Electron Loss from the Van Allen Zones due to Pitch Angle Scattering by Electromagnetic Disturbances

Abstract

Experimental observations indicate that relativistic electrons in the Van Allen zones are perturbed by physical processes that violate one or more adiabatic invariants of the electron’s motion. This paper discusses one such possible process that of pitch angle or mirror point diffusion. A qualitative explanation of pitch angle diffusion and how this process can lead to loss of trapped particles is presented, and then a Fokker-Planck type diffusion equation is introduced to describe the process mathematically. This diffusion equation is used to treat the loss of electrons from trapped orbits due to pitch angle diffusion that results from cyclotron resonant scattering of electrons by whistlers. Predictions are compared with satellite data on the loss from the Van Allen zones of electrons artificially injected by a nuclear explosion. It is found that the data indicate that pitch angle diffusion does indeed play an important role in the loss process, but the dominant mechanism producing the pitch angle scattering must be something other than cyclotron resonant scattering by whistlers since this mechanism leads to several predictions that are in disagreement with the data. These include prediction of the electron pitch angle distribution, the Variation of the pitch angle distribution with L-shell and energy, the Variation of the loss rate with energy, and the magnitude of the loss rate. Further investigation reveals that a process that, unlike whistlers, scatters electrons more or less uniformly along their spiral path would produce a pitch angle diffusion mechanism in much better accord with observation. Interaction with some type of wide band electromagnetic noise is suggested as such a process which might be of importance.