NLTE Spectral Line Formation in a Three-Dimensional Atmosphere with Velocity Fields

Abstract

A method to solve the “two-level-atom-with-overlapping-continuum” problem in a three-dimensional atmosphere is presented. The method is based on treating the radiative transfer along a number of rays through the model as separate sub-problems. In each iteration the error in the source function is evaluated along all the rays through the model and an estimate of the necessary correction is obtained for each ray. The converged solution is an exact solution to the problem. The method requires a computer time per iteration equivalent to the solution of N one-dimensional two-level atom problems, plus 2N solutions of the radiative transfer problem with given source function, where N is the number of rays = NΩNXNY. As an application, the method is used on the case of a neutral iron line in the solar photosphere. The solar photosphere is represented by u snapshot from three-dimensional hydrodynamic simulations of the solar granulation. The presence of the granular velocity field has e substantial influence on the behavior of the line source function in the upper photosphere. The resulting line source function exceeds the local Planck function in most of the upper photosphere, thus decreasing the central depth of synthesized spectral line profiles, relative to the LTE case.