Correlation between temperature structure and UV contrasts in the clouds of Venus from Venus Express VMC and VeRa data.

Venus exhibits strong and changing contrasts at ultraviolet wavelengths apparently related to the clouds and the dynamics in the cloud layer, but to date their origin continues to be unknown.

 

In this work we analyse the possibility of correlation between the UV-brightness and the temperature structure in the atmosphere using unique data from Venus Express. On the one hand these data are measurements of the temperature structure from radio occultation data (VeRa experiment) in very small areas on Venus, on the other hand they are UV-images of the same spot up to 11 hours before and a few hours after the radio occultation experiment (VMC instrument). This type of analysis has not been presented before, as no such data exists from earlier missions.

(Three images each two hours apart taking during ingress of orbit 2805 on 25 December 2013. The yellow star indicates the spot of the VeRa radio occultation that happens after 8h, 6h and 4h respectively (left to right) of the moment of the image. The red rectangles are the wind-advected latitude / longitude boxes corresponding to VeRa location advected by the zonal and meridional winds. The boundaries of the boxes are measures of the uncertainties in the zonal and meridional winds.)

The South Polar Dynamics Campaign that was done during the last month of 2013 focussed on getting images and radio occultations on each VEX orbit. In addition, we found other orbits where this occurred. In total we identified 56 orbit with suitable data for this study. We apply a phase angle correction to compensate for the changing viewing geometry between the individual images and account for the advection of clouds by zonal and meridional winds.

(Spearman’s rank correlation coefficients for UV Radiance Factor Ratio as a function of temperature (left column) and normalised temperature (right column), at levels between 65 and 75 km altitude for three latitude bins. The largest effect is at the lower and higher altitudes, where the change of temperature with latitude is strongest. The corresponding one-sided p-value for each of the correlations is shown in the bottom panels. We choose a limit of p < 0.02 to claim significance, which is indicated by the green area at the bottom of the plots. Only for 67 km altitude in the uncorrected temperature and 67 and 68 km altitude in the normalised temperature correlation does the p-value drop below the limit.)

 

(The Radiance Factor Ratio versus temperature (left) and normalised temperature (right) at 67 km altitude. Normalisation is done relative to a linear least square fit to the temperature as a function of latitude at each level between 50 and 80 km altitude.)

 

After very carefully taking into account all the sources or error and uncertainty We find a possible anti-correlation between UV-brightness and atmospheric temperature around 67 km altitude for low latitudes, with a one percent probability this finding is 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 (for example, Crisp 1986). If we assume the temperature difference we observe between UV-bright and UV-dark areas are due to this heating, then it is possible to compare this value to what is expected from the model heating rate calculations. The theoretical number would be on the order of 10-20 K, the measured value is on the order of 10 K, and thus is of the same order. This is encouraging, particularly given that this is observed only in the low latitude region, where solar heating would be expected to be most significant, and because the altitude range where the heating is observed also corresponds to that in Crisp’s model. This could be the first observational evidence for a direct link between UV-brightness and atmospheric temperature in the 65 - 70 km altitude region in the clouds of Venus.