[P-CG25] 惑星大気圏・電磁圏

[PCG25-07] Study of Molecular Ion Contribution to the Polar Plume from Mars Based on MAVEN Observations

*坂倉 孝太郎¹、関 華奈子¹、堺 正太朗¹、David Brain²、James McFadden³、Jasper Halekas⁴、Gina Dibraccio⁵、Bruce Jakosky² (1.東京大学理学部地球惑星科学専攻、2.コロラド大学、3.カリフォルニア大学、4.アイオワ大学、5.NASA)

キーワード：火星、MAVEN、大気散逸、二酸化炭素、極域プリューム

Mars once had water on its surface about 4 billion years ago, but there is no liquid water on the surface at present. Escape of the atmosphere to space is considered as the main cause of this climate change. However, the mechanism of the large amount of atmospheric loss is far from understood. Ion escape is one of the important candidates of the loss mechanism. There are three channels of the ion escape, namely, tailward escape, pickup ion, and polar plume. Polar plume ions are accelerated by solar wind electric field and escape to positive E hemisphere of the Mars-Sun-Electric field (MSE) coordinates. It is estimated by Dong et al. [2017] that the escape rate of O⁺ plume is 20-30% of the total O⁺ escape depending on the solar EUV radiation. The rate is not negligible in order to understand ion escape from Mars. Molecular ions in the polar plume should also be studied since it is shown that main escape species is O₂⁺ for tailward escape by Inui et al. [2019]. We recently reported a CO₂⁺-rich plume event on August 28, 2015. Peak flux of CO₂⁺ was 3.6x10⁶ cm^-2s^-1, which is about one order of magnitude higher than the average flux of O⁺ in the polar plume reported by Dong et al. [2017]. Such high escape flux is unexpected because CO₂⁺ is minor component at high-altitude ionosphere due to its small scale height. To fully understand the mechanism of polar plume, it is important to study the composition of the plumes.

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. [2018]. 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.

References:
Dong, Y., et al. (2017), J. Geophys. Res. Space Phys., 122, 4009–4022, doi:10.1002/2016JA023517
Inui, S., et al. (2018), Geophys. Res. Lett., 45, 5283–5289, doi:10.1029/2018GL077584
Inui, S., et al. (2019). J. Geophys. Res. Space Phys., 124, 5482–5497. doi:10.1029/2018JA026452