Stochastic diffusion of electrons interacting with whistler-mode waves in the solar wind

TL;DR

This study investigates how high amplitude and oblique whistler waves influence electron scattering in the solar wind, revealing conditions for halo formation and emphasizing the importance of initial conditions and numerical methods in modeling stochastic particle dynamics.

Contribution

It introduces a detailed simulation approach for electron interactions with whistler waves, highlighting the role of wave amplitude and angle in particle scattering and halo formation.

Findings

High amplitude, oblique whistlers can isotropize electrons at 1 AU.

Quasi-parallel whistlers can scatter electrons to form a halo near the Sun.

A phase space volume analysis improves understanding of stochastic electron motion.

Abstract

Effects of increasing whistler amplitude and propagation angle are studied through a variational test particle simulation and calculations of the resonance width. While high amplitude and oblique whistlers in typical 1 AU solar wind parameters are capable of forming an isotropic population without any additional processes, anomalous interactions with quasi-parallel whistlers may be essential to the process of halo formation near the Sun. High amplitude and quasi-parallel whistlers can scatter strahl electrons to low velocities (less than the wave phase velocity) to form a halo population, as long as their amplitude is sufficiently high. We also present in detail a careful treatment of the sensitivity to initial conditions based on calculations of the phase space volume, which is necessary for numerical calculations of highly stochastic motion due to resonant interactions with large amplitude waves. Our method ensures that the volume-preserving characteristic of the Boris algorithm is consistently applied for simulations of both stochastic and non-stochastic particle motion.