Dependence of Martian Proton Precipitation On IMF Orientation: Hybrid Simulations

The small Martian induced magnetosphere sometimes fails to completely screen the solar wind from precipitating on the exobase. The precipitating protons may serve as an additional mass, momentum and energy coupling channel between the Martian upper atmosphere and the plasma environment. Moreover, the plasma flow pattern in the interaction region is governed by the orientation of the interplanetary magnetic field (IMF) and the associated convective electric field (E = −V × B). Therefore, the IMF orientation would influence the proton precipitation situation, modulating the coupling channel. Using a hybrid plasma model (electrons as a massless fluid and ions as kinetic particles), we investigate how the proton and associated energy precipitation rates vary with upstream IMF cone angle, which is the angle between the solar wind velocity direction and the IMF direction. We quantify the precipitation rates by analyzing global maps of downward proton and energy fluxes under different IMF cone angles. Our key findings are: (1) While the total proton precipitation rate increases with altitude, the net precipitation rate remains constant. (2) Proton precipitation exhibits strong sensitivity to IMF orientation. Precipitation fluxes and rates increase progressively from perpendicular IMF through Parker spiral and cone angle 30° configurations, peaking under radial IMF conditions. (3) Under perpendicular and Parker spiral IMF orientations, the net precipitation rates stabilize at ~10²² s⁻¹ in our simulation. In contrast, the radial IMF condition drives a dramatic enhancement, with rates reaching ~10²⁴ s⁻¹—a two-order-of-magnitude increase compared to the other orientations. (4) Precipitation fluxes show distinct +E/-E hemispheric asymmetry for cone angle 30°, Parker spiral, and perpendicular IMF conditions.