Multi3D, 3D Non-Lte Radiative Transfer

Abstract

We have developed a computer program, Multi3D, for solving non-LTE radiative transfer problems in three dimensional geometry. Boundaries can be periodic or the radiation field may be prescribed. The atomic model may contain many energy levels. A short characteristic method is used for the radiative transfer and the rate equations are linearized such that a generalization to radiation hydrodynamics is straightforward. The code can be run on parallel computer architectures using MPI. The parallel speed-up is linear up to the number of nodes equal to the number of bound-bound transitions in the atom. Examples are given for the formation of the resonance lines of neutral sodium, Nal D, and singly ionized calcium, Call K, in a simulation of the solar convection using 3, 6 and 12 level model atoms and 32 x 32 x 100 points in the atmosphere.