BOSTON, Mass. - Joint observations by the Galileo and Cassini spacecraft have helped scientists better understand the complex, powerful nature of Jupiter's magnetic field.

An artist's concept of Cassini during the flyby of Jupiter. Photo: NASA/JPL

In a series of presentations this week at the American Geophysical Union spring meeting in Boston, space scientists have shown how data from the two spacecraft have revealed new features and explained old mysteries about Jupiter's magnetic field.

The data come from months of joint study of the planet and its environment performed by the Galileo spacecraft, in orbit around Jupiter for over five years, and the Cassini spacecraft, which flew by Jupiter in late December en route to its ultimate destination, Saturn.

Among the findings announced at the conference was the discovery of a tenuous cloud of atoms surrounding the planet for millions of kilometers that can trace their origins back to the volcanic moon Io. The atoms of sulfur and oxygen, as well as molecules of sulfur dioxide, were detected by instruments on the Cassini spacecraft.

"What is really there is a Jupiter 'nebula' that is fed by the volcanoes of Io," said Stamatios Krimigis, head of the space department of Johns Hopkins University's Applied Physics Lab and principal investigator of Cassini's magnetospheric imaging instrument.

The particles in this nebula are ejected from Io's many volcanoes and, as charged particles, are caught up in Jupiter's magnetic field as part of a feature known as the Io torus. They remain there until they collide with an electron and become neutral. As neutral atoms and molecules, they are no longer retained by the magnetosphere and zoom away at speeds of up to 70 kilometers per second.

The nebula of particles extends for at least 22 million kilometers from Jupiter, according to Krimigis. Some of those particles become ionized again, losing an electron after intercepting a photon, and get caught up in the solar wind.

Images from Cassini were also able to capture the interaction between Jupiter's magnetic field and Io. The images, taken by Cassini while Io was eclipse behind Jupiter, show a pair of bright blue regions on opposite sides of Io's disk caused by emissions from sulfur dioxide molecules.

The bright regions identify the regions where magnetic field lines from Jupiter are tangent with Io's surface, creating a 500,000 volt electric circuit that energize the molecules. "These bright spots are the terminals in this electrical generator," explained Paul Geissler of the University of Arizona. The regions move as predicted along the disk as Io moves in its orbit around Jupiter, changing its orientation with respect to the magnetic field.

Observations from Galileo and Cassini have also given scientists the opportunity to better understand the twists and turns in the magnetic field. They found that magnetic field lines bend in different directions depending on latitude: those near the equator would bend back as they encountered drag from a plasma disk, while those at high latitudes would bend forward as they interacted with the solar wind.

"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," explained Margaret Kivelson of UCLA, principal investgator of Galileo's magnetometer.

While Jupiter's magnetosphere can seem esoteric, studies of it can improve our understanding of the Earth's magnetic field. "By looking at a magnetosphere where the properties that control the behavior of it are vastly different from what they are on Earth, we can test our understanding of the Earth's magnetosphere and improve it," said Kivelson. "There's a very strong link to understanding Earth's magnetosphere."