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- Title
Electron Diffusion and Advection During Nonlinear Interactions With Whistler‐Mode Waves.
- Authors
Allanson, O.; Watt, C. E. J.; Allison, H. J.; Ratcliffe, H.
- Abstract
Radiation belt codes evolve electron dynamics due to resonant wave‐particle interactions. It is not known how to best incorporate electron dynamics in the case of a wave power spectrum that varies considerably on a "sub‐grid" timescale shorter than the computational time‐step of the radiation belt model ΔtRBM, particularly if the wave amplitude reaches high values. Timescales associated with the growth rate of thermal instabilities are very short, and are typically much shorter than ΔtRBM. We use a kinetic code to study electron interactions with whistler‐mode waves in the presence of a thermally anisotropic background. For "low" values of anisotropy, instabilities are not triggered and we observe similar results to those obtained in Allanson et al. (2020, https://doi.org/10.1029/2020JA027949), for which the diffusion roughly matched the quasilinear theory over short timescales. For "high" levels of anisotropy, wave growth via instability is triggered. Dynamics are not well described by the quasilinear theory when calculated using the average wave power. Strong electron diffusion and advection occur during the growth phase (≈100 ms). These dynamics "saturate" as the wave power saturates at ≈ 1 nT, and the advective motions dominate over the diffusive processes. The growth phase facilitates significant advection in pitch angle space via successive resonant interactions with waves of different frequencies. We suggest that this rapid advective transport during the wave growth phase may have a role to play in the electron microburst mechanism. This motivates future work on macroscopic effects of short‐timescale nonlinear processes in radiation belt modeling. Plain Language Summary: Naturally and anthropogenically generated electromagnetic waves interact strongly with ambient charged particles in near‐Earth space. These interactions can lead to rapid particle energization, and modern society is underpinned by the hundreds of radiation‐vulnerable satellites that orbit within this hazardous radiation environment. It is therefore important to be able to model the evolution of these charged particles to mitigate risk as well as possible. Here, we study the response of electrons to electromagnetic "whistler‐mode" waves using a fully physics‐based code. The main advance is that we consider this interaction in the context of a realistically diverse plasma composition, including components that are unstable. This allows us to consider the competing effects of incident waves, and naturally generated waves arising from the unstable plasma background. We find that for "typical/low" values of unstable plasma, the electron dynamics can be well modeled by the standard theory. However, for a more highly unstable background, we observe very different dynamics. This study motivates future work on how to incorporate the effect of an unstable plasma background in the modeling of electron dynamics. Key Points: Electron dynamics during thermal instability not reproduced by resonant diffusion quasilinear approach using average wave powerStrong and rapid (≈100 ms) advection and diffusion measured in growth phase of high amplitude (≈1 nT) whistler‐mode wavesResults suggest that nonlinear dynamics during equatorial whistler‐mode wave‐growth phase could contribute to microburst mechanism
- Subjects
ELECTRON diffusion; RADIATION belts; WAVE-particle interactions; PARTICLE interactions; ENHANCED magnetoresistance
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 5, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA028793