Effects of discreteness of chorus waves on quasilinear diffusion-based modeling of energetic electron dynamics

Abstract

Chorus waves are typically observed as a series of discrete, narrowband rising or falling tone elements, as opposed to a continuous uniform band, as it is often modeled. The effects of this discreteness on the applicability of quasilinear theory to interactions between electrons and parallel-propagating chorus waves in a dipole field are investigated using test particle simulations. Previous work indicated that quasilinear theory might not be directly applicable, because chorus elements are coherent or quasi-coherent. Nonlinear processes such as phase trapping and bunching were demonstrated by modeling a chorus element using a single wave. Here we represent a chorus wave field with a series of coherent elements with subpacket structures using a previously developed method involving test particle simulations to explore the applicability of quasilinear theory. By comparing electron distribution functions from test particle simulations and quasilinear predictions, we demonstrate that, besides the wave amplitude, the discreteness of chorus waves also affects the applicability of quasilinear theory. When chorus elements are close to each other and the wave amplitude is small, quasilinear theory can well describe the evolution of the electron distribution. However, when chorus elements are widely separated in space and time, the discreteness of chorus might reduce the possibility of resonant interactions between electrons and chorus. Nonlinear effects of chorus waves on electrons are also shown using the current model. The method presented in this work should be helpful for investigating the applicability of quasilinear theory in general situations. Our results should be important to understanding and modeling electrons dynamics due to interactions with chorus.