[PPS04-P10] Long-term steadiness of thermal tide structures in zonal and meridional wind fields at the cloud top of Venus
Keywords:Venus, Thermal tides, Super-rotation, Venus Express, Akatsuki
In this study, we investigated stability or temporal variation of wind speed structures due to the thermal tides based on observed images taken with Venus Monitoring Camera (VMC) on board Venus Express. The data period is about 7 years (from 2006 to 2013), though there were data gaps of 100 days in every Venus year (224 days) because of VMC observation condition. We confirmed characteristics of the structures due to thermal tides same as previous studies, that is, zonal winds in day-side region, for example, took a local minimum at around local noon in equator region and it was 10 m/s faster than the local minimum around local morning and evening regions. The local time of the local minimum was almost same among different observation periods, and wind speed difference between local noon and local evening/morning was also similar, although there was a clear temporal variation in dayside mean zonal wind speeds as reported in Kouyama et al. (2013) and Khatuntsev et al (2013), whose peak to peak variation was more than 20 m/s. In addition, we also investigated longitude dependence (or local topography dependence) of zonal wind speed, which was suggested in Bertaux et al (2016), in each period of dayside observation sequence of VMC (~80 days). When we averaged zonal wind speed in whole data periods, we confirmed the apparent correlation between zonal wind distribution averaged and surface topography of Venus same as Bertaux et al (2016). On the other hand, it was hard to confirm the correlation when we averaged zonal wind speed in each observation period. In addition, in any longitude zonal wind speed showed a similar temporal variation during VMC observation period, so that we consider it is hard to distinguish local topography dependence of zonal wind speed from the temporal variation of the background mean zonal wind speed and wind speed variation due to observable local time transition.