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See Saturn's twin light shows

J. Nichols / Univ. of Leicester / NASA / ESA
Click for video: Ultraviolet imagery of Saturn, captured by the Hubble Space
Telescope a year ago, reveals the planet's northern and southern auroras at the
same time. Click on the image to watch a video from the European Space Agency.

Scientists are showing off a one-of-a-kind double aurora, spotted on Saturn by the Hubble Space Telescope. The northern-southern light display is notable not only because of its rarity, but also because it shows that the giant planet's magnetic field is out of balance.

Saturn's auroral displays are created by the interaction between the planet's magnetic field and electrically charged particles zooming out from the sun. This isn't unusual, of course. Earth's auroras provide the best-known examples of the phenomenon, and scientists have seen polar lights on Jupiter as well as Mars, Uranus and Neptune.

But the peculiarities of Saturn's poles have intrigued astronomers for years. Some puzzle over a hexagonal cloud pattern circling the north pole. Others have found that Saturn's northern lights went all over the place, with dramatic ups and downs.

To learn more about the auroras, a team led by the University of Leicester's Jonathan Nichols pointed the world's best-known space telescope at the giant planet for several days last year between January and March, which was close to the time of Saturn's equinox. That's when Saturn's rings are edge-on with relation to the sun, and both the north and the south polar regionsŠcan be seen from Earth simultaneously.

Nichols noted that last year was Hubble's best opportunity to see the double feature. Probably its only opportunity, as a matter of fact. By the timeŠthe Saturnian equinox rolls around again, in 2024, the space telescope will likely be over the hill. So Nichols' team captured time-sequence images using the Advanced Camera for Surveys' ultraviolet-sensitive solar blind channel - the only part of the ACS that was working at the time.

"This sustained series of images of simultaneous north-south aurora are important scientifically, since they cannot be obtained at any other planet, including Earth," Nichols said in a university news release. "They tell us a great deal about the nature of the planet's magnetic field and the processes which generate aurorae in a way not possible at Earth. It's a great example of how planetary science can fully complement the study of the Earth."

Nichols and his colleagues reported their results late last year in Geophysical Research Letters, and the Hubble team released the double-aurora movie just today. The glowing displaysŠfrom the two auroras were seen to rise and fall pretty much in tandem. That'sŠconsistent with the idea that charged particles flow between north and south along invisible "traffic lanes" in Saturn's magnetic field.

The light shows are not completely symmetric, however. The auroral oval is slightly smalller and more intense in the north than in the south, and that implies that the magnetic field is not evenly distributed across the planet. It must be slightly stronger in the north - an effect that wouldŠaccelerateŠthe charged particles to higher energiesŠas they're fired toward the atmosphere.

So what's the reason for the asymmetry? That's not yet known, but researchers have a few possibilities they want to check out. They may well get their chance: This may be the last hurrah for Hubble's observations of a Saturnian equinox, but the Cassini orbiter is expected to send backŠobservations of Saturn's magnetic field and auroras for seven more years.

Nichols and his colleagues said the Hubble data provide a good starting point for solving Saturn's polar puzzles. "These observations will provide important context for Cassini observations as Saturn moves from southern to northern summer," they wrote.

For more about Saturn and Cassini, check out the Web resources available from NASA and the orbiter's imaging team, as well as msnbc.com's Saturn stories and Saturn slideshows. We even put together a special slideshowŠto celebrateŠSaturn's rite of spring. And let's not forget about Hubble: We have slideshows that round up Hubble's greatest hits as well as Hubble's latest greatest hits. Aw, heck ...Šyou might as well click your way through our entire space gallery while you're at it.

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