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Author name code: kneer
ADS astronomy entries on 2022-09-14
author:"Kneer, Franz J." 

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

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Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
   Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
   T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
   Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
   A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
   K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
2016A&A...596A...4F    Altcode: 2016arXiv160800513F
  Context. A significant part of the penumbral magnetic field returns
  below the surface in the very deep photosphere. For lines in the
  visible, a large portion of this return field can only be detected
  indirectly by studying its imprints on strongly asymmetric and
  three-lobed Stokes V profiles. Infrared lines probe a narrow layer
  in the very deep photosphere, providing the possibility of directly
  measuring the orientation of magnetic fields close to the solar
  surface. <BR /> Aims: We study the topology of the penumbral magnetic
  field in the lower photosphere, focusing on regions where it returns
  below the surface. <BR /> Methods: We analyzed 71 spectropolarimetric
  datasets from Hinode and from the GREGOR infrared spectrograph. We
  inferred the quality and polarimetric accuracy of the infrared data
  after applying several reduction steps. Techniques of spectral
  inversion and forward synthesis were used to test the detection
  algorithm. We compared the morphology and the fractional penumbral
  area covered by reversed-polarity and three-lobed Stokes V profiles for
  sunspots at disk center. We determined the amount of reversed-polarity
  and three-lobed Stokes V profiles in visible and infrared data of
  sunspots at various heliocentric angles. From the results, we computed
  center-to-limb variation curves, which were interpreted in the context
  of existing penumbral models. <BR /> Results: Observations in visible
  and near-infrared spectral lines yield a significant difference in the
  penumbral area covered by magnetic fields of opposite polarity. In
  the infrared, the number of reversed-polarity Stokes V profiles is
  smaller by a factor of two than in the visible. For three-lobed Stokes
  V profiles the numbers differ by up to an order of magnitude.

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

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

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Title: Probing deep photospheric layers of the quiet Sun with high
    magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
   M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
   Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
   Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
   Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
   K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
   Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
   Schmidt, D.; Kiess, C.; Nicklas, H.
2016A&A...596A...6L    Altcode: 2016arXiv160506324L
  Context. Investigations of the magnetism of the quiet Sun are hindered
  by extremely weak polarization signals in Fraunhofer spectral
  lines. Photon noise, straylight, and the systematically different
  sensitivity of the Zeeman effect to longitudinal and transversal
  magnetic fields result in controversial results in terms of the strength
  and angular distribution of the magnetic field vector. <BR /> Aims:
  The information content of Stokes measurements close to the diffraction
  limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
  spatial straylight and photon noise into account. <BR /> Methods: Highly
  sensitive full Stokes measurements of a quiet-Sun region at disk center
  in the deep photospheric Fe I lines in the 1.56 μm region were obtained
  with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
  statistics and Stokes V asymmetries were analyzed and compared to a
  similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
  diagnostics based directly on the shape and strength of the profiles
  were applied to the GRIS data. We made use of the magnetic line ratio
  technique, which was tested against realistic magneto-hydrodynamic
  simulations (MURaM). <BR /> Results: About 80% of the GRIS spectra
  of a very quiet solar region show polarimetric signals above a 3σ
  level. Area and amplitude asymmetries agree well with small-scale
  surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
  analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
  range with some concentrations of kilo-Gauss fields. <BR /> Conclusions:
  The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
  are so far unique in the combination of high spatial resolution scans
  and high magnetic field sensitivity. Nevertheless, the unavoidable
  effect of spatial straylight and the resulting dilution of the weak
  Stokes profiles means that inversion techniques still bear a high risk
  of misinterpretating the data.

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Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
   Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
   Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
   González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
   Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
   Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
   M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
   T.; Volkmer, R.
2016A&A...596A...5M    Altcode: 2018arXiv180410089M
  Context. Over the past 20 yr, the quietest areas of the solar surface
  have revealed a weak but extremely dynamic magnetism occurring
  at small scales (&lt;500 km), which may provide an important
  contribution to the dynamics and energetics of the outer layers of
  the atmosphere. Understanding this magnetism requires the inference
  of physical quantities from high-sensitivity spectro-polarimetric
  data with high spatio-temporal resolution. <BR /> Aims: We present
  high-precision spectro-polarimetric data with high spatial resolution
  (0.4”) of the very quiet Sun at 1.56 μm obtained with the GREGOR
  telescope to shed some light on this complex magnetism. <BR /> Methods:
  We used inversion techniques in two main approaches. First, we assumed
  that the observed profiles can be reproduced with a constant magnetic
  field atmosphere embedded in a field-free medium. Second, we assumed
  that the resolution element has a substructure with either two constant
  magnetic atmospheres or a single magnetic atmosphere with gradients of
  the physical quantities along the optical depth, both coexisting with
  a global stray-light component. <BR /> Results: Half of our observed
  quiet-Sun region is better explained by magnetic substructure within
  the resolution element. However, we cannot distinguish whether this
  substructure comes from gradients of the physical parameters along the
  line of sight or from horizontal gradients (across the surface). In
  these pixels, a model with two magnetic components is preferred, and
  we find two distinct magnetic field populations. The population with
  the larger filling factor has very weak ( 150 G) horizontal fields
  similar to those obtained in previous works. We demonstrate that the
  field vector of this population is not constrained by the observations,
  given the spatial resolution and polarimetric accuracy of our data. The
  topology of the other component with the smaller filling factor is
  constrained by the observations for field strengths above 250 G:
  we infer hG fields with inclinations and azimuth values compatible
  with an isotropic distribution. The filling factors are typically
  below 30%. We also find that the flux of the two polarities is not
  balanced. From the other half of the observed quiet-Sun area 50% are
  two-lobed Stokes V profiles, meaning that 23% of the field of view
  can be adequately explained with a single constant magnetic field
  embedded in a non-magnetic atmosphere. The magnetic field vector and
  filling factor are reliable inferred in only 50% based on the regular
  profiles. Therefore, 12% of the field of view harbour hG fields with
  filling factors typically below 30%. At our present spatial resolution,
  70% of the pixels apparently are non-magnetised.

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

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

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Title: On the structure and dynamics of Ellerman bombs. Detailed
    study of three events and modelling of Hα
Authors: Bello González, N.; Danilovic, S.; Kneer, F.
2013A&A...557A.102B    Altcode:
  <BR /> Aims: We study the structure and dynamics of three Ellerman
  bombs (EBs) observed in an evolving active region. <BR /> Methods: The
  active region NOAA 11271 was observed with the Vacuum Tower Telescope
  at Observatorio del Teide/Tenerife on August 18, 2011. We used the
  two-dimensional Triple Etalon SOlar Spectrometer (TESOS) to obtain time
  sequences of the active region and of EBs in Hα at a cadence of 15
  s. Simultaneously, we obtained full Stokes profiles with the Tenerife
  Infrared Polarimeter (TIP II) in the two magnetically sensitive Fe i
  infrared lines (IR) at 1.56 μ, scanning spatial sections of the area
  with cadences of 28-46 s. The Hα data were reconstructed with speckle
  methods to study the evolution of the atmospheric stratification. Two
  methods were used to extract magnetic field information from the IR
  Stokes profiles: 1) fitting of the (Q,U,V) profiles by Gaussians; and
  2) applying the Milne-Eddington approximation, assuming two separate
  magnetic structures in the resolution element and fitting by trial and
  error some profiles from the EB areas. Data from SDO-HMI and -AIA were
  also used. We performed two-dimensional (2D) non-LTE radiative transfer
  calculations of Hα in parameterised models of EBs. <BR /> Results:
  The three EBs studied in detail occurred in a complex active region near
  sunspots. They were very bright with a factor of 1.5-2.8 brighter than
  the nearby area. They lived for 1/2 h and longer. They were related to
  broadband faculae, but the latter were not the brightest features in the
  field of view. The EBs occurred in magnetic field configurations with
  opposite polarity close together. One EB was located at the outskirts
  of a penumbra of a complex sunspot and showed repeated "flaring" in
  SDO-AIA data. Another was close to a strong field patch and moved into
  this during the end of its lifetime. The third EB showed clear changes
  of field structure during the time it was observed. We obtained from
  the 2D modelling that heating and increase in Hα opacity are likely
  to occur at heights of 300-800 km. Line shifts and asymmetries can
  well be reproduced by velocities at these heights and also at much
  larger heights. <BR /> Conclusions: The three EBs occurred at sites
  with magnetic fields of opposite polarity, which were likely the cause
  of the Hα brightening upon reconnection.

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Title: The GREGOR Solar Telescope on Tenerife
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
   Solanki, S. K.; Balthasar, H.; Bello González, N.; Berkefeld, T.;
   Collados Vera, M.; Hofmann, A.; Kneer, F.; Lagg, A.; Puschmann, K. G.;
   Schmidt, D.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.
2012ASPC..463..365S    Altcode: 2012arXiv1202.4289S
  2011 was a successful year for the GREGOR project. The telescope was
  finally completed in May with the installation of the 1.5-meter primary
  mirror. The installation of the first-light focal plane instruments was
  completed by the end of the year. At the same time, the preparations
  for the installation of the high-order adaptive optics were finished,
  its integration to the telescope is scheduled for early 2012. This
  paper describes the telescope and its instrumentation in their present
  first-light configuration, and provides a brief overview of the science
  goals of GREGOR.

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Title: The GREGOR Solar Telescope
Authors: Denker, C.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Schmidt,
   W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.; Volkmer, R.; von
   der Luehe, O.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.;
   Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F.
2012IAUSS...6E.203D    Altcode:
  The 1.5-meter GREGOR solar telescope is a new facility for
  high-resolution observations of the Sun. The telescope is located at the
  Spanish Observatorio del Teide on Tenerife. The telescope incorporates
  advanced designs for a foldable-tent dome, an open steel-truss telescope
  structure, and active and passive means to minimize telescope and mirror
  seeing. Solar fine structure can be observed with a dedicated suite
  of instruments: a broad-band imaging system, the "GREGOR Fabry-Perot
  Interferometer", and the "Grating Infrared Spectrograph". All post-focus
  instruments benefit from a high-order (multi-conjugate) adaptive optics
  system, which enables observations close to the diffraction limit of
  the telescope. The inclusion of a spectrograph for stellar activity
  studies and the search for solar twins expands the scientific usage
  of the GREGOR to the nighttime domain. We report on the successful
  commissioning of the telescope until the end of 2011 and the first
  steps towards science verification in 2012.

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Title: A retrospective of the GREGOR solar telescope in scientific
    literature
Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.;
   Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.;
   Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.;
   Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa,
   M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
   Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.;
   Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann,
   T.; Wiehr, E.; Wittmann, A. D.; Woche, M.
2012AN....333..810D    Altcode: 2012arXiv1210.3167D
  In this review, we look back upon the literature, which had the
  GREGOR solar telescope project as its subject including science cases,
  telescope subsystems, and post-focus instruments. The articles date
  back to the year 2000, when the initial concepts for a new solar
  telescope on Tenerife were first presented at scientific meetings. This
  comprehensive bibliography contains literature until the year 2012,
  i.e., the final stages of commissioning and science verification. Taking
  stock of the various publications in peer-reviewed journals and
  conference proceedings also provides the “historical” context
  for the reference articles in this special issue of Astronomische
  Nachrichten/Astronomical Notes.

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Title: Hopes and expectations with GREGOR
Authors: Kneer, F.
2012AN....333..790K    Altcode:
  This contribution to the series of GREGOR inauguration articles
  addresses the history of the GREGOR telescope. It was obvious since a
  long time that the study of the atmospheric dynamics on the Sun needs
  telescopes with a large aperture. So the first plans to replace the
  40 years old Gregory-Coudé Telescope, with its 45 cm primary mirror,
  by a large, 1.5-meter telescope date back to 1997. After a positive
  review of the project by the Deutsche Forschungsgemeinschaft in 2000,
  the large financial support started in 2000. Unfortunately, the new
  technology of the Cesic mirrors was not yet ripe to produce the large
  primary mirror with this light-weight material. So, the project was
  much delayed. After recollecting for the reader several dates, I also
  go through some properties of GREGOR. I recall the aims of the project
  and discuss difficulties and ways to realise the intentions.

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Title: The 1.5 meter solar telescope GREGOR
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
   Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.;
   Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann,
   A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
   Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.;
   Strassmeier, K. G.; Waldmann , T. A.
2012AN....333..796S    Altcode:
  The 1.5 m telescope GREGOR opens a new window to the understanding
  of solar small-scale magnetism. The first light instrumentation
  includes the Gregor Fabry Pérot Interferometer (GFPI), a filter
  spectro-polarimeter for the visible wavelength range, the GRating
  Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The
  excellent performance of the first two instruments has already been
  demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest
  solar telescope and number 3 in the world. Its all-reflective Gregory
  design provides a large wavelength coverage from the near UV up to at
  least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR
  is equipped with a high-order adaptive optics system, with a subaperture
  size of 10 cm, and a deformable mirror with 256 actuators. The science
  goals are focused on, but not limited to, solar magnetism. GREGOR
  allows us to measure the emergence and disappearance of magnetic flux
  at the solar surface at spatial scales well below 100 km. Thanks to its
  spectro-polarimetric capabilities, GREGOR will measure the interaction
  between the plasma flows, different kinds of waves, and the magnetic
  field. This will foster our understanding of the processes that heat the
  chromosphere and the outer layers of the solar atmosphere. Observations
  of the surface magnetic field at very small spatial scales will shed
  light on the variability of the solar brightness.

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

---------------------------------------------------------
Title: On acoustic and gravity waves in the solar photosphere and
    their energy transport
Authors: Kneer, F.; Bello González, N.
2011A&A...532A.111K    Altcode:
  <BR /> Aims: We study acoustic and atmospheric gravity waves in the
  quiet Sun to estimate their energy transport to the chromosphere. <BR
  /> Methods: A two-dimensional time sequence from quiet Sun disc centre
  was analysed with simultaneous spectroscopic observations in Fe i 5576
  Å and Fe i 5434 Å (both with Landé factor g = 0). We calculated
  response functions of the velocities for the line minimum shifts
  and atmospheric transmissions of waves for the two lines. For this,
  NLTE line formation in granular and intergranular model atmospheres
  from numerical simulations were performed. For the interpretation
  of the observed waves and for the estimates of energy fluxes, we
  assumed adiabatic propagation of plane waves in an isothermal model
  atmosphere. Fourier analyses of intensity and velocity fluctuations
  were carried out. They yield power, phase, and coherence as functions of
  frequency ν (from temporal Fourier transforms) and in the k<SUB>h</SUB>
  - ν plane (from three-dimensional transforms). The power spectra,
  together with the mass densities at velocity formation heights, give
  then the energy fluxes. <BR /> Results: The rms velocities found here
  in the acoustic and gravity wave domains are lower by a factor ~1.5
  as in earlier work. We therefore admit a factor of 2 for an upward
  correction of the estimated fluxes. For acoustic waves we find: 1)
  upward propagating waves are present on the Sun with frequencies up to
  14-15 mHz (periods U ≈ 70 s); 2) the approximation of plane adiabatic
  waves in an isothermal atmosphere appears adequate for estimating the
  energy fluxes; 3) the acoustic energy fluxes are in the same range
  as found in our earlier work from ground-based, two-dimensional
  spectroscopy, 1500-3100 W m<SUP>-2</SUP> at an atmospheric height
  of ~380 km and 1300-2700 W m<SUP>-2</SUP> at 570 km. The energy flux
  carried by gravity waves is difficult to determine. We find: 1) phase
  and coherence spectra between continuum and velocity fluctuations
  show that convective overshoot and gravity waves are superimposed. We
  account for the convective flows using these coherence spectra. 2) At
  low frequencies, the vertical wavelength Λ<SUB>z</SUB> can be short
  (≪300 km), yielding large corrections for atmospheric transmissions
  (factors &gt; 100). We thus exclude from the flux estimates waves
  with |k<SUB>z</SUB>| &gt; 20 Mm<SUP>-1</SUP> and with vertical group
  velocities υ<SUB>gr,z</SUB> &lt; 0.3 km s<SUP>-1</SUP>. They are
  likely to be strongly reduced in amplitude by radiative damping. 3)
  With these caveats, the energy fluxes carried by gravity waves are found
  in the range of 4000 - 8200 W m<SUP>-2</SUP> at 380 km and 700-1400
  W m<SUP>-2</SUP> at 570 km. Gravity waves thus also contribute to the
  energy transport into the chromosphere.

---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere at high spatial
    resolution. II. Measurement in the Fe I 5434 Å line
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev,
   O.; Shchukina, N.
2010A&A...522A..31B    Altcode:
  <BR /> Aims: We investigate the energy supply of the solar chromosphere
  by acoustic waves. <BR /> Methods: A time sequence with high spatial
  and temporal resolution from the quiet Sun disc centre in Fe i
  5434 Å (Landé factor g = 0) is analysed. We used models from a
  numerical simulation of granular convection and apply NLTE spectral
  line transfer to determine the height of formation. For estimates of
  acoustic energy flux, we adopted wave propagation with inclinations
  of the wave vector with respect to the vertical of 0°, 30°, and
  45°. For a granular and an intergranular model, the transmissions of
  the atmosphere to high-frequency waves were determined for the three
  inclination angles. Wavelet and Fourier analyses were performed
  and the resulting power spectra were corrected for atmospheric
  transmission. <BR /> Results: We find waves with periods down to ~40
  s. They occur intermittently in space and time. The velocity signal
  is formed at a height of 500 km in the granular model and at 620 km
  in the intergranule. At periods shorter than the acoustic cutoff
  (~190 s), ~40% of the waves occur above granules and ~60% above
  intergranules. By adopting vertical propagation, we estimate total
  fluxes above granules of 2750-3360 W m<SUP>-2</SUP>, and of 910-1 000
  W m<SUP>-2</SUP> above intergranules. The weighted average is 1730-2
  060 W m<SUP>-2</SUP>. The estimates of the total fluxes increase by 15%
  when inclined wave propagation of 45° is assumed.

---------------------------------------------------------
Title: GREGOR telescope: start of commissioning
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S.;
   Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
   C.; Heidecke, F.; Hofmann, A.; Klvana, M.; Kneer, F.; Lagg, A.; Popow,
   E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K.
2010SPIE.7733E..0KV    Altcode: 2010SPIE.7733E..18V
  With the integration of a 1-meter Cesic primary mirror the GREGOR
  telescope pre-commissioning started. This is the first time, that
  the entire light path has seen sunlight. The pre-commissioning period
  includes testing of the main optics, adaptive optics, cooling system,
  and pointing system. This time was also used to install a near-infrared
  grating spectro-polarimeter and a 2D-spectropolarimeter for the visible
  range as first-light science instruments. As soon as the final 1.5
  meter primary mirror is installed, commissioning will be completed,
  and an extended phase of science verification will follow. In the near
  future, GREGOR will be equipped with a multi-conjugate adaptive optics
  system that is presently under development at KIS.

---------------------------------------------------------
Title: GREGOR solar telescope: Design and status
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.;
   Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.;
   Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.;
   Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau,
   D.; Strassmeier, K. G.
2010AN....331..624V    Altcode:
  The integration and verification phase of the GREGOR telescope
  reached an important milestone with the installation of the
  interim 1 m SolarLite primary mirror. This was the first time
  that the entire light path had seen sunlight. Since then extensive
  testing of the telescope and its subsystems has been carried out. The
  integration and verification phase will culminate with the delivery and
  installation of the final 1.5 m Zerodur primary mirror in the summer of
  2010. Observatory level tests and science verification will commence in
  the second half of 2010 and in 2011. This phase includes testing of the
  main optics, adaptive optics, cooling and pointing systems. In addition,
  assuming the viewpoint of a typical user, various observational modes
  of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared
  Spectrograph (GRIS), and high-speed camera systems will be tested to
  evaluate if they match the expectations and science requirements. This
  ensures that GREGOR will provide high-quality observations with its
  combination of (multi-conjugate) adaptive optics and advanced post-focus
  instruments. Routine observations are expected for 2012.

---------------------------------------------------------
Title: On the energy flux in acoustic waves in the solar atmosphere .
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev, O.
2010MmSAI..81..757B    Altcode:
  The energy supply for the radiative losses of the quiet solar
  chromosphere is studied. Time sequences from quiet Sun disc centre
  were obtained with the “Göttingen” Fabry-Pérot spectrometer at
  the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the
  non-magnetic Fe I 5576 Å line. The data were reconstructed with speckle
  methods. The velocities as measured at the line minimum were subjected
  to Fourier and wavelet analysis. The energy fluxes were corrected for
  the transmission of the solar atmosphere. We find an energy flux of
  ∼ 3 000 W m<SUP>-2</SUP> at a height of h=250 km. Approximately 2/3
  of it is carried by waves in the 5-10 mHz range, and 1/3 in the 10-20
  mHz band. The waves occur predominantly above inter-granular areas. We
  speculate that the acoustic flux in waves with periods shorter than the
  acoustic cutoff period (U≈190 s) can contribute to the basal heating
  of the solar chromosphere, in addition to atmospheric gravity waves.

---------------------------------------------------------
Title: Chromosphere of active regions on the Sun .
Authors: Kneer, F.
2010MmSAI..81..604K    Altcode:
  The first part of this contribution deals with the chromosphere
  of sunspots. The models of umbrae give strongly self-reversed Ca
  H and K line cores. Much `macro-turbulent' broadening is needed to
  fit the modeled line profiles to the observations. Thus, observed
  dynamics in sunspot chromospheres are discussed. In the second part,
  the chromsphere of plages is treated. We start with the relation
  of K emission vs. average magnetic field strength and continue with
  chromospheric models. Since chromospheres are most easily observed in
  Halpha , various simplifications for the interpretation of this line,
  including the cloud model, are discussed and results are given. Dynamic
  phenomena in plages, such as moustaches and magneto-atmospheric waves
  along fibrils, will be shown. Ideas about heating mechanisms of active
  chromspheres will be mentioned.

---------------------------------------------------------
Title: Faculae at the poles of the Sun revisited: infrared
    observations
Authors: Blanco Rodríguez, J.; Kneer, F.
2010A&A...509A..92B    Altcode:
  <BR /> Aims: This study extends earlier investigations on faculae and
  their small-scale magnetic fields near the solar poles (polar faculae
  - PFe) to measurements of the magnetically sensitive infrared (IR)
  Fe I lines at 1.5 μm, which provide more accurate information about
  the magnetic field than lines in the visible spectral range. <BR />
  Methods: PFe were observed with the Tenerife Infrared Polarimeter
  (TIP II) mounted at the Vacuum Tower Telescope/Observatorio del
  Teide/Tenerife. Several areas at various heliocentric angles were
  scanned. Faculae near the solar equator (equatorial faculae - EFe) were
  also observed for comparison with PFe. The full Stokes vector of the
  Fe I line pair at 1.5 μm was measured. The magnetic field properties
  were determined (1) from the centre of gravity (COG); (2) with the weak
  field approximation (WFA); (3) assuming the strong field regime (SFR);
  and (4) with inversions under the hypothesis of Milne-Eddington (ME)
  atmospheres. Line-of-sight (LOS) velocities were determined from the
  COG of I profiles and from the zero-crossing of the V profiles. <BR
  /> Results: The main findings of this work can be divided in five
  parts: (1) the detected PFe do not harbour sufficient magnetic flux
  to account for the global flux observed with other methods. (2) Near
  the solar limb, the apparent, measured transversal field components
  are most times stronger than the longitudinal components by factors of
  up to 10 for both PFe and EFe, as found from observations with HINODE
  SOT. (3) Many PFe indeed harbour kilo-G magnetic fields. Of those, more
  than 85% possess the same magnetic polarity as the global field. The
  inclinations γ of the strong fields, \vert B\vert≥ 900 G in the SFR,
  are compatible with their vertical emergence from the solar surface. (4)
  The results for weaker fields, \vert B\vert≤ 600 G from ME inversions,
  indicate a random magnetic field orientation. (5) The velocities from
  I profiles and V zero-crossings are in average 0.3 km s<SUP>-1</SUP>
  towards observer, for both PFe and EFe. The zero-crossings of V exhibit
  a large velocity dispersion, of up to 3 km s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Acoustic waves in the solar atmosphere at high spatial
    resolution
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev, O.
2009A&A...508..941B    Altcode:
  Aims. The energy supply for the radiative losses of the quiet solar
  chromosphere is studied. On the basis of high spatial resolution data,
  we investigate the amount of energy flux carried by acoustic waves in
  the solar photosphere.<BR /> Methods: Time sequences from quiet Sun disc
  centre were obtained with the “Göttingen” Fabry-Perot spectrometer
  at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in
  the non-magnetic Fe i 5576 Å line. The data were reconstructed with
  speckle methods. The velocity and intensity fluctuations at line minimum
  were subjected to Fourier and wavelet analyses. The energy fluxes at
  frequencies higher than the acoustic cutoff frequency (period U ≈
  190 s) were corrected for the transmission of the solar atmosphere,
  which reduces the signal from short-period waves.<BR /> Results:
  Both Fourier and wavelet analysis give an amount of energy flux of
  ~3000 W m<SUP>-2</SUP> at a height h = 250 km. Approximately 2/3 of
  it is carried by waves in the 5-10 mHz range, and 1/3 in the 10-20
  mHz band. Extrapolation of the flux spectra gives an energy flux
  of 230-400 W m<SUP>-2</SUP> at frequencies ν &gt; 20 mHz. We find
  that the waves occur predominantly above inter-granular areas.<BR />
  Conclusions: We conclude that the acoustic flux in waves with periods
  shorter than the acoustic cutoff period can contribute to the basal
  heating of the solar chromosphere, in addition to the atmospheric
  gravity waves found recently.

---------------------------------------------------------
Title: Full-Stokes Polarimetry with Speckle Techniques
Authors: Bello González, N.; Kneer, F.; Okunev, O.
2009ASPC..405..407B    Altcode:
  The new full-Stokes polarimeter implemented in the 2D `Göttingen'
  Fabry-Perot spectrometer, based on ferroelectric liquid crystals,
  yields magnetograms with minimised seeing induced crosstalk. A
  spatial resolution of 0."30--0."35 is achieved after applying speckle
  methods. The detection limit for the magnetic field strength is 16 G
  (≡ 3σ), yielding a polarimetric sensitivity of 7--8×10<SUP>15</SUP>
  Mx. Examples of intensity maps, Dopplergrams and magnetograms from
  quiet Sun are discussed.

---------------------------------------------------------
Title: A full-Stokes polarimeter for the GREGOR Fabry-Perot
    interferometer
Authors: Balthasar, Horst; Bello González, N.; Collados, M.; Denker,
   C.; Hofmann, A.; Kneer, F.; Puschmann, K. G.
2009IAUS..259..665B    Altcode:
  One of the first post-focus instruments of the new solar telescope
  GREGOR will be a Fabry-Perot spectrometer, which is an upgrade of the
  Göttingen Fabry-Perot interferometer at the Vacuum Tower Telescope
  (VTT) on Tenerife. This spectrometer is equipped with a full-Stokes
  polarimeter. The modulation is performed with two ferroelectric liquid
  crystals, one acting nominally as quarter-wave plate, and the other as
  half-wave plate. A modified Savart plate serves as polarimetric beam
  splitter. With the present liquid crystals, the optimum wavelength range
  of this polarimeter is between 580 and 660 nm. The spectro-polarimeter
  will benefit from the capabilities of the new telescope GREGOR which
  will provide a spatial resolution of about 0″.1 (75 km on the solar
  surface). Thus we will be able to investigate small magnetic features,
  and we will study their development with high cadence.

---------------------------------------------------------
Title: Dynamics of small-scale magnetic fields on the Sun:
    observations and numerical simulations
Authors: Bello González, N.; Yelles Chaouche, L.; Okunev, O.;
   Kneer, F.
2009A&A...494.1091B    Altcode:
  Context: Small-scale magnetic fields play an important role in the
  structure and the dynamics of the solar atmosphere. <BR />Aims: This
  study aims at revealing the evolution of magnetic fields, together
  with granular convection in the quiet Sun by means of observations
  with high spatial, spectral, and temporal resolution and of numerical
  MHD simulations. <BR />Methods: Time sequences from quiet Sun disc
  centre were obtained with the upgraded “Göttingen” Fabry-Perot
  spectropolarimeter at the Vacuum Tower Telescope, Observatorio del
  Teide/Tenerife, in the Fe I 6173 Å line. The data were reconstructed
  with speckle methods. For comparison with the observations, numerical
  simulations of granular magnetoconvection were carried out with the
  MURaM code. The intensities and Stokes vectors emerging from the
  simulation box were degraded in wavelength, spatial co-ordinates, and
  noise to the quality of the observations. <BR />Results: The noise in
  the observed magnetograms from the centre-of-gravity method is σ_B≈2
  G, yielding a polarimetric sensitivity of 3 × 10<SUP>15</SUP> Mx, at a
  cadence of 23 s with 0.33 arcsec spatial resolution in a field of view
  of ∼33 arcsec× 25 arcsec. Many of the observed V profiles in network
  and internetwork (IN) areas exhibit strong asymmetries that indicate
  strong magnetoconvection. The temporal evolutions of IN structures and
  of a bright point (BP), as seen in broadband and line-minimum images,
  in Dopplergrams, and in magnetograms, are presented. The magnetic
  field structure in the numerical MHD simulations is even more complex
  than seen in the observations. Correspondingly, the emergent Stokes
  profiles are often very abnormal. The degradation yields a reduction
  of the intrinsic field strength to the “observed” one by a factor
  4-5. The spectral resolution of the spectrometer is adequate, yet
  the limitation in spatial resolution and by noise filtering swamps
  the details seen in the non-degraded simulations. A BP was not found
  in the simulations, presumably because BP's are not so common and
  the simulated box is quiet small: only 6 Mm wide in both horizontal
  directions. <BR />Conclusions: The combination of high-resolution
  observations with numerical simulation is a highly valuable means
  for studying small-scale magnetic fields on the Sun. Two-dimensional,
  low-noise data with spectral resolution as good as achieved here and
  with spatial resolution of 0.1 arcsec and better are needed to better
  understand this important part of the solar magnetism. <P />A movie
  is only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Small-scale magnetic field dynamics on the Sun at high spatial
    and temporal resolution
Authors: Bello González, N.; Okunev, O.; Kneer, F.
2008A&A...490L..23B    Altcode:
  Aims: We present examples of the time evolution of phenomena
  occurring in the quiet Sun photosphere as seen from observations of
  high spatial and spectral resolution and with a time cadence of 23
  s. <BR />Methods: A time sequence of about 5 min has been taken with the
  Göttingen Fabry-Perot spectropolarimeter at the VTT in the Fe I 6173 Å
  line. The data were reconstructed with speckle methods. <BR />Results:
  With a polarimetric sensitivity of 2 × 10<SUP>-3</SUP>~I<SUB>c</SUB>
  (σ_B=2 G), the formation of a bright point as seen from broadband and
  line-minimum intensities, as well as in magnetograms and Dopplergrams
  can be traced in detail. Other events, like a stable mixed-polarity
  configuration in the intergranules and the appearance of magnetic
  signature above granules in both dark and bright areas are presented.

---------------------------------------------------------
Title: Evolution of Small-scale Magnetodynamics on the Sun with High
    Spatial and Temporal Resolution
Authors: Bello González, N.; Kneer, F.
2008ESPM...12..2.6B    Altcode:
  For the detection and study of small-scale magnetic fields on
  the Sun, observations with both high spatial resolution and high
  polarimetric sensitivity are required. The combination formed by
  the upgraded Göttingen FPI spectrometer (spectral resolution of
  25mÅ, at 617.3nm), full Stokes polarimeter, Adaptive Optics (KAOS)
  and speckle reconstruction techniques, provide broadband images of
  0.25 arcsec spatial resolution and magnetograms of 0.33 arcsec, with
  minimised seeing induced signals yielding a polarimetric sensitivity
  of ~0.002 Ic. A description of the optical system as well as results
  from the analysis of intensity maps, Dopplergrams and magnetograms from
  quiet and active regions on the Sun will be presented. The data were
  taken in the FeI 6173Å. We select few examples from the huge variety
  of magnetodynamic processes seen in the observations, e.g. the fast
  evolution of a bright point in broadband and line minimum intensity,
  in magnetograms, and in Dopplergrams is shown. Further examples
  indicate convective collapse, magnetic field advection, and magnetic
  field diffusion.

---------------------------------------------------------
Title: High-resolution Spectropolarimetry with the Vacuum Tower
Telescope at the Observatorio del Teide/Tenerife: History and Success
    of the Göttingen Programme
Authors: Kneer, F.; Bello Gonzalez, N.
2008ESPM...12.2.30K    Altcode:
  The efforts of the Göttingen solar physics group on high spatial
  resolution and 2D spectroscopy started in 1986. First results were
  obtained with speckle broadband imaging (de Boer 1992) and with 2D
  spectroscopy by means of one Fabry-Perot etalon (FPI) and a Universal
  Birefringent Filter (UBF). 2D Stokes V polarimetry followed soon. The
  UBF was replaced by a 2nd FPI which allowed short exposures also in
  the narrow-band channel of the spectrometer. Thus 2D spectropolari-
  metry could be combined with speckle reconstruction. The “Göttingen
  FPI” has been upgraded in 2005 for high efficiency and in 2007 for
  full Stokes polarimetry and good spectral resolution. It has become
  a fast and versatile instrument that has been, and will be, operated
  sucessfully for many scientific programmes on the atmospheric dynamics
  of the Sun.

---------------------------------------------------------
Title: Fast events and waves in an active region of the Sun observed
    in Hα with high spatial resolution
Authors: Sánchez-Andrade Nuño, B.; Bello González, N.; Blanco
   Rodríguez, J.; Kneer, F.; Puschmann, K. G.
2008A&A...486..577S    Altcode:
  Context: We study the chromosphere of an active region of the Sun in
  the Hα line. <BR />Aims: The development of new instrumentation and new
  methods of data analysis allows to scrutinize the dynamics of the solar
  chromosphere with high spatial, spectral, and temporal resolution. The
  observations we present shed light on some magneto-dynamic processes
  occurring above an active region in the chromosphere. <BR />Methods:
  We took a time series of 55 min in Hα from AR 10875 at θ≈36°. We
  used the “Göttingen” Fabry-Perot spectrometer at the Vacuum Tower
  Telescope, Observatorio del Teide/Tenerife, to obtain two-dimensional
  spectrograms in Hα. Adaptive optics and image reconstruction yielded
  a spatial resolution better than 0.5 arcsec throughout the time
  sequence. From the wealth of structures, we selected areas of interest
  to study further, in detail, some ongoing processes. <BR />Results:
  A small straight surge developed aside of a pore with upward phase
  speed of 100 km s<SUP>-1</SUP> and line-of-sight (LOS) velocity of 15
  km s<SUP>-1</SUP>. The surge retreated rapidly with LOS velocity of
  45 km s<SUP>-1</SUP> at its mouth. It underwent a rebound and fell
  back again. Two sympathetic mini-flares were observed that lasted
  only approximately 40 s, but showed strong Hα emission. We found
  magnetoacoustic waves in long fibrils as mainly short wave trains,
  short packets or pulses, i.e., solitary waves consisting of small
  (1´´-2´´) blobs. They start at either end of the fibrils and travel
  with phase speeds of 12-14 km s<SUP>-1</SUP>, i.e., close to the tube
  speed and approximately the sound velocity for sufficiently large
  magnetic field strengths. Some waves speed up to reach velocities of
  the order of 30 km s<SUP>-1</SUP>. This is much lower than the expected
  Alfvén velocity of ≥200 km s<SUP>-1</SUP> for reasonable magnetic
  field strengths and mass densities. We suggest that slow waves are not
  purely longitudinal, but possess gas velocities perpendicular to the
  direction of propagation of few km s<SUP>-1</SUP>. Also, fast waves
  travel along sinuous lines suggesting entangled magnetic fields. They
  spread out along the direction of propagation in the course of their
  evolution and often vanish. We discuss the implications.

---------------------------------------------------------
Title: Narrow-band full Stokes polarimetry of small structures on
    the Sun with speckle methods
Authors: Bello González, N.; Kneer, F.
2008A&A...480..265B    Altcode:
  Aims:For the detection and the study of small-scale magnetic fields
  on the Sun, it is important to obtain observations with both high
  spatial resolution and high polarimetric sensitivity. <BR />Methods:
  A second narrow-band etalon and a full Stokes polarimeter, based on
  ferroelectric liquid crystals, were implemented in the two-dimensional
  “Göttingen” Fabry-Perot spectrometer/polarimeter at the Vacuum
  Tower Telescope, Observatorio del Teide/Tenerife. First observations
  with the Fe I 6173 Å line and their data analysis with speckle
  methods are described. <BR />Results: The new polarimeter yields
  magnetograms of a field of view of 31 arcsec×52 arcsec with minimised
  seeing induced signals and without spurious signals as from the use of
  beam-splitting calcites. The achieved spatial and temporal resolution
  are 0.30-0.35 arcsec and 36 s, respectively. With a detection limit
  for the field strength of 16 G (\cor3σ), a polarimetric sensitivity
  of 7-8×10<SUP>15</SUP> Mx is obtained. Examples of intensity maps,
  Dopplergrams, and magnetograms from quiet and active regions on the
  Sun are discussed. Some of the results on solar magnetism are on 1)
  a pore with small-scale structure where we find a region with very
  low temperature gradient, 2) polar faculae with strong magnetic field
  signals and weaker signals in other areas surrounding them, and 3)
  small-scale inter-network magnetic fields with area fillings in the
  resolution elements of the order of 0.15.

---------------------------------------------------------
Title: On the properties of faculae at the poles of the Sun
Authors: Blanco Rodríguez, J.; Okunev, O. V.; Puschmann, K. G.;
   Kneer, F.; Sánchez-Andrade Nuño, B.
2007A&A...474..251B    Altcode:
  Context: Faculae at the poles of the Sun, or polar faculae (PFe),
  take part in the solar magnetic cycle. Their occurrence maximum is
  shifted by 5-6 years with respect to the sunspot cycle. PFe are stable
  phenomena, with lifetimes of several hours to days, and harbour magnetic
  fields of kilo-Gauss strength. Yet their role for the global magnetic
  field at the solar poles is unknown. <BR />Aims: To contribute to the
  knowledge of the physical properties of PFe and to the understanding of
  their role in the global magnetism of the Sun. <BR />Methods: PFe were
  observed on 21-24 August 2005 with the Vacuum Tower Telescope at the
  Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer
  was used with a Stokes V polarimeter to scan the Fe i 6173 Å line
  (Landé factor g = 2.5) and the Hα line in two-dimensional fields
  of view (FOVs). A large observational coverage of the polar caps was
  obtained. The data were analysed with speckle methods. Magnetic field
  strengths were determined with the weak field approximation, with the
  approximation of the strong field regime, and with the centre of gravity
  (COG) method. Velocities were measured with the COG method and from
  the zero-crossing of the Stokes V profiles. <BR />Results: PFe show
  a decrease of the continuum and broadband intensity contrast towards
  the disc centre and no decrease of contrast towards the limb, similar
  to as faculae in active regions near the equator. Extrapolating from
  the observed FOVs to the total areas of PF occurrence around the solar
  poles, we find 4 120 PFe in the northern polar cap and, asymmetrically
  to this number, 1 250 PFe near the south pole. The total area coverages
  by PFe are ~7.6×10<SUP>8</SUP> km<SUP>2</SUP> and ~3.4×10<SUP>8</SUP>
  km<SUP>2</SUP> near the solar north and south poles, respectively. Some
  of the PFe exhibit magnetic polarities opposite to the global polarity
  at the time of observation. The resulting total magnetic fluxes in PFe
  fall short by an order of magnitude from those found in the literature
  for the fluxes at the polar caps. This also holds if we include magnetic
  structures which are not related to brightenings. We conclude that
  with the present spatial resolution of 0.4 arcsec-0.5 arcsec (FWHM),
  PFe represent the “large-scale” end of a distribution of unipolar
  strands near the solar poles. The velocities in PFe show amplitudes of
  2 km s<SUP>-1</SUP>, with systematic up-flows in the Stokes I profile,
  but no average velocity measured in the V zero-crossings.

---------------------------------------------------------
Title: Spicule emission profiles observed in He i 10 830 Å
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
   Trujillo Bueno, J.; Blanco Rodríguez, J.; Kneer, F.
2007A&A...472L..51S    Altcode: 2007arXiv0707.4421S
  Aims:Off-the-limb observations with high spatial and spectral resolution
  will help us understand the physical properties of spicules in the
  solar chromosphere. <BR />Methods: Spectropolarimetric observations
  of spicules in the He i 10 830 Å multiplet were obtained with the
  Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope
  at the Observatorio del Teide (Tenerife, Spain). The analysis shows
  the variation of the off-limb emission profiles as a function of the
  distance to the visible solar limb. The ratio between the intensities
  of the blue and the red components of this triplet (R=I_blue/I_red)
  is an observational signature of the optical thickness along the light
  path, which is related to the intensity of the coronal irradiation. <BR
  />Results: We present observations of the intensity profiles of spicules
  above a quiet Sun region. The observable R as a function of the distance
  to the visible limb is also given. We have compared our observational
  results to the intensity ratio obtained from detailed radiative transfer
  calculations in semi-empirical models of the solar atmosphere assuming
  spherical geometry. The agreement is purely qualitative. We argue that
  future models of the solar chromosphere and transition region should
  account for the observational constraints presented here.

---------------------------------------------------------
Title: Modern Solar Facilities - Advanced Solar Science
Authors: Kneer, Franz; Puschmann, Klaus G.; Wittmann, Axel D.
2007msfa.conf.....K    Altcode:
  An international workshop entitled: Modern Solar Facilities - Advanced
  Solar Science was held in Göttingen Sept. 27-29, 2006. The workshop,
  which was attended by 88 participants from 24 different countries, gave
  a broad overview of the current state of solar research, with emphasis
  on telescopes and instrumentation, high-resolution and high-precision
  observations, and theory and interpretation. The book collects written
  versions of 71 papers presented at the conference.

---------------------------------------------------------
Title: GREGOR: the New German Solar Telescope
Authors: Balthasar, H.; von der Lühe, O.; Kneer, F.; Staude, J.;
   Volkmer, R.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
   C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Nicklas, H.; Popow, E.;
   Puschmann, K.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier,
   K.; Wittmann, A.
2007ASPC..368..605B    Altcode: 2007arXiv0704.2693B
  GREGOR is a new open solar telescope with an aperture of 1.5 m. It
  replaces the former 45-cm Gregory Coudé telescope on the Canary island
  Tenerife. The optical concept is that of a double Gregory system. The
  main and the elliptical mirrors are made from a silicon-carbide material
  with high thermal conductivity. This is important to keep the mirrors
  on the ambient temperature avoiding local turbulence. GREGOR will be
  equipped with an adaptive optics system. The new telescope will be ready
  for operation in 2008. Post-focus instruments in the first stage will be
  a spectrograph for polarimetry in the near infrared and a 2-dimensional
  spectrometer based on Fabry-Pérot interferometers for the visible.

---------------------------------------------------------
Title: Temporal evolution of intensity, velocity and magnetic field
    of sunspots at high spatial resolution
Authors: Bello González, N.; Kneer, F.; Puschmann, K. G.
2007msfa.conf..217B    Altcode:
  We present results of sunspot observations obtained in April 2006
  with the new "Göttingen" Fabry-Perot spectrometer. Thanks to the
  large field of view (77"x58") of the new optical setup it has been
  possible to perform 2D-spectropolarimetric observations of a small
  sunspot and ist surroundings at a heliocentric angle [Theta] ~ 40Å. A
  long time series of about one hour has been taken scanning along the
  magnetic Fe I 6173 Å and the non-magnetic Fe I 5576 Å spectral lines
  quasi-simultaneously. Hence, with the help of image reconstruction
  techniques, the temporal evolution of the sunspot fine-structure in
  intensity as well as in velocity and magnetic field is analysed at
  high spatial resolution.

---------------------------------------------------------
Title: Off-limb spectroscopy of the He I 10830 Å multiplet:
    observations vs. modelling
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
   Trujillo Bueno, J.; Kneer, F.
2007msfa.conf..177S    Altcode:
  Spectropolarimetric observations of spicules were carried out with
  the new optical setup of the Tenerife Infrared Polarimeter (TIP 2)
  at the VTT, showing the variation of the He I 10830Å multiplet
  off-limb emission profiles with increasing distance to the solar
  visible limb. The ratio between the intensities of the blue and the
  red components of this triplet (R = Iblue/Ired) is an observational
  signature of the optical thickness along the light path, which, at the
  same time, is proportional to the population of the lower (metastable)
  level that takes part in these transitions. Our observational results
  show a variation of R as a function of the distance to the limb. In
  agreement with recent theoretical results we conclude that R could be
  used as a diagnostic tool for downward UV coronal irradiance, which is
  believed to be responsible for the population of the metastable level of
  the He I 10830 Å triplet. We have compared our observational results
  with the ratio obtained from detailed radiative transfer calculations
  in semi-empirical models of the solar atmosphere (assuming spherical
  geometry) finding a fairly poor agreement. We argue that future models
  of the solar chromosphere and transition region should account for
  the observational constraints presented here.

---------------------------------------------------------
Title: From the "Göttingen" Fabry-Perot Interferometer to the
    GREGOR FPI
Authors: Puschmann, K. G.; Kneer, F.; Nicklas, H.; Wittmann, A. D.
2007msfa.conf...45P    Altcode:
  Fabry-Perot Interferometers (FPIs) have advantages over slit
  spectrographs, allowing fast two-dimensional, narrowband imaging
  and post factum image reconstruction of the spectropolarimetric data
  obtained. The resulting intensity, velocity and magnetic field maps
  are a fundamental base for the understanding of the dynamics of
  the solar atmosphere and its magnetic fields at smallest spatial
  scales. Efforts are undertaken to provide, with the Göttingen
  Fabry-Perot interferometer, an up-todate post-focus instrument for
  the German 1.5 m GREGOR solar telescope. Therefore a renewal of the
  spectrometer has been achieved during the first half of 2005. First
  observations at the German Vacuum Tower Telescope (VTT) reveal new
  scientific aspects and a very promising outlook for the future at
  GREGOR. In this contribution a general description of the upgraded
  spectrometer is given. Its final optical design at GREGOR is described
  and an optical analysis of the GREGOR FPI is outlined. Latest results
  with the new instrument obtained at the VTT are presented.

---------------------------------------------------------
Title: New high resolution solar telescope GREGOR
Authors: Volkmer, R.; von der Lühe, O.; Kneer, F.; Staude, J.;
   Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
   C.; Heidecke, F.; Hofmann, A.; Klvaña, M.; Sobotka, M.; Nicklas, H.;
   Popow, E.; Puschmann, K. G.; Schmidt, W.; Soltau, D.; Strassmeier,
   K.; Wittmann, A. D.
2007msfa.conf...39V    Altcode:
  The 1.5m solar telescope GREGOR is being constructed at Tenerife,
  Spain. Its purpose is to observe with high spatial and spectral
  resolution small-scale dynamic magnetic features on the Sun. The
  telescope is completely open with retractable dome and actively cooled
  primary mirror made of silicon carbide to minimize thermal effects
  on the image quality. After completion it will be one of the most
  powerful solar telescopes. This paper presents a general overview of
  the telescope characteristics and the current status.

---------------------------------------------------------
Title: Modern solar facilities - advanced solar science
Authors: Kneer, Franz; Puschmann, Klaus G.; Wittmann, Axel D.
2007msfa.conf....0K    Altcode:
  An international workshop entitled: Modern Solar Facilities - Advanced
  Solar Science was held in Göttingen Sept. 27-29, 2006. The workshop,
  which was attended by 88 participants from 24 different countries, gave
  a broad overview of the current state of solar research, with emphasis
  on telescopes and instrumentation, high-resolution and high-precision
  observations, and theory and interpretation. The book collects written
  versions of 71 papers presented at the conference.

---------------------------------------------------------
Title: Methods in high-resolution solar spectroscopy
Authors: Kneer, F.
2007msfa.conf...67K    Altcode:
  After few historical remarks, the needs in high-resolution spectroscopy
  are outlined. They are concerned with temporal, spatial, and wavelength
  resolution for a reasonable field of view of supergranulation cell or
  larger. At 0."1 spatial resolution ([about] 72.5 km on the Sun) the
  measurement for a snapshot should be finished within the transit time
  ttr [about] 10 s (size of resolution element/sound speed). Finally, the
  performances of grating spectrometers and Fabry-Perot interferometers
  are compared. The latter are far superior to grating spectrographs
  due to their high resolution-luminosity product.

---------------------------------------------------------
Title: Observations of a flaring active region in H[alpha]
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Kneer, F.
2007msfa.conf..273S    Altcode:
  A 40 min time series of the active region NOAA AR10875 was obtained
  on April 26th, 2006 under very good seeing conditions using the new
  "Göttingen" - Fabry-Perot interferometer at the German Vacuum Tower
  Telescope, Observatorio del Teide, Tenerife. The Hα line was scanned
  giving two-dimensional images at 21 different wavelength positions
  with a cadence of ~ 20 s per scan. After speckle reconstruction the
  spatial resolution achieved was better than 0".35. The data provide a
  unique possibility to study the temporal evolution of a pre-flaring,
  flaring and post-flaring solar active region in Hα. From the analysis
  of the present spectrometric data set we expect important advances in
  the understanding of Alfén waves, dynamics of fibrils and filaments
  and the physical nature of the sigmoid structure seen in the core of
  Hα. Here we present first results from this data set.

---------------------------------------------------------
Title: Dynamics of the quiet photosphere and its magnetic field
    under high spatial resolution
Authors: Puschmann, K. G.; Kneer, F.; Domínguez Cerdeña, I.
2007msfa.conf..151P    Altcode:
  An excellent 1 h 10 min time series of a quiet solar region near
  disk centre was acquired on 4th of May 2006 at the German Vacuum
  Tower Telescope. Using the new "Göttingen" Fabry-Perot interferometer
  together with the Kiepenheuer Adaptive Optics system spectropolarimetric
  images were obtained, scanning the Fe I 5247Å and Fe I 5250Å lines
  with a temporal cadence of 17 s each. Thanks to speckle reconstruction
  methods the achieved spatial resolution of the obtained intensity,
  velocity and magnetic field maps is ~ 0."3. First results on
  the dynamics of the quiet photosphere and magnetic fields reveal
  strengthening and weakening of the polarisation signal in interaction
  to granular convection processes indicating advection and diffusion
  of magnetic flux at smallest scales. Field structures, very elongated
  along intergranular lanes, show high dynamics, combine and split, and
  leave strong fingerprints at high photospheric layers where related
  temperatures fluctuate. Recent numerical MHD simulations come close
  to our findings. Possible indications of magnetic flux annihilation,
  flux tube evacuation or signs of upward-propagating shock fronts
  are illustrated.

---------------------------------------------------------
Title: Study of polar faculae with north pole coverage of the Sun
Authors: Blanco Rodríguez, J.; Okunev, O.; Puschmann, K. G.; Kneer, F.
2007msfa.conf..181B    Altcode:
  We present here a description of the main characteristics of faculae
  at the poles of the Sun (polar faculae PFe) observed in a series of
  observations covering the north solar pole. Statistics of a group
  of these PFe are used to estimate the contrast behaviour from [mμ]
  = 0.6 towards the limb as well as their magnetic field. Hα is also
  observed to analyse the behaviour of PFe in the chromosphere.

---------------------------------------------------------
Title: Magnetic Fields of the Quiet Sun: Distribution of Field
    Strengths
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2006ASPC..358...48D    Altcode:
  The quiet Sun photospheric plasma has a variety of magnetic field
  strengths going from zero to 1800 G. The empirical characterization of
  these field strengths requires a probability density function (PDF),
  i.e., a function P(B) describing the fraction of quiet Sun occupied by
  each field strength B. We present a method to combine magnetic field
  strength measurements based on the Zeeman effect and the Hanle effect
  in order to estimate the true P(B). The application of the method
  to real observations renders a set of possible PDFs, which outline
  the general characteristics of the quiet Sun magnetic fields. Their
  most probable field strength differs from zero. The magnetic energy
  density is a significant fraction of the kinetic energy of the granular
  motions at the base of the photosphere (larger than 15 or larger than
  2×10<SUP>3</SUP> erg cm<SUP>-3</SUP>). The unsigned flux density
  (or mean magnetic field strength) has to be between 130 G and 190
  G. A significant part of the unsigned flux (between 10% and 40%) and
  of the magnetic energy (between 40% and 80%) are provided by the field
  strengths larger than 1 kG which, however, occupy only a small fraction
  of the surface (between 1% and 4%). <P />The quiet Sun photosphere has
  far more unsigned magnetic flux and magnetic energy than the active
  regions and the network together.

---------------------------------------------------------
Title: Quiet-Sun Magnetic Fields: Simultaneous Inversion of Visible
    and IR Spectro-Polarimetric Observations
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2006ASPC..358...88D    Altcode:
  We studied the quiet-Sun magnetic fields interpreting
  spectro-polarimetric observations of infrared and visible spectral
  lines. Magnetic field strengths and filling factors were inferred by the
  simultaneous inversion of Stokes profiles of the Fe I lines at 6301.5,
  6302.5, 15648, and 15653 Å under the MISMA hypothesis. They cover a
  solar intra-network region at disk center. We analyzed Stokes profiles
  with signals above noise in both spectral ranges, which correspond
  to 40% of the field of view. Most of these 2280 profiles could only
  be inverted with a model including 3 magnetic components with very
  different field strengths, which indicates the co-existence of kG and
  sub-kG fields. We measured a total unsigned magnetic flux density
  of 9.6 G over the whole field of view. For half of the pixels the
  magnetic field has opposite polarities within the resolution element. We
  computed the probability density function of finding each magnetic field
  strength. It has an important contribution of kG field strengths, which
  concentrates most of the magnetic flux and energy. This kG contribution
  has a preferred magnetic polarity while the weak fields are balanced.

---------------------------------------------------------
Title: Quiet Sun Magnetic Fields from Simultaneous Inversions of
    Visible and Infrared Spectropolarimetric Observations
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2006ApJ...646.1421D    Altcode: 2006astro.ph..4381D
  We study the quiet Sun magnetic fields using spectropolarimetric
  observations of the infrared and visible Fe I lines at 6301.5, 6302.5,
  15648, and 15653 Å. Magnetic field strengths and filling factors
  are inferred by the simultaneous fit of the observed Stokes profiles
  under the MISMA hypothesis. The observations cover an intranetwork
  region at the solar disk center. We analyze 2280 Stokes profiles whose
  polarization signals are above noise in the two spectral ranges, which
  correspond to 40% of the field of view. Most of these profiles can
  be reproduced only with a model atmosphere including three magnetic
  components with very different field strengths, which indicates the
  coexistence of kG and sub-kG fields in our 1.5" resolution elements. We
  measure an unsigned magnetic flux density of 9.6 G considering the full
  field of view. Half of the pixels present magnetic fields with mixed
  polarities in the resolution element. The fraction of mixed polarities
  increases as the polarization weakens. We compute the probability
  density function of finding each magnetic field strength. It has a
  significant contribution of kG field strengths, which concentrates
  most of the observed magnetic flux and energy. This kG contribution
  has a preferred magnetic polarity, while the polarity of the weak
  fields is balanced.

---------------------------------------------------------
Title: The new 1.5m solar telescope GREGOR: first light and start
    of commissioning
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
   Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
   Heidecke, Frank; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald;
   Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus;
   Sobotka, Michal; Klvana, Miroslav; Collados, Manuel
2006SPIE.6267E..0WV    Altcode: 2006SPIE.6267E..29V
  The integration of the three main silicon carbide mirrors into the new
  1.5 m solar telescope GREGOR at Izana on Tenerife, Spain is planned
  during 2006. We expect first light at the end of 2006. A progress
  report about integration of the optics and mechanics and planning of
  the commissioning phase of the telescope and post focus instruments
  will be presented at the meeting. The GREGOR telescope is build by a
  consortium of the Kiepenheuer Institut fur Sonnenphysik in Freiburg,
  the Astrophysikalische Institut Potsdam, the Institut fur Astronomie
  Gottingen and additional national and international Partners.

---------------------------------------------------------
Title: The new Göttingen Fabry-Pérot spectrometer for
    two-dimensional observations of the Sun
Authors: Puschmann, K. G.; Kneer, F.; Seelemann, T.; Wittmann, A. D.
2006A&A...451.1151P    Altcode:
  Studies of small-scale dynamics and magnetic fields in the solar
  atmosphere require spectroscopy and polarimetry with high spatial
  resolution. For this purpose, spectrometers based on Fabry-Pérot
  interferometers (FPIs) have advantages over slit spectrographs. They
  possess a high throughput and allow fast two-dimensional,
  narrow-band imaging and image reconstruction of the data. In the
  present contribution we describe an upgrade, essentially renewal,
  of the Göttingen FPI spectrometer achieved during the first half of
  2005. A new etalon from IC Optical Systems Ltd. (formerly Queensgate),
  England, with 70 mm free aperture for high spectral resolution has
  been mounted. New CCD detectors from LaVision GmbH (Göttingen) with
  powerful computer hard- and software were implemented. We consider the
  product of signal-to-noise ratio, frame rate, and field of view as a
  measure of the efficiency. At low light levels, e.g. in narrow-band
  speckle applications, this product has increased by a factor ~60
  compared to the old system. In addition, several spectral regions can
  now be scanned quasi-simultaneously. We present first results obtained
  with the upgraded spectrometer. The efforts are undertaken to provide
  an up-to-date post-focus instrument for the new German 1.5 m GREGOR
  solar telescope presently under construction at the Observatorio del
  Teide on Tenerife.

---------------------------------------------------------
Title: The Distribution of Quiet Sun Magnetic Field Strengths from
    0 to 1800 G
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2006ApJ...636..496D    Altcode: 2005astro.ph..9243D; 2006ApJ...636..496C
  The quiet-Sun photospheric plasma has a variety of magnetic field
  strengths going from zero to 1800 G. The empirical characterization
  of these field strengths requires a probability density function
  (PDF), i.e., a function P(B) describing the fraction of quiet Sun
  occupied by each field strength B. We show how to combine magnetic
  field strength measurements based on the Zeeman effect and the Hanle
  effect to estimate an unbiased P(B). The application of the method
  to real observations renders a set of possible PDFs, which outline
  the general characteristics of the quiet-Sun magnetic fields. Their
  most probable field strength differs from zero. The magnetic energy
  density is a significant fraction of the kinetic energy of the
  granular motions at the base of the photosphere (larger than 15%
  or larger than 2×10<SUP>3</SUP> ergs cm<SUP>-3</SUP>). The unsigned
  flux density (or mean magnetic field strength) has to be between 130
  and 190 G. A significant part of the unsigned flux (between 10% and
  50%) and of the magnetic energy (between 45% and 85%) are provided by
  the field strengths larger than 500 G, which, however, occupy only a
  small fraction of the surface (between 1% and 10%). The fraction of kG
  fields in the quiet Sun is even smaller, but they are important for a
  number of reasons. The kG fields still trace a significant fraction of
  the total magnetic energy, they reach the high photosphere, and they
  appear in unpolarized light images. The quiet-Sun photosphere has
  far more unsigned magnetic flux and magnetic energy than the active
  regions and the network combined.

---------------------------------------------------------
Title: Chromospheric Dynamics of a Solar Active Region
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Sánchez
   Cuberes, M.; Blanco Rodríguez, J.; Kneer, F.
2005ESASP.600E..70S    Altcode: 2005dysu.confE..70S; 2005ESPM...11...70S
  No abstract at ADS

---------------------------------------------------------
Title: Study of Polar Faculae
Authors: Blanco Rodríguez, J.; Sánchez-Andrade Nuño, B.; Puschmann,
   K. G.; Kneer, F.
2005ESASP.600E..59B    Altcode: 2005dysu.confE..59B; 2005ESPM...11...59B
  No abstract at ADS

---------------------------------------------------------
Title: Synthesis of Stokes Profiles from a Two Component Penumbral
    Model
Authors: Bello González, N.; Okunev, O.; Kneer, F.
2005ESASP.600E..56B    Altcode: 2005ESPM...11...56B; 2005dysu.confE..56B
  No abstract at ADS

---------------------------------------------------------
Title: Analysis of a Wide Chromospheric Active Region
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Sánchez
   Cuberes, M.; Blanco Rodríguez, J.; Kneer, F.
2005ESASP.596E..62S    Altcode: 2005ccmf.confE..62S
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic Structures on the Sun: Osbervations with the New
    "GÖTTINGEN" Two-Dimensional Spectrometer on Tenerife
Authors: Kneer, F.; Puschmann, K. G.; Blanco Rodríguez, J.;
   Sánchez-Andrade Nuño, B.; Wittmann, A. D.
2005ESASP.596E..71K    Altcode: 2005ccmf.confE..71K
  No abstract at ADS

---------------------------------------------------------
Title: Study of Polar Faculae
Authors: Blanco Rodríguez, J.; Sánchez-Andrade Nuño, B.; Puschmann,
   K. G.; Kneer, F.
2005ESASP.596E..52B    Altcode: 2005ccmf.confE..52B
  No abstract at ADS

---------------------------------------------------------
Title: Analysis of Polarimetric Sunspot Data from Tesos/vtt/tenerife
Authors: Valdivielso Casas, L.; Bello González, N.; Puschmann, K. G.;
   Sánchez-Andrade Nuño, B.; Kneer, F.
2005ESASP.596E..67V    Altcode: 2005ccmf.confE..67V
  No abstract at ADS

---------------------------------------------------------
Title: Study of Asymmetries of Stokes Profiles from High Spatial
    Resolution Spectropolarimetry
Authors: Bello González, N.; Okunev, O.; Kneer, F.
2005ESASP.596E..50B    Altcode: 2005ccmf.confE..50B
  No abstract at ADS

---------------------------------------------------------
Title: Numerical modeling of solar faculae close to the limb
Authors: Okunev, O. V.; Kneer, F.
2005A&A...439..323O    Altcode:
  A numerical simulation was performed to interpret high-resolution
  spectropolarimetric observations of polar faculae on the Sun. A
  semi-empirical three-dimensional model of facula structures,
  controlled by several free parameters, was constructed. It consists
  of two components, the atmosphere within small-scale magnetic flux
  tubes and the exterior atmosphere. Multi-ray 1.5D radiative transfer
  calculations were performed along oblique rays passing through a highly
  inhomogeneous atmosphere of the simulation box. By the comparison of
  the properties of the calculated Stokes profiles from the synthetic
  faculae with the observed properties, a set of free parameters of
  the model (such as: size, number density of flux tubes, internal
  temperature stratification and magnetic field strength) was deduced,
  which satisfies the observational constraints. The hypothesis about
  solar faculae as a conglomerate of small-scale magnetic flux tubes is
  verified. The model reproduces all observed properties: the continuum
  contrast and its center-to-limb variation, the Stokes I and V profiles
  of Fe I and Fe II lines, the apparent magnetic field strengths, and
  the displacement towards the limb of the continuum intensity against
  the line-of-sight magnetograms.

---------------------------------------------------------
Title: The new 1.5 solar telescope GREGOR: progress report and
    results of performance tests
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
   Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
   Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wittmann, Axel;
   Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal;
   Klvana, Miroslav; Collados, Manuel
2005SPIE.5901...75V    Altcode:
  The telescope structure including control system and the complete
  retractable dome of the new 1.5 m solar telescope GREGOR were
  assembled during 2004 at Izana on Tenerife, Spain. The GREGOR
  telescope is build by a consortium of the Kiepenheuer Institut fuer
  Sonnenphysik, the Astrophysikalische Institut Potsdam, the Institut
  fuer Astrophysik Goettingen and additional national and international
  Partners. Pointing, tracking and thermal tests were made to verify
  the proposed performance. The results of these tests and a progress
  report of the project will be presented.

---------------------------------------------------------
Title: On super-resolution in astronomical imaging
Authors: Puschmann, K. G.; Kneer, F.
2005A&A...436..373P    Altcode:
  Our work is an attempt to fulfil one of the aims of astronomical
  imaging, that is, to obtain information at high angular
  resolution. Stars as point sources can be represented by
  Dirac δ-functions whose Fourier transforms contain information
  about the position and the (integrated) intensity at all angular
  frequencies. Thus, we can deconvolve unresolved images of star fields
  even at angular distances smaller than the diffraction limit of
  the telescope with which the observations are performed. We give an
  example of reconstruction of the image of two stars with an angular
  separation of a factor 2.5 less than α_Ra=1.22×λ/D. However, we
  find that super-resolution is feasible only for point sources. For
  extended objects the information about intensity fluctuations at angular
  frequencies u&gt;u_max=D/λ=1/α_min is irretrievably lost. We discuss
  the impossibility of super-resolution for the Sun using a numerically
  simulated image of solar granulation. However, one can enhance the
  contrast of solar images, though without increasing angular resolution
  beyond the diffraction limit.

---------------------------------------------------------
Title: Polarimetry of sunspot penumbrae with high spatial resolution
Authors: Bello González, N.; Okunev, O. V.; Domínguez Cerdeña,
   I.; Kneer, F.; Puschmann, K. G.
2005A&A...434..317B    Altcode:
  We present two-dimensional high-spatial-resolution spectropolarimetric
  observations of sunspot penumbrae. They were obtained in April 2002 and
  May 2003 with the "Göttingen" Fabry-Pérot spectrometer at the Vacuum
  Tower Telescope at the Observatorio del Teide (Tenerife). Speckle
  methods were applied for image reconstruction which resulted
  in a spatial resolution of 0.5 arcsec in the magnetograms of the
  penumbrae. We analysed Stokes I and V profiles of the Fe II 6149 Å
  line, which exhibits no instrumental Stokes Q/U→ V crosstalk, and of
  the Fe I line pair at 6302 Å. The main results are the following: 1)
  on scales larger than 0.5 arcsec, the intensity pattern of penumbrae
  stays the same in the continuum and core images of the 6301.5 Å line,
  which stem from 0 km and 300 km (above τ_5=1), respectively. Yet
  at scales of 0.5 arcsec and smaller the pattern in the two spectral
  features is clearly different. 2) On the limb side of sunspots the
  Evershed flow is carried by dark filaments and on the centre side by
  bright features and their somewhat weakened tails. We explain this with
  a picture in which the velocity of hot rising gas is best seen on the
  centre side, while on the limb side the horizontal outward and possibly
  downward flows are seen when the gas has cooled down. 3) The un-combed
  structure of the magnetic field is confirmed. On the limb side, the more
  horizontal fields coincide with dark fibrils or with diffuse intensity
  structures. Generally, the more horizontal fields are located at the
  positions of strong outflows. 4) Strong line-of-sight components of
  the magnetic fields are not found in bright filaments but in dark
  structures, somewhat displaced from the darkest parts. Their positions
  do not coincide with those of the strongest velocity fields. In general,
  our results are compatible with the picture of low lying flow channels
  coincident with the horizontal magnetic field, or possibly emerging and
  diving down into sub-photospheric layers, like a "sea serpent". Some
  further dynamic phenomena are discussed, which demonstrate the richness
  of processes in penumbrae, and reveal unexpected properties.

---------------------------------------------------------
Title: Quiet sun magnetic fields vs. polar faculae - local vs. global
    dynamo?
Authors: Okunev, O. V.; Domínguez Cerdeña, I.; Puschmann, K. G.;
   Kneer, F.; Sánchez Almeida, J.
2005AN....326..205O    Altcode:
  Quiet Sun magnetic fields in the internetwork are almost
  ubiquitous. Simultaneous observations in infra-red and visible lines
  and high spatial resolution (&lt; 0.5<SUP>”</SUP>) data in visible
  lines show that their field strengths range from below few hundred
  Gauss to kilo-Gauss. Most of the flux is contained in small-scale,
  strong-field features located mainly in intergranular lanes. The
  average unsigned flux density exceeds 20 Gauss. The new detections
  are confirmed by recent quiet Sun observations in the G band. The
  generation of the strong fields in the internetwork, which may be due
  to a local dynamo, poses a challenging problem. - Polar faculae (PFe)
  are small-scale magnetic features at the polar caps of the Sun. They
  take part in the solar cycle and are thus likely to be rooted deeply
  in the solar interior. They are the result of the global dynamo at
  the solar poles. PFe also possess kilo-Gauss magnetic fields which
  have the same polarity as the global magnetic field. The rôle of
  quiet Sun magnetic field structures and of PFe for the dynamics of
  the corona and for the solar wind are addressed.

---------------------------------------------------------
Title: Spectropolarimetry in a Sunspot Penumbra at High Spatial
    Resolution
Authors: Gonzalez, N. Bello; Ounev, O.; Kneer, F.
2005ASSL..320..183G    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spectropolarimetry in a sunspot penumbra at high spatial
    resolution
Authors: Bello González, N.; Okunev, O.; Kneer, F.
2005ASSL..320..183B    Altcode: 2005smp..conf..183B
  We present results from polarimetric data on sunspot penumbrae obtained
  with the Vacuum Tower Telescope at the Observatorio del Teide, Tenerife,
  using the `Göttingen' Fabry-Pérot Interferometer. Speckle image
  reconstruction has been performed giving us a spatial resolution of
  0.6”. The observations were taken in the Fe II 6149.2 Å line which,
  given its particular Zeeman splitting, has no instrumental (Q,U) to V
  crosstalk and provides us measurements of I and V Stokes profiles to
  analyse velocities and magnetic fields in penumbrae.

---------------------------------------------------------
Title: Simultaneous Visible and IR spectropolarimetry of the quiet Sun
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2005ASSL..320..175D    Altcode: 2005ASSL..320..175C; 2005smp..conf..175D
  This work presents the first quiet Sun spectropolarimetric observations
  carried out in the visible and the infrared (IR) simultaneously. The
  Fe I lines at 6301.5, 6302.5, 15648, and 15652 Å were observed
  co-spatially, and at the same time (with a time lag of only 1 minute),
  with high sensitive spectropolarimeters operated in two different
  telescopes (VTT and THEMIS at the Observatorio del Teide). We find
  Stokes V profiles above noise in 30% of the observed area, showing
  intrinsic magnetic fields of kG (traced by visible lines) co-existing
  with sub-kG fields (traced by infrared lines). We also found V profiles
  with opposite polarity in the visible and the IR in 25% of the pixels
  under study (8% of the area).

---------------------------------------------------------
Title: On the structure of polar faculae on the Sun
Authors: Okunev, O. V.; Kneer, F.
2004A&A...425..321O    Altcode:
  Faculae on the polar caps of the Sun, in short polar faculae (PFe),
  are investigated. They take part in the magnetic solar cycle. Here,
  we study the fine structures of PFe, their magnetic fields and their
  dynamics on short time scales. The observations stem from several
  periods in 2001 and 2002. They consist of spectropolarimetric data
  (Stokes I and V) taken in the Fe I 6301.5 and 6302.5 Å and Fe II
  6149.3 Å lines with the Gregory-Coudé Telescope (GCT) and the Vacuum
  Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. At the
  VTT, the “Göttingen” two-dimensional Fabry-Perot spectrometer was
  used. It allows image reconstruction with speckle methods resulting in
  spatial resolution of approximately 0.25 arcsec for broadband images
  and 0.5 arcsec for magnetograms. The application of singular value
  decomposition yielded a polarimetric detection limit of | V|≈2 ×
  10<SUP>-3</SUP>I<SUB>c</SUB>. We find that PFe, of size of 1 arcsec
  or larger, possess substantial fine structure of both brightness and
  magnetic fields. The brightness and the location of polar facular
  points change noticeably within 50 s. The facular points have strong,
  kilo-Gauss magnetic fields, they are unipolar with the same polarity
  as the global, poloidal magnetic field. The ambient areas, however,
  exhibit weak flux features of both polarities, as in the quiet Sun
  near disk center. Strong upflows of 1 km s<SUP>-1</SUP> are detected
  in the intensity profiles of PFe. The zero-crossings of the Stokes
  V profiles yield an average velocity of the magnetized plasma of
  v<SUB>FT</SUB>≈-0.4 km s<SUP>-1</SUP> (towards the observer).

---------------------------------------------------------
Title: Progress report of the 1.5 m solar telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar F.; Kneer, Franz;
   Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Schmidt,
   Wolfgang; Soltau, Dirk; Nicklas, Harald; Wiehr, Eberhardt; Wittmann,
   Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka,
   Michal; Klvana, Miroslav; Collados, Manuel
2004SPIE.5489..693V    Altcode:
  GREGOR is the new 1.5 m solar telescope assembled on Tenerife, Spain, by
  the German consortium of the Kiepenheuer Institut fur Sonnenphysik, the
  Astronomischen Institut Potsdam, the Universitats-Sternwarte Gottingen
  and other national and international Partners. The refurbishment of
  the building is almost finished. The manufacturing of the telescope
  structure and the optics is still in progress. After the integration of
  the new complete retractable dome in July 2004 the telescope structure,
  optic and post focus instruments will be assembled during the rest of
  the year. First light is planned during May 2005.

---------------------------------------------------------
Title: Dynamics of an enhanced network region observed in Hα
Authors: Al, N.; Bendlin, C.; Hirzberger, J.; Kneer, F.; Trujillo
   Bueno, J.
2004A&A...418.1131A    Altcode:
  This investigation is based on Hα observations of high spatial
  resolution. They stem from an enhanced network region near disk centre
  of the sun and consist of broad-band and narrow-band images taken with
  the two-dimensional “Göttingen” Fabry-Perot spectrometer mounted in
  the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The
  “lambdameter method” was applied to derive intensity and velocity
  maps over the two-dimensional field of view reflecting the behaviour
  of these parameters in the solar chromosphere. The determination of
  the source function, the line-of-sight (LOS) velocity, the Doppler
  width and the optical depth was based on Beckers' cloud model
  (Beckers \cite{beckers}). From the LOS velocity distribution along
  the Hα structures we conclude that ballistic motions are unlikely to
  prevail. Especially the bright Hα features cannot be explained by the
  cloud model. It is shown how, instead, two-dimensional non-LTE radiative
  transfer calculations of embedded chromospheric structures lead to
  reasonable agreement with the observed line profiles from these bright
  features. It is found that many of the bright fibril-like structures
  near dark fibrils can be explained by radiation which is blocked by
  absorbing material at large heights and escapes through less opaque
  regions. We estimate the number densities and the temperature. With
  these and with the measured velocities, the kinetic energy flux and
  the enthalpy flux related to the motions of the fine structures can
  be calculated. Both fall short by at least an order of magnitude of
  covering energy losses by radiation of active chromospheric regions.

---------------------------------------------------------
Title: Polarimetry in sunspot penumbrae at high spatial resolution.
Authors: Bello Gonzalez, N.; Okunev, O.; Dominguez Cerdena, I.;
   Kneer, F.
2004ANS...325...79B    Altcode: 2004ANS...325..P03B; 2004ANS...325a..79B
  No abstract at ADS

---------------------------------------------------------
Title: Short period waves in the solar atmosphere.
Authors: Andic, A.; Kneer, F.
2004ANS...325...96A    Altcode: 2004ANS...325a..96A; 2004ANS...325..P26A
  No abstract at ADS

---------------------------------------------------------
Title: Analysis of simultaneous visible and infrared
    spectropolarimetric observations of quiet Sun.
Authors: Dominguez Cerdena, I.; Sanchez Almeida, J.; Kneer, F.
2004ANS...325...94D    Altcode: 2004ANS...325a..94D; 2004ANS...325..P23D
  No abstract at ADS

---------------------------------------------------------
Title: Adaptive optics transfer function estimation for solar
    observations.
Authors: Sailer, M.; von der Luehe, O.; Kneer, F.
2004ANS...325..130S    Altcode: 2004ANS...325..P65S; 2004ANS...325a.130S
  No abstract at ADS

---------------------------------------------------------
Title: Structure and short-time evolution of polar faculae on the sun.
Authors: Okunev, O. V.; Kneer, F.
2004ANS...325...80O    Altcode: 2004ANS...325..P04O; 2004ANS...325a..80O
  No abstract at ADS

---------------------------------------------------------
Title: Simultaneous Visible and Infrared Spectropolarimetry of a
    Solar Internetwork Region
Authors: Sánchez Almeida, J.; Domínguez Cerdeña, I.; Kneer, F.
2003ApJ...597L.177S    Altcode: 2003astro.ph..9727A; 2003astro.ph..9727S
  We present the first simultaneous infrared (IR) and visible
  spectropolarimetric observations of a solar internetwork region. The
  Fe I lines at 6301.5, 6302.5, 15648, and 15652 Å were observed, with
  a lag of only 1 minute, using highly sensitive spectropolarimeters
  operated in two different telescopes (Vacuum Tower Telescope and THEMIS
  at the Observatorio del Teide). Some 30% of the observed region shows
  IR and visible Stokes V signals above noise. These polarization signals
  indicate the presence of kilogauss (kG) magnetic field strengths (traced
  by the visible lines) coexisting with sub-kG fields (traced by the
  infrared lines). In addition, one-quarter of the pixels with signal have
  visible and IR Stokes V profiles with opposite polarity. We estimate
  the probability density function of finding each longitudinal magnetic
  field strength in the region. It has a tail of kG field strengths that
  accounts for most of the (unsigned) magnetic flux of the region.

---------------------------------------------------------
Title: On the fractal dimension of small-scale magnetic structures
    in the Sun
Authors: Janßen, K.; Vögler, A.; Kneer, F.
2003A&A...409.1127J    Altcode:
  We compare, by means of fractal analyses, the shapes of observed
  small-scale magnetic structures on the Sun with those of magnetic
  features resulting from numerical simulations of magnetoconvection. The
  observations were obtained with the “Göttingen” Fabry-Perot
  spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide
  on Tenerife. Magnetograms with 0\farcs4-0\farcs5 spatial resolution were
  obtained from two-dimensional Stokes V polarimetry in the Fe I 6302.5 Å
  line and by image reconstruction with speckle methods. The simulations
  of magnetoconvection was performed with the MURAM code. It solves the
  time-dependent MHD equations for a compressible, partly ionized plasma
  including radiative transfer in LTE. To determine the fractal dimensions
  the perimeter-area relation is used. We discuss the influence of seeing
  and noise in the fractal dimension D of the observed magnetograms. A
  dependence of D on the distance from disk center could not be found. The
  observations give D=1.21+/-0.05 for a pixel size corresponding to
  0\farcs105, while for the numerical simulations D=1.38+/-0.07 for a
  pixel size of 20.83 km. If we use a yardstick adapted to the spatial
  resolution the observations give the dimension D=1.41+/-0.05 in close
  agreement with the simulations. This agreement is remarkable since
  the pixel sizes and spatial resolutions of the simulations and of the
  observations differ by a factor of 15. The finding supports the view
  of self-similarity of solar magnetic structures over a large range of
  scales. In addition, it demonstrates the realism of the simulations
  and suggests that all important physical processes are included. We
  discuss our results in comparison with other investigations.

---------------------------------------------------------
Title: Inter-network magnetic fields  observed with sub-arcsec
    resolution
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
2003A&A...407..741D    Altcode: 2003astro.ph..6329C; 2003astro.ph..6329D
  We analyze a time sequence of Inter-Network (IN) magnetograms
  observed at the solar disk center. Speckle reconstruction techniques
  provide a good spatial resolution (0\farcs5 cutoff frequency) yet
  maintaining a fair sensitivity (some 20 G). Patches with signal above
  noise cover 60% of the observed area, most of which corresponds to
  intergranular lanes. The large surface covered by signal renders a mean
  unsigned magnetic flux density between 17 G and 21 G (1 G equiv 1 Mx
  cm<SUP>-2</SUP>). The difference depends on the spectral line used to
  generate the magnetograms (łinetwo or łineone ). Such systematic
  difference can be understood if the magnetic structures producing
  the polarization have intrinsic field strengths exceeding 1 kG, and
  consequently, occupying only a very small fraction of the surface
  (some 2%). We observe both, magnetic signals changing in time scales
  smaller than 1 min, and a persistent pattern lasting longer than the
  duration of the sequence (17 min). The pattern resembles a network
  with a spatial scale between 5 and 10 arcsec, which we identify as the
  mesogranulation. The strong dependence of the polarization signals
  on spatial resolution and sensitivity suggests that much quiet Sun
  magnetic flux still remains undetected.

---------------------------------------------------------
Title: Current Status of the 1.5m Solar Telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
   Jürgen; Berkfeld, Thomas; Schmidt, Wolfgang; Soltau, Dirk; Nicklas,
   Harald; Wiehr, Eberhardt; Wittman, Axel; Hofmann, Axel; Sobotka,
   Michal; Klvana, Miroslav
2003ANS...324..112V    Altcode: 2003ANS...324..P19V
  No abstract at ADS

---------------------------------------------------------
Title: Transfer Function Calibration for Speckle Reconstruction
Authors: Sailer, Markus; von der Lühe, Oskar; Kneer, Franz
2003ANS...324...23S    Altcode: 2003ANS...324..C04S
  No abstract at ADS

---------------------------------------------------------
Title: Velocity and Magnetic Fields in Sunspot Penumbrae at Hight
    Spatial and Spectral Resolution
Authors: Bello González, Nazaret; Kneer, Franz
2003ANS...324...28B    Altcode: 2003ANS...324..D03B
  No abstract at ADS

---------------------------------------------------------
Title: Hausdorff-Dimension of Magnetic Structures
Authors: Janssen, Katja; Vögler, Alexander; Kneer, Franz
2003ANS...324...30J    Altcode: 2003ANS...324..D06J
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of Solar Chromospheric Finestructures in H-alpha
    observed with High Spatial Resolution
Authors: Al, Nurol; Kneer, Franz; Hirzberger, Johann
2003ANS...324..111A    Altcode: 2003ANS...324..P17A
  No abstract at ADS

---------------------------------------------------------
Title: GREGOR: the new 1.5m solar telescope on Tenerife
Authors: Volkmer, Reiner; von der Luehe, Oskar; Kneer, Franz; Staude,
   Juergen; Hofmann, Axel; Schmidt, Wolfgang; Sobotka, Michal; Soltau,
   Dirk; Wiehr, Eberhardt; Wittmann, Axel; Berkefeld, Thomas
2003SPIE.4853..360V    Altcode:
  The new 1.5 m high resolution telescope will be build up on the reused
  solar tower of the German 45 cm Gregory Coudé Telescope at the Teide
  Observatory, Izaña, Tenerife. The new telescope is a Gregory type
  with open telescope structure, alt-azimuth mount, complete retractable
  dome, and a pool of well established and new developed post focus
  instruments. An adaptive optics system provides the capability for
  diffraction limited observations at visible wavelengths and the
  polarimetry device in the secondary focus reduces the perturbation
  due to instrumental polarization in an efficient way. We describe the
  main optical characteristics and the focal plane instrumentation with
  respect to the latest status of the project.

---------------------------------------------------------
Title: Quiet-Sun Magnetic Fields at High Spatial Resolution
Authors: Domínguez Cerdeña, I.; Kneer, F.; Sánchez Almeida, J.
2003ApJ...582L..55D    Altcode: 2002astro.ph.11454C; 2002astro.ph.11454D
  We present spectropolarimetric observations of internetwork magnetic
  fields at the solar disk center. A Fabry-Pérot spectrometer was used to
  scan the two Fe I lines at 6301.5 and 6302.5 Å. High spatial resolution
  (0.5") magnetograms were obtained after speckle reconstruction. The
  patches with magnetic fields above noise cover approximately 45%
  of the observed area. Such large coverage renders a mean unsigned
  magnetic flux density of some 20 G (or 20 Mx cm<SUP>-2</SUP>), which
  exceeds all previous measurements. Magnetic signals occur predominantly
  in intergranular spaces. The systematic difference between the flux
  densities measured in the two iron lines leads to the conclusion that,
  typically, we detect structures with intrinsic field strengths larger
  than 1 kG occupying only 2% of the surface.

---------------------------------------------------------
Title: High Resolution 2D Spectro-polarimetric Observations of Polar
    Faculae and Quiet Sun in Two Iron Lines
Authors: Domínguez Cerdeña, I.; Okunev, O.; Kneer, F.; Sánchez
   Almeida, J.
2003ASPC..307..370D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Two-dimensional speckle spectroscopy of Hα features
Authors: Al, N.; Hirzberger, J.; Kneer, F.
2003AN....324..364A    Altcode:
  In May 2002, the solar chromosphere was observed with a two-dimensional
  spectrometer which is mounted in the German Vacuum Tower Telescope (VTT)
  at the Observatorio del Teide/Tenerife. The aim of this observation was
  to investigate the fine structure of the solar chromosphere seen in Hα
  . We took narrow-band filtergrams (Delta lambda ~ 72 mÅ) by scanning
  through this line. Broad-band images taken strictly simultaneously
  with the narrow-band filtergrams were restored by speckle methods. The
  instantaneous optical transfer function from this restoration procedure
  was used for the reconstruction of the narrow-band images. Some results
  of this high spatial resolution observation are presented below.

---------------------------------------------------------
Title: From the Gregory-Coudé Telescope to GREGOR: a development
    from past to future.  Summary of workshop held in Göttingen, July
    24-26, 2002
Authors: Kneer, F.; Wiehr, E.; Wittmann, A. D.
2003AN....324..283K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Study of polar faculae by means of spectro-polarimetric
    observations and radiative transfer calculations
Authors: Okunev, O.; Kneer, F.
2003AN....324..330O    Altcode:
  Polar faculae are of special interest for solar physics because of
  their close relationship to the global magnetic field of the Sun and to
  solar activity, and because of the recently found kilogauss magnetic
  fields, which are very unusual for the structures outside active
  regions at high latitudes of the Sun. The idea is that polar faculae
  can be represented by bundles of unresolved small-scale magnetic flux
  tubes, which are characterized by sizes of about 100 km and strong
  magnetic fields. High resolution spectro-polarimetric observations
  of the considered structures were performed and complemented by the
  radiation transfer calculations with oblique rays passing through an
  inhomogeneous magnetic medium. The recent results of observations and
  numerical calculations are presented.

---------------------------------------------------------
Title: Speckle spectro-polarimetry of magnetic structures
Authors: Janssen, K.; Kneer, F.
2003AN....324..328J    Altcode:
  Two-dimensional spectrograms were obtained with the Vacuum Tower
  Telescope, Tenerife, in order to study small-scale structures and
  faculae on the Sun. Using the speckle reconstruction method, we obtain
  high-resolution images and wavelength scans. Magnetic fields can be
  studied from Stokes V profiles, and velocity maps are gained by the
  Doppler shift of the center of gravity of Stokes I. Here some results
  about small-scale structures and their magnetic fields are shown.

---------------------------------------------------------
Title: High resolution 2D spectro-polarimetric observations of the
    quiet Sun in two iron lines
Authors: Domínguez Cerdeña, I.; Kneer, F.; Sánchez Almeida, J.
2003AN....324..327D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Fabry-Perot spectrometer for high-resolution observation
    of the Sun
Authors: Kneer, F.; Al, N.; Hirzberger, J.; Nicklas, H.; Puschmann,
   K. G.
2003AN....324..302K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Short-period waves in the solar atmosphere
Authors: Wunnenberg, M.; Andjić, A.; Kneer, F.
2003AN....324..356W    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Evidence for short-period acoustic waves in the solar
    atmosphere
Authors: Wunnenberg, M.; Kneer, F.; Hirzberger, J.
2002A&A...395L..51W    Altcode:
  Short-period acoustic waves are thought to supply the energy for the
  radiative losses of the non-magnetic chromosphere of the Sun and,
  in general, of late-type stars. Here, we present evidence for the
  existence of waves in the solar atmosphere with periods in the range
  of 50 s &lt;P&lt; 100 s. Two-dimensional time sequences with a cadence
  of 25 s were obtained from quiet Sun disk center in Fe I 5434 Å. The
  observations were performed with the “Göttingen” Fabry-Perot
  spectrometer in the Vacuum Tower Telescope at the Observatorio del
  Teide/Tenerife. They are subjected to speckle reconstruction and to a
  wavelet analysis. The atmospheric ranges forming the velocity signals
  are narrowed by linear combinations of Doppler maps from wavelengths
  near line center. The power in the short-period range is concentrated
  above intergranular spaces. We estimate an acoustic flux into the
  chromosphere of approximately 3*E<SUP>6</SUP> erg cm<SUP>-2</SUP>
  s<SUP>-1</SUP>, as needed for the chromospheric radiative losses.

---------------------------------------------------------
Title: Formation of granular intensity fluctuations on the Sun
Authors: Pérez Rodríguez, E.; Kneer, F.
2002A&A...395..279P    Altcode:
  We deal with granular intensity fluctuations in the Sun and how they are
  caused by temperature fluctuations. We calculate temperature response
  functions RF<SUB>T</SUB>(lambda ,z) in the solar photosphere. We
  apply the Diagonal Element Lambda Operator (DELO) method to
  calculate the response functions for the continuum (lambda lambda =
  380 nm-2.5 mu ). LTE is assumed throughout. It is demonstrated that,
  within the framework of the linear approximation, the H<SUP>-</SUP>
  continuum absorption plays an important role for the formation of the
  fluctuations in continuum images. It counteracts substantially the
  intensity fluctuations caused by variations of the Planck function. The
  temperature response functions are used to calculate, for three models
  of temperature fluctuations, the continuum intensity contrasts and
  their formation height for the above wavelength range.

---------------------------------------------------------
Title: GREGOR, a 1.5 m Gregory-type telescope for solar observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Soltau, W. Schmidt
   D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
2002NCimC..25..689K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Weak magnetic flux features on the Sun
Authors: Hartj, B.; Kneer, F.
2002A&A...385..264H    Altcode:
  We observed weak magnetic flux in enhanced Ca K network and in its
  surroundings at various positions on the solar disc (cos theta
  =1.0dots0 .1). The Gregory Coudé Telescope at the Observatorio
  del Teide on Tenerife was used together with an image scanner and a
  Stokes V polarimeter. 1/2(I<SUB>lambda</SUB> +/- V<SUB>lambda</SUB>
  ) spectrograms in the 6302 Å region were taken. A second order flat
  fielding and smoothing brought the polarimetric sensitivity to a
  detection limit of 0.001 I<SUB>c</SUB> (with I<SUB>c</SUB> = continuum
  intensity) for exposure times of 300 ms in pixels corresponding to
  0.5”x0.19” (widehat = slit width x pixel size of detector). We present
  correlations between continuum intensity, line center intensity, and
  magnetic flux and discuss Stokes V profiles. The V amplitude asymmetry
  from areas with strong flux decreases towards the limb, a change of
  sign is not identified. The magnetic fields are ordered around strong
  flux centers with decreasing mean field strength outwards from the
  centers. In the range of 1x 10<SUP>16</SUP> Mx to 5x 10<SUP>16</SUP>
  Mx (in one pixel of 5x 10<SUP>14</SUP> cm<SUP>2</SUP>) the probability
  distribution of the magnetic flux is ~ |Phi |<SUP>-q</SUP> with q~3,
  different from a Gaussian distribution. From this we derive tentativily
  a radial dependence from the flux centers of the form Phi (r) ~
  1/r. For the probability distributions a significant center-to-limb
  variation is not detected. Together with the center-to-limb behaviour
  of the asymmetries, we interpret this tentatively as an isotropic
  orientation of the weak magnetic flux elements and of the gas flows. A
  vertical orientation of the magnetic fields is also compatible with
  the measurements, though.

---------------------------------------------------------
Title: Dynamics of small features in the solar chromosphere
Authors: Al, N.; Bendlin, C.; Kneer, F.
2002A&A...383..283A    Altcode:
  A spatially highly resolved time series of a quiet region at disk
  centre taken in the Na D_2 line with the “Göttingen” spectrometer
  in the Vacuum Tower Telescope/Tenerife was used to study the dynamic
  behaviour of different chromospheric features. Two CCDs were employed
  for simultaneous exposures of narrow-band and “white-light” images
  forming 128 repetitive “scans” of 30 images each in nearly two
  hours. The data set yielded line profiles over the two-dimensional field
  of view which served to create various maps indicating, e.g., vertical
  velocities or minimum line intensities. The analysis of the features of
  interest was further based on maps displaying the horizontal velocity
  field derived from white-light images. The occurrence of various
  bright points was monitored, showing quite different properties. Some
  persistent intra-network bright points follow the horizontal flows of
  the underlying photosphere. During the time series, a “K grain” is
  seen to occur several times at the same location. The power spectrum
  of the fluctuation of the minimum intensity of its corresponding Na
  D_2 mean line profile exhibits a pronounced absolute maximum for a
  period of about 24 min. The behaviour of several peculiar intra-network
  bright points still lacks an unequivocal interpretation. Possibly, the
  signature of gravity waves has been detected. Further investigation
  revealed that the power spectra of the fluctuation of Doppler shift
  and minimum intensity calculated for one of these bright points both
  show an absolute maximum for a period of 4.3 min. In the course of
  these fluctuations, time spans of regular oscillations were noticed
  for all such points lasting from about 15 to 25 min. Power spectra
  calculated only over these times exhibit pronounced absolute maxima
  in a rather narrow frequency range.

---------------------------------------------------------
Title: GREGOR: a 1.5 m telescope for solar research
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Berkefeld, Th.;
   Kneer, F.; Staude, J.
2001AN....322..353V    Altcode:
  GREGOR will be a new 1.5 meter solar telescope at the Teide Observatory,
  Izaña, Tenerife. It will provide observations with high polarimetric
  precision at visible and infra-red wavelengths from Earth in the
  solar photosphere and chromosphere with a resolution of 70 km on the
  Sun. GREGOR replaces the 45 cm Gregory Coudé telescope.

---------------------------------------------------------
Title: Speckle spectro-polarimetry of magnetic structures
Authors: Janssen, K.; Kneer, F.
2001AN....322..383J    Altcode:
  High spatial resolution images with spectral and polarimetric
  information were obtained with the Vacuum Tower Telescope (VTT),
  Tenerife, in order to study dynamic small scale structures on
  the Sun. With the Göttingen Fabry-Perot interferometer (FPI)
  we observed faculae at different ( cos , theta ) positions on the
  Sun. Simultaneously with these narrow-band images, 150 broad-band
  images were taken from which high resolution images are calculated
  with the spectral ratio and the speckle masking methods. With the aid
  of the transfer functions, gained with the speckle recontruction,
  the narrow-band FPI images can be reconstructed as well. Here our
  first results are presented.

---------------------------------------------------------
Title: Post-focus instrumentation for GREGOR
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
   Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
2001AN....322..361K    Altcode:
  GREGOR is a high-resolution solar telescope with an aperture of 1.5
  m. It will be equipped with an Adaptive Optics system and is designed
  for high-precision measurements of magnetic fields and plasma motions
  in the solar photosphere and chromosphere with a resolution of 70
  km on the Sun. GREGOR will replace the Gregory Coudé Telescope at
  the Observatorio del Teide on Tenerife. In concert with the other
  solar telescopes at Teide Observatory it will be useful for studying
  the dynamics of the solar atmosphere and the underlying physical
  processes. GREGOR will also serve as a test bed for next generation
  solar telescopes. We discuss briefly the postfocus instrumentation
  of GREGOR.

---------------------------------------------------------
Title: A Fabry-Perot spectrometer for high-resolution observation
    of the Sun
Authors: Kneer, F.; Hirzberger, H.
2001AN....322..375K    Altcode:
  Fabry-Perot interferometers (FPIs) are powerful instruments for
  spectro-polarimetry of the Sun with high spatial resolution. They
  allow easy image reconstruction of two-dimensional fields of view. Some
  examples of high quality results obtained with the “Göttingen” FPI
  spectrometer, mounted in the Vacuum Tower Telescope at the Observatorio
  del Teide/Tenerife, are presented in a poster to this workshop. We thus
  concentrate on the design of a new instrument for the 1.5 m GREGOR solar
  telescope. We discuss the pros and cons of telecentric and collimated
  mounting and describe the expected performance, especially the spectral
  resolution, of our design.

---------------------------------------------------------
Title: 2D-spectroscopy of the Evershed flow in sunspots
Authors: Hirzberger, J.; Kneer, F.
2001A&A...378.1078H    Altcode:
  The radial variation of the Evershed flow in two small sunspots (NOAA
  8737 and NOAA 9145) is studied by means of two-dimensional spectrograms
  of high spatial resolution. We find a systematic decrease of the
  flow velocity with photospheric height and a shift of the velocity
  maximum towards larger penumbral radii in higher layers but no clear
  correlation between flow velocity and continuum intensity. At the
  outer penumbral boundaries the Evershed flow ceases abruptly and even
  downward directed flow velocities in the deepest probed photospheric
  layers were found. Furthermore, granules adjacent to the penumbral
  boundary show a systematic redshift of their spot-side parts which is
  attributed to fast, eventually supersonic, downflows between them and
  the penumbral boundary.

---------------------------------------------------------
Title: GREGOR: high resolution solar observations from 1 AU
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.;
   Staude, J.; Pailer, N.
2001ESASP.493..417V    Altcode: 2001sefs.work..417V
  No abstract at ADS

---------------------------------------------------------
Title: High resolution 2D-spectroscopy of granular dynamics
Authors: Hirzberger, J.; Koschinsky, M.; Kneer, F.; Ritter, C.
2001A&A...367.1011H    Altcode:
  Spectroscopic data with high spatial resolution are used to study the
  granular dynamics of the Sun. The observations were obtained with the
  “Göttingen” two-dimensional (2D) Fabry-Perot interferometer in the
  Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. Time
  sequences of spectral scans across the non-magnetic Fe I 5576 Å
  line were taken from disc center. The 2D spectroscopic images were
  reconstructed with speckle methods, from which a spatial resolution
  of 0\farcs4-0\farcs5 was achieved. A power and coherence analysis of
  intensity and velocity maps from different photospheric heights has been
  carried out. The coherence between intensity and velocity fluctuations
  stays high for structural scales &gt;0\farcs5, which underlines the
  high spatial resolution of the data. Furthermore, the vertical flow
  field and its time evolution within exploding granules have been
  analyzed. We find fast downflows in the dark centers of exploding
  granules with velocities up to 1.2 km s<SUP>-1</SUP>. Additionally, we
  estimated the flow velocities of so-called “dark dots”. We discuss
  indications that these structures represent a new type of downflow
  within the centers of bright granules.

---------------------------------------------------------
Title: A Fabry-Perot Spectrometer for High-Resolution Observation
    of the Sun
Authors: Kneer, F.; Hirzberger, J.
2001AGM....18S1005K    Altcode:
  Fabry-Perot interferometers (FPIs) are powerful instruments for
  spectro-polarimetry of the Sun with high spatial resolution. They
  allow easy image reconstruction of two-dimensional narrow-band fields
  of view. Some examples of high quality results obtained with the
  “Göttingen” FPI spectrometer, mounted in the Vacuum Tower Telescope
  at the Observatorio del Teide/Tenerife, are presented in a poster to
  this workshop. We thus concentrate on the design of a new instrument
  for the 1.5 m GREGOR solar telescope. We discuss the pros and cons
  of telecentric and collimated mounting and describe the expected
  performance, especially the spectral resolution, of our design.

---------------------------------------------------------
Title: Speckle Spectro-Polarimetry of Small-Scale Magnetic Structures
Authors: Janßen, K.; Koschinsky, M.; Kneer, F.
2001ASPC..236..407J    Altcode: 2001aspt.conf..407J
  No abstract at ADS

---------------------------------------------------------
Title: GREGOR, a 1.5 m Gregory-type Telescope for Solar Observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
   Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
2001AGM....18.P223K    Altcode:
  GREGOR is a high-resolution solar telescope with an aperture of 1.5
  m. It will be equipped with an Adaptive Optics system and is designed
  for high-precision measurements of magnetic fields and plasma motions
  in the solar atmosphere and chromosphere with a resolution of 70 km
  on the Sun. GREGOR will replace the Gregory Coudé Teleskope at the
  Observatorio del Teide on Tenerife. We describe the optical design
  and the focal plane instrumentation. In concert with the other solar
  telescopes at Teide Observatory it will be useful for studying the
  dynamics of the solar atmosphere and the underlying processes. GREGOR
  will also serve as a test bed for next generation solar telescopes.

---------------------------------------------------------
Title: Speckle spectro-polarimetry of solar magnetic structures
Authors: Koschinsky, M.; Kneer, F.; Hirzberger, J.
2001A&A...365..588K    Altcode:
  We present speckle observations of small-scale magnetic structures
  on the Sun. They were obtained with the “Göttingen” Fabry-Perot
  interferometer (FPI) in the Vacuum Tower Telescope at the Observatorio
  del Teide, Tenerife, from quiet and active regions close to disc
  center. A Stokes V polarimeter was added to the FPI to measure
  V profiles in the Fe I 6302.5 Å line. The setup allows image
  reconstruction with speckle methods. The achieved spatial resolution
  in the magnetograms is 0farcs4 -0farcs5 . We describe the observational
  technique and the data reduction. The results from small-scale magnetic
  flux elements in a very quiet region, in an active region with pores
  and abnormal granulation, and in a sunspot with its surroundings
  are discussed. In the quiet Sun, granular dynamics dominate the
  time evolution of the magnetic elements. Flux occurs in both bright
  intergranular points and in dark intergranular spaces. Likewise,
  with the present spatial resolution, no preference of magnetic flux in
  abnormal granulation in an active region can be found. Flux occurs in
  both bright (abnormal) small-scale granules and in the darker spaces
  in between them. The small sunspot studied has very little magnetic
  flux in its ambient quiet regions, especially no strong, conspicuous
  concentrations of returned flux, i.e. of flux with polarity opposite
  to that in the sunspot.

---------------------------------------------------------
Title: Speckle Spectro-Polarimetry of Magnetic Structures
Authors: Janßen, K.; Kneer, F.
2001AGM....18.P222J    Altcode:
  High spatial resolution images with spectral and polarimetric
  information were obtained with the Vacuum Tower Telescope (VTT),
  Tenerife, in order to study dynamic small scale structures on the
  Sun. With the Göttingen Fabry-Perot interferometers (FPIs) we observed
  faculae at different cos θ positions on the Sun. Simultaneously with
  these narrow-band images, 150 broad-band images were taken from which
  high resolution images can be calculated with the Spectral Ratio
  and the Speckle Masking methods. Once these high resolution images
  are completed, the narrow-band FPI images can be reconstructed as
  well. Here we present our first results.

---------------------------------------------------------
Title: Granular velocities of the Sun from speckle interferometry
Authors: Krieg, J.; Kneer, F.; Koschinsky, M.; Ritter, C.
2000A&A...360.1157K    Altcode:
  We present observations of granular velocities and their relation to
  the granular intensity pattern from the disc center of the Sun. They
  were obtained in June 1997 with the Vacuum Tower Telescope at the
  Observatorio del Teide on Tenerife. High spatial resolution, 0”4-0”5
  for the velocities, was achieved with speckle methods applied to
  two-dimensional narrow- band images in Na D<SUB>2</SUB> from the
  "Göttingen" Fabry- Perot interferometer. The velocity observations
  refer to a geometric height of 50-200 km (above τ<SUB>5</SUB> =
  1). Velocity amplitudes of ±2.2 km s<SUP>-1</SUP> are seen. The high
  velocity regions are small-scale and the upflows coincide frequently
  with the bright borders of granules or with small-scale brightenings. A
  statistical analysis reveals only a rough consistency to the -5/3 or
  -17/3 law of log(power) vs. log(wavenumber) expected for isotropic
  turbulence (cf. Espagnet et al. 1993). We consider an agreement with
  such power laws as accidental since the intensity and velocity power
  spectra found here decrease smoothly from flat ones at low wavenumbers
  to steep ones at high wavenumbers. The coherence of velocity and
  intensity fluctuation stays above 0.5 up to horizontal wavenumbers
  k<SUB>h</SUB> ≍ 11 Mm<SUP>-1</SUP> 0.8 " and the phase difference
  between intensity and velocity stays stable down to structures of
  0”5. While the intensity pattern exhibits a clearly non-Gaussian
  distribution, the velocity distribution can be represented by a Gaussian
  with a "macroturbulent" velocity of 0.825 km s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Die Rotation der Sonne
Authors: Janßen, Katja; Kneer, Franz
2000A&R....58...40J    Altcode:
  Sonnenspektren hoher spektraler Auflösung sind vielfach begehrt. Sie
  werden im Unterricht, in Arbeitsgemeinschaften und in Vorlesungen
  verwendet. Anhand solcher Spektren lässt sich vieles aus dem wichtigen
  Gebiet der astronomischen Spektroskopie erklären und lernen.

---------------------------------------------------------
Title: On weak magnetic flux structures of the Sun
Authors: Stolpe, F.; Kneer, F.
2000A&A...353.1094S    Altcode:
  The micro-image-scanner of the Gregory Coudé Telescope at the
  Observatorio del Teide/Tenerife is used for observations of structures
  with weak magnetic flux in the solar atmosphere. The methods for
  the data reduction are chosen carefully to achieve the highest
  possible performance, also with respect to the needs of handling
  large amounts of data. A lower detection limit of magnetic flux on
  the Sun of 2 10<SUP>8</SUP> Wb on an area of 0\farcs67x0\farcs67 was
  achieved. Magnetic structures of weak magnetic flux are observed using
  different spectral lines in network and intra-network regions at disk
  center of the Sun. The present study deals mainly with a high spatial
  resolution observation in the magnetically sensitive Fe I 630.25
  nm line. We find that weak flux may appear almost everywhere in the
  granular pattern, with a 65 %\ preference for intergranular spaces. Some
  elements have diameters of 2\arcsec-3\arcsec with substructure and
  distorted shape. Many elements are very small, &lt;1\arcsec. From the
  ubiquitous appearance of weak magnetic flux structures in the granular
  pattern, in regions of both upward and downward flow, and in regions
  of line weakening and line enhancement, we argue that their magnetic
  pressure may not exceed the granular flow pressure. This gives an
  upper limit of the amplitude of the magnetic flux density of |B|=5.5
  10<SUP>-2</SUP> T in agreement with the result by Keller et al. (1994)
  <P />1 Wb = 1 V s = 10<SUP>8</SUP> Mx <P />1 T = 1 V s m<SUP>-2</SUP>
  = 10<SUP>4</SUP> Gauss.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1999.
Authors: Kneer, F.
2000MitAG..83..339K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: GREGOR: A 1.5m Telescope for Solar Research
Authors: von der Lühe, O.; Schmidt, Wolfgang; Soltau, Dirk; Kneer,
   Franz; Staude, Jurgen
2000ESASP.463..629V    Altcode: 2000sctc.proc..629V
  No abstract at ADS

---------------------------------------------------------
Title: High Resolution 2D-Spectroscopy of the Sun
Authors: Hirzberger, J.; Kneer, F.
2000HvaOB..24...89H    Altcode:
  With the "Goettingen" Fabry-Perot interferometer which is installed
  at the Vacuum Tower Telescope in Izana, Tenerife the non-magnetic
  FeI 5576 A line was scanned in a field of quiet granulation at solar
  disk center. From the simultaneously obtained broad band images an
  instantaneous optical transfer function (OTF) of the Earth's atmosphere
  was computed with speckle interferometric methods. Restoring the narrow
  band data from this OTF the spatial resolution of the two-dimensional
  spectra was successfully enhanced by a factor of two.

---------------------------------------------------------
Title: Coordinated Observation of a Small Magnetic Region with SUMER
    and GCT
Authors: Wilken, V.; Kneer, F.; Curdt, W.
1999ESASP.446..711W    Altcode: 1999soho....8..711W
  No abstract at ADS

---------------------------------------------------------
Title: Coordinated Observation Of A Small Magnetic Region With GCT
    And SUMER
Authors: Wilken, V.; Kneer, F.; Curdt, W.
1999ESASP.446..707W    Altcode: 1999soho....8..707W
  The SUMER instrument and the Gregory-Coude-Telescope (GCT) at the
  Observatorio del Teide/Tenerife have been used for observations
  of a small magnetic region in the solar atmosphere. With the GCT
  2-dimensional spectro-polarimetry was performed and later analysed
  by applying the "centre of gravity method" (Semel, 1967) to measure
  the longitudinal magnetic flux. In this way we obtained seven
  magnetograms of the target area during the three hours campaign,
  while SUMER performed every five minutes EUV-scans in two spectral
  lines. The magnetograms show two cells with diameters of approximately
  20000 km and 26000 km. Inside the cells the measured flux within the
  resolution element of 0.76 arcsec*0.76 arcsec. was ~1*10<SUP>9</SUP> Wb
  (corresponding to 33 Gauss flux density) on average. On the other hand
  the boundary regions have fluxes up to 1*10<SUP>10</SUP> Wb. SUMER's
  intensity images of Si II (Chromosphere) and C IV (Transition region)
  show strong brightenings which very well resemble the magnetic flux
  pattern in the Photosphere. The intensities of Ne VIII (lower Corona)
  correlate with the magnetic flux as well, but far less than the former
  two lines. During the 3h observing run the coarse magnetic structure
  remains essentially unchanged. However, on scales smaller than 3
  arcsec., the temporal intensity fluctuations of the EUV-lines show
  little similarity, if any. Possibly, at small scales the different
  EUV-lines originate from different structures which are not excited
  simultaneously.

---------------------------------------------------------
Title: Sonnenspektren für den Unterricht.
Authors: Janßen, K.; Kneer, F.
1999S&W....38..454J    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Height variation of the solar granulation
Authors: Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky, M.;
   Ritter, C.
1999A&A...343..983K    Altcode:
  We analyze spectral scans of narrow-band images across the Na
  D_2 line. They were obtained from disc centre of the Sun with our
  Fabry-Perot interferometer (FPI) in the Vacuum Tower Telescope at
  the Observatorio del Teide. The FPI was set to a bandwidth of 200
  mAngstroms FWHM and the spectral stepwidth was 100 mAngstroms. Our aim
  is to study the variation of the granulation pattern with height in
  the atmosphere. To achieve high spatial resolution the simultaneously
  taken broad-band images are restored with speckle methods. With the
  knowledge of these reconstructed images we were also able to restore
  the narrow-band images. The formation heights are found from temperature
  response functions RF_T. In the wings of NaD_2, the response functions
  for different wavelengths exhibit a substantial overlap in atmospheric
  heights. Therefore we use linear combinations which allow a better
  height discrimination. Applying the same combinations to the D_2 images
  we can visualize the height variation of the granular pattern. The
  granular intensity fluctuations are a matter of the deep photosphere
  alone. They disappear at heights of about 100 km (above tau_ {5000}=1).

---------------------------------------------------------
Title: Erratum: Two-dimensional spectroscopic observations of
    chromospheric oscillations
Authors: Al, Nurol; Bendlin, Cornelia; Kneer, Franz
1999A&A...342..311A    Altcode:
  For technical reasons, Figs. ref{f8} and ref{f9} were printed with
  incorrect colour separation. To rectify this unfortunate error, the
  correct Figs. ref{f8} and ref{f9} with the captions are reprinted below.

---------------------------------------------------------
Title: Solar Observation with High Spatial and Spectral Resolution
Authors: Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky, M.; Ritter,
   C.; von der Lühe, O.
1999AGAb...15...88K    Altcode: 1999AGM....15..P03K
  The solar atmosphere is, like stellar atmospheres in general,
  highly dynamic on small scales. To understand, e.g., the generation
  of turbulent motion and waves or chromospheric and coronal heating
  processes, observations with high resolution are needed. We achieve
  both high spatial and spectral resolution with a double Fabry-Perot
  interferometer which allows image restoration with speckle methods. Our
  new results include the height variation of the granular intensity
  pattern and velocity maps of unprecedented spatial resolution.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1998.
Authors: Kneer, F.
1999MitAG..82..329K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spectral and Spatial High Resolution Observations of the Sun
    with Image Reconstruction and Deconvolution Methods
Authors: Koschinsky, M.; Krieg, J.; Kneer, F.; Ritter, C.; Wunnenberg,
   M.
1999ASPC..183..349K    Altcode: 1999hrsp.conf..349K
  No abstract at ADS

---------------------------------------------------------
Title: GREGOR, a 1.5 M Solar Telescope
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.;
   Staude, J.
1999AGAb...15....5V    Altcode: 1999AGM....15..A01V
  We present the design of a new large solar telescope which is going to
  be installed at the Observatorio del Teide, in the tower that presently
  houses the 45cm Gregory-Coudé-Telescope. The new telescope has an
  aperture of 1.5 meters and its optical design is basically a Gregorian
  configuration. It will be an open telescope in an azimuthal mount. An
  adaptive optics system is incorporated in the optical design as well as
  a polarimetry package. The feasibility of lightweight optics for the
  primary mirror has been investigated in an industrial pre-study. The
  focal plane instrumentation will include a high resolution filter
  spectrometer similar to the existing TESOS instrument at the VTT and
  a new spectro-polarimeter for the visible and the near UV. The latter
  instrument is presently being developed jointly by the KIS and the
  High Altitude Observatory in Boulder, USA. Budget permitting, detailed
  (Phase-B) planning will start in 2000, and the telescope will be
  developed and built in 2002 and 2003 with first light in spring of 2004.

---------------------------------------------------------
Title: Granular Intensities and Velocities with High Spatial
    Resolution
Authors: Ritter, C.; Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky,
   M.
1999AGAb...15....7R    Altcode: 1999AGM....15..A04R
  We present results obtained with our two-dimensional spectrometer in the
  solar Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The
  spectrometer is based on Fabry-Perot interferometers which scan in
  wavelength across spectral lines. The observations were taken in the
  Na D_2 line from a quiet region at the disc centre of the Sun. The
  data analysis uses speckle methods for image restoration. We find
  the heights in the solar atmosphere where the measured intensity
  and velocity fluctuations originate from Response Functions. The
  granular intensity pattern disappears at 100 - 150 km (above tau_5 =
  1). The granular velocities are high with amplitudes of 2.5 km s^{-1},
  while the rms velocity is 0.6 km s^{-1}. Thus, the velocity field is
  highly intermittent.

---------------------------------------------------------
Title: Coordinated Observation of a Small Magnetic Region with SUMER
    and GCT
Authors: Wilken, V.; Kneer, F.; Curdt, W.
1999AGAb...15...91W    Altcode: 1999AGM....15..P08W
  The SUMER instrument and the Gregory-Coudé-Telescope (GCT) at the
  Observatorio del Teide/Tenerife have been used for observations of
  a small magnetic region in the solar atmosphere. The GCT measured
  the longitudinal magnetic flux, while SUMER performed EUV-scans. The
  magnetograms show two cells with diameters of approximately 23 000
  km. Inside the cells the flux within the resolution element of 0.”76 x
  0.”76 was 1 ṡ 10^9 Wb (corresponding to 33 Gauss flux density). The
  boundary regions have fluxes up to 1 ṡ 10<SUP>10</SUP> Wb. SUMER's
  intensity images of Si II (Chromosphere) and C IV (Transition Region)
  show strong brightenings which very well resemble the magnetic flux
  pattern in the Photosphere. The intensities of Ne VIII (lower Corona)
  correlate with the magnetic flux far less than the former two lines. On
  small scales the temporal intensity fluctuations of the EUV lines show
  only little similarity among each other. Possibly, at small scales the
  different EUV lines originate from different structures which are not
  excited simultaneously.

---------------------------------------------------------
Title: Solar Observation with high spectral resolution and image
    restoration.
Authors: Koschinsky, M.; Kneer, F.; Wunnenberg, M.; Krieg, J.;
   Ritter, C.
1999AGAb...15....6K    Altcode: 1999AGM....15..A02K
  Observations of the Sun with both, high spectral and high spatial
  resolution are needed for an understanding of the structure, the
  dynamics and the temporal evolution of small scale features in the
  solar photosphere. To achieve this, high technical requirements on
  the optical setup have to be satisfied, and also extensive methods
  of data reduction and image restoration are needed. We present our
  approach to obtain high resolution observational data by using a
  double Fabry-Perot interferometer at the Vacuum Tower Telescope (VTT)
  on Tenerife. The spectral resolution is about 30 mÅ. Different methods
  of image reconstruction to remove distortions by seeing and telescopic
  aberrations to a high extent are presented as well.

---------------------------------------------------------
Title: Fine structure of polar faculae.
Authors: Makarov, V. I.; Okunev, O. V.; Pravdjuk, L. M.; Kneer, F.
1999joso.proc..151M    Altcode:
  Using photoheliograms with high spatial resolution (SSSO) it was found
  that polar faculae consists of a few fine structure elements with the
  sizes ranging from 200 km to 400 km (the telescopic resolution limit
  is about 180 km). Presumably, polar faculae are caused by magnetic
  flux concentrations and their shape reminds of the set of flux tubes.

---------------------------------------------------------
Title: Some remarkable bright points seen in the Na D<SUB>2</SUB>
    line.
Authors: Al, N.; Bendlin, C.; Kneer, F.
1999joso.proc..133A    Altcode:
  A spatially highly resolved time series of the solar chromosphere
  was taken with a two-dimensional spectrometer mounted in the German
  Vacuum Tower Telescope (VTT) on Tenerife. The authors obtained
  narrow-band filtergrams (Δλ = 30 mÅ) and white-light images strictly
  simultaneously with two CCDs. Repetitive tuning of a Fabry-Perot
  interferometer allowed to scan through the Na D<SUB>2</SUB> line (5890
  Å) in short intervals. In this contribution, the authors focus on
  bright points in the intra-network using different diagnostic tools
  to study their temporal and oscillatory behaviour.

---------------------------------------------------------
Title: Diagnostics and Dynamics of the Solar Chromosphere
Authors: Kneer, F.; von Uexküll, M.
1999ASSL..239...99K    Altcode: 1999msa..proc...99K
  Research of the chromosphere of the Sun is exciting, as it has been over
  more than a century. The present contribution can only give glimpses
  into the wealth of chromospheric structures and dynamics. Likewise, in
  view of the limited space, it is not possible to present in due balance
  the published work on the solar chromosphere. The reader is referred to
  the monographs by Bray and Loughhead (1974, with a historical account
  and many references to early work) and by Athay (1976) for the state
  of knowledge two decades ago. Among the conferences dealing meanwhile
  with the chromosphere we mention the proceedings edited by Bonnet and
  Delache (1976) and by Ulmschneider et al. (1991). Withbroe and Noyes
  (1977) treated the mass and energy flow in the chromosphere and corona;
  a throughout account of the Ca II K<SUB>2v</SUB> cell grains is given
  by Rutten and Uitenbroek (1991); the review by Narain and Ulmschneider
  (1996) deals with chromospheric and coronal heating mechanisms. Last
  but not least, the book “Le Soleil” by Secchi (1877) is historically
  precious and full of still viable ideas.

---------------------------------------------------------
Title: Two-dimensional spectroscopic observations of chromospheric
    oscillations
Authors: Al, Nurol; Bendlin, Cornelia; Kneer, Franz
1998A&A...336..743A    Altcode:
  The oscillatory behaviour of the solar chromosphere was studied
  from observations of a quiet region at disk centre using various
  diagnostic tools. The two-dimensional spectrometer in the Vacuum Tower
  Telescope/Tenerife (Spain) served to obtain a spatially highly resolved
  time series of “white-light” images and narrow-band filtergrams in
  the Na D_2 line. With a tunable Fabry-Perot interferometer, this line
  was scanned taking 30 images (i.e. a “scan”) around the line core
  with wavelength steps of 30 m Angstroms and a spectral resolution
  of about 200 000. From these images, line profiles were derived for
  every pixel in the field of view. With each such narrow-band scan, a
  scan of “white-light” images was taken strictly simultaneously. The
  whole time series comprises (2x) 128 scans. Every 56 s, a new pair of
  scans was started with two CCDs, thus the observation covers nearly
  two hours. Finally, after correlation and other reduction procedures,
  a field size of 69\farcs4 x 50\farcs4 remained with 0\farcs2/pixel
  on the CCD-chips. In the data reduction, new images were created
  representing the minimum intensity (I) of each line profile in the
  field of view, and also velocity (V) maps (derived from the Doppler
  shifts of the line profiles) for all 128 scans. &gt;From these images,
  power spectra and diagnostic diagrams were computed. In the subsequent
  analysis, a distinction between network and intra-network regions was
  made where this seemed appropriate. One- and two-dimensional (V-I) phase
  and coherence spectra were analysed with regard to oscillations and to
  the nature of the waves leaving their marks in these diagrams. Several
  noteworthy results also raised the question of the actual line formation
  height of Na D_2, among them being the non-detection of a chromospheric
  eigenmode. While an explanation for a conspicuous 70(deg) plateau in a
  small region of the phase spectra already exists, the suspected reason
  behind the decreasing phase difference from about -60(deg) for the
  f-mode down to ~ -120(deg) for higher modes is still subject to some
  speculation. Moreover, the data gave evidence of gravity waves, probably
  discovered for the first time in a V-I phase spectrum of Na D_2.

---------------------------------------------------------
Title: MISC, an instrument for multi-dimensional spectroscopy
Authors: Stolpe, F.; Kneer, F.
1998A&AS..131..181S    Altcode:
  A new instrument for multi-dimensional spectroscopy at the Gregory
  CoudeTelescope (GCT) at the Observatorio del Teide/Tenerife is
  presented. The system, consisting of a scanning device, a Stokes-V
  polarimeter and a suitable CCD system, performs fast spatial
  scanning. It preserves the good polarization properties of the
  telescope by using a Bowen compensator. We describe the instrument,
  the different modes of operation, and how it is controlled and give
  some performance data.

---------------------------------------------------------
Title: Oscillations of the Sun's chromosphere. VIII. Horizontal
    motions of CA II K bright points
Authors: Wellstein, S.; Kneer, F.; von Uexkuell, M.
1998A&A...335..323W    Altcode:
  We present a re-analysis of a time series of solar disc centre Ca ii
  K<SUB>2v</SUB> filtergrams taken with the Vacuum Tower Telescope at the
  Observatorio del Teide/Tenerife. We concentrate on the measurements of
  proper motions of K grains in the internetwork regions and of bright
  points in the chromospheric network. For the K grains we find horizontal
  velocities of 2-15 km s(-1) , values much lower than those deduced by
  Steffens et al. (1996) from a smaller sample, analyzed differently. In
  accord with our earlier conclusion from k-omega diagrams (Kneer &amp;
  von Uexkuell 1993) and with numerical simulations by Carlsson &amp;
  Stein (1997) high-frequency (pseudo-) p-modes can viably explain the
  K grains. Yet, the rareness of the K grains may indicate a connection
  to magnetic fields. The proper motions of the network bright points
  are non-periodic, very impulsive, with velocities of 7-10 km s(-1)
  . Estimating the energy flux if these motions are magnetic kink waves
  (cf. Choudhuri et al. 1993, Muller et al. 1994), we find it sufficient
  to heat the solar corona, but too small to cover the radiative losses
  of the chromospheric network.

---------------------------------------------------------
Title: Spectral line radiation from solar small-scale flux tubes. II
Authors: Hasan, S. S.; Kneer, F.; Kalkofen, W.
1998A&A...332.1064H    Altcode:
  We examine spectral line radiation from small-scale magnetic flux tubes
  in the solar atmosphere. This is a continuation of work by Kneer et
  al. (1996). The main difference with the previous investigation is in
  the choice of the external atmosphere. Earlier we adopted an atmosphere
  resembling the empirical quiet Sun model for the ambient medium. In the
  present study, we iteratively adjust the temperature structure of the
  external atmosphere to fit the Stokes I and V profiles and the average
  continuum intensities with those obtained from observations. Our
  models are hotter in the uppermost photospheric layers and cooler
  in the deeper layers than the quiet Sun model and agree well with
  semi-empirical flux tube models.

---------------------------------------------------------
Title: Ground-based Observation with High Spatial and Spectral
    Resolution
Authors: Krieg, J.; Kneer, F.; Koschinsky, M.; Ritter, C.; Starck,
   J. -L.
1998ESASP.417..317K    Altcode: 1998cesh.conf..317K
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of Solar Fine Structure: Observation with High
    Spatial Resolution
Authors: Al, N.; Bendlin, C.; de Boer, C. R.; Denker, C.; Kneer, F.;
   Schmitt, D.; Volkmer, R.; Wilken, V.
1998ASPC..154..553A    Altcode: 1998csss...10..553A
  The Sun is an ideal object for studying non-magnetic and magnetic
  processes in cool stars. Here, we focus on fine structures of a
  few 100 km in the solar atmosphere. Granular overshoot, motions
  and waves of magnetic elements in the quiet Sun and in plages,
  etc., all affect the atmospheric structure from the bottom of the
  photosphere up to the corona and the solar wind. Observations with
  high spatial resolution are required to reveal the dynamic behaviour
  and to understand the underlying physical processes. During the past
  five years, speckle methods have become an excellent tool to obtain
  images of solar fine structure with diffraction-limited resolution. We
  demonstrate by some examples how one can gain new insights from
  speckle interferometry. Likewise, spectroscopy of solar fine structure
  is also making rapid progress towards high spatial resolution. Our
  two-dimensional, narrow-band spectrometer (Delta\lambda = 20-30 mAA
  ), working with a scanning Fabry-Perot interferometer, proves very
  powerful in several aspects. We present some results obtained in Na D_2
  from the quiet solar chromosphere. Using suitable observing techniques
  together with image restoration, we aim at achieving diffraction-limited
  resolution also for narrow-band spectroscopy.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1997.
Authors: Kneer, F.
1998MitAG..81..287K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spectro-Polarimetry of Polar Faculae
Authors: Homann, T.; Kneer, F.; Makarov, V. I.
1997SoPh..175...81H    Altcode:
  This contribution deals with the properties of small-scale
  magnetic elements at the polar caps of the Sun. Spectro-polarimetric
  observations, obtained with high spatial resolution with the Gregory
  Coudé Telescope at the Observatorio del Teide on Tenerife, were
  analysed. We find, though with limited data sets, that polar faculae
  differ in two aspects from faculae of the network in non-active
  regions near the equator (equatorial faculae): (1) Polar faculae
  appear to have the same magnetic polarity as the general polar magnetic
  field. Presumably, the latter is rooted in the small-scale faculae. The
  equatorial faculae show both magnetic polarities. (2) Polar faculae,
  with a size of 3.5” ± 1.3”, are larger than equatorial faculae with
  2.1” ± 0.4”. Yet as for equatorial faculae, polar faculae possess
  kilogauss magnetic fields.

---------------------------------------------------------
Title: Speckle measurements of the centre-to-limb variation of the
    solar granulation.
Authors: Wilken, V.; de Boer, C. R.; Denker, C.; Kneer, F.
1997A&A...325..819W    Altcode:
  The Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife
  was used to perform speckle observations of photospheric granulation
  from disc centre to limb at λ=550+/-5nm. Images were reconstructed
  with the spectral ratio technique (von der Luehe 1984) and the
  speckle masking method (Weigelt 1977, Optics Comm. 21, 55, Weigelt
  &amp; Wirnitzer 1983, Optics Letters 8, 389, Lohmann et al. 1983,
  Applied Optics 22, 4028). The granular rms contrast relative to the
  local average intensity decreases monotonically from 13.5+/-1.0%
  at disc centre to 8-9% at cosθ=0.1. The granular images as well as
  the power spectra of the intensity fluctuations show a fading of the
  coarse granular pattern towards the limb while smaller structures
  become more pronounced off the disc centre. We identify these as the
  bright worm-like structures appearing sometimes at the granular borders
  (de Boer et al. 1992A&amp;A...257L...4D). This finding supports the
  result from simulations of granular dynamics by Steffen et al. (1994,
  Shocks in the solar photosphere and their spectroscopic signature. In:
  Schuessler M., Schmidt W. (eds.) Solar Magnetic Fields, Cambridge
  Univ. Press, p. 298). There the bright structures are locations of low
  pressure between the centres of granules and intergranular spaces with
  penetration of hot gas into the lower photosphere.

---------------------------------------------------------
Title: Flare Multi-Line 2D-SPECTROSCOPY
Authors: Mein, P.; Mein, N.; Malherbe, J. -M.; Heinzel, P.; Kneer,
   F.; von Uexkull, M.; Staiger, J.
1997SoPh..172..161M    Altcode: 1997ESPM....8..161M
  A small flare was observed at the Teide Observatory on October 5,
  1994. Simultaneous data were obtained at the German Vacuum Tower
  Telescope (VTT) with the MSDP spectrograph providing high-resolution
  imaging spectroscopy in two chromospheric lines, and the Gregory Coudé
  Telescope (GCT) providing information about the magnetic field. Basic
  flare characteristics are:

---------------------------------------------------------
Title: High spatial-resolution spectropolarimetry of small-scale
    solar magnetic fields.
Authors: Stolpe, F.; Kneer, F.
1997A&A...317..942S    Altcode:
  We present a study of the structure and the dynamics of small-scale
  magnetic elements within solar plage regions. Spectropolarimetric
  observations with high spatial and spectral resolutions have been
  obtained for three iron lines with different sensitivities to magnetic
  field strength and temperature with the Gregory Coud e Telescope at
  the Observatorio del Teide/Tenerife. The main results are: (o) With
  the present spatial resolution of 0.6-1.0", the continuum intensity
  from magnetic areas is close to the average intensity of the quiet
  Sun (+/-5%). There is a slight tendency of decreasing intensity with
  increasing magnetic flux within the resolution elements. (o) Macroscopic
  velocities with rms values of 1.5-2.0km/s as used to explain the widths
  of the Stokes V profiles are not seen with the spatial resolution
  obtained here. (o) The large observed fluctuations in the wavelength
  separation of the red and blue Stokes V maxima suggest that there may
  be a considerable variation in the properties of the flux tubes. This
  should give constraints for modeling small-scale flux tubes.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1996.
Authors: Kneer, F.
1997MitAG..80..257K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: 2d-Spectroscopy with the Image Scanner of the
    Gregory-Coude-Telescope on Tenerife
Authors: Stolpe, F.; Koschinsky, M.; Kneer, F.
1997ExA.....7..301S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Methods of Two-Dimensional Spectroscopy
Authors: Kneer, F.
1997ASPC..118..329K    Altcode: 1997fasp.conf..329K
  One of the main fields of solar research is the study of dynamic
  processes of small-scale structures. For this purpose, time sequences of
  spectroscopic and polarimetric information in two spatial dimensions
  with best achievable quality are needed. The present contribution
  deals with the ways to obtain images in small wavelength bands. Among
  these are image scanners and the MSDP (Multi-Channel Subtractive
  Double Pass Spectrograph). Further potential instruments are scanning
  Fabry-Perot interferometers (FPI). The principles of such instruments
  are discussed. The results obtained hitherto from the FPI in the Vacuum
  Tower Telescope at the Observatorio del Teide are promising. Small-band,
  two-dimensional spectroscopy with spatial resolution close to the
  telescopic diffraction limit seems possible in the near future.

---------------------------------------------------------
Title: Small-scale magnetic fields on the Sun.
Authors: Kneer, F.
1996NAWG.1996..181K    Altcode:
  Spectro-polarimetric observations with high spatial resolution of
  small-scale magnetic fields are presented. They were taken from plages
  near disc centre of the Sun with the Gregory Coudé Telescope at the
  Observatorio del Teide/Tenerife. Stokes I and V profiles calculated from
  static flux tubes are compared with the observations. The small-scale
  magnetic elements have substructure and are dynamic. They possess
  varying gas velocities and temporarily strong magnetic fields.

---------------------------------------------------------
Title: On the use of Bowen compensators for polarimetry.
Authors: Koschinsky, M.; Kneer, F.
1996A&AS..119..171K    Altcode:
  For high precision polarimetry it is very desirable to observe with a
  telescope that is free of instrumental polarization. Yet it is often
  inevitable to introduce optical devices which generate crosstalk between
  the Stokes Q,U components and the V component due to phase retardations
  upon reflection at mirror surfaces. We show that Bowen compensators
  are well suited to compensate for these phase changes. They consist
  of 2 rotatable λ/8 phase retarder plates, and it can be shown that
  they act on polarized light either by a rotation of the axes of the
  polarization ellipse followed by a prescribed, desired phase change
  or by a phase change followed by rotation of the ellipse. We present
  applications of Bowen compensators in the Gregory Coude Telescope at
  the Observatorio del Teide/Tenerife.

---------------------------------------------------------
Title: High Resolution Observations of Small-Scale Magnetic Elements
    and Interpretation
Authors: Kneer, F.; Stolpe, F.
1996SoPh..164..303K    Altcode:
  This contribution deals with the properties of small-scale magnetic
  elements in plages. Spectro-polarimetric observations, obtained with the
  highest possible spatial resolution with the German solar telescopes at
  the Observatorio del Teide on Tenerife, were analysed. We conclude from
  the spread of line parameters measured in the Stokes I and V profiles
  of Fe I and Fe II lines that a wide range of magnetic properties is
  realised in the solar atmosphere. The flow velocities in small-scale
  magnetic flux tubes, deduced from the zero-crossing of the V profiles
  at high spatial resolution, show a fluctuation of v<SUB>Doppler</SUB>
  = 580 m s<SUP>-1</SUP>. This is substantially smaller than the
  "turbulent" broadening velocities of v<SUB>Doppler</SUB> = 2 - 3 km
  s<SUP>−1</SUP> commonly derived by fitting V profiles from flux tube
  models to low spatial resolution data, e.g. from a Fourier Transform
  Spectrometer. Attempts to explain the high resolution I and V profiles
  by models of hydrostatic flux tubes are discussed. It appears impossible
  to accomplish agreement between the modeled and observed radiation of
  lines with strong and weak magnetic sensitivity at the same time. We
  suggest a scenario in which small-scale magnetic elements possess
  substructure and are dynamic, with gas flows and magnetic field
  strengths varying in space and time.

---------------------------------------------------------
Title: Spectral line radiation from solar small-scale magnetic
    flux tubes.
Authors: Kneer, F.; Hasan, S. S.; Kalkofen, W.
1996A&A...305..660K    Altcode:
  We consider spectral line radiation from small-scale magnetic model flux
  tubes in the solar atmosphere. The structure of the tube is determined
  from the magnetostatic equations in the thin flux tube approximation. We
  assume that the tube is in energy equilibrium and pressure balance
  with the ambient medium. For the latter, we construct a quiet sun model
  with an artificial heating term in order to reproduce the VAL C model,
  treating the medium as a plane-parallel atmosphere. The flux tube models
  are parameterized by the plasma β_0_ (the ratio of gas the pressure
  to the magnetic pressure), the convective efficiency parameter α,
  and the radius R_0_ at height z=0 (τ_5000_=1) in the quiet sun. The
  Stokes I and V profiles emerging from the models and averaged over
  areas that include the neighbourhood of the flux tube are calculated
  for various spectral lines with different sensitivity for magnetic
  field strength and temperature. The profiles are compared with high
  spatial resolution observations of plages near disc centre that have
  been obtained with the Gregory Coude Telescope at the Observatorio del
  Teide/Tenerife. The information contained in both I and V profiles is
  found to be very useful in constraining the theoretical models. The
  best match of models with observations is achieved for values of β_0_
  between 0.3 and 0.5. For a sufficiently wide separation of the V extrema
  of the strongly split lines, a broadening mechanism is required. Pure
  velocity (microturbulent) broadening compatible with observations
  of strongly split lines gives too much broadening for weakly split
  lines. A broadening that is proportional to the Lande factor, i.e.,
  magnetic broadening, appears to be more appropriate. This suggests
  dynamic models with temporary enhancement of the magnetic field
  strength. The continuum intensity of our models is higher and the
  absorption and V amplitude in the FeII 6149A line are stronger than
  observed. An improvement in the match between model predictions and
  observations is likely to come from models in which the ambient gas
  has a lower temperature as well as a lower temperature gradient than
  are found in the quiet, field-free sun. Such models are currently
  under development for cylindrical flux tubes.

---------------------------------------------------------
Title: Short-period waves in small-scale magnetic flux tubes on
    the Sun.
Authors: Volkmer, R.; Kneer, F.; Bendlin, C.
1995A&A...304L...1V    Altcode:
  We present a time sequence of spectro-polarimetric measurements
  of high spatial and spectral resolution. It was obtained in the
  Fe I 6302A line from a moderately active region near disc centre
  of the Sun. The observation was performed with a two-dimensional
  spectrometer which uses a Fabry-Perot interferometer for wavelength
  scanning and to which a Stokes-V polarimeter was added. This allows
  us to follow horizontal motions of structures, here of small-scale
  magnetic elements, to study structural changes, and to measure vertical
  velocities of the magnetized plasma. In an isolated magnetic feature
  we find quasi-periodic horizontal motions with 2km/s amplitude and,
  with high statistical significance, vertical oscillations of 280m/s
  rms velocity and with periods near 100s. The energy flux in these
  short-period waves is estimated at 1.6-2.3x10^7^ergxcm-2/s, yet only for
  this isolated small feature. So the energy supply appears sufficient
  for chromospheric heating in bright structures of the Ca network,
  but it falls much below the needs to balance the radiative loss in
  larger plage areas.

---------------------------------------------------------
Title: A proposal for a low instrumental polarization coude
    telescope. II. The German Gregory-Coude Telescope at the Observatorio
    del Teide.
Authors: Sanchez Almeida, J.; Martinez Pillet, V.; Kneer, F.
1995A&AS..113..359S    Altcode:
  We have put into practice the technique to minimize the instrumental
  polarization (IP) of coude telescopes proposed by Martinez Pillet &amp;
  Sanchez Almeida (1991): a λ/2-plate inserted into the optical path,
  with the proper orientation, cancels the IP. The compensation of the
  Gregory-Coude Telescope at the Observatorio del Teide turns out to
  fulfil theoretical expectations. Empirical tests at 630nm demonstrate
  that its IP decreases by a factor ~4. We show that the residual IP is
  not intrinsic to the method but it is due to the limited precision of
  the retarder presently used. In addition, observations indicate that
  the insertion of the λ/2-plate does not noticeably deteriorate the
  optical quality of the whole telescope. In short, this work proves
  the practical soundness of the λ/2-plate technique to reduce IP.

---------------------------------------------------------
Title: Speckle masking imaging of the moustache phenomenon.
Authors: Denker, C.; de Boer, C. R.; Volkmer, R.; Kneer, F.
1995A&A...296..567D    Altcode:
  We present high spatial resolution observations of a sunspot near
  the solar disc centre obtained with the Vacuum Tower Telescope at the
  Observatorio del Teide (Tenerife). Our investigation aims at a better
  comprehension of umbral and penumbral fine structures. The speckle
  masking image reconstruction technique was applied to narrow-band
  filtergrams taken in the red wing of Hα. The reconstructed images
  reveal small-scale structures close to the telescopic diffraction
  limit of 0.19". Especially, the moustache phenomenon - the intensity
  enhancement in the inner wings of strong chromospheric absorbtion lines
  - is clearly discernible. The moustaches are concentrated at the outer
  border of the penumbra and in its neighbourhood. There, they appear
  at locations coinciding with the intergranular lanes. The moustaches
  are not circular in shape. Instead they show sharp intensity peaks
  surrounded by bright areas with frayed borders. Due to the high spatial
  resolution achieved in the restored filtergrams and the correction for
  the speckle transfer function, we find much higher intensities than
  in previous investigations concerning the moustache phenomenon. The
  peak intensities are sometimes larger than 1.7 times the intensity
  of the granular background, i.e. the line profiles in the inner wing
  of Hα exhibit emission features. Attempts to restore narrow-band
  filtergrams with speckle interferometry are relatively new. In this
  paper it is shown that the speckle masking technique is capable to
  reconstruct images with a low signal-to-noise ratio taken within a
  passband of 0.05nm.

---------------------------------------------------------
Title: Oscillations of the Sun's chromosphere. VII. K grains
    revisited.
Authors: von Uexkuell, M.; Kneer, F.
1995A&A...294..252V    Altcode:
  We analyse time sequences of high spatial resolution filtergrams
  obtained simultaneously in Mg b_2_ and Ca K_2v_ from quiet Sun disc
  centre with the Vacuum Tower Telescope at Observatorio del Teide,
  Tenerife. Special attention is paid to the temporal evolution of
  the bright points, or K grains, in the interior of the chromospheric
  network. These represent the intensity maxima of brightness oscillations
  in K_2v_ with large amplitude. The oscillations last 1 to 5 periods
  of 150-240s duration and are strongly non-sinusoidal. They do not
  appear to be excited by pulses from below and then to decay. They
  rather exhibit the beat phenomenon of a horizontally extended wave
  field. The high amplitude oscillations occur only rarely, during 5-10%
  of the time, if we pose the limit that high amplitude means that the
  maximum intensity is a factor 1.5 larger than the average intensity. The
  power spectrum does not show any signature of a chromospheric "3 min"
  mode nor of a mode at the acoustic cutoff frequency (period 3.5min),
  nor do we find a power ridge at constant frequency. Instead, as in
  our earlier finding, the chromospheric oscillations are ordered in
  modes in continuation of the 5min modes. Modeling will thus need to
  adopt that the waves propagate in a three-dimensional medium with,
  at least partially, reflecting boundaries. The bright points of the
  chromospheric network behave very differently, more chaotic, like
  noise. They show most power at low frequencies which we interpret as
  the consequence of the stochastic intensity fluctuations.

---------------------------------------------------------
Title: Speckle-masking imaging of bright points (moustaches)
Authors: Denker, C.; Deboer, C. R.; Kneer, F.
1995IAUS..176P..51D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Wellen in der Sonnenchromosphäre.
Authors: Kneer, F.; von Uexküll, M.
1994S&W....33..871K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the fluctuation of wing strengths as diagnostics of the
    solar atmosphere
Authors: Kneer, F.; Nolte, U.
1994A&A...286..309K    Altcode:
  We investigate the feasibility to obtain information on pressure
  fluctuations in the solar atmosphere from the collisionally dominated
  damping wings of strong Fraunhofer lines. One expects that the
  depression in the wings of lines from neutral atoms of predominantly
  ionised species, as Na, Fe, Mg, is proportional to the pressure. Thus
  we measured the fluctuations of continuum intensity and wing strength
  of Na D_2_ and Mg b_2_ from high spatial resolution observations
  obtained with the Gregory Coude Telescope at the Observatorio del
  Teide/Tenerife. Strong fluctuations of the wing strengths are indeed
  seen on granular scales. They are uncorrelated with the continuum
  brightness. Modelling of atmospheric temperature and pressure
  fluctuations, including response functions for intensity and wing
  strength, shows that temperature fluctuations have larger effects, by
  factors 5-6, than pressure perturbations upon both continuum brightness
  and wing strength. With precise measurements of temperature fluctuations
  and analysing the wing strengths of several lines which differ in
  their pressure dependence the detection of pressure fluctuations will
  become possible.

---------------------------------------------------------
Title: Speckle observations of solar granulation
Authors: de Boer, C. R.; Kneer, F.
1994IAUS..158..398D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spectro-polarimetry of magnetic elements with high spatial
    resolution
Authors: Kneer, F.; Amer, M. A.
1994smf..conf..319K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Oscillations of the Sun's chromosphere. VI. K grains,
    resonances, and gravity waves.
Authors: Kneer, F.; von Uexkuell, M.
1993A&A...274..584K    Altcode:
  We present observations of simultaneous filtergram time sequences in
  Mg b<SUB>2</SUB>, Ca K and Hα obtained from quiet Sun disc centre with
  the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Fourier
  analyses are performed to obtain power, coherence and phase spectra in
  the k-ω plane. There, the dominant features are the wellknown ridges
  of the 5 min resonant modes. Yet in the chromosphere the ridges extend
  to high wavenumbers (wavelengths ≍ 1.3 Mm) and to high frequencies
  (periods ≍ 105 s). Neither the famous chromospheric "3 min"
  oscillations nor an oscillation at the acoustic cutoff frequency
  (period 210 s) appear exceptionally pronounced. The signature of
  gravity waves is indicated from phase relations. <P />We distinguish
  between the behaviour in the interior of the chromospheric network and
  on the boundary. The network boundary behaves less oscillatory than
  the interior. In snapshots of chromospheric intensities the K grains
  (Beckers 1964), or, synonymously the bright cell points, appear in
  the cell interior. They represent the phases of high temperature of
  a wave field with partly resonant and coherent properties. (We take
  intensity fluctuations as proxies for temperature fluctuations.) The
  waves are only partly upward propagating p-modes with a multitude of
  eigenvalues in frequency and wavenumber, like the subphotospheric
  p-modes. We suggest that an excitation mechanism acts within the
  chromosphere itself to drive the waves. This could explain the phase
  relations between intensity and velocity oscillations.

---------------------------------------------------------
Title: High spatial resolution spectro-polarimetry of small-scale
    magnetic elements on the Sun
Authors: Amer, M. A.; Kneer, F.
1993A&A...273..304A    Altcode:
  Spectro-polarimetric observations from plage regions on the Sun,
  obtained with the Gregory Coudé Telescope at Observatorio del
  Teide/Tenerife, were analysed. They consist of high spatial resolution
  data recorded with a Stokes V polarimeter and a CCD detector from
  the magnetically sensitive lines Fe II 6149 (g<SUB>eff</SUB> =
  1.33) and Fe I 6151 (g<SUB>eff</SUB> = 1.83) simultaneously with the
  non-magnetic Fe I 5576 line and from Fe I 6173 (g<SUB>eff</SUB> = 2.5)
  also simultaneously with Fe I 5576. <P />The average asymmetries of
  the V profiles are in agreement with previous measurements obtained
  with low spatial resolution, and the average wavelength positions
  of the zero crossings of the V profiles are consistent with the
  picture of zero average velocity within magnetic fluxtubes. Within the
  magnetic features and from one feature to the next, there exist strong
  fluctuations of the asymmetries of the emergent V profiles and of the
  velocities. The latter can be as large as 1 km s<SUP>-1</SUP>. Yet
  neither a correlation nor an anticorrelation between asymmetry and flow
  can be found. Siphon flows may occur across the borders of regions with
  opposite polarity. <P />On some locations with high V signal we see very
  strong line weakenings and suggest that there the area filling factor of
  the magnetic elements amounts to 30 - 50 %. At a height of about 200 km
  the tubes merge and loose their identity. The magnetic field strengths
  can be derived from the separation of the extrema of the V profiles of
  Fe I 6173. They are large, about 2500 gauss (at τ<SUB>5</SUB> = 1 in
  the tube). The V profiles observed with high spatial resolution do not
  seem to require high non-thermal, micro- or macroturbulent broadening.

---------------------------------------------------------
Title: On the photospheric temperature in small-scale magnetic flux
    concentrations
Authors: Fabiani Bendicho, P.; Kneer, F.; Trujillo Bueno, J.
1992A&A...264..229F    Altcode:
  Results are presented of 2D radiative transfer calculations performed
  for geometric configurations that simulate partly evacuated
  small-scale magnetic flux sheets embedded in the ambient solar
  atmosphere. Temperature distributions in (gray) radiative equilibrium
  at low optical depths where radiation transfer dominates the energy
  budget are obtained. Two-dimensional radiative equilibrium flux
  sheet models are calculated using a novel method which shows that the
  temperature enhancement of the upper layers of photospheric magnetic
  flux concentrations is due to the radiation channeling effect, i.e.,
  that horizontal radiative transfer tends to channel emerging radiation
  into the lower opacity regions. The walls of the flux sheets are found
  to radiate energy from subphotospheric surrounding layers, giving rise
  to a strong heating of the atmosphere of the flux sheets. Radiative
  energy migrates horizontally from the heated flux sheets towards the
  ambient medium and there it heats the atmosphere at low optical depths.

---------------------------------------------------------
Title: Speckle observations of abnormal solar granulation
Authors: de Boer, C. R.; Kneer, F.
1992A&A...264L..24D    Altcode:
  We present observations of solar granulation obtained with the Vacuum
  Tower Telescope at Observatorio del Teide, Tenerife. Speckle methods
  were applied to restore images of abnormal granulation in the vicinity
  of a sunspot near disk center. These reconstructions show nests and
  chains of bright 'points', whose diameters and spatial distances are
  close to the diffraction limit of the telescope (about 0.2 arcsec). The
  'points' are not always circular in shape but also elongated. If these
  features are identified with the footpoints of small-scale magnetic
  fluxtubes, their fast temporal evolution within one or two minutes
  and their horizontal motion with velocities up to 2 - 3 km/sec will
  influence the understanding of fluxtube dynamics.

---------------------------------------------------------
Title: Stokes V asymmetry due to stationary siphon flows
Authors: Degenhardt, D.; Kneer, F.
1992A&A...260..411D    Altcode:
  Results are presented of an investigation of theoretically derived
  Stokes I and V profiles emerging from an atmosphere which contains
  magnetic flux tubes harboring stationary siphon flows. The Stokes I and
  V profiles are calculated for three magnetically sensitive lines. The
  relative area and amplitude asymmetries of the Stokes V, as well as
  the zero-crossing wavelength of the V profile, are calculated for
  the upstream part of the flux tube arch as well as for the downstream
  part of the arch. It is shown that a wide range of physically realistic
  parameters of the simulations yield positive relative area and amplitude
  asymmetries with values comparable to the observations.

---------------------------------------------------------
Title: High resolution observations of the Evershed flow
Authors: Boerner, P.; Kneer, F.
1992A&A...259..307B    Altcode:
  Observations with high spatial resolution of the Evershed effect in
  a sunspot near the limb are presented. They were obtained with the
  Gregory Coude Telescope at Observatorio del Teide, Tenerife, and consist
  of photographic spectrograms in the wavelength region 5160-5180A and
  corresponding slit-jaw images in white light at various positions of the
  sunspot image on the slit of the spectrograph. The measurement of the
  velocity field covers a height range of approximately 100 km to 700 km
  in the Mg b2 line. The Evershed flow is inhomogeneous at all heights,
  though decreasing in amplitude with increasing height. We find maximum
  velocities of 4 km/s at the lowest layer. The flow goes clearly beyond
  the outer penumbra border. In Mg b2 the Evershed effect is inverse,
  on average, while at the 500 km level it is not. We suggest that in
  one fortunate case, with slit orientation parallel to a flow tube, we
  see siphon flows outward, reaching to the height of the Mg b2 layer
  and ending in photospheric faculae. The stationarity of the flow on
  small scales is questioned.

---------------------------------------------------------
Title: A new instrument for high resolution, two-dimensional solar
    spectroscopy.
Authors: Bendlin, C.; Volkmer, R.; Kneer, F.
1992A&A...257..817B    Altcode:
  A two-dimensional spectrometer suitable for solar observations of high
  spatial, spectral, and temporal resolution was built and successfully
  tested with the German Vacuum Tower Telescope at the Observatorio
  del Teide/Tenerife. Using a universal birefringent filter and a
  Fabry-Perot interferometer (FPI), narrow-band (⪉30 mÅ) filtergrams
  can be obtained. With this instrument, it is possible to scan through
  a Fraunhofer line at an appropriate number of wavelength settings
  within a few seconds. Here, fast scanning is accomplished by tuning
  only the FPI. The images are taken by two CCDs. One of them is coupled
  with an image intensifier to achieve short integration times. Due to
  the instrument's design, the adequate field of view has a diameter
  of about 20-40 arsec. The spectrometer can be tuned to virtually any
  wavelength in the visible spectrum.

---------------------------------------------------------
Title: Speckle observations of solar granulation.
Authors: de Boer, C. R.; Kneer, F.; Nesis, A.
1992A&A...257L...4D    Altcode:
  We present observations of solar granulation in a plage region near
  disc center obtained with the Vacuum Tower Telescope at Observatorio
  del Teide, Tenerife. Speckle methods were employed for data acquisition
  and data reduction. The images show small-scale structures of the size
  near the telescopic diffraction limit of 0.2 arcsec. We call attention
  to bright lanes at the borders between granules and intergranular
  areas. Conceivably, they are the intensity signature of strong upflows
  at the border of granules or of shocks in supersonic convection which
  are predicted by computer simulations of the granular phenomenon.

---------------------------------------------------------
Title: Small-scale waves and motions in photosphere and chromosphere
    of the Sun.
Authors: Kneer, F.
1992AnGeo..10...47K    Altcode:
  This is a review on motions and waves in the solar atmosphere. The
  discussion is limited to non-active phenomena in the solar
  atmosphere. Firstly, the gross structure - the average temperature
  profile in the photosphere, chromosphere, transition region and corona
  - is outlined and the energetic needs to maintain this structure are
  addressed. Then the various small-scale phenomena and processes in the
  photosphere and chromosphere are described. Secondary motions such
  as gravity waves and acoustic waves as well as waves involving the
  magnetic field, are excited by the granulation and may carry energy
  upward. The chromosphere shows several distinct features and dynamical
  processes. Within the cell interior of the chromospheric network,
  grains with quasi-periodic brightening are observed. At the cell
  boundaries, where magnetic fields are concentrated, less oscillatory
  motions but more stochastic variations than those seen in the cell
  interior are observed. For the boundaries as well as for the spicules,
  the magnetic field dominates the dynamics and may provide the means
  of chromospheric heating.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1991.
Authors: Kneer, F.
1992MitAG..75..165K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Sonnenbeobachtung bei hoher Auflösung.
Authors: Kneer, F.
1992GGMit..29....5K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Fluctuation of wing strengths as diagnostic tool for the
    structure of granulation.
Authors: Nolte, U.; Kneer, F.; Wiehr, E.
1992AGAb....7..150N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the structure of spectral line gap regions
Authors: Kneer, F.; von Uexkuell, M.
1991A&A...247..556K    Altcode:
  The analysis of a photographic spectrogram of the plage region near
  the disk center of the sun is undertaken to study small-scale magnetic
  features with special attention given to the structure of fluxtubes,
  and a related model is developed. Line-gap regions are identified in
  the Fe-I line which are characterized by redshifts and fluctuation
  velocity as well as an asymmetric profile indicative of a direct
  relationship between increasing downflow and increasing depth. The
  observations are compared to calculated line profiles in which a
  magnetic tube under certain conditions is present. The pressure and
  density are calculated for the tube under the assumption of hydrostatic
  equilibrium, and the observed intensity profiles can be reproduced
  when the correct parameters are used. The calculations demonstrate
  that a strong kilogauss magnetic field can be introduced to explain
  the line gap in the Fe-II 6149-A line.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1990.
Authors: Kneer, F.
1991MitAG..74..163K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: MgH as a diagnostic tool for fluctuations in the solar
    photosphere
Authors: Kulaczewski, J.; Degenhardt, D.; Kneer, F.
1990A&A...234..530K    Altcode:
  A linear perturbation method is used to calculate the behavior of an
  MgH spectral line at 5175 A as a function of temperature and pressure
  fluctuations. Particular attention is given to the MgH spectral lines of
  the A2Pi-X2Sigma(+) (0,0)-system in solar active and quiet regions. It
  is found that Mgh 5175 A is sensitive to temperature perturbations in
  the height range of 0 to 300 km with a peak in the response function
  near 100 km.

---------------------------------------------------------
Title: Stability of cool flux tubes in the solar chromosphere. II -
    Non-linear dynamical behaviour
Authors: Hassan, S. S.; Kneer, F.
1990A&A...232..536H    Altcode:
  A single vertical cool flux tube in the solar chromosphere is focused
  upon for stability studies. The analysis of a previous study by Hasan
  and Kneer (1986) is extended to the nonlinear regime with a view
  to examining the consequences of having self-exciting mechanisms of
  oscillations above the photosphere. In particular, the possibility
  of whether the motions driven by the convective instability caused by
  the presence of CO could extract sufficient energy from the radiation
  field near the Tmin region of empirical models and deposit it in
  higher layers to produce chromospheric heating is investigated. The
  time evolution of this instability is followed by solving the MHD
  equations in the thin flux tube approximation. The analysis includes
  energy exchange with the radiation field. The simulations of a flux
  tube with a transmitting upper boundary show that the average energy
  flux in the oscillations is inadequate for chromospheric heating.

---------------------------------------------------------
Title: Multidimensional radiative transfer in stratified
    atmospheres. VI - Radiative instabilities
Authors: Trujillo Bueno, J.; Kneer, F.
1990A&A...232..135T    Altcode:
  The possibility that radiative instabilities in stellar atmospheres
  are driven by transfer of energy processes between stellar gas and the
  radiation field is considered. Harmonic temperature fluctuations in
  gray radiative equilibrium atmospheres are introduced, and the linear
  response of the radiation field to the ensuing Planck-function and
  opacity fluctuations is investigated. Analytical and numerical
  calculations are performed, emphasizing the influence of the
  multidimensional radiative transfer (MRT) effects of opacity
  fluctuations on the radiative relaxation time as a function of the
  wavenumber of the perturbations. Quantitative examples are given
  for stellar atmospheres with solarlike T(eff) and gravitational
  stratification. It is concluded that, while the MRT effects of
  B fluctuations generally tend to be stabilizing, the MRT effects
  of chi fluctuations are optically important for driving radiative
  instabilities.

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1989.
Authors: Kneer, F.
1990MitAG..73...97K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Waves in active regions: observations.
Authors: Kneer, F.
1990PDHO....7..277K    Altcode:
  This review deals with oscillations in prominences, with the 5 min p
  modes in active regions, and with waves in sunspots.

---------------------------------------------------------
Title: Observations of Magnetic Features with the German Solar
    Telescopes at the Observatorio-Del / Tenerife
Authors: Kneer, F.; Soltau, D.; Wiehr, E.
1990IAUS..142..113K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Oscillations of the sun's chromosphere. V - Importance of
    network dynamics for chromospheric heating
Authors: von Uexkuell, M.; Kneer, F.; Malherbe, J. M.; Mein, P.
1989A&A...208..290V    Altcode:
  A 64-min time sequence of disk center H-alpha spectrograms taken
  with the MSDP spectrograph at the Observatoire du Pic du Midi
  is analyzed. This type of spectrograph allows spectroscopy of a
  two-dimensional field of view. From the H-alpha line profiles at each
  pixel, line-shift and minimum intensity fluctuation are determined as
  functions of spatial and temporal coordinates. A frequency analysis
  with standard Fourier techniques is performed. It is confirmed that
  in the interior of the chromospheric network cells the oscillatory
  behavior dominates, whereas at the boundaries one generally finds random
  motions on scales of 2-10 arcsec. The random behavior of the H-alpha
  structures outlines the permanent rearrangement of the magnetic field
  lines pushed around by the subphotospheric granular flow.

---------------------------------------------------------
Title: The Gregory-Coudé- Telescope at the Observatorio Del Teide
    Tenerife
Authors: Kneer, F.; Wiehr, E.
1989ASIC..263...13K    Altcode: 1989ssg..conf...13K
  No abstract at ADS

---------------------------------------------------------
Title: Universitäts-Sternwarte. Jahresbericht für 1988.
Authors: Kneer, F.
1989MitAG..72..107K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Spatially Resolved Spectra of Solar Granules
Authors: Holweger, H.; Kneer, F.
1989ASIC..263..173H    Altcode: 1989ssg..conf..173H
  No abstract at ADS

---------------------------------------------------------
Title: On the Influence of Opacity Fluctuations on the Energy Transfer
    by Radiation
Authors: Trujillo-Bueno, J.; Kneer, F.
1989ASIC..263..441T    Altcode: 1989ssg..conf..441T
  No abstract at ADS

---------------------------------------------------------
Title: Spectroscopy of the solar photosphere with high spatial
    resolution
Authors: Wiehr, E.; Kneer, F.
1988A&A...195..310W    Altcode:
  A high-resolution spectrogram from the solar photosphere obtained with
  the recently installed Gregory-Coude telescope at the Canary Islands
  is analyzed. Continuum intensity fluctuations are seen at scales down
  to 0.53 arcsec, which demonstrates the high spatial resolution. Peak
  velocities reach 1.5 km/s, while rms velocities range from 350 to
  490 m/s. It is found that, for deeply-formed spectral lines, shifts
  are well correlated with continuum intensity fluctuations at scales
  between 1.5 and 6 arcsec.

---------------------------------------------------------
Title: Two-dimensional spectroscopy of the Sun in Hα.
Authors: von Uexküll, M.; Kneer, F.; Mein, P.
1988AGAb....1...11V    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Gregory-Coudé-Telescope in the Observatorio del Teide,
    Izaña.
Authors: Kneer, F.; Wiehr, E.; Wittmann, A. D.
1988AGAb....1...46K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Multidimensional radiative transfer in stratified atmospheres
Authors: Kneer, F.; Trujillo-Bueno, J.
1987A&A...183...91K    Altcode:
  Energy transport by radiation is an important contribution to the energy
  budget in stellar atmospheric structures. In this paper, radiative
  relaxation of small-scale structures is investigated. The authors
  show in a linear analysis: (1) Already at structural lengths of 10
  opacity scale heights, horizontal photon exchange is important for the
  energy budget. (2) In atmospheric layers near continuum optical depth
  τ<SUB>c</SUB> = 1 and below, the continuum absorption and emission
  processes dominate the radiative relaxation. (3) Weak spectral lines or
  lines with σ<SUB>l</SUB>ɛ ≤ 1 have little influence on the energy
  exchange. (4) At large heights, transport in few spectral lines with
  σ<SUB>l</SUB>ɛ very large 1 can compete with continuum processes.

---------------------------------------------------------
Title: Multidimensional radiative transfer in stratified
    atmospheres. IV - Radiative cooling by LTE and non-LTE spectral lines
Authors: Trujillo-Bueno, J.; Kneer, F.
1987A&A...174..183T    Altcode:
  The question of efficiency of radiative energy losses in spectral
  lines is addressed. In a semi-infinite atmosphere with constant
  temperature, the total radiative energy loss (integrated over all
  depths) in a spectral line without continuum is infinite, in both
  LTE and non-LTE. Thus, only local energy balances may be considered
  with such models. The authors give radiative cooling functions for
  various non-LTE parameters and structural lengths of a two-dimensional
  stratified atmosphere. At the surface, cooling is less efficient in
  non-LTE than in LTE. At large optical depths, both become equal and
  are non-negligible. In these layers horizontal transfer effects become
  important for the energy balance.

---------------------------------------------------------
Title: Einige Aspekte der Erfordernisse und Möglichkeiten zeitlicher
    Auflösung in der optischen Sonnenphysik
Authors: Kneer, F.; Knölker, M.
1987MitAG..68..167K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Observations from Space vs. Ground Based Observations:
    Advantages and Disadvantages
Authors: Kneer, F.
1987LNP...292..159K    Altcode: 1987ssp..conf..159K
  A comparison of solar observations from space and from the ground
  is made.

---------------------------------------------------------
Title: Radiative Relaxation in Small Scale Structures
Authors: Trujillo-Bueno, J.; Kneer, F.
1987rfsm.conf..281T    Altcode:
  The authors discuss the effects of multidimensional radiative transfer
  on the energy exchange by radiation in small-scale structures.

---------------------------------------------------------
Title: Construction of the German Solar Telescopes at Observatorio
del Teide/Tenerife: The Gregory-Coude Telescope
Authors: Kneer, F.; Schmidt, W.; Wiehr, E.; Wittmann, A. D.
1987MitAG..68..181K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Stability of cool flux tubes in the solar chromosphere. Linear
    analysis.
Authors: Hasan, S. S.; Kneer, F.
1986A&A...158..288H    Altcode:
  The linear stability of cool flux tubes in the solar chromosphere
  which are initially in radiative equilibrium is examined. Owing to
  the presence of carbon monoxide, there exists a narrow region near the
  temperature minimum where the temperature gradient becomes steep enough
  to drive a convective instability. The thin flux tube equations are
  used and in a simple manner radiative heat exchange with the ambient
  medium are included. Initial states of constant beta (where beta is
  the ratio of gas to magnetic pressure) are considered. Results for
  various values of beta are presented. It is found that for beta less
  than 5.7 the tube is overstable with periods in the range 300-600 s. At
  beta = 5.7 a bifurcation occurs into two purely growing modes. Growth
  rates, eigenvectors of the fundamental modes are calculated and phase
  relationships are examined. It is suggested that overstable oscillations
  should invariably be associated with cool flux tubes. These oscillations
  transport energy and can thus change the thermodynamic structure of flux
  tubes. It is conjectured that the CO overstability may be responsible
  for spicules.

---------------------------------------------------------
Title: Energy Transport by Radiation
Authors: Kneer, F.
1986ssmf.conf..147K    Altcode:
  The exchange of energy by radiation is an important contribution
  to the energy budget of solar fine structures. The short horizontal
  distances necessitate a multidimensional treatment of the radiation
  transfer. Several aspects are discussed. (1) The equations and their
  properties are described in various geometries. (2) A review on
  earlier work is given. (3) Calculations of first order perturbations
  in a gravitationally stratified atmosphere are presented and compared
  with Spiegel's (1957) analytic result. (4) The radiative stability of
  a stratified atmosphere is discussed.

---------------------------------------------------------
Title: Oscillations of the sun's chromosphere. IV - Temporal evolution
    of H-alpha profile
Authors: Kneer, F.; von Uexkuell, M.
1986A&A...155..178K    Altcode:
  H-alpha observations of the quiet solar chromosphere at disk center are
  presented, including a 9 min time sequence of photographic filtergrams
  in line center and + or - 0.5 A and a 54 min sequence of photographic
  spectrograms. The very different dynamical behavior of the interior and
  boundary of the network cells is emphasized. The dominant processes
  in the interior are waves in the 5 min period range and with shorter
  periods. The energy flux in these waves is estimated to be too small to
  account for the radiative losses from standard chromospheric models. In
  the cell boundaries, the material flows down, on time average, near the
  centers of rosettes. The short time fluctuations are mainly stochastic
  while the oscillations are reduced. This suggests that the energy
  balance in the cell boundaries is determined by MHD turbulence.

---------------------------------------------------------
Title: Multidimensional Radiative Transfer in Stratified Atmospheres:
    Radiative Cooling by LTE and non-LTE Spectral Lines
Authors: Trujillo-Bueno, J.; Kneer, F.
1986MitAG..67..304T    Altcode:
  A detailed paper has been submitted to Astron. Astrophys.

---------------------------------------------------------
Title: Interpretation of High Resolution Observations
Authors: Kneer, F.
1985tphr.conf...63K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Oscillations of the sun's chromosphere. III - Simultaneous
    H-alpha observations from two sites
Authors: von Uexkuell, M.; Kneer, F.; Mattig, W.; Nesis, A.;
   Schmidt, W.
1985A&A...146..192V    Altcode:
  The authors analyze time sequences of Hα filtergrams taken
  simultaneously from two distant observatories, Capri and Izaña. By
  means of a coherence analysis the authors discriminate between
  instrumental effects including seeing and truly solar intensity
  fluctuations. Waves with periods as short as 60 s are present in the
  solar chromosphere; the lower limit is set by the time resolution of
  the observations.

---------------------------------------------------------
Title: Oscillations of the sun's chromosphere. II - H-alpha line
    centre and wing filtergram time sequences
Authors: Kneer, F.; von Uexkuell, M.
1985A&A...144..443K    Altcode:
  In order to investigate the dynamics of the solar chromosphere we
  perform a Fourier analysis of time sequences (total duration 128 min)
  of Hα photographic filtergrams taken simultaneously at disc centre
  in line centre and ±0.5 Å from the line centre. The results are: (i)
  At low frequencies (periods &gt;450 s) the brightness fluctuations are
  caused by the temporal evolution of Hα structures at the boundaries
  of the chromospheric network. We observe that much of the coarse
  chromospheric structure survives the 128 min time span. We derive
  lifetimes of 2-8 min for the small-scale structure and 5-20 min for the
  larger structures. (ii) The modal structure of the 5 min oscillation
  is clearly visible in the power spectra of the three filtergram. The
  fundamental (f) mode can be followed to high horizontal wavenumbers
  k<SUB>h</SUB> ≍3.7 Mm<SUP>-1</SUP> and follows the expected
  relationship ω<SUP>2</SUP><SUB>f</SUB> = gk<SUB>h</SUB>, where g is
  the surface gravity. (iii) A chromospheric resonant mode cannot be
  found in the k - ω plane. (iv) The higher resonant p modes reach into
  the acoustic wave domain (periods T ≍ 150 s) and thus require the
  transition zone as the upper reflecting layer. (v) We find no evidence
  for internal gravity waves. (vi) From a coherence and phase analysis we
  conclude that the brightness fluctuations of the chromospheric structure
  seen in -0.5 Å lead those in +0.5 Å by 2 min at k<SUB>h</SUB> =2 Mm
  <SUP>-1</SUP> and by 4 min at k<SUB>h</SUB> =0.5 Mm<SUP>-1</SUP>. (vii)
  From the same coherence and phase analysis we can identify acoustic
  waves in the solar atmosphere with periods as short as 80 s. They
  possess as much power as the 5 min oscillations and are seen better
  outside the chromospheric network than within the network. (viii) The
  phase difference between intensity in Hα line centre and velocity,
  constructed from the two Hα wing filtergrams, decreases from about 90°
  at low frequencies and high wavenumbers to 0° at high frequencies and
  low wavenumbers. Tentatively we interpret this as a change from mainly
  standing waves for the low order p modes to mainly upward propagating
  waves in the acoustic wave domain. Acoustic waves are thus candidates
  for the heating process of the inter-network regions.

---------------------------------------------------------
Title: Report of the working group on future observations of the sun.
Authors: Dere, K.; Kneer, F.; Landman, D.
1985cdm..proc...14D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Radiative equilibrium models and heating requirements.
Authors: Kneer, F.
1985cdm..proc..252K    Altcode:
  Strong cooling in the IR rotation-vibration bands of carbon monoxide
  makes the solar atmosphere convectively unstable near the temperature
  minimum of the empirical models. Some of the calculated radiative
  equilibrium and heated models are bistable. The author speculates that
  the instability due to molecule formation contributes to the production
  of chromospheres.

---------------------------------------------------------
Title: Waves in the chromosphere observed in Hα.
Authors: Kneer, F.; von Uexküll, M.
1985cdm..proc..282K    Altcode:
  The authors analyze Hα filtergram time sequences and emphasize the
  further development and use of filter spectroscopy for studies on the
  dynamics of the solar atmosphere.

---------------------------------------------------------
Title: Interpretation of high-resolution observations.
Authors: Kneer, F.
1985MPARp.212...63K    Altcode:
  The path on which one arrives from observation to knowledge is highly
  recursive. This contribution is intended to draw attention to the
  ways of specification of the assumptions necessarily made to solve
  the diagnostic problem. Both observational conditions and earlier
  experience enter the assumptions about the complexity, initial and
  boundary conditions of the process under consideration. High-resolution
  observations reveal a wealth of small-scale, dynamic phenomena whose
  properties must be taken into account when diagnosing by way of
  simulation and calculation of emergent intensities.

---------------------------------------------------------
Title: Chromospheric umbral oscillations.
Authors: Mattig, W.; von Uexküll, M.; Kneer, F.
1984ESASP.220...59M    Altcode: 1984ESPM....4...59M
  No abstract at ADS

---------------------------------------------------------
Title: Interpretation of High-Resolution Measurements, Selected
    Problems (Keynote)
Authors: Kneer, F.
1984ssdp.conf..110K    Altcode:
  This review deals with four problems bearing on small-scale dynamical
  processes: (1) small-scale intensity fluctuations, (2) small-scale
  velocities, (3) average models of the solar atmosphere, and (4)
  small-scale magnetic fields.

---------------------------------------------------------
Title: A possible explanation of the Wilson-Bappu relation and the
    chromospheric temperature rise in late-type stars.
Authors: Kneer, F.
1983A&A...128..311K    Altcode:
  IR vibration-rotation bands of CO are noted to cause a radiative
  instability due to the extreme temperature sensitivity of molecule
  formation, in the course of an attempt to construct stellar atmospheres
  which are in radiative equilibrium for the case of a T(eff) of 5800 K,
  which approximates the T(eff) of the sun. It is accordingly suggested
  that such stellar atmospheres, in principle, may not exist. CO
  molecule-based arguments can be extended to other molecular species
  which should destabilize the atmospheres of cooler stars. This implies
  that, for the sun, the instability occurs at the mass column density
  of the temperature minimum deduced from observations. For sunspot
  chromospheres, the predicted critical mass column density agrees with
  that of the onset of the chromospheric temperature rise in current
  sunspot models. It is further suggested that the chromospheres of
  late-type stars are produced inevitably, and that their seed is located
  at their base.

---------------------------------------------------------
Title: The chromosphere above sunspot umbrae. IV. Frequency analysis
    of umbral oscillations.
Authors: von Uexküll, M.; Kneer, F.; Mattig, W.
1983A&A...123..263V    Altcode:
  The authors analyse time sequences of photographic spectrograms in Ca II
  H, K, 8498 Å, and 8542 Å, in Sodium D<SUB>1</SUB> and D<SUB>2</SUB>,
  in Ni I 5893 Å and Hα, obtained from two sunspots. The frequency
  analysis of line intensities and shifts for the umbral chromospheric
  oscillations in the period range 110 s-200 s is discussed.

---------------------------------------------------------
Title: The chromosphere above sunspot umbrae. IV - Frequency analysis
    of umbral oscillations
Authors: Uexkuell, M. V.; Kneer, F.; Mattig, W.
1983A&A...123..263U    Altcode:
  An analysis is made of time sequences of photographic spectrograms in Ca
  II H, K, 8498 A, and 8542 A, in sodium D1 and D2, and in Ni I 5893 A and
  H-alpha obtained from two sunspots with the Vacuum Tower Telescope at
  Sacramento Peak Observatory. The two sunspot chromospheres are found to
  behave similarly with regard to their phase relationships but somewhat
  differently with regard to the oscillation resonant frequencies. In one
  and the same umbra, oscillation modes having different frequencies may
  be excited. The vertical velocity of propagation of the phase in umbral
  chromospheres, which is 10-25 km/s, decreases with increasing frequency
  and is much lower than the Alfven speed. The umbral chromospheric
  oscillations are thus slow mode waves. The downward motion enhances
  the intensity by some 65 deg (H-alpha) to 85 deg (D1), indicating
  dissipation of mechanical energy. It is also found that the mechanical
  energy flux of the umbral chromospheric oscillations is not sufficient
  to account for the chromospheric radiative losses.

---------------------------------------------------------
Title: Brightness oscillations of the sun's chromosphere in K
    and H-alpha
Authors: Kneer, F.; von Uexkuell, M.
1983A&A...119..124K    Altcode:
  The authors have performed a power analysis of time sequences of
  photographic K and Hα filtergrams from disc center of the sun. The
  results are: In the k-ω-diagram from the Ca II K filtergrams, the modal
  structure of the 5-min-oscillation is clearly seen. The 5-min modes
  are difficult to detect in the k-ω-diagrams from the Hα sequences. In
  both lines, K and Hα, the authors observe ridges in the k-ω-diagrams
  running parallel to the k<SUB>h</SUB>-axis. Their periods correspond to
  the chromospheric 3-min-oscillation. But power appears also, in discrete
  ridges, at shorter periods P = 60 s...150 s, where the lower limit is
  given by the Nyquist frequency of the observations. After subtraction
  of the time averaged intensity at each position in the filtergrams,
  the rms fluctuations are 3.5% in K and 4.0% in Hα.

---------------------------------------------------------
Title: Zur Aufheizung der Chromosphäre später Sterne
Authors: Kneer, F.
1983MitAG..60..301K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: New features of the oscillation spectrum of the sun
Authors: Kneer, F.; von Uexkuell, M.; Newkirk, G., Jr.
1982A&A...113..129K    Altcode:
  The results of a Fourier analysis of observations of brightness
  fluctuations in a sequence of CN 3883 Angstrom filtergrams are
  presented. From the 75 min time interval and field of view of 170 x
  220 sq arcsec, a 40 sec sampling interval was examined, maintaining
  a spatial resolution as high as 1600 km. The data is filtered for a
  high wavenumber power spectrum, and an average power coefficient is
  calculated by examining power spectra for each of the sub-arrays. It
  is concluded that the 5 min velocity oscillation modal structure is
  easily detected in brightness fluctuations in the CN band, the surface
  gravity mode is the strongest single feature aside from the zero
  frequency ridge caused by convection, and the trapped chromospheric
  mode may be represented by a constant frequency ridge. Improved broad
  band filtergrams may be used to analyze little explored regions of
  the oscillation spectrum and to interpret intensity fluctuations as
  temperature fluctuations.

---------------------------------------------------------
Title: Zur Struktur der Chromosphäre über Sonnenflecken
Authors: Kneer, F.; Mattig, W.; von Uexküll, M.
1982MitAG..55...65K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Chromosphärische Helligkeits-oszillationen
Authors: Kneer, F.; Newkirk, G.; von Uexküll, M.
1982MitAG..55...70K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The analysis of solar limb observations. II - Geometrical
    smearing
Authors: Durrant, C. J.; Kneer, F.; Maluck, G.
1981A&A...104..211D    Altcode:
  Center-of-disk and limb observations of photospheric brightness
  fluctuations reveal a systematic suppression of the values at the
  limb. This is a geometrical effect arising from the line of sight at the
  limb passing through various structures across the surface. This effect
  is examined here in the light of recent granular temperature models
  using a simple model incorporating the dominant granular scale. The
  geometry is found to have little influence on the deep-seated granular
  brightness field but to strongly suppress the brightness fluctuations
  caused by the upper photospheric temperature field. The significance
  of these results for the restoration of limb observations is discussed.

---------------------------------------------------------
Title: The chromosphere above sunspot umbrae. III - Spatial and
    temporal variations of chromospheric lines
Authors: Kneer, F.; Mattig, W.; v. Uexkuell, M.
1981A&A...102..147K    Altcode:
  The lines Ca II H, K, 8542 A, Na I D(1) and D(2), Ni I 5893 A, and
  H-alpha, were analyzed in photographic spectrograms obtained from a
  sunspot. The following results are found for the umbra: (1) the H and K
  lines are highly correlated, with an emission cores maximum-intensities
  ratio of 1.2; (2) the K intensities are poorly correlated with those
  of 8542, D(2), and H-alpha; this behavior is attributed to atmospheric
  structure horizontal fluctuations having little correlations between
  those in high and low layers, (3) the periodic temporal fluctuations
  of the K emission core with umbral flashes are identified, and found to
  occur throughout the umbral area covered by the spectrograph slit; and
  (4) fluctuations in the photospheric Ni I 5893 A line cannot be matched
  to the chromospheric fluctuations, in accord with Giovanelli (1978).

---------------------------------------------------------
Title: OSO-8 Observations of CAII H and K MGII H and K Lyman-Alpha
    and Lyman-Beta above a Sunspot
Authors: Kneer, F.; Scharmer, G.; Mattig, W.; Wyller, A.; Artzner,
   G.; Lemaire, P.; Vial, J. C.
1981SoPh...69..289K    Altcode:
  Observations with the French (L.P.S.P.) experiment on board OSO-8 of
  a sunspot and nearby plage region are described. The behaviour of the
  emission cores of the Ca II H and K and Mg II h and k resonance lines is
  very similar and the correspondence in intensity between the four lines
  persists in all observed features. In contrast, the Lyman lines show
  little correlation with the other lines. Their emission regions appear
  broader in the spectroheliograms than the underlying sunspot structure
  and must not necessarily possess a counterpart in lower layers. From
  the central intensity of Lα above the umbra an electron density of
  4.3 × 10<SUP>10</SUP> cm<SUP>-3</SUP> ≲n<SUB>e</SUB><SUP>*</SUP>
  ≲2.3 × 10<SUP>11</SUP> cm<SUP>-3</SUP> at 20 000 K is estimated.

---------------------------------------------------------
Title: The Chromosphere above Sunspot Umbrae - Part Three - Spatial
    and Temporal Variations of Chromospheric Lines
Authors: Kneer, F.; Mattig, W.; Uexkull, M. V.
1981phss.conf..318K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Multidimensional radiative transfer in stratified
    atmospheres. III - Non-LTE line formation
Authors: Kneer, F.
1981A&A....93..387K    Altcode:
  Multidimensional radiative transfer under non-LTE conditions is
  investigated. Line formation strictly by two-level atoms is treated
  with both coherent scattering and complete redistribution and with
  non-LTE parameters ranging from epsilon 0.1 to 0.000001. Lateral
  transfer effects increase with increasing non-LTE conditions and may,
  in the solar case, compete with observational limitation of angular
  resolution. Calculations of line transfer in simple model structures
  in the solar chromosphere show that horizontal migration of photons
  may diminish the contrast by an order of magnitude compared to that
  expected in a one-dimensional treatment.

---------------------------------------------------------
Title: Sunspot chromospheres and their relation to the chromospheres
    of laten main sequence stars.
Authors: Mattig, W.; Kneer, F.
1981A&A....93...20M    Altcode:
  The sunspot umbra parameters such as spectral type, effective
  temperature, and chromospheric Ca II H and K line emission are related
  to the same parameters of luminosity class V stars of spectral type
  G 8 to M 2. The sunspots follow the Wilson-Bappu relation. It is
  concluded that the strong magnetic fields in umbrae may control the
  nonradiative energy supply of the sunspot chromosphere; however, the
  heating mechanism is not reflected in the average appearance of the
  H and K line cores.

---------------------------------------------------------
Title: Coherence Analysis of Granular Intensity
Authors: Kneer, F. J.; Mattig, W.; Nesis, A.; Werner, W.
1980SoPh...68...31K    Altcode:
  A high resolution spectrogram of the Mg b<SUB>2</SUB> line from the
  quiet Sun disc centre is subjected to a coherence analysis. We find
  that the coherence between intensity fluctuations in the continuum
  and the wings of the line breaks down at a distance Δλ = 0.35
  Å from line centre. From this and the r.m.s. intensity contrast
  as a function of Δλ we are led to the following simple model of
  temperature fluctuation δT in the solar photosphere: A lower part
  (below 50 km, or τ<SUB>5000</SUB> &gt; 0.25) with strongly inward
  increasing δT and an upper part (above 50 km) with constant δT =
  75 K. The two parts are supposed to fluctuate incoherently.

---------------------------------------------------------
Title: On time-dependent ionization in stellar chromospheres
Authors: Kneer, F.
1980A&A....87..229K    Altcode:
  This investigation deals with the question of whether ionization
  equilibrium in stellar chromospheres is adjusted quickly or slowly
  compared with dynamical time scales. It is shown that the adjustment
  time for ionization of hydrogen may be of the order of 10 to the
  2nd s (middle solar chromosphere) to 10 to the 3rd s (upper solar
  chromosphere). This has consequences on the structure of chromospheres;
  for instance, stationary flows - which are limited to subsonic speed
  in plane-parallel geometry - drastically change the electron density
  in comparison to the static case. Unless confirmed by consistent
  dynamical calculations, chromospheric models based on the assumption
  of statistical steady state should be taken as rough estimates of
  chromospheric structure.

---------------------------------------------------------
Title: Observations of Chromospheric Lines from OSO-8
Authors: Grossmann-Doerth, U.; Kneer, F.; Uexkuell, M.; Artzner,
   G. E.; Vial, J. C.
1980SoPh...66....3G    Altcode:
  The line profiles of Lα, Ca II K and Mg IIk were measured with the
  spectrometer of the `Laboratoire de Physique Stellaire et Planétaire'
  on board of OSO-8. The results of these measurements are presented.

---------------------------------------------------------
Title: Report from the discussion group on speckle interferometry
    and adaptive optics
Authors: Kneer, F.
1980fsoo.conf..298K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: What are the resolution limits given by the solar atmosphere
    itself?
Authors: Kneer, F.
1980fsoo.conf..204K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Multidimensional radiative transfer in stratified atmospheres:
    gray radiative equilibrium.
Authors: Kneer, F.; Heasley, J. N.
1979A&A....79...14K    Altcode:
  This paper tests the validity of the multidimensional Eddington,
  or diffusion, approximation in radiative transfer in a gray
  radiative-equilibrium atmosphere with opacity increasing exponentially
  toward the stellar interior. The diffusion approximation is unacceptable
  at small optical depths. The height dependence of intensity fluctuations
  is studied systematically by adopting the above simplified model
  atmosphere for the solar photosphere. Lateral radiative exchange is
  efficient and drastically damps the fluctuations in the uppermost
  layers.

---------------------------------------------------------
Title: Mehrdimensionaler Strahlungstransport im
    Strahlungsgleichgewicht
Authors: Kneer, F.
1979MitAG..45...95K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Report from discussion group on speckle interferometry and
    adaptive optics.
Authors: Kneer, F.
1979MmArc.106..298K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: What are the resolution limits given by the solar atmosphere
    itself?
Authors: Kneer, F.
1979MmArc.106..204K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Chromosphere above Sunspot Umbrae II. The Interpretation
    of the H, K and IR Lines of CA II
Authors: Kneer, F.; Mattig, W.
1978A&A....65...17K    Altcode:
  Summary. The chromospheres above sunspot umbrae are investigated
  by comparing calculated and observed profiles of the Ca ii H, K,
  and infra-red lines. Statistical steady state is assumed for the
  level populations. We test several model chromospheres in hydrostatic
  equilibrium. We distinguish between chromospheres which are optically
  thick and optically thin at the centre of the K line. In view of the
  observed intensity ratio 1K3/1H3 1.15 we are forced to adopt a thick
  model as reference chromosphere. Key words: chromosphere - sunspot
  umbra - Ca ii lines

---------------------------------------------------------
Title: The chromosphere above sunspot umbrae. I. Observations of
    the emission cores in the Ca II H- and K-lines.
Authors: Mattig, W.; Kneer, F.
1978A&A....65...11M    Altcode:
  Summary. Photographic spectra of the Ca ii H- and K- lines in sunspot
  umbrae are analysed. The emission features in the line profiles may be
  classified into minimum emission profiles that also lack self-reversal,
  and profiles which change rapidly in time and exhibit self-reversals
  (umbral flashes) that are most often strongly asymmetric. Average
  intensity profiles from the minimum emission parts of the umbra and
  characteristic intensities of the more active parts of the umbra are
  given. Key words: chromosphere - sunspot umbra - Ca ii lines

---------------------------------------------------------
Title: Chromosphärenmodell von Sonnenflecken
Authors: Kneer, F.; Mattig, W.
1978MitAG..43..141K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Report from the discussion group on speckle interferometry
    and adaptive optics
Authors: Kneer, F.
1978fsoo.conf..298K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: What are the resolution limits given by the solar atmosphere
    itself?
Authors: Kneer, F.
1978fsoo.conf..204K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The effects of partial redistribution on facular K line
    profiles.
Authors: Heasley, J. N.; Kneer, F.; Chapman, G. A.
1977SoPh...52..309H    Altcode:
  We present theoretical Ca II K-line profiles and filtergram contrasts
  for several recent models of solar faculae. The line profiles vary
  greatly between models and between complete and partial frequency
  redistribution non-LTE calculations for any given model. The filtergram
  contrasts are relatively insensitive to the line formation theory which
  greatly simplifies the calculation for comparison with observations. All
  of the models considered exhibit K-line contrasts smaller than the
  mean value observed by Mehltretter.

---------------------------------------------------------
Title: The sun's chromospheric velocity field as inferred from the
    Ca II K line.
Authors: Durrant, C. J.; Grossmann-Doerth, U.; Kneer, F. J.
1976A&A....51...95D    Altcode:
  Theory and observation of the solar Ca II K line under high spatial
  resolution are briefly reviewed. It is shown that contrary to popular
  belief, present theory is not capable of explaining the observed
  features of the K line. An attempt is made to develop a schematic model
  of the temporal behavior of the spatially resolved K line, based on
  Leibacher's (1971) investigation of waves in the solar atmosphere

---------------------------------------------------------
Title: Formation of spectral lines with partial frequency
    redistribution.
Authors: Heasley, J. N.; Kneer, F.
1976ApJ...203..660H    Altcode:
  A method for treating the effects of partial frequency redistribution
  (PRD) in non-LTE radiative-transfer problems is presented in which the
  rate equations may be kept in their usual form and the PRD effects may
  be included by altering the form of the radiative-transfer equation. The
  required modifications of the transfer and statistical-equilibrium
  equations are outlined along with the changes these necessitate in
  the complete linearization method. The formulation is compared with
  that of Milkey et al. (1973), and sample PRD calculations are given
  for the solar chromospheric Ly-alpha and Ca II K line profiles. The
  results are found to be in excellent quantitative agreement with
  previous computations.

---------------------------------------------------------
Title: Radiative hydrodynamics of chromospheric transients.
Authors: Kneer, F.; Nakagawa, Y.
1976A&A....47...65K    Altcode:
  A self-consistent method for the numerical modeling of transient
  phenomena in stellar atmospheres is developed. The nonlinear equations
  of hydrodynamics are solved together with the equations of radiative
  transfer and non-LTE ionization equilibrium, and consideration is given
  to one-dimensional (vertical) motions in a plane-parallel atmosphere
  which is initially in hydrostatic and statistical equilibrium. The
  radiative-transfer problem is treated for a two-level-plus-continuum
  hydrogen atom. The coupled partial differential equations are solved
  through complete linearization of the corresponding difference
  equations. The effects of a thermal pulse introduced at the bottom of
  a model solar chromosphere are calculated. The temperature perturbation
  produces a pressure pulse which runs upwards through the atmosphere and
  generates complex variations of temperature, velocity, and radiation.

---------------------------------------------------------
Title: Wechselwirkung zwischen Strahlungstransport und Gasdynamik
    in der Sonnenchromosphäre.
Authors: Kneer, F.; Nakagawa, Y.
1976MitAG..38..203K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Comments on the redistribution function of Jefferies and White.
Authors: Kneer, F.
1975ApJ...200..367K    Altcode:
  The simplified form of the angle-averaged redistribution function R"(v',
  v) for coherent scattering in the atom's frame, suggested by Jefferies
  and White (1960), is neither normalized nor symmetric. It is shown
  how these requirements can be met by a simple change, retaining the
  idea that photons are completely redistributed in the Doppler core and
  coherently scattered in the line wings. Subject headings: functions -
  line formation - radiative transfer

---------------------------------------------------------
Title: Properties of the solar Ca  ii K-Line at high spatial
    resolution
Authors: Grossmann-Doerth, U.; Kneer, F.; Uexküll, M. v.
1974SoPh...37...85G    Altcode:
  The analysis of three Ca II K-spectra with spatial resolution of
  ∼1″ is described and its results presented. The comparison of
  the observed single peak line profiles with model computation leads
  to some conclusions regarding the non-uniform large-scale velocity
  fields in the chromosphere.

---------------------------------------------------------
Title: On Some Characteristics of Umbral Fine Structure
Authors: Kneer, F.
1973SoPh...28..361K    Altcode:
  Photographic spectra of the umbra of a sunspot (1971, August 24,
  Rome No. 6205) around 6150 Å show fine bright threads which were
  identified as the spectra of a lightbridge, of the bright end of a
  penumbral filament and of umbral dots, respectively. It was found,
  in agreement with the results of other authors, that the magnetic
  field in bright structures is considerably weaker than in dark umbral
  material. Analysis of line profiles of Fe II 6149.2 Å in umbral dots
  indicates (a) a fieldstrength reduced by a factor ≈2 compared to
  the surroundings, (b) an outflow with v≈3.0 km s<SUP>−1</SUP>
  relative to the penumbra and (c) possibly photospheric temperatures
  in umbral dots.

---------------------------------------------------------
Title: Profiles of Magnetically Split Lines in Sunspots
Authors: Kneer, F.
1972A&A....18...47K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Line Profiles in Sunspots
Authors: Kneer, F.
1972A&A....18...39K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Linienprofile in Sonnenflecken
Authors: Kneer, F.
1972MitAG..31..149K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Linienprofile in Sonnenflecken : ein Beitrag zur Aufstellung
von Umbramodellen 

---------------------------------------------------------
Title: Linienprofile in Sonnenflecken : ein Beitrag
zur Aufstellung von Umbramodellen 

---------------------------------------------------------
Title: Line profiles in sunspots :
    a contribution to the establishment of umbra models;
Authors: Kneer, Franz
1970PhDT........99K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Sunspot Intensities and their Correction for Scattered Light
Authors: Kneer, F.; Mattig, W.
1968SoPh....5...42K    Altcode:
  Photoelectric measurements of monochromatic spot intensities obtained
  with the domeless Coudé refractor in Anacapri are given. The scattered
  light superimposed on the spot, as deduced from measurements outside
  the sun's limb, amounts on the average to about 4% of the photospheric
  intensity. The accuracy of the derived spot intensities is better than
  10%. Two of four investigated spots yielded temperatures below 3900K
  (ΔΘ &gt; 0.5).