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US scientists unveil secrets of Saturn's polar light
Updated: 2005-02-17 16:20

A team of US astronomers said Wednesday they have found that Saturn's aurora behave differently than scientists have believed for the last 25 years.

An image taken by the Cassini spacecraft on July 3, 2004 and received on Earth on the same day shows the planet Saturn casting a shadow over its rings. [file photo]

Based on the data from NASA's Cassini spacecraft and Hubble Space Telescope, the research group led by John Clarke, a professor at Boston University, overturned theories about how Saturn's magnetosphere behaves and how its aurora are generated.

Their findings will be published in the Feb. 17 issue of Nature.

By choreographing the instruments aboard the Saturn-bound Cassini spacecraft and the Hubble Space Telescope circling Earth to look at Saturn's southern polar region, Clarke's team found that the planet's aurora are fundamentally unlike those observed on Earth or Jupiter.

Scientists had long believed Saturn's aurora are influenced by the solar wind, like Earth's, and assumed they are influenced by a ring of ions and charged particles encircling the planet, like Jupiter's.

But the team found that when Saturn's aurora become brighter and more powerful, the ring of energy encircling the pole shrinks in diameter. By contrast, when Earth's aurora become brighter, the polar region fills with light for several minutes, then dims, and the ring of light expands.

Jupiter's aurora, in comparison, are only weakly influenced by the solar wind, becoming brighter about once a month.

Saturn's auroral displays also become brighter on the sector of the planet where night turns to day as the storms increase in intensity, unlike either of the other two planets. At certain times, Saturn's auroral ring was more like a spiral, its ends not connected as the energy storm circled the pole.

The observations showed that Saturn's aurora differ in character from day to day, as they do on Earth, moving around on some days and stationary on others. But compared to Earth's auroral displays, which last only about 10 minutes, Saturn's aurora can last days.

The observations also indicated that the sun's magnetic field and solar wind may play a much larger role in Saturn's aurora than previously suspected.

Hubble images show that some displays remain stationary as the planet rotates beneath, as happens on Earth, but also show that, as on Jupiter, the aurora sometimes move along with Saturn as it spins on its axis.

This difference suggests that Saturn's aurora are driven in an unexpected manner by the sun's magnetic field and the solar wind, and that the planet's aurora possibly have different physical states at different times, the researchers said.

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