It has often been stated that Saturn’s magnetosphere and aurorae
are intermediate between those of Earth, where the dominant
processes are solar wind driven1, and those of Jupiter, where
processes are driven by a large source of internal plasma2–4. But
this view is based on information about Saturn that is far inferior
to what is now available. Here we report ultraviolet images of
Saturn, which, when combined with simultaneous Cassini
measurements of the solar wind5 and Saturn kilometric radio
emission6, demonstrate that its aurorae differ morphologically
from those of both Earth and Jupiter. Saturn’s auroral emissions
vary slowly; some features appear in partial corotation whereas
others are fixed to the solar wind direction; the auroral oval shifts
quickly in latitude; and the aurora is often not centred on the
magnetic pole nor closed on itself. In response to a large increase
in solar wind dynamic pressure5 Saturn’s aurora brightened
dramatically, the brightest auroral emissions moved to higher
latitudes, and the dawn side polar regions were filled with intense
emissions. The brightening is reminiscent of terrestrial aurorae,
but the other two variations are not. Rather than being intermediate
between the Earth and Jupiter, Saturn’s auroral emissions
behave fundamentally differently from those at the other
planets.