Particle diffusion, flows, and NLTE calculations

Particle diffusion plays an important role in the transition-region between the chromosphere and the corona of the Sun and solar-type stars. Also, mass flows are often observed and these in combination with diffusion make it necessary to perform physically consistent calculations to determine the elemental ionization and atom/ion level populations. Such calculations require the solution of NLTE radiative transfer equations to determine the ionization and level populations, and need to include the effects of particle transport processes. This paper discusses the basis and implementation of the method used in the Solar Radiation Physical Modeling (SRPM) project, also implemented for H and He in the PANDORA code.