[P-CG21] 惑星大気圏・電磁圏

[PCG21-P05] Venus upper atmosphere as observed by Hisaki: Dayglow and ion tail

*Kei Masunaga¹、Kanako Seki¹、Naoki Terada²、Fuminori Tsuchiya³、Tomoki Kimura⁴、Kazuo Yoshioka¹、Go Murakami⁵、Atsushi Yamazaki⁵、Chihiro Tao⁶、Ichiro Yoshikawa⁷ (1.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo、2.Department of Geophysics, Graduate School of Science, Tohoku University、3.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University、4.Nishina Center for Accelerator Based Sciecne, RIKEN、5.Institute of Space and Astronoutical Science, Japan Aerospace Exploration Agency、6.National Institute of Information and Communications Technology、7.Department of Complexity Science and Engineering, University of Tokyo)

キーワード：Hisaki, EXCEED, Venus, Dayglow, Ion escape

Hisaki was launched on September 14, 2013. Since then, we have conducted quasi-continuous observations of the upper atmosphere of Venus by using the Extreme Ultraviolet Spectroscope for Exospheric Dynamics (EXCEED) [Yoshioka et al., 2013; Yoshikawa et al., 2014] on-board Hisaki. Our observation aims to see variations of the EUV dayglow of Venus and to detect ions, such as O⁺ and C⁺, escaping from Venus.
From three quasi-continuous observations in 2014, we found characteristic periodic variations of oxygen EUV dayglow [Masunaga et al., 2015]. In 2015, we conducted new observations of Venus upper atmosphere covering different local times or different latitude from those in 2014. Our result shows that the ~4 day periodicity is observed on the dawn side of Venus. We suggest that gravity waves may propagate from the middle atmosphere of Venus, where the atmosphere is super-rotating with a 4-day period, to the upper atmosphere of Venus. The local time difference of the ~4-day periodicity of the dayglow suggests that there is a local time difference in wave propagation altitudes or wave filtering structures.
Using the 10” slit of EXCEED, we also observed Venus tail region to detect ion tail escaping from Venus. The 10” slit has a better pointing accuracy than that of the 60” slit. The result shows that C⁺ ion tail (CII 133.5 nm) is detected with SNR > 1. We discuss the escape rate of C⁺ ions from Venus by assuming cold C⁺ ions are escaping from Venus.