MULTILEVEL NLTE RADIATIVE-TRANSFER IN ISOLATED ATMOSPHERIC STRUCTURES- IMPLEMENTATION OF THE MALI-TECHNIQUE

We have developed and extensively tested a new multilevel NLTE transfe r code for isolated solar atmospheric structures (loops, prominences, spicules etc.). The code is based on the MALI approach of Rybicki and Hummer (1991, 1992) to multilevel accelerated lambda iterations. It is demonstrated that this method is fully capable of treating a difficul t problem of NLTE hydrogen excitation and ionization equilibrium, prov ided that we linearize the preconditioned statistical equilibrium equa tions with respect to atomic level populations and the electron densit y. With this generalization of the original MALI approach, the numeric al code is robust and stable. As compared to the standard linearizatio n method of Auer and Mihalas (1969), the new MALI code designed for 1D slabs is more than one order of magnitude faster and its accuracy is quite satisfactory. We discuss several details of our implementation o f the MALI technique to isolated, externally irradiated, 1D structures and finally draw some future prospects.