Saturn's Aurora Surprises Team Led By SwRI.

SAN ANTONIO -- Scientists have long known that energetic bursts of charged particles create bright auroras in the upper atmosphere of the Earth's polar regions and also in the polar regions of Jupiter and Saturn.

At Earth, the auroral particles can originate both in the solar wind and in the Earth's atmosphere. They then gain energy indirectly from the interaction of the solar wind with the Earth's magnetic field, with most of the acceleration occurring at relatively low altitudes of a few thousand kilometers above the aurora.

At Jupiter, the energization is derived mainly from planetary rotation, while the particles originate primarily in the atmosphere of the planet and its moons (principally Io), with the solar wind playing an apparently minor role.

In a paper published in the February 17 issue of the journal Nature, a team of scientists led by Dr. Frank Crary, a physicist at Southwest Research Institute(R) (SwRI(R)) in San Antonio, Texas, reports that Saturn's aurora responds to the solar wind in a surprisingly different way from that of either Earth or Jupiter.

The team used the Hubble Space Telescope and instruments on board NASA's Cassini spacecraft to measure solar wind interactions during a one-month period in 2004 as Cassini approached Saturn for a multi-year orbital mission. The resulting measurements confirmed that, like Earth, Saturn's magnetosphere is actively driven by the solar wind. However, while Earth's auroras are strongly influenced by the direction of the solar-wind magnetic field, Saturn's appear to be independent of that orientation.

"The auroras of Earth and Saturn both are driven by shock waves in the solar wind, and induced electric fields," said Crary, a senior research scientist in SwRI's Space Science and Engineering Division and a Cassini co-Investigator. "One big surprise was that the magnetic field imbedded in the solar wind plays a smaller role at Saturn."

The Earth's aurora is very sensitive to the direction of the solar wind's magnetic field; when the solar wind field points southward, it can interconnect with the mainly northward Earth's magnetic field, allowing the solar wind to affect its magnetosphere directly. Near Saturn, the solar wind's magnetic field is rarely northward or southward, and its Earth-like role is not evident. But like the Earth, observations from Cassini show dramatic changes in the aurora that were directly driven by solar wind shocks.

Solar wind observations upstream of Saturn relied primarily on the Cassini Plasma Spectrometer (CAPS), developed by an international team headed by Dr. David Young of SwRI. SwRI researchers are also active on Cassini's Imaging Science Subsystem, Composite Infrared Spectrometer and Ion and Neutral Mass Spectrometer teams. Cassini, launched in 1997, carries 12 scientific instruments to image the saturnian system at infrared, ultraviolet and visible wavelengths and with radar, and to sample directly the charged particle, dust, neutral gas and plasma wave environments. The spacecraft also carried the European Space Agency's (ESA) Huygens probe, which recently descended to the surface of Saturn's largest moon, Titan.

Cassini-Huygens is a cooperative mission of NASA, ESA and the Italian Space Agency. The Jet Propulsion Laboratory, operated by the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C. NASA's Hubble Space Telescope is a cooperative program with the European Space Agency and is operated by the Space Telescope Science Institute in Baltimore.

For more information contact Joe Fohn, 210-522-4630, or [e-mail]. For the latest images and information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, with more than 2,800 employees and an annual research volume of more than $399 million.