Temporal variations in the cloud cover of Venus as detected from Venus Monitoring Camera Images on Venus Express Orbiter
Abstract:The Venus Monitoring Camera (VMC) on Venus Express  has been collecting images of the planet since orbit insertion in April 2006 through four narrow band pass (50 nm halfwidth) with center wavelengths of 365, 550, 950 and 1050 nm . With varying range to the planet during the spacecraft’s elliptical, near polar orbit, VMC obtains views of the day side southern hemisphere ( ~ 72,500 km) and the limb when it is furthest away from the planet, and can see a fraction of the planet’s sun-lit limb northern latitudes when the spacecraft is closer to the planet ( >~ 25,000 km). We use these images to look at the temporal behavior of the normalized intensity and unit slant optical depth (location of the bright limb) at four wavelengths during April 2006 – March 2014. We detect correlated changes in the normalized brightness and the altitude of the unit optical depth over this period.
Images were normalized using Minnaert function to account for the varying scattering geometry in order to detect changes in the reflectivity of the cloud cover at selected locations in local solar time. The unit optical depth was determined from the location of the planet's bright limb, taken to be where the brightness gradient is maximum along the bright limb azimuth. The changes observed appear to be quasi periodic.
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