Jupiter Particles' Escape Route Found

May 31, 2001


Guy Webster/JPL (818) 354-6278

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.

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. A new color movie clip of the
images is available at:





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.

Additional information about Cassini-Huygens is online at http://saturn.jpl.nasa.gov.

The Cassini spacecraft is scheduled to arrive at Saturn in July 2004 to
begin a four-year exploration of the ringed planet and its moons. The
Cassini mission is managed by NASA's Jet Propulsion Laboratory in Pasadena,
Calif., for NASA's Office of Space Science, Washington, D.C. JPL is a
division of the California Institute of Technology in Pasadena.

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Jet Propulsion Laboratory

California Institute of Technology

National Aeronautics and Space Administration

Pasadena, Calif. 91109.

Telephone (818) 354-5011

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