Circulation of the Venus atmosphere from two years of Akatsuki UVI camera observations

The UVI camera onboard the JAXA Akatsuki mission provided long-term series of images of the Venus cloud tops at 283 nm and 365 nm – two wavelength that correspond to the spectral bands of gaseous sulfur dioxide and UV absorber. We used the automated correlation method to track motions of the cloud features and derive wind speed and its variations for two observation intervals of one Venusian year each:  October 2019 – April 2020 (S07) and April 2022 – September 2022 (S11). The mean zonal velocity derived from 283 nm images at noon is by up to 5 m/s higher than the speed measured at 365 nm. Also the afternoon peak of zonal velocity at 283 nm is shifted towards evening terminator with respect to that measured at 365 nm. Zonal velocity increases with phase angle that implies positive altitude gradient of the wind velocity. This might suggest that the radiation at 283 nm forms in slightly higher layers than that at 365 nm that can explain the difference in velocity measured in two spectral bands. We found a dependence of the atmospheric dynamics and planet albedo on the position of Y feature. The strongest correlation within pairs of images in 283 nm and 365 nm is observed in the heading part of the Y feature resulting in closer wind speeds measured in these channels.