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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.
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.
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Title: VizieR Online Data Catalog: Center-to-limb variation of quiet
Sun (Allende+, 2004)
Authors: Allende Prieto, C.; Asplund, M.; Fabiani Bendicho, P.
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).
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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.
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
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Title: Basic Multidimensional Radiative Transfer
Authors: Fabiani Bendicho, P.
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.
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Title: Three-dimensional radiative transfer with multilevel atoms
Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.
1999ASSL..243..219F Altcode: 1999sopo.conf..219F
No abstract at ADS
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Title: Multidimensional radiative transfer with multilevel
atoms. II. The non-linear multigrid method.
Authors: Fabiani Bendicho, P.; Trujillo Bueno, J.; Auer, L.
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 (<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 & 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.
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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.
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, & 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.
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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.
