From the complete set of observations, it has become clear that
Saturn’s auroral emissions vary on timescales down to tens of
minutes, similar to Jupiter’s main oval but far slower than its
polar emissions. The images from 8 January (Fig. 1) show the
presence of a localized bright emission region along the main oval at
130–1708 l SLS (Saturn longitude system), rotating in the direction
of the planet, becoming fainter with time, and shifting to higher
latitudes. This emission region may correspond to earlier reports of
an active sector in both the ultraviolet and Saturn kilometric
radiation (SKR) emissions based on Voyager measurements7, and
might correspond to the source of SKR emission6. The brightest
ultraviolet emissions (concentrating on emissions that appear stable
over time) move at 30–65% of Saturn’s rotation speed, whereas the
motion of the overall emission pattern appears to be best fitted by
75% of the corotation rate. This would imply that the emissions
map along Saturn’s magnetic field to a region where the plasma is
moving at 30–75% of the corotation rate in Saturn’s magnetic
field. As the plasma generally rotates at a lower angular velocity
with increasing distance, the rotational motions of auroral emissions
can be compared with plasma flow speeds to estimate the
equatorial distance from the planet connecting to these auroral
emissions. From earlier Voyager measurements of the plasma
flow9, this could be consistent with a wide range of distances
from the planet, from 6–7 Saturn radii (RS) out to the magnetopause
boundary. Plasma flow measurements from Cassini orbiting
Saturn, in comparison with auroral images, will be helpful in
determining the distance to which the main auroral oval maps.
This observation of partially corotating emissions contrasts with
earlier HST images of Saturn’s auroral emissions10–12, which
indicated that the emissions consistently brightest in the dawn
sector—that is, fixed in local time. This raises the possibility of
different states of Saturn’s magnetosphere, leading to different
auroral emission distributions. Such states could be related to the
orientation of Saturnwith respect to, or variations in, the solar wind
and IMF