Electron temperature anisotropy in an expanding plasma: Particle-in-Cell simulations

TL;DR

This study uses particle-in-cell simulations to investigate how electron temperature anisotropy develops in an expanding plasma, relevant to stellar winds, balancing expansion effects and electromagnetic instabilities.

Contribution

It provides the first self-consistent simulation analysis of electron temperature anisotropy evolution in expanding plasmas, considering different flow regimes.

Findings

Expansion increases parallel temperature anisotropy.

Firehose instability reduces anisotropy.

Results applicable to solar and stellar wind modeling.

Abstract

We perform fully-kinetic particle-in-cell simulations of an hot plasma that expands radially in a cylindrical geometry. The aim of the paper is to study the consequent development of the electron temperature anisotropy in an expanding plasma flow as found in a collisionless stellar wind. Kinetic plasma theory and simulations have shown that the electron temperature anisotropy is controlled by fluctuations driven by electromagnetic kinetic instabilities. In this study the temperature anisotropy is driven self-consistently by the expansion. While the expansion favors an increase of parallel anisotropy ($T_\parallel>T_\perp$), the onset of the firehose instability will tend to decrease it. We show the results for a supersonic, subsonic, and static expansion flows, and suggest possible applications of the results for the solar wind and other stellar winds.