Martian Ionospheric Cold Hydrogen Ion Escape at Solar Maximum: First Observations

The water escape from Mars to space could be in the form of hydrogen and oxygen ions as driven by solar wind–Mars interactions. Although oxygen ion escape has been extensively investigated, the H⁺ escape rate was measured only at solar minimum. To determine the impacts of solar activity on the ionospheric H⁺ escape rate, we report the observational results from the Tianwen-1 spacecraft at solar maximum. The cold dense ionospheric ion outflows through the magnetotail have an equal energy acceleration process, consistent with the characteristics of ambipolar electric field acceleration. The escape rate of planetary cold H⁺ through the magnetotail is estimated to be ∼2 × 10²³ s⁻¹, a value substantially lower than the neutral hydrogen escape rate, and the H/O ratio (∼0.3) of the tailward escaping ions (H⁺, O⁺ and O₂⁺) is below the stoichiometric ratio of water. These results indicate the ionospheric H⁺ outflow plays a minimal role for the water loss on Mars across solar cycles. To assess the contribution of H⁺ escape to total hydrogen loss, future analysis must target the pickup H⁺ escape rate within the magnetosheath.