Electron diffusion and advection during nonlinear interactions with whistler‐mode waves

Allanson, Oliver, Watt, Clare, Allison, H. J. and Ratcliffe, H. (2021) Electron diffusion and advection during nonlinear interactions with whistler‐mode waves. Journal of Geophysical Research: Space Physics, 126 (5). e2020JA028793. ISSN 2169-9380

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Official URL: https://doi.org/10.1029/2020JA028793

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 occurs during the growth phase ( ≈ 100ms). These dynamics ‘saturate' as the wave power saturates at ≈ 1nT, 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 modelling.

Item Type: Article
Additional Information: Funding information: The authors gratefully acknowledge Sarah Glauert (British Antarctic Survey) for the use of the PADIE software. This research was supported by the Natural Environment Research Council (NERC) High light Topic Grants #NE/P017274/1 (Rad-Sat, OA and CEJW). This work was in part funded by the UK EPSRC grants EP/G054950/1, EP/G056803/1, EP/G055165/1 and EP/ M022463/1 (the EPOCH code). This work was in part performed using the Cambridge Service for Data Driven Discovery (CSD3), part of which is operated by the University of Cambridge Research Computing on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The DiRAC component of CSD3 was funded by BEIS capital funding via STFC capital grants ST/P002307/1 and ST/R002452/1 and STFC operations grant ST/R00689X/1. DiRAC is part of the National e-Infrastructure. This work was in part performed using the Reading Academic Computing Cluster (RACC) at the University of Reading. This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk).
Subjects: F300 Physics
F500 Astronomy
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: Elena Carlaw
Date Deposited: 12 May 2021 11:53
Last Modified: 31 May 2021 14:38
URI: http://nrl.northumbria.ac.uk/id/eprint/46151

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