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) images obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 over the period of 1994-1997. These images probe the stratospheric aerosols at pressures of a few tens of millibars. We discovered a UV-darkened segment in the polar shading just equatorward of the north polar hood that remained aligned with the southern-most portion of the north auroral oval. The darkened segment generally extended over 100°-260° System III longitude, and 39°-53°N planetocentric latitude in the F160W, F218W, and F255W images. No single type of feature in the F218W and F255W images appeared consistently aligned within the outline of the north auroral oval, but several transient features were observed. It is not certain whether these transient features were associated with auroral processes, or just coincidentally aligned. A "south dark patch" was consistently observed within the outline of the south auroral oval and was temporally variable in size and shape. By tracking selected features, we obtained the first direct measurements of the zonal motions in Jupiter's high-latitude stratosphere. Primarily retrograde motions were observed from 41°N to 60°N. Both prograde and retrograde motions appeared from 48°S to 71°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 stratosphere. Enhanced meridional mixing would tend to be more effective at dispersing the polar aerosols to lower latitudes in the north than in the south.