PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers

摘要

The interpretation of the intensity and polarization of the spectral lineradiation produced in the atmosphere of the Sun and of other stars requiressolving a radiative transfer problem that can be very complex, especially whenthe main interest lies in modeling the spectral line polarization produced byscattering processes and the Hanle and Zeeman effects. One of the difficultiesis that the plasma of a stellar atmosphere can be highly inhomogeneous anddynamic, which implies the need to solve the non-equilibrium problem of thegeneration and transfer of polarized radiation in realistic three-dimensional(3D) stellar atmospheric models. Here we present PORTA, an efficient multilevelradiative transfer code we have developed for the simulation of the spectralline polarization caused by scattering processes and the Hanle and Zeemaneffects in 3D models of stellar atmospheres. The numerical method of solutionis based on the non-linear multigrid iterative method and on a novelshort-characteristics formal solver of the Stokes-vector transfer equationwhich uses monotonic B\'ezier interpolation. Therefore, with PORTA thecomputing time needed to obtain at each spatial grid point the self-consistentvalues of the atomic density matrix (which quantifies the excitation state ofthe atomic system) scales linearly with the total number of grid points.Another crucial feature of PORTA is its parallelization strategy, which allowsus to speed up the numerical solution of complicated 3D problems by severalorders of magnitude with respect to sequential radiative transfer approaches,given its excellent linear scaling with the number of available processors. ThePORTA code can also be conveniently applied to solve the simpler 3D radiativetransfer problem of unpolarized radiation in multilevel systems.