Mysterious new aurora

on Saturn
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 13 November, 2008

An infrared camera on the Cassini spacecraft has detected a unique aurora at Saturn’s north polar cap, unlike any other known in the Solar System.

"We've never seen an aurora like this elsewhere," says Tom Stallard of the University of Leicester, and lead author of a paper released today in the journal Nature. "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."

The image shows both a bright ring, as seen from Earth, as well as an example of bright auroral emission within the polar cap that had been undetected until the advent of Cassini.  Silhouetted by the glow (shown here in red) of the hot interior of Saturn are the clouds and haze that underlie this auroral region. Image: NASA/JPL/University of Arizona.

Auroras are caused by the collision of charged particles streaming along the magnetic field lines of a planet into its atmosphere, and give rise to the spectacular Northern Lights or the aurora borealis on Earth (and similarly the aurora australis in the south polar regions, too). Most aurorae appear green and red due to emissions from atomic oxygen.

For the giant planets Jupiter and Saturn, the majority of aurora are caused by particles trapped within the magnetic environments of those planets. Jupiter's main auroral ring, caused by interactions internal to Jupiter's magnetic environment, is constant in size, while Saturn's main aurora, which is caused by the solar wind, undergoes dramatic size changes as the wind varies. The newly observed aurora at Saturn's north pole, however, does not 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," says Nick Achilleos, Cassini scientist based 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."

Energetic particles, crashing into the upper atmosphere cause Saturn’s aurora (shown in blue in the image presented here) to glow brightly in infrared. This aurora, which defies past predictions of what was expected, is constantly changing, even disappearing within a 45 minute-period. Cassini’s infrared eyes found that the aurora sometimes fills the region from around 82 degrees north all the way over the pole.

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