Author name code: beck
ADS astronomy entries on 2022-09-14
author:"Beck, Christian" AND (aff:"Freiburg" OR aff:"Solar")
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Title: The magnetic topology of the inverse Evershed flow
Authors: Prasad, A.; Ranganathan, M.; Beck, C.; Choudhary, D. P.;
   Hu, Q.
Bibcode: 2022A&A...662A..25P
Altcode: 2022arXiv220302702P
  Context. The inverse Evershed flow (IEF) is a mass motion towards
  sunspots at chromospheric heights. <BR /> Aims: We combined
  high-resolution observations of NOAA 12418 from the Dunn Solar
  Telescope and vector magnetic field measurements from the Helioseismic
  and Magnetic Imager (HMI) to determine the driver of the IEF. <BR />
  Methods: We derived chromospheric line-of-sight (LOS) velocities from
  spectra of Hα and Ca II IR. The HMI data were used in a non-force-free
  magnetic field extrapolation to track closed field lines near the
  sunspot in the active region. We determined their length and height,
  located their inner and outer foot points, and derived flow velocities
  along them. <BR /> Results: The magnetic field lines related to the
  IEF reach on average a height of 3 megameter (Mm) over a length
  of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9)
  sunspot radii. The average field strength difference ΔB between inner
  and outer foot points is +400 G. The temperature difference ΔT is
  anti-correlated with ΔB with an average value of −100 K. The pressure
  difference Δp is dominated by ΔB and is primarily positive with a
  driving force towards the inner foot points of 1.7 kPa on average. The
  velocities predicted from Δp reproduce the LOS velocities of 2-10 km
  s<SUP>−1</SUP> with a square-root dependence. <BR /> Conclusions:
  We find that the IEF is driven along magnetic field lines connecting
  network elements with the outer penumbra by a gas pressure difference
  that results from a difference in field strength as predicted by the
  classical siphon flow scenario.

Title: Velocities of an Erupting Filament
Authors: Wang, Shuo; Jenkins, Jack M.; Muglach, Karin; Martinez Pillet,
   Valentin; Beck, Christian; Long, David M.; Choudhary, Debi Prasad;
   McAteer, James
Bibcode: 2022ApJ...926...18W
Altcode: 2021arXiv211107830W
  Solar filaments exist as stable structures for extended periods of
  time before many of them form the core of a coronal mass ejection
  (CME). We examine the properties of an erupting filament on 2017 May
  29-30 with high-resolution He I 10830 Å and Hα spectra from the
  Dunn Solar Telescope, full-disk Dopplergrams of He I 10830 Å from
  the Chromospheric Telescope, and EUV and coronograph data from SDO
  and STEREO. Pre-eruption line-of-sight velocities from an inversion
  of He I with the HAZEL code exhibit coherent patches of 5 Mm extent
  that indicate counter-streaming and/or buoyant behavior. During the
  eruption, individual, aligned threads appear in the He I velocity
  maps. The distribution of velocities evolves from Gaussian to strongly
  asymmetric. The maximal optical depth of He I 10830 Å decreased from
  τ = 1.75 to 0.25, the temperature increased by 13 kK, and the average
  speed and width of the filament increased from 0 to 25 km s<SUP>-1</SUP>
  and 10 to 20 Mm, respectively. All data sources agree that the filament
  rose with an exponential acceleration reaching 7.4 m s<SUP>-2</SUP>
  that increased to a final velocity of 430 km s<SUP>-1</SUP> at 22:24
  UT; a CME was associated with this filament eruption. The properties
  during the eruption favor a kink/torus instability, which requires
  the existence of a flux rope. We conclude that full-disk chromospheric
  Dopplergrams can be used to trace the initial phase of on-disk filament
  eruptions in real time, which might potentially be useful for modeling
  the source of any subsequent CMEs.

Title: The Visible Spectro-Polarimeter of the Daniel K. Inouye
    Solar Telescope
Authors: de Wijn, A. G.; Casini, R.; Carlile, A.; Lecinski, A. R.;
   Sewell, S.; Zmarzly, P.; Eigenbrot, A. D.; Beck, C.; Wöger, F.;
   Knölker, M.
Bibcode: 2022SoPh..297...22D
Altcode: 2022arXiv220300117D
  The Daniel K. Inouye Solar Telescope (DKIST) Visible Spectro-Polarimeter
  (ViSP) is a traditional slit-scanning spectrograph with the ability
  to observe solar regions up to a 120 ×78 arcsec<SUP>2</SUP> area. The
  design implements dual-beam polarimetry, a polychromatic polarization
  modulator, a high-dispersion echelle grating, and three spectral
  channels that can be automatically positioned. A defining feature of
  the instrument is its capability to tune anywhere within the 380 - 900
  nm range of the solar spectrum, allowing for a virtually infinite number
  of combinations of three wavelengths to be observed simultaneously. This
  enables the ViSP user to pursue well-established spectro-polarimetric
  studies of the magnetic structure and plasma dynamics of the solar
  atmosphere, as well as completely novel investigations of the solar
  spectrum. Within the suite of first-generation instruments at the DKIST,
  ViSP is the only wavelength-versatile spectro-polarimeter available to
  the scientific community. It was specifically designed as a discovery
  instrument to explore new spectroscopic and polarimetric diagnostics
  and test improved models of polarized line formation through high
  spatial-, spectral-, and temporal-resolution observations of the Sun's
  polarized spectrum. In this instrument article, we describe the science
  requirements and design drivers of ViSP and present preliminary science
  data collected during the commissioning of the instrument.

Title: Heating of the solar atmosphere by electric currents
Authors: Choudhary, Debi Prasad; Louis, Rohan; Prasad, Avijeet; Beck,
   Christian; Yalim, Mehmet
Bibcode: 2021AGUFMSH12B..07C
Altcode:
  We present direct evidence of Ohmic dissipation of electric currents
  that leads to heating of the solar chromosphere above a light bridge
  in a sunspot by combining high-resolution spectroscopic Ca II IR data
  from the Dunn Solar Telescope with vector magnetic field observations
  from HMI. An extrapolation of the photospheric magnetic field from
  HMI to the corona using a non-force-free field technique provided the
  three-dimensional distribution of electric currents at locations of
  magnetic discontinuities, while the inversion of the chromospheric
  Ca II IR line spectra with the CAISAR code delivered the temperature
  stratifications from the photosphere to the chromosphere. The comparison
  of these results clearly shows that the light bridge is a site of
  strong electric currents of about 0.3 A m2 at the bottom boundary, which
  extend to about 0.7 Mm while decreasing monotonically with height. The
  dissipation of these currents produces a chromospheric temperature
  excess of about 600800 K relative to the umbra. Only the light bridge,
  where relatively weak and highly inclined magnetic fields emerge
  over a duration of 13 hrs, shows a spatial coincidence of thermal
  enhancements and electric currents. The temperature enhancements
  and the Cowling heating are primarily confined to a height range
  of 0.40.7 Mm above the light bridge. The corresponding increase in
  internal energy of 200 J m3 can be supplied by the heating in about
  10 min. This heating process by Ohmic dissipation can happen at any
  place in the solar atmosphere where large electric currents co-exist
  with non-zero resistivity. Large-scale corona loops embedded in a more
  vertical background magnetic field could experience the same effect
  leading to a localized heating of coronal plasma.

Title: Heating of the solar chromosphere in a sunspot light bridge
    by electric currents
Authors: Louis, Rohan E.; Prasad, Avijeet; Beck, Christian; Choudhary,
   Debi P.; Yalim, Mehmet S.
Bibcode: 2021A&A...652L...4L
Altcode: 2021arXiv210712066L
  Context. Resistive Ohmic dissipation has been suggested as a mechanism
  for heating the solar chromosphere, but few studies have established
  this association. <BR /> Aims: We aim to determine how Ohmic dissipation
  by electric currents can heat the solar chromosphere. <BR /> Methods:
  We combine high-resolution spectroscopic Ca II data from the Dunn Solar
  Telescope and vector magnetic field observations from the Helioseismic
  and Magnetic Imager (HMI) to investigate thermal enhancements in a
  sunspot light bridge. The photospheric magnetic field from HMI was
  extrapolated to the corona using a non-force-free field technique that
  provided the three-dimensional distribution of electric currents, while
  an inversion of the chromospheric Ca II line with a local thermodynamic
  equilibrium and a nonlocal thermodynamic equilibrium spectral archive
  delivered the temperature stratifications from the photosphere to the
  chromosphere. <BR /> Results: We find that the light bridge is a site of
  strong electric currents, of about 0.3 A m<SUP>−2</SUP> at the bottom
  boundary, which extend to about 0.7 Mm while decreasing monotonically
  with height. These currents produce a chromospheric temperature excess
  of about 600−800 K relative to the umbra. Only the light bridge,
  where relatively weak and highly inclined magnetic fields emerge over a
  duration of 13 h, shows a spatial coincidence of thermal enhancements
  and electric currents. The temperature enhancements and the Cowling
  heating are primarily confined to a height range of 0.4−0.7 Mm above
  the light bridge. The corresponding increase in internal energy of 200 J
  m<SUP>−3</SUP> can be supplied by the heating in about 10 min. <BR />
  Conclusions: Our results provide direct evidence for currents heating
  the lower solar chromosphere through Ohmic dissipation.

Title: The National Science Foundation's Daniel K. Inouye Solar
    Telescope — Status Update
Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn,
   A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.;
   Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.;
   Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.;
   Mathioudakis, M.; Dkist Team
Bibcode: 2021AAS...23810601R
Altcode:
  The National Science Foundation's 4m Daniel K. Inouye Solar Telescope
  (DKIST) on Haleakala, Maui is now the largest solar telescope in the
  world. DKIST's superb resolution and polarimetric sensitivity will
  enable astronomers to unravel many of the mysteries the Sun presents,
  including the origin of solar magnetism, the mechanisms of coronal
  heating and drivers of flares and coronal mass ejections. Five
  instruments, four of which provide highly sensitive measurements
  of solar magnetic fields, including the illusive magnetic field of
  the faint solar corona. The DKIST instruments will produce large and
  complex data sets, which will be distributed through the NSO/DKIST Data
  Center. DKIST has achieved first engineering solar light in December
  of 2019. Due to COVID the start of the operations commissioning phase
  is delayed and is now expected for fall of 2021. We present a status
  update for the construction effort and progress with the operations
  commissioning phase.

Title: Spectropolarimetric Measurements Of The Inverse Evershed Flow
Authors: Choudhary, D. P.; Beck, C.; Prasad, A.; Monankrishna, R.;
   Dhara, S.
Bibcode: 2021AAS...23831319C
Altcode:
  We use high spatial resolution observations with spectropolarimeters
  at the Dunn Solar Telescope in multiple spectral lines originating
  in the chromosphere to study inverse Evershed flow channels that
  connect the outer penumbra and the moat region around sunspots at
  various heliocentric positions. The measurements were combined with
  extrapolated magnetic field lines to determine the three-dimensional
  topology of the flow channels. The magnetic field lines guiding the
  flows reach on average a height of 2-5 Mm over a length of 10-20 Mm,
  with cold inner (hot outer) foot points located at 1.2 (1.9) sunspot
  radii. The average difference in field strength between the inner and
  outer foot point is about +400 G, while the temperature difference is
  about -100 K. This configuration can drive and sustain a siphon flow
  from the moat boundary towards the penumbra over the lifetime of the
  flow channels of about one hour. This study clearly demonstrates the
  inverse Evershed flow to be a siphon flow along arched chromospheric
  loops.

Title: Magnetoacoustic wave energy dissipation in the atmosphere of
    solar pores
Authors: Gilchrist-Millar, Caitlin A.; Jess, David B.; Grant, Samuel
   D. T.; Keys, Peter H.; Beck, Christian; Jafarzadeh, Shahin; Riedl,
   Julia M.; Van Doorsselaere, Tom; Ruiz Cobo, Basilio
Bibcode: 2021RSPTA.37900172G
Altcode: 2020arXiv200711594G
  The suitability of solar pores as magnetic wave guides has been a key
  topic of discussion in recent years. Here, we present observational
  evidence of propagating magnetohydrodynamic wave activity in a group
  of five photospheric solar pores. Employing data obtained by the
  Facility Infrared Spectropolarimeter at the Dunn Solar Telescope,
  oscillations with periods of the order of 5 min were detected at
  varying atmospheric heights by examining Si ɪ 10827 Å line bisector
  velocities. Spectropolarimetric inversions, coupled with the spatially
  resolved root mean square bisector velocities, allowed the wave energy
  fluxes to be estimated as a function of atmospheric height for each
  pore. We find propagating magnetoacoustic sausage mode waves with
  energy fluxes on the order of 30 kW m<SUP>-2</SUP> at an atmospheric
  height of 100 km, dropping to approximately 2 kW m<SUP>-2</SUP>
  at an atmospheric height of around 500 km. The cross-sectional
  structuring of the energy fluxes reveals the presence of both body-
  and surface-mode sausage waves. Examination of the energy flux decay
  with atmospheric height provides an estimate of the damping length,
  found to have an average value across all five pores of L<SUB>d</SUB>
  ≈ 268 km, similar to the photospheric density scale height. We find
  the damping lengths are longer for body mode waves, suggesting that
  surface mode sausage oscillations are able to more readily dissipate
  their embedded wave energies. This work verifies the suitability of
  solar pores to act as efficient conduits when guiding magnetoacoustic
  wave energy upwards into the outer solar atmosphere. <P />This article
  is part of the Theo Murphy meeting issue `High-resolution wave dynamics
  in the lower solar atmosphere'.

Title: The Formation of an Atypical Sunspot Light Bridge as a Result
    of Large-scale Flux Emergence
Authors: Louis, Rohan E.; Beck, Christian; Choudhary, Debi P.
Bibcode: 2020ApJ...905..153L
Altcode: 2020arXiv201014085L
  We use a combination of full-disk data from the Solar Dynamics
  Observatory and high-resolution data from the Dunn Solar Telescope (DST)
  to study the formation, structure, and evolution of an atypical light
  bridge (LB) in a regular sunspot. The LB results from the emergence of
  magnetic flux with one footpoint rooted in a pore outside the parent
  sunspot that appears about 17 hr before the LB. The pore has a polarity
  opposite to that of the sunspot and recedes from it at a speed of
  about 0.4 km s<SUP>-1</SUP>. This is accompanied by the development of
  an elongated magnetic channel in the outer penumbra that triggers the
  formation of the LB when it reaches the inner penumbral boundary. The
  LB is a nearly horizontal structure with a field strength of about
  1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85
  km s<SUP>-1</SUP> along its entire length. The emergence of the LB
  leads to dynamic surges in the chromosphere and transition region
  about 13 minutes later. We derive the photospheric and chromospheric
  structure of the LB in the DST data from spectral line parameters and
  inversions of He I at 1083 nm, Si I at 1082.7 nm, Ca II IR at 854
  nm, and H<SUB>α</SUB> at 656 nm and speckle-reconstructed imaging
  at 700 and 430 nm. The LB shows an elongated filamentary shape in
  the photosphere without lateral extrusions. The thermal inversion
  of Ca II IR reveals the LB to be about 600-800 K hotter than the
  umbra. Different sections of the LB are elevated to heights between 400
  and 700 km. Our results indicate that LB formation is part of a flux
  emergence event with the LB envelope reaching a height of about 29 Mm
  before dissolving after about 13 hr. We conclude that the existence
  of persistent, large-scale photospheric blueshifts in LBs is the most
  likely criterion for distinguishing between flux emergence events and
  overturning convection in field-free umbral intrusions.

Title: The Daniel K. Inouye Solar Telescope - Observatory Overview
Authors: Rimmele, Thomas R.; Warner, Mark; Keil, Stephen L.; Goode,
   Philip R.; Knölker, Michael; Kuhn, Jeffrey R.; Rosner, Robert R.;
   McMullin, Joseph P.; Casini, Roberto; Lin, Haosheng; Wöger, Friedrich;
   von der Lühe, Oskar; Tritschler, Alexandra; Davey, Alisdair; de Wijn,
   Alfred; Elmore, David F.; Fehlmann, André; Harrington, David M.;
   Jaeggli, Sarah A.; Rast, Mark P.; Schad, Thomas A.; Schmidt, Wolfgang;
   Mathioudakis, Mihalis; Mickey, Donald L.; Anan, Tetsu; Beck, Christian;
   Marshall, Heather K.; Jeffers, Paul F.; Oschmann, Jacobus M.; Beard,
   Andrew; Berst, David C.; Cowan, Bruce A.; Craig, Simon C.; Cross,
   Eric; Cummings, Bryan K.; Donnelly, Colleen; de Vanssay, Jean-Benoit;
   Eigenbrot, Arthur D.; Ferayorni, Andrew; Foster, Christopher; Galapon,
   Chriselle Ann; Gedrites, Christopher; Gonzales, Kerry; Goodrich, Bret
   D.; Gregory, Brian S.; Guzman, Stephanie S.; Guzzo, Stephen; Hegwer,
   Steve; Hubbard, Robert P.; Hubbard, John R.; Johansson, Erik M.;
   Johnson, Luke C.; Liang, Chen; Liang, Mary; McQuillen, Isaac; Mayer,
   Christopher; Newman, Karl; Onodera, Brialyn; Phelps, LeEllen; Puentes,
   Myles M.; Richards, Christopher; Rimmele, Lukas M.; Sekulic, Predrag;
   Shimko, Stephan R.; Simison, Brett E.; Smith, Brett; Starman, Erik;
   Sueoka, Stacey R.; Summers, Richard T.; Szabo, Aimee; Szabo, Louis;
   Wampler, Stephen B.; Williams, Timothy R.; White, Charles
Bibcode: 2020SoPh..295..172R
Altcode:
  We present an overview of the National Science Foundation's Daniel
  K. Inouye Solar Telescope (DKIST), its instruments, and support
  facilities. The 4 m aperture DKIST provides the highest-resolution
  observations of the Sun ever achieved. The large aperture of
  DKIST combined with state-of-the-art instrumentation provide the
  sensitivity to measure the vector magnetic field in the chromosphere
  and in the faint corona, i.e. for the first time with DKIST we will
  be able to measure and study the most important free-energy source
  in the outer solar atmosphere - the coronal magnetic field. Over its
  operational lifetime DKIST will advance our knowledge of fundamental
  astronomical processes, including highly dynamic solar eruptions
  that are at the source of space-weather events that impact our
  technological society. Design and construction of DKIST took over two
  decades. DKIST implements a fast (f/2), off-axis Gregorian optical
  design. The maximum available field-of-view is 5 arcmin. A complex
  thermal-control system was implemented in order to remove at prime
  focus the majority of the 13 kW collected by the primary mirror and
  to keep optical surfaces and structures at ambient temperature, thus
  avoiding self-induced local seeing. A high-order adaptive-optics
  system with 1600 actuators corrects atmospheric seeing enabling
  diffraction limited imaging and spectroscopy. Five instruments, four
  of which are polarimeters, provide powerful diagnostic capability
  over a broad wavelength range covering the visible, near-infrared,
  and mid-infrared spectrum. New polarization-calibration strategies
  were developed to achieve the stringent polarization accuracy
  requirement of 5×10<SUP>−4</SUP>. Instruments can be combined and
  operated simultaneously in order to obtain a maximum of observational
  information. Observing time on DKIST is allocated through an open,
  merit-based proposal process. DKIST will be operated primarily in
  "service mode" and is expected to on average produce 3 PB of raw
  data per year. A newly developed data center located at the NSO
  Headquarters in Boulder will initially serve fully calibrated data to
  the international users community. Higher-level data products, such as
  physical parameters obtained from inversions of spectro-polarimetric
  data will be added as resources allow.

Title: Center-to-limb Variation of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.; Ranganathan, M.
Bibcode: 2020ApJ...902...30B
Altcode: 2020arXiv200812748B
  We present the properties of the inverse Evershed flow (IEF)
  based on the center-to-limb variation of the plasma speed and loop
  geometry of chromospheric superpenumbral fibrils in eleven sunspots
  that were located at a wide range of heliocentric angles from 12° to
  79°. The observations were acquired at the Dunn Solar Telescope in the
  spectral line of Hα at 656 nm to determine chromospheric flows and the
  photospheric Si I line at 1082.7 nm to estimate the photospheric umbral
  magnetic field strength. All sunspots display opposite line-of-sight
  (LOS) velocities on the limb and center side with a distinct shock
  signature near the outer penumbral edge. We developed a simplified
  flexible sunspot model assuming axisymmetry and prescribing the radial
  flow speed profile at a known loop geometry to replicate the observed
  two-dimensional IEF patterns under different viewing angles. The
  simulated flow maps match the observations for chromospheric loops with
  10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of
  1-3 Mm, and a flow speed of 2-9 km s<SUP>-1</SUP>. We find on average
  a good agreement of the simulated velocities and the observations
  on elliptical annuli around the sunspot. Individual IEF channels
  show a significant range of variation in their properties and reach
  maximal LOS speeds of up to 12 km s<SUP>-1</SUP>. Upwards or downwards
  directed flows do not show a change of sign in the LOS velocities for
  heliocentric angles above 30°. Our results are consistent with the IEF
  being caused by a siphon flow mechanism driving a flow at about sonic
  speed along elevated loops with a flattened top in the chromosphere.

Title: Magnetic Structure of an Erupting Filament
Authors: Wang, Shuo; Jenkins, Jack M.; Martinez Pillet, Valentin;
   Beck, Christian; Long, David M.; Prasad Choudhary, Debi; Muglach,
   Karin; McAteer, James
Bibcode: 2020ApJ...892...75W
Altcode: 2020arXiv200202104W
  The full 3D vector magnetic field of a solar filament prior to
  eruption is presented. The filament was observed with the Facility
  Infrared Spectropolarimeter at the Dunn Solar Telescope in the
  chromospheric He I line at 10830 Å on 2017 May 29 and 30. We inverted
  the spectropolarimetric observations with the Hanle and Zeeman Light
  code to obtain the chromospheric magnetic field. A bimodal distribution
  of field strength was found in or near the filament. The average field
  strength was 24 Gauss, but prior to the eruption we find the 90th
  percentile of field strength was 435 Gauss for the observations on May
  29. The field inclination was about 67° from the solar vertical. The
  field azimuth made an angle of about 47°-65° to the spine axis. The
  results suggest an inverse configuration indicative of a flux rope
  topology. He I intensity threads were found to be coaligned with the
  magnetic field direction. The filament had a sinistral configuration
  as expected for the southern hemisphere. The filament was stable on
  2017 May 29 and started to rise during two observations on May 30,
  before erupting and causing a minor coronal mass ejection. There
  was no obvious change of the magnetic topology during the eruption
  process. Such information on the magnetic topology of erupting filaments
  could improve the prediction of the geoeffectiveness of solar storms.

Title: Temporal Evolution of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.
Bibcode: 2020ApJ...891..119B
Altcode: 2020arXiv200204560B
  The inverse Evershed flow (IEF) is an inflow of material into the
  penumbra of sunspots in the solar chromosphere that occurs along dark,
  elongated super-penumbral fibrils extending from about the outer edge
  of the moat cell to the sunspot. The IEF channels exhibit brightenings
  in the penumbra, where the supersonic IEF descends to the photosphere
  causing shock fronts with localized heating. We used an 1 hr time series
  of spectroscopic observations of the chromospheric spectral lines of
  Ca II IR at 854 nm and Hα at 656 nm taken with the Interferometric
  Bidimensional Spectrometer at the Dunn Solar Telescope to investigate
  the temporal evolution of IEF channels. Complementary information on
  the photospheric magnetic field was obtained from observations with the
  Facility Infrared Spectropolarimeter at 1083 nm and the Helioseismic
  and Magnetic Imager. We find that individual IEF channels are long-lived
  (10-60 minutes) and only show minor changes in position and flow speed
  during their lifetime. Initiation and termination of IEF channels
  takes several minutes. The IEF channels with line-of-sight velocities
  of about 10 km s<SUP>-1</SUP> show no lasting impact from transient or
  oscillatory phenomena with maximal velocity amplitudes of only about 1
  km s<SUP>-1</SUP> that run along them. We could not detect any clear
  correlation of the location and evolution of IEF channels to local
  magnetic field properties in the photosphere in the penumbra or moving
  magnetic features in the sunspot moat. Our results support a picture
  of the IEF as a field-aligned siphon flow along arched loops. From our
  data we cannot determine if their evolution is controlled by events
  at the outer end in the moat or at the inner end in the penumbra.

Title: Stokes Line Parameters as Possible Indicators of Flaring
Activity: A Comparison of Flaring and Non-Flaring Active Regions
Authors: Romich, K.; Cadavid, A. C.; Choudhary, D. P.; Beck, C.
Bibcode: 2019AGUFMSH31D3338R
Altcode:
  While the association between solar active regions (ARs) and solar
  flares is well-established, there is currently no reliable means
  of determining when (or if) a given AR will flare. Much of flare
  forecasting is based on the application of machine-learning statistical
  techniques that use parameters derived from the local magnetic field
  as predictors; these are commonly obtained through spectropolarimetric
  inversions using Stokes profiles from the observed radiation. The
  standard inversion codes, such as those based on the Milne-Eddington
  approximation, yield the average magnetic field values at or near flare
  locations. However, this fails to utilize the rich information contained
  in the shape of Stokes profiles, such as the existence of line-of-sight
  (LOS) magnetic and velocity gradients and multiple magnetic components
  along the LOS or within the resolving element. The resulting loss
  of information can potentially lead to inaccurate forecasts. We
  propose a novel approach, in which line parameters derived from
  spectrally-resolved Stokes profiles are considered as possible
  precursors to flare events. Using data from the spectropolarimeter
  onboard the Hinode satellite, we examine the amplitude asymmetry, net
  circular polarization, and degree of complexity of Stokes V profiles
  from several flaring and non-flaring ARs. Particular attention is given
  to regions near the polarity inversion line (PIL) due to its documented
  role in flare initiation. We define the PIL using the magnetic field
  vector relative to the solar surface, which we calculate from the Stokes
  Q, U, and V profiles; this corrects for projection effects arising
  from LOS observations of magnetic flux from ARs off disc center. In
  light of the shortcomings of existing methods, we hope to expand the
  set of viable indicators of flaring activity with the long-term goal
  of improving flare forecasting models.

Title: Time dependent properties of Inverse Evershed Flow and
    Perspectives with Daniel K. Inouye Solar Telescope (DKIST)
Authors: Choudhary, D. P.; Beck, C.
Bibcode: 2019AGUFMSH41F3334C
Altcode:
  We observed isolated leading sunspot, located at a heliocentric angle
  of 43 degrees, of a decaying active region (AR) NOAA 12418 on 16
  September 2015 with the Interferometric Bidimensional, the Facility
  InfraRed Spectropolarimeter at the Dunn Solar Telescope to study the
  time dependent chromospheric flow properties along the fibril structure,
  which is widely known as Inverse Evershed Flow. The observations were
  carried out in spectral lines of H_alpha at 656 nm and Ca ii IR at
  854.2 nm, photospheric Zeeman-sensitive Si i line at 1082.7 nm and the
  chromospheric He i line at 1083 nm. Our data is complemented with the
  Milne-Eddington inversion results for the photospheric magnetic field
  obtained with the derived from observations by the Helioseismic and
  Magnetic Imager on-board the Solar Dynamics Observatory. We find that
  all three chromospheric lines show a very similar behavior in their
  line-core intensity and the LOS velocity with matching spatial and
  temporal properties. We find that individual IEF channels persist for a
  few ten minutes to more than one hour. IEF channels that disappear are
  often rapidly replaced by a new channel at the about the same location
  after a short time. The IEF channels show little radial or lateral
  motion and usually end in the mid to outer penumbra. Initiation of
  the flow takes about 10min, while the termination is faster and takes
  only about 5min. The IEF channels seem to appear at preferred azimuth
  angles that are spaced at about 10◦ distance. The transient events
  were found to have almost no effect on the overall Inverse Evershed
  Flow pattern. In this paper, we shall present the results from our
  current study and perspectives for further observational study with
  upcoming 4 meter Daniel K. Inouye Solar Telescope.

Title: An Erupting Solar Filament Observed at the DST
Authors: Wang, S.; Jenkins, J. M.; Pillet, V. M.; Beck, C.; Long,
   D.; Choudhary, D. P.; McAteer, J.
Bibcode: 2019AGUFMSH33B3376W
Altcode:
  An erupting filament that lead to a coronal mass ejection (CME) was
  observed at the Dunn Solar Telescope (DST). We present HAZEL inversions
  of spectropolarimetric observations of the quiescent filament acquired
  with the Facility Infrared Spectropolarimeter at the DST. This study
  includes three observations of the He I triplet at 10830 Å on May 29
  and 30, 2017. The filament was stable on May 29, and was observed in
  the process of rising at speeds of 20-30 km/s during the two spatial
  maps taken on May 30. Vector magnetic fields along the filament were
  obtained that show an inverse configuration indicative of a flux rope
  topology, including co-aligned threads. To take advantage of the better
  spatial and temporal resolution of the Daniel K. Inouye Solar Telescope
  (DKIST), future collaborations of the DKIST and the DST to study solar
  filaments are discussed.

Title: Filament Magnetic Fields at the DST and DKIST
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
   Long, David; Choudhary, Debi Prasad; McAteer, James
Bibcode: 2019AAS...23422603W
Altcode:
  Observations from the 0.8-m Dunn Solar Telescope (DST) are
  qualitatively similar to data that will be produced by the 4-m
  Daniel K. Inouye Solar Telescope (DKIST), albeit at a lower spatial
  resolution and polarimetric sensitivity. We present HAZEL inversions
  of spectropolarimetric observations of a quiescent filament acquired
  with the Facility Infrared Spectropolarimeter at the DST. This study
  includes three observations of the He I triplet at 10830 Å on May 29
  and 30, 2017. The filament was stable on May 29, and was observed in
  the process of rising at speeds of 20-30 km/s during the two spatial
  maps taken on May 30. Vector magnetic fields along the filament were
  obtained that show an inverse configuration indicative of a flux rope
  topology, including co-aligned threads. To take advantage of the better
  spatial and temporal resolution of the DKIST, future collaborations
  of the DKIST and the DST to study solar filaments are discussed. We
  propose to further study the evolution of solar filaments that erupt
  and lead to Coronal Mass Ejections using interspaced observations from
  the DKIST and DST spectropolarimeters. While the DST observations will
  give information about the global evolution of physical properties
  leading to the destabilization, the DKIST observations will provide the
  information about the physical conditions in the small-scale structures
  that support the filament material.

Title: Fast Inversion of Solar Ca II Spectra in Non-local
    Thermodynamic Equilibrium
Authors: Beck, C.; Gosain, S.; Kiessner, C.
Bibcode: 2019ApJ...878...60B
Altcode: 2019arXiv190411843B
  Present-day solar imaging spectrometers typically yield a few hundred
  million spectra in one hour of observing time. This number will increase
  by an order of magnitude for future instruments with larger 4k ×
  4k sensors, such as those planned to be used for the upcoming Daniel
  K. Inouye Solar Telescope. A fast quantitative analysis of such huge
  data volumes can be done by comparing the observations to an archive of
  pre-calculated synthetic spectra to infer the thermodynamic properties
  of the atmosphere. To analyze intensity spectra of the Ca II IR line at
  854 nm in the solar atmosphere, we generated an archive with 2,000,000
  spectra under the assumption of non-local thermodynamic equilibrium
  (NLTE) with the NICOLE code. We tested its performance by inverting 60
  spectral scans of Ca II IR at 854 nm in the magnetically quiet Sun with
  700,000 profiles each. Based on the inversion results obtained using
  the full archive, we constructed a smaller archive by keeping only
  the 70,000 archive profiles that were actually used. We can reproduce
  the observed intensity spectra to within a few percent using either
  the full or the small archive. For spectra with 30 wavelength points,
  this NLTE inversion approach takes 0.02 (0.35) s per profile to obtain a
  temperature stratification when using the small (full) archive, i.e.,
  it can invert a single spectral scan in about 4 (68) hr. The code
  is able to simultaneously deal with an arbitrary number of spectral
  lines. This makes it a promising tool for deriving thermodynamic
  properties of the solar atmosphere from current or future solar
  high-resolution observations of photospheric and chromospheric lines.

Title: NLTE inversions of Ca II 854.2 nm spectra from SOLIS/VSM:
    Temperature diagnostics
Authors: Gusain, Sanjay; Beck, Christian
Bibcode: 2019AAS...23430203G
Altcode:
  We present non-local thermodynamic equilibrium (NLTE) inversions
  of Ca II 854.2 nm spectra from SOLIS/VSM observations. We show that
  precomputed spectra from a database of model atmospheres can be applied
  for a reliable inference of temperature stratifications in the solar
  atmosphere. We show examples of such inferences in different solar
  structures such as the umbra, penumbra, circum-facular regions and
  the quiet Sun. We also fit full Stokes profiles assuming a simple
  magnetic model with an exponential dependence with optical depth to
  derive vector magnetic field information and compare with the values
  derived from the weak-field approximation.

Title: Magnetic Flux Density in 3D MHD Simulations and Observations
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
Bibcode: 2019ASPC..526..191B
Altcode:
  We compare the polarization signals induced in three-dimensional (3D)
  magneto-hydrodynamical (MHD) simulations by the Zeeman effect in the
  presence of photospheric magnetic fields to those in observations
  at disc centre. We consider quantities determined from Stokes vector
  profiles of observations of photospheric spectral lines in the visible
  and near-infrared, and in corresponding synthetic spectra obtained
  from numerical 3D MHD simulations with an average magnetic flux
  density of 20-200 G. We match the spatial resolution of observations
  by degrading the spectra of the simulations. We find that the total
  unsigned vertical magnetic flux density in the simulation should
  be less than 50 G to reproduce the observed polarization signals in
  the quiet Sun internetwork. A value of ∼30 G best agrees with all
  observations we employed.

Title: Magnetic Structures of a Quiecent Filament
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
   Long, David; Choudhary, Debi; McAteer, James
Bibcode: 2019shin.confE..52W
Altcode:
  A quiecent filament is observed from the Dunn Solar Telescope (DST) on
  May 29 and 30, 2017. We present HAZEL inversions of spectropolarimetric
  observations acquired with the Facility Infrared Spectropolarimeter
  at the DST. This study includes three observations of the He I triplet
  at 1083.0 nm.

Title: Magnetic Properties and Flow Angle of the Inverse Evershed
    Flow at Its Downflow Points
Authors: Beck, C.; Choudhary, D. P.
Bibcode: 2019ApJ...874....6B
Altcode: 2019arXiv190204660B
  We determined the direction and strength of the photospheric and lower
  chromospheric magnetic field in the umbra and penumbra of a sunspot from
  inversions of spectropolarimetric observations of photospheric lines
  at 617 nm and 1565 nm and the chromospheric Ca II IR line at 854 nm,
  respectively. We compare the magnetic field vector with the direction
  of 75 flow channels that harbor the chromospheric inverse Evershed
  effect (IEF) near their downflow points (DFPs) in the sunspot’s
  penumbra. The azimuth and inclination of the IEF channels to the line
  of sight (LOS) were derived from spatial maps of the LOS velocity and
  line-core intensity of the Ca II IR line and a thermal inversion of the
  Ca II IR spectra to obtain temperature cubes. We find that the flow
  direction of the IEF near the DFPs is aligned with the photospheric
  magnetic field to within about ±15°. The IEF flow fibrils make
  an angle of 30°-90° to the local vertical with an average value
  of about 65°. The average field strength at the DFPs is about 1.3
  kG. Our findings suggest that the IEF in the lower chromosphere is a
  field-aligned siphon flow, where the larger field strength at the inner
  footpoints together with the lower temperature in the penumbra causes
  the necessary gas pressure difference relative to the outer footpoints
  in the hotter quiet Sun with lower magnetic field strength. The IEF
  connects to magnetic field lines that are not, like in the case of the
  regular Evershed flow, but which continue upward into the chromosphere,
  indicating an “uncombed” penumbral structure.

Title: The First Solar Seeing Profile Measurement with Two Apertures
    and Multiple Guide Regions
Authors: Ren, Deqing; Zhao, Gang; Wang, Xin; Beck, Christian;
   Broadfoot, Robert
Bibcode: 2019SoPh..294....1R
Altcode: 2018SoPh..294....1R
  Ground-based observations suffer from atmospheric turbulence
  perturbations, which seriously degrade the image quality. The seeing
  profile associated with the turbulence is critical to characterize an
  astronomical site. The optimal design and performance estimation of
  future solar ground-layer adaptive optics (GLAO) and multi-conjugate
  adaptive optics (MCAO) heavily rely on our knowledge of the seeing
  profile at a specific site. Many current optical seeing profile
  measurement techniques require one to use a large solar telescope for
  that purpose. The development of a portable instrument to measure and
  characterize the seeing profile is thus highly needed, in particular
  for testing potential new sites or for the regularly monitoring of
  the seeing condition at existing sites. Recently, we proposed the
  Advanced Multiple Aperture Seeing Profiler (A-MASP), which uses multiple
  small telescopes and multiple regions of interest (ROIs) on the solar
  surface to measure the seeing profile up to an altitude of 30 km. Here,
  we report our recent proof-of-concept observation run of the A-MASP
  technique with the Dunn Solar Telescope (DST) of the National Solar
  Observatory (NSO). We found that the Fried parameter, r<SUB>0</SUB>, was
  about 12 cm at the observed wavelength of 630 nm in the early morning
  and that there were three main turbulence layers. The strongest one
  was the mix layer near the ground. We observed the evolution of the
  top of the mix layer and found that it can rise to about 1.5 km in
  about 18 min, which is consistent with the theory of daytime boundary
  layer evolution. Another turbulence layer was observed from 8 to 15 km,
  which is at the top of the convective layer. Comparing an instrument
  with two sub-apertures with a real A-MASP instrument, we found that
  they should lead to similar results except for the altitude h =0.

Title: Inferring telescope polarization properties through spectral
    lines without linear polarization
Authors: Derks, A.; Beck, C.; Martínez Pillet, V.
Bibcode: 2018A&A...615A..22D
Altcode: 2018arXiv180401153D
  Context. Polarimetric observations taken with ground- or space-based
  telescopes usually need to be corrected for changes of the polarization
  state in the optical path. <BR /> Aims: We present a technique
  to determine the polarization properties of a telescope through
  observations of spectral lines that have no or negligible intrinsic
  linear polarization signals. For such spectral lines, any observed
  linear polarization must be induced by the telescope optics. We apply
  the technique to observations taken with the Spectropolarimeter for
  Infrared and Optical Regions (SPINOR) at the Dunn Solar Telescope
  (DST) and demonstrate that we can retrieve the characteristic
  polarization properties of the DST at three wavelengths of 459,
  526, and 615 nm. <BR /> Methods: We determine the amount of crosstalk
  between the intensity Stokes I and the linear and circular polarization
  states Stokes Q, U, and V, and between Stokes V and Stokes Q and U in
  spectropolarimetric observations of active regions. We fit a set of
  parameters that describe the polarization properties of the DST to
  the observed crosstalk values. We compare our results to parameters
  that were derived using a conventional telescope calibration unit
  (TCU). <BR /> Results: The values for the ratio of reflectivities X =
  r<SUB>s</SUB>/r<SUB>p</SUB> and the retardance τ of the DST turret
  mirrors from the analysis of the crosstalk match those derived with
  the TCU within the error bars. We find a negligible contribution of
  retardance from the entrance and exit windows of the evacuated part
  of the DST. Residual crosstalk after applying a correction for the
  telescope polarization stays at a level of 3-10% regardless of which
  parameter set is used, but with an rms fluctuation in the input data
  of already a few percent. The accuracy in the determination of the
  telescope properties is thus more limited by the quality of the input
  data than the method itself. <BR /> Conclusions: It is possible to
  derive the parameters that describe the polarization properties of a
  telescope from observations of spectral lines without intrinsic linear
  polarization signal. Such spectral lines have a dense coverage (about
  50 nm separation) in the visible part of the spectrum (400-615 nm),
  but none were found at longer wavelengths. Using spectral lines without
  intrinsic linear polarization is a promising tool for the polarimetric
  calibration of current or future solar telescopes such as the Daniel
  K. Inouye Solar Telescope (DKIST).

Title: Status of the Daniel K. Inouye Solar Telescope: unraveling
    the mysteries the Sun.
Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip
   R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini,
   Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich;
   Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt,
   Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka,
   Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin,
   Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team
Bibcode: 2018AAS...23231601R
Altcode:
  The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under
  construction on Haleakala, Maui will be the world’s largest solar
  telescope. Designed to meet the needs of critical high resolution and
  high sensitivity spectral and polarimetric observations of the sun,
  this facility will perform key observations of our nearest star that
  matters most to humankind. DKIST’s superb resolution and sensitivity
  will enable astronomers to address many of the fundamental problems
  in solar and stellar astrophysics, including the origin of stellar
  magnetism, the mechanisms of coronal heating and drivers of the
  solar wind, flares, coronal mass ejections and variability in solar
  and stellar output. DKIST will also address basic research aspects of
  Space Weather and help improve predictive capabilities. In combination
  with synoptic observations and theoretical modeling DKIST will unravel
  the many remaining mysteries of the Sun.The construction of DKIST is
  progressing on schedule with 80% of the facility complete. Operations
  are scheduled to begin early 2020. DKIST will replace the NSO
  facilities on Kitt Peak and Sac Peak with a national facility with
  worldwide unique capabilities. The design allows DKIST to operate as
  a coronagraph. Taking advantage of its large aperture and infrared
  polarimeters DKIST will be capable to routinely measure the currently
  illusive coronal magnetic fields. The state-of-the-art adaptive optics
  system provides diffraction limited imaging and the ability to resolve
  features approximately 20 km on the Sun. Achieving this resolution
  is critical for the ability to observe magnetic structures at their
  intrinsic, fundamental scales. Five instruments will be available at
  the start of operations, four of which will provide highly sensitive
  measurements of solar magnetic fields throughout the solar atmosphere
  - from the photosphere to the corona. The data from these instruments
  will be distributed to the world wide community via the NSO/DKIST data
  center located in Boulder. We present examples of science objectives
  and provide an overview of the facility and project status, including
  the ongoing efforts of the community to develop the critical science
  plan for the first 2-3 years of operations.

Title: The Magnetic Response of the Solar Atmosphere to Umbral Flashes
Authors: Houston, S. J.; Jess, D. B.; Asensio Ramos, A.; Grant,
   S. D. T.; Beck, C.; Norton, A. A.; Krishna Prasad, S.
Bibcode: 2018ApJ...860...28H
Altcode: 2018arXiv180300018H
  Chromospheric observations of sunspot umbrae offer an exceptional
  view of magnetoacoustic shock phenomena and the impact they have on
  the surrounding magnetically dominated plasma. We employ simultaneous
  slit-based spectro-polarimetry and spectral imaging observations of
  the chromospheric He I 10830 Å and Ca II 8542 Å lines to examine
  fluctuations in the umbral magnetic field caused by the steepening of
  magnetoacoustic waves into umbral flashes. Following the application
  of modern inversion routines, we find evidence to support the scenario
  that umbral shock events cause expansion of the embedded magnetic
  field lines due to the increased adiabatic pressure. The large number
  statistics employed allow us to calculate the adiabatic index, γ =
  1.12 ± 0.01, for chromospheric umbral locations. Examination of
  the vector magnetic field fluctuations perpendicular to the solar
  normal revealed changes up to ∼200 G at the locations of umbral
  flashes. Such transversal magnetic field fluctuations have not been
  described before. Through comparisons with nonlinear force-free field
  extrapolations, we find that the perturbations of the transverse field
  components are oriented in the same direction as the quiescent field
  geometries. This implies that magnetic field enhancements produced by
  umbral flashes are directed along the motion path of the developing
  shock, hence producing relatively small changes, up to a maximum
  of ∼8°, in the inclination and/or azimuthal directions of the
  magnetic field. Importantly, this work highlights that umbral flashes
  are able to modify the full vector magnetic field, with the detection
  of the weaker transverse magnetic field components made possible by
  high-resolution data combined with modern inversion routines.

Title: Thermodynamic Properties of the Inverse Evershed Flow at Its
    Downflow Points
Authors: Choudhary, D. P.; Beck, C.
Bibcode: 2018ApJ...859..139C
Altcode:
  We used spectropolarimetric observations of a sunspot in the active
  region NOAA 11809 in the Ca II line at 854.2 nm taken with the
  SpectroPolarimeter for Optical and Infrared Regions at the Dunn Solar
  Telescope to infer thermodynamic parameters along 100 super-penumbral
  fibrils that harbor the inverse Evershed flow. The fibrils were
  identified in line-of-sight (LOS) velocity and line-core intensity
  maps. The chromospheric LOS velocity abruptly decreases from 3 to 15 km
  s<SUP>-1</SUP> to zero at the inner footpoints of the fibrils that are
  located from the mid penumbra to about 1.4 spot radii. The spectra often
  show multiple absorption components, indicating spatially or vertically
  unresolved structures. Synthetic spectra with a 100% fill factor
  of a flow channel in the upper atmosphere yield strongly asymmetric
  profiles but no multiple separate components. The line-core intensity
  always peaks slightly closer to the umbra than the LOS velocity. Using
  the CAlcium Inversion using a Spectral ARchive code, we find that the
  fibrils make an angle of 30°-60° to the local vertical away from the
  umbra. The temperature near the downflow points is enhanced by 200 K
  at log τ ∼ -2 and up to 2000 K at log τ ∼ (-6) compared to the
  quiet Sun, without any signature in the low photosphere. Our results are
  consistent with a critical, i.e., sonic, or supersonic siphon flow along
  super-penumbral flux tubes in which accelerating plasma abruptly attains
  subcritical velocity through a standing shock in or near the penumbra.

Title: Inferring Telescope Polarization Properties Through Spectral
    Lines Without Linear Polarization
Authors: Derks, Alysa; Beck, Christian; Martinez Pillet, Valentin
Bibcode: 2018tess.conf21060D
Altcode:
  We present a technique to determine the polarization properties
  of a telescope through observations of spectral lines that have
  no or negligible intrinsic linear polarization signals. For such
  spectral lines, any observed linear polarization must be induced by
  the telescope optics. We apply the technique to observations taken
  with SPINOR at the Dunn Solar Telescope (DST) and demonstrate that we
  can retrieve the characteristic polarization properties of the DST at
  three wavelengths of 459, 526, and 615 nm. The values for the ratio
  of reflectivities X = rs/rp and the retardance τ of the DST turret
  mirrors from the analysis of the crosstalk match those derived with
  the TCU within the error bars. <P />We conclude that it is possible
  to derive the parameters that describe the polarization properties
  of a telescope from observations of spectral lines without intrinsic
  linear polarization signal. Such spectral lines have a dense coverage
  (about 50 nm separation) in the visible part of the spectrum (400-615
  nm), but none were found at longer wavelengths. Using spectral lines
  without intrinsic linear polarization is a promising tool for the
  polarimetric calibration of current or future solar telescopes such
  as the Daniel K. Inouye Solar Telescope (DKIST).

Title: Alfvén wave dissipation in the solar chromosphere
Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz
   V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.;
   Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt,
   Rebecca L.
Bibcode: 2018NatPh..14..480G
Altcode: 2018arXiv181007712G
  Magnetohydrodynamic Alfvén waves<SUP>1</SUP> have been a focus of
  laboratory plasma physics<SUP>2</SUP> and astrophysics<SUP>3</SUP>
  for over half a century. Their unique nature makes them ideal energy
  transporters, and while the solar atmosphere provides preferential
  conditions for their existence<SUP>4</SUP>, direct detection has proved
  difficult as a result of their evolving and dynamic observational
  signatures. The viability of Alfvén waves as a heating mechanism relies
  upon the efficient dissipation and thermalization of the wave energy,
  with direct evidence remaining elusive until now. Here we provide the
  first observational evidence of Alfvén waves heating chromospheric
  plasma in a sunspot umbra through the formation of shock fronts. The
  magnetic field configuration of the shock environment, alongside the
  tangential velocity signatures, distinguish them from conventional
  umbral flashes<SUP>5</SUP>. Observed local temperature enhancements
  of 5% are consistent with the dissipation of mode-converted Alfvén
  waves driven by upwardly propagating magneto-acoustic oscillations,
  providing an unprecedented insight into the behaviour of Alfvén waves
  in the solar atmosphere and beyond.

Title: High-resolution Observations of Hα Spectra with a Subtractive
    Double Pass
Authors: Beck, C.; Rezaei, R.; Choudhary, D. P.; Gosain, S.;
   Tritschler, A.; Louis, R. E.
Bibcode: 2018SoPh..293...36B
Altcode: 2017arXiv171207077B
  High-resolution imaging spectroscopy in solar physics has relied on
  Fabry-Pérot interferometers (FPIs) in recent years. FPI systems,
  however, become technically challenging and expensive for telescopes
  larger than the 1 m class. A conventional slit spectrograph with a
  diffraction-limited performance over a large field of view (FOV) can
  be built at much lower cost and effort. It can be converted into an
  imaging spectro(polari)meter using the concept of a subtractive double
  pass (SDP). We demonstrate that an SDP system can reach a similar
  performance as FPI-based systems with a high spatial and moderate
  spectral resolution across a FOV of 100<SUP>″</SUP>×100<SUP>″</SUP>
  with a spectral coverage of 1 nm. We use Hα spectra taken with an SDP
  system at the Dunn Solar Telescope and complementary full-disc data to
  infer the properties of small-scale superpenumbral filaments. We find
  that the majority of all filaments end in patches of opposite-polarity
  fields. The internal fine-structure in the line-core intensity of Hα
  at spatial scales of about 0.″5 exceeds that in other parameters
  such as the line width, indicating small-scale opacity effects in a
  larger-scale structure with common properties. We conclude that SDP
  systems in combination with (multi-conjugate) adaptive optics are a
  valid alternative to FPI systems when high spatial resolution and a
  large FOV are required. They can also reach a cadence that is comparable
  to that of FPI systems, while providing a much larger spectral range
  and a simultaneous multi-line capability.

Title: The Polarization Signature of Photospheric Magnetic Fields
    in 3D MHD Simulations and Observations at Disk Center
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
Bibcode: 2017ApJ...842...37B
Altcode: 2017arXiv170506812B
  Before using three-dimensional (3D) magnetohydrodynamical (MHD)
  simulations of the solar photosphere in the determination of elemental
  abundances, one has to ensure that the correct amount of magnetic
  flux is present in the simulations. The presence of magnetic flux
  modifies the thermal structure of the solar photosphere, which affects
  abundance determinations and the solar spectral irradiance. The amount
  of magnetic flux in the solar photosphere also constrains any possible
  heating in the outer solar atmosphere through magnetic reconnection. We
  compare the polarization signals in disk-center observations of the
  solar photosphere in quiet-Sun regions with those in Stokes spectra
  computed on the basis of 3D MHD simulations having average magnetic
  flux densities of about 20, 56, 112, and 224 G. This approach allows
  us to find the simulation run that best matches the observations. The
  observations were taken with the Hinode SpectroPolarimeter (SP),
  the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow
  Spectrograph (POLIS), and the GREGOR Fabry-Pèrot Interferometer
  (GFPI), respectively. We determine characteristic quantities of full
  Stokes profiles in a few photospheric spectral lines in the visible
  (630 nm) and near-infrared (1083 and 1565 nm). We find that the
  appearance of abnormal granulation in intensity maps of degraded
  simulations can be traced back to an initially regular granulation
  pattern with numerous bright points in the intergranular lanes
  before the spatial degradation. The linear polarization signals in
  the simulations are almost exclusively related to canopies of strong
  magnetic flux concentrations and not to transient events of magnetic
  flux emergence. We find that the average vertical magnetic flux density
  in the simulation should be less than 50 G to reproduce the observed
  polarization signals in the quiet-Sun internetwork. A value of about 35
  G gives the best match across the SP, TIP, POLIS, and GFPI observations.

Title: Design and calibration of a high-sensitivity and high-accuracy
    polarimeter based on liquid crystal variable retarders
Authors: Guo, Jing; Ren, De-Qing; Liu, Cheng-Chao; Zhu, Yong-Tian;
   Dou, Jiang-Pei; Zhang, Xi; Beck, Christian
Bibcode: 2017RAA....17....8G
Altcode:
  Polarimetry plays an important role in the measurement of solar magnetic
  fields. We developed a high-sensitivity and high-accuracy polarimeter
  (HHP) based on nematic liquid crystal variable retarders (LCVRs), which
  has a compact setup and no mechanical moving parts. The system design
  and calibration methods are discussed in detail. The azimuth error of
  the transmission axis of the polarizer as well as the fast axes of the
  two LCVRs and the quarter-wave plate were determined using dedicated
  procedures. Linearly and circularly polarized light were employed to
  evaluate the performance of the HHP. The experimental results indicate
  that a polarimetric sensitivity of better than \[5.7 × {10<SUP> -
  3</SUP>}\] can be achieved by using a single short-exposure image,
  while an accuracy on the order of 10<SUP>-5</SUP> can be reached by
  using a large number of short-exposure images. This makes the HHP a
  high-performance system that can be used with a ground-based solar
  telescope for high-precision solar magnetic field investigations.

Title: Absorption by Mercury's Exosphere During the May 9th, 2016
    Solar Transit.
Authors: Schmidt, C.; Reardon, K.; Killen, R. M.; Gary, D. E.; Ahn, K.;
   Leblanc, F.; Baumgardner, J. L.; Mendillo, M.; Beck, C.; Mangano, V.
Bibcode: 2016AGUFM.P53B2198S
Altcode:
  Observations of Mercury during a solar transit have the unique property
  that line absorption may be used to retrieve the exosphere's column
  density at all points above the terminator simultaneously. We report
  on measurements during the 9 May 2016 transit with the Dunn Solar
  Telescope (Interferometric BIdimensional Spectropolarimeter: IBIS &amp;
  Horizontal Spectrograph: HSG) and the Big Bear Solar Observatory (Fast
  Imaging Solar Spectrograph: FISS). The sodium exosphere was observed
  via Fabry-Perot imaging with IBIS in 9 mA increments, and with FISS at
  a dispersion of 17 mA/pixel by scanning the spectrograph slit over the
  planet's disk. A search for potassium D line absorption was performed
  using slit spectroscopy with HSG at a resolution of R 270,000. In each
  instrument, exposures of 20-40 ms and adaptive optics enable spatial
  structure to be resolved on sub-arcsecond scales. The line profiles at
  every spatial bin are divided by a shifted and scaled reference spectrum
  in order to isolate the exosphere's absorption from line absorption in
  the solar atmosphere and structures inherent to granulation. Analysis
  of these data sets is ongoing, but preliminary findings clearly show
  the densest column of sodium near the poles and the content at dawn
  enhanced several times with respect to dusk. Such is consistent with
  2003 transit results taken at the same Mercury season (Schleicher et
  al., 2004), however the data volumes herein permit a more in-depth
  study in which time-dependence of the exosphere may be considered.

Title: Spectroscopy at the Solar Limb: II. Are Spicules Heated to
    Coronal Temperatures?
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.; Fabbian, D.
Bibcode: 2016SoPh..291.2281B
Altcode: 2016arXiv160606132B; 2016SoPh..tmp..132B
  Spicules of the so-called type II were suggested to be relevant for
  coronal heating because of their ubiquity on the solar surface and
  their eventual extension into the corona. We investigate whether solar
  spicules are heated to transition-region or coronal temperatures and
  reach coronal heights (≫6 Mm) using multiwavelength observations
  of limb spicules in different chromospheric spectral lines (Ca II H,
  Hε , Hα , Ca II IR at 854.2 nm, He I at 1083 nm) taken with slit
  spectrographs and imaging spectrometers. We determine the line width
  of spectrally resolved line profiles in individual spicules and
  throughout the field of view, and estimate the maximal height that
  different types of off-limb features reach. We derive estimates of
  the kinetic temperature and the non-thermal velocity from the line
  width of spectral lines from different chemical elements. We find that
  most regular, i.e. thin and elongated, spicules reach a height of at
  most about 6 Mm above the solar limb. The majority of features found
  at larger heights are irregularly shaped with a significantly larger
  lateral extension, of up to a few Mm, than spicules. Both individual and
  average line profiles in all spectral lines show a decrease in their
  line width with height above the limb with very few exceptions. The
  kinetic temperature and the non-thermal velocity decrease with height
  above the limb. We find no indications that the spicules in our data
  reach coronal heights or transition-region or coronal temperatures.

Title: On the Magnetism and Dynamics of Prominence Legs Hosting
    Tornadoes
Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.;
   Collados, M.; Beck, C.; de la Cruz Rodríguez, J.
Bibcode: 2016ApJ...825..119M
Altcode: 2016arXiv160501183M
  Solar tornadoes are dark vertical filamentary structures observed
  in the extreme ultraviolet associated with prominence legs and
  filament barbs. Their true nature and relationship to prominences
  requires an understanding of their magnetic structure and dynamic
  properties. Recently, a controversy has arisen: is the magnetic field
  organized forming vertical, helical structures or is it dominantly
  horizontal? And concerning their dynamics, are tornadoes really rotating
  or is it just a visual illusion? Here we analyze four consecutive
  spectro-polarimetric scans of a prominence hosting tornadoes on its
  legs, which helps us shed some light on their magnetic and dynamical
  properties. We show that the magnetic field is very smooth in all the
  prominence, which is probably an intrinsic property of the coronal
  field. The prominence legs have vertical helical fields that show
  slow temporal variation that is probably related to the motion of
  the fibrils. Concerning the dynamics, we argue that (1) if rotation
  exists, it is intermittent, lasting no more than one hour, and (2)
  the observed velocity pattern is also consistent with an oscillatory
  velocity pattern (waves).

Title: Three-Dimensional Chromospheric Thermal Structure of Sunspots
Authors: Prasad Choudhary, Debi; Beck, Christian
Bibcode: 2016SPD....47.0401P
Altcode:
  We have observed several sunspots using the Spectropolarimeter for
  Infrared and Optical wavelength Ranges at the Dunn Solar Telescope
  during 29 July to 4 August 2013. The data consists of full Stokes
  profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion
  of these Stokes spectra provides the magnetic, thermal and velocity
  structure at photospheric and chromospheric heights of sunspots. In this
  contribution, we present the results on the 3D thermal structure in the
  super-penumbral canopy of a round sunspot, derived by a novel approach
  for the inversion of Ca II IR spectra. Tracing individual fibrils
  in the super-penumbral canopy, we find that about half of them form
  only short loops of a a few Mm length that return to the photosphere
  in the close surroundings of the sunspot instead of connecting to more
  remote magnetic network at the outer end of the moat flow. We also find
  indications for standing shocks at the inner foot points of the flow
  channels that are compatible with a supersonic siphon flow scenario.

Title: Cross-Calibrating Sunspot Magnetic Field Strength Measurements
    from the McMath-Pierce Solar Telescope and the Dunn Solar Telescope
Authors: Watson, Fraser T.; Beck, Christian; Penn, Matthew J.;
   Tritschler, Alexandra; Pillet, Valentín Martinez; Livingston,
   William C.
Bibcode: 2015SoPh..290.3267W
Altcode: 2015arXiv151107315W; 2015SoPh..tmp..158W
  In this article we describe a recent effort to cross-calibrate data
  from an infrared detector at the McMath-Pierce Solar Telescope and
  the Facility InfraRed Spectropolarimeter (FIRS) at the Dunn Solar
  Telescope. A synoptic observation program at the McMath-Pierce has
  measured umbral magnetic field strengths since 1998, and this data set
  has recently been compared with umbral magnetic field observations from
  SOHO/MDI and SDO/HMI. To further improve on the data from McMath-Pierce,
  we compared the data with measurements taken at the Dunn Solar Telescope
  with far greater spectral resolution than has been possible with
  space instrumentation. To minimise potential disruption to the study,
  concurrent umbral measurements were made so that the relationship
  between the two datasets can be most accurately characterised. We find
  that there is a strong agreement between the umbral magnetic field
  strengths recorded by each instrument, and we reduced the FIRS data
  in two different ways to successfully test this correlation further.

Title: Multiwavelength spectropolarimetric observations of an
    Ellerman bomb
Authors: Rezaei, R.; Beck, C.
Bibcode: 2015A&A...582A.104R
Altcode:
  Context. Ellerman bombs (EBs) are enhanced emission in the wings of the
  Hα line in the solar spectrum. <BR /> Aims: We study the structure of
  an EB in the photosphere and chromosphere. <BR /> Methods: We analyze
  simultaneous observations of four chromospheric lines (Hα, Ca ii H,
  Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines
  (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170
  nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the
  Helioseismic and Magnetic Imager (HMI) are used to trace the temporal
  evolution of the magnetic structure. <BR /> Results: We identify the
  EB by excess emission in the wings of the Hα line, a brightening in
  the UV continuum, and large emission peaks in the core of the two Ca
  ii lines. The EB shows a blueshift in all chromospheric lines, while
  no shifts are observed in the photospheric lines. The blueshift in
  the chromospheric layer causes very asymmetric emission peaks in the
  Ca ii H line. The photospheric Si i spectral line shows a shallower
  line depth at the location of the EB. The UV continuum maps show that
  the EB was substantially brighter than its surroundings for about 30
  min. The continuum contrast of the EB from 170 nm to 1080 nm shows a
  power-law dependency on the wavelength. The temperature enhancement
  amounts to 130 K in the low photosphere and 400 K at the temperature
  minimum level. This temperature excess is also seen in an LTE inversion
  of the Ca ii spectra. The total thermal and radiative energy content
  of the EB is about 10<SUP>20</SUP> J and 10<SUP>18</SUP> J in the
  photosphere and chromosphere, respectively. The HMI data hints at a
  photospheric magnetic flux cancellation as the driver of the EB. <BR
  /> Conclusions: Ellerman bombs release the energy in a height range
  of several pressure scale heights around the temperature minimum such
  that they affect both the photosphere and the lower chromosphere.

Title: Variation in sunspot properties between 1999 and 2014
Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt,
   W.; Collados, M.
Bibcode: 2015A&A...578A..43R
Altcode:
  <BR /> Aims: We study the variation in the magnetic field strength,
  area, and continuum intensity of umbrae in solar cycles 23 and 24. <BR
  /> Methods: We analyzed a sample of 374 sunspots observed from 1999
  until 2014 with the Tenerife Infrared Polarimeter at the German
  Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter
  at the Dunn Solar Telescope. The sample of field strength, area,
  and intensities was used to trace any long-term or cyclic trend of
  umbral properties in the last 15 years. <BR /> Results: Sunspots
  are systematically weaker, that is, have a weaker field strength and
  stronger continuum intensity, toward the end of cycle 23 than they
  had at the maximum of cycle 23. The linear trend reverses with the
  onset of cycle 24. We find that the field strength decreases in the
  declining phase of cycle 23 by about 112 (± 16) G yr<SUP>-1</SUP>,
  while it increases in the rising phase of cycle 24 by about 138 (±
  72) G yr<SUP>-1</SUP>. The umbral intensity shows the opposite trend:
  the intensity increases with a rate of 0.7 (± 0.3)% of I<SUB>c</SUB>
  yr<SUP>-1</SUP> toward the end of cycle 23 and decreases with a rate
  of 3.8 (± 1.5)% of I<SUB>c</SUB> yr<SUP>-1</SUP> toward the maximum of
  cycle 24. The distribution of the umbral maximum field strength in cycle
  24 is similar to that of cycle 23, but is slightly shifted toward lower
  values by about 80 G, corresponding to a possible long-term gradient in
  umbral field strength of about 7 ± 4 G yr<SUP>-1</SUP>. If instead of
  the maximum umbral field we consider the average value over the entire
  umbra, the distribution shifts by about 44 Gauss. <BR /> Conclusions:
  The umbral brightness decreases in the rising stage of a solar cycle,
  but increases from maximum toward the end of the cycle. Our results
  do not indicate a drastic change of the solar cycle toward a grand
  minimum in the near future.

Title: Three Dimensional Chromospheric Thermal Structure of Sunspot
Authors: Prasad Choudhary, Debi; Beck, Christian; Rezaei, R.
Bibcode: 2015TESS....131201P
Altcode:
  We have observed sunspots using the Spectropolarimeter for infrared
  and optical wavelength ranges at the Dunn Solar Telescope during
  29 July to 4 August 2013. The data consists of full Stokes profiles
  in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
  these Stokes spectra provides the magnetic, thermal and velocity
  structure at photospheric and chromospheric heights of sunspots. In
  this contribution, we present the results on the 3D thermal structure
  in the super-penumbral canopy of a well rounded sunspot, derived
  by a novel approach for the inversion of Ca II IR spectra. Tracing
  individual fibrils in the super-penumbral canopy, we find that about
  half of them form only short loops of a a few Mm length that return
  to the photosphere in the close surroundings of the sunspot instead
  of connecting to more remote magnetic network at the outer end of the
  moat flow.

Title: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields
    in Solar Prominence Feet
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
   A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J.
Bibcode: 2015ApJ...802....3M
Altcode: 2015arXiv150103295M
  Solar prominences are clouds of cool plasma levitating above
  the solar surface and insulated from the million-degree corona by
  magnetic fields. They form in regions of complex magnetic topology,
  characterized by non-potential fields, which can evolve abruptly,
  disintegrating the prominence and ejecting magnetized material
  into the heliosphere. However, their physics is not yet fully
  understood because mapping such complex magnetic configurations
  and their evolution is extremely challenging, and must often be
  guessed by proxy from photometric observations. Using state-of-the-art
  spectro-polarimetric data, we reconstruct the structure of the magnetic
  field in a prominence. We find that prominence feet harbor helical
  magnetic fields connecting the prominence to the solar surface below.

Title: Fast Inversion of Solar Ca II Spectra
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.; Louis, R. E.
Bibcode: 2015ApJ...798..100B
Altcode: 2014arXiv1410.8451B
  We present a fast (Lt1 s per profile) inversion code for solar Ca
  II lines. The code uses an archive of spectra that are synthesized
  prior to the inversion under the assumption of local thermodynamic
  equilibrium (LTE). We show that it can be successfully applied to
  spectrograph data or more sparsely sampled spectra from two-dimensional
  spectrometers. From a comparison to a non-LTE inversion of the same
  set of spectra, we derive a first-order non-LTE correction to the
  temperature stratifications derived in the LTE approach. The correction
  factor is close to unity up to log τ ~ -3 and increases to values of
  2.5 and 4 at log τ = -6 in the quiet Sun and the umbra, respectively.

Title: The relation between umbral magnetic field strength and area
    density of umbral dots
Authors: Ferguson, Sierra; Beck, Christian
Bibcode: 2015AAS...22513704F
Altcode:
  Interiors of sunspots are modeled either as a monolithic block of
  magnetic flux or groups of flux bundles. Umbral dots (UDs) - small
  bright grains inside the dark umbra - are harder to explain in the
  monolithic model, but they fit into the group of flux bundle model
  as field-free intrusions that push magnetic field lines aside due to
  their convective nature. The goal of this project was to determine if
  there is a relationship between the UD density and the area of the
  umbra as well as the magnetic field strength of the umbra. Routines
  were developed in IDL that located the UDs and then would find the
  respective areas and magnetic field strength of the umbrae. These
  routines can be applied to both numerical simulations and observational
  data. Our findings show that there is an indication of correlation
  between the UD density and the umbral area, but there is no clear
  indication of a correlation with the UD density and the magnetic field
  strength. The simulations show a significantly larger UD density than
  the observations This work is carried out through the National Solar
  Observatory Summer Research Assistantship (SRA) Program. The National
  Solar Observatory is operated by the Association of Universities for
  Research in Astronomy, Inc. (AURA) under cooperative agreement with
  the National Science Foundation.

Title: Three Dimensional Chromospheric Temperature Structure of
    Sunspot
Authors: Choudhary, D. P.; Beck, C.; Rezaei, R.
Bibcode: 2014AGUFMSH41B4132C
Altcode:
  We have observed sunspots using the Spectropolarimeter for infrared
  and optical wavelength ranges at the Dunn Solar Telescope during
  29 July to 4 August 2013. The data consists of full Stokes profiles
  in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
  these Stokes spectra provides the magnetic, thermal and velocity
  structure at photospheric and chromospheric heights of sunspots. In
  this contribution, we present the results on the 3D thermal structure
  in the super-penumbral canopy of a well rounded sunspot, derived
  by a novel approach for the inversion of Ca II IR spectra. Tracing
  individual fibrils in the super-penumbral canopy, we find that about
  half of them form only short loops of a a few Mm length that return
  to the photosphere in the close surroundings of the sunspot instead
  of connecting to more remote magnetic network at the outer end of the
  moat flow.

Title: Anomalous flows in a sunspot penumbra
Authors: Louis, Rohan E.; Beck, Christian; Mathew, Shibu K.;
   Venkatakrishnan, P.
Bibcode: 2014A&A...570A..92L
Altcode: 2014arXiv1408.6690L
  Context. The photospheric Evershed flow is a distinct characteristic of
  penumbrae that is closely associated with the photometric and magnetic
  structure of sunspots. <BR /> Aims: We analyse the properties of an
  anomalous flow in the photosphere in a sunspot penumbra and compare it
  with those of the regular Evershed flow. <BR /> Methods: High-resolution
  spectropolarimetric observations of active region NOAA 11271 were
  obtained with the spectro-polarimeter (SP) on board Hinode. We used the
  magnetic field properties derived by a Milne-Eddington inversion. In
  addition, we used Ca II H images obtained by the broad-band filter
  instrument to study the low chromospheric response to the photospheric
  structure and Dopplergrams from the Helioseismic and Magnetic Imager
  to follow the evolution of the photospheric flows. <BR /> Results:
  We detect a blueshifted feature that appeared on the limb-side
  penumbra of a sunspot and that was present intermittently during
  the next two hours. It exhibited a maximum blueshift of 1.6 km
  s<SUP>-1</SUP>, an area of 5.2 arcsec<SUP>2</SUP>, and a maximum
  uninterrupted lifetime of 1 h. The blueshifted feature, when present,
  lies parallel to redshifts. Both blue- and redshifts flank a highly
  inclined or horizontal magnetic structure that is radially oriented
  in the penumbra. The low-cadence SP maps reveal changes in size,
  radial position in the penumbra, and line-of-sight (LOS) velocity
  of the blueshifted feature, from one scan to the next. There was an
  increase of nearly 500 G in the field strength with the onset of the
  blueshifts, particularly when the LOS velocity in the feature exceeded
  1.5 km s<SUP>-1</SUP>. There was only a marginal reduction in the
  field inclination of about 10° with the increase in blueshifts. In the
  chromosphere, intense, arc-shaped brightenings were observed close to
  the location of the photospheric blueshifts, which extend from the edge
  of the umbral core to the penumbra-quiet Sun boundary. The intensity
  of these brightenings exceeds the background intensity by 30% to 65%
  with the strongest and largest brightenings observed about 30 min after
  the strongest blueshifts were detected at the photosphere. The close
  spatial proximity of the two phenomenon strongly suggests a causal
  relationship. <BR /> Conclusions: The blueshifted feature represents
  plasma motion that could be related to a magnetic structure that rises
  in the solar atmosphere and subsequently reconnects with the ambient
  chromospheric magnetic field of the sunspot or an inverse Evershed flow,
  which would be unique in the photosphere. This transient phenomena is
  presumably related to the dynamic stability of the sunspot because the
  corresponding umbral core separated two days later at the location of
  the blueshifts and fragmented subsequently.

Title: Small-scale chromospheric jets above a sunspot light bridge
Authors: Louis, Rohan E.; Beck, Christian; Ichimoto, Kiyoshi
Bibcode: 2014A&A...567A..96L
Altcode: 2014arXiv1406.0103L
  Context. The chromosphere above sunspot umbrae and penumbrae shows
  several different types of fast dynamic events such as running
  penumbral waves, umbral flashes, and penumbral microjets. <BR /> Aims:
  The aim of this paper is to identify the physical driver responsible
  for the dynamic and small-scale chromospheric jets above a sunspot
  light bridge. <BR /> Methods: High-resolution broadband filtergrams
  of active region NOAA 11271 in Ca ii H and G band were obtained with
  the Solar Optical Telescope on board Hinode. We identified the jets in
  the Ca ii H images using a semi-automatic routine and determined their
  length and orientation. We applied local correlation tracking (LCT)
  to the G-band images to obtain the photospheric horizontal velocity
  field. The magnetic field topology was derived from a Milne-Eddington
  inversion of a simultaneous scan with the Spectropolarimeter. <BR />
  Results: The chromospheric jets consist of a bright, triangular-shaped
  blob that lies on the light bridge, while the apex of this blob extends
  into a spike-like structure that is bright against the dark umbral
  background. Most of the jets have apparent lengths of less than 1000
  km and about 30% of the jets have lengths between 1000-1600 km. The
  jets are oriented within ±35° to the normal of the spine of the light
  bridge. Most of them are clustered near the central part of the light
  bridge within a 2'' area. The jets are seen to move rapidly along the
  light bridge and many of them cannot be identified in successive images
  taken with a 2 min cadence. The jets are primarily located on one side
  of the light bridge and are directed into the umbral core. The Stokes
  profiles at or close to the location of the blobs on the LB exhibit
  both a significant net circular polarization and multiple components,
  including opposite-polarity lobes. The magnetic field diverges from the
  light bridge towards the umbral cores that it separates. The LCT reveals
  that in the photosphere there is a predominantly uni-directional flow
  with speeds of 100-150 m s<SUP>-1</SUP> along the light bridge. This
  unidirectional flow is interrupted by a patch of weak or very small
  motions on the light bridge which also moves along the light bridge. <BR
  /> Conclusions: The dynamic short-lived chromospheric jets above the LB
  seem to be guided by the magnetic field lines. Reconnection events are
  a likely trigger for such phenomenon since they occur at locations where
  the magnetic field changes orientation sharply and where we also observe
  isolated patches of opposite-polarity magnetic components. We find no
  clear relation between the jets and the photospheric flow pattern.

Title: A Three-dimensional View of the Thermal Structure in a
    Super-penumbral Canopy
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.
Bibcode: 2014ApJ...788..183B
Altcode: 2014arXiv1405.1473B
  We investigate the three-dimensional (3D) thermal topology in a
  super-penumbral canopy of an active region (AR). We derive temperature
  stratifications in the AR by an inversion of the Ca II IR line at 854.2
  nm, assuming local thermal equilibrium. We find that about half of the
  radially oriented fibrils in the super-penumbral canopy form short,
  low-lying (h &lt; 1 Mm) loops in the 3D temperature cube. These closed
  loops connect from bright grains in or close to the penumbra to the
  photosphere a few Mms away from the sunspot. The other half of the
  fibrils monotonically rise with distance from the sunspot. Many of
  the fibrils show a central dark core and two lateral brightenings in
  line-core intensity images. The corresponding velocity image shows
  fibrils that are as wide as the fibrils seen in intensity without a
  lateral substructure. Additionally, we study a feature from a different
  class of structures without prominent mass flows. Its 3D topology is
  formed by two parallel, closed loops that connect patches of opposite
  polarity. We present evidence that the inverse Evershed flow into the
  sunspot in the lower chromosphere is the consequence of siphon flows
  along short loops that connect photospheric foot points. The dark-cored
  structure of the chromospheric fibrils cannot have a convective origin
  because of their location above regular granulation. The dark core
  most likely results from an opacity difference between the central
  axis and the lateral edges caused by the significant flow speed along
  the fibrils.

Title: A Spectro-polarimetric Analysis of Sunspot Umbrae
Authors: Watson, Fraser; Tritschler, Alexandra; Penn, Matthew J.;
   Beck, Christian; Livingston, William; Martinez Pillet, Valentin
Bibcode: 2014AAS...22411202W
Altcode:
  The recent quiet solar cycle has invited new questions as to
  the nature of the solar magnetic field and how it changes over
  time. To investigate this, we use the National Solar Observatory’s
  McMath-Pierce Solar Telescope Facility (McMP) and Dunn Solar Telescope
  (DST) to compare measurements of sunspots from five active regions
  observed in 2013. Both BABO at the McMP and FIRS at the DST were
  used to provide spectra of the Fe 1564.8nm line, which is affected
  by the presence of magnetic fields. The magnetic field is derived
  from Zeeman splitting in Stokes-I by BABO, and by inversion of the
  Stokes parameters from FIRS data allowing for comparisons of sunspot
  properties between the two instruments.. We present the first results
  from this study including the magnetic fields in sunspot umbrae from
  five active regions measured simultaneously by BABO and FIRS.

Title: Near-infrared spectropolarimetry of a δ-spot
Authors: Balthasar, H.; Beck, C.; Louis, R. E.; Verma, M.; Denker, C.
Bibcode: 2014A&A...562L...6B
Altcode: 2014arXiv1401.4386B
  Sunspots harboring umbrae of both magnetic polarities within a common
  penumbra (δ-spots) are often but not always related to flares. We
  present first near-infrared observations (Fe i λ1078.3 nm and Si i
  λ1078.6 nm spectra) obtained with the Tenerife Infrared Polarimeter at
  the Vacuum Tower Telescope in Tenerife on 2012 June 17, which afford
  accurate and sensitive diagnostics to scrutinize the complex fields
  along the magnetic neutral line of a δ-spot within active region NOAA
  11504. We examined the vector magnetic field, line-of-sight (LOS)
  velocities, and horizontal proper motions of this rather inactive
  δ-spot. We find a smooth transition of the magnetic vector field
  from the main umbra to that of opposite polarity (δ-umbra), but a
  discontinuity of the horizontal magnetic field at some distance from the
  δ-umbra on the polarity inversion line. The magnetic field decreases
  faster with height by a factor of two above the δ-umbra. The latter is
  surrounded by its own Evershed flow. The Evershed flow coming from the
  main umbra ends at a line dividing the spot into two parts. This line
  is marked by the occurrence of central emission in the Ca iiλ854.2 nm
  line. Along this line, high chromospheric LOS-velocities of both signs
  appear. We detect a shear flow within the horizontal flux transport
  velocities parallel to the dividing line.

Title: Thermodynamic fluctuations in solar photospheric
    three-dimensional convection simulations and observations
    (Corrigendum)
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
   Rezaei, R.
Bibcode: 2013A&A...559C...1B
Altcode:
  No abstract at ADS

Title: Thermodynamic fluctuations in solar photospheric
    three-dimensional convection simulations and observations
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
   Rezaei, R.
Bibcode: 2013A&A...557A.109B
Altcode: 2013arXiv1306.6093B
  Context. Numerical three-dimensional (3D) radiative
  (magneto-)hydrodynamical [(M)HD] simulations of solar convection
  are nowadays used to understand the physical properties of the solar
  photosphere and convective envelope, and, in particular, to determine
  the Sun's photospheric chemical abundances. To validate this approach,
  it is important to check that no excessive thermodynamic fluctuations
  arise as a consequence of the partially incomplete treatment of
  radiative transfer causing radiative damping that is too modest. <BR
  /> Aims: We investigate the realism of the thermodynamics in recent
  state-of-the-art 3D convection simulations of the solar atmosphere
  carried out with the Stagger code. <BR /> Methods: We compared the
  characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565
  nm) and one Si i line at 1082.7 nm in solar disc-centre observations
  of different spatial resolution with spectra synthesized from 3D
  convection simulations. The observations were taken with ground-based
  (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI),
  POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all
  at the Vacuum Tower Telescope on Tenerife) and space-based instruments
  (Hinode/Spectropolarimeter). We degraded the synthetic spectra to
  the spatial resolution of the observations, based on the distribution
  of the continuum intensity I<SUB>c</SUB>. We estimated the spectral
  degradation to be applied to the simulation results by comparing atlas
  spectra with averaged observed spectra. In addition to deriving a set
  of line parameters directly from the intensity profiles, we used the
  SIR (Stokes Inversion based on Response functions) code to invert
  the spectra. <BR /> Results: The spatial degradation kernels yield
  a similar generic spatial stray-light contamination of about 30%
  for all instruments. The spectral stray light inside the different
  spectrometers is found to be between 2% and 20%. Most of the line
  parameters from the observational data are matched by the degraded
  HD simulation spectra. The inversions predict a macroturbulent
  velocity v<SUB>mac</SUB> below 10 m s<SUP>-1</SUP> for the HD
  simulation spectra at full spatial resolution, whereas they yield
  v<SUB>mac</SUB> ≲ 1000 m s<SUP>-1</SUP> at a spatial resolution of
  0.″3. The temperature fluctuations in the inversion of the degraded
  HD simulation spectra do not exceed those from the observational data
  (of the order of 100-200 K rms for -2 ⪉ log τ<SUB>500 nm</SUB>
  ⪉ -0.5). The comparison of line parameters in spatially averaged
  profiles with the averaged values of line parameters in spatially
  resolved profiles indicates a significant change in (average) line
  properties on a spatial scale between 0.″13 and 0.″3. <BR />
  Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra),
  we find no indications of excessive thermodynamic fluctuations
  in the 3D HD simulation. To definitely confirm that simulations
  without spatial degradation contain fully realistic thermodynamic
  fluctuations requires observations at even higher spatial resolution
  (i.e. &lt;0.″13). <P />Appendices A and B are available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: GREGOR Fabry-Pérot interferometer and its companion the blue
    imaging solar spectrometer
Authors: Puschmann, Klaus G.; Denker, Carsten; Balthasar, Horst; Louis,
   Rohan E.; Popow, Emil; Woche, Manfred; Beck, Christian; Seelemann,
   Thomas; Volkmer, Reiner
Bibcode: 2013OptEn..52h1606P
Altcode: 2013arXiv1302.7157P
  The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
  first-light instruments of the German 1.5-m GREGOR solar telescope
  at the Observatorio del Teide, Tenerife, Spain. The GFPI allows
  fast narrow-band imaging and postfactum image restoration. The
  retrieved physical parameters will be a fundamental building block
  for understanding the dynamic sun and its magnetic field at spatial
  scales down to ∼50 km on the solar surface. The GFPI is a tunable
  dual-etalon system in a collimated mounting. It is designed for
  spectrometric and spectropolarimetric observations between 530-860
  nm and 580-660 nm, respectively, and possesses a theoretical spectral
  resolution of R≈250,000. Large-format, high-cadence charged coupled
  device detectors with sophisticated computer hard- and software enable
  the scanning of spectral lines in time-spans equivalent to the evolution
  time of solar features. The field-of-view (FOV) of 50″×38″ covers
  a significant fraction of the typical area of active regions in the
  spectroscopic mode. In case of Stokes-vector spectropolarimetry,
  the FOV reduces to 25″×38″. The main characteristics of the GFPI
  including advanced and automated calibration and observing procedures
  are presented. Improvements in the optical design of the instrument
  are discussed and first observational results are shown. Finally,
  the first concrete ideas for the integration of a second FPI, the blue
  imaging solar spectrometer, are laid out, which will explore the blue
  spectral region below 530 nm.

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

Title: First Results from the EUNIS 2013 Sounding Rocket Campaign
Authors: Daw, Adrian N.; Rabin, D. M.; Brosius, J. W.; Haas, J. P.;
   Plummer, T.; Cauzzi, G.; Reardon, K. P.; Beck, C.
Bibcode: 2013SPD....4410501D
Altcode:
  The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS)
  sounding rocket launched 23 April 2013 at 17:30 UT, as part of a
  campaign including co-ordinated observations with the Dunn Solar
  Telescope/IBIS, Hinode/EIS, SoHO/CDS, RHESSI and SDO. EUNIS obtained
  the highest-resolution observations of the solar spectrum from 52-63 nm
  observed to date, as well as observations with the previously-flown
  waveband from 30-37 nm. The broad spectral coverage of the EUV
  observations includes emission lines of ionization stages from He I to
  Fe XIX, and thus a wide temperature range of 0.025 to 10 MK. Absolute
  radiometric calibration of EUNIS provides underflight calibration of
  CDS, EIS and AIA. Spectra were obtained with a 1.3 s cadence as the
  660-arcsec long slit was rastered across two different regions. The
  observations captured a B-class flare in active region NOAA 11726 as
  well as active regions 11723, 11724, off-limb, quiet sun and a coronal
  hole. We discuss first results from anaysis of this rich and extensive
  data set.

Title: The energy of waves in the photosphere and lower
    chromosphere. IV. Inversion results of Ca II H spectra
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
Bibcode: 2013A&A...553A..73B
Altcode: 2013arXiv1302.6936B
  Context. Most semi-empirical static one-dimensional (1D) models of
  the solar atmosphere in the magnetically quiet Sun (QS) predict an
  increase in temperature at chromospheric layers. Numerical simulations
  of the solar chromosphere with a variable degree of sophistication,
  i.e. from 1D to three-dimensional (3D) simulations; assuming local
  thermal equilibrium (LTE) or non-LTE (NLTE), on the other hand,
  only yielded an increase in the brightness temperature without any
  stationary increase in the gas temperature. <BR /> Aims: We investigate
  the thermal structure in the solar chromosphere as derived from an
  LTE inversion of observed Ca ii H spectra in QS and active regions
  (ARs). <BR /> Methods: We applied an inversion strategy based on the
  SIR (Stokes inversion by response functions) code to Ca ii H spectra to
  obtain 1D temperature stratifications. We investigated the temperature
  stratifications on differences between magnetic and field-free regions
  in the QS, and on differences between QS and ARs. We determined the
  energy content of individual calcium bright grains (BGs) as specific
  candidates of chromospheric heating events. We compared observed with
  synthetic NLTE spectra to estimate the significance of the LTE inversion
  results. <BR /> Results: The fluctuations of observed intensities yield
  a variable temperature structure with spatio-temporal rms fluctuations
  below 100 K in the photosphere and between 200 and 300 K in the QS
  chromosphere. The average temperature stratification in the QS does not
  exhibit a clear chromospheric temperature rise, unlike the AR case. We
  find a characteristic energy content of about 7 × 10<SUP>18</SUP> J
  for BGs that repeat with a cadence of about 160 s. The precursors of BGs
  have a vertical extent of about 200 km and a horizontal extent of about
  1 Mm. The comparison of observed with synthetic NLTE profiles partly
  confirms the results of the LTE inversion that the solar chromosphere
  in the QS oscillates between an atmosphere in radiative equilibrium and
  one with a moderate chromospheric temperature rise. Two-dimensional x -
  z temperature maps exhibit nearly horizontal canopy-like structures with
  an extent of a few Mm around photospheric magnetic field concentrations
  at a height of about 600 km. <BR /> Conclusions: The large difference
  between QS regions and ARs and the better match of AR and NLTE reference
  spectra suggest that magnetic heating processes are more important
  than commonly assumed. The temperature fluctuations in QS derived by
  the LTE inversion do not suffice on average to maintain a stationary
  chromospheric temperature rise. The spatially and vertically resolved
  information on the temperature structure allows one to investigate
  in detail the topology and evolution of the thermal structure in the
  lower solar atmosphere. <P />Appendix A is available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Formation of a penumbra in a decaying sunspot
Authors: Louis, R. E.; Mathew, S. K.; Puschmann, K. G.; Beck, C.;
   Balthasar, H.
Bibcode: 2013A&A...552L...7L
Altcode: 2013arXiv1303.3599L
  Context. Penumbrae are an important characteristic of sunspots, whose
  formation is intricately related to the nature of sub-photospheric
  magnetic fields. <BR /> Aims: We study the formation of a penumbra in
  a decaying sunspot and compare its properties with those seen during
  the development of a proto-spot. <BR /> Methods: High-resolution
  spectropolarimetric observations of active region NOAA 11283 were
  obtained from the spectro-polarimeter on board Hinode. These were
  complemented with full-disk filtergrams of continuum intensity,
  line-of-sight magnetograms, and dopplergrams from the Helioseismic and
  Magnetic Imager at high cadence. <BR /> Results: The formation of a
  penumbra in the decaying sunspot occurs after the coalescence of the
  sunspot with a magnetic fragment/pore, which initially formed in the
  quiet Sun close to an emerging flux region. At first, a smaller set of
  penumbral filaments develop near the location of the merger with very
  bright penumbral grains with intensities of 1.2 I<SUB>QS</SUB>, upflows
  of 4 km s<SUP>-1</SUP>, and a lifetime of 10 h. During the decay of
  these filaments, a larger segment of a penumbra forms at the location
  of the coalescence. These new filaments are characterized by nearly
  supersonic downflows of 6.5 km s<SUP>-1</SUP> that change to a regular
  Evershed flow nearly 3 h later. <BR /> Conclusions: The coalescence of
  the pore with the decaying sunspot provided sufficient magnetic flux for
  the penumbra to form in the sunspot. The emerging flux region could have
  played a decisive role in this process because the formation occurred
  at the location of the merger and not on the opposite side of the
  sunspot. <P />An animation of the HMI data is available in electronic
  form at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: The energy of waves in the photosphere and lower
    chromosphere. III. Inversion setup for Ca II H spectra in local
    thermal equilibrium
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
Bibcode: 2013A&A...549A..24B
Altcode: 2012arXiv1209.6194B
  Context. The Ca II H line is one of the strongest lines in the solar
  spectrum, and it provides continuous information on the solar atmosphere
  from the photosphere to the lower chromosphere. <BR /> Aims: We describe
  an inversion approach that reproduces observed Ca II H spectra by
  assuming local thermal equilibrium (LTE). <BR /> Methods: We developed
  an inversion strategy based on the SIR code that reproduces Ca II H
  spectra in the LTE approximation. The approach uses a two-step procedure
  with an archive of pre-calculated spectra to fit the line core and a
  subsequent iterative modification to improve the fit mainly in the line
  wing. Simultaneous spectra in the 630 nm range can optionally be used
  to fix the continuum temperature. The method retrieves one-dimensional
  (1D) temperature stratifications while neglecting lateral radiative
  transport. Line-of-sight velocities are included post facto with
  an empirical approach. <BR /> Results: An archive of about 300 000
  pre-calculated spectra is more than sufficient to reproduce the line
  core of observed Ca II H spectra both in the quiet Sun and in active
  regions. The subsequent iterative adjustment of the thermodynamical
  stratification matches observed and best-fit spectra to a level of about
  0.5% of I<SUB>c</SUB> in the line wing and about 1% of I<SUB>c</SUB>
  in the line core. <BR /> Conclusions: The successful application of
  the LTE inversion strategy suggests that inversion schemes based on
  pre-calculated spectra allow a reliable and relatively fast retrieval of
  solar properties from observed chromospheric spectra. The approach can
  be easily extended to a 1D non-LTE (NLTE) case by a simple exchange
  of the pre-calculated archive spectra. Using synthetic NLTE spectra
  from numerical three-dimensional (3D) simulations instead will
  finally allow one to extend the approach from the static 1D-case
  to dynamical atmosphere models, including the complete 3D radiative
  transport. <P />The animation is available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: The GREGOR Fabry-Pérot Interferometer
Authors: Puschmann, K. G.; Denker, C.; Kneer, F.; Al Erdogan, N.;
   Balthasar, H.; Bauer, S. M.; Beck, C.; Bello González, N.; Collados,
   M.; Hahn, T.; Hirzberger, J.; Hofmann, A.; Louis, R. E.; Nicklas, H.;
   Okunev, O.; Martínez Pillet, V.; Popow, E.; Seelemann, T.; Volkmer,
   R.; Wittmann, A. D.; Woche, M.
Bibcode: 2012AN....333..880P
Altcode: 2012arXiv1210.2921P
  The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
  first-light instruments of the German 1.5-meter GREGOR solar telescope
  at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two
  tunable etalons in collimated mounting. Thanks to its large-format,
  high-cadence CCD detectors with sophisticated computer hard- and
  software it is capable of scanning spectral lines with a cadence
  that is sufficient to capture the dynamic evolution of the solar
  atmosphere. The field-of-view (FOV) of 50 arcsec × 38 arcsec is well
  suited for quiet Sun and sunspot observations. However, in the vector
  spectropolarimetric mode the FOV reduces to 25 arcsec × 38 arcsec. The
  spectral coverage in the spectroscopic mode extends from 530-860 nm
  with a theoretical spectral resolution of R ≈ 250,000, whereas in
  the vector spectropolarimetric mode the wavelength range is at present
  limited to 580-660 nm. The combination of fast narrow-band imaging and
  post-factum image restoration has the potential for discovery science
  concerning the dynamic Sun and its magnetic field at spatial scales
  down to ∼50 km on the solar surface.

Title: The GREGOR Fabry-Perot interferometer: status report and
    prospects
Authors: Puschmann, Klaus G.; Balthasar, Horst; Beck, Christian;
   Louis, Rohan E.; Popow, Emil; Seelemann, Thomas; Volkmer, Reiner;
   Woche, Manfred; Denker, Carsten
Bibcode: 2012SPIE.8446E..79P
Altcode: 2012arXiv1207.2084P
  The GREGOR Fabry-Ṕerot Interferometer (GFPI) is one of three
  first-light instruments of the German 1.5-meter GREGOR solar telescope
  at the Observatorio del Teide, Tenerife, Spain. The GFPI allows
  fast narrow-band imaging and post-factum image restoration. The
  retrieved physical parameters will be a fundamental building block
  for understanding the dynamic Sun and its magnetic field at spatial
  scales down to 50 km on the solar surface. The GFPI is a tunable
  dual-etalon system in a collimated mounting. It is designed for
  spectropolarimetric observations over the wavelength range from 530-860
  nm with a theoretical spectral resolution of R ≍ 250,000. The GFPI is
  equipped with a full-Stokes polarimeter. Large-format, high-cadence CCD
  detectors with powerful computer hard- and software enable the scanning
  of spectral lines in time spans equivalent to the evolution time of
  solar features. The field-of-view of 50''×38'' covers a significant
  fraction of the typical area of active regions. We present the main
  characteristics of the GFPI including advanced and automated calibration
  and observing procedures. We discuss improvements in the optical design
  of the instrument and show first observational results. Finally, we
  lay out first concrete ideas for the integration of a second FPI, the
  Blue Imaging Solar Spectrometer, which will explore the blue spectral
  region below 530 nm.

Title: The energy of waves in the photosphere and lower
    chromosphere. II. Intensity statistics
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
Bibcode: 2012A&A...544A..46B
Altcode: 2012arXiv1206.1759B
  Context. The energy source powering the solar chromosphere is still
  undetermined, but leaves its traces in observed intensities. <BR />
  Aims: We investigate the statistics of the intensity distributions
  as a function of the wavelength for Ca ii H and the Ca ii IR line at
  854.2 nm to estimate the energy content in the observed intensity
  fluctuations. <BR /> Methods: We derived the intensity variations
  at different heights of the solar atmosphere, as traced by the line
  wings and line cores of the two spectral lines. We converted the
  observed intensities to absolute energy units employing reference
  profiles calculated in non-local thermal equilibrium (NLTE). We also
  converted the intensity fluctuations to corresponding brightness
  temperatures assuming LTE. <BR /> Results: The root-mean-square
  (rms) fluctuations of the emitted intensity are about 0.6 (1.2)
  W m<SUP>-2</SUP> ster<SUP>-1</SUP> pm<SUP>-1</SUP> near the core of
  the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative
  intensity fluctuations of about 20% (30%). For the line wing,
  we find rms values of about 0.3 W m<SUP>-2</SUP> ster<SUP>-1</SUP>
  pm<SUP>-1</SUP> for both lines, corresponding to relative fluctuations
  below 5%. The relative rms values show a local minimum for wavelengths
  forming at a height of about 130 km, but otherwise increase smoothly
  from the wing to the core, i.e., from photosphere to chromosphere. The
  corresponding rms brightness temperature fluctuations are below 100 K
  for the photosphere and up to 500 K in the chromosphere. The skewness
  of the intensity distributions is close to zero in the outer line
  wing and positive throughout the rest of the line spectrum, owing to
  the frequent occurrence of high-intensity events. The skewness shows a
  pronounced local maximum at locations with photospheric magnetic fields
  for wavelengths in-between those of the line wing and the line core
  (z ≈ 150-300 km), and a global maximum at the very core (z ≈ 1000
  km) for both magnetic and field-free locations. <BR /> Conclusions:
  The energy content of the intensity fluctuations is insufficient to
  create a chromospheric temperature rise that would be similar to the
  one in most reference models of the solar atmosphere. The increase in
  the rms fluctuations with height indicates the presence of upwardly
  propagating acoustic waves of increasing oscillation amplitude. The
  intensity and temperature variations indicate that there is a
  clear increase in dynamical activity from photosphere towards the
  chromosphere, but the variations fall short of the magnitude predicted
  by fully dynamical chromospheric models by a factor of about five. The
  enhanced skewness between the photosphere and lower solar chromosphere
  at magnetic locations is indicative of a mechanism that acts solely
  on magnetized plasma. <P />Appendices are available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Variation in sunspot properties between 1999 and 2011 as
    observed with the Tenerife Infrared Polarimeter
Authors: Rezaei, R.; Beck, C.; Schmidt, W.
Bibcode: 2012A&A...541A..60R
Altcode: 2012arXiv1203.1444R
  <BR /> Aims: We study the variation in the magnetic field strength
  and the umbral intensity of sunspots during the declining phase of
  the solar cycle No. 23 and in the beginning of cycle No. 24. <BR />
  Methods: We analyze a sample of 183 sunspots observed from 1999 until
  2011 with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
  Tower Telescope (VTT). The magnetic field strength is derived from the
  Zeeman splitting of the Stokes-V signal in one near-infrared spectral
  line, either Fe i 1564.8 nm, Fe i 1089.6 nm, or Si i 1082.7 nm. This
  avoids the effects of the unpolarized stray light from the field-free
  quiet Sun surroundings that can affect the splitting seen in Stokes-I
  in the umbra. The minimum umbral continuum intensity and umbral area are
  also measured. <BR /> Results: We find that there is a systematic trend
  for sunspots in the late stage of the solar cycle No. 23 to be weaker,
  i.e., to have a smaller maximum magnetic field strength than those at
  the start of the cycle. The decrease in the field strength with time of
  about 94 Gyr<SUP>-1</SUP> is well beyond the statistical fluctuations
  that would be expected because of the larger number of sunspots close
  to cycle maximum (14 Gyr<SUP>-1</SUP>). In the same time interval, the
  continuum intensity of the umbra increases with a rate of 1.3 (±0.4)%
  of I<SUB>c</SUB> yr<SUP>-1</SUP>, while the umbral area does not show
  any trend above the statistical variance. Sunspots in the new cycle
  No. 24 show higher field strengths and lower continuum intensities
  than those at the end of cycle No. 23, interrupting the trend. <BR />
  Conclusions: Sunspots have an intrinsically weaker field strength and
  brighter umbrae at the late stages of solar cycles compared to their
  initial stages, without any significant change in their area. The abrupt
  increase in field strength in sunspots of the new cycle suggests that
  the cyclic variations are dominating over any long-term trend that
  continues across cycles. We find a slight decrease in field strength
  and an increase in intensity as a long-term trend across the cycles.

Title: Siphon flow in a cool magnetic loop
Authors: Bethge, C.; Beck, C.; Peter, H.; Lagg, A.
Bibcode: 2012A&A...537A.130B
Altcode: 2011arXiv1111.5564B
  Context. Siphon flows that are driven by a gas pressure difference
  between two photospheric footpoints of different magnetic field strength
  connected by magnetic field lines are a well-studied phenomenon in
  theory, but observational evidence is scarce. Aims. We investigate the
  properties of a structure in the solar chromosphere in an active region
  to find out whether the feature is consistent with a siphon flow in
  a magnetic loop filled with chromospheric material. <P />Methods. We
  derived the line-of-sight (LOS) velocity of several photospheric
  spectral lines and two chromospheric spectral lines, Ca II H 3968.5
  *Aring; and He I 10830 Å, in spectropolarimetric observations of
  NOAA 10978 done with the Tenerife Infrared Polarimeter (TIP-II) and
  the POlarimetric LIttrow Spectrograph (POLIS). The structure can be
  clearly traced in the LOS velocity maps and the absorption depth of
  He I. The magnetic field configuration in the photosphere is inferred
  directly from the observed Stokes parameters and from inversions with
  the HELIX<SUP>+</SUP> code. Data from the full-disk Chromospheric
  Telescope (ChroTel) in He I in intensity and LOS velocity are used for
  tracking the temporal evolution of the flow, along with TRACE Fe IX/X
  171 Å data for additional information about coronal regions related to
  the structure under investigation. <P />Results. The inner end of the
  structure is located in the penumbra of a sunspot. It shows downflows
  whose strength decreases with decreasing height in the atmosphere. The
  flow velocity in He I falls abruptly from above 40 km s<SUP>-1</SUP>
  to about zero further into the penumbra. A slight increase of emission
  is seen in the Ca II H spectra at the endpoint. At the outer end of the
  structure, the photospheric lines that form higher up in the atmosphere
  show upflows that accelerate with height. The polarization signal near
  the outer end shows a polarity opposite to that of the sunspot, the
  magnetic field strength of 580 G is roughly half as large as at the
  inner end. The structure exists for about 90 min. Its appearance is
  preceeded by a brightening in its middle in the coronal TRACE data. <P
  />Conclusions. The observed flows match theoretical predictions of
  chromospheric and coronal siphon flows, with accelerating upflowing
  plasma at one footpoint with low field strength and decelerating
  downflowing plasma at the other end. A tube shock at the inner end is
  probable, but the evidence is not conclusive. The TRACE data suggest
  that the structure forms because of a reorganization of field lines
  after a reconnection event.

Title: Stray-light contamination and spatial deconvolution of
    slit-spectrograph observations
Authors: Beck, C.; Rezaei, R.; Fabbian, D.
Bibcode: 2011A&A...535A.129B
Altcode: 2011arXiv1109.2421B
  Context. Stray light caused by scattering on optical surfaces
  and in the Earth's atmosphere degrades the spatial resolution
  of observations. Whereas post-facto reconstruction techniques
  are common for 2D imaging and spectroscopy, similar options for
  slit-spectrograph data are rarely applied. <BR /> Aims: We study the
  contribution of stray light to the two channels of the POlarimetric
  LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of
  a slit-spectrograph instrument. We test the performance of different
  methods of stray-light correction and spatial deconvolution to improve
  the spatial resolution post-facto. <BR /> Methods: We model the stray
  light as having two components: a spectrally dispersed component and
  a "parasitic" component of spectrally undispersed light caused by
  scattering inside the spectrograph. We used several measurements to
  estimate the two contributions: a) observations with a (partly) blocked
  field of view (FOV); b) a convolution of the FTS spectral atlas; c)
  imaging of the spider mounting in the pupil plane; d) umbral profiles;
  and e) spurious polarization signal in telluric spectral lines. The
  measurements with a partly blocked FOV in the focal plane allowed us
  to estimate the spatial point spread function (PSF) of POLIS and the
  main spectrograph of the German Vacuum Tower Telescope (VTT). We then
  used the obtained PSF for a deconvolution of both spectroscopic and
  spectropolarimetric data and investigated the effect on the spectra. <BR
  /> Results: The parasitic contribution can be directly and accurately
  determined for POLIS, amounting to about 5% (0.3%) of the (continuum)
  intensity at 396 nm (630 nm). The spectrally dispersed stray light is
  less accessible because of its many contributing sources. We estimate
  a lower limit of about 10% across the full FOV for the dispersed stray
  light from umbral profiles. In quiet Sun regions, the stray-light level
  from the close surroundings (d &lt; 2'') of a given spatial point is
  about 20%. The stray light reduces to below 2% at a distance of 20''
  from a lit area for both POLIS and the main spectrograph. The spatial
  deconvolution using the PSF obtained improves the spatial resolution
  and increases the contrast, with a minor amplification of noise. <BR
  /> Conclusions: A two-component model of the stray-light contributions
  seems to be sufficient for a basic correction of observed spectra. The
  instrumental PSF obtained can be used to model the off-limb stray light,
  to determine the stray-light contamination accurately for observation
  targets with large spatial intensity gradients such as sunspots,
  and also to improve the spatial resolution of observations post-facto.

Title: The Chromospheric Telescope
Authors: Bethge, C.; Peter, H.; Kentischer, T. J.; Halbgewachs, C.;
   Elmore, D. F.; Beck, C.
Bibcode: 2011A&A...534A.105B
Altcode: 2011arXiv1108.4880B
  <BR /> Aims: We introduce the Chromospheric Telescope (ChroTel) at the
  Observatorio del Teide in Izaña on Tenerife as a new multi-wavelength
  imaging telescope for full-disk synoptic observations of the solar
  chromosphere. We describe the design of the instrument and summarize
  its performance during the first one and a half years of operation. We
  present a method to derive line-of-sight velocity maps of the full
  solar disk from filtergrams taken in and near the He i infrared line
  at 10 830 Å. <BR /> Methods: ChroTel observations are conducted
  using Lyot-type filters for the chromospheric lines of Ca ii K,
  Hα, and He i 10 830 Å. The instrument operates autonomically and
  gathers imaging data in all three channels with a cadence of down to
  one minute. The use of a tunable filter for the He i line allows us
  to determine line-shifts by calibrating the line-of-sight velocity
  maps derived from the filtergram intensities with spectrographic data
  from the Tenerife Infrared Polarimeter at high spatial and spectral
  resolution. <BR /> Results: The robotic operation and automated data
  reduction have proven to operate reliably in the first one and and
  half years. The achieved spatial resolution of the data is close to
  the theoretical limit of 2 arcsec in Hα and Ca ii K and 3 arcsec in He
  i. Line-of-sight velocities in He i can be determined with a precision
  of better than 3-4 km s<SUP>-1</SUP> when co-temporal spectrographic
  maps are available for calibration. <BR /> Conclusions: ChroTel offers a
  unique combination of imaging in the most important chromospheric lines,
  along with the possibility to determine line-of-sight velocities in
  one of the lines. This is of interest for scientific investigations
  of large-scale structures in the solar chromosphere, as well as for
  context imaging of high-resolution solar observations.

Title: Spectroscopy at the solar limb. I. Average off-limb profiles
    and Doppler shifts of Ca II H
Authors: Beck, C. A. R.; Rezaei, R.
Bibcode: 2011A&A...531A.173B
Altcode: 2011arXiv1106.0646B
  <BR /> Aims: We present constraints on the thermodynamical structure
  of the chromosphere from ground-based observations of the Ca
  ii H line profile near and off the solar limb. <BR /> Methods:
  We obtained a slit-spectrograph data set of the Ca ii H line with
  a high signal-to-noise ratio in a field of view extending 20''
  across the limb. We analyzed the spectra for the characteristic
  properties of average and individual off-limb spectra. We used
  various tracers of the Doppler shifts, such as the location of the
  absorption core, the ratio of the two emission peaks H<SUB>2V</SUB>
  and H<SUB>2R</SUB>, and intensity images at a fixed wavelength. <BR
  /> Results: The average off-limb profiles show a smooth variation
  with increasing limb distance. The line width increases up to a
  height of about 2 Mm above the limb. The profile shape is fairly
  symmetric with nearly identical H<SUB>2V</SUB> and H<SUB>2R</SUB>
  intensities; at a height of 5 Mm, it changes into a single Gaussian
  without emission peaks. We find that all off-limb spectra show large
  Doppler shifts that fluctuate on the smallest resolved spatial
  scales. The variation is more prominent in cuts parallel to the
  solar limb than on those perpendicular to it. As far as individual
  structures can be unequivocally identified at our spatial resolution,
  we find a specific relation between intensity enhancements and Doppler
  shifts: elongated brightenings are often flanked all along their
  extension by velocities in opposite directions. <BR /> Conclusions:
  The average off-limb spectra of Ca ii H present a good opportunity
  to test static chromospheric atmosphere models because they lack the
  photospheric contribution that is present in disk-center spectra. We
  suggest that the observed relation between intensity enhancements
  and Doppler shifts could be caused by waves propagating along the
  surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave
  propagation inclined to the tube axis would cause a helical shape of
  the Doppler excursions, visible as opposite velocity at the sides of
  the flux tube. Spectroscopic data allow one to distinguish this from
  a sausage-mode oscillation where the maximum Doppler shift and the
  tube axis would coincide. <P />Appendices are available in electronic
  form at <A href="http://www.andaa.org">http://www.andaa.org</A>The
  Data set is available in electronic form at the CDS via
  anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173</A>

Title: Two-dimensional solar spectropolarimetry with the KIS/IAA
    Visible Imaging Polarimeter
Authors: Beck, C.; Bellot Rubio, L. R.; Kentischer, T. J.; Tritschler,
   A.; Del Toro Iniesta, J. C.
Bibcode: 2010A&A...520A.115B
Altcode: 2010arXiv1007.1153B
  Context. Spectropolarimetry at high spatial and spectral resolution
  is a basic tool to characterize the magnetic properties of the solar
  atmosphere. <BR /> Aims: We introduce the KIS/IAA Visible Imaging
  Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS
  spectrometer at the German Vacuum Tower Telescope (VTT) into a full
  vector polarimeter. VIP is a collaboration between the Kiepenheuer
  Institut für Sonnenphysik (KIS) and the Instituto de Astrofísica
  de Andalucía (IAA-CSIC). <BR /> Methods: We describe the optical
  setup of VIP, the data acquisition procedure, and the calibration of
  the spectropolarimetric measurements. We show examples of data taken
  between 2005 and 2008 to illustrate the potential of the instrument. <BR
  /> Results: VIP is capable of measuring the four Stokes profiles of
  spectral lines in the range from 420 to 700 nm with a spatial resolution
  better than 0farcs5. Lines can be sampled at 40 wavelength positions
  in 60 s, achieving a noise level of about 2 × 10<SUP>-3</SUP> with
  exposure times of 300 ms and pixel sizes of 0farcs17 × 0farcs17 (2
  × 2 binning). The polarization modulation is stable over periods of a
  few days, ensuring high polarimetric accuracy. The excellent spectral
  resolution of TESOS allows the use of sophisticated data analysis
  techniques such as Stokes inversions. One of the first scientific
  results of VIP presented here is that the ribbon-like magnetic
  structures of the network are associated with a distinct pattern of
  net circular polarization away from disk center. <BR /> Conclusions:
  VIP performs spectropolarimetric measurements of solar magnetic fields
  at a spatial resolution that is only slightly worse than that of the
  Hinode spectropolarimeter, while providing a 2D field field of view and
  the possibility to observe up to four spectral regions sequentially
  with high cadence. VIP can be used as a stand-alone instrument or in
  combination with other spectropolarimeters and imaging systems of the
  VTT for extended wavelength coverage.

Title: A chromospheric dark-cored fibril in Ca II IR spectra
Authors: Beck, C.; Tritschler, A.; Wöger, F.
Bibcode: 2010AN....331..574B
Altcode:
  We investigate the thermodynamical and magnetic properties of a
  ``dark-cored" fibril seen in the chromospheric Ca II IR line at 854.2
  nm to determine the physical process behind its appearance. We analyse
  a time series of spectropolarimetric observations obtained in the Ca
  II IR line at 854.2 nm and the photospheric Fe I line at 630.25 nm. We
  simultaneously invert the spectra in both wavelength ranges with the
  SIR code to obtain the temperature and velocity stratification with
  height in the solar atmosphere and the magnetic field properties in the
  photosphere. The structure can be clearly traced in the line-of-sight
  (LOS) velocity and the temperature maps. It connects from a small
  pore with kG fields to a region with lower field strength. The flow
  velocity and the temperature indicate that the height of the structure
  increases with increasing distance from the inner footpoint. The Stokes
  V signal of 854.2 nm shows a Doppler-shifted polarization signal with
  the same displacement as in the intensity profile, indicating that the
  supersonic flow seen in the LOS velocity is located within magnetized
  plasma. We conclude that the chromospheric dark-cored fibril traces
  a siphon flow along magnetic field lines, driven by the gas pressure
  difference caused by the higher magnetic field strength at the inner
  footpoint. We suggest that fast flows guided by the magnetic field lead
  to the appearance of ``dark-cored" fibrils in intensity images. Although
  the observations included the determination of the polarization signal
  in the chromospheric Ca II IR line, the signal could not be analysed
  quantitatively due to the low S/N. Chromospheric polarimetry will thus
  require telescopes of larger aperture able to collect a sufficient
  number of photons for a reliable determination of polarization in deep
  and only weakly polarized spectral lines.

Title: Linear wavelength correlation matrices of photospheric and
    chromospheric spectral lines. I. Observations vs. modeling
Authors: Beck, C. A. R.; Rammacher, W.
Bibcode: 2010A&A...510A..66B
Altcode: 2009arXiv0909.2224B
  Context. The process that heats the solar chromosphere is a difficult
  target for observational studies because the assumption of local
  thermal equilibrium (LTE) is not valid in the upper solar atmosphere,
  which complicates the analysis of spectra. <BR /> Aims: We investigate
  the linear correlation coefficient between the intensities at different
  wavelengths in photospheric and chromospheric spectral lines because the
  correlation can be determined directly for any spectra from observations
  or modeling. Waves which propagate vertically through the stratified
  solar atmosphere affect different wavelengths at different times when
  the contribution functions for each wavelength peak in different
  layers. This leads to a characteristic pattern of (non-)coherence
  of the intensity at various wavelengths with respect to each other
  which carries information on the physical processes. <BR /> Methods:
  We derived the correlation matrices for several photospheric and
  chromospheric spectral lines from observations. We separated locations
  with a significant photospheric polarization signal and thus magnetic
  fields from those without a polarization signal. For comparison with
  the observations, we calculated correlation matrices for spectra from
  simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD
  simulation run. We applied the correlation method also to temperature
  maps at different optical depth layers derived from a LTE inversion
  of Ca II H spectra. <BR /> Results: We find that all photospheric
  spectral lines show a similar pattern: a pronounced asymmetry of
  the correlation between line core and red or blue wing. The pattern
  cannot be reproduced with a simulation of the granulation pattern, but
  with waves that travel upwards through the formation heights of the
  lines. The correct asymmetry between red and blue wing only appears
  when a temperature enhancement occurs simultaneously with a downflow
  velocity in the wave simulation. All chromospheric spectral lines show
  a more complex pattern. The 1-D NLTE simulations of monochromatic
  waves produce a correlation matrix that qualitatively matches the
  observations near the very core of the Ca II H line. The photospheric
  signature is well reproduced in the correlation matrix derived from
  the 3-D MHD simulation. <BR /> Conclusions: The correlation matrices
  of observed photospheric and chromospheric spectral lines are highly
  structured with characteristic and different patterns in every spectral
  line. The comparison with matrices derived from simulations and simple
  modeling suggests that the main driver of the detected patterns are
  upwards propagating waves. Application of the correlation method to
  3-D temperature cubes seems to be a promising tool for a detailed
  comparison of simulation results and observations in future studies.

Title: Magnetic loop emergence within a granule
Authors: Gömöry, P.; Beck, C.; Balthasar, H.; Rybák, J.; Kučera,
   A.; Koza, J.; Wöhl, H.
Bibcode: 2010A&A...511A..14G
Altcode: 2009arXiv0910.4449G
  <BR /> Aims: We investigate the temporal evolution of magnetic
  flux emerging within a granule in the quiet-Sun internetwork at
  disk center. <BR /> Methods: We combined IR spectropolarimetry of
  high angular resolution performed in two Fe i lines at 1565 nm with
  speckle-reconstructed G-band imaging. We determined the magnetic
  field parameters by a LTE inversion of the full Stokes vector using
  the SIR code, and followed their evolution in time. To interpret
  the observations, we created a geometrical model of a rising loop in
  3D. The relevant parameters of the loop were matched to the observations
  where possible. We then synthesized spectra from the 3D model for a
  comparison to the observations. <BR /> Results: We found signatures of
  magnetic flux emergence within a growing granule. In the early phases,
  a horizontal magnetic field with a distinct linear polarization signal
  dominated the emerging flux. Later on, two patches of opposite circular
  polarization signal appeared symmetrically on either side of the linear
  polarization patch, indicating a small loop-like structure. The mean
  magnetic flux density of this loop was roughly 450 G, with a total
  magnetic flux of around 3 × 10<SUP>17</SUP> Mx. During the ~12 min
  episode of loop occurrence, the spatial extent of the loop increased
  from about 1 to 2 arcsec. The middle part of the appearing feature
  was blueshifted during its occurrence, supporting the scenario of an
  emerging loop. There is also clear evidence for the interaction of
  one loop footpoint with a preexisting magnetic structure of opposite
  polarity. The temporal evolution of the observed spectra is reproduced
  to first order by the spectra derived from the geometrical model. During
  the phase of clearest visibility of the loop in the observations,
  the observed and synthetic spectra match quantitatively. <BR />
  Conclusions: The observed event can be explained as a case of flux
  emergence in the shape of a small-scale loop. The fast disappearance
  of the loop at the end could possibly be due to magnetic reconnection.

Title: The energy of waves in the photosphere and lower
    chromosphere. I. Velocity statistics
Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M.
Bibcode: 2009A&A...507..453B
Altcode: 2009arXiv0905.1011B
  Context: Acoustic waves are one of the primary suspects besides magnetic
  fields for the chromospheric heating process to temperatures above
  radiative equilibrium (RE). <BR />Aims: We derived the mechanical
  wave energy as seen in line-core velocities on disc centre to obtain
  a measure of mechanical energy flux with height for a comparison
  with the energy requirements in a semi-empirical atmosphere model,
  the Harvard-Smithsonian reference atmosphere (HSRA). <BR />Methods: We
  analyzed a 1-hour time series and a large-area map of Ca II H spectra
  on the traces of propagating waves. We analyzed the velocity statistics
  of several spectral lines in the wing of Ca II H, and the line-core
  velocity of Ca II H. We converted the velocity amplitudes into volume
  (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we
  used the increase of internal energy (∝ R ρ Δ T) necessary to lift
  a RE atmosphere to the HSRA temperature stratification. <BR />Results:
  We find that the velocity amplitude grows in agreement with linear
  wave theory and thus slower with height than predicted from energy
  conservation. The mechanical energy of the waves above around z ~ 500 km
  is insufficient to maintain on a long-term average the chromospheric
  temperature rise in the semi-empirical HSRA model. The intensity
  variations of the Ca line core (z ~ 1000 km) can, however, be traced
  back to the velocity variations of the lowermost forming spectral line
  considered (z ~ 250 km). <BR />Conclusions: The chromospheric intensity,
  and hence, (radiation) temperature variations are seen to be induced by
  passing waves originating in the photosphere. The wave energy is found
  to be insufficient to maintain the temperature stratification of the
  semi-empirical HSRA model above 500 km. We will in a following paper of
  this series investigate the energy contained in the intensity variations
  to see if the semi-empirical model is appropriate for the spectra.

Title: The magnetic flux of the quiet Sun internetwork as observed
    with the Tenerife infrared polarimeter
Authors: Beck, C.; Rezaei, R.
Bibcode: 2009A&A...502..969B
Altcode: 2009arXiv0903.3158B
  Context: Observations made with the spectropolarimeter onboard the
  HINODE satellite have detected abundant horizontal magnetic fields in
  the internetwork quiet Sun. <BR />Aims: We compare the results for the
  horizontal fields obtained at 630 nm with ground-based observations at
  1.56 μm, where the sensitivity to magnetic fields is higher than in the
  visible. <BR />Methods: We obtained 30-s integrated spectropolarimetric
  data of the quiet Sun on disc centre during a period of extremely
  stable and good seeing. The data have a rms noise in polarization of
  around 2 × 10<SUP>-4</SUP> of the continuum intensity. The low noise
  level allows the spectra to be inverted with the SIR code. We compare
  the inversion results with proxies to determine the magnetic flux. <BR
  />Results: We confirm the presence of the horizontal fields in the
  quiet Sun internetwork as reported for the satellite data, including
  voids without linear polarization signal that extend over an area of
  a few granules. Voids in the circular polarization signal are only
  of granular scale. More than 60% of the surface show polarization
  signals of above four times the rms noise level. We find that the
  total magnetic flux contained in the more inclined to horizontal fields
  (γ &gt; 45°) is lower by a factor of around 2 than that of the less
  inclined fields. The proxies for flux determination are strongly
  affected by the thermodynamic state of the atmosphere, and hence,
  seem to be unreliable. <BR />Conclusions: During spells of good seeing
  conditions, adaptive optics can render ground-based slit-spectrograph
  observations at a 70-cm telescope equivalent to the seeing-free
  space-based data of half-meter class telescopes. We suggest that the
  difference in the ratio of horizontal to transversal flux between the
  ground-based infrared data and the satellite-based visible data is
  due to the different formation heights of the respective spectral
  lines. We emphasize that the true amount of magnetic flux cannot
  be derived directly from the spectra. For purely horizontal flux,
  one would need its vertical extension that has to be estimated by
  explicit modeling, using the observed spectra as boundary conditions,
  or be taken from MHD simulations. Time-series of the evolution of the
  magnetic flux and chromospheric diagnostics are needed to address its
  possible contribution to chromospheric heating. <P />Appendices A and
  B are only available in electronic form at http://www.aanda.org

Title: Reversal-free Ca II H profiles: a challenge for solar
    chromosphere modeling in quiet inter-network
Authors: Rezaei, R.; Bruls, J. H. M. J.; Schmidt, W.; Beck, C.;
   Kalkofen, W.; Schlichenmaier, R.
Bibcode: 2008A&A...484..503R
Altcode: 2008arXiv0804.2325R
  Aims: We study chromospheric emission to understand the temperature
  stratification in the solar chromosphere. <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: A 3D sunspot model derived from an inversion of
    spectropolarimetric observations and its implications for the
    penumbral heating
Authors: Beck, C.
Bibcode: 2008A&A...480..825B
Altcode: 2007arXiv0712.3168B
  Aims:I deduced a 3D sunspot model that is in full agreement with
  spectropolarimetric observations, in order to address the question of
  a possible penumbral heating process by the repetitive rise of hot flow
  channels. <BR />Methods: I performed inversions of spectropolarimetric
  data taken simultaneously in infrared (1.5 μm) and visible (630 nm)
  spectral lines. I used two independent magnetic components inside each
  pixel to reproduce the irregular Stokes profiles in the penumbra and
  studied the averaged and individual properties of the two components. By
  integrating the field inclination to the surface, I developed a 3D
  model of the spot from inversion results without intrinsic height
  information. <BR />Results: I find that the Evershed flow is harbored
  by the weaker of the two field components. This component forms flow
  channels that show upstreams in the inner and mid penumbra, continue
  almost horizontally as slightly elevated loops throughout the penumbra,
  and finally bend down in the outer penumbra. I find several examples
  where two or more flow channels are found along a radial cut from the
  umbra to the outer boundary of the spot. <BR />Conclusions: I find
  that a model of horizontal flow channels in a static background field
  is in good agreement with the observed spectra. The properties of the
  flow channels correspond very well to the moving tube simulations of
  Schlichenmaier et al. (1998, A&amp;A, 337, 897). From the temporal
  evolution in intensity images and the properties of the flow channels
  in the inversion, I conclude that interchange convection of rising
  hot flux tubes in a thick penumbra still seems a possible mechanism
  for maintaining the penumbral energy balance.

Title: The signature of chromospheric heating in Ca II H spectra
Authors: Beck, C.; Schmidt, W.; Rezaei, R.; Rammacher, W.
Bibcode: 2008A&A...479..213B
Altcode: 2007arXiv0712.2538B
  Context: The heating process that balances the solar chromospheric
  energy losses has not yet been determined. Conflicting views exist on
  the source of the energy and the influence of photospheric magnetic
  fields on chromospheric heating. <BR />Aims: We analyze a 1-h time
  series of cospatial Ca II H intensity spectra and photospheric
  polarimetric spectra around 630 nm to derive the signature of the
  chromospheric heating process in the spectra and to investigate its
  relation to photospheric magnetic fields. The data were taken in a
  quiet Sun area on disc center without strong magnetic activity. <BR
  />Methods: We have derived several characteristic quantities of Ca
  II H to define the chromospheric atmosphere properties. We study the
  power of the Fourier transform at different wavelengths and the phase
  relations between them. We perform local thermodynamic equilibrium (LTE)
  inversions of the spectropolarimetric data to obtain the photospheric
  magnetic field, once including the Ca intensity spectra. <BR />Results:
  We find that the emission in the Ca II H line core at locations
  without detectable photospheric polarization signal is due to waves
  that propagate in around 100 s from low forming continuum layers in
  the line wing up to the line core. The phase differences of intensity
  oscillations at different wavelengths indicate standing waves for ν
  &lt; 2 mHz and propagating waves for higher frequencies. The waves
  steepen into shocks in the chromosphere. On average, shocks are both
  preceded and followed by intensity reductions. In field-free regions,
  the profiles show emission about half of the time. The correlation
  between wavelengths and the decorrelation time is significantly higher
  in the presence of magnetic fields than for field-free areas. The
  average Ca II H profile in the presence of magnetic fields contains
  emission features symmetric to the line core and an asymmetric
  contribution, where mainly the blue H2V emission peak is increased
  (shock signature). <BR />Conclusions: We find that acoustic waves
  steepening into shocks are responsible for the emission in the Ca II H
  line core for locations without photospheric magnetic fields. We suggest
  using wavelengths in the line wing of Ca II H, where LTE still applies,
  to compare theoretical heating models with observations. <P />Appendices
  are only available in electronic form at http://www.aanda.org

Title: Magnetic properties of G-band bright points in a sunspot moat
Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.;
   Sütterlin, P.
Bibcode: 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: Relation between photospheric magnetic field and chromospheric
    emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C. A. R.; Bruls,
   J. H. M. J.; Schmidt, W.
Bibcode: 2007A&A...466.1131R
Altcode: 2007astro.ph..1896R
  Aims: We investigate the relationship between the photospheric
  magnetic field and the emission of the mid chromosphere of the
  Sun. <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: 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.
Bibcode: 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: Evershed Clouds as Precursors of Moving Magnetic Features
    around Sunspots
Authors: Cabrera Solana, D.; Bellot Rubio, L. R.; Beck, C.; del Toro
   Iniesta, J. C.
Bibcode: 2006ApJ...649L..41C
Altcode: 2006astro.ph..9108C
  The relation between the Evershed flow and moving magnetic features
  (MMFs) is studied using high-cadence, simultaneous spectropolarimetric
  measurements of a sunspot in visible (630.2 nm) and near-infrared
  (1565 nm) lines. Doppler velocities, magnetograms, and total linear
  polarization maps are calculated from the observed Stokes profiles. We
  follow the temporal evolution of two Evershed clouds that move radially
  outward along the same penumbral filament. Eventually, the clouds
  cross the visible border of the spot and enter the moat region, where
  they become MMFs. The flux patch farther from the sunspot has the same
  polarity of the spot, while the MMF closer to it has opposite polarity
  and exhibits abnormal circular polarization profiles. Our results
  provide strong evidence that at least some MMFs are the continuation
  of the penumbral Evershed flow into the moat. This, in turn, suggests
  that MMFs are magnetically connected to sunspots.

Title: The flow field in the sunspot canopy
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R.
Bibcode: 2006A&A...454..975R
Altcode: 2006astro.ph..4301R
  Aims.We investigate the flow field in the sunspot canopy using
  simultaneous Stokes vector spectropolarimetry of three sunspots
  (θ=27°, 50°, 75°) and their surroundings in visible (630.15 and
  630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron
  lines.<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: VIP - 2D Vector Spectropolarimetry of the Solar Atmosphere
    near the Diffraction Limit
Authors: Bellot Rubio, L. R.; Tritschler, A.; Kentischer, T.; Beck,
   C.; Del Toro Iniesta, J. C.
Bibcode: 2006IAUJD...3E..58B
Altcode:
  The KIS/IAA Vector Imaging Polarimeter (VIP) is a new instrument for
  two-dimensional spectropolarimetry of the solar atmosphere. It is used
  with TESOS, the triple etalon Fabry-Perot interferometer installed at
  the German Vacuum Tower Telescope of Observatorio del Teide (Tenerife,
  Spain). The polarimeter is based on a pair of nematic liquid crystal
  retarders and a Wollaston prism. VIP and TESOS are able to observe
  any spectral line in the range from 450 nm to 750 nm with a spectral
  resolving power of about 250000 and spatial resolutions better than
  0.5" (thanks to the Kiepenheuer Adaptive Optics System). Typically,
  the four Stokes parameters of a line can be measured at 40 wavelength
  points in less than 60 s, with a noise level of 1-2 x 10^-3 and a
  pixel size of 0.18" x 0.18" (2x2 binning). The modulation matrix of
  VIP is derived using the polarimetric calibration unit installed at
  the telescope. Here we present first-light observations of VIP carried
  out in November 2005. We recorded the full Stokes profiles of the 630.1
  and 630.2 nm Fe I lines emerging from a solar pore and its surroundings
  at a spatial resolution of ~0.4". Based on these data, we discuss the
  performance and capabilities of VIP as a powerful instrument for high
  spatial and temporal resolution measurements of vector magnetic fields
  in the solar atmosphere.

Title: Observational Evidence for the "Hot Wall" Effect in Small
    Magnetic Flux Concentrations
Authors: Mikurda, K.; Beck, C.
Bibcode: 2006IAUJD...3E..91M
Altcode:
  Introduction: When lacking polarimetric observations, the Bright Points
  (BPs) visible in the G-band at 430 nm are commonly used as tracers
  for magnetic fields. Methods: Observations presented in this paper
  were taken on October 11, 2005 at the German Vacuum Tower Telescope
  and involved the Tenerife Infrared Polarimeter (TIP) at 1.5 micron,
  the Telecentric Etalon Solar Spectrometer (TESOS) in the Fe I spectral
  line at 557.6 nm, and a speckle setup in G-band. The area scanned by
  TIP was 75''x33'' and covered a pore surrounded by network. The TIP
  spectra were inverted with the SIR (Stokes Inversion based on Response
  functions) code to retrieve the magnetic field vector. Results: We find
  that G-band BPs are not cospatial with the central part of the flux
  concentrations. Even at the small heliocentric angle of 12 degree,
  the BPs appear projected on the limb side walls of the granules
  (Fig. 1), whereas the fields are concentrated in the intergranular
  lanes. Discussion: Our findings indicate that the G-band Bright Points
  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, where CH dissociates, in the granules next to the
  field concentrations. Thus, information drawn from the observations of
  BPs cannot be used to conclude on the actual variation of the magnetic
  field structure, as only the outer parts of the flux concentrations
  are seen in the BPs. Figure 1. Non-cospatiality of BPs and field
  concentrations. Top row, left to right: line-core intensity, LOS
  velocity (range =+/- 1 km/s), magnetic flux , polarity, LOS velocity
  from the inversion (range =+/- 1.5 km/s), field azimuth. Bottom
  row, left to right: polarization degree, G-band intensity, continuum
  intensity at 1.5 micron, BP mask, field strength, field inclination. The
  white arrow in the G-band image denotes the limb direction, the cross
  marks the center of the 6.7 x 6.7 Mm field-of-view shown.

Title: The 3-D topology of magnetic fields in and around sunspots
Authors: Beck, Christian
Bibcode: 2006PhDT........30B
Altcode:
  The most prominent features on the surface of the solar disc are
  the sunspots, which have been studied since their detection in
  the 17th century. Despite this years and centuries of scientific
  investigations, surprisingly many facets of sunspots are not well
  understood. In this thesis, the properties of a sunspots' penumbra
  are derived from the observed spectra by means of an inversion,
  a forward modeling technique. A method for the construction of a
  3-dimensional model of the field topology from the inversion results
  is presented. It offers the possibility to build a toy-model of the
  spot. This allows to investigate the relationships between the geometry
  and other properties of the sunspot like flow velocities, intensity,
  temperature, or field strength. The application of the procedure to
  time series of observations will allow to study the temporal evolution
  of the geometry and the other properties, to determine the physical
  processes happening and their respective drivers. <P />A second
  part of the thesis is devoted to the analysis of isolated field
  concentrations in the surroundings of the sunspot. These isolated
  magnetic elements are assumed to be the elementary building blocks
  of the solar surface magnetism. In the turbulent surroundings of
  the solar atmosphere, only relatively strong concentrations with a
  field strength above 1 kG are assumed to be stable at all. Such field
  concentrations produce brightenings in the G band, where a lot of
  spectral lines of the CH molecule are present. Inside magnetic fields,
  the density is reduced, as the magnetic pressure contributes to the
  pressure balance equation. Thus, the reduced density allows to see
  to deeper layers of the atmosphere with higher temperature. If the
  CH-molecules dissociate due to the higher temperature, the spectral
  lines disappear and the intensity increases strongly. Due to the
  small size of the field concentration, the brightenings are strongly
  localized (''G-band Bright Points''). In this study, we find a much
  larger variability of the Bright Point properties than expected, which
  puts their elementary nature in some doubt. <P />In the conclusions
  of this work, the resulting 3-D topology of the sunspot is used in
  an attempt to develop a consistent picture of the development and the
  fine structure of sunspots.

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.
Bibcode: 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: Polarimetric Littrow Spectrograph - instrument calibration
    and first measurements
Authors: Beck, C.; Schmidt, W.; Kentischer, T.; Elmore, D.
Bibcode: 2005A&A...437.1159B
Altcode:
  We present first measurements and the calibration procedure for the
  Polarimetric Littrow Spectrograph (POLIS) operated at the Vacuum Tower
  Telescope on Tenerife, together with a brief summary of the technical
  characteristics of the instrument. In its present configuration,
  we achieve a polarimetric accuracy of about 3 × 10<SUP>-3</SUP>
  in the visible channel (630 nm) of the instrument. The accuracy is
  limited by cross talk among the different polarization states. The
  detection limit for polarized light is about 2 × 10<SUP>-3</SUP>
  for a 7 s exposure. Polarimetric measurements in the blue channel (Ca
  II H line, 396.7 nm) are strongly limited by the low photon flux. At
  this wavelength we present Stokes-V maps with a spatial resolution of
  about 0.5 arcs. The polarimetric quality of any spectropolarimeter is
  limited by the precision of the instrument calibration. We present
  a new method for self-calibration that reduces cross talk among the
  polarization components to 0.1%. This improvement results from a
  measurement of the retardance of the calibration waveplate with an
  accuracy of 0.1°. We demonstrate the capability of the simultaneous
  use of POLIS and the Tenerife Infrared Polarimeter which is integrated
  in the main spectrograph of the Vacuum Tower Telescope.

Title: Magnetic flux in the internetwork quiet Sun
Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.;
   Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C.
Bibcode: 2005A&A...436L..27K
Altcode:
  We report a direct comparison of the amplitudes of Stokes spectra of the
  Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations
  with simultaneous observations in the same spectral regions. The
  Stokes spectra were synthesized in snapshots with a mixed polarity
  magnetic field having a spatially averaged strength, &lt; B &gt;,
  between 10 and 30 G. The distribution of Stokes V amplitudes depends
  sensitively on &lt; B &gt;. A quiet inter-network region was observed
  at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630
  nm). We find that the Stokes V amplitudes of both infrared and visible
  observations are best reproduced by the simulation snapshot with &lt;
  B &gt; = 20 G. In observations with 1 resolution, up to 2/3 of the
  magnetic flux can remain undetected.

Title: Magnetic Flux Cancellation in the Moat of Sunspots: Results
    from Simultaneous Vector Spectropolarimetry in the Visible and
    the Infrared
Authors: Bellot Rubio, Luis R.; Beck, Christian
Bibcode: 2005ApJ...626L.125B
Altcode:
  We analyze multiwavelength observations of the cancellation of a
  moving magnetic feature and a plage element at the outer edge of
  the moat of an isolated, round sunspot. The event lasted for some 35
  minutes until the smaller flux concentration disappeared completely
  from the photosphere. The data set consists of high-resolution, full
  vector spectropolarimetric measurements of four visible lines and
  two near-infrared lines, along with speckle-reconstructed G-band and
  Ca II H filtergrams. The observations reveal strong chromospheric
  emission at the neutral line separating the two magnetic poles;
  it becomes visible 18 minutes after the cancellation has started
  and persists for 25 minutes. We have carried out an inversion of
  the observed Stokes profiles to determine the variation of the
  vector magnetic field, temperature, and line-of-sight velocity
  during the cancellation. No significant changes in field strength,
  field inclination, or temperature are observed in either of the two
  opposite-polarity patches. The decrease in magnetic flux is primarily
  due to a decrease in magnetic filling factor, which is accompanied by
  strong upflows (of at least 1.1 km s<SUP>-1</SUP>) in the smaller flux
  concentration. These results suggest that the cancellation is due to
  magnetic reconnection in the photosphere.

Title: POLIS: A spectropolarimeter for the VTT and for GREGOR
Authors: Schmidt, W.; Beck, C.; Kentischer, T.; Elmore, D.; Lites, B.
Bibcode: 2003AN....324..300S
Altcode:
  The polarimetric Littrow Spectrograph POLIS is designed for vector
  polarimetry at high angular and spectral resolution. It measures the
  magnetic field simultaneously in the photosphere and the chromosphere
  of the sun. Both branches of the polarimetry unit are dual beam systems
  with a single rotating modulator for both wavelengths and polarizing
  beam splitters in front of each CCD camera. POLIS has been installed
  at the VTT on Tenerife and has seen First Light on 17 May 2002. A
  modified version of POLIS will be developed for the balloon mission
  Sunrise. That version will have UV capabilities down to 200 nm.

