Plasma acceleration and boundary compression in the magnetosheath of Mars under low-Alfvén-Mach-number solar wind.

Abstract

When the upstream Alfvén Mach number is low, the magnetic field may play a dominant role in the solar-wind interaction with Mars, yet the resulting magnetospheric dynamics and associated energy conversion remain poorly constrained. Bulk proton acceleration reaching about 150% of the solar-wind speed is detected jointly by MAVEN, Tianwen-1, and MEX missions in the magnetosheath. The acceleration regions exhibit pronounced asymmetry between the two electric-field hemispheres, defined by the solar-wind electric field, while the shape of the adjacent magnetotail shows axial and hemispheric asymmetries. We demonstrate that the large-scale J×B force, with J denoting the electric current density and B the magnetic field, is the primary driver of acceleration and axial asymmetry. Mass loading of planetary ions introduces strong hemispheric asymmetries-features unique to unmagnetized bodies with atmospheres. Here, we reveal a pathway by which magnetic field converges and redistributes solar-wind energy, and infer that plasma acceleration under low upstream Alfvén Mach number operates not only at Earth but also at Mars and other bodies with magnetosheaths.