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Statistical analysis of the reflection of incident O+ pickup ions at Mars: MAVEN observations

Analyzing ~1.3 year data set of O+ ion velocity distribution functions obtained from the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we statistically investigate reflections of incident O+ pickup ions (>10 keV) occurring below the Martian bow shock. To quantitatively evaluate importance of the reflection of incident O+ pickup ions, we estimate reflection ratios by calculating average inward and outward O+ ion fluxes above the Martian bow shock. Our result shows that an average reflection ratio is 14.1 ± 6.7%. We also investigate dependences of the reflection ratio on the solar wind, solar EUV flux, and southern hemispheric crustal magnetic field on Mars. We find that the reflection ratio strongly depends on the magnitude of the interplanetary magnetic field (IMF), solar wind dynamic pressure, and gyroradius of O+ pickup ions in the solar wind, rather than the solar EUV flux and the crustal magnetic field. We suggest that the reflection ratio is determined by the magnetic field both in the solar wind and below the bow shock that mainly controls gyroradius of incident O+ pickup ions in these regions. We also find that the reflection ratio increases to 38.9 ± 10.2% as these parameters become extreme conditions (i.e., IMF magnitude >6 nT). Since incident O+ pickup ions are known to be a major source of atmospheric sputtering escape from Mars, our result suggests that ion reflections may have a role to reduce the sputtering escape from Mars under the extreme solar wind condition.