Beta‐Dependent Constraints on Ion Temperature Anisotropy in Jupiter's Magnetosheath. Issue 15 (5th August 2022)

• Record Type: Journal Article
• Authors: Bandyopadhyay, R.; Begley, L. J.; Maruca, B. A.; McComas, D. J.; Szalay, J. R.; Allegrini, F.; Ebert, R. W.; Gershman, D. J.; Connerney, J. E. P.; Bolton, S. J.
• Is Part Of: Geophysical research letters. Volume 49:Issue 15(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-05
• Subjects: instabilities -- plasmas -- planetary magnetospheres -- solar wind -- waves -- turbulence; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL098053
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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. Plain Language Summary: Thermal or random motion can be quantified by temperature. In most systems that we encounter in daily life, thermal motion has no preferred direction, and therefore, most of these systems can be quantified by a scalar temperature. In space plasmas, however, due to low collision rates and presence of a large‐scale mean magnetic field, the strength of thermal motion of the charged particles parallel and perpendicular to the background magnetic field, quantified by parallel and perpendicular temperature, are typically different. Using measurement from NASA's Juno spacecraft in Jupiter's magnetosheath, we present a statistical survey of the ion temperature anisotropy. A theoretical model approximately matches the variation of the anisotropy parameter. This phenomenon has been previously observed in various other space plasmas—including the solar wind (at various distances from the Sun) and in Earth's magnetosheath—which suggests that it plays a key role in energy dynamics across the solar system. Key Points: Juno observations in Jupiter's magnetosheath show a broad range of ion temperature anisotropy A narrower range of ion temperature anisotropy values is observed as the parallel ion beta increases Contours of constant growth rates, computed from linear Vlasov theory, approximately constrain the data distribution