Cassini keeps amazing me, even when I start to think I'm getting used to incredible images beamed back from Saturn.
But this is new: it took several images of Saturn's aurora, and strung them together to make a video! Here's a still frame:
The outline of Saturn is there, along with some latitude lines for reference (the bright thin wedge is the daylit side of the planet itself). You can see stars blurred in the background, with the aurora the orangey glow rising about the Saturnian cloud deck. This frame is cool, but for frak's sake go watch the video! It's mesmerizing. I'd put it here, but it's 3 Mb and that would destroy the Hive Overmind servers.
The images put together to create the animation were taken over four days. Interestingly (well, to me anyway) these images were taken in visible light; we already know from Hubble observations that Saturn's aurorae are bright in UV.
These northern lights on Saturn are formed in pretty much the same way they are on Earth: particles in the Sun's solar wind get captured in the magnetic field of the planet, and are swept to the poles where they slam into molecules in the atmosphere, lighting it up (literally) like a neon sign. The details are far more complicated, of course, but that's the basic picture. The images from Cassini don't show color, which is too bad: the color of the glow can tell you what chemical elements are in the atmosphere, since different atoms emit at different wavelengths. Still, there is much to be learned about Saturn's magnetic field from images like these.
And may I add: wow. Aurorae, from a billion kilometers away. Incredible!