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- Title
Particle‐in‐Cell Experiments Examine Electron Diffusion by Whistler‐Mode Waves: 2. Quasi‐Linear and Nonlinear Dynamics.
- Authors
Allanson, O.; Watt, C. E. J.; Ratcliffe, H.; Allison, H. J.; Meredith, N. P.; Bentley, S. N.; Ross, J. P. J.; Glauert, S. A.
- Abstract
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. However there is significant evidence indicating that higher amplitude waves cause electron dynamics not adequately described using quasi‐linear theory. Using the 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 (Bw,rms/B0)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 nondiffusive dynamics as well as diffusive dynamics. When electron dynamics are extracted and analyzed over time scales shorter than T, we are able to isolate both the diffusive and nondiffusive (advective) dynamics. Interestingly, when considered over these 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 (Bw,rms/B0)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. Plain Language Summary: Electromagnetic waves interact strongly with charged particles in the Earth's inner magnetosphere. It is important to be able to model the evolution of these particles, since we rely on the many satellites that orbit within this hazardous radiation environment. Particle dynamics within the Earth's outer radiation belt are usually modelled using a long‐established theory fundamentally based on diffusive dynamics. We effectively benchmark this treatment for some individual cases in which one would expect agreement, that is, lower amplitude waves. We then utilize our novel method to probe cases for which the application of the standard diffusive model is questionable. We find that the resonant interactions with higher amplitude waves result in advective dynamics as well as expected diffusive behavior. When the problem is properly considered, the diffusive component of the dynamics does in fact agree well with the predictions of quasi‐linear theory. However, this is only one component of the dynamics, and one should also consider the advective component. This work motivates the use of model equations that incorporate both diffusive and other nondiffusive terms. Key Points: Whistler‐mode waves (Bw,rms/B0)2∼O(10−10)−O(10−6) in uniform B give diffusive and advective dynamicsOver appropriately short time scales the diffusive component of the dynamics agrees with quasilinear theory even for highest amplitude wavesThese time scales range from thousands to tens of gyroperiods for the lowest and highest amplitude waves, respectively
- Subjects
PARTICLE dynamics; ELECTRON diffusion; WAVE-particle interactions; ELECTROMAGNETIC waves; NONLINEAR dynamical systems
- Publication
Journal of Geophysical Research. Space Physics, 2020, Vol 125, Issue 7, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA027949