In the Dec. 13 issue of the journal Nature, Krimigis' team describes how Saturn's ring current changes over time; it's a dynamic system, doughnut shaped but sometimes appearing like someone took a bite out of it. They also found that Saturn's ring current is persistently asymmetric - unlike Earth's - and it rotates closely in-step with Saturn itself.

Ring currents form when hot ionized gas (known as plasma) becomes trapped on a planet's magnetic field lines. The main source of the plasma that forms Saturn's ring current is material from the gas vented by geysers on the moon Enceladus.

At Earth, ring currents form during large solar wind-driven magnetic storms, although they fade quickly as the driving solar wind disturbance recedes into deep space. At Saturn, the Magnetospheric Imaging Instrument (MIMI) observed that the ring current's intensity seemed only weakly related to solar activity.

"We might get a more intense reading when a solar wind pressure spike passes by," says Dr. Donald Mitchell, a MIMI co-investigator from APL. "But the surprise is that Saturn's ring current didn't become symmetric or dissipate as it does at Earth. It stayed lumpy and rotated around the planet several times. We don't know exactly why that happens, but we have seen it exhibit this behavior repeatedly."

The presence of a ring current around Saturn was first suggested in the early 1980s from magnetic anomalies observed by NASA's Pioneer 11 and Voyager 1 and 2 spacecraft. But Saturn's ring current had never been mapped on a global scale; only small areas were mapped previously, and not in this detail. MIMI was designed for just this purpose; developed by an APL-led international team, MIMI has three distinct sensors, one of which contributed the images for this work.

False-color images accompanying the Nature article were taken by MIMI's ion and neutral camera and show the intensity of the energetic neutral atoms emitted from the ring current through a process called charge exchange. This happens when a trapped energetic ion steals an electron from a cold gas atom, becomes neutral and escapes the magnetic field.

Scientists are using these images to create a map of the invisible ring current, which is roughly five times farther from Saturn than its famous icy rings.

MIMI gathered the images for the Nature paper in March 2007 as Cassini looped nearly 1.5 million kilometers (920,000 miles) over Saturn's poles, giving the instrument a bird's eye view of the magnetic activity swirling around the planet.

The Cassini-Huygens mission is a cooperative project 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 Science Mission Directorate, Washington, D.C. APL's Krimigis is the principal investigator for MIMI, which was designed and built and is operated by an Applied Physics Laboratory-led team. The University of Maryland and the Max Planck Institute for Solar Physics in Germany contributed two of the three sensors. Part of the analyses for this work was performed as a collaborative effort with the Academy of Athens in Greece.