Statistical Investigation of Deformation of Electron Pitch Angle Distributions Associated With Chorus Waves Observed by the Arase Satellite

Whistler-mode chorus waves play important roles in the development of energetic electron populations in the Earth's inner magnetosphere. We have statistically analyzed rapid changes in the electron flux associated with chorus waves using data from the Arase satellite. The Arase satellite observations obtained from 23 March 2017 to 12 October 2018 show that the rapid changes are concentrated near the magnetic equator from nightside to dawnside. We compared the energy and pitch angle range of the rapid changes in the electron flux with the region bounded by the resonance energy curve of whistler mode waves which are calculated from properties of the observed chorus waves in 46 events. This comparison shows that, for most of the events, the energy and pitch angle range of the rapid changes in the electron flux can be explained by the first-order cyclotron resonance with the observed chorus waves. We also found that the timescale for the change in the electron pitch angle distribution ranges from several seconds to a few tens of seconds. This timescale is much faster than that expected by quasi-linear diffusion theory, suggesting that nonlinear wave-particle interactions play important roles in the deformation of the electron pitch angle distributions.