Non-adiabatic motion of charged particles traversing a weak magnetic field: Pitch angle scattering

Abstract

A charged particle moves with velocity v in a constant non-uniform magnetic field H , spiralling with Larmor radius R . If R is small compared with the scale length L of the field, the magnetic moment associated with the Larmor motion of the particle is nearly constant. Consequently θ, the (‘pitch’) angle between v and H , varies as arcsin H 1/2 . Hence θ in such adiabatic motion is approximately the same at points on the path where H has the same value. But the magnetic moment and the pitch angle may differ materially at two such points, each in the region where R/L is small, if between them the particle traverses a region where R/L is not small. This region of non-adiabatic motion ‘scatters’ the pitch angles.