Electron Diffusion and Advection During Nonlinear Interactions With Whistler‐Mode Waves

O Allanson,H J Allison,C E J Watt,H Ratcliffe

doi:10.1029/2020ja028793

Abstract

AbstractRadiation 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.

Highlights

IntroductionWhistler-mode waves play an important role in the dynamics of electrons in energy and pitch-angle space in the Earth's inner magnetosphere (e.g., see Artemyev et al, 2016; Horne et al, 2005; Thorne, 2010), as well as a variety of laboratory, solar, space and astrophysical plasmas (e.g., see Jeong et al, 2020; Kuzichev et al, 2019; Podesta & Gary, 2011; Riquelme & Spitkovsky, 2011; Stawicki et al, 2001; Stenzel, 1999; Tong et al, 2019; Verscharen & Chandran, 2013)
Whistler-mode waves play an important role in the dynamics of electrons in energy and pitch-angle space in the Earth's inner magnetosphere, as well as a variety of laboratory, solar, space and astrophysical plasmas
Our experiments demonstrate/confirm that for “high” levels of anisotropy: (i) the wave power can reach “high” amplitudes ≈ 1 nT such that Bw ∼ O(B0/100); (ii) the high amplitude of the final saturation state is essentially unaffected by the lower amplitude incident wave power; (iii) but that saturation is reached more quickly for a higher amplitude of incident wave; (iv) wave power can rapidly spread over wider regions of frequency space over the wave-growth period, but stop spreading as the wave power saturates

Summary

Introduction

Whistler-mode waves play an important role in the dynamics of electrons in energy and pitch-angle space in the Earth's inner magnetosphere (e.g., see Artemyev et al, 2016; Horne et al, 2005; Thorne, 2010), as well as a variety of laboratory, solar, space and astrophysical plasmas (e.g., see Jeong et al, 2020; Kuzichev et al, 2019; Podesta & Gary, 2011; Riquelme & Spitkovsky, 2011; Stawicki et al, 2001; Stenzel, 1999; Tong et al, 2019; Verscharen & Chandran, 2013). Two of the core assumptions of this theory are that: (i) the wave amplitude is a small perturbation to the background field; (ii) and that the wave spectrum is constant in time over the duration for which the diffusion coefficient is calculated — the so-called “limit of resonant diffusion” (Kennel & Engelmann, 1966) (i.e., electromagnetic waves have zero growth rate). Up to 30% of long-duration chorus waves possess properties that indicate that the quasilinear treatment should fail (Zhang et al, 2018, 2019). These are termed “nonlinear wave-particle interactions.” Despite the fact that the waves evident from these observations appear to violate the above assumption, diffusion codes based on the quasilinear theory can yield very good results (e.g., see Glauert et al, 2018; W. Li et al, 2014; Thorne et al, 2013)

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