[P-CG19] 惑星大気圏・電磁圏

[PCG19-P04] CIRイベント時における火星からのイオン散逸に関する統計的研究

*亀井 りま¹、関 華奈子¹、桂華 邦裕¹、ラムスタッド ロビン²、ブレイン デイヴィッド²、原 拓也³、マクファーデン ジェームズ³、ハンレー グウェン³、クリストファー ファウラー⁴、ハレカス ジャスパー⁵、ディブラシオ ジーナ⁶、カリー シャノン² (1.東京大学大学院 理学系研究科 地球惑星科学専攻、2.コロラド大学 大気宇宙物理学研究所、3.カリフォルニア大学 宇宙科学研究所、4.ウェストバージニア大学 物理・天文学科、5.アイオワ大学 物理・天文学科、6.NASA ゴダード宇宙飛行センター)

キーワード：火星、イオン散逸

It is important to clarify responses of each atmospheric escape mechanism to solar activities for understanding of the planetary atmospheric evolution. This is especially significant for Mars, which lacks an intrinsic magnetic field, allowing direct interaction between the solar wind and its atmosphere. While various mechanisms can contribute to the atmospheric escape, we here focus on the ion escape driven by solar activity events, such as Interplanetary Coronal Mass Ejections (ICMEs) and Corotating Interaction Regions (CIRs). ICMEs occur when large amounts of plasma are ejected into interplanetary space following solar flares, while CIRs are formed when fast solar wind overtakes slower one, creating interaction regions. Both ICMEs and CIRs often facilitate high solar wind dynamic pressure condition and disturb the Martian induces magnetosphere and influence atmospheric escape. MAVEN observations and their comparison with global MHD simulation results show that atmospheric escape rates increased significantly during an ICME event in March 2015 [1]. On the other hand, Ramstad and Barabash (2021) pointed out that the ion escape rate from Mars does not have clear dependence on the solar wind dynamic pressure based on statistical analysis [2]. These contradictory observations indicate the need for careful investigation of effects of ICMEs and CIRs on the ion loss from Mars. Observationally there are two major escape channels for ions from Mars: polar plumes accelerated by the convective electric field of the solar wind and the tailward escape, a bulk ion outflow through Martian magnetotail [2]. Statistical studies of the polar plumes [3] and tailward escape [4] both indicate that the spatial distributions of the ion escape flux are highly localized in terms of the MSE coordinates determined by the direction of the solar wind electric field. In this study, we aim to evaluate the impact of solar wind on the ion escape from Mars especially during CIRs by carefully investigating the localization effects of both ion escape channels.
Utilizing simultaneous observations by Mars Express and MAVEN from 2015 to 2019, we identified CIRs. The data satisfying the following criteria were selected: (1) the maximum daily solar wind density exceeded 15 cm^-3, and (2) the difference in velocity over two days was greater than 100 km/s. (3)Among the selected data, events where velocity increased following a density rise were classified as CIRs. As a result, we found 120 CIRs over the five-year period.
We used the Supra-Thermal And Thermal Ion Composition (STAIC) onboard MAVEN to investigate spatial distributions of escaping ions. Distributions of inward and outward heavy ion fluxes are separately examined both in the Mars-Solar-Orbital (MSO) coordinates and the Mars-Solar-Electric field (MSE) coordinates to differentiate the effects of the crustal magnetic field and acceleration by solar wind electric field. The fluxes of planetary ions moving toward and away from Mars are mapped to a spherical shell that ranged from 1.3 to 3.0 Mars radii. The results show the flues of heavy ions moving toward Mars increased more than 50 % on night side regions in the northern MSE hemisphere. From a comparison between before and after the arrival of solar events(ICMEs and CIRs) based on the statistical results, effects of CIRs on the ion loss from Mars are discussed.

References
[1] Jakosky et al. (2015), Science, 350, aad0210, doi:10.1126/science.aad0210.
[2] Ramstad and Barabash (2021), Space Sci. Rev., 217, 36, doi:10.1007/s11214-021-00791-1.
[3] Dong et al. (2015), Geophys. Res. Lett., 42, 8942–8950, doi:10.1002/2015GL065346.
[4] Inui et al. (2019), J. Geophys. Res., 124, 5482–5497, doi:10.1029/ 2018JA026452.