Venus Atmospheric Circulation from Two Years of Observations by the UVI/Akatsuki Ultraviolet Camera

Abstract — The 283-nm and 365-nm spectral channels of the Ultraviolet Imager on board the Akatsuki spacecraft of the Japan Aerospace Exploration Agency (JAXA) enabled observations of features at two altitude levels in the atmosphere of Venus and estimate wind speeds from the displacement of these features. The main absorber in the 283-nm channel is sulfur dioxide SO 2 , present in the upper haze. An unknown UV absorber located at the cloud top (70 ± 2 km) is responsible for the formation of contrasting features in the 365-nm channel. Analysis of wind speed fields observed in both channels allows us to estimate the relative vertical position of both observation levels. Based on a series of sequential UV images (283 and 365-nm) of the cloud cover of Venus, wind velocity fields were obtained for two periods, each covering a Venusian year (October 2019–April 2020 and April 2022–September 2022). The article provides a comparative analysis of the results. Zonal velocities demonstrate dependence on local time and phase angle of observations. In equatorial latitudes, minimum of the average zonal velocity is shifted toward the evening terminator. The average zonal velocities in the 283-nm channel are systematically larger than those in the 365-nm channel, indicating that the 283-nm channel provides information from a relatively higher altitude. A dependence of the dynamic parameters of the atmosphere and image brightness on the position of Y-feature was discovered. In the frontal part of the Y‑feature, the greatest correlation is observed between simultaneous images obtained in different spectral channels, which leads to a convergence of wind speeds in these channels.