Short Chorus Packets in Radiation Belts: Statistics and Role in Energetic Electron Acceleration

Whistler-mode chorus waves contribute significantly to electron acceleration in Earth's radiation belts. It is unclear, however, whether the observed acceleration can be well described by quasi-linear theory alone, or if this acceleration is due to intense waves that require a nonlinear treatment. This paper reports on a comprehensive statistical analysis of 8 years of lower-band chorus wave packets measured by the Van Allen Probes and THEMIS spacecraft, performed to examine whether, when, and where these waves are above the theoretical threshold for nonlinear resonant wave-particle interaction. We find that ∼ 5–30% of all chorus waves may interact nonlinearly with ∼ 30–300 keV electrons. Such considerable occurrence rates of nonlinear interaction imply that the evolution of energetic electron fluxes should be dominated by nonlinear effects, rather than by quasi-linear diffusion as commonly assumed. However, we also find that only 15% of the wave power is carried by long packets considered in classical nonlinear models of wave-particle interaction, whereas 85% of the wave power comes from short packets with large frequency variations. We show that observed frequency fluctuations significantly reduce acceleration rates to realistic, moderate levels. Our results explain why global diffusive models of wave-particle interaction may statistically explain radiation belt dynamics, despite the fact that most observed wave amplitudes well exceed the maximum amplitudes for a safe application of the quasi-linear theory.