Sweep-by-sweep implicit lambda iteration for non-LTE radiative transfer in 2d cartesian coordinates

Abstract

We have recently developed a novel method for solving non-LTE radiative transfer problem in 2D media. The method can be considered as a generalization of the Forth-and-back implicit lambda iteration (FBILI), originally developed for 1D atmospheric models. In this so-called Sweep-by-sweep implicit lambda iteration short characteristics approach is used for the formal solution of the radiative transfer equation, whereas an extremely high convergence rate is achieved by means of multiple source function updates within one iteration and thanks to the use of quasi-invariant iteration factors. In this paper we briefly present its implementation in the case of two-level atom line transfer with complete frequency redistribution in a 2D Cartesian grid and we compare its convergence properties with those of the iterative methods currently in use (Jacobi and Gauss-Seidel iteration). We show that Sweep-by-sweep ILl converges 6-7 times faster than the Jacobi method and scales better with grid resolution.