Hydrogen exosphere at Mars: Pickup protons and their acceleration at the bow shock

Pickup protons (PIs) with a ring-beam distribution originating from the extended hydrogen exosphere of Mars are observed by the ASPERA-3 experiment onboard the Mars Express spacecraft at solar minimum conditions. PIs can contribute similar to 1% to the solar wind number density at close distances to the flank bow shock. The observed fluxes of PIs indicate the existence of a dense hydrogen corona. The height profile of it is evaluated and compared to the models. At the bow shock, pickup protons experience an efficient reflection accompanied by ion energization in the motional electric field. Despite the small scale of the Martian bow shock the acceleration of PIs is rather effective and can operate for injection of pre-energized ions into a subsequent acceleration process. The injection efficiency is derived to be about >= 20%. These results imply that similar processes at shocks with much larger scales ( e. g. the termination shock) can be important for ion acceleration.