Magnetic Reconnection inside a Martian Ion Plume: MAVEN Observations

Abstract The Martian ion plume is an important escape channel that influences the long-term evolution of the planet’s atmosphere, yet the small-scale dynamics within plumes remain poorly constrained by in situ observations. Using magnetic-field and plasma measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we report the first identification of a collisionless magnetic reconnection event within a molecular oxygen ion ( O 2 + ) plume at low altitude (∼600–750 km). The observations show a clear current sheet embedded in the plume. Minimum variance analysis of the magnetic field indicates that the sheet is quasi-two-dimensional, and a characteristic bipolar Hall magnetic-field signature is present. Concurrently, O 2 + ions undergo substantial heating and acceleration, forming a field-aligned ion beam that is distinctly different from the classic pickup ion populations. Walén tests further provide evidence for quasi-Alfvénic reconnection outflows. This event reveals a new dynamical pathway in the solar-wind–plume interaction. Plume reconnection may redistribute local heavy-ion transport between plume-like, tailward, and possibly return-flow pathways, and thus may influence the net escape in a localized manner. These results indicate that Martian ion plume escape involves complex dynamical and kinetic coupling.