Strongly Subisothermal Polytropic Behavior of Ions in the Martian Magnetosheath Revealed by MAVEN Observations

Abstract Energy redistribution during plasma compression or expansion remains poorly understood, particularly within induced magnetospheric systems. The polytropic index serves as an important diagnostic of this process by characterizing the thermodynamic response of temperature and pressure to density variations. This study presents the first statistical determination of the ion polytropic index in the Martian magnetosheath, leveraging 9 yr of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Derived indices largely fall within the subisothermal range, yielding a median value of 0.42. Such polytropic behavior signifies systematic ion cooling during local compression—a thermodynamic response that departs significantly from both the adiabatic condition and the near-isothermal trends typically observed in Earth’s magnetosheath. Beyond this characteristic, a well-defined spatial trend is also identified, with the index increasing radially from the bow shock toward the induced magnetosphere boundary. The polytropic index displays a clear anticorrelation with the amplitude of low-frequency magnetic field fluctuations, indicating a close association between wave activity and ion thermodynamics in the Martian magnetosheath. In summary, this study provides a pioneering quantitative characterization of ion polytropic behavior at Mars, offering a critical benchmark for comparative analyses between induced and intrinsic magnetospheric environments.