PARTIAL REDISTRIBUTION IN MULTILEVEL ATOMS .1. METHOD AND APPLICATIONTO THE SOLAR HYDROGEN LINE FORMATION

We present a robust method for solution of multilevel non-LTE line tra nsfer problems including the effects of partial frequency redistributi on (PRD). This method allows the self-consistent solution for redistri bution of scattered line photons simultaneously in multiple transition s of a model atom, including the effects of resonant Raman scattering (''cross-redistribution'') among lines sharing common upper levels. Th e method is incorporated into the framework of the widely used non-LTE complete redistribution code MULTI. We have applied this method to th e problem of transfer in hydrogen lines in a plane-parallel solar mode l atmosphere, including cross-redistribution between the H alpha and L beta, using general redistribution functions for the L alpha and L be ta lines which are not restricted by the impact approximation. The con vergence properties of this method are demonstrated to be comparable t o that of the equivalent complete redistribution problem. In this sola r model, PRD in the L alpha line produces the dominant influence on th e level populations. It changes considerably the populations of the ex cited states of hydrogen, as well as the proton number density, in the middle and upper chromosphere, owing to modification of the L alpha w ing radiation. The population of the hydrogen ground state undergoes o nly modest changes, however. The influence of cross-redistribution and PRD in L beta has a much smaller influence on the level populations b ut a considerable influence on the wing intensity of the L beta line.