Wave-particle interactions with parallel whistler waves: nonlinear and time-dependent effects revealed by Particle-in-Cell simulations

TL;DR

This study uses Particle-in-Cell simulations to reveal nonlinear and time-dependent effects in wave-particle interactions in Earth's radiation belt, challenging traditional quasi-linear models and highlighting the importance of nonlinear dynamics.

Contribution

It demonstrates the significance of nonlinear and time-dependent effects in wave-particle interactions, which are often neglected in standard quasi-linear theories used in radiation belt modeling.

Findings

Pitch angle diffusion is enhanced during linear growth and saturates quickly.

Saturation is linked to bounded diffusion domains and the 90° diffusion barrier.

Traditional bounce-averaged models may underestimate rapid diffusion effects.

Abstract

We present a self-consistent Particle-in-Cell simulation of the resonant interactions between anisotropic energetic electrons and a population of whistler waves, with parameters relevant to the Earths radiation belt. By tracking PIC particles, and comparing with test-particle simulations we emphasize the importance of including nonlinear effects and time evolution in the modeling of wave-particle interactions, which are excluded in the resonant limit of quasi- linear theory routinely used in radiation belt studies. In particular we show that pitch angle diffusion is enhanced during the linear growth phase, and it rapidly saturates well before a single bounce period. This calls into question the widely used bounce average performed in most radiation belt diffusion calculations. Furthermore we discuss how the saturation is related to the fact that the domain in which the,particles pitch angle diffuse is bounded, and to the well-known problem of $90^\circ$ diffusion barrier diffusion barrier.