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L. A. Sromovsky, P. M. Fry (University of Wisconsin - Madison), T. E. Dowling (University of Louisville), S. S. Limaye (University of Wisconsin - Madison), K. H. Baines (Jet Propulsion Laboratory)
HST imaging of Neptune with WFPC2 since 1994 provides an extensive data set for investigating recent characteristics of Neptune's atmospheric circulation. We find that cloud-top motions from 1994 through 1996 can be segregated into essentially two distinct components: one that closely matches the 1989 Voyager mean profile of Sromovsky et al. (1993. Icarus \bf 105, 110-141) and another associated with waves that appear to originate from a northern dark spot (first identified by Hammel et al. (1994. Science \bf 268, 1740-1742). The complex of bright clouds associated with this dark spot extends from 40o N across the equator to ~ 20o S. The complex varies substantially over a several month period and has component elements with rapid meridional motions. It might be caused by a type of Kelvin wave, which has the correct propagation direction after Doppler shifting the winds relative to the equator. At 0.89 \mum, the hemisphere that contains the bright complex is 39% brighter (0.36 mag) than the opposite hemisphere, a ratio comparable to the 0.44 mag ratio obtained for the peak hemispheric difference of the 1986-1989 ``outburst'' reported by Hammel et al. 1992). The dark spot seen in 1996 at 33o N is very likely the same one seen in 1994 and 1995. It seems to have a varying longitudinal drift rate, but no observable drift in latitude, unlike the Voyager GDS, which drifted equatorward at a rate 1o/month (Sromovsky et al. 1993). A wavenumber-one cloud band near 60o S latitude, most evident in 1996 images, was also present in 1989 Voyager images. A persistent wave number-two pattern of clouds at 45o S, exhibits strong temporal variations in brightness. Zonal bands that were densely populated with discrete cloud features during 1994-1996 are at different latitudes from those observed during the 1989 Voyager encounter.