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

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. Mars sometimes encounters low-Alfvén-Mach-number solar wind, allowing magnetic fields to control plasma flow. Here, the authors show that this accelerates magnetosheath protons and compresses the Martian magnetotail.