Nonlinear interactions between whistler mode chorus waves and energetic electrons in the Earth’s radiation belts

Abstract:  Plasma waves are key drivers of the highly variable electron dynamics in Earth’s outer radiation belts. Particularly, whistler-mode chorus waves, which are commonly observed with intense wave amplitudes, are known to be a key driver of rapid electron acceleration and precipitation observed by many recent satellite (e.g., ARASE, ELFIN, THEMIS, and Van Allen Probes), and balloon missions (BARREL). However, quantitative understanding of the electron acceleration and precipitation due to the nonlinear interactions with chorus waves is limited. Research presented in this talk systematically evaluates the nonlinear effects of chorus waves in the full electron pitch angle-energy space using test particle simulations, quasilinear models, as well as satellite observations. More specifically, the dependences of nonlinear effects on the chorus wave amplitude modulation (waveform structures), as well as wave amplitude and frequency bandwidth (spectrum structures), are quantified over a wide range of wave parameters. The results show that realistic chorus wave structures tend to limit the nonlinear effects on energetic electrons. The system can be described by a diffusion model similar to quasilinear theory, but nonlinear effects alter the diffusion coefficients from quasilinear ones. I further demonstrate that nonlinear phase trapping by the upper-band chorus waves can efficiently accelerate electrons to form the distinct butterfly pitch angle distribution within 30 seconds, using an intriguing event observed by the Van Allen Probes. The effects of nonlinear interaction (Landau trapping) on electron precipitation are also evaluated during a bursty electron precipitation event observed by the ELFIN CubeSats, in association with very oblique chorus waves observed by THEMIS near the equatorial plane. The test particle simulation results provide the first direct evidence of rapid (~5 s) electron precipitation driven by high-order resonances due to chorus waves. Overall, this presentation provides a full quantification of nonlinear effects and their dependences on various electron and chorus wave parameters. The findings are crucial to our fundamental understanding of wave-particle interactions, particularly on a short timescale in the Earth’s radiation belts, as well as in other space plasma environments, such as solar wind and other planets, as well as astrophysical and laboratory plasmas.

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