[P-CG21] 惑星大気圏・電磁圏

[PCG21-P08] C II emission processes on Mars revealed by the MAVEN IUVS observations and feasibility study of C II observations by LAPYUTA

*堺 正太朗^1,2、中川 広務¹、Deighan Justin³、益永 圭⁴、土屋 史紀²、Jain Sonal³、寺田 直樹¹、Schneider Nick³ (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.宇宙航空研究開発機構 宇宙科学研究所)

キーワード：火星、炭素、炭素イオン、発光過程、紫外線観測

It is believed that the Martian climate was warm and humid more than 4 billion years ago because Mars maintained a thick atmosphere and kept the liquid water. On the other hand, present-day Mars leaves only a thin atmosphere and there is no liquid water on the surface. It means that Mars has experienced a massive atmospheric escape to space and associated atmospheric evolution over 4.6 billion years. The key process in understanding atmospheric evolution is the escape of carbon dioxide (CO₂) and carbon monoxide (CO), which dominate the Martian atmosphere. CO₂ and CO escape into space 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 indicates that C⁺ is a good tracer of the CO₂ and CO escape in the upper atmosphere. However, the physical parameters of C⁺, such as density and flux, have not been accurately understood on both the dayside and nightside because, unlike O⁺, there are difficulties in observation and modeling.
One candidate for C⁺ observation is the detection of ultraviolet (UV) emission, referred to as C II emission, which is driven by the dissociative ionization (DI) of CO₂ and CO, the photoionization (PI) of C, the electron impact (EI) of CO₂, CO, and C, and the resonant scattering (RS) of C⁺. The C II emission was observed on the nightside at the auroral event by the Imaging Ultraviolet Spectrograph (IUVS) instrument onboard MAVEN (Lillis et al., 2022). The energetic electrons likely contribute to this emission at this event. If these emission processes can be isolated from UV observations, it might be possible to identify the C⁺ origin and understand whether the time variation in C⁺ distribution is internal (e.g., dust storms and atmospheric waves) or external (e.g., solar wind) in origin. In this study, we aim to resolve the C II emission processes based on comprehensive observations by Mars Atmosphere and Volatile EvolutioN (MAVEN) and their analyses.
The brightness due to the emission by EI and RS is investigated using observation data from Solar Wind Electron Analyzer and Neutral Gas and Ion Mass Spectrometer onboard MAVEN. The emission by EI is found to be more than 50 R at altitudes below 150 km on the dayside. IUVS onboard MAVEN has also observed C II emission of ~30 R/nm on the dayside, while less than 10 R/nm on the nightside. The combination of in-situ and the IUVS observations suggests that electron impact, dissociative ionization, and photoionization contribute to the C II emission on the dayside. Note that since the DI and PI cross sections are not available, it is difficult to theoretically estimate the brightness by both DI and PI. The differences in the emission processes with altitude are discussed in the presentation. We also discuss the brightness of the C II emission on the nightside as well as on the dayside, comparing the results of Lillis et al. (2022), and the estimation of the DI and PI cross sections. We furthermore present a feasibility study of the C II observation on Mars and Venus for the LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly) mission, a future UV telescope mission that has been selected as one of the candidates for the 6^th JAXA M-class mission.


Reference:
Lillis, R. J., Deighan, J., Brain, D., Fillingim, M., Jain, S., Chaffin, M., et al. (2022). First synoptic images of FUV discrete aurora and discovery of sinuous aurora at Mars by EMM EMUS. Geophys. Res. Lett., 49, e2022GL099820. https://doi.org/10.1029/2022GL099820