Influence of the Martian crustal magnetic fields on oxygen ion escape at Mars

The escape of oxygen ions from Mars has played a crucial role in the planet’s long-term atmospheric evolution and habitability. The crustal magnetic fields influence ion escape, but the exact role remains debated. Previous studies have presented contrasting conclusions, suggesting that the crustal fields may either suppress or enhance oxygen ion escape. To date, the extent and mechanisms of this influence remain insufficiently understood. This study aims to investigate the influence of the Martian crustal magnetic fields on the oxygen ion escape at Mars. Several groups of 3D global hybrid simulations of Mars-solar wind interaction were performed, with the escaping oxygen ion trajectories traced. The results from the simulations with or without the crustal fields and under different interplanetary magnetic field conditions were then compared. The simulation results show that the presence of crustal fields enhances the ionospheric oxygen ion escape, while the exospheric oxygen ion escape rate remains largely unaffected. The crustal magnetic fields alter the local electric and magnetic environments and, subsequently, modify the local oxygen ion density and flow direction in the ionosphere. First, the steep magnetic inclination and large magnetic strength in crustal field regions increase the density of low-altitude ionospheric oxygen ions and facilitate their outward transport, thereby promoting ion escape. Second, the crustal fields modify the local electric field structure, which also affects ion acceleration and escape. When strong crustal fields are located on the dayside, their obstruction of the upstream plasma flow weakens the dayside radial electric field at low altitudes in the southern hemisphere. The weakened electric field tends to assist or reduce ion escape, depending on whether it points toward or away from Mars, respectively. In any case, the influence of the magnetic field topology change (the steep magnetic inclination and large magnetic strength) in crustal field regions dominates the effect of weakened electric field, resulting in a higher escape rate than that without the crustal fields. Additionally, when strong crustal fields are on the nightside, the dayside moderate crustal fields still enhance the local density and outward transport of ionospheric oxygen ions, while their impact on the local electric field remains limited. The net effect is enhanced ion escape over the +E hemisphere where the solar wind motional electric field points away from Mars.