Jupiter's polar regions in the ultraviolet as imaged by HST/WFPC2: Auroral-aligned features and zonal motions

Jupiter's polar regions are examined using ultraviolet (UV, 120-320 nm) ima ges obtained with the Hubble Space Telescope Wide Field and Planetary Camer a 2 over the period of 1994-1997, These images probe the stratospheric aero sols at pressures of a few tens of millibars. We discovered a UV-darkened s egment in the polar shading just equatorward of the north polar hood that r emained aligned with the southern-most portion of the north auroral oval. T he darkened segment generally extended over 100 degrees-260 degrees System III longitude, and 39 degrees-53 degrees N planetocentric latitude in the F 160W, F218W, and F255W images. No single type of feature in the F218W and F 255W images appeared consistently aligned within the outline of the north a uroral oval, but several transient features were observed. It is not certai n whether these transient features were associated with auroral processes, or just coincidentally aligned. A "south dark patch" was consistently obser ved within the outline of the south amoral oval and was temporally variable in size and shape. By tracking selected features, we obtained the first di rect measurements of the zonal motions in Jupiter's high-latitude stratosph ere. Primarily retrograde motions were observed from 41 degrees N to 60 deg rees N. Both prograde and retrograde motions appeared from 48 degrees S to 71 degrees S. We propose that the asymmetric appearance of the polar hoods (R. A. West 1979, Icarus 38, 12-33; R. Wagener and J. Caldwell 1988, Icarus 74, 141-152) is influenced, in part, by meridional mixing in the stratosph ere. Enhanced meridional mixing would tend to be more effective at dispersi ng the polar aerosols to lower latitudes in the north than in the south. (C ) 2000 Academic Press.