Statistical Analysis of Ion Properties in the Martian Magnetosheath Based on MAVEN Observations: A Comparison of Core and Total Ion Populations

Abstract This study statistically compares ion properties between the thermal core and total ion populations in the Martian magnetosheath, based on observations collected by the Mars Atmosphere and Volatile EvolutioN (or MAVEN) spacecraft from 2015 to 2023. Ion moments for the core population are derived by fitting measured three-dimensional ion velocity distributions with bi-Maxwellian functions, whereas those for the total population are obtained by directly computing weighted sums over the distributions. The core population accounts for a median of approximately 80% of the total number density and dominates the bulk flow of the total population. In contrast, the total population exhibits significantly higher temperature, thermal pressure, and plasma beta, primarily due to the presence of suprathermal ions. Both populations exhibit greater temperature anisotropies downstream of quasi-perpendicular shocks compared to quasi-parallel shocks, consistent with stronger shock compression and a greater contribution from gyrating reflected ions under quasi-perpendicular conditions. Notably, the core population consistently exhibits significantly higher temperature anisotropy than the total population under both shock geometries, suggesting the presence of additional perpendicular heating mechanisms acting on the core population beyond shock compression. One plausible mechanism is stochastic energization by kinetic Alfvén waves. Overall, this systematic comparison of ion parameters between the core and total populations provides new insights into the thermodynamic structure and governing processes of the Martian magnetosheath.