Context images for Venus Express radio occultation measurements: A search for a correlation between temperature structure and UV contrasts in the clouds of Venus

Context. Venus exhibits strong and changing contrasts at ultraviolet wavelengths. They appear to be related to the clouds and the dynamics in the cloud layer, but to date their origin continues to be unknown. Aims. We investigate the nature of the UV contrasts exhibited by Venus’ clouds by examining possible correlations between the thermal structure inferred from radio occultation data and UV brightness from imagery data, both observed with Venus Express. Methods. We analysed Venus Express images obtained from 11 hours before to a few hours after the time of radio occultation measurements of the same area. We accounted for the advection of clouds by zonal and meridional winds and applied a phase angle correction to compensate for the changing viewing geometry. Results. We find a possible anti-correlation between UV brightness and atmospheric temperature around an altitude of 67 km for low latitudes, with a one percent probability of this finding being due to chance (p value = 0.01). Heating in this altitude and latitude region due to an increase in the UV absorber has been predicted by radiative forcing studies. The predictions roughly match our observed temperature amplitude between UV-dark and UV-bright regions. Conclusions. This could be the first observational evidence of a direct link between UV brightness and atmospheric temperature in the 65–70 km altitude region in the clouds of Venus.