@article{MartinezGonzalezPastorYabarLaggetal.2016,
  author    = {Martinez Gonzalez, M. J. and Pastor Yabar, A. and Lagg, A. and Asensio Ramos, A. and Collados Vera, M. and Solanki, S. K. and Balthasar, H. and Berkefeld, T. and Denker, Carsten and Doerr, H. P. and Feller, A. and Franz, M. and Gonz{\´a}lez Manrique, Sergio Javier and Hofmann, A. and Kneer, F. and Kuckein, Christoph and Louis, R. and von der L{\"u}he, O. and Nicklas, H. and Orozco, D. and Rezaei, R. and Schlichenmaier, R. and Schmidt, D. and Schmidt, W. and Sigwarth, M. and Sobotka, M. and Soltau, D. and Staude, J. and Strassmeier, Klaus G. and Verma, Meetu and Waldman, T. and Volkmer, R.},
  title     = {Inference of magnetic fields in the very quiet Sun},
  series = {Journal of geophysical research : Earth surface},
  volume    = {596},
  journal   = {Journal of geophysical research : Earth surface},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201628449},
  pages     = {11},
  year      = {2016},
  abstract  = {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 mu m 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 (similar to 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 similar to 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.},
  language  = {en}
}
@article{BalthasarGoemoeryGonzalezManriqueetal.2016,
  author    = {Balthasar, H. and G{\"o}m{\"o}ry, P. and Gonz{\´a}lez Manrique, Sergio Javier and Kuckein, Christoph and Kavka, J. and Kucera, A. and Schwartz, P. and Vaskova, R. and Berkefeld, T. and Collados Vera, M. and Denker, Carsten and Feller, A. and Hofmann, A. and Lagg, A. and Nicklas, H. and Suarez, D. and Pastor Yabar, A. and Rezaei, R. and Schlichenmaier, R. and Schmidt, D. and Schmidt, W. and Sigwarth, M. and Sobotka, M. and Solanki, S. K. and Soltau, D. and Staude, J. and Strassmeier, Klaus G. and Volkmer, R. and von der L{\"u}he, O. and Waldmann, T.},
  title     = {Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {337},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201612432},
  pages     = {1050 -- 1056},
  year      = {2016},
  abstract  = {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 SiI lambda 1082.7 nm, He I lambda 1083.0 nm, and Ca I lambda 1083.9 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. (C) 2016 WILEY-VCH Verlag GmbH\& Co. KGaA, Weinheim},
  language  = {en}
}
@article{VermaDenkerBoehmetal.2016,
  author    = {Verma, Meetu and Denker, Carsten and B{\"o}hm, F. and Balthasar, H. and Fischer, C. E. and Kuckein, Christoph and Gonzalez, N. Bello and Berkefeld, T. and Collados Vera, M. and Diercke, Andrea and Feller, A. and Gonzalez Manrique, Sergio Javier and Hofmann, A. and Lagg, A. and Nicklas, H. and Orozco Suarez, D. and Pator Yabar, A. and Rezaei, R. and Schlichenmaier, R. and Schmidt, D. and Schmidt, W. and Sigwarth, M. and Sobotka, M. and Solanki, S. K. and Soltau, D. and Staude, J. and Strassmeier, Klaus G. and Volkmer, R. and von der L{\"u}he, O. and Waldmann, T.},
  title     = {Flow and magnetic field properties in the trailing sunspots of active region NOAA 12396},
  series = {Astronomische Nachrichten = Astronomical notes},
  volume    = {337},
  journal   = {Astronomische Nachrichten = Astronomical notes},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {0004-6337},
  doi       = {10.1002/asna.201612447},
  pages     = {1090 -- 1098},
  year      = {2016},
  abstract  = {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.1082.7 nm line, the chromospheric He I lambda 1083.0 nm triplet, and the photospheric Ca I lambda 1083.9 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. (C) 2016 WILEY-VCH Verlag GmbH\& Co.KGaA, Weinheim},
  language  = {en}
}
@article{VermaDenkerBalthasaretal.2018,
  author    = {Verma, Meetu and Denker, Carsten and Balthasar, H. and Kuckein, Christoph and Rezaei, R. and Sobotka, Michal and Deng, N. and Wang, Haimin and Tritschler, A. and Collados, M. and Diercke, Andrea and Gonz{\´a}lez Manrique, Sergio Javier},
  title     = {High-resolution imaging and near-infrared spectroscopy of penumbral decay},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {614},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201731801},
  pages     = {14},
  year      = {2018},
  abstract  = {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 degrees 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.},
  language  = {en}
}