On the time-dependent radiative transfer in photospheric plasmas

The paper is the first in a series investigating time-dependent radiative transfer processes of x rays in photospheric plasmas at non-relativistic temperatures. An analytical random walk model is presented describing the diffusion of photons in a homogeneous spherical geometry. The starting point is a set of moment equations derived earlier from the relativistic Boltzmann equation. Green functions for every individual scattering order are derived. The total Green function is the sum over all scattering orders and it solves the hyperbolic diffusion equation. An important element of flexibility-arises from the introduction of the so-called characteristic parameter chi which is basically the inverse of the characteristic velocity. By adjusting chi for the low scattering orders, agreement with numerical simulations is obtained. (author)