September 14, 2009
Scientists using the Cassini spacecraft's Magnetospheric Imaging Instrument (MIMI) have detected a new, temporary radiation belt at Saturn, located around the orbit of its moon Dione at about 234,000 miles (377,000 kilometers) from the center of the planet.
Radiation belts, like Earth's Van Allen belts, have been discovered at Jupiter, Saturn, Uranus, and Neptune. However, to date, it only has been possible to observe the variability of their intensity at Earth and Jupiter. Now that Cassini has been orbiting Saturn for more than 5 years, it has been possible to assess changes in Saturn's radiation belts.
An international team of astronomers made the discovery analyzing data from the MIMI's Low Energy Magnetospheric Measurement System Sensor (LEMMS), which measures the energy and angular distribution of charged particles in the magnetic bubble that surrounds Saturn.
"The most dramatic changes have been observed as sudden increases in the intensity of high energy charged particles in the inner part of Saturn's magnetosphere, in the vicinity of the moons Dione and Tethys," said Roussos. "These intensifications, which could create temporary satellite atmospheres around these moons, occurred three times in 2005 as a response to an equal number of solar storms that hit Saturn's magnetosphere and formed a new, temporary component to Saturn's radiation belts," he added.
The new belt, which has been named "the Dione belt," was only detected by MIMI/LEMMS for a few weeks after each of its three appearances. The team believes that newly formed charged particles in the Dione belt were gradually absorbed by Dione and another nearby moon, named Tethys, which lies slightly closer to Saturn at an orbit of 183,000 miles (295,000 km).
Unlike the Van Allen belts around the Earth, Saturn's radiation belts inside the orbit of Tethys are very stable, showing negligible response to solar storm occurrences and no variability over the 5 years that they have been monitored by Cassini.
Interestingly, it was found that the transient Dione belt was only detected outside the orbit of Tethys. It appeared to be clearly separated from the inner belts by a permanent radiation gap along the orbit of Tethys.
"Our observations suggest that Tethys acts as a barrier against inward transport of energetic particles and is shielding the planet's inner radiation belts from solar wind influences. That makes the inner, ionic radiation belts of Saturn the most isolated magnetospheric structure in our solar system," said Roussos.
The radiation belts within Tethys' orbit probably arise from the interaction of the planet's main rings and atmosphere and galactic cosmic ray particles that, unlike the solar wind, have the high energies needed to penetrate the innermost Saturnian magnetosphere. This means that the inner radiation belts will only vary if the cosmic ray intensities at the distance of Saturn change significantly.
"Outside the orbit of Tethys, the variability of Saturn's radiation belt might be enhanced in the coming years as we start approaching the solar maximum," Roussos said. "If solar storms occur frequently in the new solar cycle, the Dione belt might become a permanent, although highly variable, component of Saturn's magnetosphere, which could affect Saturn's global magnetospheric dynamics."