Mass Composition of the Escaping Flux at Mars: MEX Observations

Abstract

Using more than 7 years of ion measurements from the ASPERA-3/IMA instrument onboard the Mars Express mission, we analyze the mass composition of the two main outflowing heavy ion species O₂⁺ and O⁺. Using an improved multispecies fitting technique, we discriminate the molecular from atomic oxygen ion population. We find that the low-energy ions (3.2-100 eV) dominate the heavy ion population downstream of the induced magnetospheric boundary, where the mean ratio of O₂⁺ relative to O⁺ is 0.76 (averaged over the low-energy range and all observed directions). The spatial distribution of the O₂⁺/O⁺ ratio for the near-Mars environment shows that very close to the planet O₂⁺ is somewhat more abundant in part of the dayside. The same trend is even clearer in the central tail up to distances of 1.2R_M from the center of the tail. In contrast, O⁺ is more abundant in the terminator region and the mantle just inside the induced magnetospheric boundary. O⁺ is typically more abundant in the magnetosheath.