Quasilinear and Nonlinear Electron Interactions with Whistler-Mode Waves

Abstract:

Numerous recent spacecraft observations indicate the prevalence of large amplitude whistler-mode waves within the Earth’s inner magnetosphere. Individual electron dynamics due to interactions with these so-called nonlinear waves are known in principle to be radically different to those that are consistent with the current standard quasi-linear modelling approach. It is important to try and understand when and how the quasi-linear or nonlinear regime is dominant. Test particle codes indicate that electron dynamics due to interactions with low amplitude incoherent whistler mode‐waves can be adequately described by quasi‐linear theory. Using the particle-in-cell method that was introduced in Allanson et al. (2019, https://doi.org/10.1029/2019JA027088), we track the dynamical response of electrons due to interactions with incoherent whistler‐mode waves, across all energy and pitch angle space. We conduct five experiments each with different values of the electromagnetic wave amplitude. We find that the electron dynamics agree well with the quasi‐linear theory diffusion coefficients for low amplitude incoherent waves with (δB/B)² ≈ 3.7·10^-10 , over a time scale T of the order of 1,000 gyroperiods. However, the resonant interactions with higher amplitude waves cause significant non-diffusive (advective) dynamics as well as diffusive dynamics in energy and pitch-angle space. When considered over appropriately shorter time scales (of the order of hundreds or tens of gyroperiods), the diffusive component of the dynamics agrees well with the predictions of quasi‐linear theory, even for wave amplitudes up to (δB/B)² ≈ 5.8·10^-6. Quasi‐linear theory is based on fundamentally diffusive dynamics, but the evidence presented herein also indicates the existence of a distinct advective component. Therefore, the proper description of electron dynamics in response to wave‐particle interactions with higher amplitude whistler‐mode waves may require Fokker‐Planck equations that incorporate diffusive and advective terms.

Allanson, O. et al. (2020). Particle‐in‐cell experiments examine electron diffusion by whistler‐mode waves: 2. Quasilinear and nonlinear dynamics. Journal of Geophysical Research: Space Physics, 125. https://doi.org/10.1029/2020JA027949