The use of iteration factors in the solution of the NLTE line transfer problem—I. Two-level atom

Abstract

From the original idea of the monochromatic variable Eddington factors and through a critical revision of the straightforward Λ iteration scheme we have developed an improved iterative method to solve the line formation problem. Our study deals with the computational aspects of the method when applied to the solution of the two-level-atom line transfer problem. At each iteration step, angle and frequency averaged depth-dependent factors defined as the ratios of the relevant intensity moments are computed form the current values of the radiation field. These factors are then used to close the system of the radiative transfer equation moments. Due to the quasi-invariance of the factors, the exact solution of the system is achieved within only a few iterations, even under physical conditions very far from LTE.