, A.; Norton, A.; Socas-Navarro, H.; Tomczyk, S. Bibcode: 2002AAS...200.5611G Altcode: 2002BAAS...34..736G We investigate the diagnostic potential of polarimetric measurements with filtergraph instruments. Numerical simulations are used to explore the possibility of inferring the magnetic field vector, its filling factor, and the thermodynamics of model atmospheres when only a few wavelength measurements are available. The simulations assume the magnetic sun to be represented by Milne-Eddington (ME) atmospheres. The results indicate that two wavelength measurements are insufficient to reliably determine the magnetic parameters, regardless of whether magnetograph techniques or least-squares fitting inversions are used. However, as few as four measurements analyzed with the inversion technique provide enough information to retrieve the intrinsic magnetic field with an accuracy better than 10 generated by more general models and ASP data passed through a simulated filtergraph. Title: Magnetic Properties of Photospheric Regions with Very Low Magnetic Flux Authors: Socas-Navarro, H.; Sánchez Almeida, J. Bibcode: 2002ApJ...565.1323S Altcode: 2001astro.ph.10025S The magnetic properties of the quiet Sun are investigated using a novel inversion code, FATIMA, based on the Principal Component Analysis of the observed Stokes profiles. The stability and relatively low noise sensitivity of this inversion procedure allows for the systematic inversion of large data sets with a very weak polarization signal. Its application to quiet-Sun observations of network and internetwork regions reveals that a significant fraction of the quiet-Sun contains kilogauss fields (usually with very small filling factors) and confirms that the pixels with weak polarization account for most of the magnetic flux. Mixed polarities in the resolution element are also found to occur more likely as the polarization weakens. Title: Pattern recognition techniques and the measurement of solar magnetic fields Authors: Lopez Ariste, Arturo; Rees, David E.; Socas-Navarro, Hector; Lites, Bruce W. Bibcode: 2001SPIE.4477...96L Altcode: Measuring vector magnetic fields in the solar atmosphere using the profiles of the Stokes parameters of polarized spectral lines split by the Zeeman effect is known as Stokes Inversion. This inverse problem is usually solved by least-squares fitting of the Stokes profiles. However least-squares inversion is too slow for the new generation of solar instruments (THEMIS, SOLIS, Solar-B, ...) which will produce an ever-growing flood of spectral data. The solar community urgently requires a new approach capable of handling this information explosion, preferably in real-time. We have successfully applied pattern recognition and machine learning techniques to tackle this problem. For example, we have developed PCA-inversion, a database search technique based on Principal Component Analysis of the Stokes profiles. Search is fast because it is carried out in low dimensional PCA feature space, rather than the high dimensional space of the spectral signals. Such a data compression approach has been widely used for search and retrieval in many areas of data mining. PCA-inversion is the basis of a new inversion code called FATIMA (Fast Analysis Technique for the Inversion of Magnetic Atmospheres). Tests on data from HAO's Advanced Stokes Polarimeter show that FATIMA isover two orders of magnitude faster than least squares inversion. Initial tests on an alternative code (DIANNE - Direct Inversion based on Artificial Neural NEtworks) show great promise of achieving real-time performance. In this paper we present the latest achievements of FATIMA and DIANNE, two powerful examples of how pattern recognition techniques can revolutionize data analysis in astronomy. Title: Fast Inversion of Spectral Lines Using Principal Components Analysis. II. Inversion of Real Stokes Data Authors: Socas-Navarro, H.; López Ariste, A.; Lites, B. W. Bibcode: 2001ApJ...553..949S Altcode: The principal components analysis (PCA) technique is used to develop an extremely fast and stable Stokes inversion code, suitable for application to large data sets from instruments that operate under standard conditions. This paper provides some tips on reducing the dimensionality of the problem, which are then used to develop a practical implementation of PCA for the automatic analysis of Stokes profiles. The resulting code is tested using real spectropolarimetric observations of the pair of Fe I lines at 6302 Å at a sunspot. Title: Observation of Linear Polarization in the Infrared Ca II Triplet Lines during Umbral Flashes Authors: López Ariste, A.; Socas-Navarro, H.; Molodij, G. Bibcode: 2001ApJ...552..871L Altcode: We report on Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires (THEMIS)2 observations of linear polarization events associated with umbral flashes observed in the Ca II infrared (IR) triplet lines. The observed signals are usually delayed in time and shifted in space when compared to the intensity and circular polarization signals from the flash. The observations are compatible with a scenario whereby flashes are produced by a perturbation propagating along the magnetic field lines as they bend out toward the penumbra. Only a fraction of the resolution element appears to be emitting flashlike profiles, as if the waves were propagating only within localized magnetic field lines. This localization, however, does not impede the apparent propagation of the perturbation horizontally within the umbra. Title: A Time-dependent Semiempirical Model of the Chromospheric Umbral Oscillation Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2001ApJ...550.1102S Altcode: We present a time-dependent semiempirical model of the chromospheric umbral oscillation in sunspots. This model has been obtained by applying recently developed non-LTE inversion techniques to a time series of spectropolarimetric observations. The model consists of two optically thick unresolved atmospheric components: a ``quiet'' component with downward velocities that covers most of the resolution element and an ``active'' component with upward velocities as high as 10 km s-1 that covers a smaller filling factor and has a higher temperature at the same chromospheric optical depth. This semiempirical model accounts for all the observational signatures of the chromospheric oscillation when the filling factor of the active component oscillates between a few percent and 20% of the resolution element. We discuss a plausible physical scenario in which upward-propagating waves in a downflowing magnetized environment lead to periodic mass ejections from the atmospheric layers where the waves become nonlinear. Based on observations obtained with the Gregory Coudé Telescope, operated on the island of Tenerife by the Observatory of Göttingen University in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Title: Converging Flows in the Penumbra of a δ-Sunspot Authors: Lites, B. W.; Socas-Navarro, H.; Skumanich, A.; Shimizu, T. Bibcode: 2001ASPC..248..143L Altcode: 2001mfah.conf..143L No abstract at ADS Title: Convergent Flows in the Penumbra of a δ-Sunspot Authors: Lites, B. W.; Socas-Navarro, H. Bibcode: 2001ASPC..236..329L Altcode: 2001aspt.conf..329L No abstract at ADS Title: Stokes Inversion Techniques: Recent Achievements and Future Horizons Authors: Socas-Navarro, H. Bibcode: 2001ASPC..236..487S Altcode: 2001aspt.conf..487S No abstract at ADS Title: Non-LTE inversion of spectral lines and Stokes profiles (Invited Review) Authors: Socas-Navarro, H. Bibcode: 2001hsa..conf..233S Altcode: No abstract at ADS Title: Anomalous Circular Polarization Profiles in Sunspot Chromospheres Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000ApJ...544.1141S Altcode: This paper presents a detailed description, analysis, and interpretation of the spectropolarimetric observations recently reported by Socas-Navarro, Trujillo Bueno, & Ruiz Cobo. These observations consist of time series of Stokes I and V profiles above a sunspot umbra. The spectral lines observed simultaneously are the Ca II chromospheric lines at 8498 and 8542 Å and the photospheric Fe I line at 8497 Å. These spectropolarimetric observations unveil an intriguing time-dependent behavior of the Stokes V profiles in the chromospheric lines. This behavior should be considered as an observational reference for future radiation magnetohydrodynamic simulations of sunspot chromospheres. The analysis of the observed time series shows that a ``normal,'' nearly antisymmetric V profile rapidly evolves toward an ``anomalous,'' completely asymmetric profile, returning later to the normal state. The occurrence of such anomalous circular polarization profiles repeats itself with a periodicity of ~150 s. After giving arguments to discard other scenarios, we are able to interpret the anomalous V profiles as a consequence of the development of a second unresolved atmospheric component. This unresolved component seems to be the same that produces the umbral flashes observed in other sunspots, where it is present with a larger filling factor. Based on observations obtained with the Gregory Coudé Telescope, operated on the island of Tenerife by the Observatory of Göttingen University, in the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. Title: Untangling Sunspot Penumbrae: New Stokes Profile Analyses Authors: Leka, K. D.; Socas-Navarro, H. Bibcode: 2000SPD....31.0119L Altcode: 2000BAAS...32..804L We present preliminary analyses of Stokes [I, Q, U, V] spectra of a sunspot penumbra using two different analysis/inversion procedures. It has long been known that the Stokes spectra from sunspot penumbrae, even at disk center, commonly show multiple lobes and asymmetries. Hence, the magnetic/thermodynamic maps obtained from Stokes spectra using Milne-Eddington approaches are good first approximations, but will not uncover any further details of penumbral physics. Penumbrae are known to be very structured, hence we first present the results of an inversion using a recent augmentation to the HAO inversion routine: the ability to model the observed spectra using two magnetic atmospheres in addition to the non-magnetic atmosphere. Such a 'three-component' approach is useful to interpret unresolved structures which contribute to signals within the resolution elements. Second, we present the results of an inversion using the new "LILIA" code, currently under development at HAO. LILIA is the "community Stokes inversion code" component of the Solar Magnetism Initiative (SMI), and is based on the strategy of Ruiz Cobo and del Toro Iniesta (1992, ApJ, 398, 375). We describe the strengths and limitations of the two approaches, with attention to their application for the community at large to interpret data from both current and future ground-based and space-based instruments. Title: Anomalous Polarization Profiles in Sunspots: Possible Origin of Umbral Flashes Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000Sci...288.1396S Altcode: 2000Sci...288.1398S We present time-series spectropolarimetric observations of sunspots in the Ca II infrared triplet lines, which show a periodic occurrence of anomalous, asymmetric, circular polarization profiles in the umbral chromosphere. The profiles may be caused by the periodic development of an unresolved atmospheric component in a downward flowing magnetized environment. This active component with upward directed velocities as high as 10 kilometers per second is connected to the umbral flash (UF) phenomenon. We can explain the observations with a semiempirical model of the chromospheric oscillation and of the sunspot magnetized atmospheric plasma during a UF event. Title: Non-LTE Inversion of Stokes Profiles Induced by the Zeeman Effect Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 2000ApJ...530..977S Altcode: This paper presents a new diagnostic tool for the inference of the thermal, dynamic, and magnetic properties of the solar chromosphere. It consists of a non-LTE inversion code of Stokes profiles induced by the Zeeman effect in magnetized stellar atmospheres. This code is the generalization, to the non-LTE Stokes transfer case, of the inversion code for unpolarized line profiles of Socas-Navarro, Ruiz Cobo, & Trujillo Bueno. It is based upon a full non-LTE multilevel treatment of Zeeman line transfer in which the thermal, magnetic, and dynamic properties of the atmospheric model are adjusted automatically by means of nonlinear least-squares-fitting techniques until a best fit to the observed Stokes profiles is obtained. Our non-LTE inversion approach is based on the concept of response functions, which measure the emergent Stokes profiles' first-order reaction to changes in the atmospheric parameters. We generalize our fixed departure coefficients (FDC) approximation in order to allow fast computation of such response functions in the present non-LTE Zeeman line transfer context. We present several numerical tests showing the reliability of our inversion method for retrieving the information about the thermodynamics and the magnetic field vector that is contained in the polarization state of the chosen spectral lines. We also explore the limitations of the inversion code by applying it to simulated observations where the physical hypotheses on which it is based on are not met. Finally, we apply our non-LTE Stokes inversion code to real spectropolarimetric observations of a sunspot observed in the IR triplet lines of Ca II. As a result, a new mean model of the sunspot chromosphere is provided. Title: Calculations of Self-Absorption Curves of Metallic Emission Lines in Emission Line Stars Authors: Socas-Navarro, H.; Israelian, G.; Friedjung, M. Bibcode: 2000ASPC..204..241S Altcode: 2000tiaf.conf..241S No abstract at ADS Title: A new diagnostic tool for the solar chromosphere Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B. Bibcode: 1999ASSL..243..263S Altcode: 1999sopo.conf..263S No abstract at ADS Title: NLTE Inversion of Spectral Lines and Stokes Profiles Authors: Socas-Navarro, H. Bibcode: 1999PhDT.......278S Altcode: No abstract at ADS Title: Non-LTE inversion of spectral lines and Stokes profiles Authors: Socas Navarro, Héctor Bibcode: 1999PhDT.......540S Altcode: No abstract at ADS Title: Non-LTE Inversion of Line Profiles Authors: Socas-Navarro, H.; Ruiz Cobo, B.; Trujillo Bueno, J. Bibcode: 1998ApJ...507..470S Altcode: In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the atmospheric conditions. We introduce the fixed departure coefficients (FDC) approximation in order to compute these response functions in a fast and straightforward manner. The accuracy of this approximation is checked comparing FDC response functions with those obtained from full NLTE computations. An NLTE inversion code based on these response functions has been developed and extensively tested. Reference synthetic profiles, similar to those expected from real observations, are given as input to the inversion algorithm and the recovered models are shown to be compatible with the reference models within the error bars. Our NLTE inversion code thus provides a new tool for the investigation of the chromospheres of the Sun and other stars. Title: Linearization versus Preconditioning: Which Approach Is Best for Solving Multilevel Transfer Problems? Authors: Socas-Navarro, H.; Trujillo Bueno, J. Bibcode: 1997ApJ...490..383S Altcode: We present a critical analysis of linearization and preconditioning, the two most used approaches proposed for achieving the required linearity in the iterative solution of the multilevel transfer problem. By distinguishing from the outset between the response of the radiation field to the source function and opacity perturbations, we are able to demonstrate that if the linearization strategy, on which the local approximate Λ-operator option of the multilevel transfer code MULTI is based, is applied neglecting the terms coming from the response of the radiation field to the opacity perturbations, one then recovers the same equations obtained using the preconditioning technique of Rybicki & Hummer. It is also shown that if this preconditioning technique is applied taking into account the response of the radiation field to both the source function and opacity variations, one then ends up with the same equations found via the linearization method. Thus these two approaches to the numerical solution of the multilevel transfer problem turn out to be essentially the same, because similar equations are obtained if the same information is taken into account. Finally, it is pointed out that, if one wishes to guarantee positivity for the atomic level populations, it is necessary to neglect the terms associated with the response of the radiation field to the opacity perturbations. Neglecting such terms does not deteriorate the convergence rate of multilevel transfer methods that make use of a local approximate operator. Title: Are NLTE effects important for the inversion of iron lines? Authors: Socas-Navarro, H.; Trujillo Bueno, J.; Ruiz Cobo, B.; Shchukina, N. G. Bibcode: 1997joso.proc...86S Altcode: No abstract at ADS Title: Star Formation in Normal and Barred Cluster Spirals Authors: Moss, C.; Whittle, M.; Pesce, J. E.; Socas-Navarro, H. Bibcode: 1995ApL&C..31..215M Altcode: 1995astro.ph..3084M; 1995ApL....31..215M An objective prism H alpha survey has shown that there is a population of early type spiral galaxies in nearby clusters with strong central bursts of star formation which could be due to galaxy--galaxy tidal interactions. Such galaxies are rarely found in the field.