Observational evidence of alpha‐particle capture at Mars

Abstract

Solar wind alpha‐particles are suggested to be the most important source of helium in the Martian atmosphere. Recent hybrid simulations show that as much as 30% of the alpha‐particles passing through the planetary cross section can be lost from the solar wind due to charge‐exchange processes associated with the Mars/solar wind interaction. We use ion data from the ASPERA‐3 instrument on Mars Express to investigate the penetration of alpha‐particles into the Martian atmosphere. We conclude that solar wind alpha‐particles contribute to the helium observed in the atmosphere. In accordance with simulations we see an asymmetry in the capture efficiency related to the solar wind convection electric field.