Long-term variations of the UV contrast on Venus observed by the Venus Monitoring Camera on board Venus Express

•We analyze temporal variations of Venus UV contrast observed by Venus Express.•We assess photometric correction methods for global UV images.•UV contrast shows clear temporal variations of up to ∼40%.•Mesospheric SO2 abundance is correlated with the UV contrast.•Phase angle dependence of UV contrasts varied, suggesting dynamical influence. We analyze the Venus ultraviolet (UV) reflectivity as observed by the Venus Monitoring Camera on board Venus Express over 2000 orbits in the years 2006–2011. We compare several laws for the photometric correction of global images of Venus, and find that the combined law of Lambert and Lommel-Seeliger is most suitable for our study. Our analysis of the corrected images reveals strong fluctuations in the reflectivity contrast between low and high latitude regions of up to 40%, that follow variations of the SO2 abundance above the cloud top. Additionally, the phase angle dependence of the contrast gradually change from weak to strong, which may be related with the vertical distribution of the unknown UV absorber and the overlaying upper haze layer. We suggest that these variations result from a combination of two processes. One is the meridional transport of SO2, which forms sulfuric acid aerosol particles at high latitudes. The other is the presence of vertical winds near the cloud top level, which control the vertical mixing of the unknown UV absorber and the upper haze.