Dependence of Thermodynamic Processes on Upstream Interplanetary Magnetic Field Conditions for Magnetosheath Ions

Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations over a 10-year period from 2008 to 2017, we statistically investigate the thermodynamic properties for magnetosheath ions and their dependence on upstream interplanetary magnetic field (IMF) conditions. The thermodynamic properties for magnetosheath ions are estimated by using the polytropic index averaged over the subinterval that belongs to the same streamline ((Formula presented.)). The THEMIS observations show that the probability distribution of (Formula presented.) for magnetosheath ions has a major peak at (Formula presented.) ~1 (quasi-isothermal conditions) with a longer left tail down to (Formula presented.) ~0 (quasi-isobaric conditions). The spatial distributions of (Formula presented.) for two different types according to IMF spiral angle (i.e., Parker spiral and ortho-Parker spiral IMF orientations) reveal that the ions in the downstream of a quasi-perpendicular shock (quasi-perpendicular magnetosheath) exhibit quasi-isothermal processes, while those in the downstream of a quasi-parallel shock (quasi-parallel magnetosheath) show (Formula presented.) lower than unity (down to (Formula presented.) ~0.8) implying the anticorrelation between the ion temperature and the ion number density variations. Moreover, (Formula presented.) in the quasi-parallel magnetosheath tends to decrease with increasing magnetic local time distance from the magnetic local noon. These results indicate that the thermodynamic properties for magnetosheath ions depend on the bow shock geometry (quasi-perpendicular bow shocks versus quasi-parallel bow shocks) and are presumably controlled by a variety of instabilities, waves, and turbulence in the magnetosheath.