Radiative Transfer of Doppler Broadened Resonance Lines

Abstract

The radiative transfer of Doppler broadened resonance lines in a plasma which is not in local thermodynamic equilibrium is studied for a simple model of an infinite, uniform plane parallel plasma. A two-level atomic model is used in which electrons are transferred between the levels by electron excitation and de-excitation, spontaneous emission and photo-excitation. Stimulated emission is neglected. It is assumed that the source function is independent of frequency and that the spontaneous emission is isotropic and unpolarized. The radiation is emitted in a Doppler broadened profile in which natural and pressure broadening may be neglected. In the calculations the population of the excited level is assumed to be much smaller than the ground level. The distribution of excited atoms in a steady state with their own resonance radiation has been calculated, and from this the rate of loss of energy, and the line profiles and intensities are obtained. The rate of loss of energy is compared with a model of diffusion of photons in frequency space. The line profiles are self-reversed although the calculations are for a uniform plasma. The validity of the assumptions is examined and the restrictions they place on the range of application are considered with particular reference to Lyman α radiation.