Author name code: fabiani-bendicho ADS astronomy entries on 2022-09-14 author:"Fabiani Bendicho, Pena" ------------------------------------------------------------------------ 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).

We secured spectra for 8 spectral setups in 6 different positions across the solar disk, as summarized in Table 1.

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).

(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 NH/Ne 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 χ2 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.

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 (<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. 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.

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.

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.

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.