[P-CG25] 惑星大気圏・電磁圏

[PCG25-13] Seasonal variation of the dayside N₂/CO₂ at 140 km altitude derived from MAVEN/IUVS

*吉田 奈央¹、中川 広務¹、寺田 直樹¹、Nicholas Schnider²、Scott Evans³、Sonal Jain²、藤原 均⁴、今村 剛⁵、Justin Deighan²、Ian Stewart²、Michael Stevens⁶、Roger Yelle⁷ (1.東北大学大学院 理学研究科、2.Laboratory for Atmospheric and Space Physics, Boulder, USA、3.Computational Physics, Inc., USA、4.成蹊大学 理工学部、5.東京大学大学院 新領域創成科学研究科、6.Naval Research Laboratory, Washington, USA、7.Department of Planetary Sciences, University of Arizona)

キーワード：熱圏、混合比、均質圏界面、MAVEN、散逸

It is believed that liquid water was abundant on early Mars, despite Mars being too cold today to sustain significant amounts of liquid water. The most likely explanation is that early Mars had a more-effective greenhouse atmosphere and most part of atmosphere and water have lost to space. Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was designed to explore the loss of gas to space at the present. MAVEN spacecraft revealed highly variable nature of upper atmosphere in density, temperature, and atmospheric compounds (e.g. Mahaffy et al., 2015; Elrod et al., 2017; Stone et al., 2018; Slipski et al., 2018). However, the controlling factor of the variations has yet to be fully characterized due to the limited spatial- and temporal- observing geometry by in-situ measurements. In addition to the effects by the external force, such as solar wind and solar EUV on the upper atmosphere, the effect of lower atmospheric phenomena is also required to account for the substantial variations of upper atmosphere. For instance, the homopause (~120 km altitude) influences the upper thermospheric composition, thereby the species escaping to space [Imamura et al., 2016]. The fractionation between the homopause and exobase determines the relative abundance of species to escape to space [Jakosky et al., 2017]. For this purpose, further investigation around atmospheric boundary from the middle atmosphere to lower thermosphere is crucial for understanding regional coupling between lower and upper atmosphere.
We have investigated the seasonal variation of the N₂/CO₂ ratio at 140 km altitude derived from ultraviolet spectroscopy remote-sensing measurements by Imaging Ultraviolet Spectrograph (IUVS) aboard MAVEN. We used the dataset of level 2 version 13 revision 1 data provided by the Planetary Data System, which includes retrieved CO₂ and N₂ number density profiles derived from dayglow emissions. We analyzed N₂ and CO₂ number densities observed from October 2014 to May 2018. Observations cover in the dayside from 7 to 19 hr. The observations covered almost all solar longitudes within the two Martian Years. The retrieved CO₂ density has small uncertainty but the retrieved N₂ density has relatively larger uncertainty in particular above ~170 km due to the dimmer emission intensity. For precise analysis of the N₂/CO₂ ratio, we confine our analysis to the data at 140 km altitude where N₂/CO₂ has uncertainty less than 50%. We found that the N₂/CO₂ ratio at 140 km altitude significantly varies in the range of 0.02 to 0.20, which shows an annual sinusoidal trend. The higher ratio appears during aphelion and the lower ratio appears during perihelion. CO₂ and N₂ number densities also have similar annual variations. It is noted that the CO₂ density varies by a factor of 100, while N₂ density by a factor of 10. This large CO₂ variation affects the N₂/CO₂ ratio at 140 km. The potential sources of the seasonal variation we found are variations (1) of the surface mixing ratio, (2) of the homopause altitude, and (3) of the thermospheric temperature. In this paper, the effect of surface mixing ratio is discussed using Mars Climate Database version 5.3 [Forget et al., 1999; Lewis et al., 1999]. We also address the effects of other sources by considering the seasonal variation of homopause altitude [Slipski et al., 2018] and background temperature [Bougher et al., 2017; Stone et al., 2018].