1,4,5,2,9,10,6,- 1Faculty of Environment and Information Studies, Keio University, Fujisawa, Kanagawa, Japan (shotaro@sfc.keio.ac.jp)
- 2Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA
- 3Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan
- 4Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Sendai, Japan
- 5Institute of Arts and Sciences, Yamagata University, Yamagata, Yamagata, Japan
- 6Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA
- 7Center for Space Sciences and Technology, University of Maryland, Baltimore County, Baltimore, MD, USA
- 8Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
- 9Center for Space Physics, Boston University, Boston, MA, USA
- 10University of Toulouse, CNES, CNRS, IRAP, Toulouse, France
- 11Research Center for Advanced Science and Technology, University of Tokyo, Meguro-ku, Tokyo, Japan
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 variability on Mars. Sakai et al. (2024) investigated the emission mechanisms of 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 variability but rather as an indicator of variability in the Martian thermosphere, where CO2 and CO dominate.
In contrast, C I emission has the potential to be a more direct tracer of Martian ionospheric variability. Lo et al. (2022) demonstrated that C I 156.1 nm emission is primarily 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 may play a key role in determining this emission. Gérard et al. (2026) further showed that electron impacts with both CO2 and CO contribute significantly to C I 156.1 nm emission. The present 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 variability 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 were used, focusing on deep dip campaigns #2 (17 – 22 April 2015), #8 (16 – 23 October 2017), and #9 (14 – 30 April 2018) as well as the aerobraking campaign (12 February – 29 March 2019). 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, and the CO2 density rather than to altitude. In contrast, Imaging Ultraviolet Spectrograph (IUVS) onboard MAVEN has observed C I emission brightness approximately one order of magnitude greater than that inferred from in-situ observations. Even when observations are made at the same altitude, differences in CO2 density can alter the relative contributions of multiple C I emission processes; therefore, the brightness observed by IUVS may be higher than that observed in situ. Electron impact on CO can contribute up to about 30% of the C I emission relative to that produced by electron impact on CO2. However, at altitudes where CO2 density exceeds 1010 cm-3, electron-impact C I emission remains dominant, indicating that this emission can serve as a tracer of suprathermal electrons in the Martian ionosphere.
References:
Gérard, J. C., Soret, L., Hubert, B., Lillis, R., Jain, S., & Deighan, J. (2026). Far ultraviolet carbon emissions in the Mars aurora: Brightness, intensity ratios and seasonal dependence. Icarus, 448, 116919. https://doi.org/10.1016/j.icarus.2025.116919
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
How to cite: Sakai, S., Deighan, J., Nakagawa, H., Tsuchiya, F., Masunaga, K., Mitchell, D., Benna, M., Schneider, N., Terada, N., Jain, S., Mayyasi, M., Mazelle, C., Lillis, R., Curry, S., and Seki, K.: Electron-impact C I 156.1 nm emission as a probe of Martian ionospheric variability, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-165, https://doi.org/10.5194/epsc2026-165, 2026.
