Sulfur Dioxide Distribution at the Venusian Cloud-Top Retrieved From Akatsuki UV Images

Abstract

$S{O}_2$ and the unidentified UV absorber are major absorbers in the near-ultraviolet region in the Venusian atmosphere and influence the climate system. We developed a method to retrieve $S{O}_2$ and the imaginary part of the refractive index $n_i$, a proxy for the unidentified absorber, from 15,002 pairs of 283 and $365nm$ images taken by the Ultraviolet imager (UVI) onboard Akatsuki. We obtained the distributions of $S{O}_2$ and the unidentified absorber with high spatial resolution every 2 hours for about 6.5 years. We analyzed the local time and latitudinal distributions of $S{O}_2$ and the unidentified absorber. The $S{O}_2$ mixing ratio at the cloud top around $70km$ altitude depends on latitude and local time, ranging from 80 to $200ppb$, within the range of previous observations. $S{O}_2$ and the unidentified absorber are abundant at low latitudes and decrease toward mid-latitudes, which is attributed to vertical transport by the Hadley circulation. The $S{O}_2$ abundance is maximized around 14:30 at low latitudes; the distribution is explained by the vertical transport by the semi-diurnal tide and consistent with the cloud-top velocity distribution. The local time and latitude dependence of $S{O}_2$ obtained is consistent with a three-dimensional general circulation model involving photochemical reactions. We also derived the long-term variations in $S{O}_2$ and the unidentified absorber. $S{O}_2$ increased from 2016 to 2019, showed no clear trend until early 2021, and decreased through 2022. No clear correlation was seen between the unidentified absorber and the mean super rotation speed, although it contributes to the solar heating rate.