[PCG25-07] Study of Molecular Ion Contribution to the Polar Plume from Mars Based on MAVEN Observations
Keywords:Mars, MAVEN, atmospheric escape, carbon dioxide, polar plume
In order to investigate relationship between CO2+-rich plume event and solar wind conditions, we conducted a statistical study. We analyzed data obtained by STATIC (Supra Thermal and Thermal Ion Composition), MAG (magnetometer) and SWIA (Solar Wind Ion Analyzer) onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) from Dec. 11, 2014 to Feb. 23, 2019. STATIC can measure ion distribution functions with mass discrimination. In order to derive CO2+ number density, we use the fitting method invented by Inui et al. . By fitting a log-normal distribution to O2+ count data, we remove O2+ contamination in the CO2+ mass range. We defined CO2+ plume event as the time period when the observed maximum flux of CO2+ in an MAVEN orbit is higher than 2.5x105 cm-2s-1. In order to remove false CO2+ plume events due to high O2+ counts, we set another requirement that CO2+ has evident peak which is at least comparable to O2+ contamination in the CO2+ mass range. The results show that the CO2+ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This result is consistent with hypothesis that CO2+ plume is caused by deep penetration of the solar wind electric field due to the high solar wind dynamic pressure. On one hand, detection probability of O2+ plume events don't show such dependences on the solar wind parameters. This is probably because O2+ is abundant near the ionopause enough to create dense (>2.5x105 cm-2s-1) O2+ plumes regardless of the solar wind conditions. On the other hand, the escape flux of O2+ due to the polar plume has weak positive correlation with the solar wind dynamic pressure. This result might reflect high density of O2+ near the ionopause under the high solar wind dynamic pressure conditions.
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