An inexplicable new broad region of auroral light has been photographed at Saturn's polar cap.

"We've never seen an aurora like this elsewhere," said Tom Stallard, an RCUK Academic Fellow working with Cassini data at the University of Leicester. "It's not just a ring of aurorae like those we've seen at Jupiter or Earth.

"This one covers an enormous area across the pole. Our current ideas on what forms Saturn's aurorae predict that this region should be empty, so finding such a bright one here is a fantastic surprise."

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These colorful atmospheric light shows are caused when charged particles stream along the magnetic field of a planet and into its atmosphere.

On Earth the charged particles come from the solar wind — a stream of particles that emanates from the sun. Skywatchers at high latitudes know the resulting displays as the Northern Lights.

Jupiter's main auroral ring, caused by interactions internal to Jupiter's magnetic environment, is constant in size. Saturn's main aurora, which is caused by the solar wind, changes size dramatically as the wind varies.

The newly observed aurora at Saturn, however, doesn't fit into either category.

"Saturn's unique auroral features are telling us there is something special and unforeseen about this planet's magnetosphere and the way it interacts with the solar wind and the planet's atmosphere," said Nick Achilleos, a scientist on the Cassini magnetometer team at the University College London. "Trying to explain its origin will no doubt lead us to physics which uniquely operates in the environment of Saturn."

The new image, in infrared, was imaged by the Cassini spacecraft. It is reported in the Nov. 13 issue of the journal Nature.

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