For the first time, the Cassini spacecraft has captured visible light images of the rippling northern lights on Saturn, which claim the title of tallest auroral curtains in the Solar System.

The movie consists of 472 images taken during an 81 hour period by Cassini's narrow-angle camera between 5-8 October 2009 and reveals changes in Saturn's aurora every few minutes. The images show a previously unseen vertical profile to the auroras, which ripple like tall curtains, reaching more than 1,200 kilometres above the edge of the planet’s northern hemisphere. By comparison, Earth’s auroras tend to flare only about 100-500 kilometres above the surface.

“The auroras have put on a dazzling show, shape-shifting rapidly and exposing curtains that we suspected were there, but hadn’t seen on Saturn before,” says Andrew Ingersoll, a member of the Cassini imaging team that processed the new video. “Seeing these things on another planet helps us understand them a little better when we see them on Earth.”

Aurorae appear mostly in the high latitudes near a planet’s magnetic poles when charged particles plunge into the planet’s upper atmosphere, causing it to glow in colours that reflect the chemical content of the atmosphere. Earth's aurorae mostly appear red and green thanks to oxygen and nitrogen, but scientists are still working on determining the true colours of Saturn's aurora (they are shown in false colour orange in these images to make them stand out from the background), created in an atmosphere dominated by hydrogen.

This light hydrogen gas allows Saturn's atmosphere and auroral curtains to reach much farther out than Earth's displays but the speed at which they change is comparable to some of those on Earth. Scientists are still working to understand the processes that produce these rapid changes but the heights will help them learn how much energy is required to light up the auroras. The curtain shapes also show the paths that the charged particles take as they flow along the lines of the magnetic field between the magnetosphere and the uppermost part of the atmosphere.

“I was wowed when I saw these images and the curtain,” says Tamas Gombosi, chair of Cassini’s magnetosphere and plasma science working group. “Put this together with the other data Cassini has collected on the auroras so far, and you really get a new science.”

The images also capture the myriad background stars.

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