Multidimensional non-LTE radiative transfer. I. A universal two-dimensional short-characteristics scheme for Cartesian, spherical, and cylindrical coordinate systems

We have developed an efficient and robust two-dimensional non-LTE radiation transfer solver appropriate for line transfer in the equivalent two-level atom formalism. The numerical method applies the accelerated lambda iteration technique together with the short-characteristics scheme. The code presented in this paper incorporates all three standard geometries (Cartesian, cylindrical, and spherical) in a transparent way while allowing for arbitrary (three-dimensional) velocity fields. The geometry-specific parts of the radiative transfer solver are modularized so that the change of geometry is accomplished by simply setting the appropriate switch. We have also developed a parallel version of the code, in which we use a parallelization in spatial sub-domains, and showed that such a scheme is sufficiently robust. We have performed a number of tests of the performance of the solver in all three geometries. Finally, we discuss the internal accuracy of the transfer solutions depending on the number of spatial, angular, and frequency grid points.