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Saturn's mysterious auroras explained.

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Saturn's Auroras.

In some ways, the bright auroras at Saturn's poles are very similar to our own Northern/Southern lights here on Earth. But in other ways, they're very different, and it's a mystery that has been puzzling Astronomers since they were first discovered 25 years ago. Saturn's auroras can brighten for days (compared to minutes here Earth), and can stay still while the planet rotates underneath. Now scientists have used observations from Hubble and the Cassini spacecraft to develop a new theory about how Saturn's magnetic field interacts with the solar wind to produce its unusual auroras.

Scientists studying data from NASA's Cassini spacecraft and Hubble Space Telescope have found that Saturn's auroras behave differently than scientists have believed for the last 25 years.

The researchers, led by John Clarke of Boston University, found the planet's auroras, long thought of as a cross between those of Earth and Jupiter, are fundamentally unlike those observed on either of the other two planets. The team analyzing Cassini data includes Dr. Frank Crary, a research scientist at Southwest Research Institute in San Antonio, Texas, and Dr. William Kurth, a research scientist at the University of Iowa, Iowa City.

Hubble snapped ultraviolet pictures of Saturn's auroras over several weeks, while Cassini's radio and plasma wave science instrument recorded the boost in radio emissions from the same regions, and the Cassini plasma spectrometer and magnetometer instruments measured the intensity of the aurora with the pressure of the solar wind. These sets of measurements were combined to yield the most accurate glimpse yet of Saturn's auroras and the role of the solar wind in generating them. The results will be published in the February 17 issue of the journal Nature.

The findings show that Saturn's auroras vary from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared to Earth, where dramatic brightening of the auroras lasts only about 10 minutes, Saturn's can last for days.

The observations also show that the Sun's magnetic field and solar wind may play a much larger role in Saturn's auroras than previously suspected. Hubble images show that auroras sometimes stay still as the planet rotates beneath, like on Earth, but also show that the auroras sometimes move along with Saturn as it spins on its axis, like on Jupiter. This difference suggests that Saturn's auroras are driven in an unexpected manner by the Sun's magnetic field and the solar wind, not by the direction of the solar wind's magnetic field.

"Both Earth's and Saturn's auroras are driven by shock waves in the solar wind and induced electric fields," said Crary. "One big surprise was that the magnetic field imbedded in the solar wind plays a smaller role at Saturn."

At Earth, when the solar wind's magnetic field points southward (opposite to the direction of the Earth's magnetic field), the magnetic fields partially cancel out, and the magnetosphere is "open". This lets the solar wind pressure and electric fields in, and allows them to have a strong effect on the aurora. If the solar wind's magnetic field isn't southward, the magnetosphere is "closed'' and solar wind pressure and electric fields can't get in. "Near Saturn, we saw a solar wind magnetic field that was never strongly north or south. The direction of the solar wind magnetic field didn't have much effect on the aurora. Despite this, the solar wind pressure and electric field were still strongly affecting auroral activity," added Crary. Seen from space, an aurora appears as a ring of energy circling a planet's polar region. Auroral displays are spurred when charged particles in space interact with a planet's magnetosphere and stream into the upper atmosphere. Collisions with atoms and molecules produce flashes of Radiant energy in the form of light. Radio waves are generated by electrons as they fall toward the planet.

The team observed that even though Saturn's auroras do share characteristics with the other planets, they are fundamentally unlike those on either Earth or Jupiter. When Saturn's auroras become brighter and thus more powerful, the ring of energy encircling the pole shrinks in diameter. At Saturn, unlike either of the other two planets, auroras become brighter on the day-night boundary of the planet which is also where magnetic storms increase in intensity. At certain times, Saturn's auroral ring is more like a spiral, its ends not connected as the magnetic storm circles the pole.

The new results do show some similarities between Saturn's and Earth's auroras: Radio waves appear to be tied to the brightest auroral spots. "We know that at Earth, similar radio waves come from bright auroral arcs, and the same appears to be true at Saturn," said Kurth. "This similarity tells us that, on the smallest scales, the physics that generate these radio waves are just like what goes on at Earth, in spite of the differences in the location and behavior of the aurora."

Now with Cassini in orbit around Saturn, the team will be able to take a more direct look at the how the planet's auroras are generated. They will next probe how the Sun's magnetic field may fuel Saturn's auroras and learn more details about what role the solar wind may play. Understanding Saturn's magnetosphere is one of the major science goals of the Cassini mission.

For the latest images and information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

The Cassini-Huygens mission is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.

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