MMS Observations of Beta-dependent Constraints on Ion Temperature Anisotropy in Earth’s Magnetosheath

Protons (ionized hydrogen) in the solar wind frequently exhibit distinct temperatures ( T ⊥p and T ∥p ) perpendicular and parallel to the plasma’s background magnetic field. Numerous prior studies of the interplanetary solar wind have shown that, as proton beta ( β ∥p ) increases, a narrower range of proton temperature anisotropy ( R p ≡ T ⊥p / T ∥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 to explore such β ∥p -dependent limits on R p in Earth’s magnetosheath. The distribution of these data across the ( β ∥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 is consistent with these instabilities acting to limit extremes of proton temperature anisotropy in the magnetosheath.