[P-CG19] 惑星大気圏・電磁圏

[PCG19-02] Martian ionospheric variations revealed through C I 156.1 nm emission induced by electron impact

*堺 正太朗^1,2、Deighan Justin³、Mitchell David⁴、中川 広務¹、Jain Sonal³、益永 圭⁵、土屋 史紀²、Schneider Nicholas³、Benna Mehdi^6,7、寺田 直樹¹、Mayyasi Majd⁸、Mazelle Christian⁹、Lillis Robert⁴、Curry Shannon³、関 華奈子¹⁰ (1.東北大学大学院理学研究科地球物理学専攻、2.東北大学大学院理学研究科惑星プラズマ・大気研究センター、3.コロラド大学ボールダー校 LASP、4.カリフォルニア大学バークレー校 宇宙科学研究所、5.山形大学学士課程基盤教育院、6.メリーランド大学ボルチモア・カウンティ校 CSST、7.NASAゴダード宇宙飛行センター、8.ボストン大学宇宙物理学センター、9.IRAP, CNRS, トゥールーズ大学, UPS-OMP, CNES、10.東京大学大学院理学系研究科地球惑星科学専攻)

キーワード：火星、火星探査、電離圏、炭素、炭素発光、紫外線観測

It is widely believed that Mars had a warm and humid climate more than 4 billion years ago, as it maintained a thick atmosphere and sustained liquid water. In contrast, present-day Mars has only a thin atmosphere, and no liquid water remains on its surface. This indicates that Mars has undergone significant atmospheric escape into space and associated atmospheric evolution over the past 4.6 billion years. A key process in understanding this evolution is the escape of carbon dioxide (CO₂) and carbon monoxide (CO), the dominant components of the Martian atmosphere. These gases escape not only as molecules but also as carbon (C) and oxygen (O) atoms or, particularly in the upper atmosphere, as ions through dissociative ionization. This suggests that carbon is a useful tracer of thermospheric CO₂ and CO variation, as well as ionospheric variation on Mars. Sakai et al. (2024) investigated the emission mechanisms for C II 133.5 nm and found that C II emission is driven by both dissociative ionization and electron impact ionization of CO₂ and CO in the lower ionosphere. Their findings suggest that C II emission may not serve as a tracer of ionospheric variation but rather as an indicator of variations in the Martian thermosphere, where CO₂ and CO dominate.
In contrast, C I emission has the potential to be a tracer of Martian ionospheric variations. Lo et al. (2022) demonstrated that C I 156.1 nm emission is primally driven by electron impact below an altitude of 140 km, where the emission is most intense. This suggests that suprathermal electrons associated with the ionosphere might play a key role in determining this emission. The current study aims to elucidate the relationship between suprathermal electrons and C I 156.1 nm emission. Furthermore, an investigation is conducted into whether C I emission can serve as a diagnostic tool for ionospheric variation by estimating electron flux from the intensity of C I emission.
For this analysis, data from Solar Wind Electron Analyzer (SWEA) and Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have been used, focusing on deep dip campaigns #2, #8, and #9 as well as the aerobraking campaign. The results have indicated that C I 156 nm emission is proportional to the integrated electron flux, particularly in the energy range of 25 – 400 eV. This presentation will further discuss comparisons with observations from Imaging Ultraviolet Spectrograph (IUVS) onboard MAVEN and assess whether this emission can be utilized as a diagnostic tool for Martian ionospheric variation.

References:
Lo, D. Y., Yelle, R. V., Deighan, J. I., Jain, S. K., Evans, J. S., Stevens, M. H., et al. (2022). MAVEN/IUVS observations of C I 156.1 nm and 165.7 nm dayglow: Direct detection of carbon and implications on photochemical escape. Icarus, 371, 114664. https://doi.org/10.1016/j.icarus.2021.114664

Sakai, S., Nakagawa, H., Deighan, J., Jain, S. K., Masunaga, K., Tsuchiya, F., et al. (2024). C⁺ 133.5 nm emission mechanisms on Mars revealed by the MAVEN observations. Ap. J., 977:226. https://doi.org/10.3847/1538-4357/ad8e35