Scientists simultaneously using a combination of NASA
spacecraft have seen into the workings of an invisible
whirling bubble of charged particles surrounding Jupiter.
That bubble, Jupiter’s magnetosphere, is the biggest
object with distinct boundaries within our solar system, more
than 100 times wider than Jupiter itself. It contracts in
response to shock waves from the Sun, according to one report
appearing in the journal Nature tomorrow. In all, seven
reports appearing together will detail various results from a
concerted research campaign that took advantage of the Saturn-
bound Cassini spacecraft’s flyby of Jupiter 14 months ago.
The campaign found extremely energetic electrons
traveling near the speed of light close to Jupiter, as well as
a vast nebula of neutral atoms, and triggers for glowing
auroras near Jupiter’s north and south poles.
"We’re seeing results from a remarkable opportunity,"
said Dr. Scott Bolton, a physicist at NASA’s Jet Propulsion
Laboratory, Pasadena, Calif., and a co-author of three of the
reports.
"We had one spacecraft, Galileo, inside the magnetosphere
monitoring what was happening there at the same time another
spacecraft, Cassini, was outside the magnetosphere monitoring
the solar wind just upstream," Bolton said. The solar wind is
particles from the Sun flowing outward through the solar
system. Jupiter’s magnetosphere, like Earth’s, deflects the
solar wind but gets pushed around by its gusts.
On Jan. 10, 2001, when Cassini and Galileo were more than
20 times farther from each other than Earth is from the Moon,
each spacecraft encountered the boundary of Jupiter’s
magnetosphere while the bubble was contracting in response to
an increase in solar-wind pressure.
"This is the first two-point measurement of the Jovian
system actually responding to the solar wind," said Dr.
William Kurth, physicist at the University of Iowa, Iowa City,
and lead author of the Nature report on these results. "The
combined observations of Galileo and Cassini help show us the
relative importance of the influence of the solar wind and the
factors affecting the magnetosphere from within — primarily
the energy from Jupiter’s rotation and the supply of material
from volcanoes on the moon Io." The Jupiter observations
strengthen confidence in our understanding about Earth’s
protective magnetosphere.
Shock waves from outbursts on the Sun, carried outward on
the solar wind and detected by Cassini, also stimulated radio
emissions from deep within Jupiter’s magnetosphere and
brightened auroras at Jupiter’s poles, Dr. Donald Gurnett of
the University of Iowa reports. Those effects suggest that
electron density and electric currents in the magnetosphere
increase when it is compacted by the shock wave.
Besides Galileo, which has been orbiting Jupiter since
1995, and Cassini, scientists used two Earth orbiters — the
Hubble Space Telescope and Chandra X-ray Observatory – plus
radio telescopes in New Mexico and Arizona to examine
Jupiter’s surroundings while Cassini was there.
Hubble images show patches of Jupiter’s aurora stimulated
by an event Galileo detected within the magnetosphere, reports
Dr. Barry Mauk of Johns Hopkins University’s Applied Physics
Laboratory, Laurel, Md. The event is a surge of charged
particles toward the planet, apparently analogous to similar
aurora-triggering surges that release pent-up energy in
Earth’s magnetosphere. Some other features in Jupiter’s aurora
are "footprints" of currents flowing through the magnetosphere
from three of the planet’s large moons, reports Dr. John
Clarke of Boston University. Dr. Randall Gladstone of the
Southwest Research Institute, San Antonio, Texas, describes a
45-minute rhythm in auroras at X-ray wavelengths, likely
linked to a still-unidentified stimulus in the outer portion
of the magnetosphere.
Cassini carries a type of magnetosphere-imaging
instrument no previous interplanetary spacecraft has had. The
instrument not only showed some structural detail of Jupiter’s
magnetosphere, it also detected a cloud of neutral atoms
stretching away from the planet as a "hot neutral wind,"
reports Dr. Stamatios Krimigis of Hopkins’ Applied Physics
Laboratory. The magnetic field holds charged particles in, but
neutral ones escape to create a nebula of particles that
extends beyond the magnetosphere.
High-energy electrons in radiation belts close to Jupiter
emit radio waves that have been monitored from Earth for
years. JPL’s Bolton and other scientists used Cassini while it
was near Jupiter to map details never seen before in those
belts. About 2,300 students at high schools and middle schools
across the country participated in a program of radio-
telescope observations that aided interpretation of those
Cassini observations.
Cassini is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. JPL manages Cassini
and Galileo for NASA’s Office of Space Science, Washington,
D.C. JPL is a division of the California Institute of
Technology in Pasadena.
