A&A Supplement series, Vol. 127, February I 1998, 607-618
Received July 29; accepted May 27, 1997
Send offprint request: T. Nejezchleba
Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-25165 Ondrejov, Czech Republic
A method of calculating NLTE models of a plan-parallel solar flare atmosphere with stationary plasma flows is presented. To solve the radiative transfer equation, equations of statistical equilibrium (ESE) and the stationary momentum equation, we use the multilevel approximate lambda iteration (MALI) approach. The numerical code is based on the method recently developed by Rybicky & Hummer (1991, 1992) and allows to take into account macroscopic velocity fields in observer's frame formulation. The preconditioned ESE, constructed by this method, are finally linearized with respect to level populations and electron densities to treat the the hydrogen ionization balance. The numerical code based on this method is used to compute a grid of NLTE flare models with various velocity fields in order to show the influence of the velocity fields on the -line asymmetries.
The analysis of the results lead to conclusions that would improve interpretations of flare line asymmetries: 1) The velocities affect the level populations, 2) The type of asymmetry depends on the changes in the optical depth scales and on the run of the source function in the atmosphere. 3) A monotonous velocity affects only one wing of the line profile. 4) To get the velocity field from an observed profile the bisector method should be modified.
keywords: line: formation -- line: profiles -- radiative transfer -- methods: numerical -- Sun: flares