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Carolina
Martinez (818) 354-9382
Jet
Propulsion Laboratory, Pasadena, Calif.
Ann
Marie Menting (617) 353-2240
Boston
University, Mass.
Donna Weaver (410) 338-4493
Space Telescope Science
Institute, Baltimore, Md.
Dolores Beasley (202)
358-1753
NASA Headquarters,
Washington
News Release: 2005-028
February 16, 2005
NASA
Spacecraft
Help Solve Saturn's Mysterious Auroras
Scientists
studying data from NASA's Cassini spacecraft and Hubble Space Telescope
have found that Saturn's auroras behave differently than scientists have
believed for the last 25 years.
The researchers, led by John Clarke of Boston
University, found the planet's auroras, long thought of as a cross between
those of Earth and Jupiter, are fundamentally unlike those observed on
either of the other two planets. The
team analyzing Cassini data includes Dr. Frank Crary, a research scientist
at Southwest Research Institute in San Antonio, Texas, and Dr. William
Kurth, a research scientist at the University of Iowa, Iowa
City.
Hubble
snapped ultraviolet pictures of Saturn's auroras over several weeks, while
Cassini's radio and plasma wave science instrument recorded the boost in
radio emissions from the same regions, and the Cassini plasma spectrometer
and magnetometer instruments measured the intensity of the aurora with the
pressure of the solar wind. These sets of measurements were combined
to yield the most accurate glimpse yet of Saturn's auroras and the role of
the solar wind in generating them. The results will be
published in the February 17 issue of the journal Nature.
The
findings show that Saturn's auroras vary from day to day, as they do on
Earth, moving around on some days and remaining stationary on others.
But compared to Earth, where dramatic brightening of the auroras
lasts only about 10 minutes, Saturn's can last for days.
The
observations also show that the Sun's magnetic field and solar wind may
play a much larger role in Saturn's auroras than previously suspected.
Hubble images show that auroras sometimes stay still as the planet
rotates beneath, like on Earth, but also show that the auroras sometimes
move along with Saturn as it spins on its axis, like on Jupiter. This difference suggests that
Saturn's auroras are driven in an unexpected manner by the Sun's magnetic
field and the solar wind, not by the direction of the solar wind's
magnetic field.
"Both
Earth's and Saturn's auroras are driven by shock waves in the solar wind
and induced electric fields," said Crary. "One big surprise was that the
magnetic field imbedded in the solar wind plays a smaller role at
Saturn.''
At
Earth, when the solar wind's magnetic field points southward (opposite to
the direction of the Earth's magnetic field), the magnetic fields
partially cancel out, and the magnetosphere is "open". This lets the solar
wind pressure and electric fields in, and allows them to have a strong
effect on the aurora. If the solar wind's magnetic field isn't southward,
the magnetosphere is "closed'' and solar wind pressure and electric fields
can't get in. "Near Saturn, we saw a solar wind magnetic field that
was never strongly north or south. The direction of the solar wind
magnetic field didn't have much effect on the aurora. Despite this, the solar wind
pressure and electric field were still strongly affecting auroral
activity," added Crary.
Seen
from space, an aurora appears as a ring of energy circling a planet's
polar region. Auroral displays are spurred when charged particles in space
interact with a planet's magnetosphere and stream into the upper
atmosphere. Collisions with atoms and molecules produce flashes of
radiant energy in the form of light. Radio waves are generated by
electrons as they fall toward the planet.
The
team observed that even though Saturn's auroras do share characteristics
with the other planets, they are fundamentally unlike those on either
Earth or Jupiter. When Saturn's auroras become brighter and thus
more powerful, the ring of energy encircling the pole shrinks in diameter.
At Saturn, unlike either of the other two planets, auroras become
brighter on the day-night boundary of the planet which is also where
magnetic storms increase in intensity. At certain times, Saturn's
auroral ring is more like a spiral, its ends not connected as the magnetic
storm circles the pole.
The new results do show some
similarities between Saturn's and Earth's auroras: Radio waves appear to be tied to
the brightest auroral spots. "We know that at Earth, similar radio waves
come from bright auroral arcs, and the same appears to be true at Saturn,"
said Kurth. "This similarity
tells us that, on the smallest scales, the physics that generate these
radio waves are just like what goes on at Earth, in spite of the
differences in the location and behavior of the aurora."
Now
with Cassini in orbit around Saturn, the team will be able to take a more
direct look at the how the planet's auroras are generated. They will
next probe how the Sun's magnetic field may fuel Saturn's auroras and
learn more details about what role the solar wind may play. Understanding Saturn's
magnetosphere is one of the major science goals of the Cassini
mission.
For
the latest images and information about the Cassini-Huygens mission, visit
http://saturn.jpl.nasa.gov
and http://www.nasa.gov/cassini
.
The
Cassini-Huygens mission is a cooperative
mission of NASA, the European Space Agency and the Italian Space
Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the mission for NASA's Office of Space Science,
Washington, D.C.
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