Author name code: fabiani-bendicho
ADS astronomy entries on 2022-09-14
author:"Fabiani Bendicho, Pena" 
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Title: Three-dimensional Radiative Transfer with Multilevel Atoms
Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.
Bibcode: 2007arXiv0710.5427F
Altcode:
  The efficient numerical solution of Non-LTE multilevel transfer problems
  requires the combination of highly convergent iterative schemes with
  fast and accurate formal solution methods of the radiative transfer
  (RT) equation. This contribution begins presenting a method for the
  formal solution of the RT equation in three-dimensional (3D) media
  with horizontal periodic boundary conditions. This formal solver is
  suitable for both, unpolarized and polarized 3D radiative transfer
  and it can be easily combined with the iterative schemes for solving
  non-LTE multilevel transfer problems that we have developed over
  the last few years. We demonstrate this by showing some schematic
  3D multilevel calculations that illustrate the physical effects of
  horizontal radiative transfer. These Non-LTE calculations have been
  carried out with our code MUGA 3D, a 3D multilevel Non-LTE code based
  on the Gauss-Seidel iterative scheme that Trujillo Bueno and Fabiani
  Bendicho (1995) developed for RT applications.

Title: VizieR Online Data Catalog: Center-to-limb variation of quiet
    Sun (Allende+, 2004)
Authors: Allende Prieto, C.; Asplund, M.; Fabiani Bendicho, P.
Bibcode: 2005yCat..34231109A
Altcode:
  Solar observations of the center-to-limb variation of several spectral
  lines were carried out in October 22-23, 1997, with the Gregory Coude
  Telescope (GCT) and its Czerny-Turner echelle spectrograph at the
  Observatorio del Teide (Tenerife, Spain). <P />We secured spectra
  for 8 spectral setups in 6 different positions across the solar
  disk, as summarized in Table 1. <P />Positions #1 to #5 were always
  at heliocentric angles theta = 0, 15, 30, 45, and 60 degrees (mu =
  cos(theta) = 1.00, 0.97, 0.87, 0.71, and 0.50) along a straight line
  crossing the center of the solar disk. Position #6 was also selected
  along the same direction, sometimes at theta = 75 degrees and others
  at 80 degrees (mu = 0.26 or 0.17). <P />(2 data files).

Title: Center-to-limb variation of solar line profiles as a test of
    NLTE line formation calculations
Authors: Allende Prieto, C.; Asplund, M.; Fabiani Bendicho, P.
Bibcode: 2004A&A...423.1109A
Altcode: 2004astro.ph..5154A; 2004astro.ph..5154P
  We present new observations of the center-to-limb variation
  of spectral lines in the quiet Sun. Our long-slit spectra are
  corrected for scattered light, which amounts to 4-8% of the continuum
  intensity, by comparison with a Fourier transform spectrum of the disk
  center. Different spectral lines exhibit different behaviors, depending
  on their sensitivity to the physical conditions in the photosphere and
  the range of depths they probe as a function of the observing angle,
  providing a rich database to test models of the solar photosphere and
  line formation. We examine the effect of inelastic collisions with
  neutral hydrogen in NLTE line formation calculations of the oxygen
  infrared triplet, and the Na I λ6160.8 line. Adopting a classical
  one-dimensional theoretical model atmosphere, we find that the sodium
  transition, formed in higher layers, is more effectively thermalized
  by hydrogen collisions than the high-excitation oxygen lines. This
  result appears as a simple consequence of the decrease of the ratio
  N<SUB>H</SUB>/N<SUB>e</SUB> with depth in the solar photosphere. The
  center-to-limb variation of the selected lines is studied both under
  LTE and NLTE conditions. In the NLTE analysis, inelastic collisions
  with hydrogen atoms are considered with a simple approximation or
  neglected, in an attempt to test the validity of such approximation. For
  the sodium line studied, the best agreement between theory and
  observation happens when NLTE is considered and inelastic collisions
  with hydrogen are neglected in the rate equations. The analysis of
  the oxygen triplet benefits from a very detailed calculation using an
  LTE three-dimensional model atmosphere and NLTE line formation. The
  χ<SUP>2</SUP> statistics favors including hydrogen collisions with
  the approximation adopted, but the oxygen abundance derived in that
  case is significantly higher than the value derived from OH infrared
  transitions. <P />GCT spectra are only available in electronic form
  at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)
  or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/1109

Title: Basic Multidimensional Radiative Transfer
Authors: Fabiani Bendicho, P.
Bibcode: 2003ASPC..288..419F
Altcode: 2003sam..conf..419F
  In the last years, the improvement in the observations and
  the increasing spatial resolution obtained open a wide range of
  questions related to the diagnostic and simulation of multidimensional
  plasmas. This contribution focuses on the development and implementation
  of efficient 2D and 3D radiative transfer (RT) methods that allow
  Non-LTE effects in inhomogeneous astrophysical plasmas to be rigorously
  investigated. We discuss the optimal way to solve the multidimensional
  RT problem with emphasis on the numerical difficulties arising from
  interpolation and boundary questions. We present some 3D formal solvers
  that are suitable for both, unpolarized and polarized RT. Finally we
  show the power of current multidimensional codes with some illustrative
  and realistic Non-LTE multilevel calculations in 2D and 3D schematic
  models of stellar atmospheres.

Title: Three-dimensional radiative transfer with multilevel atoms
Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.
Bibcode: 1999ASSL..243..219F
Altcode: 1999sopo.conf..219F
  No abstract at ADS

Title: Multidimensional radiative transfer with multilevel
    atoms. II. The non-linear multigrid method.
Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.; Auer, L.
Bibcode: 1997A&A...324..161F
Altcode:
  A new iterative method for solving non-LTE multilevel radiative
  transfer (RT) problems in 1D, 2D or 3D geometries is presented. The
  scheme obtains the self-consistent solution of the kinetic and
  RT equations at the cost of only a few (&lt;10) formal solutions
  of the RT equation. It combines, for the first time, non-linear
  multigrid iteration (Brandt, 1977, Math. Comp. 31, 333; Hackbush,
  1985, Multi-Grid Methods and Applications, springer-Verlag, Berlin),
  an efficient multilevel RT scheme based on Gauss-Seidel iterations
  (cf. Trujillo Bueno &amp; Fabiani Bendicho, 1995ApJ...455..646T),
  and accurate short-characteristics formal solution techniques. By
  combining a valid stopping criterion with a nested-grid strategy
  a converged solution with the desired true error is automatically
  guaranteed. Contrary to the current operator splitting methods the very
  high convergence speed of the new RT method does not deteriorate when
  the grid spatial resolution is increased. With this non-linear multigrid
  method non-LTE problems discretized on N grid points are solved in O(N)
  operations. The nested multigrid RT method presented here is, thus,
  particularly attractive in complicated multilevel transfer problems
  where small grid-sizes are required. The properties of the method are
  analyzed both analytically and with illustrative multilevel calculations
  for Ca II in 1D and 2D schematic model atmospheres.

Title: A Novel Iterative Scheme for the Very Fast and Accurate
    Solution of Non-LTE Radiative Transfer Problems
Authors: Trujillo Bueno, J.; Fabiani Bendicho, P.
Bibcode: 1995ApJ...455..646T
Altcode:
  Iterative schemes based on Gauss-Seidel (G-S) and optimal successive
  over-relaxation (SOR) iteration are shown to provide a dramatic
  increase in the speed with which non-LTE radiation transfer (RT)
  problems can be solved. The convergence rates of these new RT methods
  are identical to those of upper triangular nonlocal approximate
  operator splitting techniques, but the computing time per iteration
  and the memory requirements are similar to those of a local operator
  splitting method. In addition to these properties, both methods are
  particularly suitable for multidimensional geometry, since they neither
  require the actual construction of nonlocal approximate operators nor
  the application of any matrix inversion procedure. <P />Compared with
  the currently used Jacobi technique, which is based on the optimal
  local approximate operator (see Olson, Auer, &amp; Buchler 1986), the
  G-S method presented here is faster by a factor 2. It gives excellent
  smoothing of the high-frequency error components, which makes it the
  iterative scheme of choice for multigrid radiative transfer. This
  G-S method can also be suitably combined with standard acceleration
  techniques to achieve even higher performance. <P />Although the
  convergence rate of the optimal SOR scheme developed here for solving
  non-LTE RT problems is much higher than G-S, the computing time per
  iteration is also minimal, i.e., virtually identical to that of a local
  operator splitting method. While the conventional optimal local operator
  scheme provides the converged solution after a total CPU time (measured
  in arbitrary units) approximately equal to the number n of points per
  decade of optical depth, the time needed by this new method based on the
  optimal SOR iterations is only √n/2√2. This method is competitive
  with those that result from combining the above-mentioned Jacobi and
  G-S schemes with the best acceleration techniques. <P />Contrary to
  what happens with the local operator splitting strategy currently in
  use, these novel methods remain effective even under extreme non-LTE
  conditions in very fine grids.

Title: On the photospheric temperature in small-scale magnetic flux
    concentrations
Authors: Fabiani Bendicho, P.; Kneer, F.; Trujillo Bueno, J.
Bibcode: 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.

