Vertical Coupling Between the Cloud-Level Atmosphere and the Thermosphere of Venus Inferred From the Simultaneous Observations by Hisaki and Akatsuki

The vertical coupling between the cloud-level atmosphere and the thermosphere of Venus was investigated using 365 nm images obtained by the Ultraviolet Imager on board Akatsuki and oxygen atom 135.6 nm dayglow intensities obtained by the Extreme Ultraviolet Spectroscope for Exospheric Dynamics on board the space telescope Hisaki. Simultaneous observations revealed a common periodicity of ~3.6 Earth days in the cloud-tracked velocity, the cloud brightness, and the airglow intensity. The oscillation at the cloud level is attributed to a planetary-scale Kelvin wave. A one-dimensional linear wave model showed that the Kelvin wave cannot propagate from the cloud level to the thermosphere because of radiative damping. The modeling also revealed that a small-scale gravity waves having wavelengths of <1,000 km can reach the thermosphere on the dawnside and that they are strongly attenuated on the duskside because of the difference in the background winds with vertical shears. We further developed a one-dimensional photochemical model to investigate the response of the airglow intensity to the change of the eddy diffusion coefficient, which is expected to increase with the wave amplitude. The model showed that a 3.6-day oscillation of the diffusion coefficient with an amplitude of ~300 m² s⁻¹ explains the observed airglow intensity variation. A possible cause of the 3.6-day period is the filtering of gravity wave fluxes by the Kelvin wave-induced wind.