Rapid Frequency Variations within Intense Chorus Wave Packets: Constraints on the Nonlinear Resonant Electron Acceleration

Abstract:

Whistler-mode chorus waves are responsible for electron acceleration in Earth’s radiation belts. It is unclear, however, whether the observed acceleration can be well described by quasi-linear theory, or if this acceleration is due to intense waves that require nonlinear treatment. We perform a comprehensive statistical analysis of intense lower-band chorus wave packets to investigate the relationships between wave frequency variations, packet length, and wave amplitude, and their temporal variability. We find that 15% of the wave power is carried by long packets, with low frequency sweep rates (linear trend in time) that agree with the nonlinear theory of chorus wave growth. Eighty-five percent of the wave power, however, comes from short packets with large frequency variations around the linear trend. The kappa-like probability distribution of these variations is consistent with random superposition of different waves. We examine effects of these complex wave fields on electron nonlinear scattering and acceleration. Rapid variations of the wave frequency could result in a destruction of nonlinear resonant interaction and reduction of the electron acceleration efficiency.