Longest Lightning Storm: Saturn Sets Record
The radio signals generated by the immense lightning storm on Saturn, indicated in the picture. Saturn's moon Tethys sits between the Cassini spacecraft (which took the image) and Saturn.
Credit: RPWS Team/NASA/JPL/Space Science Institute

A powerful lightning storm brewing in Saturn's atmosphere since January has become the solar system's longest continuously observed thunderstorm, astronomers have announced.

The storm breaks the record duration of 7.5 months set by another thunderstorm observed on Saturn by NASA?s Cassini spacecraft between November 2007 and July 2008.

The current thunderstorm on Saturn is the ninth that has been measured since Cassini swung into orbit around Saturn in July 2004.

Lightning discharges in Saturn?s atmosphere emit very powerful radio waves, which are measured by the antennas and receivers of the Cassini Radio and Plasma Wave Science (RPWS) instrument. The radio waves are about 10,000 times stronger than their terrestrial counterparts and originate from huge thunderstorms in Saturn?s atmosphere with diameters of about 1,900 miles (3,000 km).

The observations of this latest thunderstorm will be presented by Georg Fischer of the Austrian Academy of Sciences at the European Planetary Science Congress in Potsdam, Germany, on Sept. 15.

"These lightning storms are not only astonishing for their power and longevity, the radio waves that they emit are also useful for studying Saturn?s ionosphere, the charged layer that surrounds the planet a few thousand kilometers above the cloud tops," Fischer said. "The radio waves have to cross the ionosphere to get to Cassini and thereby act as a natural tool to probe the structure of the layer and the levels of ionization in different regions."

Results from Cassini's RPWS instrument have confirmed previous studies of the Voyager spacecraft indicating that levels of ionization are approximately 100 times higher on the day-side than the night side of Saturn?s ionosphere.

Lightning storms on Saturn usually occur in a region that nicknamed ?Storm Alley? by scientists. The region lies 35 degrees south of Saturn?s equator.

"The reason why we see lightning in this peculiar location is not completely clear," Fischer said. "It could be that this latitude is one of the few places in Saturn?s atmosphere that allow large-scale vertical convection of water clouds, which is necessary for thunderstorms to develop. However, it may be a seasonal effect. Voyager observed lightning storms near the equator, so now that Saturn has passed its equinox on Aug. 11, we may see the storms move back to equatorial latitudes."

The lightning's presence was confirmed in another way: During Cassini?s last close flyby of Titan on Aug. 25, Cassini?s view of Saturn was obscured by Titan for a half-hour, and no lightning was observed.

"Although we know from Cassini images where Saturn lightning comes from, this unique event was another nice proof for their origin," Fischer said.