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Communication Dans Un Congrès Année : 2020

Day-night variation of O2/CO2 in Mars lower thermosphere

Résumé

We report detection of the homopause, for O2/CO2 profiles on Mars made by the Mars Atmosphere and Volalite EvolutioN/Imaging Ultraviolet Spectrograph. The O2/CO2 ratio below the homopause is equal to the values in the well-mixed homosphere; while, the O2/CO2 ratio above the homopause is enhanced by diffusive separation. The altitudes of the homopause substantially varies with latitude, season, and local-time in the range between 100 and 150 km. The predictions of the Mars Climate Database, however, systematically underestimate the altitudes of homopause by ~20 km. At a certain pressure level, we find that the variation of homopause is not obvious between day and night in the first half of the Martian Year. This suggests that inflation and contraction of the lower atmosphere can explain the variation of homopause. Meanwhile, the variation of homopause in the latter half of the Martian Year is noticeably difference between day and night. The inferred eddy diffusion coefficients at homopause are in the range between 107 and 108 cm2 s-1. This reasonably agrees with the extrapolated results by previous studies (Izakov, 1978; Slipski et al., 2018). Time constant of diffusion must be order of 104 to 105 s (hours to days). The day-night variation can be explained as a signature of the global circulation in the upper atmosphere, in addition to the difference of eddy diffusion coefficient. Our result may imply a strong upwelling on the dayside and subsidence on the nightside. This enhances more fractions of lighter species on the nightside. This process is well known on the Earth, and especially Venus, where the nightside `bulges' in H and He are factors of hundreds and thousands in the thermosphere. Similar physics can work on Mars, as reported by Elrod et al (2016) which represented the He bulge in Mars thermosphere. The classical diffusion calculations are made to demonstrate the considerations discussed above.
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Dates et versions

insu-04488964 , version 1 (04-03-2024)

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H. Nakagawa, R. V. Yelle, F. Jiang, S. Jain, Loïc Verdier, et al.. Day-night variation of O2/CO2 in Mars lower thermosphere. American Geophysical Union, Fall Meeting 2020, Dec 2020, Online, Unknown Region. pp.abstract #P040-04. ⟨insu-04488964⟩
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