Electron velocity distribution instability in magnetized plasma wakes and artificial electron mass

TL;DR

This paper investigates electron velocity distribution instabilities in plasma wakes behind large objects in space, revealing how artificial electron mass in simulations can lead to unphysical instability results.

Contribution

It demonstrates the formation of a bump-on-tail instability in electron distributions within plasma wakes and analyzes the effects of artificially increased electron mass on instability.

Findings

Electron bump-on-tail instability occurs in plasma wakes due to non-conservation of parallel energy.

Electron plasma turbulence peaks near the wake axis.

Artificially increased electron mass in simulations causes exaggerated, unphysical instabilities.

Abstract

The wake behind a large object (such as the moon) moving rapidly through a plasma (such as the solar wind) contains a region of depleted density, into which the plasma expands along the magnetic field, transverse to the flow. It is shown here that (in addition to any ion instability) a bump-on-tail which is unstable appears on the electrons' parallel velocity distribution function because of the convective non-conservation of parallel energy. It arises regardless of any non-thermal features on the external electron velocity distribution. The detailed electron distribution function throughout the wake is calculated by integration along orbits; and the substantial energy level of resulting electron plasma (Langmuir) turbulence is evaluated quasilinearly. It peaks near the wake axis. If the mass of the electrons is artificially enhanced, for example in order to make numerical simulation feasible, then much more unstable electron distributions arise; but these are caused by the unphysical mass ratio.