Resonant and non-resonant whistlers-particle interaction in the radiation belts

TL;DR

This study compares PIC simulations and quasi-linear diffusion models of wave-particle interactions in the radiation belts, revealing significant discrepancies and highlighting the need for nonlinear theories to accurately describe non-resonant scattering processes.

Contribution

It provides the first direct comparison between PIC simulations and quasi-linear diffusion models for whistler-electron interactions in the radiation belts, emphasizing the limitations of linear approaches.

Findings

Quantitative mismatch between PIC and diffusion results in non-resonant regions.

Overestimation of scattering near the loss cone by the diffusion code.

Nonlinear effects are necessary for accurate modeling of wave-particle interactions.

Abstract

We study the wave-particle interactions between lower band chorus whistlers and an anisotropic tenuous population of relativistic electrons. We present the first direct comparison of first-principle Particle-in-Cell (PIC) simulations with a quasi-linear diffusion code, in this context. In the PIC approach, the waves are self-consistently generated by a temperature anisotropy instability that quickly saturates and relaxes the system towards marginal stability. We show that the quasi-linear diffusion and PIC results have significant quantitative mismatch in regions of energy/pitch angle where the resonance condition is not satisfied. Moreover, for pitch angles close to the loss cone the diffusion code overestimates the scattering, particularly at low energies. This suggest that higher order nonlinear theories should be taken in consideration in order to capture non-resonant interactions, resonance broadening, and to account for scattering at angles close to $90^\circ$.