Tianwen-1 observations of Martian oxygen ion plumes

Mars does not possess an intrinsic magnetic field to shield it from solar wind invasion. Therefore, the ionized particles in the top atmosphere can thus interact with the solar wind and escape into interplanetary space. The solar wind convection electric field E_SW can accelerate the oxygen ions in the Martian ionosphere and pick them up into space. These pickup ions can form an important escape channel, namely "ion plume", due to their large gyro radius compared to the Mars radius. China's first Mars mission, the Tianwen-1 spacecraft, observed strong ion plume flux at its apoapsis ~4 R_M. We found their energy of peak flux can be well estimated by the E_SW acceleration. Based on the E_SW acceleration mechanism, a backward tracing using in-situ measurement data is performed to trace their source locations. We found these ion plumes with strong flux mainly come from dayside low-middle MSE (Mars-Solar-Electric) latitude ionosphere in the +E hemisphere (the hemisphere where E_SW points from the planet to the solar wind), confirming the results in previous simulations.