The morphology of Saturn's ultraviolet aurora

摘要

Spectrographic images in the far ultraviolet, obtained from observations made from 2007 to 2009, are used to quantify and compare the morphology of Saturn's northern and southern aurora in local time. Made by the UVIS imager on the Cassini spacecraft, these observations differ from previous UV observations of Saturn's aurora by offering complete local time coverage of the aurora from small zenith angles. A statistical picture emerges after combining over 10 ~5 spectrographic images of the north and south aurora. From its composite, the northern aurora exhibits an intensity peak of ～40 kR near 05 h local time, while the southern aurora peaks at ～60 kR near the same local time. Aurora intensities in the post-noon sectors are essentially constant at ～10 kR. The half-power points of latitudinal profiles reveal widths of ～4° near dawn and ～8° near dusk in both north and south. The aurora has an approximately circular shape (not oval!), although harmonic fits suggest the northern aurora is more nearly circular than the southern. The center of the northern aurora has an offset of ～1-2° from the pole, while the southern center is offset by ～2-3°, with both offsets in the midnight-to-dawn quadrant. When mapped to the polar regions, precipitation of field-aligned energetic electrons(>20 keV)generally lies equatorward of the main auroral intensity at all local times.