Non-Maxwellianity of electron distributions near Earth's magnetopause

TL;DR

This study quantifies how electron distributions near Earth's magnetopause deviate from Maxwellian shapes, revealing the influence of plasma processes and identifying regions with significant non-Maxwellian behavior.

Contribution

It provides a comprehensive statistical analysis of electron distribution deviations using MMS data, highlighting the primary sources and regions of non-Maxwellianity in Earth's near-space environment.

Findings

Large non-Maxwellianities are linked to hot and cold electron components.

Significant deviations occur in bowshock, magnetosphere, and reconnection regions.

Enhanced non-Maxwellianities are intermittently observed in the turbulent magnetosheath.

Abstract

Plasmas in Earth's outer magnetosphere, magnetosheath, and solar wind are essentially collisionless. This means particle distributions are not typically in thermodynamic equilibrium and deviate significantly from Maxwellian distributions. The deviations of these distributions can be further enhanced by plasma processes, such as shocks, turbulence, and magnetic reconnection. Such distributions can be unstable to a wide variety of kinetic plasma instabilities, which in turn modify the electron distributions. In this paper the deviations of the observed electron distributions from a bi-Maxwellian distribution function is calculated and quantified using data from the Magnetospheric Multiscale (MMS) spacecraft. A statistical study from tens of millions of electron distributions shows that the primary source of the observed non-Maxwellianity are electron distributions consisting of distinct hot and cold components in Earth's low-density magnetosphere. This results in large non-Maxwellianities in at low densities. However, after performing a stastical study we find regions where large non-Maxwellianities are observed for a given density. Highly non-Maxwellian distributions are routinely found are Earth's bowshock, in Earth's outer magnetosphere, and in the electron diffusion regions of magnetic reconnection. Enhanced non-Maxwellianities are observed in the turbulent magnetosheath, but are intermittent and are not correlated with local processes. The causes of enhanced non-Maxwellianities are investigated.