On the validity of weak particle-field interaction theory for the description of cosmic-ray particle diffusion in random magnetic fields

Abstract

Since considerable debate has arisen recently about the applicability of the weak-interaction theory for the description of cosmic-ray particle diffusion in random magnetic fields, a critical discussion of the conditions of validity of this theory is presented. It is pointed out that the singular behaviour of the diffusion coefficient at pitch angles of θ=90° does not lead to a breakdown of the weak-interaction theory, though an extension of pitch-angle scattering analysis to higher order may be necessary.

In particular, an investigation of the δ-function contribution to the diffusion coefficient at pitch angles of 90° discovered by Fisket al. (1974) indicates that it has no influence on either pitch-angle scattering or the associated problem of particle diffusion parallel to the mean magnetic field. Although an infinite diffusion coefficient at 90° pitch angles implies a singualr breakdown of the weak-interaction theory at that point, the effects are purely local and are negligible when considering averaged particle properties.