Survey of Mars Energetic O+ Ions Beyond the Induced Magnetospheric Boundary

Abstract:

High-energy (ions > 2 keV), high-count rate (> 200 per measurement interval) observations of planetary ions above Mars' induced magnetic boundary (IMB) are surveyed and analyzed. A systematic search of the Mars Express (MEX) ion data using an orbit filtering criteria is described, using magnetometer data from Mars Global Surveyor (MGS) to determine the solar wind motional electric field (Esw) direction. Two levels of statistical survey are presented, one focused on times when the MEX orbit was directly in line with the Esw and another for all angles between the MEX location and the Esw. For the first study, within the three-year overlap of MGS and MEX, 9 orbit intervals were found with clear and unambiguous high-energy O⁺ observations, all with flows roughly Esw directed. The second survey used a point-by-point determination of MEX relative to +Esw and contained many thousands of 192-s measurements. This study yielded only a weak indication for an Esw-aligned plume. Furthermore, the y-z components of the weighted average velocities in the bins of this y-z spatial domain survey do not systematically point in the Esw direction. The first survey implies the existence of this plume and shows that its characteristics are seemingly consistent with the expected energy and flight direction from numerical studies, while the second study softens the finding and demonstrates that there are many planetary ions beyond the IMB moving in unexpected directions. Several possible explanations for this discrepancy are discussed.