Escape route found for Jupiter's particles
Posted: June 1, 2001

Jupiter's magnetosphere, an ionized-gas bubble encasing the planet, is lopsided and leaky, with an unexpected abundance of high-energy particles bleeding out of one side, according to recent measurements by NASA's Cassini spacecraft.

Those escaping electrons and ions might be riding magnetic field lines that are attached to Jupiter at one end and waving loose on the other, unlike more common lines that loop between Jupiter's north and south hemispheres closer to the planet.

On January 15, 17 days after it passed its closest approach to Jupiter, NASA's Cassini spacecraft looked back to see the giant planet as a thinning crescent. This image is a color mosaic from that day, shot from a distance of 18.3 million kilometers (11.4 million miles). The smallest visible features are roughly 110 kilometers (70 miles) across. The solar phase angle, the angle from the spacecraft to the planet to the Sun, is 120 degrees. A crescent Io, innermost of Jupiter's four large moons, appears to the left of Jupiter. Photo: NASA/University of Arizona
Deciphering the process could advance understanding of the protective magnetic field around Earth, as well as the much greater one around Jupiter, said Dr. Dennis Matson, Cassini project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Jupiter's magnetosphere is so vast that if it shined at wavelengths visible to the eye, it would appear from Earth to be two to three times wider than the disc of the Sun, even though it is more than four times as far away.

"The dusk flank of Jupiter's magnetosphere is a surprising contrast to the dawn flank," said Dr. Stamatios (Tom) Krimigis, a Cassini scientist who heads the space department of the Johns Hopkins University's Applied Physics Laboratory, Laurel, Md. Cassini spent most of January and February skating along the magnetosphere's dusk flank, which is on the side of the planet turning away from the Sun. Other spacecraft, such as Voyager, previously sampled the opposite flank, corresponding to Jupiter's dawn side.

Cassini was flying past Jupiter last winter for a gravity boost to reach Saturn. Researchers grabbed the opportunity to study the giant planet from different vantage points by also using NASA's Galileo spacecraft, which is orbiting Jupiter, plus other spacecraft and ground-based telescopes, in coordination with Cassini's Jupiter observations. More than 20 scientists are presenting some preliminary results from that campaign during meetings of the American Geophysical Union in Boston this week.

The electrons Cassini caught escaping may answer a puzzle. Scientists had figured that some electrons were getting out of Jupiter's magnetosphere, sometimes even reaching Earth's neighborhood, but they didn't know the primary route. "It appears we've found where they're coming from," Krimigis said.

Dr. John Clarke of the University of Michigan, Ann Arbor, used a movie taken by NASA's Hubble Space Telescope of Jupiter's auroras while Cassini and Galileo were monitoring Jupiter's magnetosphere and the solar wind, a flow of particles speeding away from the Sun and deflected around the magnetic fields of planets. Clarke said that movements of the auroral glows indicate which features in them are linked to the magnetosphere, because they follow the rotation of the magnetic field, and which are linked to solar-wind effects, because their positions stay oriented with respect to the direction toward the Sun.

The timing and location of one patch of auroral brightening captured by Hubble corresponded to a pulse of electrons detected by Galileo in the magnetosphere. That pulse appears to have been a type that also occurs in Earth's magnetosphere, said Dr. Barry Mauk of Johns Hopkins University's Applied Physics Lab, Laurel, Md., team member on the energetic particle detector experiment on Galileo. "Energy builds up in the system, pulling the magnetic field lines outward like rubber bands, but eventually these rubber bands can snap back toward the planet," Mauk said. The snapping back brings an injection of high-energy electrons, he said.

Having Galileo inside Jupiter's magnetosphere at the same time Cassini was just outside of it in the solar wind gave scientists a chance to see whether such injections are triggered by fluctuations in the solar wind, as can happen at Earth. No obvious solar wind event corresponded to the injections seen by Galileo. "It appears injections can happen without being externally stimulated," Mauk said.

The solar wind does appear to have tipped features of Jupiter's magnetosphere northward part of the time during the Galileo and Cassini joint studies, said Dr. Margaret Kivelson of the University of California, Los Angeles, principal investigator for Galileo's magnetometer instrument. That gave Galileo a taste of conditions that are usually farther south, and it found that magnetic field lines there twist differently than they do near the equatorial plane.

"It's as if a hula dancer had a skirt made of ribbons that fly out as she twirls, but at one layer the ribbons twirl in one direction and at a different layer they twirl in the other direction," Kivelson said.

Jupiter's moon Io has its own auroras, which Cassini captured in images taken while Io was in Jupiter's shadow. "We could see that bright blue emissions near the equator move around in a way that tells us their source," said Dr. Paul Geissler of the University of Arizona, Tucson. The electron flow causing gases to glow there comes from an electrical current running between Io and Jupiter, he said.

In addition, Io's volcanoes put out about a ton per second of gases such as oxygen and sulfur. These are spun out of Jupiter's magnetosphere and form a "Jovian nebula" that extends tens of millions of kilometers or miles away from Jupiter, Krimigis found with one of Cassini's sensors. "We have even detected sulfur dioxide a long way from Jupiter," he said.

JPL, a division of the California Institute of Technology in Pasadena, manages Cassini and Galileo for NASA's Office of Space Science, Washington, D.C. Cassini is a joint project of NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore, Md., manages Hubble.

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The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.