13 - Instabilities due to anisotropic fast particles

Summary

Resonant scattering

The scattering rate due to Coulomb interactions between a fast particle (speed v) and thermal particles decreases with increasing v as v⁻³. Thus one might expect that fast particles are scattered very ineffectively. The reverse is the case in the low density (low β) plasmas in the magnetosphere, and in space and astrophysical plasmas generally. Fast particles are scattered very efficiently due to resonant interaction with low frequency waves, called resonant scattering.

The evidence which led to the initial development of the theory of resonant scattering came from the properties of the trapped particles in the magnetosphere. By the early 1960's it was clear that for the stability of the distributions of trapped magnetospheric particles (in the terrestrial ‘radiation’ or ‘van Allen’ belts) to be consistent with the observations of precipitation of these particles, both the fast electrons and the fast ions must be scattered very efficiently. The development of the theory of resonant scattering led to a satisfactory qualitative and semi-quantitative explanation for these magnetospheric observations. Resonant scattering also offered ways of resolving serious difficulties connected with the scattering and acceleration of fast particles in astrophysical plasmas. The most obvious of these concerns the confinement of galactic cosmic rays (§13.4). Another serious difficulty was with the acceleration of fast particles: early theories for the acceleration required (either implicitly or explicitly) very efficient scattering.