THE ROLE OF THE LINE-PROFILE IN RADIATIVE-TRANSFER IN EXPANDING PLASMAS

The Sobolev approximation is frequently used in order to simplify the problem of line radiation transport in differentially expanding atmospheres. The only condition required is that the velocity gradient be large enough to ensure that the Sobolev length is much smaller than any spatial length scales in the problem (the infinite medium approximation). In this limit, the specific form of the line profile is not included within the Sobolev theory. We investigate, using a Monte-Carlo calculation, the interplay between the line profile, the Sobolev length (or velocity gradient) and the spatial variation within a non-uniform, expanding plasma. We also discuss the difficulties involved in the definition of the Sobolev length parameter for various different line profiles.