Beta-Dependent Constraints on Ion Temperature Anisotropy in Jupiter’s Magnetosheath

Most space plasmas are weakly collisional. Due to the low collision rate, the plasmas are routinely out of local thermal equilibrium and often exhibit non-Maxwellian velocity distributions. The charged particles typically exhibit anisotropy in temperature measurements, that is, distinct temperatures are observed perpendicular and parallel (T_⊥i and T_||i) to the local magnetic field. Numerous prior studies have shown that for increasing values of parallel ion beta (β_||i), the range of ion temperature anisotropy (R_i = T_⊥i/T_||i) values becomes narrower. Conventionally, this behavior has been attributed to the actions of kinetic microinstabilities. This study is the first to explore such β_||i-dependent limits on ion temperature anisotropy in Jupiter's magnetosheath. We use linear Vlasov theory to compute contours of constant growth rate for different instability thresholds, which closely align with the limits of the data distribution, supporting that these instabilities are acting to limit extremes of ion temperature anisotropy in the Jovian magnetosheath.