Martian ion currents and escape driven by interplanetary magnetic field orientation based on hybrid simulations

As a planet lacking a global magnetic field, Mars interacts directly with the solar wind, forming an induced magnetosphere that mediates energy transfer and atmospheric ion loss. The topology of this interaction and the resulting atmospheric ion escape are strongly influenced by the orientation of the interplanetary magnetic field (IMF). In this study, we utilize a hybrid model to investigate how variations in the IMF orientation shape ion current systems and atmospheric ion escape rates of O⁺, O₂⁺, CO₂⁺. We first perform simulations with a constant |B_sw|, where varying the IMF cone angle results in different strengths of the convective electric field (E_sw = V_sw × B_sw). Our results suggest that the spatial morphology of ion plumes undergoes a substantial evolution, forming a distinct cross-flow plume as the IMF rotates from perpendicular to parallel. These ion plumes exhibit a mass-dependent deflection, where heavier CO₂⁺ travel farther with larger gyroradii than lighter O⁺, acting as an asymmetric obstacle in the –Y_MSE hemisphere (where MSE is the Mars solar electric coordinate frame). In turn, the solar wind proton current develops pronounced asymmetries under a parallel IMF, becoming largely diffused in the −Y_MSE hemisphere because of the interaction with the additional plume obstacles. Consequently, the ion escape rates exhibit a nonmonotonic dependence on the IMF orientation, peaking under a parallel  IMF as escape shifts from a tail- to plume-dominated flow with substantial upstream enhancement. To decouple the effects of IMF geometry from those of the convective electric field, we further conduct a comparative simulation with constant  B_y  (hence constant |E_sw|), where the cone angle is varied by changing the B_x component while allowing |B_sw| to vary. With increasing B_x toward a parallel orientation, the total field magnitude grows, causing the Alfvén Mach number (M_A) to decrease from super-Alfvénic to trans-Alfvénic and ultimately to sub-Alfvénic values. Within the range from perpendicular to a 30° cone angle, where the system remains in the super-Alfvénic regime, ion escape is largely insensitive to the growing B_x component. This finding indicates that the magnetic barrier maintains its shielding efficiency under the super-Alfvénic regime.