explanation      blue bibcodes open ADS page with paths to full text
Author name code: schlichenmaier
ADS astronomy entries on 2022-09-14
author:"Schlichenmaier, Rolf" 

---------------------------------------------------------
Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
   Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
   C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
   Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
   M.; the EST team
2022arXiv220710905Q    Altcode:
  The European Solar Telescope (EST) is a project aimed at studying
  the magnetic connectivity of the solar atmosphere, from the deep
  photosphere to the upper chromosphere. Its design combines the knowledge
  and expertise gathered by the European solar physics community during
  the construction and operation of state-of-the-art solar telescopes
  operating in visible and near-infrared wavelengths: the Swedish 1m Solar
  Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
  the French Télescope Héliographique pour l'Étude du Magnétisme
  et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
  (DOT). With its 4.2 m primary mirror and an open configuration, EST will
  become the most powerful European ground-based facility to study the Sun
  in the coming decades in the visible and near-infrared bands. EST uses
  the most innovative technological advances: the first adaptive secondary
  mirror ever used in a solar telescope, a complex multi-conjugate
  adaptive optics with deformable mirrors that form part of the optical
  design in a natural way, a polarimetrically compensated telescope design
  that eliminates the complex temporal variation and wavelength dependence
  of the telescope Mueller matrix, and an instrument suite containing
  several (etalon-based) tunable imaging spectropolarimeters and several
  integral field unit spectropolarimeters. This publication summarises
  some fundamental science questions that can be addressed with the
  telescope, together with a complete description of its major subsystems.

---------------------------------------------------------
Title: Characterization of magneto-convection in sunspots. The
    Gough-Tayler stability criterion in MURaM sunspot simulations
Authors: Schmassmann, M.; Rempel, M.; Bello González, N.;
   Schlichenmaier, R.; Jurčák, J.
2021A&A...656A..92S    Altcode:
  Context. Observations have shown that in stable sunspots, the umbral
  boundary is outlined by a critical value of the vertical magnetic
  field component. However, the nature of the distinct magnetoconvection
  regimes in the umbra and penumbra is still unclear. <BR /> Aims: We
  analyse a sunspot simulation in an effort to understand the origin
  of the convective instabilities giving rise to the penumbral and
  umbral distinct regimes. <BR /> Methods: We applied the criterion
  from Gough &amp; Tayler (1966, MNRAS, 133, 85), accounting for the
  stabilising effect of the vertical magnetic field, to investigate
  the convective instabilities in a MURaM sunspot simulation. <BR />
  Results: We find: (1) a highly unstable shallow layer right beneath the
  surface extending all over the simulation box in which convection is
  triggered by radiative cooling in the photosphere; (2) a deep umbral
  core (beneath −5 Mm) stabilised against overturning convection
  that underlies a region with stable background values permeated
  by slender instabilities coupled to umbral dots; (3) filamentary
  instabilities below the penumbra nearly parallel to the surface and
  undulating instabilities coupled to the penumbra which originate
  in the deep layers. These deep-rooted instabilities result in the
  vigorous magneto-convection regime characteristic of the penumbra; (4)
  convective downdrafts in the granulation, penumbra, and umbra develop
  at about 2 km s<SUP>−1</SUP>, 1 km s<SUP>−1</SUP>, and 0.1 km
  s<SUP>−1</SUP>, respectively, indicating that the granular regime of
  convection is more vigorous than the penumbra convection regime, which,
  in turn, is more vigorous than the close-to-steady umbra; (5) the GT
  criterion outlines both the sunspot magnetopause and peripatopause,
  highlighting the tripartite nature of the sub-photospheric layers
  of magnetohydrodynamic (MHD) sunspot models; and, finally, (6)
  the Jurčák criterion is the photospheric counterpart of the GT
  criterion in deep layers. <BR /> Conclusions: The GT criterion as a
  diagnostic tool reveals the tripartite nature of sunspot structure
  with distinct regimes of magneto-convection in the umbra, penumbra,
  and granulation operating in realistic MHD simulations. <P
  />Movies associated with Figs. 2 and 3 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141607/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: On the (in)stability of sunspots
Authors: Strecker, H.; Schmidt, W.; Schlichenmaier, R.; Rempel, M.
2021A&A...649A.123S    Altcode: 2021arXiv210311487S
  Context. The stability of sunspots is one of the long-standing unsolved
  puzzles in the field of solar magnetism and the solar cycle. The thermal
  and magnetic structure of the sunspot beneath the solar surface is
  not accessible through observations, thus processes in these regions
  that contribute to the decay of sunspots can only be studied through
  theoretical and numerical studies. <BR /> Aims: We study the effects
  that destabilise and stabilise the flux tube of a simulated sunspot
  in the upper convection zone. The depth-varying effects of fluting
  instability, buoyancy forces, and timescales on the flux tube are
  analysed. <BR /> Methods: We analysed a numerical simulation of a
  sunspot calculated with the MURaM code. The simulation domain has a
  lateral extension of more than 98 Mm × 98 Mm and extends almost 18
  Mm below the solar surface. The analysed data set of 30 hours shows a
  stable sunspot at the solar surface. We studied the evolution of the
  flux tube at defined horizontal layers (1) by means of the relative
  change in perimeter and area, that is, its compactness; and (2) with
  a linear stability analysis. <BR /> Results: The simulation shows a
  corrugation along the perimeter of the flux tube (sunspot) that proceeds
  fastest at a depth of about 8 Mm below the solar surface. Towards
  the surface and towards deeper layers, the decrease in compactness is
  damped. From the stability analysis, we find that above a depth of 2
  Mm, the sunspot is stabilised by buoyancy forces. The spot is least
  stable at a depth of about 3 Mm because of the fluting instability. In
  deeper layers, the flux tube is marginally unstable. The stability
  of the sunspot at the surface affects the behaviour of the field
  lines in deeper layers by magnetic tension. Therefore the fluting
  instability is damped at depths of about 3 Mm, and the decrease in
  compactness is strongest at a depth of about 8 Mm. The more vertical
  orientation of the magnetic field and the longer convective timescale
  lead to slower evolution of the corrugation process in layers deeper
  than 10 Mm. <BR /> Conclusions: The formation of large intrusions of
  field-free plasma below the surface destabilises the flux tube of
  the sunspot. This process is not visible at the surface, where the
  sunspot is stabilised by buoyancy forces. The onset of sunspot decay
  occurs in deeper layers, while the sunspot still appears stable in
  the photosphere. The intrusions eventually lead to the disruption
  and decay of the sunspot. <P />The animation is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202040199/olm">https://www.aanda.org</A>
  <P />This paper is mainly based on Part I of
  the Ph.D. thesis "On the decay of sunspots", <A
  href="https://freidok.uni-freiburg.de/data/165760">https://freidok.uni-freiburg.de/data/165760</A>

---------------------------------------------------------
Title: A distinct magnetic property of the inner penumbral
    boundary. III. Analysis of simulated sunspots
Authors: Jurčák, Jan; Schmassmann, Markus; Rempel, Matthias; Bello
   González, Nazaret; Schlichenmaier, Rolf
2020A&A...638A..28J    Altcode: 2020arXiv200403940J
  Context. Analyses of sunspot observations revealed a fundamental
  magnetic property of the umbral boundary: the invariance of the
  vertical component of the magnetic field. <BR /> Aims: We analyse
  the magnetic properties of the umbra-penumbra boundary in simulated
  sunspots and thus assess their similarity to observed sunspots. We
  also aim to investigate the role of the plasma β and the ratio of
  kinetic to magnetic energy in simulated sunspots in the convective
  motions because these quantities cannot be reliably determined from
  observations. <BR /> Methods: We used a set of non-gray simulation
  runs of sunspots with the MURaM code. The setups differed in terms
  of subsurface magnetic field structure and magnetic field boundary
  imposed at the top of the simulation domain. These data were used to
  synthesize the Stokes profiles, which were then degraded to the Hinode
  spectropolarimeter-like observations. Then, the data were treated
  like real Hinode observations of a sunspot, and magnetic properties
  at the umbral boundaries were determined. <BR /> Results: Simulations
  with potential field extrapolation produce a realistic magnetic field
  configuration on the umbral boundaries of the sunspots. Two simulations
  with a potential field upper boundary, but different subsurface
  magnetic field structures, differ significantly in the extent of their
  penumbrae. Increasing the penumbra width by forcing more horizontal
  magnetic fields at the upper boundary results in magnetic properties
  that are not consistent with observations. This implies that the size of
  the penumbra is given by the subsurface structure of the magnetic field,
  that is, by the depth and inclination of the magnetopause, which is
  shaped by the expansion of the sunspot flux rope with height. None of
  the sunspot simulations is consistent with the observed properties of
  the magnetic field and the direction of the Evershed flow at the same
  time. Strong outward-directed Evershed flows are only found in setups
  with an artificially enhanced horizontal component of the magnetic
  field at the top boundary that are not consistent with the observed
  magnetic field properties at the umbra-penumbra boundary. We stress
  that the photospheric boundary of simulated sunspots is defined by a
  magnetic field strength of equipartition field value.

---------------------------------------------------------
Title: Characterization of the umbra-penumbra boundary by the vertical
    component of the magnetic field. Analysis of ground-based data from
    the GREGOR Infrared Spectrograph
Authors: Lindner, P.; Schlichenmaier, R.; Bello González, N.
2020A&A...638A..25L    Altcode: 2020arXiv200409956L
  Context. The vertical component of the magnetic field was found to
  reach a constant value at the boundary between penumbra and umbra of
  stable sunspots in a recent statistical study of Hinode/SP data. This
  finding has profound implications as it can serve as a criterion to
  distinguish between fundamentally different magneto-convective modes
  operating in the sun. <BR /> Aims: The objective of this work is to
  verify the existence of a constant value for the vertical component of
  the magnetic field (B<SUB>⊥</SUB>) at the boundary between umbra and
  penumbra from ground-based data in the near-infrared wavelengths and to
  determine its value for the GREGOR Infrared Spectrograph (GRIS@GREGOR)
  data. This is the first statistical study on the Jurčák criterion with
  ground-based data, and we compare it with the results from space-based
  data (Hinode/SP and SDO/HMI). <BR /> Methods: Eleven spectropolarimetric
  data sets from the GRIS@GREGOR slit-spectograph containing fully-fledged
  stable sunspots were selected from the GRIS archive. SIR inversions
  including a polarimetric straylight correction are used to produce
  maps of the magnetic field vector using the Fe I 15648 Å and 15662 Å
  lines. Averages of B<SUB>⊥</SUB> along the contours between penumbra
  and umbra are analyzed for the 11 data sets. In addition, contours at
  the resulting B<SUB>⊥</SUB><SUP>const</SUP> are drawn onto maps and
  compared to intensity contours. The geometric difference between these
  contours, ΔP, is calculated for each data set. <BR /> Results: Averaged
  over the 11 sunspots, we find a value of B<SUB>⊥</SUB><SUP>const</SUP>
  = (1787 ± 100) gauss. The difference from the values previously derived
  from Hinode/SP and SDO/HMI data is explained by instrumental differences
  and by the formation characteristics of the respective lines that were
  used. Contours at B<SUB>⊥</SUB> = B<SUB>⊥</SUB><SUP>const</SUP> and
  contours calculated in intensity maps match from a visual inspection
  and the geometric distance ΔP was found to be on the order of 2
  pixels. Furthermore, the standard deviation between different data sets
  of averages along umbra-penumbra contours is smaller for B<SUB>⊥</SUB>
  than for B<SUB>∥</SUB> by a factor of 2.4. <BR /> Conclusions: Our
  results provide further support to the Jurčák criterion with the
  existence of an invariable value B<SUB>⊥</SUB><SUP>const</SUP> at the
  umbra-penumbra boundary. This fundamental property of sunspots can act
  as a constraining parameter in the calibration of analysis techniques
  that calculate magnetic fields. It also serves as a requirement for
  numerical simulations to be realistic. Furthermore, it is found
  that the geometric difference, ΔP, between intensity contours
  and contours at B<SUB>⊥</SUB> = B<SUB>⊥</SUB><SUP>const</SUP>
  acts as an index of stability for sunspots. <P />The data from
  the GRIS instrument is publicly available in the archive at <A
  href="http://sdc.leibniz-kis.de">http://sdc.leibniz-kis.de</A>.

---------------------------------------------------------
Title: Science Requirement Document (SRD) for the European Solar
    Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
   Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
   Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
   K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
   Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
   T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
   J.; Utz, D.; Zuccarello, F.
2019arXiv191208650S    Altcode:
  The European Solar Telescope (EST) is a research infrastructure
  for solar physics. It is planned to be an on-axis solar telescope
  with an aperture of 4 m and equipped with an innovative suite of
  spectro-polarimetric and imaging post-focus instrumentation. The EST
  project was initiated and is driven by EAST, the European Association
  for Solar Telescopes. EAST was founded in 2006 as an association
  of 14 European countries. Today, as of December 2019, EAST consists
  of 26 European research institutes from 18 European countries. The
  Preliminary Design Phase of EST was accomplished between 2008 and
  2011. During this phase, in 2010, the first version of the EST Science
  Requirement Document (SRD) was published. After EST became a project
  on the ESFRI roadmap 2016, the preparatory phase started. The goal
  of the preparatory phase is to accomplish a final design for the
  telescope and the legal governance structure of EST. A major milestone
  on this path is to revisit and update the Science Requirement Document
  (SRD). The EST Science Advisory Group (SAG) has been constituted by
  EAST and the Board of the PRE-EST EU project in November 2017 and has
  been charged with the task of providing with a final statement on the
  science requirements for EST. Based on the conceptual design, the SRD
  update takes into account recent technical and scientific developments,
  to ensure that EST provides significant advancement beyond the current
  state-of-the-art. The present update of the EST SRD has been developed
  and discussed during a series of EST SAG meetings. The SRD develops
  the top-level science objectives of EST into individual science
  cases. Identifying critical science requirements is one of its main
  goals. Those requirements will define the capabilities of EST and the
  post-focus instrument suite. The technical requirements for the final
  design of EST will be derived from the SRD.

---------------------------------------------------------
Title: Spectropolarimetric Observations of an Arch Filament System
    with GREGOR
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
   Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.;
   Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt,
   D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau,
   D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O.
2019ASPC..526..217B    Altcode: 2018arXiv180401789B
  We observed an arch filament system (AFS) in a sunspot group with the
  GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The
  AFS was located between the leading sunspot of negative polarity and
  several pores of positive polarity forming the following part of the
  sunspot group. We recorded five spectro-polarimetric scans of this
  region. The spectral range included the spectral lines Si I 1082.7
  nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate
  on the silicon line which is formed in the upper photosphere. The
  line profiles are inverted with the code 'Stokes Inversion based
  on Response functions' to obtain the magnetic field vector. The
  line-of-sight velocities are determined independently with a Fourier
  phase method. Maximum velocities are found close to the ends of AFS
  fibrils. These maximum values amount to 2.4 km s<SUP>-1</SUP> next
  to the pores and to 4 km s<SUP>-1</SUP> at the sunspot side. Between
  the following pores, we encounter an area of negative polarity that
  is decreasing during the five scans. We interpret this by new emerging
  positive flux in this area canceling out the negative flux. In summary,
  our findings confirm the scenario that rising magnetic flux tubes
  cause the AFS.

---------------------------------------------------------
Title: Photospheric Magnetic Fields of the Trailing Sunspots in
    Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
   C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
   González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
   Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
   A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
   M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
   R.; von der Lühe, O.; Waldmann, T.
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: New Insights on Penumbra Magneto-Convection
Authors: Bello González, N.; Jurčák, J.; Schlichenmaier, R.;
   Rezaei, R.
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: Convective blueshifts in the solar
    atmosphere. III. High-accuracy observations of spectral lines in
    the visible
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
   Steinmetz, T.; Holzwarth, R.
2019A&A...624A..57L    Altcode: 2019arXiv190107606L
  Context. Convective motions in the solar atmosphere cause spectral lines
  to become asymmetric and shifted in wavelength. For photospheric lines,
  this differential Doppler shift varies from the solar disk center
  to the limb. <BR /> Aims: Precise and comprehensive observations of
  the convective blueshift and its center-to-limb variation improve
  our understanding of the atmospheric hydrodynamics and ensuing line
  formation, and provide the basis to refine 3D models of the solar
  atmosphere. <BR /> Methods: We performed systematical spectroscopic
  measurements of the convective blueshift of the quiet Sun with the
  Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
  Tower Telescope. The spatial scanning of the solar disk covered
  11 heliocentric positions each along four radial (meridional and
  equatorial) axes. The high-resolution spectra of 26 photospheric
  to chromospheric lines in the visible range were calibrated with a
  laser frequency comb to absolute wavelengths at the 1 m s<SUP>-1</SUP>
  accuracy. Applying ephemeris and reference corrections, the bisector
  analysis provided line asymmetries and Doppler shifts with an
  uncertainty of only few m s<SUP>-1</SUP>. To allow for a comparison
  with other observations, we convolved the results to lower spectral
  resolutions. <BR /> Results: All spectral line bisectors exhibit
  a systematic center-to-limb variation. Typically, a blueshifted
  "C"-shaped curve at disk center transforms into a less blueshifted
  "\"-shape toward the solar limb. The comparison of all lines reveals
  the systematic dependence of the convective blueshift on the line
  depth. The blueshift of the line minima describe a linear decrease
  with increasing line depths. The slope of the center-to-limb variation
  develops a reversal point at heliocentric positions between μ = 0.7
  and 0.85, seen as the effect of horizontal granular flows in the mid
  photosphere. Line minima formed in the upper photosphere to chromosphere
  exhibit hardly any blueshift or even a slight redshift. Synthetic
  models yield considerable deviations from the observed center-to-limb
  variation. <BR /> Conclusions: The obtained Doppler shifts of the
  quiet Sun can serve as an absolute reference for other observations,
  the relative calibration of Dopplergrams, and the necessary refinement
  of atmospheric models. Based on this, the development of high-precision
  models of stellar surface convection will advance the detection of
  (potentially habitable) exoplanets by radial velocity measurements. <P
  />The reduced spectra are only available at the CDS via anonymous ftp to
  <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A57">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A57</A>

---------------------------------------------------------
Title: VizieR Online Data Catalog: Convective blueshifts in solar
    atmos. (Loehner-Boettcher+, 2019)
Authors: Loehner-Boettcher, J.; Schmidt, W.; Schlichenmaier, R.;
   Steinmetz, T.; Holzwarth, R.
2019yCat..36240057L    Altcode:
  Solar spectra were observed in eight different spectral regions (525nm,
  538nm, 543nm, 557nm, 589nm, 614nm, 617nm, 630nm) with the Laser Absolute
  Reference Spectrograph (LARS) at the VTT on Tenerife. The observations
  were performed at different heliocentric position mu=cos(alpha)
  from disk center (mu=1.0) to near the solar limb (mu=0.3,0.2). These
  20min-time-averaged observations were performed along four radial
  axes (north, south, east, west). The calibration of the solar spectra
  with a laser frequency comb results in an absolute wavelength scale
  with an Doppler accuracy of 1m/s. The spectra were recorded with the
  spectrographs (resolution greater than 700000) CCD camera. <P />Vacuum
  Tower Telescope (VTT), Laser Absolute Reference Spectrograph (LARS),
  May 2016 to May 2018. <P />IF Integrated sunlight of 10arcsec field
  of view AND Spectral resolution greater than 700000, Temporal average
  over 20min at the respective heliocentric position mu. <P />FITS files
  contain the average solar spectrum in the same form throughout: *: 2048
  pixel, dimension [*,0]: spectral intensity, dimension [*,1]: standard
  deviation, dimension [*,2]: absolute wavelength in Angstroem. <P
  />Data naming includes the instrument, date, time, spectral region,
  heliocentric position mu, and abbreviated radial axis (except at disk
  center) <P />(2 data files).

---------------------------------------------------------
Title: Recent advancements in the EST project
Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort,
   Michiel; Schlichenmaier, Rolf
2019AdSpR..63.1389J    Altcode: 2018arXiv181100851J
  The European Solar Telescope (EST) is a project of a new-generation
  solar telescope. It has a large aperture of 4 m, which is necessary for
  achieving high spatial and temporal resolution. The high polarimetric
  sensitivity of the EST will allow to measure the magnetic field in the
  solar atmosphere with unprecedented precision. Here, we summarise the
  recent advancements in the realisation of the EST project regarding
  the hardware development and the refinement of the science requirements.

---------------------------------------------------------
Title: Magnetic properties of a long-lived sunspot. Vertical magnetic
    field at the umbral boundary
Authors: Schmassmann, M.; Schlichenmaier, R.; Bello González, N.
2018A&A...620A.104S    Altcode: 2018arXiv181009358S
  Context. In a recent statistical study of sunspots in 79 active regions,
  the vertical magnetic field component B<SUB>ver</SUB> averaged along the
  umbral boundary is found to be independent of sunspot size. The authors
  of that study conclude that the absolute value of B<SUB>ver</SUB> at the
  umbral boundary is the same for all spots. <BR /> Aims: We investigate
  the temporal evolution of B<SUB>ver</SUB> averaged along the umbral
  boundary of one long-lived sunspot during its stable phase. <BR />
  Methods: We analysed data from the HMI instrument on-board SDO. Contours
  of continuum intensity at I<SUB>c</SUB> = 0.5I<SUB>qs</SUB>, whereby
  I<SUB>qs</SUB> refers to the average over the quiet sun areas, are used
  to extract the magnetic field along the umbral boundary. Projection
  effects due to different formation heights of the Fe I 617.3 nm line
  and continuum are taken into account. To avoid limb artefacts, the
  spot is only analysed for heliocentric angles smaller than 60°. <BR />
  Results: During the first disc passage, NOAA AR 11591, B<SUB>ver</SUB>
  remains constant at 1693 G with a root-mean-square deviation of 15 G,
  whereas the magnetic field strength varies substantially (mean 2171 G,
  rms of 48 G) and shows a long term variation. Compensating for formation
  height has little influence on the mean value along each contour, but
  reduces the variations along the contour when away from disc centre,
  yielding a better match between the contours of B<SUB>ver</SUB> =
  1693 G and I<SUB>c</SUB> = 0.5I<SUB>qs</SUB>. <BR /> Conclusions:
  During the disc passage of a stable sunspot, its umbral boundary can
  equivalently be defined by using the continuum intensity I<SUB>c</SUB>
  or the vertical magnetic field component B<SUB>ver</SUB>. Contours of
  fixed magnetic field strength fail to outline the umbral boundary. <P
  />Movies associated with Figs. 3 and 5 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201833441/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Absolute velocity measurements in sunspot umbrae
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
   Doerr, H. -P.; Steinmetz, T.; Holzwarth, R.
2018A&A...617A..19L    Altcode: 2018arXiv180408304L
  Context. In sunspot umbrae, convection is largely suppressed by the
  strong magnetic field. Previous measurements reported on negligible
  convective flows in umbral cores. Based on this, numerous studies have
  taken the umbra as zero reference to calculate Doppler velocities of
  the ambient active region. <BR /> Aims: We aim to clarify the amount of
  convective motion in the darkest part of umbrae, by directly measuring
  Doppler velocities with an unprecedented accuracy and precision. <BR />
  Methods: We performed spectroscopic observations of sunspot umbrae with
  the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
  Tower Telescope. A laser frequency comb enabled the calibration of
  the high-resolution spectrograph and absolute wavelength positions for
  13 observation sequences. A thorough spectral calibration, including
  the measurement of the reference wavelength, yielded Doppler shifts
  of the spectral line Ti I 5713.9 Å with an uncertainty of around 5
  m s<SUP>-1</SUP>. A bisector analysis gave the depth-dependent line
  asymmetry. <BR /> Results: The measured Doppler shifts are a composition
  of umbral convection and magneto-acoustic waves. For the analysis of
  convective shifts, we temporally averaged each sequence to reduce the
  superimposed wave signal. Compared to convective blueshifts of up to
  -350 m s<SUP>-1</SUP> in the quiet Sun, sunspot umbrae yield strongly
  reduced convective blueshifts around -30 m s<SUP>-1</SUP>. We find
  that the velocity in a sunspot umbra correlates significantly with the
  magnetic field strength, but also with the umbral temperature defining
  the depth of the Ti I 5713.9 Å line. The vertical upward motion
  decreases with increasing field strength. Extrapolating the linear
  approximation to zero magnetic field reproduces the measured quiet Sun
  blueshift. In the same manner, we find that the convective blueshift
  decreases as a function of increasing line depth. <BR /> Conclusions:
  Simply taking the sunspot umbra as a zero velocity reference for the
  calculation of photospheric Dopplergrams can imply a systematic velocity
  error reaching 100 m s<SUP>-1</SUP>, or more. Setting up a relationship
  between vertical velocities and magnetic field strength provides a
  remedy for solar spectropolarimetry. We propose a novel approach of
  substantially increasing the accuracy of the Doppler velocities of a
  sunspot region by including the magnetic field information to define
  the umbral reference velocity.

---------------------------------------------------------
Title: The magnetic nature of umbra-penumbra boundary in sunspots
Authors: Jurčák, J.; Rezaei, R.; González, N. Bello; Schlichenmaier,
   R.; Vomlel, J.
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. <BR /> 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. <BR /> 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. <BR /> 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: Flows along arch filaments observed in the GRIS `very fast
    spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
   Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
   Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
   R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
   A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
   W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
   J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2017IAUS..327...28G    Altcode: 2017arXiv170102206G
  A new generation of solar instruments provides improved spectral,
  spatial, and temporal resolution, thus facilitating a better
  understanding of dynamic processes on the Sun. High-resolution
  observations often reveal multiple-component spectral line profiles,
  e.g., in the near-infrared He i 10830 Å triplet, which provides
  information about the chromospheric velocity and magnetic fine
  structure. We observed an emerging flux region, including two small
  pores and an arch filament system, on 2015 April 17 with the `very
  fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
  situated at the 1.5-meter GREGOR solar telescope at Observatorio del
  Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
  per minute) spectral scans of the solar surface and its potential to
  follow dynamic processes on the Sun. We demonstrate the performance
  of the `very fast spectroscopic mode' by tracking chromospheric
  high-velocity features in the arch filament system.

---------------------------------------------------------
Title: Penumbral thermal structure below the visible surface
Authors: Borrero, J. M.; Franz, M.; Schlichenmaier, R.; Collados,
   M.; Asensio Ramos, A.
2017A&A...601L...8B    Altcode: 2017arXiv170502832B
  Context. The thermal structure of the penumbra below its visible
  surface (I.e., τ<SUB>5</SUB> ≥ 1) has important implications for
  our present understanding of sunspots and their penumbrae: their
  brightness and energy transport, mode conversion of magneto-acoustic
  waves, sunspot seismology, and so forth. <BR /> Aims: We aim at
  determining the thermal stratification in the layers immediately
  beneath the visible surface of the penumbra: τ<SUB>5</SUB> ∈ [1,3]
  (≈70-80 km below the visible continuum-forming layer) <BR /> Methods:
  We analyzed spectropolarimetric data (I.e., Stokes profiles) in three Fe
  I lines located at 1565 nm observed with the GRIS instrument attached
  to the 1.5-m solar telescope GREGOR. The data are corrected for the
  smearing effects of wide-angle scattered light and then subjected
  to an inversion code for the radiative transfer equation in order
  to retrieve, among others, the temperature as a function of optical
  depth T(τ<SUB>5</SUB>). <BR /> Results: We find that the temperature
  gradient below the visible surface of the penumbra is smaller than
  in the quiet Sun. This implies that in the region τ<SUB>5</SUB> ≥
  1 the penumbral temperature diverges from that of the quiet Sun. The
  same result is obtained when focusing only on the thermal structure
  below the surface of bright penumbral filaments. <BR /> Conclusions:
  We interpret these results as evidence of a thick penumbra, whereby
  the magnetopause is not located near its visible surface. In addition,
  we find that the temperature gradient in bright penumbral filaments is
  lower than in granules. This can be explained in terms of the limited
  expansion of a hot upflow inside a penumbral filament relative to
  a granular upflow, as magnetic pressure and tension forces from the
  surrounding penumbral magnetic field hinder an expansion like this.

---------------------------------------------------------
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.
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, B<SUP>stable</SUP><SUB>ver</SUB>. 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
  B<SUB>ver</SUB>&lt;B<SUP>stable</SUP><SUB>ver</SUB>, as well as
  in granular convective areas. Eventually, a stable umbra-penumbra
  boundary settles at B<SUP>stable</SUP><SUB>ver</SUB>. <BR /> 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. <BR /> 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. <BR /> 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 B<SUP>stable</SUP><SUB>ver</SUB>
  ≈ 1.8 kG. <BR /> Conclusions: Our findings are in an agreement
  with the need of B<SUP>stable</SUP><SUB>ver</SUB> for establishing
  a stable umbra-penumbra boundary: while B<SUB>ver</SUB> in
  the pore is smaller than B<SUP>stable</SUP><SUB>ver</SUB>, 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. <P />The movie associated to Fig. 1 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628547/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Canonical Bver value on umbra/penumbra boundaries
Authors: Jurcak, Jan; Bello González, Nazaret; Schlichenmaier, Rolf;
   Rezaei, Reza
2017psio.confE.112J    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Slipping reconnection in a solar flare observed in high
    resolution with the GREGOR solar telescope
Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák,
   J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann,
   A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.;
   Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
   M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.;
   Volkmer, R.; Waldmann, T.
2016A&A...596A...1S    Altcode: 2016arXiv160500464S
  A small flare ribbon above a sunspot umbra in active region 12205 was
  observed on November 7, 2014, at 12:00 UT in the blue imaging channel
  of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference
  filter. Context observations from the Atmospheric Imaging Assembly
  (AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical
  Telescope (SOT) onboard Hinode, and the Interface Region Imaging
  Spectrograph (IRIS) show that this ribbon is part of a larger one
  that extends through the neighboring positive polarities and also
  participates in several other flares within the active region. We
  reconstructed a time series of 140 s of Ca II H images by means of the
  multiframe blind deconvolution method, which resulted in spatial and
  temporal resolutions of 0.1″ and 1 s. Light curves and horizontal
  velocities of small-scale bright knots in the observed flare ribbon
  were measured. Some knots are stationary, but three move along the
  ribbon with speeds of 7-11 km s<SUP>-1</SUP>. Two of them move in the
  opposite direction and exhibit highly correlated intensity changes,
  which provides evidence of a slipping reconnection at small spatial
  scales. <P />Movies associated to Figs. 1 and 2 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201527966/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
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. <BR /> Aims:
  We aim to confirm or refute the existence of weak-field regions in
  the deepest photospheric layers of the penumbra. <BR /> Methods:
  We investigated the magnetic field at log τ<SUB>5</SUB> = 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. <BR /> Results: At log τ<SUB>5</SUB>
  = 0 we find no evidence of regions with dynamically weak (B&lt;
  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.
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<SUP>-1</SUP> 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.
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. <BR /> Aims: We study the topology of the penumbral magnetic
  field in the lower photosphere, focusing on regions where it returns
  below the surface. <BR /> 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. <BR /> Results: Observations in visible
  and near-infrared spectral lines yield a significant difference in the
  penumbral area covered by magnetic fields of opposite polarity. In
  the infrared, the number of reversed-polarity Stokes V profiles is
  smaller by a factor of two than in the visible. For three-lobed Stokes
  V profiles the numbers differ by up to an order of magnitude.

---------------------------------------------------------
Title: Horizontal flow fields in and around a small active region. The
    transition period between flux emergence and decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
   Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
   A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
   A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
   Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
   M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
   K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016A&A...596A...3V    Altcode: 2016arXiv160507462V
  Context. The solar magnetic field is responsible for all aspects
  of solar activity. Thus, emergence of magnetic flux at the surface
  is the first manifestation of the ensuing solar activity. <BR />
  Aims: Combining high-resolution and synoptic observations aims to
  provide a comprehensive description of flux emergence at photospheric
  level and of the growth process that eventually leads to a mature
  active region. <BR /> Methods: The small active region NOAA 12118
  emerged on 2014 July 17 and was observed one day later with the 1.5-m
  GREGOR solar telescope on 2014 July 18. High-resolution time-series
  of blue continuum and G-band images acquired in the blue imaging
  channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were
  complemented by synoptic line-of-sight magnetograms and continuum
  images obtained with the Helioseismic and Magnetic Imager (HMI) onboard
  the Solar Dynamics Observatory (SDO). Horizontal proper motions and
  horizontal plasma velocities were computed with local correlation
  tracking (LCT) and the differential affine velocity estimator (DAVE),
  respectively. Morphological image processing was employed to measure
  the photometric and magnetic area, magnetic flux, and the separation
  profile of the emerging flux region during its evolution. <BR />
  Results: The computed growth rates for photometric area, magnetic
  area, and magnetic flux are about twice as high as the respective
  decay rates. The space-time diagram using HMI magnetograms of five days
  provides a comprehensive view of growth and decay. It traces a leaf-like
  structure, which is determined by the initial separation of the two
  polarities, a rapid expansion phase, a time when the spread stalls,
  and a period when the region slowly shrinks again. The separation
  rate of 0.26 km s<SUP>-1</SUP> is highest in the initial stage, and
  it decreases when the separation comes to a halt. Horizontal plasma
  velocities computed at four evolutionary stages indicate a changing
  pattern of inflows. In LCT maps we find persistent flow patterns such
  as outward motions in the outer part of the two major pores, a diverging
  feature near the trailing pore marking the site of upwelling plasma and
  flux emergence, and low velocities in the interior of dark pores. We
  detected many elongated rapidly expanding granules between the two
  major polarities, with dimensions twice as large as the normal granules.

---------------------------------------------------------
Title: 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.
2016A&A...596A...8J    Altcode: 2016arXiv160801988J
  <BR /> 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. <BR /> 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. <BR /> 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. <BR /> Conclusions: Thanks to the high spatial resolution
  of the observations that is possible with the GREGOR telescope at 1.08
  microns, we find that the prominent small-scale fluctuations in the
  magnetic field inclination, which are a salient part of the property
  of sunspot penumbral photospheres, also persist in the chromosphere,
  although at somewhat reduced amplitudes. Such a complex magnetic
  configuration may facilitate penumbral chromospheric dynamic phenomena,
  such as penumbral micro-jets or transient bright dots.

---------------------------------------------------------
Title: Active region fine structure observed at 0.08 arcsec resolution
Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.;
   Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.;
   Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.;
   Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann,
   T.; Kneer, F.; Nicklas, H.; Sobotka, M.
2016A&A...596A...7S    Altcode: 2016arXiv160707094S
  Context. The various mechanisms of magneto-convective energy transport
  determine the structure of sunspots and active regions. <BR />
  Aims: We characterise the appearance of light bridges and other
  fine-structure details and elaborate on their magneto-convective
  nature. <BR /> Methods: We present speckle-reconstructed images taken
  with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the
  486 nm and 589 nm bands. We estimate the spatial resolution from the
  noise characteristics of the image bursts and obtain 0.08″ at 589
  nm. We describe structure details in individual best images as well
  as the temporal evolution of selected features. <BR /> Results: We
  find branched dark lanes extending along thin (≈1″) light bridges
  in sunspots at various heliocentric angles. In thick (≳ 2″) light
  bridges the branches are disconnected from the central lane and have a Y
  shape with a bright grain toward the umbra. The images reveal that light
  bridges exist on varying intensity levels and that their small-scale
  features evolve on timescales of minutes. Faint light bridges show
  dark lanes outlined by the surrounding bright features. Dark lanes are
  very common and are also found in the boundary of pores. They have a
  characteristic width of 0.1″ or smaller. Intergranular dark lanes of
  that width are seen in active region granulation. <BR /> Conclusions: We
  interpret our images in the context of magneto-convective simulations
  and findings: while central dark lanes in thin light bridges are
  elevated and associated with a density increase above upflows, the dark
  lane branches correspond to locations of downflows and are depressed
  relative to the adjacent bright plasma. Thick light bridges with central
  dark lanes show no projection effect. They have a flat elevated plateau
  that falls off steeply at the umbral boundary. There, Y-shaped filaments
  form as they do in the inner penumbra. This indicates the presence of
  inclined magnetic fields, meaning that the umbral magnetic field is
  wrapped around the convective light bridge.

---------------------------------------------------------
Title: Probing deep photospheric layers of the quiet Sun with high
    magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
   M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
   Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
   Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
   Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
   K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
   Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
   Schmidt, D.; Kiess, C.; Nicklas, H.
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. <BR /> 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. <BR /> 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). <BR /> 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. <BR /> 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.
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.
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. <BR />
  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. <BR /> 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. <BR /> 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. <P />Movies associated to Figs. 2 and 13
  are available at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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.
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 (&lt;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. <BR /> 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. <BR /> 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. <BR /> Results: Half of our observed
  quiet-Sun region is better explained by magnetic substructure within
  the resolution element. However, we cannot distinguish whether this
  substructure comes from gradients of the physical parameters along the
  line of sight or from horizontal gradients (across the surface). In
  these pixels, a model with two magnetic components is preferred, and
  we find two distinct magnetic field populations. The population with
  the larger filling factor has very weak ( 150 G) horizontal fields
  similar to those obtained in previous works. We demonstrate that the
  field vector of this population is not constrained by the observations,
  given the spatial resolution and polarimetric accuracy of our data. The
  topology of the other component with the smaller filling factor is
  constrained by the observations for field strengths above 250 G:
  we infer hG fields with inclinations and azimuth values compatible
  with an isotropic distribution. The filling factors are typically
  below 30%. We also find that the flux of the two polarities is not
  balanced. From the other half of the observed quiet-Sun area 50% are
  two-lobed Stokes V profiles, meaning that 23% of the field of view
  can be adequately explained with a single constant magnetic field
  embedded in a non-magnetic atmosphere. The magnetic field vector and
  filling factor are reliable inferred in only 50% based on the regular
  profiles. Therefore, 12% of the field of view harbour hG fields with
  filling factors typically below 30%. At our present spatial resolution,
  70% of the pixels apparently are non-magnetised.

---------------------------------------------------------
Title: Fitting peculiar spectral profiles in He I 10830Å absorption
    features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
   Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
   Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
   Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
   Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
   M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
   M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1057G    Altcode: 2016arXiv160300679G
  The new generation of solar instruments provides better
  spectral, spatial, and temporal resolution for a better
  understanding of the physical processes that take place on the
  Sun. Multiple-component profiles are more commonly observed with these
  instruments. Particularly, the He I 10830 Å triplet presents such
  peculiar spectral profiles, which give information on the velocity
  and magnetic fine structure of the upper chromosphere. The purpose
  of this investigation is to describe a technique to efficiently fit
  the two blended components of the He I 10830 Å triplet, which are
  commonly observed when two atmospheric components are located within
  the same resolution element. The observations used in this study were
  taken on 2015 April 17 with the very fast spectroscopic mode of the
  GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
  telescope, located at the Observatorio del Teide, Tenerife, Spain. We
  apply a double-Lorentzian fitting technique using Levenberg-Marquardt
  least-squares minimization. This technique is very simple and much
  faster than inversion codes. Line-of-sight Doppler velocities can
  be inferred for a whole map of pixels within just a few minutes. Our
  results show sub- and supersonic downflow velocities of up to 32 km
  s<SUP>-1</SUP> for the fast component in the vicinity of footpoints of
  filamentary structures. The slow component presents velocities close
  to rest.

---------------------------------------------------------
Title: Flows in and around Active Region NOAA12118 Observed with
    the GREGOR Solar Telescope and SDO/HMI
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
   Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
   A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
   A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
   Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
   M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
   K.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016ASPC..504...29V    Altcode: 2016arXiv160301109V
  Accurate measurements of magnetic and velocity fields in and around
  solar active regions are key to unlocking the mysteries of the
  formation and the decay of sunspots. High spatial resolution images
  and spectral sequences with a high cadence obtained with the GREGOR
  solar telescope give us an opportunity to scrutinize 3-D flow fields
  with local correlation tracking and imaging spectroscopy. We present
  GREGOR early science data acquired in 2014 July - August with the GREGOR
  Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series
  of blue continuum (λ 450.6 nm) images of the small active region
  NOAA 12118 were restored with the speckle masking technique to derive
  horizontal proper motions and to track the evolution of morphological
  changes. In addition, high-resolution observations are discussed in
  the context of synoptic data from the Solar Dynamics Observatory.

---------------------------------------------------------
Title: 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.
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, B<SUP>stable</SUP><SUB>ver</SUB>, was found
  along the umbra-penumbra boundary of developed sunspots. <BR /> Aims: We
  study the changing value of B<SUB>ver</SUB> as the penumbra forms and as
  it reaches a stable state. We compare this with the corresponding value
  in fully developed penumbrae. <BR /> 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. <BR /> Results: The vertical component of the magnetic
  field, B<SUB>ver</SUB>, 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 B<SUB>ver</SUB> value comparable to that of
  umbra-penumbra boundaries of fully fledged sunspots. <BR /> 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 B<SUB>ver</SUB><SUP>stable</SUP>. During
  the penumbra formation process, the inner penumbra extends
  into regions where the umbra previously prevailed. Hence, in
  areas where B<SUB>ver</SUB>&lt;B<SUP>stable</SUP><SUB>ver</SUB>,
  the magneto-convection mode operating in the umbra turns into a
  penumbral mode. Eventually, the inner penumbra boundary settles at
  B<SUB>ver</SUB><SUP>stable</SUP>, which hints toward the role of
  B<SUB>ver</SUB><SUP>stable</SUP> as inhibitor of the penumbral mode
  of magneto-convection.

---------------------------------------------------------
Title: A distinct magnetic property of the inner penumbral boundary
Authors: Jurčák, Jan; Bello Gonzalez, Nazaret; Schlichenmaier,
   Rolf; Rezaei, Reza
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 $B_{\rm ver} &lt;
  B^{\rm stable}_{\rm ver}$, 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: Evolution of magnetic field inclination in a forming penumbra
Authors: Jurčák, Jan; Bello González, Nazaret; Schlichenmaier,
   Rolf; Rezaei, Reza
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: Stray-light correction in 2D spectroscopy
Authors: Schlichenmaier, R.; Franz, M.
2013A&A...555A..84S    Altcode:
  Context. In solar physics, spectropolarimeters based on Fabry-Pérot
  interferometers are commonly used for high spatial resolution
  observations. In the data pipeline, corrections for scattered light may
  be performed on each narrow band image. <BR /> Aims: We elaborate on
  the effects of stray-light corrections on Doppler maps. <BR /> Methods:
  First, we demonstrate the basic correction effect in a simplified
  situation of two profiles that suffer from stray light. Then, we study
  the correction effects on velocity maps by transforming a Hinode SP
  map into a two-dimensional spectroscopic data set with i(x,y) at each
  wavelength point, which mimicks narrow band images. Velocity maps are
  inferred from line profiles of original and stray-light corrected
  data. <BR /> Results: The correction of scattered light in narrow
  band images affects the inferred Doppler velocity maps: relative red
  shifts always become more red, and relative blue shifts become more
  blue. This trend is independent of whether downflows have dark or bright
  intensities. As a result, the effects of overcorrection produce both
  downflows and upflows. <BR /> Conclusions: In 2D spectropolarimetry,
  corrections for scattered light can improve the image intensity and
  velocity contrast but inherently produce downflow signatures in the
  penumbra. Hence, such corrections are justified only if the properties
  of the stray light (seeing, telescope, and instrument) are well known.

---------------------------------------------------------
Title: Correlations between sunspots and their moat flows
Authors: Löhner-Böttcher, J.; Schlichenmaier, R.
2013A&A...551A.105L    Altcode: 2013arXiv1301.2434L
  Context. The presence of the moat flow around sunspots is intimately
  linked to the mere existence of sunspots. <BR /> Aims: We characterize
  the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our
  knowledge of sunspot structures and photospheric flow properties. <BR
  /> Methods: We calibrated HMI synoptic Doppler maps and used them
  to analyze 3 h time averages of 31 circular, stable, and fully
  developed sunspots at heliocentric angles of some 50°. Assuming
  axially symmetrical flow fields, we infer the azimuthally averaged
  horizontal velocity component of the MF and EF from 51 velocity maps. We
  studied the MF properties (velocity and extension) and elaborate on
  how these components depend on sunspot parameters (sunspot size and
  EF velocity). To explore the weekly and monthly evolution of MFs, we
  compare spots rotating from the eastern to western limbs and spots that
  reappear on the eastern limb. <BR /> Results: Our calibration procedure
  of HMI Doppler maps yields reliable and consistent results. In 3 h
  averages, we find the MF decreases on average from some 1000 ± 200 m/s
  just outside the spot boundary to 500 m/s after an additional 4 Mm. The
  average MF extension lies at 9.2 ± 5 Mm, where the velocity drops
  below some 180 m/s. Neither the MF velocity nor its extension depend
  significantly on the sunspot size or EF velocity. But, the EF velocity
  does show a tendency to be enhanced with sunspot size. On a time scale
  of a week and a month, we find decreasing MF extensions and a tendency
  for the MF velocity to increase for strongly decaying sunspots, whereas
  the changing EF velocity has no impact on the MF. <BR /> Conclusions:
  On 3 h averages, the EF velocity scales with the size of sunspots, while
  the MF properties show no significant correlation with the EF or with
  the sunspot size. This we interpret as a hint that the physical origins
  of EF and MF are distinct. <P />Appendix A is available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: The velocity field of sunspot penumbrae. II. Return flow and
    magnetic fields of opposite polarity
Authors: Franz, M.; Schlichenmaier, R.
2013A&A...550A..97F    Altcode: 2012arXiv1212.4732F
  <BR /> Aims: We search for penumbral magnetic fields of opposite
  polarity and for their correspondence with downflows. <BR /> Methods:
  We used spectropolarimetric HINODE data of a spot very close to
  disk center to suppress the horizontal velocity components as much
  as possible. We focus our study on 3-lobe Stokes V profiles. <BR />
  Results: From forward modeling and inversions, we show that 3-lobe
  profiles testify to the presence of opposite magnetic fields. They occur
  predominately in the mid and outer penumbra and are associated with
  downflows in the deep layers of the photosphere. <BR /> Conclusions:
  Standard magnetograms show that only 4% of the penumbral area harbors
  magnetic fields of opposite polarity. If 3-lobe profiles are included
  in the analysis, this number increases to 17%.

---------------------------------------------------------
Title: 2nd ATST-EAST Workshop in Solar Physics: Magnetic Fields from
    the Photosphere to the Corona
Authors: Rimmele, T. R.; Tritschler, A.; Wöger, F.; Collados Vera,
   M.; Socas-Navarro, H.; Schlichenmaier, R.; Carlsson, M.; Berger, T.;
   Cadavid, A.; Gilbert, P. R.; Goode, P. R.; Knölker, M.
2012ASPC..463.....R    Altcode:
  No abstract at ADS

---------------------------------------------------------
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.
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: GRIS: The GREGOR Infrared Spectrograph
Authors: Collados, M.; López, R.; Páez, E.; Hernández, E.; Reyes,
   M.; Calcines, A.; Ballesteros, E.; Díaz, J. J.; Denker, C.; Lagg,
   A.; Schlichenmaier, R.; Schmidt, W.; Solanki, S. K.; Strassmeier,
   K. G.; von der Lühe, O.; Volkmer, R.
2012AN....333..872C    Altcode:
  This paper describes the main characteristics of GRIS (GREGOR Infrared
  Spectrograph), the grating spectrograph installed in the recently
  inaugurated (May 2012) 1.5-meter GREGOR telescope located at the
  Observatorio del Teide in Tenerife. The spectrograph has a standard
  Czerny-Turner configuration with parabolic collimator and camera mirrors
  that belong to the same conic surface. Although nothing prevents its
  use at visible wavelengths, the spectrograph will be initially used
  in combination with the infrared detector of the Tenerife Infrared
  Polarimeter (TIP-II) in standard spectroscopic mode as well as for
  spectropolarimetric measurements.

---------------------------------------------------------
Title: Supersonic Magnetic Flows in the Quiet Sun Observed with
    SUNRISE/IMaX
Authors: Borrero, J. M.; Pillet, V. M.; Schlichenmaier, R.; Schmidt,
   W.; Berkefeld, T.; Solanki, S. K.; Bonet, J. A.; Iniesta, J. C. d. T.;
   Domingo, V.; Barthol, P.; Gandorfer, A.
2012ASPC..455..155B    Altcode: 2012arXiv1202.4354B
  In this contribution we describe some recent observations of high-speed
  magnetized flows in the quiet Sun granulation. These observations
  were carried out with the Imaging Magnetograph eXperiment (IMaX)
  onboard the stratospheric balloon SUNRISE, and possess an unprecedented
  spatial resolution and temporal cadence. These flows were identified as
  highly shifted circular polarization (Stokes V) signals. We estimate
  the LOS velocity responsible for these shifts to be larger than 6 km
  s<SUP>-1</SUP>, and therefore we refer to them as supersonic magnetic
  flows. The average lifetime of the detected events is 81.3 s and
  they occupy an average area of about 23 000 km<SUP>2</SUP>. Most of
  the events occur within granular cells and correspond therefore to
  upflows. However some others occur in intergranular lanes or bear no
  clear relation to the convective velocity pattern. We analyze a number
  of representative examples and discuss them in terms of magnetic loops,
  reconnection events, and convective collapse.

---------------------------------------------------------
Title: On the Formation of Penumbrae as Observed with the German
    VTT SOHO/MDI, and SDO/HMI
Authors: Schlichenmaier, R.; Rezaei, R.; González, N. B.
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: Shear and vortex motions in a forming sunspot . Twist
    relaxation in magnetic flux ropes
Authors: Bello González, N.; Kneer, F.; Schlichenmaier, R.
2012A&A...538A..62B    Altcode:
  <BR /> Aims: We measure proper motions of fine structures in a forming
  sunspot to infer information about the dynamics of flux emergence at
  the sub-photospheric level. <BR /> Methods: The active region NOAA
  11024 was observed with the Vacuum Tower Telescope at Observatorio del
  Teide/Tenerife over several days in July 2009. Here, we concentrate
  on a two-hour sequence taken on July 4, when the leading spot was
  at an early stage of its evolution. Speckle reconstructions from Ca
  ii K images and polarimetric data in Fe i λ6173 allow us to study
  proper motions of umbral fine structures. <BR /> Results: We detect
  three prominent features: (1) A light bridge, divided by a dark lane
  along its axis, shows proper motions in opposing directions on its
  sides, with velocities of ~100-500 m s<SUP>-1</SUP>. The flows are
  seen in both the Ca ii K and the broadband time sequences. (2) Umbral
  dots in one umbral region outline a vortex with speeds of up to 550
  m s<SUP>-1</SUP>. The direction of the motion of the umbral dots is
  different from that in the light bridge. (3) At one rim of the umbra,
  the fine structure of the magnetic field moves horizontally with typical
  velocities of 250-300 m s<SUP>-1</SUP>, prior to the formation of the
  penumbra. <BR /> Conclusions: We report on shear and vortex motions in
  a forming sunspot and interpret them as tracers of twist relaxation
  in magnetic flux ropes. We suggest that the forming sunspot contains
  detached magnetic flux ropes that emerge at the surface with different
  amounts of twist. As they merge to form a sunspot, they untwist giving
  rise to the observed shear and vortex motions.

---------------------------------------------------------
Title: The formation of sunspot penumbra. Magnetic field properties
Authors: Rezaei, R.; Bello González, N.; Schlichenmaier, R.
2012A&A...537A..19R    Altcode: 2011arXiv1111.3189R
  <BR /> Aims: We study the magnetic flux emergence and formation of
  a sunspot penumbra in the active region NOAA 11024. <BR /> 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). <BR /> 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 × 10<SUP>20</SUP>
  to 18.2 × 10<SUP>20</SUP> Mx, while the magnetic flux of the umbra
  remains constant at ~3.8 × 10<SUP>20</SUP> 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 × 10<SUP>18</SUP> Mx. The spot's flux increase rate of 4.2 ×
  10<SUP>16</SUP> Mx s<SUP>-1</SUP> 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. <BR /> 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: Sunspot Modeling: From Simplified Models to Radiative MHD
    Simulations
Authors: Rempel, Matthias; Schlichenmaier, Rolf
2011LRSP....8....3R    Altcode:
  We review our current understanding of sunspots from the scales of their
  fine structure to their large scale (global) structure including the
  processes of their formation and decay. Recently, sunspot models have
  undergone a dramatic change. In the past, several aspects of sunspot
  structure have been addressed by static MHD models with parametrized
  energy transport. Models of sunspot fine structure have been relying
  heavily on strong assumptions about flow and field geometry (e.g.,
  flux-tubes, "gaps", convective rolls), which were motivated in
  part by the observed filamentary structure of penumbrae or the
  necessity of explaining the substantial energy transport required
  to maintain the penumbral brightness. However, none of these models
  could self-consistently explain all aspects of penumbral structure
  (energy transport, filamentation, Evershed flow). In recent years,
  3D radiative MHD simulations have been advanced dramatically to the
  point at which models of complete sunspots with sufficient resolution
  to capture sunspot fine structure are feasible. Here, overturning
  convection is the central element responsible for energy transport,
  filamentation leading to fine structure, and the driving of strong
  outflows. On the larger scale these models are also in the progress
  of addressing the subsurface structure of sunspots as well as sunspot
  formation. With this shift in modeling capabilities and the recent
  advances in high resolution observations, the future research will be
  guided by comparing observation and theory.

---------------------------------------------------------
Title: The formation of a penumbra as observed with the German VTT
    and SoHO/MDI
Authors: Schlichenmaier, Rolf; González, Nazaret Bello; Rezaei, Reza
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. <P />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. <P />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: Searching for Overturning Convection in Penumbral Filaments:
    Slit Spectroscopy at 0farcs2 Resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Langhans, K.
2010ApJ...725...11B    Altcode: 2010arXiv1009.5650B
  Recent numerical simulations of sunspots suggest that overturning
  convection is responsible for the existence of penumbral filaments
  and the Evershed flow, but there is little observational evidence
  of this process. Here, we carry out a spectroscopic search for
  small-scale convective motions in the penumbra of a sunspot located
  5° away from the disk center. The position of the spot is very
  favorable for the detection of overturning downflows at the edges
  of penumbral filaments. Our analysis is based on measurements of
  the Fe I 709.0 nm line taken with the Littrow spectrograph of the
  Swedish 1 m Solar Telescope under excellent seeing conditions. We
  compute line bisectors at different intensity levels and derive
  Doppler velocities from them. The velocities are calibrated using
  a nearby telluric line, with systematic errors smaller than 150 m
  s<SUP>-1</SUP>. Deep in the photosphere, as sampled by the bisectors
  at the 80%-88% intensity levels, we always observe blueshifts or
  zero velocities. The maximum blueshifts reach 1.2 km s<SUP>-1</SUP>
  and tend to be cospatial with bright penumbral filaments. In the line
  core, we detect blueshifts for the most part, with small velocities
  not exceeding 300 m s<SUP>-1</SUP>. Redshifts also occur, but at the
  level of 100-150 m s<SUP>-1</SUP>, and only occasionally. The fact
  that they are visible in high layers casts doubts on their convective
  origin. Overall, we do not find indications of downflows that could be
  associated with overturning convection at our detection limit of 150
  m s<SUP>-1</SUP>. Either no downflows exist, or we have been unable
  to observe them because they occur beneath τ = 1 or the spatial
  resolution/height resolution of the measurements is still insufficient.

---------------------------------------------------------
Title: Supersonic Magnetic Upflows in Granular Cells Observed with
    SUNRISE/IMAX
Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.;
   Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.;
   Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M.
2010ApJ...723L.144B    Altcode: 2010arXiv1009.1227B
  Using the IMaX instrument on board the SUNRISE stratospheric balloon
  telescope, we have detected extremely shifted polarization signals
  around the Fe I 5250.217 Å spectral line within granules in the solar
  photosphere. We interpret the velocities associated with these events
  as corresponding to supersonic and magnetic upflows. In addition, they
  are also related to the appearance of opposite polarities and highly
  inclined magnetic fields. This suggests that they are produced by the
  reconnection of emerging magnetic loops through granular upflows. The
  events occupy an average area of 0.046 arcsec<SUP>2</SUP> and last for
  about 80 s, with larger events having longer lifetimes. These supersonic
  events occur at a rate of 1.3 × 10<SUP>-5</SUP> occurrences per second
  per arcsec<SUP>2</SUP>.

---------------------------------------------------------
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
   R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
   B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
   M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
   Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
2010SoPh..267....1M    Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
  While sunspots are easily observed at the solar surface, determining
  their subsurface structure is not trivial. There are two main
  hypotheses for the subsurface structure of sunspots: the monolithic
  model and the cluster model. Local helioseismology is the only means
  by which we can investigate subphotospheric structure. However, as
  current linear inversion techniques do not yet allow helioseismology to
  probe the internal structure with sufficient confidence to distinguish
  between the monolith and cluster models, the development of physically
  realistic sunspot models are a priority for helioseismologists. This
  is because they are not only important indicators of the variety of
  physical effects that may influence helioseismic inferences in active
  regions, but they also enable detailed assessments of the validity of
  helioseismic interpretations through numerical forward modeling. In
  this article, we provide a critical review of the existing sunspot
  models and an overview of numerical methods employed to model wave
  propagation through model sunspots. We then carry out a helioseismic
  analysis of the sunspot in Active Region 9787 and address the serious
  inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
  this sunspot is most probably associated with a shallow, positive
  wave-speed perturbation (unlike the traditional two-layer model)
  and that travel-time measurements are consistent with a horizontal
  outflow in the surrounding moat.

---------------------------------------------------------
Title: The role of emerging bipoles in the formation of a sunspot
    penumbra
Authors: Schlichenmaier, R.; Bello González, N.; Rezaei, R.; Waldmann,
   T. A.
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: Center to limb variation of penumbral Stokes V profiles
Authors: Franz, M.; Schlichenmaier, R.
2010AN....331..570F    Altcode: 2010arXiv1008.2346F
  We investigated the horizontal and the vertical component of the
  Evershed flow (EF). To this end, we computed average Stokes V profiles
  for various velocity classes in penumbrae at different heliocentric
  angles. Our results show that for blueshifted profiles an additional
  lobe with the same polarity as the spot is present in the blue side
  of the average Stokes V profile. The amplitude of the additional
  lobe grows with increasing blueshift and with increasing heliocentric
  angle. For small redshifts, the profiles show an additional lobe with
  the opposite polarity as the spot on the red side of the average Stokes
  V profile. Even at disk center, the original polarity of the average
  Stokes V profile is reversed for strong redshifts. The transition
  between the different types of Stokes V profiles is continuous and
  indicates that not only the vertical, but also the horizontal EF is
  a magnetized stream of plasma in a magnetic background field.

---------------------------------------------------------
Title: The formation of a sunspot penumbra
Authors: Schlichenmaier, R.; Rezaei, R.; Bello González, N.; Waldmann,
   T. A.
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. <BR />
  Aims: We aim to enhance our knowledge of how a sunspot penumbra forms
  and how sunspots grow in size. <BR /> 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. <BR /> 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 arcsec<SUP>2</SUP> with only a few penumbral filaments and
  then grows to a size of 360 arcsec<SUP>2</SUP>. 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. <BR /> 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. <P />Movies are only available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Small-Scale Velocities in Sunspot Penumbrae
Authors: Franz, M.; Schlichenmaier, R.; Schmidt, W.
2010ASSP...19..510F    Altcode: 2010mcia.conf..510F
  To investigate the penumbral plasma flow at small scales, we used
  spectropolarimetric data of sunspots recorded by Hinode at 07:00UT
  and 16:00UT on 14 November 2008.We computed maps of apparent Doppler
  velocities by comparing the spectral position of the Fe I 630.15nm line
  with the position of the line core of an average quiet Sun profile. We
  evaluated the bisector of the line wing to investigate the flow pattern
  in the deep photosphere.

---------------------------------------------------------
Title: Spectral Analysis of Sunspot Penumbrae Observed with Hinode
Authors: Franz, M.; Schlichenmaier, R.
2009ASPC..415..369F    Altcode: 2010arXiv1008.2391F
  To investigate the penumbral plasma flow on a small scale,
  spectropolarimetric data of sunspots recorded by Hinode was used. Maps
  of Doppler velocities were created by evaluating the bisector in
  the line-wing, thereby visualizing the flow pattern in the low
  photosphere where the Evershed effect is most pronounced. <P />In
  penumbrae close to the disk center, the vertical component of the
  Evershed flow dominates. The latter consists of a series of elongated
  up-flow patterns extending radially through the entire center-side
  penumbra at a constant azimuth. Along this structure, strong up-flows
  appear in concentrated patches separated by weaker up-flows or even
  down-flows. The strong up-flows appear at the bright heads and the
  umbral side of the dark-core of the filament, while the down-flows
  are rather located at the penumbral side of the filament. Projection
  effects lead to an overall red-shift of the limb-side penumbra, but
  the described pattern of up- and down-flows is still ascertainable.

---------------------------------------------------------
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.
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 velocity field of sunspot penumbrae. I. A global view
Authors: Franz, M.; Schlichenmaier, R.
2009A&A...508.1453F    Altcode: 2009arXiv0909.4744F
  Aims. We investigated the vertical penumbral plasma flow on small
  spatial scales using data recorded by the spectropolarimeter of the
  solar optical telescope onboard Hinode.<BR /> Methods: We computed maps
  of apparent Doppler velocities by comparing the spectral positions
  of the Fe I 630.15 nm &amp; Fe I 630.25 nm lines with the averaged
  line profiles of the quiet Sun. To visualize the flow pattern in the
  low photosphere, we used a bisector of the wing of the absorption
  lines. The small heliocentric angle (3° ≤ Θ ≤ 9°) of our data
  sets means that the horizontal component of the Evershed flow (EF)
  does not contribute significantly to the line shift.<BR /> Results: We
  found that in the quiet Sun (QS), the area showing upflows is always
  larger than the one exhibiting downflows. In the penumbra, upflows
  dominate only at low velocities |v_dop| ≤ 0.4 km s<SUP>-1</SUP>,
  while at higher velocities |v_dop| ≥ 0.6 km s<SUP>-1</SUP> downflows
  prevail. Additionally, the maximal upflow velocity in penumbrae is
  lower, while the maximal downflow velocity is larger with respect to
  the QS velocities. Furthermore, on a spatial average, the penumbra
  shows a redshift, corresponding to a downflow of more than 0.1 km
  s<SUP>-1</SUP>. Upflows are elongated and appear predominately in
  the inner penumbra. Strong downflows with velocities of up to 9 km
  s<SUP>-1</SUP> are concentrated at the penumbra-QS boundary. They are
  magnetized and are rather round. The inner penumbra shows an average
  upflow, which turns into a mean downflow in the outer penumbra. The
  upflow patches in the inner penumbra and the downflow locations in the
  outer penumbra could be interpreted as the sources and the sinks of
  the EF. We did not find any indication of roll-type convection within
  penumbral filaments.<BR />

---------------------------------------------------------
Title: Temporal Evolution of Magnetic Elements
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Beck, C.
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: Sunspots: From Small-Scale Inhomogeneities Towards a Global
    Theory
Authors: Schlichenmaier, Rolf
2009SSRv..144..213S    Altcode: 2008SSRv..tmp..187S; 2008arXiv0811.2747S
  The penumbra of a sunspot is a fascinating phenomenon featuring complex
  velocity and magnetic fields. It challenges both our understanding
  of radiative magneto-convection and our means to measure and derive
  the actual geometry of the magnetic and velocity fields. In this
  contribution we attempt to summarize the present state-of-the-art
  from an observational and a theoretical perspective. We describe
  spectro-polarimetric measurements which reveal that the penumbra is
  inhomogeneous, changing the modulus and the direction of the velocity,
  and the strength and the inclination of the magnetic field with
  depth, i.e., along the line-of-sight, and on spatial scales below
  0.5 arcsec. Yet, many details of the small-scale geometry of the
  fields are still unclear such that the small scale inhomogeneities
  await a consistent explanation. A simple model which relies on
  magnetic flux tubes evolving in a penumbral “background”
  reproduces some properties of sunspot inhomogeneities, like its
  filamentation, its strong (Evershed-) outflows, and its uncombed
  geometry, but it encounters some problems in explaining the penumbral
  heat transport. Another model approach, which can explain the heat
  transport and long bright filaments, but fails to explain the Evershed
  flow, relies on elongated convective cells, either field-free as in
  the gappy penumbra or filled with horizontal magnetic field as in
  Danielson’s convective rolls. Such simplified models fail to give a
  consistent picture of all observational aspects, and it is clear that
  we need a more sophisticated description of the penumbra, that must
  result from simulations of radiative magneto-convection in inclined
  magnetic fields. First results of such simulations are discussed. The
  understanding of the small-scales will then be the key to understand
  the global structure and the large-scale stability of sunspots.

---------------------------------------------------------
Title: Sunspots: From Small-Scale Inhomogeneities Towards a Global
    Theory
Authors: Schlichenmaier, Rolf
2009odsm.book..213S    Altcode:
  The penumbra of a sunspot is a fascinating phenomenon featuring complex
  velocity and magnetic fields. It challenges both our understanding
  of radiative magneto-convection and our means to measure and derive
  the actual geometry of the magnetic and velocity fields. In this
  contribution we attempt to summarize the present state-of-the-art from
  an observational and a theoretical perspective.

---------------------------------------------------------
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.
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 Small Scale Flow Field of a Sunspot Penumbra
Authors: Schlichenmaier, R.; Franz, M.
2008ESPM...12.2.28S    Altcode:
  A sunspot is a coherent phenomenon on large spatial and temporal
  scales, but it consists of an ensemble of small-scale and dynamic
  features. It seems crucial to realize that a sunspot is not static,
  but manifests a dynamic equilibrium: The dynamic fine structure forms
  a globally stable sunspot, and it is the goal of sunspot physics to
  understand how an ensemble of short living features on small scales is
  organized to form a coherently large and long living sunspot. To this
  end it is a necessity to investigate the nature of the fine structure:
  its small-scale flow field, the topology of the magnetic field, and the
  radiative interactions that form the intensity fine structure. We know
  that the energy transport has to be of convective nature, but up to now
  the underlying convective processes remain unclear. Is it convection in
  magnetic field free gaps that exist in the space separating a strong
  magnetic field which is more or less static? Is it convective flows
  that are channeled by magnetic flux tube? Or is it dissipative turbulent
  magneto-convection? <P />In this contribution we study the small-scale
  flow field in order to learn about the convective flow pattern. We
  take advantage of the high spatial and high spectral resolution data
  from Hinode. The satellite has acquired spectroscopic data from disk
  passages of about 10 sunspots in 2007. Taking into account the varying
  viewing angle according to the location of the sunspot on the disk,
  we investigate the small-scale flow pattern. Thereby, we aim at an
  understanding of the type of convection that is responsible for the
  Evershed flow, for the small-scale penumbral structure, and for the
  energy transport in penumbrae.

---------------------------------------------------------
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.
2008A&A...484..503R    Altcode: 2008arXiv0804.2325R
  Aims: We study chromospheric emission to understand the temperature
  stratification in the solar chromosphere. <BR />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 10<SUP>5</SUP> 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. <BR />Results: A fraction of about
  25% of the observed Ca II H line profiles do not show a measurable
  emission peak in H<SUB>2v</SUB> and H<SUB>2r</SUB> 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. <BR />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: Fine Structure of the Net Circular Polarization in a Sunspot
    Penumbra
Authors: Tritschler, A.; Müller, D. A. N.; Schlichenmaier, R.;
   Hagenaar, H. J.
2007ApJ...671L..85T    Altcode: 2007arXiv0710.4545T
  We present novel evidence for fine structure observed in the
  net circular polarization (NCP) of a sunspot penumbra based on
  spectropolarimetric measurements utilizing the Zeeman-sensitive Fe
  I 630.2 nm line. For the first time we detect filamentary organized
  fine structure of the NCP on spatial scales that are similar to the
  inhomogeneities found in the penumbral flow field. We also observe an
  additional property of the visible NCP, a zero-crossing of the NCP
  in the outer parts of the center-side penumbra, which has not been
  recognized before. In order to interpret the observations we solve the
  radiative transfer equations for polarized light in a model penumbra
  with embedded magnetic flux tubes. We demonstrate that the observed
  zero-crossing of the NCP can be explained by an increased magnetic
  field strength inside magnetic flux tubes in the outer penumbra combined
  with a decreased magnetic field strength in the background field. Our
  results strongly support the concept of the uncombed penumbra.

---------------------------------------------------------
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.
2007A&A...476L..33R    Altcode: 2007arXiv0711.0408R
  Aims:We study the structure of the magnetic elements in network-cell
  interiors. <BR />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. <BR
  />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. <BR
  />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: Magnetic properties of G-band bright points in a sunspot moat
Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.;
   Sütterlin, P.
2007A&A...472..607B    Altcode: 2007arXiv0707.1232B
  We present simultaneous spectropolarimetric observations of four
  visible (630 nm) and three infrared (1565 nm) spectral lines from the
  German Vacuum Tower Telescope, together with speckle-reconstructed
  filtergrams in the G-band and the Ca II H line core from the Dutch
  Open Telescope. After alignment of the data sets, we used the G-band
  intensity to locate bright points (BPs) in the moat of a regular
  sunspot. With the cospatial and cotemporal information provided by the
  polarimetric data, we characterize the magnetic, kinematic, and thermal
  properties of the BPs. We find that (a) 94% of the BPs are associated
  with magnetic fields; (b) their field strengths range between 500 and
  1400 G, with a rather flat distribution; (c) the contrast of BPs in the
  G-band depends on the angle between the vector magnetic field and the
  line of sight; (d) the BPs harbor downflows of magnetized plasma and
  exhibit Stokes V profiles with large area and amplitude asymmetries;
  (e) the magnetic interior of BPs is hotter than the immediate field-free
  surroundings by about 1000 K at equal optical depth; and (f) the mean
  effective diameter of BPs in our data set is 150 km, with very few
  BPs larger than 300 km. Most of these properties can be explained
  by the classical magnetic flux tube model. However, the wide range
  of BP parameters found in this study indicates that not all G-band
  BPs are identical to stable long-lived flux tubes or sheets of kG
  strength. <P />Appendices A-C are only available in electronic form
  at http://www.aanda.org

---------------------------------------------------------
Title: Variation of the Stokes-V area asymmetry across magnetic
    elements
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
   R.; Lites, B. W.
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.
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. <BR
  />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<SUP>-3</SUP> I_c. <BR />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. <BR
  />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.
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. <BR />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, H<SUB>non</SUB>,
  and a magnetically, H<SUB>mag</SUB>, heated component from a
  non-heated component, H<SUB>co</SUB> in the H-index. <BR />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<SUP>-2</SUP>. <BR />Conclusions:
  We find that a dominant fraction of the calcium emission caused by
  the heated atmosphere in the magnetic network has non-magnetic origin
  (H<SUB>mag</SUB>≈2 pm, H<SUB>non</SUB>≈3 pm). Considering the effect
  of straylight, the contribution from an atmosphere with no temperature
  rise to the H-index (H<SUB>co</SUB>≈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: Disentangling The Magnetic Field Structure Of Sunspots -
    Stereoscopic Polarimetry With Solar Orbiter
Authors: Müller, D. A. N.,; Schlichenmaier, R.; Fleck, B.; Fritz, G.
2007ESASP.641E..32M    Altcode:
  Sunspots exhibit complex, highly structured magnetic fields and
  flows. Disentangling the atmospheric structure of sunspots is a
  great challenge, and can only be achieved by the combination of
  spectropolarimetry at high spatial resoultion and detailed modeling
  efforts. We use a generalized 3D the embeds magnetic flux tuber in
  a stratified atmosphere and calculates the emerging polarization
  of spectrail lines for arbitrary viewing angles. The resulting
  polarization maps are a very efficient tool to distinguish between
  different atmospheric scenarios and determine the 3D structure of the
  magnetic field and the flow field. In this contribution, we present
  synthetic maps of the net circular polarication (NCP) as a function of
  the heliocentric angle for different spectral lines of interest. Among
  these are the Fe I 617.3 nm line which would be observed by the VIM
  instrument abard Solar Orbiter and the Fe I 630.2 nm line which will
  be observed by Hinode (formerly known as Solar-B).

---------------------------------------------------------
Title: Photospheric magnetic field and chromospheric emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Schmidt, W.
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: On the inhomogeneities of the sunspot penumbra
Authors: Schlichenmaier, R.; Müller, D. A. N.; Beck, C.
2007msfa.conf..233S    Altcode: 2007astro.ph..3021S
  The penumbra is ideally suited to challenge our understanding
  of magnetohydrodynamics. The energy transport takes place as
  magnetoconvection in inclined magnetic fields under the effect of
  strong radiative cooling at the surface. The relevant processes
  happen at small spatial scales. In this contribution we describe
  and elaborate on these small-scale inhomogeneities of a sunspot
  penumbra. We describe the penumbral properties inferred from imaging,
  spectroscopic and spectropolarimetric data, and discuss the question
  of how these observations can be understood in terms of proposed models
  and theoretical concepts.

---------------------------------------------------------
Title: Magnetic properties of G-band bright points
Authors: Beck, C.; Mikurda, K.; Bellot Rubio, L. R.; Schlichenmaier,
   R.; Sütterlin, P.
2007msfa.conf..165B    Altcode:
  Bright points (BPs) visible in the G band at 430 nm are commonly used
  as tracers of magnetic fields, indicating the location of kG flux
  concentrations. To study the actual magnetic properties of G-band BPs,
  we took observations in 2003 and 2005, employing simultaneously a
  speckle setup in the G band and vector spectropolarimetry to derive
  the magnetic field vector. From the analysis of the co-aligned
  polarimetric data we find that the BPs show a broad range of field
  strengths, magnetic fluxes, and field inclinations. Many G-band
  BPs are not co-spatial with the central part of the nearby flux
  concentrations. Even at the small heliocentric angle of only 12°,
  the BPs appear projected on adjacent granules, whereas the magnetic
  field is concentrated in the intergranular lanes. Our findings support
  the view that the G-band BPs are a result of the "hot wall effect". The
  downward shift of the optical depth scale in the presence of magnetic
  fields allows to see deeper and hotter layers in the hot granules next
  to the field concentrations, where CH dissociates. Thus, information
  drawn from imaging observations of BPs has limited use to investigate
  the actual magnetic field structure, when the BPs are not co-spatial
  with the central part of the flux concentrations.

---------------------------------------------------------
Title: Polarimetric Observations of the Formation of a G-Band
    Bright Point
Authors: Beck, C.; Schmidt, W.; Bellot Rubio, L. R.; Schlichenmaier,
   R.; Sütterlin, P.; Lites, B. W.
2006ASPC..358...72B    Altcode:
  We investigate the kinematic and magnetic properties of G-band bright
  points in the moat of a regular sunspot. The analysis is based on vector
  polarimetric measurements made at the German Vacuum Tower Telescope in
  visible (630 nm) and infrared (1565 nm) spectral lines, complemented
  by high-resolution filtergrams in the G-band at 430.6 nm and the core
  of the Ca II H line at 396.7 nm from the Dutch Open Telescope. The
  spectro-polarimetric data has been inverted to derive the magnetic
  field properties of the observed region. We witness the formation of a
  G-band bright point from a patch of diffuse flux with an initial field
  strength of 0.4 kG. The magnetic field strength increases to 0.9 kG in
  the course of several minutes, accompanied by a downflow of magnetized
  plasma. A few minutes after the field intensification, a G-band bright
  point is seen at the location of the flux concentration. The formation
  of the bright point shows the signatures of convective collapse.

---------------------------------------------------------
Title: The multi-component field topology of sunspot penumbrae. A
    diagnostic tool for spectropolarimetric measurements
Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C.
2006A&A...460..925M    Altcode: 2006astro.ph..9632M
  Context: .Sunspot penumbrae harbor highly structured magnetic fields
  and flows. The moving flux tube model offers an explanation for several
  observed phenomena, e.g. the Evershed effect and bright penumbral
  grains.<BR /> Aims: .A wealth of information can be extracted from
  spectropolarimetric observations. In order to deduce the structure of
  the magnetic field in sunspot penumbrae, detailed forward modeling is
  necessary. On the one hand, it gives insight into the sensitivity of
  various spectral lines to different physical scenarios. On the other
  hand, it is a very useful tool to guide inversion techniques. In this
  work, we present a generalized 3D geometrical model that embeds an
  arbitrarily shaped flux tube in a stratified magnetized atmosphere.<BR
  /> Methods: .The new semi-analytical geometric model serves as a
  frontend for a polarized radiative transfer code. The advantage of
  this model is that it preserves the discontinuities of the physical
  parameters across the flux tube boundaries. This is important for the
  detailed shape of the emerging Stokes Profiles and the resulting net
  circular polarization (NCP).<BR /> Results: .(a) The inclination of
  downflows in the outer penumbra must be shallower than approximately
  15° (b) observing the limb-side NCP of sunspots in the Fe I 1564.8
  nm line offers a promising way to identify a reduced magnetic field
  strength in flow channels; (c) the choice of the background atmosphere
  can significantly influence the shape of the Stokes profiles, but does
  not change the global characteristics of the resulting NCP curves for
  the tested atmospheric models.<BR />

---------------------------------------------------------
Title: The flow field in the sunspot canopy
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R.
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.<BR /> 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<SUP>-1</SUP>). We study the radial
  dependence of Stokes V profiles in the directions of disk center and
  limb side. <BR /> 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.<BR /> 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.

---------------------------------------------------------
Title: The fine-structure of a Sunspot penumbra
Authors: Schlichenmaier, R.
2006IAUJD...3E..88S    Altcode:
  The fine-structure of the penumbra is an ideal target to study the
  effects of radiative magnetoconvection in inclined magnetic field,
  and to compare theoretical models with observed properties. I will
  present spectroscopic and spectro-polarimetric measurements, which are
  interpreted to reconstruct the thermal stratification and the complex
  topology of the magnetic field and the flow field. Such measurements
  reveal that the line-of-sight velocity and the magnetic field depend on
  depth in the atmosphere and exhibit gradients or discontinuities. The
  magnetic field is found to be uncombed, with an essentially horizontal
  component that carries a radial outward flow (Evershed flow), and a
  less inclined magnetic field component. I will report on the discovery
  of dark-cored bright filaments, and present spectroscopic measurements,
  which demonstrate that the dark cores are associated with the Evershed
  flow. The observational findings will be interpreted in the framework
  of theoretical models and ideas.

---------------------------------------------------------
Title: Net Circular Polarization of Sunspot Penumbrae- A Versatile
    Tool for Diagnosing Magnetic Field Structure
Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C.
2006ESASP.617E..72M    Altcode: 2006soho...17E..72M
  No abstract at ADS

---------------------------------------------------------
Title: Two-dimensional spectroscopy of a sunspot. III. Thermal and
    kinematic structure of the penumbra at 0.5 arcsec resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Tritschler, A.
2006A&A...453.1117B    Altcode: 2006astro.ph..1423B
  We investigate the thermal and kinematic configuration of a sunspot
  penumbra using high spectral and spatial resolution intensity profiles
  of the non-magnetic Fe I 557.6 nm line. The data set was acquired
  with the 2D solar spectrometer TESOS. The profiles are inverted using
  a one-component model atmosphere with gradients of the physical
  quantities. From this inversion we obtain the stratification with
  depth of temperature, line-of-sight velocity, and microturbulence
  across the penumbra. Our results suggest that the physical mechanism(s)
  responsible for the penumbral filaments operate preferentially in the
  lower photosphere. The spot, located at an heliocentric angle of 23°,
  exhibits larger continuum intensities in the center-side penumbra
  as compared with the limb side, which translates into an average
  temperature difference of 100-150 K at log τ<SUB>500</SUB> = 0. We
  investigate the nature of the bright ring that appears in the inner
  penumbra when sunspots are observed in the wing of spectral lines. It
  is suggested that the bright ring does not reflect a temperature
  enhancement in the mid photospheric layers. The line-of-sight velocities
  retrieved from the inversion are used to determine the flow geometry
  at different heights in the photosphere. Both the flow speed and
  flow angle increase with optical depth and radial distance. Downflows
  are detected in the mid and outer penumbra, but only in deep layers
  (log τ<SUB>500</SUB> ≥ -1.4). We demonstrate that the velocity
  stratifications retrieved from the inversion are consistent with the
  idea of penumbral flux tubes channeling the Evershed flow. Finally, we
  show that larger Evershed flows are associated with brighter continuum
  intensities in the inner center-side penumbra. Dark structures,
  however, are also associated with significant Evershed flows. This
  leads us to suggest that the bright and dark filaments seen at 0.5
  arcsec resolution are not individual flow channels, but a collection
  of them. Our analysis highlights the importance of very high spatial
  resolution spectroscopic and spectropolarimetric measurements for a
  better understanding of sunspot penumbrae.

---------------------------------------------------------
Title: Net Circular Polarization Of Sunspot Penumbrae - A Versatile
    Model For Diagnosing Magnetic Field Structure
Authors: Mueller, Daniel; Schlichenmaier, R.; Fritz, G.; Beck, C.
2006SPD....37.0707M    Altcode: 2006BAAS...38..229M
  Sunspot penumbrae harbor highly structured magnetic fields and
  flows. The moving flux tube model offers an explanation for several
  observed phenomena, e.g. the Evershed effect and bright penumbral
  grains. In this work, we present a generalized 3D model that embeds an
  arbitrarily shaped flux tube in a stratified magnetized atmosphere. The
  new model is a versatile tool to calculate the spectral signature of
  flux tubes in the penumbra and especially make predictions about the
  flow speed and tube inclination from observed maps of the net circular
  polarization (NCP). As a first result, we find that the inclination
  of downflows in the outer penumbra must be shallower than approx. 15°.

---------------------------------------------------------
Title: Prospects of Solar Physics from the Ground
Authors: Schlichenmaier, Rolf
2006IAUS..233..427S    Altcode:
  The solar magnetism, its origin, and its impact on the earth are of
  primary interest for solar physicists. The understanding of the solar
  dynamo in the convection zone and the coupling of the magnetic fields
  up to the corona and the heliosphere calls for synoptic as well as
  for high spatial resolution observations of the Sun. Understanding
  the interactions between radiative and magneto-convective processes at
  the interface between the solar interior and the atmosphere requires
  spectro-polarimetric observations at high spatial and spectral
  resolution with high polarimetric accuracy. Thus large-aperture
  telescopes are needed to resolve the small scales and to collect
  enough photons to study the evolution of the magnetic processes. For
  assembling the mosaic of the solar dynamo and its magnetic coupling
  out to the heliosphere, large scale properties and hence synoptic
  observations play a crucial role. I present my personal perspective
  of the prospects in ground-based solar physics, and comment on the
  planned and upcoming new facilities including SOLIS, GREGOR, NST,
  SUNRISE, and ATST, as well as ALMA and FASR, but also mention the
  upcoming space missions HMI@SDO and SOLAR-B.

---------------------------------------------------------
Title: A polarization model for the German Vacuum Tower Telescope
    from in situ and laboratory measurements
Authors: Beck, C.; Schlichenmaier, R.; Collados, M.; Bellot Rubio,
   L.; Kentischer, T.
2005A&A...443.1047B    Altcode:
  It is essential to properly calibrate the polarimetric properties of
  telescopes, if one wants to take advantage of the capabilities of high
  precision spectro-polarimeters. We have constructed a model for the
  German Vacuum Tower Telescope (VTT) that describes its time-dependent
  polarization properties. Since the coelostat of the telescope changes
  the polarization state of the light by introducing cross talk among
  different polarization states, such a model is necessary to correct the
  measurements, in order to retrieve the true polarization as emitted
  from the Sun. The telescope model is quantified by a time-dependent
  Mueller matrix that depends on the geometry of the light beam through
  the telescope, and on material properties: the refractive indices of the
  coelostat mirrors, and the birefringence of the entrance window to the
  vacuum tube. These material properties were determined experimentally
  in-situ by feeding the telescope with known states of polarization
  (including unpolarized light) and by measuring its response, and from
  measurements of an aluminum-coated sample in the laboratory. Accuracy
  can in our case be determined only for the combination of telescope
  and spectro-polarimeter used; for the instrument POLIS at the VTT,
  we estimate an accuracy of ±4-5× 10<SUP>-3</SUP> for the cross talk
  correction coefficients.

---------------------------------------------------------
Title: Multi-line spectroscopy of dark-cored penumbral filaments
Authors: Bellot Rubio, L. R.; Langhans, K.; Schlichenmaier, R.
2005A&A...443L...7B    Altcode:
  Dark-cored filaments could be the basic building blocks of sunspot
  penumbrae. Yet, their nature and physical conditions are unknown. In an
  attempt to improve this situation, we present the first high-resolution
  spectra of dark-cored penumbral filaments. Several such filaments
  were observed near the umbra/penumbra boundary of a sunspot located at
  heliocentric angles of 5° and 20°. Our data reveal (a) significantly
  larger Doppler shifts in the dark cores as compared to their lateral
  brightenings; (b) Doppler shifts that increase with depth in the
  photosphere, up to 1.5 km s<SUP>-1</SUP>; and (c) Doppler shifts that
  increase with increasing heliocentric distance. The Doppler velocities
  measured in the dark cores are almost certainly produced by upflows. In
  addition, dark-cored penumbral filaments exhibit weaker fields than
  their surroundings (by 100-300 G). These results provide new constraints
  for models of dark-cored penumbral filaments.

---------------------------------------------------------
Title: Flow filaments linking bright and dark filaments in a sunspot
    penumbra
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rubio, L. R.
2005AGUSMSP11A..08T    Altcode:
  We present two-dimensional spectroscopic sunspot observations of
  high spatial (≍ 0.5 arcsec) and high spectral resolution (λ/Δλ
  = 250000). The observations were taken with the Telecentric Solar
  Spectrometer (TESOS) operated at the German Vacuum Tower Telescope on
  Tenerife. We examine a single scan taken in the popular non-magnetic
  neutral iron line at 557.6 nm and concentrate our analysis on the
  unsettled issue of the relation between the Evershed flow and the
  intensity structure in a sunspot penumbra. At the end of the 20th
  century, observers concluded that the highest flow velocities are
  connected to the dark filaments which harbour more horizontal magnetic
  fields than the bright filaments. Based on a correlation analysis we
  find that the correlation between flows and intensity varies from the
  inner to the outer penumbra, from the center-side to the limb-side
  penumbra, and depends on the length of the trace used to perform
  the correlation. The line-of-sight velocity maps reveal that the
  Evershed flow on the center-side penumbra appears highly organised in
  narrow flow filaments, while the flows in the red-shifted limb-side
  penumbra do not show a filamentary fine-structure. A high correlation
  between flow speed and intensity is only observed over small spatial
  scales, i.e. considering short traces cutting individual features. The
  correlation is positive in the inner centre and limb-side penumbra, and
  tends to be negative in the outer penumbra. Our results imply that the
  Evershed flow is present in bright and dark filaments. In individual
  cases we find that flow filaments connect bright and dark filaments
  supporting the moving tube model for the penumbral fine structure.

---------------------------------------------------------
Title: On the relation between penumbral intensity and flow filaments
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Tritschler, A.
2005AN....326..301S    Altcode:
  Taking advantage of high spatial (≈ 0.5 arcsec) and high spectral
  (λ/δλ = 250 000) resolution observations obtained with the 2D
  spectrometer TESOS, we analyze a sunspot located at a heliocentric angle
  of 23<SUP>o</SUP>. We elaborate on the issue of a correlation between
  dark filaments and the Evershed flow in sunspot penumbrae. Controversies
  on the existence of such a correlation are resolved: It varies from
  the inner to the outer penumbra, from the center-side to the limb-side
  penumbra, and depends on the length of the trace which is used to
  perform the correlation. The flow map exhibits flow filaments in the
  center-side penumbra while the red-shifted limb-side penumbra does not
  show filamentary fine-structure. High correlation coefficients, |C|
  ≈ 0.9 are only found if small scales, i.e. short traces cutting
  individual features are considered. C is positive in the inner
  center and limb-side penumbra, and tends to be negative in the outer
  penumbra. Our results imply that the Evershed flow is present in bright
  and dark filaments. In individual cases we find that bright and dark
  intensities are connected by a flow filament supporting the moving
  tube model for the penumbral fine structure.

---------------------------------------------------------
Title: Penumbral Line Asymmetries Using KAOS
Authors: Soltau, Dirk; Berkefeld, Thomas; Schlichenmaier, Rolf;
   Tritschler, Alexandra; Rubio, Luis Ramon Bellot
2005sao..conf..129S    Altcode:
  We analyse and interpret the line asymmetries in a sunspot penumbra
  at a heliocentric angle of 23°. The data, acquired with TESOS [1]
  and KAOS [2] at the VTT, is of high spatial (0.5") and high spectral (
  λ/∆λ = 250 000) resolution. We observed in Fe I 557.6 nm (g=0) [3].

---------------------------------------------------------
Title: Asymmetrical appearance of dark-cored filaments in sunspot
    penumbrae
Authors: Sütterlin, P.; Bellot Rubio, L. R.; Schlichenmaier, R.
2004A&A...424.1049S    Altcode:
  Recent sunspot observations at unprecedented resolution have led to the
  discovery of dark cores in the bright filaments that form the penumbra
  (\citealt{scharmer02_Nat420}). The discovery paper considered spots
  at disk center only, so the properties of the dark-cored filaments
  remain largely unknown. Here we analyze a speckle-reconstructed time
  series of G-band and blue continuum images of a sunspot acquired with
  the Dutch Open Telescope. The target was located at an heliocentric
  angle of 27 deg. We confirm the existence of dark-cored penumbral
  filaments also in spots outside the disk center, and report on distinct
  differences between the center and limb-side penumbra. In the inner
  center-side penumbra, filaments are detected as two narrow bright
  streaks separated by a central obscuration. These structures move
  together as a single entity. On the limb side, dark cores are hardly
  seen. The time series is used to determine the sizes (∼200-250 km),
  proper motions (∼280 m s<SUP>-1</SUP>), and lifetimes (⪉45 min)
  of typical dark-cored filaments.

---------------------------------------------------------
Title: Two-dimensional spectroscopy of a sunspot. II. Penumbral
    line asymmetries
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Tritschler, A.
2004A&A...415..731S    Altcode:
  We present, analyse, and interpret line asymmetries from Fe I 557.6
  nm of a sunspot penumbra at a heliocentric angle of 23<SUP>o</SUP>
  with high spatial (0.5 arcsec) and spectral (λ/\triangleλ=250 000)
  resolution. The data set is described and presented in the first paper
  of this series \citep{tritschler+etal2003}. Line bisectors are used
  to quantify the line asymmetries. Our findings are: (1) For averaged
  limb and center side bisectors the shift increases linearly with the
  bisector intensity level, but the limb side bisector is more inclined
  than the center side bisector. (2) Individual bisectors exhibit kinks,
  such that the bisector at high intensity levels is shifted towards the
  red for both, limb and center side bisectors. Some of the kinks produce
  bisector reversals in the outer center side penumbra. The bisector
  properties and their intriguing differences between center and limb side
  can be explained if one assumes downflows in deep atmospheric layers
  (\log τ &gt; -1). This is demonstrated by synthetic bisectors. The
  differences between the two penumbral sides are due to projection
  effects of non-horizontal flow channels. Our findings also imply that
  bisectors reversals are not due to elevated channels, but due to the
  presence of downflows. Along a specific center side flow filament the
  bisector shift is found to be largest in the line wing, except for the
  outer end of the filament, where a kink at high bisector intensities
  toward the red is found. This is consistent with an upflow at the
  inner footpoint, a deep lying horizontal flow, and, after a spatial
  distance of 4 arcsec, with a downflow at the end of the flow filament.

---------------------------------------------------------
Title: Two-dimensional spectroscopy of a sunspot. I. Properties of
    the penumbral fine structure
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rubio, L. R.;
   KAOS Team; Berkefeld, T.; Schelenz, T.
2004A&A...415..717T    Altcode:
  We investigate the properties of the fine structure of a sunspot
  penumbra based on spectroscopic measurements with high spectral
  (λ/δλ=250 000) and high spatial (≈0.5 arcsec) resolution. The
  magnetically insensitive Fe I 557.6 nm line is used to probe the
  penumbral atmosphere. The data was taken at the German Vacuum Tower
  Telescope with the 2D-spectrometer TESOS, taking advantage of the
  recently installed Kiepenheuer Adaptive Optics System (KAOS). The field
  of view covers a sunspot located at 23<SUP>o</SUP> off the disk center
  and its immediate surroundings. The penumbral structure is studied
  by means of maps computed for the line-of-sight velocity, the line
  width, the equivalent width and the line depression. Line-of-sight
  velocities are derived from the Doppler shifts at different bisector
  levels. From these maps we infer the flow field geometry and study the
  azimuthal and radial dependences of the line parameters. Our findings
  can be summarized as follows: (a) the flow pattern has a conspicuous
  filamentary structure in the deep photospheric layers and is rather
  diffuse in the high layers. (b) The flow field slightly spreads and
  fans out with height. (c) The flow geometry confirms the presence of
  an upflow component in the inner penumbra and a downflow component in
  the middle and outer penumbra. (d) We find an enhanced brightness of
  the mid-penumbra (“bright ring”) in the line wings, but not in the
  continuum or line core. (e) The azimuthal average of the equivalent
  width, the line width and the absolute flow velocity increase with
  radial distance within the penumbra. (f) Small-scale variations of
  the equivalent width and the line width on the center-side penumbra
  are co-spatial and correlated with (blue-shifted) fluctuations in
  the line-of-sight velocity. (g) Inner limb-side penumbral grains are
  associated with blue-shifts of v≤-400 m s<SUP>-1</SUP>, indicating
  upflows. (h) One umbral dot in our sample is associated with a
  blue-shift of v=-200 m s<SUP>-1</SUP>.

---------------------------------------------------------
Title: On the heat transport in a sunspot penumbra
Authors: Schlichenmaier, R.; Solanki, S. K.
2003A&A...411..257S    Altcode:
  The penumbra radiates an energy flux that is roughly 75% of the
  quiet-sun value. One mechanism proposed to bring this flux to the
  surface is interchange convection of magnetic flux tubes according
  to which hot flux tubes rise to the surface, cool off their heat
  by radiation and sink down again. Another way to deposit heat in
  the penumbral photosphere is by steady upflows along magnetic flux
  tubes. We discuss these two mechanisms and elaborate on consequences
  that can be compared with and constrained by observations. We estimate
  the time scales for variations of the intensity and the magnetic field
  pattern. By comparing them with the corresponding observed time scales,
  we find that pure interchange convection is unable to account for the
  observed penumbral heat flux. Heating the penumbra by steady upflows
  along magnetic flux tubes, however, turns out to be sufficient to
  explain the penumbral brightness, under the condition that significant
  magnetic return flux is present within the penumbra. Associated with
  the magnetic return flux, downflows within the penumbra should be
  present, in accordance with recent observational findings of such
  downflows. Exploring other possible heating mechanisms, we find that
  dissipation of magnetic energy is negligible, while dissipation of the
  kinetic energy of the Evershed flow could contribute significantly to
  the brightness of the penumbra.

---------------------------------------------------------
Title: Thermal Kinematic Structure of a Sunspot at 0.5 arcsec
    Resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Tritschler, A.
2003ANS...324..104B    Altcode: 2003ANS...324..P10B
  No abstract at ADS

---------------------------------------------------------
Title: 2D Spectroscopy with a Triple Gabry-Perot Spectrometer and
    Adaptive Optics
Authors: Tritschler, A.; Schlichenmaier, R.; Bellot Rutbio, L.
2003ANS...324...21T    Altcode: 2003ANS...324..C02T
  No abstract at ADS

---------------------------------------------------------
Title: Field-aligned Evershed flows in the photosphere  of a sunspot
    penumbra
Authors: Bellot Rubio, L. R.; Balthasar, H.; Collados, M.;
   Schlichenmaier, R.
2003A&A...403L..47B    Altcode:
  We determine the inclinations of the vector magnetic field and flow
  velocity in a sunspot penumbra by interpreting full Stokes profiles of
  three infrared lines observed with the Tenerife Infrared Polarimeter. It
  is shown that analyses based on one-component atmospheres deliver flow
  velocities which are more horizontal than the average magnetic field
  by up to 10 deg. This apparent violation of the concept of frozen-in
  magnetic fields is solved as soon as two magnetic atmospheres are
  allowed to coexist in the resolution element. The magnetic field and
  velocity in the atmospheric component carrying the Evershed flow are
  found to be aligned to within +/- 2 deg all the way from the inner
  to the outer penumbra. This is the first observational confirmation
  of magnetic fields being frozen into the plasma in sunspots. Our
  results indicate that sunspot penumbrae can be understood in terms of
  inclined flux tubes embedded in a more vertical background field. The
  flux tubes carry most of the Evershed flows and return to the solar
  surface in the middle penumbra and beyond. The background atmosphere
  is essentially at rest in the inner penumbra, and harbors small flows
  in the outer penumbra.

---------------------------------------------------------
Title: 2D Solar Spectroscopy with a Triple Fabry-Perot Filtergraph
Authors: Bellot Rubio, Luis R.; Tritschler, Alexandra; Schlichenmaier,
   Rolf
2003IAUJD..20E..27B    Altcode:
  TESOS is a tunable narrow-band filter based on three Fabry-Perot etalons
  operated in a telecentric configuration. Installed at the German Vacuum
  Tower Telescope (VTT) of Teide Observatory (Spain) it is used regularly
  for 2D spectroscopy of the solar atmosphere. Its spectral resolution
  of 250000 is similar to that of grating spectrographs. In the high
  resolution mode the field of view of TESOS is 42 arcsec in diameter and
  the image scale is 0.09 arcsec/pixel. Due to the high quantum efficiency
  of the detector above 500 nm exposure times of a few tens of msec can
  be used. As a result TESOS is able to measure the intensity profiles
  of a given spectral line across the field of view in less than 40 sec
  (assuming 100 wavelength positions). Recently TESOS has been combined
  with KAOS the adaptive optics system of the VTT to improve the spatial
  resolution of the observations up to 0.4 arcsec. Here we present the
  first science results of TESOS+KAOS based on observations of a sunspot
  penumbra. Maps of various spectroscopic parameters are computed and
  the line asymmetries induced by the Evershed flows are investigated
  by means of a bisector analysis of the FeI line at 557.6 nm.

---------------------------------------------------------
Title: The Sunspot Penumbra: New Developments (Invited review)
Authors: Schlichenmaier, R.
2003ASPC..286..211S    Altcode: 2003ctmf.conf..211S
  No abstract at ADS

---------------------------------------------------------
Title: Spectral signature of magnetic flux tubes in sunspot penumbrae
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
2002A&A...393..305M    Altcode:
  We study the polarization of spectral lines in the penumbra by
  integrating the radiative transfer equation of polarized light for a
  three-dimensional model atmosphere of a sunspot. In this model, the
  Evershed flow is confined to magnetic flux tubes which are embedded
  in a static background atmosphere, in accordance with the moving tube
  model of Schlichenmaier et al. (\cite{Schlichenmaier1998apjl},b). The
  gradients and/or discontinuities in the fluid velocity and the
  magnetic field at the flux tube boundaries give rise to asymmetric
  Stokes profiles. We concentrate on the Stokes-V profiles and study the
  net circular polarization (NCP) of two photospheric spectral lines of
  neutral iron, Fe I 630.25 nm and Fe I 1564.8 nm. The different behavior
  of these two lines, which are exemplary for atomic spectral lines with
  a large Landé factor and significantly different wavelength, is based
  on the difference in excitation potential of the corresponding atomic
  transitions and the fact that the wavelength dependence of the Doppler
  shift is linear, while that of the Zeeman splitting is quadratic. We
  find that the azimuthal variation of the NCP, N(psi, is a predominantly
  antisymmetric function of psi with respect to the line connecting
  disk center and spot center (line-of-symmetry) for the infrared line
  of Fe I 1564.8 nm, while the variation is predominantly symmetric for
  Fe I 630.25 nm. We show that the antisymmetric variation is caused
  by anomalous dispersion (Faraday pulsation) and the discontinuity
  in the azimuthal angle of the magnetic field, which is due to the
  relative inclination between flux tube and background field. We
  then compute synthetic NCP maps of a sunspot and compare them with
  observational results. Finally, the center-to-limb variation of the
  NCP, N(theta ), of these spectral lines is investigated. We show
  that the location of the zero-crossing point of N(theta ) on the
  center side of the line-of-symmetry represents a diagnostic tool to
  determine the inclination angle of the Evershed flow: A vanishing NCP
  on the center-side of the line-of-symmetry is an indirect evidence of
  downflows in the penumbra.

---------------------------------------------------------
Title: Penumbral fine structure: Theoretical understanding
Authors: Schlichenmaier, R.
2002AN....323..303S    Altcode:
  To understand the dynamic fine structure and the Evershed effect of
  the sunspot penumbra, we have carried out time-dependent simulations
  of a thin magnetic flux tube embedded in a static sunspot model
  (Schlichenmaier et al. 1998a,b). Here, we present new simulation results
  of the moving tube model which reveal flux tubes that, instead of lying
  horizontal in the penumbral photosphere, develop waves (photospheric
  serpent) that reproduce not only the inward migration of penumbral
  grains, but also their observed outward migration, and which exhibit
  downflow arches in the outer penumbra. During its evolution the
  flux tube exhibits quasi-steady states. These states are compared to
  time-independent solutions of siphon flow models, which have been used
  to explain the Evershed flow (Meyer &amp; Schmidt 1968, Degenhardt 1991,
  Montesinos &amp; Thomas 1997).

---------------------------------------------------------
Title: Net circular polarization of sunspot penumbrae - symmetry
    breaking by anomalous dispersion
Authors: Müller, D. A. N.; Schlichenmaier, R.; Steiner, O.; Stix, M.
2002ESASP.508..141M    Altcode: 2002soho...11..141M
  We examine the polarization of spectral lines in the penumbra of
  sunspots by solving the radiative transfer equation of polarized
  light for a three-dimensional axially symmetric model atmosphere
  of a sunspot. The Evershed flow is confined to horizontal magnetic
  flux tubes obtained from MHD calculations. These are embedded in
  an inclined background magnetic field. In this work, we concentrate
  on the Stokes-V profiles and examine the net circular polarization
  (NCP), N = ∫V(λ)dλ, of two photospheric spectral lines of neutral
  iron, Fe I 630.25 nm and Fe I 1564.8 nm. Analyzing spectra at a fixed
  distance from the spot center, we find that the azimuthal variation
  of N, N(ψ), is an antisymmetric function of ψ w.r.t. to the line
  connecting disk center and spot center for Fe I 1564.8 nm, while
  the variation is predominantly symmetric for Fe I 630.25 nm. We show
  that the antisymmetric variation is caused by anomalous dispersion
  (rotation of the polarization vector in a magnetized plasma). The
  different inclination angles lead to a discontinuity in the azimuth
  of the magnetic field along the line-of-sight. We show that this
  discontinuity together with the effect of anomalous dispersion produced
  an antisymmetric component in N(ψ) which outweighs the symmetric
  component from the discontinuity for Fe I 1564.8 nm, while it is
  negligible for Fe I 630.25 nm. We finally compute synthetic NCP maps of
  a sunspot which offer an explanation for recent observational results.

---------------------------------------------------------
Title: Spectropolarimetry in a sunspot penumbra. Spatial dependence
    of Stokes asymmetries in Fe I 1564.8 nm
Authors: Schlichenmaier, R.; Collados, M.
2002A&A...381..668S    Altcode:
  Stokes profiles of sunspot penumbrae show distinct asymmetries, which
  point to gradients in the velocity field and in the magnetic field. We
  present spectropolarimetric measurements of the Stokes vector in the
  neutral iron triplet at 1564.8 nm taken with the Tenerife Infrared
  Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) in
  Tenerife. We report on the peculiarities of the profiles of circularly
  and linearly polarized light for spots at different heliocentric
  angles. We elaborate on the spatial dependence of Stokes asymmetries
  within the penumbra and find for profiles of circularly polarized light:
  (1) In the center-side penumbra the amplitude difference of Stokes-V
  exhibits a sign reversal on a radial cut, i.e., in the inner (outer)
  penumbra the red (blue) lobe is broader and shows a smaller amplitude
  than the blue (red) lobe. (2) In the outer limb-side penumbra (beyond
  the magnetic neutral line) the red lobe is broader and of less amplitude
  than the blue lobe. (3) Along the magnetic neutral line we find abnormal
  Stokes-V profiles, which consist of more than 2 lobes. This indicates
  the presence of two polarities. For small heliocentric angles abnormal
  profiles are also seen beyond the magnetic neutral line in the outer
  penumbra. (4) Maps of the net circular polarization have the tendency to
  be antisymmetric with respect to the axis that connects disk center with
  spot center. This finding is striking, because corresponding maps for Fe
  I 630.25 are symmetric. For linearly polarized profiles we extract the
  following features: (5) On the center-side penumbra at a heliocentric
  angle of 56<SUP>o</SUP> a Doppler-shift as high as 5 km s<SUP>-1</SUP>
  can be directly measured by the splitting of the pi -component of
  the linearly polarized component. (6) In limb-side penumbrae, the
  profiles of the pi -component show the typical asymmetry properties of
  the Evershed flow as observed in Stokes-I of magnetically insensitive
  lines. (7) In the outer center- and limb-side penumbrae the center of
  the pi -component is blue-shifted relative to the zero-crossing of
  the V-profile. Motivated by the moving tube model of Schlichenmaier
  et al. (\cite{schlichenmaier+jahn+schmidt1998b}), we construct simple
  model atmospheres featuring hot upflows and cool outflows and calculate
  corresponding synthetic V-profiles. These profiles are compared with
  our measured ones and with observed V-profiles in Fe I 630.25 from
  other authors. We find that the synthetic V-profiles can reproduce
  all essential characteristics of observed V-profiles for both lines.

---------------------------------------------------------
Title: Net circular polarization of sunspot penumbrae. Symmetry
    breaking through anomalous dispersion
Authors: Schlichenmaier, R.; Müller, D. A. N.; Steiner, O.; Stix, M.
2002A&A...381L..77S    Altcode:
  The net circular polarization, N, is used as a measure for the
  asymmetry of Stokes-V profiles: Nequiv int V(lambda ) d lambda ,
  integrated over an absorption line. Exemplary for Fe I 630.2 nm and
  Fe I 1564.8 nm, we synthesize penumbral V-profiles that stem from a
  model atmosphere in which the Evershed flow is confined to horizontal
  flux tubes which are embedded in a magnetic field that has the same
  magnetic field strength as the flow channel, but is less inclined
  w.r.t. the surface normal. At the two points where a line-of-sight
  enters and leaves the flow channel, discontinuities in the inclination,
  gamma , the velocity v, and the azimuth, phi , of the magnetic field
  vector w.r.t. the plane perpendicular to the line-of-sight produce
  V-asymmetries. Assuming an axially symmetric penumbra, we investigate
  the azimuthal dependence N(psi ) for a mid-penumbral radius. We find:
  (1) Without including anomalous dispersion, N(psi ) is symmetric
  w.r.t. the line that connects disk center to the center of the spot. (2)
  Including anomalous dispersion, this symmetry is broken. We demonstrate
  that this is due to the difference in azimuth, triangle phi (psi ),
  between the flow channel and the background that varies along the
  penumbral circle. For Fe I 630.2 nm this effect is found to be of
  minor relevance leading to essentially symmetric N-maps, whereas strong
  asymmetries are predicted for Fe I 1564.8 nm. Our results provide an
  explanation for recent observational findings.

---------------------------------------------------------
Title: Time-slice diagrams of solar granulation
Authors: Müller, D. A. N.; Steiner, O.; Schlichenmaier, R.; Brandt,
   P. N.
2001SoPh..203..211M    Altcode:
  From a series of 1400 white-light images of solar granulation spanning
  a time period of 8.2 hours, skeletal plots of time-slice diagrams are
  derived showing intergranular lane positions as a function of time. The
  diagrams permit to automatically track, classify, and relate 42 186
  granules. Recurrently fragmenting granules are found that survive
  by means of their descendants for more than 3 hours. Such long-lived
  active granules tend to have a mean spatial distance along the slice
  of about 10 Mm. This distance decreases with decreasing minimal
  required lifetime. Since active granules are expected to generate a
  steadily divergent flow over a long period of time, it is suggested
  to identify them as a source of the mesogranular flow. Deficiencies
  of the time-slice analysis are discussed. The relative frequency of
  different types of granules and the granule decay time as derived
  from the time-slice diagrams are compared with corresponding results
  of previous works.

---------------------------------------------------------
Title: Small-Scale Flow Field in a Sunspot Penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
2001ASPC..236..289S    Altcode: 2001aspt.conf..289S
  No abstract at ADS

---------------------------------------------------------
Title: Penumbral Stokes-V Asymmetries of Fe I 1564.8 nm
Authors: Schlichenmaier, R.; Soltau, D.; Lühe, O. V. D.; Collados, M.
2001ASPC..236..579S    Altcode: 2001aspt.conf..579S
  No abstract at ADS

---------------------------------------------------------
Title: Small-scale flow field in a sunspot penumbra
Authors: Schmidt, W.; Schlichenmaier, R.
2000A&A...364..829S    Altcode:
  We have measured velocity maps of a round sunspot near the center of
  the solar disk. We derive the small-scale velocity field of the penumbra
  in the deep photosphere. Superimposed on a radial outflow, we find up-
  and downflows, which are associated with bright and dark features: The
  flow in the dark component of the penumbra has a larger inclination
  angle with respect to the surface normal than the bright component,
  everywhere in the penumbra. The maximum inclination difference is
  8°. In the inner and mid penumbra, bright features (penumbral grains)
  are associated with an upflow of about 0.6 km s<SUP>-1</SUP>. At the
  outer spot boundary, the flow is inclined downwards predominantly in
  the dark component (-7degr ).

---------------------------------------------------------
Title: Flow geometry in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
2000A&A...358.1122S    Altcode:
  We have measured the material flow in the penumbra of a large symmetric
  sunspot during the passage of the spot across the solar meridian. The
  line-of-sight velocity field has been obtained from Doppler measurements
  in a Fe II line using a filter spectrometer with a large field of
  view. From data sets taken on different days, i.e. at different view
  angles, we have reconstructed the magnitude and orientation of the
  penumbral flow field in the deep photosphere. We find upflows near
  the inner and downflows at the outer boundary of the penumbra with
  nearly horizontal outflow in between. From our measurements we derive
  the following flow geometry: narrow upflow channels rise at different
  penumbral radii, they bend outwards with a nearly horizontal outflow,
  and are finally tilted a few degrees downwards at the outer penumbral
  boundary, but still inside the spot. The flow reaches its maximum speed
  of about 3.5 km s<SUP>-1</SUP> in the outer part of the penumbra. Our
  findings are a significant step towards an understanding of the mass
  balance of the Evershed flow. The proposed geometry is consistent with
  recent numerical models of penumbral filaments.

---------------------------------------------------------
Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
1999A&A...349..961S    Altcode:
  Radiative energy transport is of key importance for the dynamics of
  slender magnetic flux tubes in the solar atmosphere, particularly
  so in connection with the filamentation of the sunspot penumbra. In
  investigations using the thin-flux-tube approximation of the MHD
  equations, the radiative exchange with the surrounding atmosphere has
  hitherto been described by the relaxation-time approach, also called
  `Newton's law of cooling'. The strongly nonlinear temperature-dependence
  of the radiative absorption coefficient and large temperature
  differences between the tube and its environment render this concept
  questionable. As a simple model of a bright penumbral filament we
  consider the cooling of a hot horizontal flux tube with a longitudinal
  flow, embedded in a non-stratified, homogeneous atmosphere at 4 800
  K. We compare the results of the relaxation-time approach and of
  a nonlinear diffusion approximation with the numerical solution of
  the equation of (grey) radiative transfer. We find that the cooling
  times given by the relaxation-time method compare well with the
  results from radiative transfer as long as the initial temperature
  of the tube is below 7 500 K and its lateral optical depth does not
  exceed unity. Under these conditions, the tube cools more or less
  homogeneously over its cross section. For hotter and optically thick
  tubes, the strong temperature-dependence of the absorption coefficient
  leads to the formation of a cooling front, which migrates radially
  inward at approximately constant speed. Such inhomogeneous cooling
  is well represented by the nonlinear diffusion approximation. The
  self-similar evolution of the cooling front permits an analytical
  estimate of the cooling time, which provides a reasonable approximation
  of the result of the radiative transfer calculation. This estimate can
  be used to derive an improved radiative cooling term in the framework
  of the thin-flux-tube approximation, so that both optically thin and
  optically thick flux tubes can be treated adequately. The results of
  the radiative transfer calculations are applied to obtain an estimate
  of the length and brightness of penumbral bright grains.

---------------------------------------------------------
Title: Vertical mass flux in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
1999A&A...349L..37S    Altcode:
  We present the first direct measurement of vertical motion in the
  deepest atmospheric layers of a penumbra, obtained at hitherto
  unreached spatial resolution. Isolated hot upflows in the inner
  penumbra feed the horizontal Evershed flow that is observed in the
  mid and outer penumbra. We discover cool downflows which surround the
  hot upflows in the inner penumbra. At the outer edge of the penumbra,
  the Evershed flow terminates in a ring of downflow channels. The
  upflows transport a sufficient amount of energy to account for the
  observed brightness of the penumbra. These measurements have therefore
  significant implications for understanding the mass balance and the
  energy transport in a sunspot penumbra.

---------------------------------------------------------
Title: A Model for Penumbral Phenomena
Authors: Schlichenmaier, R.
1999ASPC..183...91S    Altcode: 1999hrsp.conf...91S
  No abstract at ADS

---------------------------------------------------------
Title: Vertical and horizontal mass flux in a sunspot penumbra.
Authors: Schlichenmaier, R.; Schmidt, W.
1999AGAb...15....8S    Altcode: 1999AGM....15..A06S
  We present direct measurements of vertical and horizontal motions in
  the deepest atmospheric layers of a sunspot penumbra with a spatial
  resolution of about 0.5 arcsec. The observations have been made with
  the Vacuum Tower Telescope of the Kiepenheuer-Institut, operated
  at the Observatorio del Teide on Tenerife, using the Telecentric
  Solar Spectrometer (TESOS), which is based on two Fabry-Perot
  interferometers. &gt;From two-dimensional spectra we infer the
  line-of-sight velocities for the weak C I line at 538.0 nm, which forms
  in the deepest photospheric layers, and for the Fe II 542.5 nm line,
  a rather temperature sensitive line which forms in the mid-photosphere
  at a height of about 100 km above the continuum level. We have observed
  a fairly round sunspot (NOAA 8578) with a diameter of 32 000 km on its
  passage through the central meridian on four consecutive days. We find
  that isolated hot upflows in the inner penumbra feed the horizontal
  Evershed flow that is observed in the mid and outer penumbra. At the
  outer edge of the penumbra, the Evershed flow terminates in a ring of
  downflow channels. Measurements at various position angles allow us
  to derive the inclination angles of these flows. Based on our measured
  upflow velocities, we estimate that the heat flux that emerges from our
  brightest feature compares with the observed brightness of penumbral
  grains. These measurements have therefore profound implications for
  understanding the mass balance and the energy transport in a sunspot
  penumbra.

---------------------------------------------------------
Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
1999AGAb...15Q..75S    Altcode: 1999AGM....15..J16S
  Radiative energy transport is of key importance for the dynamics of
  slender magnetic flux tubes in the solar atmosphere. In investigations
  using the thin-flux-tube approximation of the MHD equations, the
  radiative exchange with the surrounding atmosphere has hitherto been
  described by the relaxation-time approach, also called `Newton's law of
  cooling'. The strongly nonlinear temperature-dependence of the radiative
  absorption coefficient and large temperature differences between
  the tube and its environment render this concept questionable. As a
  simple model of a bright penumbral filament we consider the cooling
  of a hot horizontal flux tube with a longitudinal flow, embedded in
  a non-stratified, homogeneous atmosphere at 4 800 K. We compare the
  results of the relaxation-time approach and of a nonlinear diffusion
  approximation with the numerical solution of the equation of (grey)
  radiative transfer. We find that the cooling times given by the
  relaxation-time method compare well with the results from radiative
  transfer as long as the initial temperature of the tube is below 7
  500 K and its lateral optical depth does not exceed unity. Under these
  conditions, the tube cools homogeneously over its cross section. For
  hotter and optically thick tubes, the strong temperature-dependence of
  the absorption coefficient leads to the formation of a cooling front,
  which migrates radially inward at approximately constant speed. Such
  inhomogeneous cooling is well represented by the nonlinear diffusion
  approximation. The self-similar evolution of the cooling front permits
  an analytical estimate of the cooling time, which provides a reasonable
  approximation of the result of the radiative transfer calculation. This
  estimate can be used to derive an improved radiative cooling term in
  the framework of the thin-flux-tube approximation.

---------------------------------------------------------
Title: Magnetic flux tubes evolving in sunspots. A model for the
    penumbral fine structure and the Evershed flow
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
1998A&A...337..897S    Altcode: 1998astro.ph..7036S
  Assuming that the interchange convection of magnetic flux elements
  is the physical cause for the existence of filamentary penumbrae in
  sunspots, we investigate the behavior of an individual fibril embedded
  in the deep penumbra. The fibril is approximated by a thin magnetic flux
  tube which evolves dynamically in the environment given by the global
  magnetostatic model of a sunspot. Our simulation shows that the flux
  tube, initially positioned at the penumbra-quiet Sun boundary in the
  model, will rise through its deep penumbra developing a flow along the
  tube that points upward beneath the photosphere, and radially outward
  above the photosphere. Our results suggest that a bright filament may
  be formed by an extended tail of a penumbral grain. Such filaments
  are optically thick, hotter than the surroundings, and elevated above
  a darker background. An upflow in penumbral grains bends horizontally
  outwards above the photosphere and gradually cools down due to radiative
  losses leading to a tail that gradually darkens. The plasma flow inside
  the flux tube then becomes transparent and the tube constitutes a
  thin elevated flow channel, that can reproduce the observed features
  of the Evershed effect. We present also a new acceleration mechanism
  for the Evershed flow. It is demonstrated that a local surplus of gas
  pressure develops inside the tube as it rises through the specific
  (superadiabatic and magnetized) penumbral background. The resulting
  gradient of the gas pressure can drive the flow along the tube.

---------------------------------------------------------
Title: A Dynamical Model for the Penumbral Fine Structure and the
    Evershed Effect in Sunspots
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
1998ApJ...493L.121S    Altcode: 1997astro.ph.12029S
  Relying on the assumption that the interchange convection of magnetic
  flux tubes is the physical cause for the existence of sunspot penumbrae,
  we propose a model in which the dynamical evolution of a thin magnetic
  flux tube reproduces the Evershed effect and the penumbral fine
  structure such as bright and dark filaments and penumbral grains. <P
  />According to our model, penumbral grains are the manifestation of
  the footpoints of magnetic flux tubes, along which hot subphotospheric
  plasma flows upward with a few km s<SUP>-1</SUP>. Above the photosphere
  the hot plasma inside the tube is cooled by radiative losses as it
  flows horizontally outward. As long as the flowing plasma is hotter
  than the surroundings, it constitutes a bright radial filament. The
  flow confined to a thin elevated channel reaches the temperature
  equilibrium with the surrounding atmosphere and becomes optically
  thin near the outer edge of the penumbra. Here the tube has a height
  of approximately 100 km above the continuum, and the flow velocity
  reaches up to 14 km s<SUP>-1</SUP>. Such a flow channel can reproduce
  the observed signatures of the Evershed effect.

---------------------------------------------------------
Title: Die Dynamik magnetischer Flußröhren im Sonnenfleck : ein
Modell für den Evershed-Effekt und die penumbrale Feinstruktur Title:
Die Dynamik magnetischer Flußröhren im Sonnenfleck : ein Modell
für den Evershed-Effekt und die penumbrale Feinstruktur Title:
The dynamics of magnetic flux tubes in sunspots : a model for the
    Evershed effect and penumbral fine structure;
Authors: Schlichenmaier, Rolf
1997PhDT........34S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of a Thin Magnetic Flux Tube in the Penumbra
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
1997ASPC..118..140S    Altcode: 1997fasp.conf..140S
  The concept of the interchange convection has been proposed as a
  hypothetic explanation of peculiarities of sunspot penumbrae. In order
  to investigate this concept, we performed a numerical simulation in
  which a thin magnetic flux tube embedded in a sunspot model evolves
  dynamically. The simulations offer a consistent explanation for
  penumbral fine structures such as bright filaments, penumbral grains,
  and probably for the Evershed effect.

---------------------------------------------------------
Title: Energy transport in the penumbra.
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
1996NAWG.1996..202S    Altcode:
  The problem of energy transport in sunspot penumbrae is addressed. This
  paper presents the mathematical tools to investigate numerically the
  dynamics of a thin flux tube, which evolves in a 2D background formed
  by a magneto-static sunspot model. The goal is to study the role of
  interchange convection in penumbral energy transport and to examine
  the usefullness of thin flux tubes as a physical entity to explain
  observed penumbral features such as penumbral grains, bright and dark
  filaments and the Evershed flow.

---------------------------------------------------------
Title: Evolution of a Magnetic Flux Tube in a Sunspot Penumbra
Authors: Jahn, K.; Schlichenmaier, R.; Schmidt, H. U.
1996ApL&C..34...59J    Altcode: 1995astro.ph..8128J
  The motion of an individual magnetic flux tube inside the penumbra
  of a sunspot is studied numerically. Here, we present preliminary
  results. The thin flux tube approximation together with a simplified
  radiative heat exchange with the surroundings is used to study the
  evolution of a flux tube embedded into a background given by a global
  magneto-static sunspot model. The investigation is undertaken in order
  to verify the conjecture that convection in sunspot penumbrae occurs
  by an interchange of magnetic flux tubes. The code being developed
  can be used to study dynamic aspects of filamentary structure in the
  penumbra: the temporal and spatial fluctuations of the temperature
  and the magnetic field, the motion of bright penumbral grains, or the
  Evershed effect. Here we present the evolution of a wave formed by the
  tube whose fragment emerges in the penumbral photosphere and migrates
  towards the umbra. The properties of this wave show qualitative
  features of the observed bright penumbral grains with corresponding
  upward velocity and its correlation with brightness and the inclination
  of the magnetic field, and also of the Evershed effect.

---------------------------------------------------------
Title: Dynamics of a magnetic flux tube in the penumbra.
Authors: Schlichenmaier, R.; Jahn, K.; Schmidt, H. U.
1996AGAb...12...90S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Numerical Study of a Thin Magnetic Flux Tubes' Migration in
    a Sunspots' Penumbra
Authors: Jahn, K.; Schlichenmaier, R.; Schmidt, H. U.
1996pas..meet...73J    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The phase of the radial mean field in the solar dynamo.
Authors: Schlichenmaier, R.; Stix, M.
1995A&A...302..264S    Altcode:
  Observations indicate that the radial and azimuthal components of the
  mean solar magnetic field oscillate with a phase shift of approximately
  180deg during the 22-year cycle. In order to calculate such phase shifts
  we construct a simple two-dimensional, nonlinear α^2<SUP>OMEGA</SUP>
  dynamo, which operates in the overshoot region beneath the convection
  zone. Like previous models, our model predicts an almost in-phase
  oscillation for most parameter choices. Special configurations, in which
  the two components of the mean field have different distributions in
  latitude, may resolve the dilemma. Alternative conclusions are that our
  knowledge of the α effect is insufficient, or that the observational
  result is not reliable.

---------------------------------------------------------
Title: The phase of the radial field in the solar dynamo
Authors: Schlichenmaier, R.; Stix, M.
1994smf..conf..107S    Altcode:
  No abstract at ADS