Phase Decoherence Within Intense Chorus Wave Packets Constrains the Efficiency of Nonlinear Resonant Electron Acceleration

Abstract

Electron resonant interaction with whistler mode waves is traditionally considered as one of the main drivers of radiation belt dynamics. The two main theoretical concepts available for its description are quasi-linear theory of electron scattering by low-amplitude waves and nonlinear theory of electron resonant trapping and phase bunching by intense waves. Both concepts successfully describe some aspects of wave-particle interactions but predict significantly different timescales of relativistic electron acceleration. In this study, we investigate effects that can reduce the efficiency of nonlinear interactions and bridge the gap between the predictions of these two types of models. We examine the effects of random wave phase and frequency variations observed inside whistler mode wave packets on nonlinear interactions. Our results show that phase coherence and frequency fluctuations should be taken into account to accurately model electron nonlinear resonant acceleration and that, along with wave amplitude modulation, they may reduce acceleration rates to realistic, moderate levels.

Gyroresonant wave-particle interactions with chorus waves during extreme depletions of plasma density in the Van Allen radiation belts

“…Despite being inherently a nonlinear phenomenon, quasi-linear simulations are often successful in reproducing the observed effects of chorus wave-particle interactions [e.g., (21)], when individual waves are not large enough to induce nonlinear particle behavior such as phase trapping and nonlinear scattering (22,23). Other work has recently shown that, even for intense chorus waves, a quasi-linear approach can be a good approximation, as phase decoherence can mitigate nonlinear effects (24). Quasi-linear theory (25)(26)(27) describes local acceleration as diffusion in momentum space, from low energy to high energy.…”

Generation of Realistic Short Chorus Wave Packets

“…Such short wave packets and their huge sweep rates apparently cannot be explained by nonlinear trapping in a single wave. Instead, they could simply result from a superposition of two waves, leading to wave amplitude modulation at the frequency difference Δ ω (Tao et al., 2013; Zhang, Mourenas, et al., 2020).…”

Long-term dynamics driven by resonant wave–particle interactions: from Hamiltonian resonance theory to phase space mapping

“…2018; Zhang et al. 2020 a ). Such a chaotization of wave fields is likely partly driven by currents of resonant electrons (Nunn et al.…”

“…2020 a ; Zhang et al. 2020 a ). Thus, an important further development of the mapping technique for nonlinear wave–particle interaction would require modifications of the phase trapping model.…”

“…2018, 2019), accompanied by fast, strong and random variations of wave frequency and phase (Zhang et al. 2020 a , b ), often resulting in the formation of almost independent short wave packets or subpackets (Mourenas et al. 2018; Zhang et al.…”