A Review of ULF Interactions With Radiation Belt Electrons

Energetic particle fluxes in the outer zone radiation belts can vary over orders of magnitude on a variety of timescales. Power at ULF frequencies, on the order of a few millihertz, have been associated with changes in flux levels among relativistic electrons comprising the outer zone of the radiation belts. Power in this part of the spectrum may occur as a result of a number of processes, including internally-generated waves induced by plasma instabilities, and externally generated processes such as shear instabilities at the flanks or compressive variations in the solar wind. Changes in the large-scale convective motion of the magnetosphere are another important class of externally driven variations with power at ULF wavelengths. The mechanism for interaction between ULF variations and the radiation belts may result in (or require) pitch angle scattering, or may conserve the first two adiabatic invariants of particle motion. Of the latter class of interactions, radial diffusion describes the result when ULF variations lead to stochastic motion among the particle populations, and has been studied extensively as a description of radial transport within the belts. Rates of radial diffusion depend strongly on the characteristics of the driving ULF waves. This work is intended as a non-exhaustive review of radiation belt interactions with ULF waves, outlining the current theories and methods in studying the interaction, and describing pertinent wave properties.