MMS Observations of Beta-Dependent Constraints on Ion Temperature-Anisotropy in Earth's Magnetosheath

TL;DR

This study uses MMS data to investigate how plasma beta influences ion temperature anisotropy limits in Earth's magnetosheath, revealing that kinetic instabilities likely regulate these anisotropies.

Contribution

First to analyze beta-dependent ion temperature anisotropy constraints in Earth's magnetosheath using MMS data, linking observed limits to kinetic microinstability thresholds.

Findings

Data distribution aligns with instability thresholds for various microinstabilities.

Both $R_p>1$ and $R_p<1$ anisotropy limits are observed.

Instability thresholds closely match the observed anisotropy boundaries.

Abstract

Protons (ionized hydrogen) in the solar wind frequently exhibit distinct temperatures ($T_{\perp p}$ and $T_{\parallel p}$) perpendicular and parallel to the plasma's background magnetic-field. Numerous prior studies of the interplanetary solar-wind have shown that, as plasma beta ($\beta_{\parallel p}$) increases, a narrower range of temperature-anisotropy ($R_p\equiv T_{\perp p}\,/\,T_{\parallel p}$) values is observed. Conventionally, this effect has been ascribed to the actions of kinetic microinstabilities. This study is the first to use data from the Magnetospheric Multiscale Mission (MMS) to explore such $\beta_{\parallel p}$-dependent limits on $R_p$ in Earth's magnetosheath. The distribution of these data across the $(\beta_{\parallel p},R_p)$-plane reveals limits on both $R_p>1$ and $R_p<1$. Linear Vlasov theory is used to compute contours of constant growth-rate for the ion-cyclotron, mirror, parallel-firehose, and oblique-firehose instabilities. These instability thresholds closely align with the contours of the data distribution, which suggests a strong association of instabilities with extremes of ion temperature anisotropy in the magnetosheath. The potential for instabilities to regulate temperature anisotropy is discussed.