Equatorial jet in the lower to middle cloud layer of Venus revealed by Akatsuki

Self Cite

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.

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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

Horinouchi

Kouyama

Lee

et al. 2018

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“…The method is based on the template matching, but unlike in earlier studies , it utilizes image combinations from more than two images. We utilized a measure of precision based on the sharpness of the cross-correlation surfaces (Ikegawa and Horinouchi 2016), so that the results can be shown in the figures only when the estimated precision is better (smaller) than 10 m/s; a complete description of their derivation is available in the online supplement of Horinouchi et al (2017b). In equatorial latitudes, the zonal wind exhibits the superrotation at around 100 m/s.…”

Section: Cloud Trackingmentioning

confidence: 99%

Ultraviolet imager on Venus orbiter Akatsuki and its initial results

Yamazaki

Yamada

Lee

et al. 2018

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The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the 283 and 365 nm wavelengths. There are absorption bands of SO 2 and unknown absorbers in these wavelength regions. The UV images provide the spatial distribution of SO 2 and the unknown absorber around cloud top altitudes. The images also allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with 2-h intervals are used to understand the dynamics of the Venusian atmosphere by estimating the wind vectors at the cloud top altitude, as well as the mass transportation of UV absorbers. The UVI is equipped with off-axial catadioptric optics, two bandpass filters, a diffuser installed in a filter wheel moving with a step motor, and a high sensitivity charge-coupled device with UV coating. The UVI images have spatial resolutions ranging from 200 m to 86 km at sub-spacecraft points. The UVI has been kept in good condition during the extended interplanetary cruise by carefully designed operations that have maintained its temperature maintenance and avoided solar radiation damage. The images have signal-to-noise ratios of over 100 after onboard desmear processing. Keywords: Venus orbiter

“…One way to do so is to compare cloud-tracking results from multiple combinations of successively obtained images. See the online supplement ("Methods" section) of Horinouchi et al (2017b) for more details. Note that those authors used a different algorithm to correct pointing and succeeded in reducing the jumps.…”

Section: Interpretation Of the Estimated Accuracymentioning

confidence: 99%

Overview of Akatsuki data products: definition of data levels, method and accuracy of geometric correction

Ogohara

Takagi

Murakami

et al. 2017

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We provide an overview of data products from observations by the Japanese Venus Climate Orbiter, Akatsuki, and describe the definition and content of each data-processing level. Levels 1 and 2 consist of non-calibrated and calibrated radiance (or brightness temperature), respectively, as well as geometry information (e.g., illumination angles). Level 3 data are global-grid data in the regular longitude-latitude coordinate system, produced from the contents of Level 2. Non-negligible errors in navigational data and instrumental alignment can result in serious errors in the geometry calculations. Such errors cause mismapping of the data and lead to inconsistencies between radiances and illumination angles, along with errors in cloud-motion vectors. Thus, we carefully correct the boresight pointing of each camera by fitting an ellipse to the observed Venusian limb to provide improved longitude-latitude maps for Level 3 products, if possible. The accuracy of the pointing correction is also estimated statistically by simulating observed limb distributions. The results show that our algorithm successfully corrects instrumental pointing and will enable a variety of studies on the Venusian atmosphere using Akatsuki data.