A host of new studies reveal wildmagnetic and electric activity surrounding Jupiter, all wrapped up in a gianttapered bubble of influence that is the largest single structure within thesolar system.

The studies were made possible when theCassini spacecraft passed nearly 6 million miles above Jupiter in January 2001,using the giant planet's gravity for a boost toward Saturn. Jupiter'smagnetosphere, as it is called, surprisingly reached out and enveloped Cassinion two separate occasions.

The events shed new light on Jupiter'smagnetosphere, now said to be 20 times wider than the Sun and stretching outseveral times the distance from Earth to the Sun.

Relativisticallyspeaking

Among the findings, in research led byScott Bolton of NASA's Jet Propulsion Laboratory, was the first proof thatJupiter's magnetic influence propels electrons to ultra-relativistic speeds,meaning they travel at a significant fraction of the speed of light.

Bolton's team found electrons packing50 million electron volts (MeV) of energy and travelling at 99 percent thespeed of light.

Earlier research had suggested suchrelativistic electrons might exist near Jupiter, but none had been measuredabove 20 MeV. These 20 MeV electrons also travel at about 99 percent the speedof light. The new measurements, however, detected radio emission from electronsmore than twice as energetic as ever before seen, Bolton said.

"Some scientists believed thatthere may have been a natural cutoff to the energy level that the electronscould be energized," Bolton told SPACE.com."Our data suggests that if there is a cutoff, it is likely to be muchhigher than 50 MeV."

Some of these electrons may collidewith Jupiter's clouds and affect the planet's atmosphere, Bolton said. Theyalso represent a threat to spacecraft.

"Measuring the electrons withCassini will lead to improved models of Jupiter's radiation belts," Boltonsays, "and thus [future] spacecraft will be better equipped to handle theharsh radiation environment at Jupiter."

Moonprints

The Cassini data, along with othercoordinated observations involving 82 researchers, led to seven new studies,all explained in the Feb. 28 issue of the journal Nature.

Researchers also found that Jupiter'smagnetosphere displays some of its frenetic activity in the planet'satmosphere, in the form of transient, colorful lights known as aurora. They aresimilar to the Northern Lights on Earth.

Curiously, prior observations foundthat the volcanically active moon Io leaves its own footprint in the giantJovian magnetic field, which appears as a bright spot in Jupiter's atmospherethat looks much like the aurora. The spot is caused by powerful electriccurrents running between the moon and Jupiter.

Another new study, led by John T.Clarke of Boston University, found that Io's footprint, seen in ultravioletlight, extended behind it like a tail. Clarke and his colleagues discoveredthat the moons Europa and Ganymede also leave auroral footprints in Jupiter'satmosphere.

"The ultraviolet emissions fromthe magnetic footprints of the three moons ofJupiter are generated by high-energy particles, most likely electrons,which are accelerated at the moons themselves, then sent down alongJupiter's magnetic field to theplanet," Clarke explained.

The footprints from Europa and Ganymedewere not unexpected, Clarke said. But the emissions downstream from Io"were a complete surprise."

Ganymede's footprint may be caused by aminiature magnetosphere of its own, which would interact with the largermagnetosphere of Jupiter. The other large moon of Jupiter, Callisto, may alsoleave a footprint, but it could not be detected.

Jupitervs. the Sun

Jupiter's magnetosphere varies wildlyin size and shape, other new studies showed. The dynamics involve abattleground of pressures, as detailed in research by William Kurth at theUniversity of Iowa.

The planet's internally generatedmagnetic field, powered by Jupiter's rapid rotation, pushes outward. It'sbuoyed by hot gas called plasma that is trapped within it, all fed by a ton ofmaterial coming from the moon Io, Kurth said. From outside comes a constantpush of charged particles riding outward from the Sun on what's known as thesolar wind.

Around Earth, a similar battleground isdominated almost entirely by the solar wind. Scientists have sought tounderstand what effect, if any, the solar wind had on giant Jupiter and itsmore musclebound internal forces.

"The Cassini-Galileo combinedobservations have shown that the solar wind has at least some effect,"Kurth said.

Greattiming

Cassini's observations were coordinatedwith measurements made by the Galileo probe, then at Jupiter, as well as withstudies done from a distance with the Hubble and Chandra telescopes and theground-based Very Large Array.

"Jupiter is our 'local' laboratoryfor high energy space physics," says NASA's Bolton. "The conditionsinside Jupiter's radiation belts may tell us a lot about the conditions thatmay be present in distant astrophysical regions, such as near star formingregions, pulsars and neutron stars."

A little luck played a role in allthese observations, explains Thomas W. Hill, who wrote an analysis of thefindings in an accompanying article in Nature.

For one thing, Hill says, Jupiter'smagnetosphere was unusually large during the Cassini flyby. Further, a shockwave of solar energy -- essentially an enhanced burst of solar wind -- providedadditional fodder for study.

Three earlier shock waves, also studiedby both spacecraft, each resulted in a brightening of Jupiter's aurora,according to a study led by the University of Iowa's Donald Gurnett. A similarbrightening occurs above Earth when space storms hit, but researchers had longdebated whether such an event would brighten or diminish Jupiter's aurora.

"This result settles along-running debate," Hill said.

The Galileo probe is near the end ofits service, set to plunge into Jupiter next year.

Cassini, meanwhile, should arrive atSaturn in 2004 and only then will its primary mission begin. The craft willexplore Saturn's rings, atmosphere and magnetosphere. It will also deploy aprobe to the surface of Titan, Saturn's largest moon, to investigate its bizarreatmosphere, which in many ways resembles Earth's atmosphere.

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