Variability of the cloud top wind speed from the UVI/ Akatsuki imaging at 283 and 365 nm

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 unknown 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 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.