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

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.

arXiv commentSubmitted to The Astrophysical Journal Letters

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Cited by in corpus (8)

• The multi-scale nature of the solar wind
• Linear Stability in the Inner Heliosphere: Helios Reëvaluated
• Observational evidence for solar wind proton heating by ion-scale turbulence
• Intermittency and Ion Temperature-Anisotropy Instabilities: Simulation and Magnetosheath Observation
• Interplay of Turbulence and Proton-Microinstability Growth in Space Plasmas
• Fast Ion Isotropization by Current Sheet Scattering in Magnetic Reconnection Jets
• Spontaneous Magnetic Fluctuations and Collisionless Regulation of Turbulence in the Earth's Magnetotail
• Numerical simulations of temperature anisotropy instabilities stimulated by suprathermal protons