[P-CG19] 惑星大気圏・電磁圏

[PCG19-03] Variations in vertical CO/CO₂ profiles in the Martian mesosphere and lower thermosphere measured by ExoMars TGO/NOMAD: Implications of variations in eddy diffusion coefficient

*吉田 奈央¹、中川 広務¹、青木 翔平^2,3、Erwin Justin³、Vandaele Ann Carine³、Daerden Frank³、Thomas Ian³、Trompet Loïc³、小山 俊吾¹、寺田 直樹¹、Neary Lori³、村田 功¹、Villanueva Geronimo⁴、Liuzzi Giuliano^4,5、Lopez-Valverde Miguel Angel⁶、Brines Adian⁶、Modak Ashim⁶、笠羽 康正¹、Ristic Bojan³、Patel Manish⁷、Bellucci Giancarlo⁸、López-Moreno José Juan⁶ (1.東北大学大学院　理学研究科　地球物理学専攻、2.宇宙航空研究開発機構　宇宙科学研究所、3.Royal Belgian Institute for Space Aeronomy (BIRA-IASB)、4.NASA Goddard Space Flight Center、5.Department of Physics, American University、6.Instituto de Astrofisica de Andalucia (IAA/CSIC)、7.School of Physical Sciences, The Open University、8.Institute di Astrofisica e Planetologia Spaziali (IAPS/INAF))

キーワード：火星、渦拡散係数、Trace Gas Orbiter、微量大気、一次元光化学モデル

CO is produced by the photodissociation of CO₂ and recycled to CO₂ by the catalytic cycle involving HOx in the Martian atmosphere [e.g., McElroy & Donahue, 1972]. The photochemical lifetime of CO is ~6 years in the lower atmosphere [Krasnopolsky, 2007]. While, in the middle and upper atmosphere (> ~50 km), the photochemical lifetime of CO becomes even much longer due to the decrease in the HOx species density and longer than the characteristic times of production and eddy diffusion. It suggests that CO profiles are determined by the production and eddy diffusion at those regions. The eddy diffusion coefficient is used for parameterizing the efficiency of vertical diffusion, however, estimated values have a large scatter between 40 and 90 km altitude [Rodrigo et al., 1990]. Recently, a substantial variation in the eddy diffusion coefficient at the homopause altitude has been suggested [Slipski et al., 2018]. It implies that CO profiles in the middle and upper atmosphere vary with variations in the eddy diffusion coefficient. ExoMars Trace Gas Orbiter (TGO) can measure the vertical profiles of CO in the mesosphere and lower thermosphere. Olsen et al. (2021) reported the vertical distribution of CO and its variation during a dust storm, however, the effects of change in the eddy diffusion coefficient on the profile of CO mixing ratio have not been investigated. In this study, we use Nadir and Occultation for MArs Discovery (NOMAD) aboard TGO to retrieve the vertical CO/CO₂ profile and to investigate the variability of the eddy diffusion coefficient.
We applied the equivalent width technique [Chamberlain and Hunten, 1987; Krasnopolsky, 1986] to derive CO and CO₂column densities. In the case that the optical depth of the absorption line is not saturated, the slant column density is given by W = SN, where W is the area of absorption, S is the line intensity, and N is the slant column density in the line of sight. We derived the slant column density using 4288.2 and 4291.5 cm^-1 for CO and 3355.7, 3357.2, 3358.7, and 3360.3 cm^-1 for CO₂. The CO/CO₂ ratio is derived between 75 and ~105 km altitudes. We use only the orbits which measure CO spectra (in order 190, 4269.95 – 4303.99 cm^-1) and CO₂ spectra (in order 149, 3348.54 – 3375.23 cm^-1) simultaneously in MY 35, corresponding from 25th March 2019 to 6th February 2021. The total number of orbits used in this study is 649.
We found that the retrieved CO/CO₂ ratio between 75 and ~105 km shows a significant seasonal variation in the southern hemisphere, which decreases near perihelion and increases near aphelion between ~1500 and ~5000 ppm at 85 km. The slope of CO/CO₂profiles becomes steep near perihelion in the southern hemisphere. To investigate the contribution of the variability of the eddy diffusion coefficient in each hemisphere and season, we calculated the CO/CO₂ by a 1D photochemical model [Koyama et al. 2021] with two cases: (1) the eddy diffusion coefficients are uniform in vertical; (2) the vertical profile of eddy diffusion coefficient is given by K = An^-1/2, where A is constant, and n is total number density [cf. Lindzen, 1971]. Our estimation shows that the altitude-dependent eddy diffusion coefficient is better than the vertically-uniform eddy diffusion coefficients to reproduce the observed profiles. In addition, our observation firstly suggested the variation of the eddy diffusion coefficient. In the southern hemisphere, K = 4.25×10¹³n^-1/2 for L_s = 90 – 120 and K = 1.5×10¹⁴n^-1/2 for L_s = 240 – 270. Throughout the altitude range, the eddy diffusion coefficient in L_s = 240 – 270 is larger by a factor of ~2 than that in L_s = 90 – 120 in the southern hemisphere. On the other hand, the estimated eddy diffusion coefficient in the northern hemisphere is comparable between both L_s ranges; K = 7×10¹³n^-1/2 for L_s = 90 – 120 and K = 1.25×10¹⁴n^-1/2 for L_s = 240 – 270. That would suggest the efficiency of the vertical diffusion varies with season and latitude.