Saturn’s auroral oval appears at higher magnetic latitudes than
Jupiter’s, it has long been believed7 to map close to the boundary
of open and closed field lines, and on theoretical grounds8 it has
been considered more likely to be controlled by the solar wind
than Jupiter’s. At the same time, Saturn’s magnetosphere is much
larger than the Earth’s (Table 1), and Saturn’s rapid rotation and
internal plasma sources suggest that jovian-like processes should
also be active. The approach of the Cassini spacecraft to Saturn
has provided a unique opportunity to examine the nature of
Saturn’s auroral emissions. A series of five Hubble Space Telescope
(HST) orbits on 8 January 2004 provided measurements of
the longitude variations of the auroral emissions over 70% of a
Saturn rotation, while single orbit observations on 12 other days
were obtained to compare the auroral activity with varying solar
wind conditions. Twelve image sub-frames are shown in Fig. 1,
obtained with the HST Space Telescope Imaging Spectrograph
(STIS).