Mean winds at the cloud top of Venus obtained from two-wavelength UV imaging by Akatsuki

Abstract

Venus is covered with thick clouds. Ultraviolet (UV) images at 0.3-0.4 microns show detailed cloud features at the cloud-top level at about 70 km, which are created by an unknown UV-absorbing substance. Images acquired in this wavelength range have traditionally been used to measure winds at the cloud top. In this study, we report low-latitude winds obtained from the images taken by the UV imager, UVI, onboard the Akatsuki orbiter from December 2015 to March 2017. UVI provides images with two filters centered at 365 and 283 nm. While the 365-nm images enable continuation of traditional Venus observations, the 283-nm images visualize cloud features at an SO 2 absorption band, which is novel. We used a sophisticated automated cloud-tracking method and thorough quality control to estimate winds with high precision. Horizontal winds obtained from the 283-nm images are generally similar to those from the 365-nm images, but in many cases, westward winds from the former are faster than the latter by a few m/s. From previous studies, one can argue that the 283-nm images likely reflect cloud features at higher altitude than the 365-nm images. If this is the case, the superrotation of the Venusian atmosphere generally increases with height at the cloudtop level, where it has been thought to roughly peak. The mean winds obtained from the 365-nm images exhibit local time dependence consistent with known tidal features. Mean zonal winds exhibit asymmetry with respect to the equator in the latter half of the analysis period, significantly at 365 nm and weakly at 283 nm. This contrast indicates that the relative altitude may vary with time and latitude, and so are the observed altitudes. In contrast, mean meridional winds do not exhibit much long-term variability. A previous study suggested that the geographic distribution of temporal mean zonal winds obtained from UV images from the Venus Express orbiter during 2006-2012 can be interpreted as forced by topographically induced stationary gravity waves. However, the geographic distribution of temporal mean zonal winds we obtained is not consistent with that distribution, which suggests that the distribution may not be persistent.

Stationary Features at the Cloud Top of Venus Observed by Ultraviolet Imager Onboard Akatsuki

“…The scale height is calculated to be H ~ 5.1 km. The zonal wind speed at the cloud top is assumed to be 100 m/s (e.g., Horinouchi et al, ), meaning that the intrinsic phase velocity is also 100 m/s in the opposite direction. From these parameters and the observed horizontal wavelength of ~510 km (section ), the wave period is 1.4 hr, corresponding to ω ~ 0.0012 s ‐1 .…”

“…Oscillations of the vertical wind that are attributed to topographic gravity waves have been observed by the VEGA balloons over Aphrodite Terra, which has a top altitude of 3–4 km (Blamont et al, ). Recently, Bertaux et al () suggested, based on cloud tracking using cloud images taken by the Venus Monitoring Camera onboard Venus Express, that the zonal wind speed decreases above Aphrodite Terra, probably due to momentum deposition by topographic gravity waves, although the result of cloud tracking using Akatsuki UVI data does not support this result (Horinouchi et al, ). A large number of small structures that are considered to be manifestations of topographic gravity waves were also found in the thermal maps of the cloud top on the nightside by the Visible and InfraRed Thermal Imaging Spectrometer onboard Venus Express (Peralta et al, ).…”

Global Structure of Thermal Tides in the Upper Cloud Layer of Venus Revealed by LIR on Board Akatsuki

“…Whole components showed almost hemispherical symmetry, but the amplitude of the semidiurnal tide in the northern hemisphere was slightly stronger than that in the southern hemisphere. Recently, an asymmetric structure was reported in zonal winds at the cloud top (Horinouchi et al, ). There could be a mechanism that induces the asymmetric condition in the Venusian atmosphere whose impact is worth investigating with a numerical approach.…”

Morphology and Dynamics of Venus's Middle Clouds With Akatsuki/IR1

“…At the upper and middle clouds of Venus, the zonal superrotation exhibits the largest speeds and vertical shear (Sánchez‐Lavega et al, ) and is also where most of the solar energy is deposited (Titov et al, ). The upper clouds' top at ∼70 km (Ignatiev et al, ) can be observed with ultraviolet (UV) and violet wavelengths (∼360–480 nm), with their morphology and dynamics been extensively studied for decades (Belton et al, ; Horinouchi et al, ; Khatuntsev et al, ; Limaye et al, ; Rossow et al, , ; Titov et al, ), thanks to the strong contrasts caused by an unknown absorber (Lee et al, ; Pérez‐Hoyos et al, ).…”