Interaction of a spatially uniform electron beam with a rotational magnetic hole in a form of a Harris current sheet

TL;DR

This study uses PIC simulations to explore how the width of a Harris current sheet influences electron beam relaxation, revealing that narrower sheets hinder relaxation and maintain positive velocity distribution slopes, explaining solar wind observations.

Contribution

It demonstrates that narrow Harris current sheets prevent quasi-linear relaxation of electron beams due to magnetic moment non-conservation, a novel insight into solar wind electron dynamics.

Findings

Narrower current sheets hinder electron beam relaxation.

Positive slope in electron VDF persists in narrow sheets.

Explains long-distance travel of electron beams in solar wind.

Abstract

In this work we use particle-in-cell (PIC) numerical simulations to study interaction of a spatially uniform electron beam with a rotational magnetic hole in a form of a Harris current sheet. We vary width of the Harris current sheet to investigate how this affects the quasi-linear relaxation, i.e. plateau formation of the bump-on-tail unstable electron beam. We find that when width of the Harris current sheet approaches and becomes smaller than the electron gyro-radius, quasi-linear relaxation becomes hampered and a positive slope in the electron velocity distribution function (VDF) persists. We explain this by the effects of non-conservation of electron magnetic moment, which, as recent works suggest, can maintain the positive slope of the VDF. In part, this can explain why some electron beams (the ones that interact with narrow magnetic holes with sharp boundaries, represented in our study by a Harris current sheet) in the solar wind travel much longer distances than predicted by the quasi-linear theory, at least in those cases when the electron beams slide along the current sheets that are abundant when the different-speed solar wind streams interact with each other.