Author name code: rezaei ADS astronomy entries on 2022-09-14 author:"Rezaei, Reza" ------------------------------------------------------------------------ Title: Observation of a small-scale magnetic vortex associated with a chromospheric swirl: signatures of a small-scale magnetic tornado Authors: Milena Diaz Castillo, Saida; Steiner, Oskar; Fischer, Catherine; Berdyugina, Svetlana; Rezaei, Reza Bibcode: 2022cosp...44.2521M Altcode: \newcommand{\ion}[2]{#1\,{\textsc{#2}}} High-resolution solar observations revealed the existence of small-scale swirling vortices in chromospheric intensity maps and velocity diagnostics. These events are commonly localized in the quiet sun intergranular space and are often related to small-scale magnetic flux concentrations at the solar surface. Frequently, vortices have been observed in the vicinity of magnetic flux concentrations, indicating a link between swirls and the evolution of the small-scale magnetic fields. Vortices were also studied with MHD numerical simulations of the solar atmosphere, revealing their complexity, dynamics, and magnetic nature. In particular, it has been suggested that the chromospheric swirling plasma motion is due to a coherently rotating magnetic field structure, which again is driven by a photospheric vortex flow at its footpoint. In this contribution, we present a comprehensive description of the evolution of an isolated small-scale magnetic element interacting with a vortex flow, which in turn is related to a chromospheric swirl. We study observations taken with the CRisp Imaging SpectroPolarimeter (CRISP) instrument and the CHROMospheric Imaging Spectrometer (CHROMIS) at the 1m Swedish Solar Telescope (SST) in April 2019 as part of a SOLARNET access program. The data were taken at quiet-Sun disk-center, recording full Stokes photospheric maps in the \ion{Fe}{i} line at 617\,nm, full Stokes data in the \ion{Ca}{ii} infrared triplet line at 854\,nm, and spectroscopic maps in the H$\alpha$ 656\,nm, \ion{Ca}{ii} K 393\,nm, and \ion{Ca}{ii} H 396\,nm lines. Utilizing the multi-wavelength data and applying height-dependent Stokes inversion and local correlation tracking methods, we are able to analyse the magnetic field dynamics in the presence of vortex structures at photospheric and chromospheric layers. The temporal evolution of the magnetic element shows an appreciable increase in the magnetic field strength during the interaction with the vortex flow, reaching kG values for a few minutes. We also find a clear evidence of a Rapid Blue-shift Excursion (RBE) associated with the magnetic field intensification event propagating along the chromospheric vortex. In addition, we explore the polarization signatures in the photosphere to reveal the intrinsic structure of the magnetic element. Marginal but consistent detection of linear polarization signals in the surroundings of the magnetic element before intensification suggests a magnetic field torsion. Our analysis indicates that we have observed a rotating magnetic object reaching from the photosphere to the chromosphere, resembling a small-scale magnetic tornado. Title: Photospheric Magnetic Fields of the Trailing Sunspots in Active Region NOAA 12396 Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer, C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2019ASPC..526..291V Altcode: 2018arXiv180507752V The solar magnetic field is responsible for all aspects of solar activity. Sunspots are the main manifestation of the ensuing solar activity. Combining high-resolution and synoptic observations has the ambition to provide a comprehensive description of the sunspot growth and decay processes. Active region NOAA 12396 emerged on 2015 August 3 and was observed three days later with the 1.5-meter GREGOR solar telescope on 2015 August 6. High-resolution spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together with the chromospheric He I λ1083.0 nm triplet. These near-infrared spectropolarimetric observations were complemented by synoptic line-of-sight magnetograms and continuum images of the Helioseismic and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Title: Magnetic Flux Density in 3D MHD Simulations and Observations Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G. Bibcode: 2019ASPC..526..191B Altcode: We compare the polarization signals induced in three-dimensional (3D) magneto-hydrodynamical (MHD) simulations by the Zeeman effect in the presence of photospheric magnetic fields to those in observations at disc centre. We consider quantities determined from Stokes vector profiles of observations of photospheric spectral lines in the visible and near-infrared, and in corresponding synthetic spectra obtained from numerical 3D MHD simulations with an average magnetic flux density of 20-200 G. We match the spatial resolution of observations by degrading the spectra of the simulations. We find that the total unsigned vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet Sun internetwork. A value of ∼30 G best agrees with all observations we employed. Title: New Insights on Penumbra Magneto-Convection Authors: Bello González, N.; Jurčák, J.; Schlichenmaier, R.; Rezaei, R. Bibcode: 2019ASPC..526..261B Altcode: Fully-fledged penumbrae are a well characterised phenomenon from an observational point of view. Also, MHD simulations reproduce the observed characteristics and provide us with insights on the physical mechanisms possibly running behind the observed processes. Yet, how this penumbral magneto-convection sets in is still an open question. Due to the fact that penumbra formation is a relatively fast process (of the order of hours), it has eluded its observation with sufficient spatial resolution by both space- and ground-based solar observatories. Only recently, some researchers have witnessed the onset of both orphan and sunspot penumbrae in detail. We are one of those. In July 2009, we observed the early stages of the NOAA 11024 AR leading sunspot while developing its penumbra. The spectro-polarimetric dataset lead us to new observational findings. In this contribution, we put into context our and other authors' results to draw the overall picture of sunspot formation. Most important, the comparison on the properties of different types of penumbrae lead us to the conclusion that the formation of penumbrae is not just one mechanism. While the sole cause necessary for penumbral magneto-convection is a stably inclined magnetic field, observations show that inclined fields can be caused by flux emergence, to form orphan penumbrae, or by field lines transported down from upper photospheric layers, to form sunspot penumbra. This conclusion, together with the recent findings by Jur\čák and collaborators on a canonical value of the vertical component of the magnetic field blocking the action of penumbral magneto-convection in umbral areas, is a crucial step forward towards the understanding of the coupling of solar plasmas and magnetic fields in penumbral atmospheres. Title: The solar chromosphere at millimetre and ultraviolet wavelengths. I. Radiation temperatures and a detailed comparison Authors: Jafarzadeh, S.; Wedemeyer, S.; Szydlarski, M.; De Pontieu, B.; Rezaei, R.; Carlsson, M. Bibcode: 2019A&A...622A.150J Altcode: 2019arXiv190105763J Solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA) provide us with direct measurements of the brightness temperature in the solar chromosphere. We study the temperature distributions obtained with ALMA Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of, a sunspot, comprising quasi-quiet and active regions with different amounts of underlying magnetic fields. We compare these temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths (and with the line-core intensities of the optically-thin far-UV spectra), co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer. These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition region. Splitting the ALMA sub-bands resulted in an slight increase of spatial resolution in individual temperature maps, thus, resolving smaller-scale structures compared to those produced with the standard averaging routines. We find that the radiation temperatures have different, though somewhat overlapping, distributions in different wavelengths and in the various magnetic regions. Comparison of the ALMA temperatures with those of the UV diagnostics should, however, be interpreted with great caution, the former is formed under the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE. The mean radiation temperature of the ALMA Band 6 is similar to that extracted from the IRIS C II line in all areas with exception of the sunspot and pores where the C II poses higher radiation temperatures. In all magnetic regions, the Mg II lines associate with the lowest mean radiation temperatures in our sample. These will provide constraints for future numerical models. Title: High-resolution imaging and near-infrared spectroscopy of penumbral decay Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; Rezaei, R.; Sobotka, M.; Deng, N.; Wang, H.; Tritschler, A.; Collados, M.; Diercke, A.; González Manrique, S. J. Bibcode: 2018A&A...614A...2V Altcode: 2018arXiv180103686V
Aims: Combining high-resolution spectropolarimetric and imaging data is key to understanding the decay process of sunspots as it allows us to scrutinize the velocity and magnetic fields of sunspots and their surroundings.
Methods: Active region NOAA 12597 was observed on 2016 September 24 with the 1.5-meter GREGOR solar telescope using high-spatial-resolution imaging as well as imaging spectroscopy and near-infrared (NIR) spectropolarimetry. Horizontal proper motions were estimated with local correlation tracking, whereas line-of-sight (LOS) velocities were computed with spectral line fitting methods. The magnetic field properties were inferred with the "Stokes Inversions based on Response functions" (SIR) code for the Si I and Ca I NIR lines.
Results: At the time of the GREGOR observations, the leading sunspot had two light bridges indicating the onset of its decay. One of the light bridges disappeared, and an elongated, dark umbral core at its edge appeared in a decaying penumbral sector facing the newly emerging flux. The flow and magnetic field properties of this penumbral sector exhibited weak Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap adjacent to the elongated umbral core and the penumbra in that penumbral sector displayed LOS velocities similar to granulation. The separating polarities of a new flux system interacted with the leading and central part of the already established active region. As a consequence, the leading spot rotated 55° clockwise over 12 h.
Conclusions: In the high-resolution observations of a decaying sunspot, the penumbral filaments facing the flux emergence site contained a darkened area resembling an umbral core filled with umbral dots. This umbral core had velocity and magnetic field properties similar to the sunspot umbra. This implies that the horizontal magnetic fields in the decaying penumbra became vertical as observed in flare-induced rapid penumbral decay, but on a very different time-scale. Title: The magnetic nature of umbra-penumbra boundary in sunspots Authors: Jurčák, J.; Rezaei, R.; González, N. Bello; Schlichenmaier, R.; Vomlel, J. Bibcode: 2018A&A...611L...4J Altcode: 2018arXiv180108983J Context. Sunspots are the longest-known manifestation of solar activity, and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold. Aim. Here, we aim at studying the magnetic nature of umbra-penumbra boundaries in sunspots of different sizes, morphologies, evolutionary stages, and phases of the solar cycle.
Methods: We used a sample of 88 scans of the Hinode/SOT spectropolarimeter to infer the magnetic field properties in at the umbral boundaries. We defined these umbra-penumbra boundaries by an intensity threshold and performed a statistical analysis of the magnetic field properties on these boundaries.
Results: We statistically prove that the umbra-penumbra boundary in stable sunspots is characterised by an invariant value of the vertical magnetic field component: the vertical component of the magnetic field strength does not depend on the umbra size, its morphology, and phase of the solar cycle. With the statistical Bayesian inference, we find that the strength of the vertical magnetic field component is, with a likelihood of 99%, in the range of 1849-1885 G with the most probable value of 1867 G. In contrast, the magnetic field strength and inclination averaged along individual boundaries are found to be dependent on the umbral size: the larger the umbra, the stronger and more horizontal the magnetic field at its boundary.
Conclusions: The umbra and penumbra of sunspots are separated by a boundary that has hitherto been defined by an intensity threshold. We now unveil the empirical law of the magnetic nature of the umbra-penumbra boundary in stable sunspots: it is an invariant vertical component of the magnetic field. Title: High-resolution Observations of Hα Spectra with a Subtractive Double Pass Authors: Beck, C.; Rezaei, R.; Choudhary, D. P.; Gosain, S.; Tritschler, A.; Louis, R. E. Bibcode: 2018SoPh..293...36B Altcode: 2017arXiv171207077B High-resolution imaging spectroscopy in solar physics has relied on Fabry-Pérot interferometers (FPIs) in recent years. FPI systems, however, become technically challenging and expensive for telescopes larger than the 1 m class. A conventional slit spectrograph with a diffraction-limited performance over a large field of view (FOV) can be built at much lower cost and effort. It can be converted into an imaging spectro(polari)meter using the concept of a subtractive double pass (SDP). We demonstrate that an SDP system can reach a similar performance as FPI-based systems with a high spatial and moderate spectral resolution across a FOV of 100″×100″ with a spectral coverage of 1 nm. We use Hα spectra taken with an SDP system at the Dunn Solar Telescope and complementary full-disc data to infer the properties of small-scale superpenumbral filaments. We find that the majority of all filaments end in patches of opposite-polarity fields. The internal fine-structure in the line-core intensity of Hα at spatial scales of about 0.″5 exceeds that in other parameters such as the line width, indicating small-scale opacity effects in a larger-scale structure with common properties. We conclude that SDP systems in combination with (multi-conjugate) adaptive optics are a valid alternative to FPI systems when high spatial resolution and a large FOV are required. They can also reach a cadence that is comparable to that of FPI systems, while providing a much larger spectral range and a simultaneous multi-line capability. Title: Structure of sunspot light bridges in the chromosphere and transition region Authors: Rezaei, R. Bibcode: 2018A&A...609A..73R Altcode: 2017arXiv171110229R Context. Light bridges (LBs) are elongated structures with enhanced intensity embedded in sunspot umbra and pores.
Aims: We studied the properties of a sample of 60 LBs observed with the Interface Region Imaging Spectrograph (IRIS).
Methods: Using IRIS near- and far-ultraviolet spectra, we measured the line intensity, width, and Doppler shift; followed traces of LBs in the chromosphere and transition region (TR); and compared LB parameters with umbra and quiet Sun.
Results: There is a systematic emission enhancement in LBs compared to nearby umbra from the photosphere up to the TR. Light bridges are systematically displaced toward the solar limb at higher layers: the amount of the displacement at one solar radius compares well with the typical height of the chromosphere and TR. The intensity of the LB sample compared to the umbra sample peaks at the middle/upper chromosphere where they are almost permanently bright. Spectral lines emerging from the LBs are broader than the nearby umbra. The systematic redshift of the Si IV line in the LB sample is reduced compared to the quiet Sun sample. We found a significant correlation between the line width of ions arising at temperatures from 3 × 104 to 1.5 × 105 K as there is also a strong spatial correlation among the line and continuum intensities. In addition, the intensity-line width relation holds for all spectral lines in this study. The correlations indicate that the cool and hot plasma in LBs are coupled.
Conclusions: Light bridges comprise multi-temperature and multi-disciplinary structures extending up to the TR. Diverse heating sources supply the energy and momentum to different layers, resulting in distinct dynamics in the photosphere, chromosphere, and TR. Title: The Temperature - Magnetic Field Relation in Observed and Simulated Sunspots Authors: Sobotka, Michal; Rezaei, Reza Bibcode: 2017SoPh..292..188S Altcode: 2017arXiv171109821S Observations of the relation between continuum intensity and magnetic field strength in sunspots have been made for nearly five decades. This work presents full-Stokes measurements of the full-split (g =3 ) line Fe I 1564.85 nm with a spatial resolution of 0.5″ obtained with the GREGOR Infrared Spectrograph in three large sunspots. The continuum intensity is corrected for instrumental scattered light, and the brightness temperature is calculated. Magnetic field strength and inclination are derived directly from the line split and the ratio of Stokes components. The continuum intensity (temperature) relations to the field strength are studied separately in the umbra, light bridges, and penumbra. The results are consistent with previous studies, and it was found that the scatter of values in the relations increases with increasing spatial resolution thanks to resolved fine structures. The observed relations show trends common for the umbra, light bridges, and the inner penumbra, while the outer penumbra has a weaker magnetic field than the inner penumbra at equal continuum intensities. This fact can be interpreted in terms of the interlocking comb magnetic structure of the penumbra. A comparison with data obtained from numerical simulations was made. The simulated data generally have a stronger magnetic field and a weaker continuum intensity than the observations, which may be explained by stray light and limited spatial resolution of the observations, and also by photometric inaccuracies of the simulations. Title: Chromospheric impact of an exploding solar granule Authors: Fischer, C. E.; Bello González, N.; Rezaei, R. Bibcode: 2017A&A...602L..12F Altcode: 2017arXiv170600770F Context. Observations of multi-wavelength and therefore height-dependent information following events throughout the solar atmosphere and unambiguously assigning a relation between these rapidly evolving layers are rare and difficult to obtain. Yet, they are crucial for our understanding of the physical processes that couple the different regimes in the solar atmosphere.
Aims: We characterize the exploding granule event with simultaneous observations of Hinode spectroplarimetric data in the solar photosphere and Hinode broadband Ca II H images combined with Interface Region Imaging Spectrograph (IRIS) slit spectra. We follow the evolution of an exploding granule and its connectivity throughout the atmosphere and analyze the dynamics of a magnetic element that has been affected by the abnormal granule.
Methods: In addition to magnetic flux maps we use a local correlation tracking method to infer the horizontal velocity flows in the photosphere and apply a wavelet analysis on several IRIS chromospheric emission features such as Mg II k2v and Mg II k3 to detect oscillatory phenomena indicating wave propagation.
Results: During the vigorous expansion of the abnormal granule we detect radially outward horizontal flows, causing, together with the horizontal flows from the surrounding granules, the magnetic elements in the bordering intergranular lanes to be squeezed and elongated. In reaction to the squeezing, we detect a chromospheric intensity and velocity oscillation pulse which we identify as an upward traveling hot shock front propagating clearly through the IRIS spectral line diagnostics of Mg II h&k.
Conclusions: Exploding granules can trigger upward-propagating shock fronts that dissipate in the chromosphere.

Movies associated to Figs. A.1 and A.2 are available in electronic form at http://www.aanda.org Title: The Polarization Signature of Photospheric Magnetic Fields in 3D MHD Simulations and Observations at Disk Center Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G. Bibcode: 2017ApJ...842...37B Altcode: 2017arXiv170506812B Before using three-dimensional (3D) magnetohydrodynamical (MHD) simulations of the solar photosphere in the determination of elemental abundances, one has to ensure that the correct amount of magnetic flux is present in the simulations. The presence of magnetic flux modifies the thermal structure of the solar photosphere, which affects abundance determinations and the solar spectral irradiance. The amount of magnetic flux in the solar photosphere also constrains any possible heating in the outer solar atmosphere through magnetic reconnection. We compare the polarization signals in disk-center observations of the solar photosphere in quiet-Sun regions with those in Stokes spectra computed on the basis of 3D MHD simulations having average magnetic flux densities of about 20, 56, 112, and 224 G. This approach allows us to find the simulation run that best matches the observations. The observations were taken with the Hinode SpectroPolarimeter (SP), the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow Spectrograph (POLIS), and the GREGOR Fabry-Pèrot Interferometer (GFPI), respectively. We determine characteristic quantities of full Stokes profiles in a few photospheric spectral lines in the visible (630 nm) and near-infrared (1083 and 1565 nm). We find that the appearance of abnormal granulation in intensity maps of degraded simulations can be traced back to an initially regular granulation pattern with numerous bright points in the intergranular lanes before the spatial degradation. The linear polarization signals in the simulations are almost exclusively related to canopies of strong magnetic flux concentrations and not to transient events of magnetic flux emergence. We find that the average vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet-Sun internetwork. A value of about 35 G gives the best match across the SP, TIP, POLIS, and GFPI observations. Title: A distinct magnetic property of the inner penumbral boundary. II. Formation of a penumbra at the expense of a pore Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.; Rezaei, R. Bibcode: 2017A&A...597A..60J Altcode: 2016arXiv161201745J; 2016A&A...597A..60J Context. We recently presented evidence that stable umbra-penumbra boundaries are characterised by a distinct canonical value of the vertical component of the magnetic field, Bstablever. In order to trigger the formation of a penumbra, large inclinations in the magnetic field are necessary. In sunspots, the penumbra develops and establishes by colonising both umbral areas and granulation, that is, penumbral magneto-convection takes over in umbral regions with Bver<Bstablever, as well as in granular convective areas. Eventually, a stable umbra-penumbra boundary settles at Bstablever.
Aims: Here, we aim to study the development of a penumbra initiated at the boundary of a pore, where the penumbra colonises the entire pore ultimately.
Methods: We have used Hinode/SOT G-band images to study the evolution of the penumbra. Hinode/SOT spectropolarimetric data were used to infer the magnetic field properties in the studied region.
Results: The penumbra forms at the boundary of a pore located close to the polarity inversion line of NOAA 10960. As the penumbral bright grains protrude into the pore, the magnetic flux in the forming penumbra increases at the expense of the pore magnetic flux. Consequently, the pore disappears completely giving rise to an orphan penumbra. At all times, the vertical component of the magnetic field in the pore is smaller than Bstablever ≈ 1.8 kG.
Conclusions: Our findings are in an agreement with the need of Bstablever for establishing a stable umbra-penumbra boundary: while Bver in the pore is smaller than Bstablever, the protrusion of penumbral grains into the pore area is not blocked, a stable pore-penumbra boundary does not establish, and the pore is fully overtaken by the penumbral magneto-convective mode. This scenario could also be one of the mechanisms giving rise to orphan penumbrae.

The movie associated to Fig. 1 is available at http://www.aanda.org Title: MOSiC: an analysis tool for IRIS spectral data Authors: Rezaei, Reza Bibcode: 2017arXiv170104421R Altcode: This is a manual for the MOSiC package. MOSiC is a collection of IDL programs for profile analysis and Gaussian fitting of the Mg II h/k lines along with Gaussian fitting of the C II 133.5 nm line pair, the O I 135.6, the Cl I 135.2, the Si IV 139.7 and 140.3 and the O IV 140.0 nm lines observed with the IRIS near UV and far UV spectrograph. It was tested by analyzing over a hundred different IRIS data sets (quiet Sun, sunspot, ...). It works for off limb data, although it is still experimental. MOSiC analyzes different spectral lines separately and returns line intensity, width, and velocity for each line. A few sample profiles and maps are included in this manual. Title: Canonical Bver value on umbra/penumbra boundaries Authors: Jurcak, Jan; Bello González, Nazaret; Schlichenmaier, Rolf; Rezaei, Reza Bibcode: 2017psio.confE.112J Altcode: No abstract at ADS Title: Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.; Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.; Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe, O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.; Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H. Bibcode: 2016A&A...596A...2B Altcode: 2016arXiv160708165B Context. Some models for the topology of the magnetic field in sunspot penumbrae predict regions free of magnetic fields or with only dynamically weak fields in the deep photosphere.
Aims: We aim to confirm or refute the existence of weak-field regions in the deepest photospheric layers of the penumbra.
Methods: We investigated the magnetic field at log τ5 = 0 is by inverting spectropolarimetric data of two different sunspots located very close to disk center with a spatial resolution of approximately 0.4-0.45''. The data have been recorded using the GRIS instrument attached to the 1.5-m solar telescope GREGOR at the El Teide observatory. The data include three Fe I lines around 1565 nm, whose sensitivity to the magnetic field peaks half a pressure scale height deeper than the sensitivity of the widely used Fe I spectral line pair at 630 nm. Before the inversion, the data were corrected for the effects of scattered light using a deconvolution method with several point spread functions.
Results: At log τ5 = 0 we find no evidence of regions with dynamically weak (B< 500 Gauss) magnetic fields in sunspot penumbrae. This result is much more reliable than previous investigations made on Fe I lines at 630 nm. Moreover, the result is independent of the number of nodes employed in the inversion, is independent of the point spread function used to deconvolve the data, and does not depend on the amount of stray light (I.e., wide-angle scattered light) considered. Title: Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.; Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.; Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1050B Altcode: 2016arXiv160901514B Arch filament systems occur in active sunspot groups, where a fibril structure connects areas of opposite magnetic polarity, in contrast to active region filaments that follow the polarity inversion line. We used the GREGOR Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0 nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line to investigate the photospheric magnetic field and velocities, and use the line core intensities and velocities of the helium line to study the chromospheric plasma. The individual fibrils of the arch filament system connect the sunspot with patches of magnetic polarity opposite to that of the spot. These patches do not necessarily coincide with pores, where the magnetic field is strongest. Instead, areas are preferred not far from the polarity inversion line. These areas exhibit photospheric downflows of moderate velocity, but significantly higher downflows of up to 30 km s-1 in the chromospheric helium line. Our findings can be explained with new emerging flux where the matter flows downward along the field lines of rising flux tubes, in agreement with earlier results. Title: Magnetic fields of opposite polarity in sunspot penumbrae Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.; Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld, T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.; Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar, A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier, K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M. Bibcode: 2016A&A...596A...4F Altcode: 2016arXiv160800513F Context. A significant part of the penumbral magnetic field returns below the surface in the very deep photosphere. For lines in the visible, a large portion of this return field can only be detected indirectly by studying its imprints on strongly asymmetric and three-lobed Stokes V profiles. Infrared lines probe a narrow layer in the very deep photosphere, providing the possibility of directly measuring the orientation of magnetic fields close to the solar surface.
Aims: We study the topology of the penumbral magnetic field in the lower photosphere, focusing on regions where it returns below the surface.
Methods: We analyzed 71 spectropolarimetric datasets from Hinode and from the GREGOR infrared spectrograph. We inferred the quality and polarimetric accuracy of the infrared data after applying several reduction steps. Techniques of spectral inversion and forward synthesis were used to test the detection algorithm. We compared the morphology and the fractional penumbral area covered by reversed-polarity and three-lobed Stokes V profiles for sunspots at disk center. We determined the amount of reversed-polarity and three-lobed Stokes V profiles in visible and infrared data of sunspots at various heliocentric angles. From the results, we computed center-to-limb variation curves, which were interpreted in the context of existing penumbral models.
Results: Observations in visible and near-infrared spectral lines yield a significant difference in the penumbral area covered by magnetic fields of opposite polarity. In the infrared, the number of reversed-polarity Stokes V profiles is smaller by a factor of two than in the visible. For three-lobed Stokes V profiles the numbers differ by up to an order of magnitude. Title: Upper chromospheric magnetic field of a sunspot penumbra: observations of fine structure Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados, M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar, H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.; Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A...8J Altcode: 2016arXiv160801988J
Aims: The fine-structure of the magnetic field in a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere.
Methods: Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic field.
Results: We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°-15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed penumbra does not vary on small spatial scales in the upper chromosphere.
Conclusions: Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate penumbral chromospheric dynamic phenomena, such as penumbral micro-jets or transient bright dots. Title: Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González, M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.; Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.; Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.; Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier, K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.; Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.; Schmidt, D.; Kiess, C.; Nicklas, H. Bibcode: 2016A&A...596A...6L Altcode: 2016arXiv160506324L Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector.
Aims: The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account.
Methods: Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against realistic magneto-hydrodynamic simulations (MURaM).
Results: About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields.
Conclusions: The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4 are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data. Title: Flow and magnetic field properties in the trailing sunspots of active region NOAA 12396 Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer, C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados, M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016AN....337.1090V Altcode: Improved measurements of the photospheric and chromospheric three-dimensional magnetic and flow fields are crucial for a precise determination of the origin and evolution of active regions. We present an illustrative sample of multi-instrument data acquired during a two-week coordinated observing campaign in August 2015 involving, among others, the GREGOR solar telescope (imaging and near-infrared spectroscopy) and the space missions Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS). The observations focused on the trailing part of active region NOAA 12396 with complex polarity inversion lines and strong intrusions of opposite polarity flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV spectral profiles in the photospheric Si I λ1082.7 nm line, the chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I λ1083.9 nm line. Carefully calibrated GRIS scans of the active region provided maps of Doppler velocity and magnetic field at different atmospheric heights. We compare quick-look maps with those obtained with the ``Stokes Inversions based on Response functions'' (SIR) code, which furnishes deeper insight into the magnetic properties of the region. We find supporting evidence that newly emerging flux and intruding opposite polarity flux are hampering the formation of penumbrae, i.e., a penumbra fully surrounding a sunspot is only expected after cessation of flux emergence in proximity to the sunspots. Title: Three-dimensional structure of a sunspot light bridge Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.; Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T. Bibcode: 2016A&A...596A..59F Altcode: 2016arXiv161104803F Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay.
Methods: We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory.
Results: The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution.

Movies associated to Figs. 2 and 13 are available at http://www.aanda.org Title: Inference of magnetic fields in the very quiet Sun Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.; Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.; Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.; González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.; Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman, T.; Volkmer, R. Bibcode: 2016A&A...596A...5M Altcode: 2018arXiv180410089M Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution.
Aims: We present high-precision spectro-polarimetric data with high spatial resolution (0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR telescope to shed some light on this complex magnetism.
Methods: We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component.
Results: Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak ( 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised. Title: Spectroscopy at the Solar Limb: II. Are Spicules Heated to Coronal Temperatures? Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.; Fabbian, D. Bibcode: 2016SoPh..291.2281B Altcode: 2016arXiv160606132B; 2016SoPh..tmp..132B Spicules of the so-called type II were suggested to be relevant for coronal heating because of their ubiquity on the solar surface and their eventual extension into the corona. We investigate whether solar spicules are heated to transition-region or coronal temperatures and reach coronal heights (≫6 Mm) using multiwavelength observations of limb spicules in different chromospheric spectral lines (Ca II H, Hε , Hα , Ca II IR at 854.2 nm, He I at 1083 nm) taken with slit spectrographs and imaging spectrometers. We determine the line width of spectrally resolved line profiles in individual spicules and throughout the field of view, and estimate the maximal height that different types of off-limb features reach. We derive estimates of the kinetic temperature and the non-thermal velocity from the line width of spectral lines from different chemical elements. We find that most regular, i.e. thin and elongated, spicules reach a height of at most about 6 Mm above the solar limb. The majority of features found at larger heights are irregularly shaped with a significantly larger lateral extension, of up to a few Mm, than spicules. Both individual and average line profiles in all spectral lines show a decrease in their line width with height above the limb with very few exceptions. The kinetic temperature and the non-thermal velocity decrease with height above the limb. We find no indications that the spicules in our data reach coronal heights or transition-region or coronal temperatures. Title: Quiet Sun Magnetic Field Evolution Observed with Hinode SOT and IRIS Authors: Fischer, C. E.; Bello González, N.; Rezaei, R. Bibcode: 2016ASPC..504...19F Altcode: We study two physical processes that can be commonly observed in the quiet sun and involve temporal evolution of the magnetic field: convective collapse and flux cancellation. The aim is to investigate the response of the chromosphere to the magnetic events in the photosphere below. We have calibrated and aligned a co-spatial and co-temporal 3 hour quiet sun time series observed with the Hinode SOT (Solar Optical Telescope) and the IRIS (Interface Region Imaging Spectrograph) satellites. Convective collapse events are identified in the photosphere by inverting spectropolarimetric data and searching for magnetic field intensification, preceded by a downflow and accompanied by the development of a bright point in Ca II H images. We find a corresponding downflow in the low chromosphere as deduced from IRIS Mg II k and h spectra and an ensuing oscillatory velocity pattern. We use magnetograms in the high photosphere to study pairs of magnetic elements involved in flux cancellation and find an increase in the entire quasi-continuum of the IRIS Mg II k and h spectrum following the flux cancellation process and indicating a substantial energy deposit into the lower atmosphere. Title: Multiwavelength spectropolarimetric observations of an Ellerman bomb Authors: Rezaei, R.; Beck, C. Bibcode: 2015A&A...582A.104R Altcode: Context. Ellerman bombs (EBs) are enhanced emission in the wings of the Hα line in the solar spectrum.
Aims: We study the structure of an EB in the photosphere and chromosphere.
Methods: We analyze simultaneous observations of four chromospheric lines (Hα, Ca ii H, Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170 nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the Helioseismic and Magnetic Imager (HMI) are used to trace the temporal evolution of the magnetic structure.
Results: We identify the EB by excess emission in the wings of the Hα line, a brightening in the UV continuum, and large emission peaks in the core of the two Ca ii lines. The EB shows a blueshift in all chromospheric lines, while no shifts are observed in the photospheric lines. The blueshift in the chromospheric layer causes very asymmetric emission peaks in the Ca ii H line. The photospheric Si i spectral line shows a shallower line depth at the location of the EB. The UV continuum maps show that the EB was substantially brighter than its surroundings for about 30 min. The continuum contrast of the EB from 170 nm to 1080 nm shows a power-law dependency on the wavelength. The temperature enhancement amounts to 130 K in the low photosphere and 400 K at the temperature minimum level. This temperature excess is also seen in an LTE inversion of the Ca ii spectra. The total thermal and radiative energy content of the EB is about 1020 J and 1018 J in the photosphere and chromosphere, respectively. The HMI data hints at a photospheric magnetic flux cancellation as the driver of the EB.
Conclusions: Ellerman bombs release the energy in a height range of several pressure scale heights around the temperature minimum such that they affect both the photosphere and the lower chromosphere. Title: A distinct magnetic property of the inner penumbral boundary. Formation of a stable umbra-penumbra boundary in a sunspot Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.; Rezaei, R. Bibcode: 2015A&A...580L...1J Altcode: Context. A sunspot emanates from a growing pore or protospot. In order to trigger the formation of a penumbra, large inclinations at the outskirts of the protospot are necessary. The penumbra develops and establishes by colonising both umbral areas and granulation. Evidence for a unique stable boundary value for the vertical component of the magnetic field strength, Bstablever, was found along the umbra-penumbra boundary of developed sunspots.
Aims: We study the changing value of Bver as the penumbra forms and as it reaches a stable state. We compare this with the corresponding value in fully developed penumbrae.
Methods: We use broadband G-band images and spectropolarimetric GFPI/VTT data to study the evolution of and the vertical component of the magnetic field on a forming umbra-penumbra boundary. For comparison with stable sunspots, we also analyse the two maps observed by Hinode/SP on the same spot after the penumbra formed.
Results: The vertical component of the magnetic field, Bver, at the umbra-penumbra boundary increases during penumbra formation owing to the incursion of the penumbra into umbral areas. After 2.5 h, the penumbra reaches a stable state as shown by the GFPI data. At this stable stage, the simultaneous Hinode/SP observations show a Bver value comparable to that of umbra-penumbra boundaries of fully fledged sunspots.
Conclusions: We confirm that the umbra-penumbra boundary, traditionally defined by an intensity threshold, is also characterised by a distinct canonical magnetic property, namely by Bverstable. During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where Bver<Bstablever, the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at Bverstable, which hints toward the role of Bverstable as inhibitor of the penumbral mode of magneto-convection. Title: Variation in sunspot properties between 1999 and 2014 Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt, W.; Collados, M. Bibcode: 2015A&A...578A..43R Altcode:
Aims: We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24.
Methods: We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years.
Results: Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (± 16) G yr-1, while it increases in the rising phase of cycle 24 by about 138 (± 72) G yr-1. The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (± 0.3)% of Ic yr-1 toward the end of cycle 23 and decreases with a rate of 3.8 (± 1.5)% of Ic yr-1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4 G yr-1. If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss.
Conclusions: The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future. Title: A distinct magnetic property of the inner penumbral boundary Authors: Jurčák, Jan; Bello Gonzalez, Nazaret; Schlichenmaier, Rolf; Rezaei, Reza Bibcode: 2015arXiv150608574J Altcode: A sunspot emanates from a growing pore or protospot. In order to trigger the formation of a penumbra, large inclinations at the outskirts of the protospot are necessary. The penumbra develops and establishes by colonising both umbral areas and granulation. Evidence for a unique stable boundary value for the vertical component of the magnetic field strength, $B^{\rm stable}_{\rm ver}$, was found along the umbra-penumbra boundary of developed sunspots. We use broadband G-band images and spectropolarimetric GFPI/VTT data to study the evolution of and the vertical component of the magnetic field on a forming umbra-penumbra boundary. For comparison with stable sunspots, we also analyse the two maps observed by Hinode/SP on the same spot after the penumbra formed. The vertical component of the magnetic field, $B_{\rm ver}$, at the umbra-penumbra boundary increases during penumbra formation owing to the incursion of the penumbra into umbral areas. After 2.5 hours, the penumbra reaches a stable state as shown by the GFPI data. At this stable stage, the simultaneous Hinode/SP observations show a $B_{\rm ver}$ value comparable to that of umbra-penumbra boundaries of fully fledged sunspots. We confirm that the umbra-penumbra boundary, traditionally defined by an intensity threshold, is also characterised by a distinct canonical magnetic property, namely by $B^{\rm stable}_{\rm ver}$. During the penumbra formation process, the inner penumbra extends into regions where the umbra previously prevailed. Hence, in areas where $B_{\rm ver} < B^{\rm stable}_{\rm ver}$, the magneto-convection mode operating in the umbra turns into a penumbral mode. Eventually, the inner penumbra boundary settles at $B^{\rm stable}_{\rm ver}$, which hints toward the role of $B_{\rm ver}^{\rm stable}$ as inhibitor of the penumbral mode of magneto-convection. Title: Three Dimensional Chromospheric Thermal Structure of Sunspot Authors: Prasad Choudhary, Debi; Beck, Christian; Rezaei, R. Bibcode: 2015TESS....131201P Altcode: We have observed sunspots using the Spectropolarimeter for infrared and optical wavelength ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the results on the 3D thermal structure in the super-penumbral canopy of a well rounded sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. Title: Fast Inversion of Solar Ca II Spectra Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.; Louis, R. E. Bibcode: 2015ApJ...798..100B Altcode: 2014arXiv1410.8451B We present a fast (Lt1 s per profile) inversion code for solar Ca II lines. The code uses an archive of spectra that are synthesized prior to the inversion under the assumption of local thermodynamic equilibrium (LTE). We show that it can be successfully applied to spectrograph data or more sparsely sampled spectra from two-dimensional spectrometers. From a comparison to a non-LTE inversion of the same set of spectra, we derive a first-order non-LTE correction to the temperature stratifications derived in the LTE approach. The correction factor is close to unity up to log τ ~ -3 and increases to values of 2.5 and 4 at log τ = -6 in the quiet Sun and the umbra, respectively. Title: Evolution of magnetic field inclination in a forming penumbra Authors: Jurčák, Jan; Bello González, Nazaret; Schlichenmaier, Rolf; Rezaei, Reza Bibcode: 2014PASJ...66S...3J Altcode: 2014PASJ..tmp...93J As a sunspot penumbra forms, the magnetic field vector at the outer boundary of the protospot undergoes a transformation. We study the changes of the magnetic field vector at this boundary as a penumbral segment forms. We analyze a set of spectropolarimetric maps covering 2 hr during the formation of a sunspot in NOAA 11024. The data were recorded with the GFPI instrument attached to the German VTT. We observe a stationary umbra/quiet Sun boundary, where the magnetic field becomes more horizontal with time. The magnetic field inclination increases by 5°, reaching a maximum value of about 59°. The maximum inclination coincides with the onset of filament formation. In time, the penumbra filaments become longer and the penumbral bright grains protrude into the umbra, where the magnetic field is stronger and more vertical. Consequently, we observe a decrease in the magnetic field inclination at the boundary as the penumbra grows. In summary, in order to initiate the formation of the penumbra, the magnetic field at the umbral (protospot) boundary becomes more inclined. As the penumbra grows, the umbra/penumbra boundary migrates inwards, and at this boundary the magnetic field turns more vertical again, while it remains inclined in the outer penumbra. Title: Comparison of inversion codes for polarized line formation in MHD simulations. I. Milne-Eddington codes Authors: Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M. Bibcode: 2014A&A...572A..54B Altcode: 2014arXiv1409.3376B Milne-Eddington (M-E) inversion codes for the radiative transfer equation are the most widely used tools to infer the magnetic field from observations of the polarization signals in photospheric and chromospheric spectral lines. Unfortunately, a comprehensive comparison between the different M-E codes available to the solar physics community is still missing, and so is a physical interpretation of their inferences. In this contribution we offer a comparison between three of those codes (VFISV, ASP/HAO, and HeLIx+). These codes are used to invert synthetic Stokes profiles that were previously obtained from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD) simulations. The results of the inversion are compared with each other and with those from the MHD simulations. In the first case, the M-E codes retrieve values for the magnetic field strength, inclination and line-of-sight velocity that agree with each other within σB ≤ 35 (Gauss), σγ ≤ 1.2°, and σv ≤ 10 m s-1, respectively. Additionally, M-E inversion codes agree with the numerical simulations, when compared at a fixed optical depth, within σB ≤ 130 (Gauss), σγ ≤ 5°, and σv ≤ 320 m s-1. Finally, we show that employing generalized response functions to determine the height at which M-E codes measure physical parameters is more meaningful than comparing at a fixed geometrical height or optical depth. In this case the differences between M-E inferences and the 3D MHD simulations decrease to σB ≤ 90 (Gauss), σγ ≤ 3°, and σv ≤ 90 m s-1. Title: Three Dimensional Chromospheric Temperature Structure of Sunspot Authors: Choudhary, D. P.; Beck, C.; Rezaei, R. Bibcode: 2014AGUFMSH41B4132C Altcode: We have observed sunspots using the Spectropolarimeter for infrared and optical wavelength ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the results on the 3D thermal structure in the super-penumbral canopy of a well rounded sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. Title: A Three-dimensional View of the Thermal Structure in a Super-penumbral Canopy Authors: Beck, C.; Choudhary, D. P.; Rezaei, R. Bibcode: 2014ApJ...788..183B Altcode: 2014arXiv1405.1473B We investigate the three-dimensional (3D) thermal topology in a super-penumbral canopy of an active region (AR). We derive temperature stratifications in the AR by an inversion of the Ca II IR line at 854.2 nm, assuming local thermal equilibrium. We find that about half of the radially oriented fibrils in the super-penumbral canopy form short, low-lying (h < 1 Mm) loops in the 3D temperature cube. These closed loops connect from bright grains in or close to the penumbra to the photosphere a few Mms away from the sunspot. The other half of the fibrils monotonically rise with distance from the sunspot. Many of the fibrils show a central dark core and two lateral brightenings in line-core intensity images. The corresponding velocity image shows fibrils that are as wide as the fibrils seen in intensity without a lateral substructure. Additionally, we study a feature from a different class of structures without prominent mass flows. Its 3D topology is formed by two parallel, closed loops that connect patches of opposite polarity. We present evidence that the inverse Evershed flow into the sunspot in the lower chromosphere is the consequence of siphon flows along short loops that connect photospheric foot points. The dark-cored structure of the chromospheric fibrils cannot have a convective origin because of their location above regular granulation. The dark core most likely results from an opacity difference between the central axis and the lateral edges caused by the significant flow speed along the fibrils. Title: Properties of sunspot umbrae observed in cycle 24 Authors: Kiess, Christoph; Rezaei, Reza; Schmidt, Wolfgang Bibcode: 2014A&A...565A..52K Altcode: 2014arXiv1402.2881K
Aims: There is an ongoing debate whether the solar activity cycle is overlaid with a long-term decline that may lead to another grand minimum in the near future. We used the size, intensity, and magnetic field strength of sunspot umbrae to compare the present cycle 24 with the previous one.
Methods: We used data of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and selected all sunspots between May 2010 and October 2012, using one image per day. We created two subsets of this dataset with a manual tracking algorithm, both without duplication. One contains each sunspot (910 umbrae within 488 spots) and was used to analyze the distribution of umbral areas, selected with an automated thresholding method. The other subset contains 205 fully evolved sunspots. We estimated their magnetic field and the total magnetic flux and discuss the relations between umbral size, minimum continuum intensity, maximum field strength, and total magnetic flux.
Results: We find non-linear relations between umbral minimum intensity and size and between maximum magnetic field strength and size. The field strength scales linearly with the intensity and the umbral size scales roughly linearly with the total magnetic flux, while the size and field strength level off with stronger flux. When separated into hemispheres and averaged temporally, the southern umbrae show a temporal increase in size and the northern umbrae remain constant. We detected no temporal variation in the umbral mean intensity. The probability density function of the umbral area in the ascending phase of the current solar cycle is similar to that of the last solar cycle.
Conclusions: From our investigation of umbral area, magnetic field, magnetic flux, and umbral intensity of the sunspots of the rising phase of cycle 24, we do not find a significant difference to the previous cycle, and hence no indication for a long-term decline of solar activity. Title: Comparison of sunspot properties in cycles 23 and 24 Authors: Rezaei, Reza; Schmidt, Wolfgang; Beck, Christian Bibcode: 2014cosp...40E2740R Altcode: Sunspots form by coalescence of small-scale magnetic elements and pores in magnetic flux emergence areas. By observing recently formed sunspots just after their initial growth and before substantial decay, one samples a magnetic signal which has been least disturbed by granulation. Properties of the emergence events have a direct impact on the results. Failed active regions, e.g. the ones which cannot form a sunspot, are a clear example: in several cases, they would harbor enough magnetic flux to form a small sunspot but fail to do so. Another way to evaluate secular variations of flux emergence events is to quantify long-term trends of sunspot properties. The 11-year solar magnetic activity cycle has been known for centuries. During this time the activity level changed dramatically from the Maunder minimum (1650-1700) to the Modern maximum in mid 20-th century. The extended minimum of the last solar cycle alerted solar physicist about possible long-term variation in the solar magnetic activity. While some argue that the Sun was unusually active in mid 20-th century, others find it unusually inactive now. This caused speculations whether the solar activity cycle is overlaid with a long-term decline that may lead to another grand minimum in the near future. Some extrapolations predicted that there will be no sunspots in the next cycle. Detailed observations of sunspot properties were performed only in the last few cycles. Such spectropolarimetric observations enable us to accurately derive the magnetic field strengths of spots and their physical properties. We present measurements of sunspot intensity, area, and magnetic field strength and compare the present cycle 24 with the previous one. We analyze a sample of about 400 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope as well as with the Facility Infrared Spectropolarimeter of the Dunn Solar Telescope of the NSO. The magnetic field strength is derived from the Zeeman splitting of the Stokes-V signal in a near-infrared spectral line. We take into account the center-to-limb variation of umbral intensities and apply a correction to compensate the variation of magnetic sensitivity of different spectral lines. There is a systematic trend for sunspots in the late stage of the solar cycle to have a smaller maximum magnetic field strength than those at the start of the cycle. At the same time, the continuum intensity of umbrae gradually increases with phase of the solar cycle, while the umbral area does not show any trend above the statistical variance. Sunspots in cycle 24 show higher field strengths and lower continuum intensities than those at the end of cycle 23. There is a slight decrease in field strength and an increase in intensity as a long-term trend across the cycles. We find that the cyclic variations are dominating over any long-term trend that continues across cycles. From our investigation of umbral area, magnetic field, and umbral intensity of the sunspots of cycle 23 and the first half of cycle 24, we do not find a significant indication of variation in either sunspot physical properties. This indicates that if a long-term trend exists, its amplitude is smaller than the cyclic variation of umbral properties. Title: Three Dimensional Chromospheric Structure of Sunspot Authors: Choudhary, Debi Prasad; Rezaei, Reza; Beck, Christian Bibcode: 2014cosp...40E.544C Altcode: We have observed sunspots using the Spectropolarimeter for infrared and optical wavelength ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the first results on the 3D thermal structure in the super-penumbral canopy of a well rounded sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations (Corrigendum) Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...559C...1B Altcode: No abstract at ADS Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...557A.109B Altcode: 2013arXiv1306.6093B Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest.
Aims: We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.
Methods: We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity Ic. We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra.
Results: The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity vmac below 10 m s-1 for the HD simulation spectra at full spatial resolution, whereas they yield vmac ≲ 1000 m s-1 at a spatial resolution of 0.″3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100-200 K rms for -2 ⪉ log τ500 nm ⪉ -0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0.″13 and 0.″3.
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0.″13).

Appendices A and B are available in electronic form at http://www.aanda.org Title: Can spicules be detected at disc centre in broad-band Ca ii H filter imaging data? Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2013A&A...556A.127B Altcode: 2013arXiv1306.5199B Context. Recently, a possible identification of type II spicules in broad-band (full-width at half-maximum (FWHM) of ~0.3 nm) filter imaging data in Ca ii H on the solar disc was reported.
Aims: We estimate the formation height range contributing to broad-band and narrow-band filter imaging data in Ca ii H to investigate whether spicules can be detected in such observations at the centre of the solar disc.
Methods: We applied spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca ii H line profiles to simulate Ca imaging data. We used observations across the limb to estimate the relative intensity contributions of off-limb and on-disc structures. We compared the synthetic Ca filter imaging data with intensity maps of Ca spectra at different wavelengths and temperature maps at different optical depths obtained by an inversion of these spectra. In addition, we determined the intensity response function for the wavelengths covered by the filters of different FWHM.
Results: In broad-band (FWHM = 0.3 nm) Ca imaging data, the intensity emitted off the solar limb is about 5% of the intensity at disc centre. For a 0.3-nm-wide filter centred at the Ca ii H line core, up to about one third of the off-limb intensity comes from emission in Hɛ. On the disc, only about 10 to 15% of the intensity transmitted through a broad-band filter comes from the line-core region between the H1 minima (396.824 to 396.874 nm). No traces of elongated fibrillar structures are visible in the synthetic Ca broad-band imaging data at disc centre, in contrast to the line-core images of the Ca spectra. The intensity-weighted response function for a 0.3-nm-wide filter centred at the Ca ii H line core peaks at about log τ ~ -2 (z ~ 200 km). Relative contributions from atmospheric layers above 800 km are about 10%. The inversion results suggest that the slightly enhanced emission around the photospheric magnetic network in broad-band Ca imaging data is caused by a thermal canopy at a height of about 600 km.
Conclusions: Broad-band (~0.3 nm) Ca ii H imaging data do not trace upper chromospheric structures such as spicules in observations at the solar disc because of the too small relative contribution of the line core to the total wavelength-integrated filter intensity. The faint haze around network elements in broad-band Ca imaging observations at disc centre presumably traces thermal canopies in the vicinity of magnetic flux concentrations instead.

Appendix A is available in electronic form at http://www.aanda.org Title: The energy of waves in the photosphere and lower chromosphere. IV. Inversion results of Ca II H spectra Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2013A&A...553A..73B Altcode: 2013arXiv1302.6936B Context. Most semi-empirical static one-dimensional (1D) models of the solar atmosphere in the magnetically quiet Sun (QS) predict an increase in temperature at chromospheric layers. Numerical simulations of the solar chromosphere with a variable degree of sophistication, i.e. from 1D to three-dimensional (3D) simulations; assuming local thermal equilibrium (LTE) or non-LTE (NLTE), on the other hand, only yielded an increase in the brightness temperature without any stationary increase in the gas temperature.
Aims: We investigate the thermal structure in the solar chromosphere as derived from an LTE inversion of observed Ca ii H spectra in QS and active regions (ARs).
Methods: We applied an inversion strategy based on the SIR (Stokes inversion by response functions) code to Ca ii H spectra to obtain 1D temperature stratifications. We investigated the temperature stratifications on differences between magnetic and field-free regions in the QS, and on differences between QS and ARs. We determined the energy content of individual calcium bright grains (BGs) as specific candidates of chromospheric heating events. We compared observed with synthetic NLTE spectra to estimate the significance of the LTE inversion results.
Results: The fluctuations of observed intensities yield a variable temperature structure with spatio-temporal rms fluctuations below 100 K in the photosphere and between 200 and 300 K in the QS chromosphere. The average temperature stratification in the QS does not exhibit a clear chromospheric temperature rise, unlike the AR case. We find a characteristic energy content of about 7 × 1018 J for BGs that repeat with a cadence of about 160 s. The precursors of BGs have a vertical extent of about 200 km and a horizontal extent of about 1 Mm. The comparison of observed with synthetic NLTE profiles partly confirms the results of the LTE inversion that the solar chromosphere in the QS oscillates between an atmosphere in radiative equilibrium and one with a moderate chromospheric temperature rise. Two-dimensional x - z temperature maps exhibit nearly horizontal canopy-like structures with an extent of a few Mm around photospheric magnetic field concentrations at a height of about 600 km.
Conclusions: The large difference between QS regions and ARs and the better match of AR and NLTE reference spectra suggest that magnetic heating processes are more important than commonly assumed. The temperature fluctuations in QS derived by the LTE inversion do not suffice on average to maintain a stationary chromospheric temperature rise. The spatially and vertically resolved information on the temperature structure allows one to investigate in detail the topology and evolution of the thermal structure in the lower solar atmosphere.

Appendix A is available in electronic form at http://www.aanda.org Title: On the Effects of the SDO Orbital Motion on the HMI Vector Magnetic Field Measurements Authors: Fleck, B.; Centeno, R.; Cheung, M.; Couvidat, S.; Hayashi, K.; Rezaei, R.; Steiner, O.; Straus, T. Bibcode: 2013enss.confE.145F Altcode: In a previous study we have investigated the magnetic field diagnostics potential of SDO/HMI. We have used the output of high-resolution 3D, time-dependent, radiative magneto-hydrodynamics simulations to calculate Stokes profiles for the Fe I 6173 Å line. From these we constructed Stokes filtergrams using a representative set of HMI filter response functions. The magnetic field vector (x,y) and line-of-sight Doppler velocities V(x,y) were determined from these filtergrams using a simplified version of the HMI magnetic field processing pipeline, and the reconstructed magnetic field (x,y) and line-of-sight velocity V(x,y) were compared to the actual magnetic field (x,y,z) and vertical velocity V0(x,y,z) in the simulations. The present investigation expands this analysis to include the effects of the significant orbital motions of SDO, which, given the limited wavelength range of the HMI filter profiles, affects the outer wing measurements and therefore might impact the magnetic field measurements. We find that the effects of the orbital movement of SDO are noticeable, in particular for the strongest fields (B > 3 kG) and the maximum wavelength shift of 5.5 km/s (3.5 km/s orbital movement + 2 km/s solar rotation). Saturation effects for strong fields (B > 3 kG) are already visible for wavelength shifts of 3.2 km/s (orbital movement, disk center). The measurements of inclination and vertical velocity are more robust. Compared to other factors of uncertainty in the inversion of HMI Stokes measurements the orbital movement is not a major concern or source of error. Title: The energy of waves in the photosphere and lower chromosphere. III. Inversion setup for Ca II H spectra in local thermal equilibrium Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2013A&A...549A..24B Altcode: 2012arXiv1209.6194B Context. The Ca II H line is one of the strongest lines in the solar spectrum, and it provides continuous information on the solar atmosphere from the photosphere to the lower chromosphere.
Aims: We describe an inversion approach that reproduces observed Ca II H spectra by assuming local thermal equilibrium (LTE).
Methods: We developed an inversion strategy based on the SIR code that reproduces Ca II H spectra in the LTE approximation. The approach uses a two-step procedure with an archive of pre-calculated spectra to fit the line core and a subsequent iterative modification to improve the fit mainly in the line wing. Simultaneous spectra in the 630 nm range can optionally be used to fix the continuum temperature. The method retrieves one-dimensional (1D) temperature stratifications while neglecting lateral radiative transport. Line-of-sight velocities are included post facto with an empirical approach.
Results: An archive of about 300 000 pre-calculated spectra is more than sufficient to reproduce the line core of observed Ca II H spectra both in the quiet Sun and in active regions. The subsequent iterative adjustment of the thermodynamical stratification matches observed and best-fit spectra to a level of about 0.5% of Ic in the line wing and about 1% of Ic in the line core.
Conclusions: The successful application of the LTE inversion strategy suggests that inversion schemes based on pre-calculated spectra allow a reliable and relatively fast retrieval of solar properties from observed chromospheric spectra. The approach can be easily extended to a 1D non-LTE (NLTE) case by a simple exchange of the pre-calculated archive spectra. Using synthetic NLTE spectra from numerical three-dimensional (3D) simulations instead will finally allow one to extend the approach from the static 1D-case to dynamical atmosphere models, including the complete 3D radiative transport.

The animation is available in electronic form at http://www.aanda.org Title: Chromospheric Multi-Wavelength Observations near the Solar Limb: Techniques and Prospects Authors: Beck, C.; Rezaei, R. Bibcode: 2012ASPC..463..257B Altcode: 2012arXiv1203.2114B Observations of chromospheric spectral lines near and beyond the solar limb provide information on the solar chromosphere without any photospheric contamination. For ground-based observations near and off the limb with real-time image correction by adaptive optics (AO), some technical requirements have to be met, such as an AO lock point that is independent of the location of the field of view observed by the science instruments, both for 1D and 2D instruments. We show how to obtain simultaneous AO-corrected spectra in Ca II H, Hα, Ca II IR at 854 nm, and He I at 1083 nm with the instrumentation at the German Vacuum Tower Telescope in Izaña, Tenerife. We determined the spectral properties of an active-region macrospicule inside the field of view in the four chromospheric lines, including its signature in polarization in He I at 1083 nm. Compared to the line-core intensities, the Doppler shifts of the lines change on a smaller spatial scale in the direction parallel to the limb, suggesting the presence of coherent rotating structures or the passage of upwards propagating helical waves on the surfaces of expanding flux tubes. Title: Comparing Simultaneous Measurements of two High-Resolution Imaging Spectropolarimeters: The `Göttingen' FPI@VTT and CRISP@SST Authors: Bello González, N.; Bellot Rubio, L. R.; Ortiz, A.; Rezaei, R.; Rouppe van der Voort, L.; Schlichenmaier, R. Bibcode: 2012ASPC..463..251B Altcode: 2012arXiv1204.1023B In July 2009, the leading spot of the active region NOAA11024 was observed simultaneously and independently with the ‘Göttingen’ FPI at VTT and CRISP at SST, i.e., at two different sites, telescopes, instruments and using different spectral lines. The data processing and data analysis have been carried out independently with different techniques. Maps of physical parameters retrieved from 2D spectro-polarimetric data observed with ‘Göttingen’ FPI and CRISP show an impressive agreement. In addition, the ‘Göttingen’ FPI maps also exhibit a notable resemblance with simultaneous TIP (spectrographic) observations. The consistency in the results demonstrates the excellent capabilities of these observing facilities. Besides, it confirms the solar origin of the detected signals and the reliability of FPI-based spectro-polarimeters. Title: The energy of waves in the photosphere and lower chromosphere. II. Intensity statistics Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2012A&A...544A..46B Altcode: 2012arXiv1206.1759B Context. The energy source powering the solar chromosphere is still undetermined, but leaves its traces in observed intensities.
Aims: We investigate the statistics of the intensity distributions as a function of the wavelength for Ca ii H and the Ca ii IR line at 854.2 nm to estimate the energy content in the observed intensity fluctuations.
Methods: We derived the intensity variations at different heights of the solar atmosphere, as traced by the line wings and line cores of the two spectral lines. We converted the observed intensities to absolute energy units employing reference profiles calculated in non-local thermal equilibrium (NLTE). We also converted the intensity fluctuations to corresponding brightness temperatures assuming LTE.
Results: The root-mean-square (rms) fluctuations of the emitted intensity are about 0.6 (1.2) W m-2 ster-1 pm-1 near the core of the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative intensity fluctuations of about 20% (30%). For the line wing, we find rms values of about 0.3 W m-2 ster-1 pm-1 for both lines, corresponding to relative fluctuations below 5%. The relative rms values show a local minimum for wavelengths forming at a height of about 130 km, but otherwise increase smoothly from the wing to the core, i.e., from photosphere to chromosphere. The corresponding rms brightness temperature fluctuations are below 100 K for the photosphere and up to 500 K in the chromosphere. The skewness of the intensity distributions is close to zero in the outer line wing and positive throughout the rest of the line spectrum, owing to the frequent occurrence of high-intensity events. The skewness shows a pronounced local maximum at locations with photospheric magnetic fields for wavelengths in-between those of the line wing and the line core (z ≈ 150-300 km), and a global maximum at the very core (z ≈ 1000 km) for both magnetic and field-free locations.
Conclusions: The energy content of the intensity fluctuations is insufficient to create a chromospheric temperature rise that would be similar to the one in most reference models of the solar atmosphere. The increase in the rms fluctuations with height indicates the presence of upwardly propagating acoustic waves of increasing oscillation amplitude. The intensity and temperature variations indicate that there is a clear increase in dynamical activity from photosphere towards the chromosphere, but the variations fall short of the magnitude predicted by fully dynamical chromospheric models by a factor of about five. The enhanced skewness between the photosphere and lower solar chromosphere at magnetic locations is indicative of a mechanism that acts solely on magnetized plasma.

Appendices are available in electronic form at http://www.aanda.org Title: Recent Advances in the Exploration of the Small-Scale Structure of the Quiet Solar Atmosphere: Vortex Flows, the Horizontal Magnetic Field, and the Stokes- V Line-Ratio Method Authors: Steiner, O.; Rezaei, R. Bibcode: 2012ASPC..456....3S Altcode: 2012arXiv1202.4040S We review (i) observations and numerical simulations of vortical flows in the solar atmosphere and (ii) measurements of the horizontal magnetic field in quiet Sun regions. First, we discuss various manifestations of vortical flows and emphasize the role of magnetic fields in mediating swirling motion created near the solar surface to the higher layers of the photosphere and to the chromosphere. We reexamine existing simulation runs of solar surface magnetoconvection with regard to vortical flows and compare to previously obtained results. Second, we reviews contradictory results and problems associated with measuring the angular distribution of the magnetic field in quiet Sun regions. Furthermore, we review the Stokes-V-amplitude ratio method for the lines Fe i λλ 630.15 and 630.25 nm. We come to the conclusion that the recently discovered two distinct populations in scatter plots of this ratio must not bee interpreted in terms of “uncollapsed'' and “collapsed'' fields but stem from weak granular magnetic fields and weak canopy fields located at the boundaries between granules and the intergranular space. Based on new simulation runs, we reaffirm earlier findings of a predominance of the horizontal field components over the vertical one, particularly in the upper photosphere and at the base of the chromosphere. Title: On The Magnetic-Field Diagnostics Potential of SDO/HMI Authors: Fleck, Bernard; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B. Bibcode: 2012AAS...22020701F Altcode: The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe I absorption line at 6173 Å. We use the output of three high-resolution 3D, time-dependent, radiative magneto-hydrodynamics simulations (two based on the MURaM code, one on the CO5BOLD code) to calculate Stokes profiles for the Fe I 6173 Å line for snapshots of a sunspot, a plage area and an enhanced network region. Stokes filtergrams are constructed for the 6 nominal HMI wavelengths by multiplying the Stokes profiles with a representative set of HMI filter response functions. The magnetic field vector B(x,y) and line-of-sight Doppler velocities V(x,y) are determined from these filtergrams using a simplified version of the HMI magnetic field processing pipeline. Finally, the reconstructed magnetic field B(x,y) and line-of-sight velocity V(x,y) are compared to the actual magnetic field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations. Title: Detection of Vortex Tubes in Solar Granulation from Observations SUNRISE Authors: Steiner, O.; Franz, M.; González, N. B.; Nutto, C.; Rezaei, R.; Pillet, V. M.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2012ASPC..455...35S Altcode: We investigated a time series of continuum intensity maps and Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. We conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. This paper is a summary and update of the results previously presented in Steiner et al. (2010). Title: Variation Of Sunspot Properties Between 1999 And 2011 Authors: Schmidt, Wolfgang; Rezaei, R.; Beck, C. Bibcode: 2012AAS...22020608S Altcode: We study the magnetic field and the umbral intensity of sunspots for the period 1999 to 2011. We analyze full Stokes spectra of 183 spots observed with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope on Tenerife. We derive the magnetic field strength in the umbra from the Zeeman splitting of a near-infrared spectral line. This procedure eliminates the influence of non-magnetic stray light from the spot surroundings. The systematic decrease of umbral magnetic field strength observed during the declining phase of cycle 23 does not continue into cycle 24, instead, we observe a significant increase of magnetic field strength in spots of the new cycle. This indicates that the observed variations of the magnetic field strength are dominated by a cyclic effect rather than by a long-term trend. Title: On the Formation of Penumbrae as Observed with the German VTT SOHO/MDI, and SDO/HMI Authors: Schlichenmaier, R.; Rezaei, R.; González, N. B. Bibcode: 2012ASPC..455...61S Altcode: 2011arXiv1102.0965S Solar magnetic fields are generated in the solar interior and pop up at the solar surface to form active regions. As the magnetic field appears on the surface, it forms various structures like small magnetic elements, pores, and sunspots. The nature of these formation processes is largely unknown. In this contribution we elaborate on the formation of sunspots and particularly on the formation of penumbrae. We report on observations that we obtained at the German Vacuum Tower Telescope (VTT) on July 4, 2009 on the formation of the spot in AR 11024. This data set is complemented with data from the Michelson Doppler Imager (MDI) aboard SOHO, which offers an entire time coverage. Moreover, the evolution of AR 11024 is compared with a particular event of penumbra formation in AR 11124 around November 13, 2010, using intensity images from the Helioseismic and Magnetic Imager (HMI) onboard SDO. We conclude that two processes contribute to the increase of the magnetic flux of a sunspot: (1) merging pores, and (2) emerging bipoles of which the spot polarity migrates towards and merges with the spot. As the penumbra forms, the area, magnetic flux, and maximum field strength in the umbra stay constant or increase slightly, i.e., the formation of the penumbra is associated with flux emergence and flux increase of the proto-spot. If two pores merge or if a pore merges with a proto-spot a light bridge is created. This initial light bridge dissolves in the further evolution. Title: Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter Authors: Rezaei, R.; Beck, C.; Schmidt, W. Bibcode: 2012A&A...541A..60R Altcode: 2012arXiv1203.1444R
Aims: We study the variation in the magnetic field strength and the umbral intensity of sunspots during the declining phase of the solar cycle No. 23 and in the beginning of cycle No. 24.
Methods: We analyze a sample of 183 sunspots observed from 1999 until 2011 with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). The magnetic field strength is derived from the Zeeman splitting of the Stokes-V signal in one near-infrared spectral line, either Fe i 1564.8 nm, Fe i 1089.6 nm, or Si i 1082.7 nm. This avoids the effects of the unpolarized stray light from the field-free quiet Sun surroundings that can affect the splitting seen in Stokes-I in the umbra. The minimum umbral continuum intensity and umbral area are also measured.
Results: We find that there is a systematic trend for sunspots in the late stage of the solar cycle No. 23 to be weaker, i.e., to have a smaller maximum magnetic field strength than those at the start of the cycle. The decrease in the field strength with time of about 94 Gyr-1 is well beyond the statistical fluctuations that would be expected because of the larger number of sunspots close to cycle maximum (14 Gyr-1). In the same time interval, the continuum intensity of the umbra increases with a rate of 1.3 (±0.4)% of Ic yr-1, while the umbral area does not show any trend above the statistical variance. Sunspots in the new cycle No. 24 show higher field strengths and lower continuum intensities than those at the end of cycle No. 23, interrupting the trend.
Conclusions: Sunspots have an intrinsically weaker field strength and brighter umbrae at the late stages of solar cycles compared to their initial stages, without any significant change in their area. The abrupt increase in field strength in sunspots of the new cycle suggests that the cyclic variations are dominating over any long-term trend that continues across cycles. We find a slight decrease in field strength and an increase in intensity as a long-term trend across the cycles. Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B. Bibcode: 2012decs.confE.104F Altcode: The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe I absorption line at 6173 Å. We use the output of three high-resolution 3D, time-dependent, radiative magneto-hydrodynamics simulations (two based on the MURaM code, one on the CO5BOLD code) to calculate Stokes profiles Fi(λ,x,y; i=I, V, Q, U) for the Fe I 6173 Å line for snapshots of a sunspot, a plage area and an enhanced network region. Stokes filtergrams are constructed for the 6 nominal HMI wavelengths by multiplying the Stokes profiles with a representative set of HMI filter response functions. The magnetic field vector B(x,y) and line-of-sight Doppler velocities V(x,y) are determined from these filtergrams using a simplified version of the HMI magnetic field processing pipeline. Finally, the reconstructed magnetic field B(x,y) and line-of-sight velocity V(x,y) are compared to the actual magnetic field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations. Title: The formation of sunspot penumbra. Magnetic field properties Authors: Rezaei, R.; Bello González, N.; Schlichenmaier, R. Bibcode: 2012A&A...537A..19R Altcode: 2011arXiv1111.3189R
Aims: We study the magnetic flux emergence and formation of a sunspot penumbra in the active region NOAA 11024.
Methods: We simultaneously observed the Stokes parameters of the photospheric iron lines at 1089.6 nm with the TIP and 617.3 nm with the GFPI spectropolarimeters along with broad-band images using G-band and Ca ii K filters at the German VTT. The photospheric magnetic field vector was reconstructed from an inversion of the measured Stokes profiles. Using the AZAM code, we converted the inclination from line-of-sight (LOS) to the local reference frame (LRF).
Results: Individual filaments are resolved in maps of magnetic parameters. The formation of the penumbra is intimately related to the inclined magnetic field. No penumbra forms in areas with strong magnetic field strength and small inclination. Within 4.5 h observing time, the LRF magnetic flux of the penumbra increases from 9.7 × 1020 to 18.2 × 1020 Mx, while the magnetic flux of the umbra remains constant at ~3.8 × 1020 Mx. Magnetic flux in the immediate surroundings is incorporated into the spot, and new flux is supplied via small flux patches (SFPs), which on average have a flux of 2-3 × 1018 Mx. The spot's flux increase rate of 4.2 × 1016 Mx s-1 corresponds to the merging of one SFP per minute. We also find that, during the formation of the spot penumbra, a) the maximum magnetic field strength of the umbra does not change; b) the magnetic neutral line keeps the same position relative to the umbra; c) the new flux arrives on the emergence side of the spot while the penumbra forms on the opposite side; d) the average LRF inclination of the light bridges decreases from 50° to 37°; and e) as the penumbra develops, the mean magnetic field strength at the spot border decreases from 1.0 to 0.8 kG.
Conclusions: The SFPs associated with elongated granules are the building blocks of structure formation in active regions. During the sunspot formation, their contribution is comparable to the coalescence of pores. Besides a set of critical parameters for the magnetic field, a quiet environment in the surroundings is important for penumbral formation. As remnants of trapped granulation between merging pores, the light bridges are found to play a crucial role in the formation process. They seem to channel the magnetic flux through the spot during its formation. Light bridges are also the locations where the first penumbral filaments form. Title: Stray-light contamination and spatial deconvolution of slit-spectrograph observations Authors: Beck, C.; Rezaei, R.; Fabbian, D. Bibcode: 2011A&A...535A.129B Altcode: 2011arXiv1109.2421B Context. Stray light caused by scattering on optical surfaces and in the Earth's atmosphere degrades the spatial resolution of observations. Whereas post-facto reconstruction techniques are common for 2D imaging and spectroscopy, similar options for slit-spectrograph data are rarely applied.
Aims: We study the contribution of stray light to the two channels of the POlarimetric LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of a slit-spectrograph instrument. We test the performance of different methods of stray-light correction and spatial deconvolution to improve the spatial resolution post-facto.
Methods: We model the stray light as having two components: a spectrally dispersed component and a "parasitic" component of spectrally undispersed light caused by scattering inside the spectrograph. We used several measurements to estimate the two contributions: a) observations with a (partly) blocked field of view (FOV); b) a convolution of the FTS spectral atlas; c) imaging of the spider mounting in the pupil plane; d) umbral profiles; and e) spurious polarization signal in telluric spectral lines. The measurements with a partly blocked FOV in the focal plane allowed us to estimate the spatial point spread function (PSF) of POLIS and the main spectrograph of the German Vacuum Tower Telescope (VTT). We then used the obtained PSF for a deconvolution of both spectroscopic and spectropolarimetric data and investigated the effect on the spectra.
Results: The parasitic contribution can be directly and accurately determined for POLIS, amounting to about 5% (0.3%) of the (continuum) intensity at 396 nm (630 nm). The spectrally dispersed stray light is less accessible because of its many contributing sources. We estimate a lower limit of about 10% across the full FOV for the dispersed stray light from umbral profiles. In quiet Sun regions, the stray-light level from the close surroundings (d < 2'') of a given spatial point is about 20%. The stray light reduces to below 2% at a distance of 20'' from a lit area for both POLIS and the main spectrograph. The spatial deconvolution using the PSF obtained improves the spatial resolution and increases the contrast, with a minor amplification of noise.
Conclusions: A two-component model of the stray-light contributions seems to be sufficient for a basic correction of observed spectra. The instrumental PSF obtained can be used to model the off-limb stray light, to determine the stray-light contamination accurately for observation targets with large spatial intensity gradients such as sunspots, and also to improve the spatial resolution of observations post-facto. Title: VizieR Online Data Catalog: Spectroscopy at the solar limb. I. Average off-limb profiles and Doppler shifts of Ca II H. Authors: Beck, C.; Rezaei, R. Bibcode: 2011yCat..35310173B Altcode: 2011yCat..35319173B The data used in the publication consist of a set of CaII H spectra taken near and beyond the solar limb on 25/08/2009 at UT 08:43-09:15. The data were obtained with the POLIS instrument at the German VTT. The wavelength range covers the core and the blue wing of the CaII H line from 396.332nm to 396.969nm in 326 steps of 1.96pm. The observations were done by moving the solar image across the slit of the spectrograph with a step width of 0.3-arcsec, yielding in total 134 slit spectra of 326 wavelength points on 244 CCD rows along the slit. The spatial sampling along the slit was 0.3-arcsec. The center of the field-of-view was located at (x,y)=(+37",+920") relative to the center of the solar disk. The data have been corrected for stray-light with the methods described in the article and are normalized to the continuum intensity on disc center. The corresponding data file data.fit is organized as floating array (x,y,wavelength)=(134x244x326) pixels.

(1 data file). Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B. Bibcode: 2011sdmi.confE..74F Altcode: The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the magnetic field in the solar photosphere. It observes the full solar disk in the Fe I 6173 absorption line. We use the output of two high-resolution 3D, time-dependent, radiative magneto-hydrodynamics simulations (one based on the MURAM code, the other one on the COBOLD code) to calculate Stokes profiles for the Fe I 6173 line for a snapshot of a plage region and a snapshot of an enhanced network region. After spatially degrading the Stokes profiles to HMI resolution, they are multiplied by a representative set of HMI filter response functions and Stokes filtergrams are constructed for the 6 nominal HMI wavelengths. The magnetic field vector and line-of-sight Doppler velocities are determined from these filtergrams using a simplified version of the HMI magnetic field processing pipeline. Finally, the reconstructed magnetic field is compared to the actual magnetic field in the simulation. Title: The formation of a penumbra as observed with the German VTT and SoHO/MDI Authors: Schlichenmaier, Rolf; González, Nazaret Bello; Rezaei, Reza Bibcode: 2011IAUS..273..134S Altcode: 2010arXiv1009.4457S The generation of magnetic flux in the solar interior and its transport to the outer solar atmosphere will be in the focus of solar physics research for the next decades. One key-ingredient is the process of magnetic flux emergence into the solar photosphere, and the reorganization to form the magnetic phenomena of active regions like sunspots and pores.

On July 4, 2009, we observed a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra within some 4.5 hours. This process is documented by multi-wavelength observations at the German VTT: (a) imaging, (b) data with high resolution and temporal cadence acquired in Fe I 617.3 nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps and magnetograms are used to follow the formation of the proto-spot, and the subsequent evolution of the entire active region.

During the formation of the penumbra, the area and the magnetic flux of the spot increases. The additional magnetic flux is supplied by the adjacent region of emerging magnetic flux: As emerging bipole separate, the poles of the spot polarity migrate towards the spot, and finally merge with it. As more and more flux is accumulated, a penumbra forms. From inversions we infer maps for the magnetic field and the Doppler velocity (being constant along the line-of-sight). We calculate the magnetic flux of the forming spot and of the bipole footpoints that merge with the proto-spot. We witness the onset of the Evershed flow and the associated enhance of the field inclination as individual penumbral filaments form. Prior to the formation of individual penumbral sectors we detect the existence of `counter' Evershed flows. These in-flows turn into the classical radial Evershed outflows as stable penumbra segments form. Title: Spectroscopy at the solar limb. I. Average off-limb profiles and Doppler shifts of Ca II H Authors: Beck, C. A. R.; Rezaei, R. Bibcode: 2011A&A...531A.173B Altcode: 2011arXiv1106.0646B
Aims: We present constraints on the thermodynamical structure of the chromosphere from ground-based observations of the Ca ii H line profile near and off the solar limb.
Methods: We obtained a slit-spectrograph data set of the Ca ii H line with a high signal-to-noise ratio in a field of view extending 20'' across the limb. We analyzed the spectra for the characteristic properties of average and individual off-limb spectra. We used various tracers of the Doppler shifts, such as the location of the absorption core, the ratio of the two emission peaks H2V and H2R, and intensity images at a fixed wavelength.
Results: The average off-limb profiles show a smooth variation with increasing limb distance. The line width increases up to a height of about 2 Mm above the limb. The profile shape is fairly symmetric with nearly identical H2V and H2R intensities; at a height of 5 Mm, it changes into a single Gaussian without emission peaks. We find that all off-limb spectra show large Doppler shifts that fluctuate on the smallest resolved spatial scales. The variation is more prominent in cuts parallel to the solar limb than on those perpendicular to it. As far as individual structures can be unequivocally identified at our spatial resolution, we find a specific relation between intensity enhancements and Doppler shifts: elongated brightenings are often flanked all along their extension by velocities in opposite directions.
Conclusions: The average off-limb spectra of Ca ii H present a good opportunity to test static chromospheric atmosphere models because they lack the photospheric contribution that is present in disk-center spectra. We suggest that the observed relation between intensity enhancements and Doppler shifts could be caused by waves propagating along the surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave propagation inclined to the tube axis would cause a helical shape of the Doppler excursions, visible as opposite velocity at the sides of the flux tube. Spectroscopic data allow one to distinguish this from a sausage-mode oscillation where the maximum Doppler shift and the tube axis would coincide.

Appendices are available in electronic form at http://www.andaa.orgThe Data set is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173 Title: Detection of Vortex Tubes in Solar Granulation from Observations with SUNRISE Authors: Steiner, O.; Franz, M.; Bello González, N.; Nutto, Ch.; Rezaei, R.; Martínez Pillet, V.; Bonet Navarro, J. A.; del Toro Iniesta, J. C.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A. Bibcode: 2010ApJ...723L.180S Altcode: 2010arXiv1009.4723S We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface. Title: The role of emerging bipoles in the formation of a sunspot penumbra Authors: Schlichenmaier, R.; Bello González, N.; Rezaei, R.; Waldmann, T. A. Bibcode: 2010AN....331..563S Altcode: 2010arXiv1003.1313S The generation of magnetic flux in the solar interior and its transport from the convection zone into the photosphere, the chromosphere, and the corona will be in the focus of solar physics research for the next decades. With 4 m class telescopes, one plans to measure essential processes of radiative magneto-hydrodynamics that are needed to understand the nature of solar magnetic fields. One key-ingredient to understand the behavior of solar magnetic field is the process of flux emergence into the solar photosphere, and how the magnetic flux reorganizes to form the magnetic phenomena of active regions like sunspots and pores. Here, we present a spectropolarimetric and imaging data set from a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra. During the formation of the penumbra the area and the magnetic flux of the spot increases. First results of our data analysis demonstrate that the additional magnetic flux, which contributes to the increasing area of the penumbra, is supplied by the region of emerging magnetic flux. We observe emerging bipoles that are aligned such that the spot polarity is closer to the spot. As an emerging bipole separates, the pole of the spot polarity migrates towards the spot, and finally merges with it. We speculate that this is a fundamental process, which makes the sunspot accumulate magnetic flux. As more and more flux is accumulated a penumbra forms and transforms the proto-spot into a full-fledged sunspot. Title: The formation of a sunspot penumbra Authors: Schlichenmaier, R.; Rezaei, R.; Bello González, N.; Waldmann, T. A. Bibcode: 2010A&A...512L...1S Altcode: Context. The formation of a penumbra is crucial for our understanding of solar magnetism, but it has not been observed in detail.
Aims: We aim to enhance our knowledge of how a sunspot penumbra forms and how sunspots grow in size.
Methods: We present a data set of the active region NOAA 11024 acquired at the German VTT with speckle-reconstructed images in the G-band and Ca ii K. The data set includes spectropolarimetric profiles from GFPI in Fe i 617.3 nm and TIP in Fe i 1089.6 nm.
Results: On 2009 July 4, at 08:30 UT, a leading spot without penumbra and pores of opposite polarity were present in the active region. For the next 4:40 h, we observed the formation of a penumbra in the leading spot at a cadence of 5 images per second. We produced speckle reconstructed images of 0.3 arcsec spatial resolution or better, interrupted by one large gap of 35 min and a few more small gaps of about 10 min. The leading spot initially has a size of 230 arcsec2 with only a few penumbral filaments and then grows to a size of 360 arcsec2. The penumbra forms in segments, and it takes about 4 h until it encircles half of the umbra, on the side opposite the following polarity. On the side towards the following polarity, elongated granules mark a region of magnetic flux emergence.
Conclusions: This ongoing emergence appears to prevent a steady penumbra from forming on this side. While the penumbra forms, the umbral area is constant; i.e., the increase in the total spot area is caused exclusively by the growth of the penumbra. From this we conclude that the umbra has reached an upper size limit and that any new magnetic flux that joins the spot is linked to the process of penumbral formation.

Movies are only available in electronic form at http://www.aanda.org Title: The Horizontal Magnetic Field of the Quiet Sun: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schlichenmaier, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2009ASPC..415...67S Altcode: 2009arXiv0904.2030S Three-dimensional magnetohydrodynamic simulations of the surface layers of the Sun intrinsically produce a predominantly horizontal magnetic field in the photosphere. This is a robust result in the sense that it arises from simulations with largely different initial and boundary conditions for the magnetic field. While the disk-center synthetic circular and linear polarization signals agree with measurements from Hinode, their center-to-limb variation sensitively depends on the height variation of the horizontal and the vertical field component and they seem to be at variance with the observed behavior. Title: The energy of waves in the photosphere and lower chromosphere. I. Velocity statistics Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M. Bibcode: 2009A&A...507..453B Altcode: 2009arXiv0905.1011B Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE).
Aims: We derived the mechanical wave energy as seen in line-core velocities on disc centre to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model, the Harvard-Smithsonian reference atmosphere (HSRA).
Methods: We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we used the increase of internal energy (∝ R ρ Δ T) necessary to lift a RE atmosphere to the HSRA temperature stratification.
Results: We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z ~ 500 km is insufficient to maintain on a long-term average the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z ~ 1000 km) can, however, be traced back to the velocity variations of the lowermost forming spectral line considered (z ~ 250 km).
Conclusions: The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. The wave energy is found to be insufficient to maintain the temperature stratification of the semi-empirical HSRA model above 500 km. We will in a following paper of this series investigate the energy contained in the intensity variations to see if the semi-empirical model is appropriate for the spectra. Title: The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter Authors: Beck, C.; Rezaei, R. Bibcode: 2009A&A...502..969B Altcode: 2009arXiv0903.3158B Context: Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun.
Aims: We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 μm, where the sensitivity to magnetic fields is higher than in the visible.
Methods: We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10-4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux.
Results: We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (γ > 45°) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable.
Conclusions: During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating.

Appendices A and B are only available in electronic form at http://www.aanda.org Title: Freeware Solutions for Spectropolarimetric Data Reduction Authors: Paletou, F.; Rezaei, R.; Léger, L. Bibcode: 2009ASPC..405...51P Altcode: Most of the solar physicists use very expensive software for data reduction and visualization. We present hereafter a reliable freeware solution based on the Python language. This is made possible by the association of the latter with a small set of additional libraries developed in the scientific community. It provides then a very powerful and economical alternative to other interactive data languages. Although it can also be used for any kind of post-processing of data, we demonstrate the capabilities of such a set of freeware tools using THéMIS observations of the second solar spectrum. Title: Temporal Evolution of Magnetic Elements Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Beck, C. Bibcode: 2009ASPC..405..195R Altcode: 2007arXiv0712.0234R We study the structure and evolution of the magnetic field of the quiet Sun by investigating weak spectro-polarimetric signals. To this end, we observed a quiet region close to the disk center with the German VTT in Tenerife, July 07, 2006. We recorded 38 scans of the same area. Each scan was eight arcsec wide and observed within about 100 seconds. We used POLIS to simultaneously observe Stokes profiles of the neutral iron lines at 630.15 and 630.25 nm, the Stokes-I profile of the Ca II H line at 396.8 nm, and a continuum speckle channel at 500 nm. We witness two examples of magnetic flux cancellation of small-scale opposite-polarity patches, followed by an enhanced chromospheric emission. In each case, the two opposite-polarity patches gradually became smaller and, within a few minutes, the smaller one completely disappeared. The larger patch also diminished significantly. We provide evidence for a cancellation scenario in the photosphere which leaves minor traces at the chromospheric level. Title: Magnetic coupling of the solar photosphere and chromosphere Authors: Rezaei, R. Bibcode: 2008PhDT........16R Altcode: The solar surface outside sunspots and active regions, i.e., the quiet Sun, shows the ubiquitous pattern of granulation in the photosphere. The quiet solar photosphere harbors small-scale magnetic structures inside and between granules. This thesis presents thermodynamic properties of the small-scale magnetic flux concentrations in the quiet Sun using high spatial and temporal resolution observations along with numerical simulations. Spectral line profiles of the Fe I 630 nm pair and Ca II H were used to trace the photospheric and chromospheric layers of the magnetic elements.

In the presence of magnetic field spectral lines split and are polarized via the Zeeman effect. The difference of a spectral line profile, measured in left and right circular polarized light, is a Stokes-V profile with two lobes. In the absence of any gradients of velocity or magnetic field along the line of sight, Stokes-V profiles are anti-symmetric. The different area of the two lobes, the Stokes-V area asymmetry, provides information about the gradients of the magnetic and velocity fields along the line of sight.

Comparing high resolution spectropolarimetric data with synthetic maps of a 3D MHD simulation, we found several magnetic elements in the photosphere showing a central region of negative Stokes-V area asymmetry surrounded by a peripheral region with larger positive asymmetry. This finding was the first observational confirmation of the existence of a sharp boundary layer between magnetic elements and their immediate surroundings. Such boundary layers had been theoretically predicted ten years go. Furthermore, we found for the first time two Stokes-V profiles of the Fe I 630 nm line pair in a single spectrum showing opposite magnetic polarities. These two lines form in slightly different layers, so they trace the magnetic field in different geometrical heights.

The temporal evolution of these profiles showed a magnetic flux cancellation, suggesting a magnetic reconnection in the photosphere. A 1D numerical model that reproduced the observed profiles was interpreted as an indication for the magnetic reconnection. To verify the existence of vigorous gradients in the magnetic and thermal properties of the atmosphere as suggested by the cancellation event, extreme cases of asymmetry in Stokes-V profiles, i.e., profiles with only one lobe instead of two, were found in a large sample of data. We find strong evidence for concentrated magnetic flux structures with sharp boundaries sustaining a strong gradient or a discontinuity in thermodynamic parameters. In other words, we find current sheets in boundary layers which separate magnetic from non-magnetic plasma. This supports the existence of structured magnetic entities like flux tubes.

The second part of this thesis was devoted to study the thermal structure of the chromosphere and its relation to underlying photospheric magnetic flux concentrations. The Ca II H line is one of the strongest absorption lines in the visible solar spectrum. Generally, there are two emission peaks (H2v and H2r) on either side of the line core which form in the chromosphere. The existence of the emission peaks implies a temperature rise in the chromosphere. Parts of the line outside these two emission peaks mainly form in the photosphere. The integrated intensity in a 0.1 nm band around the core which contains the two emission peaks is a measure for the chromospheric emission. The minimum chromospheric emission is as large as half of the average emission. We found that the chromospheric emission in excess to the minimum emission scales with the magnetic flux density. To establish a relation between the chromospheric emission and temperature, we looked for the Ca II H line profiles without any emission peak at H2v and H2r wavelengths. A quarter of the observed calcium profiles did not show any emission peak. A comparison of these profiles with synthesized calcium profiles of one-dimensional time-independent models provides strong indication for chromospheric temperatures cooler than that of the spatially and temporally averaged quiet Sun. Our results suggest that a large fraction of the solar chromosphere consists of hot plasma, T > 10000 K. Beside this, there is a smaller fraction which is cooler than the underlying photosphere, T < 5000 K, but cannot be much cooler than, e.g., 2000 K. Title: On the Possible Sources of Chromospheric Heating Authors: Beck, C.; Collados, M. Vera; Khomenko, E.; Rezaei, R. Bibcode: 2008ESPM...12.2.14B Altcode: The chromospheric temperature rise to values above the photospheric temperature cannot be due to radiative energy transport alone. We will outline different possibilities for the additional energy transport in the solar atmosphere by processes that require (or exclude) the presence of magnetic fields. We will discuss which of them could be identified and studied in detail using current data. To find the signature of the different heating processes and derive quantitative estimates of their efficiency, we analyzed simultaneous spectropolarimetric observations of photospheric magnetic fields (@630 nm) and intensity spectra of the chromospheric Ca II H line (396 nm). The mechanical energy flux at several height layers was derived from the velocity amplitudes of propagating acoustic waves seen in different spectral lines. The enhancement of chromospheric (radiation) temperature above the radiative equilibrium values was taken from an inversion of the Ca II H spectra with the SIR code assuming local thermal equilibrium (LTE) and complete redistribution (CRD). We compare the obtained energy values with each other and with the energy requirements demanded by theoretical/semi-empirical atmospheric models.

We find that the most important agent of chromospheric heating are propagating (magneto-)acoustic waves, which suffice to explain the brightenings in Ca II H spectra and their corresponding temperature enhancements. The energy contained in these intensity variations of the Ca II H line, however, is found to be insufficient to maintain a full-time and full-volume "hot" chromosphere. Additional energy transport mechanisms without a signature in the Ca II H spectra are thus necessary. Finally, we will outline which improvements are to be expected with future observations of higher quality (spatial resolution, enhanced polarimetric sensitivity, temporal cadence, other spectral lines) to be achieved with new ground-based telescopes like GREGOR or EST. Title: Reversal-free Ca II H Profiles: a Challenge for Solar Chromosphere Modeling in Quiet Inter-Network Authors: Rezaei, R.; Bruls, J.; Beck, C.; Schmidt, W.; Kalkofen, W.; Schlichenmaier, R. Bibcode: 2008ESPM...12.2.13R Altcode: There is no agreement on the thermal structure of the solar chromosphere. While results of the CO observations and 3D MHD simulations suggest very cool structures in the upper atmosphere, SUMER observations of UV spectral lines is interpreted as signature of a full-time hot chromosphere. We tried to look for cool structures in the solar chromosphere. We observed the intensity profile of the Ca II H line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 10^5 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium(NLTE) radiative transfer code. A fraction of about 25% of the observed Ca II H line profiles do not show a measurable emission peak in H2v and H2r wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile. The observed reversal-free profiles, at a spatial resolution of 1 arcs and a temporal resolution of 5 s, indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere (e.g., FALC), but also with a very cool chromosphere as found in some dynamic simulations. Title: The Horizontal Internetwork Magnetic Field: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2008ESPM...12.3.22S Altcode: Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas it is negative in the convectively unstable layer below it. Title: Reversal-free Ca II H profiles: a challenge for solar chromosphere modeling in quiet inter-network Authors: Rezaei, R.; Bruls, J. H. M. J.; Schmidt, W.; Beck, C.; Kalkofen, W.; Schlichenmaier, R. Bibcode: 2008A&A...484..503R Altcode: 2008arXiv0804.2325R Aims: We study chromospheric emission to understand the temperature stratification in the solar chromosphere.
Methods: We observed the intensity profile of the Ca II H line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 105 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative transfer code.
Results: A fraction of about 25% of the observed Ca II H line profiles do not show a measurable emission peak in H2v and H2r wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile.
Conclusions: The observed reversal-free profiles indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere, but also with a very cool chromosphere as found in some dynamic simulations. Title: The Horizontal Internetwork Magnetic Field: Numerical Simulations in Comparison to Observations with Hinode Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm, S. Bibcode: 2008ApJ...680L..85S Altcode: 2008arXiv0801.4915S Observations with the Hinode space observatory led to the discovery of predominantly horizontal magnetic fields in the photosphere of the quiet internetwork region. Here we investigate realistic numerical simulations of the surface layers of the Sun with respect to horizontal magnetic fields and compute the corresponding polarimetric response in the Fe I 630 nm line pair. We find a local maximum in the mean strength of the horizontal field component at a height of around 500 km in the photosphere, where, depending on the initial state or the boundary condition, it surpasses the vertical component by a factor of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean horizontal field component that is 1.6 or 4.3 times stronger than the vertical component, depending on the initial state or the boundary condition. This is a consequence of both the intrinsically stronger flux density of and the larger area occupied by the horizontal fields. We find that convective overshooting expels horizontal fields to the upper photosphere, making the Poynting flux positive in the photosphere, whereas the Poynting flux is negative in the convectively unstable layer below it. Title: The signature of chromospheric heating in Ca II H spectra Authors: Beck, C.; Schmidt, W.; Rezaei, R.; Rammacher, W. Bibcode: 2008A&A...479..213B Altcode: 2007arXiv0712.2538B Context: The heating process that balances the solar chromospheric energy losses has not yet been determined. Conflicting views exist on the source of the energy and the influence of photospheric magnetic fields on chromospheric heating.
Aims: We analyze a 1-h time series of cospatial Ca II H intensity spectra and photospheric polarimetric spectra around 630 nm to derive the signature of the chromospheric heating process in the spectra and to investigate its relation to photospheric magnetic fields. The data were taken in a quiet Sun area on disc center without strong magnetic activity.
Methods: We have derived several characteristic quantities of Ca II H to define the chromospheric atmosphere properties. We study the power of the Fourier transform at different wavelengths and the phase relations between them. We perform local thermodynamic equilibrium (LTE) inversions of the spectropolarimetric data to obtain the photospheric magnetic field, once including the Ca intensity spectra.
Results: We find that the emission in the Ca II H line core at locations without detectable photospheric polarization signal is due to waves that propagate in around 100 s from low forming continuum layers in the line wing up to the line core. The phase differences of intensity oscillations at different wavelengths indicate standing waves for ν < 2 mHz and propagating waves for higher frequencies. The waves steepen into shocks in the chromosphere. On average, shocks are both preceded and followed by intensity reductions. In field-free regions, the profiles show emission about half of the time. The correlation between wavelengths and the decorrelation time is significantly higher in the presence of magnetic fields than for field-free areas. The average Ca II H profile in the presence of magnetic fields contains emission features symmetric to the line core and an asymmetric contribution, where mainly the blue H2V emission peak is increased (shock signature).
Conclusions: We find that acoustic waves steepening into shocks are responsible for the emission in the Ca II H line core for locations without photospheric magnetic fields. We suggest using wavelengths in the line wing of Ca II H, where LTE still applies, to compare theoretical heating models with observations.

Appendices are only available in electronic form at http://www.aanda.org Title: Freeware solutions for spectropolarimetric data reduction Authors: Paletou, F.; Rezaei, R.; Leger, L. Bibcode: 2007arXiv0712.1522P Altcode: Most of the solar physicists use very expensive software for data reduction and visualization. We present hereafter a reliable freeware solution based on the Python language. This is made possible by the association of the latter with a small set of additional libraries developed in the scientific community. It provides then a very powerful and economical alternative to other interactive data languages. Although it can also be used for any kind of post-processing of data, we demonstrate the capabities of such a set of freeware tools using THeMIS observations of the second solar spectrum. Title: Hinode observations reveal boundary layers of magnetic elements in the solar photosphere Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier, R.; Schmidt, W.; Lites, B. W. Bibcode: 2007A&A...476L..33R Altcode: 2007arXiv0711.0408R Aims:We study the structure of the magnetic elements in network-cell interiors.
Methods: A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board the Hinode space mission, which yielded the best spatial resolution ever achieved in polarimetric data of the Fe I 630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation.
Results: We find several examples of magnetic elements, either roundish (tube) or elongated (sheet), which show a central area of negative Stokes-V area asymmetry framed or surrounded by a peripheral area with larger positive asymmetry. This pattern was predicted some eight years ago on the basis of numerical simulations. Here, we observationally confirm its existence for the first time.
Conclusions: We gather convincing evidence that this pattern of Stokes-V area asymmetry is caused by the funnel-shaped boundary of magnetic elements that separates the flux concentration from the weak-field environment. On this basis, we conclude that electric current sheets induced by such magnetic boundary layers are common in the photosphere. Title: Variation of the Stokes-V area asymmetry across magnetic elements Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier, R.; Lites, B. W. Bibcode: 2007AN....328..706R Altcode: No abstract at ADS Title: Opposite magnetic polarity of two photospheric lines in single spectrum of the quiet Sun Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Steiner, O. Bibcode: 2007A&A...469L...9R Altcode: 2007arXiv0704.3135R Aims:We study the structure of the photospheric magnetic field of the quiet Sun by investigating weak spectro-polarimetric signals.
Methods: We took a sequence of Stokes spectra of the Fe I 630.15 nm and 630.25 nm lines in a region of quiet Sun near the disk center, using the POLIS spectro-polarimeter at the German VTT on Tenerife. The line cores of these two lines form at different heights in the atmosphere. The 3σ noise level of the data is about 1.8 × 10-3 I_c.
Results: We present co-temporal and co-spatial Stokes-V profiles of the Fe I 630 nm line pair, where the two lines show opposite polarities in a single spectrum. We compute synthetic line profiles and reproduce these spectra with a two-component model atmosphere: a non-magnetic component and a magnetic component. The magnetic component consists of two magnetic layers with opposite polarity: the upper one moves upwards while the lower one moves downward. In-between, there is a region of enhanced temperature.
Conclusions: The Stokes-V line pair of opposite polarity in a single spectrum can be understood as a magnetic reconnection event in the solar photosphere. We demonstrate that such a scenario is realistic, but the solution may not be unique. Title: Relation between photospheric magnetic field and chromospheric emission Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C. A. R.; Bruls, J. H. M. J.; Schmidt, W. Bibcode: 2007A&A...466.1131R Altcode: 2007astro.ph..1896R Aims: We investigate the relationship between the photospheric magnetic field and the emission of the mid chromosphere of the Sun.
Methods: We simultaneously observed the Stokes parameters of the photospheric iron line pair at 630.2 nm and the intensity profile of the chromospheric Ca II H line at 396.8 nm in a quiet Sun region at a heliocentric angle of 53°. Various line parameters have been deduced from the Ca II H line profile. The photospheric magnetic field vector has been reconstructed from an inversion of the measured Stokes profiles. After alignment of the Ca and Fe maps, a common mask has been created to define network and inter-network regions. We perform a statistical analysis of network and inter-network properties. The H-index is the integrated emission in a 0.1 nm band around the Ca core. We separate a non-magnetically, Hnon, and a magnetically, Hmag, heated component from a non-heated component, Hco in the H-index.
Results: The average network and inter-network H-indices are equal to 12 and 10 pm, respectively. The emission in the network is correlated with the magnetic flux density, approaching a value of H ≈ 10 pm for vanishing flux. The inter-network magnetic field is dominated by weak field strengths with values down to 200 G and has a mean absolute flux density of about 11 Mx cm-2.
Conclusions: We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network has non-magnetic origin (Hmag≈2 pm, Hnon≈3 pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (Hco≈6 pm) is about half of the observed H-index in the inter-network. The H-index in the inter-network is not correlated to any property of the photospheric magnetic field, suggesting that magnetic flux concentrations have a negligible role in the chromospheric heating in this region. The height range of the thermal coupling between the photosphere and low/mid chromosphere increases in presence of magnetic field. In addition, we demonstrate that a poor signal-to-noise level in the Stokes profiles leads to a significant over-estimation of the magnetic field strength. Title: Photospheric magnetic field and chromospheric emission Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Schmidt, W. Bibcode: 2007msfa.conf..169R Altcode: 2007astro.ph..1681R We present a statistical analysis of network and internetwork properties in the photosphere and the chromosphere. For the first time we simultaneously observed (a) the four Stokes parameters of the photospheric iron line pair at 630.2 nm and (b) the intensity profile of the Ca H line at 396.8 nm. The vector magnetic field was inferred from the inversion of the iron lines. We aim at an understanding of the coupling between photospheric magnetic field and chromospheric emission. Title: The flow field in the sunspot canopy Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R. Bibcode: 2006A&A...454..975R Altcode: 2006astro.ph..4301R Aims.We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolarimetry of three sunspots (θ=27°, 50°, 75°) and their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron lines.
Methods: .To calibrate the Doppler shifts, we compare an absolute velocity calibration using the telluric O_2-line at 630.20 nm and a relative velocity calibration using the Doppler shift of Stokes V profiles in the umbra under the assumption that the umbra is at rest. Both methods yield the same result within the calibration uncertainties (~150 m s-1). We study the radial dependence of Stokes V profiles in the directions of disk center and limb side.
Results: .Maps of Stokes V profile shifts, polarity, amplitude asymmetry, field strength and magnetic field azimuth provide strong evidence for the presence of a magnetic canopy and for the existence of a radial outflow in the canopy.
Conclusions: .Our findings indicate that the Evershed flow does not cease abruptly at the white-light spot boundary, but that at least a part of the penumbral Evershed flow continues into the magnetic canopy.