THE USE OF ITERATION FACTORS IN THE SOLUTION OF THE NLTE LINE TRANSFER PROBLEM .1. 2-LEVEL ATOM

From the original idea of the monochromatic variable Eddington factors and through a critical revision of the straightforward A iteration sc heme we have developed an improved iterative method to solve the line formation problem. Our study deals with the computational aspects of t he method when applied to the solution of the two-level-atom line tran sfer problem. At each iteration step, angle and frequency averaged dep th-dependent factors defined as the ratios of the relevant intensity m oments are computed form the current values of the radiation field. Th ese factors are then used to close the system of the radiative transfe r equation moments. Due to the quasi-invariance of the factors, the ex act solution of the system is achieved within only a few iterations, e ven under physical conditions very far from LTE.