Electron mirror branch: Observational evidence from "historical" AMPTE-IRM and Equator-S measurements

TL;DR

This study presents observational evidence from historical spacecraft data supporting the existence of an electron mirror branch in the magnetosheath, indicating that mirror modes may operate in a weak turbulence state due to electron temperature anisotropy.

Contribution

It provides the first observational hints of the electron mirror branch in the magnetosheath, aligning with recent theoretical predictions and highlighting the potential for mirror modes to escape quasilinear saturation.

Findings

Signatures of electron effects on mirror modes observed in historical data.

Mirror mode amplitudes suggest weak kinetic plasma turbulence.

Evidence supports the existence of a separate electron mirror branch.

Abstract

Based on now historical magnetic and plasma data and available wave spectra from the AMPTE-IRM spacecraft, and on as well historical Equator-S high-cadence magnetic field observations of mirror modes in the magnetosheath near the dayside magnetopause, we present some observational evidence for a recent theoretical evaluation by Noreen et al. (2017) of the contribution of a global electron temperature anisotropy to the evolution of mirror modes in the high-temperature anisotropic collisionless plasma of the magnetosheath causing a separate electron mirror branch. These old data most probably indicate that signatures of this electron effect on mirror modes had indeed been observed already long ago in magnetic and wave data though had not been recognised as such. Unfortunately either poor time resolution or complete lack of plasma data would have inhibited the confirmation of the notoriously required pressure balance in the electron branch for unambiguous confirmation of a separate electron mirror mode. If confirmed by future high-resolution observations, in both cases the large mirror mode amplitudes suggest that mirror modes escape quasilinear saturation being in a state of weak kinetic plasma turbulence.