Temporal and Spatial Variations of Venus Cloud Properties
J. W. Hovenier, C. J. Braak, W. J. J. Knibbe, J. F. de Haan (Free University, Amsterdam), L. D. Travis (Goddard Institute for Space Studies)
Polarization observations made by the Pioneer Venus Orbiter early in its mission could not be explained solely by the micron sized particles deduced by Hansen and Hovenier  from groundbased polarimetry. In addition to these particles, Kawabata et al.  needed to assume the presence of smaller particles. These submicron sized particles are located above and within the main cloud deck.
Using Pioneer Venus polarimetry data covering the entire mission, we analyzed the disk-averaged polarization to study temporal variations of the amount of submicron sized particles. We show that this amount decreased during the 1980's, while anomalous behavior occurred in 1983. The method used for this analysis is based on Newton-Raphson iteration and multiple scattering calculations. These calculations were performed using the adding/doubling method.
We also present a method to conduct a similar analysis on a pixel-wise basis, in order to explore spatial variations of Venus' cloud properties. In this analysis, we pay attention to the relation between Venus' ultraviolet brightness variations and the amount og submicron sized particles. Preliminary results of this analysis are shown.
Hansen, J. E., and J. W. Hovenier, "Interpretation of the polarization of Venus", J. Atmos. Sci. 31, 1137-1160, 1974
Kawabata, K., D. L. Coffeen, J. E. Hansen, W. A. Lane, M. Sato, and L. D. Travis, "Cloud and haze properties from Pioneer Venus polarimetry", J. Geophys. Res. 85, 8129-8140, 1980