Electron-driven instabilities in the solar wind

TL;DR

This paper reviews electron-driven micro-instabilities in the solar wind, developing a semi-graphical quasi-linear framework to describe various resonant and non-resonant instabilities caused by non-equilibrium electron features.

Contribution

It introduces a semi-graphical quasi-linear framework for analyzing electron-driven instabilities in the solar wind, covering a wide range of resonant and non-resonant mechanisms.

Findings

Describes resonant instabilities like whistler and firehose driven by temperature anisotropies.

Details instabilities caused by reflectional asymmetries, including Langmuir and electron-beam.

Briefly discusses non-resonant instabilities such as mirror-mode and firehose.

Abstract

The electrons are an essential particle species in the solar wind. They often exhibit non-equilibrium features in their velocity distribution function. These include temperature anisotropies, tails (kurtosis), and reflectional asymmetries (skewness), which contribute a significant heat flux to the solar wind. If these non-equilibrium features are sufficiently strong, they drive kinetic micro-instabilities. We develop a semi-graphical framework based on the equations of quasi-linear theory to describe electron-driven instabilities in the solar wind. We apply our framework to resonant instabilities driven by temperature anisotropies. These include the electron whistler anisotropy instability and the propagating electron firehose instability. We then describe resonant instabilities driven by reflectional asymmetries in the electron distribution function. These include the electron/ion-acoustic, kinetic Alfv\'en heat-flux, Langmuir, electron-beam, electron/ion-cyclotron, electron/electron-acoustic, whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid fan, and electron-deficit whistler instability. We briefly comment on non-resonant instabilities driven by electron temperature anisotropies such as the mirror-mode and the non-propagating firehose instability. We conclude our review with a list of open research topics in the field of electron-driven instabilities in the solar wind.