Absorption of radiation by electrons in intense magnetic fields

Abstract

This paper presents a quantum-mechanical treatment of photon absorption by a homogeneous electron plasma in a uniform magnetic field, in the anisotropic limit for which the electron energies are concentrated in their motion parallel to the field. The results are intended to be appropriate especially for intense fields (in which quantization effects and highly anisotropic plasmas are to be expected). The electrons are here specifically assumed to be initially in their lowest orbital states, although their parallel momenta are taken to be given by a continuous but otherwise arbitrary distribution function. It is found that the kinematical restrictions on the absorption process lead to interesting selection effects in both frequencies and polarizations, and that the photon attenuation coefficients (considered as functions of frequency and angle of incidence) can exhibit discontinuities and singularities as well as continuous featurs whose form and range are determined by both the field strength and the electron momentum distribution. Limiting cases of small angles and low frequencies are established, and numerical examples are postulated to illustrate these effects and suggest their significance in pulsar magnetospheres. The conditions determining the validity of the first-order approximation derived here are also investigated, and in this context the combined first-order processes of absorption and subsequent reemission are explicitly related to a corresponding resonance effect in Compton scattering.