University of Iowa researchers and colleagues are tracking the Saturn storm using an instrument aboard the Cassini spacecraft. The Radio and Plasma Wave Science instrument detects radio emissions in the same way a car radio picks up the crackle and pop of a summer thunderstorm.

"It is clear that this is the strongest lightning activity that we've seen yet with Cassini since it has arrived at Saturn," said Donald Gurnett, leader of the RPWS team. He said that at about the same time the spacecraft began detecting the radio emissions on Jan. 23, amateur astronomers reported a storm that had appeared in the planet's southern hemisphere at minus 35 degrees latitude.

by Jan. 25, Cassini was in the wrong place in its orbit to take good images of the storm on the day side, because the planet showed only a thin crescent to the spacecraft, but night-side imaging was possible using light from the rings. Saturn's period of rotation is 10 hours, 13 minutes, 59 seconds.

"With Cassini we have learned that lightning storms can emerge suddenly and last for several weeks or even a month," said RPWS team member Georg Fischer. "On the other hand, we have only observed a single smaller lightning storm throughout 2005, which is remarkably different compared to what we know about terrestrial thunderstorms."

The origin of such storms is unknown, but they could be related to Saturn's warm interior, Gurnett said, adding that the team hopes to locate the storm with greater precision in the coming weeks when Cassini is scheduled to fly closer to the planet. He said the lightning flash rate exceeds what was observed by the Voyager 1 spacecraft back in 1980, and the intensities are at least as large, if not larger.

The latitude of the new storm matches that of a storm called the Dragon, a powerful emitter of radio noise that Cassini imaged in 2004 during its approach. It lies in a region of the southern hemisphere referred to as "storm alley" by mission scientists, because of the high level of activity observed there. The storm's north-south dimension is about 3,500 kilometers (2,175 miles).

"It's really the only large storm on the whole planet," said Andrew Ingersoll, a member of the Cassini imaging team. "It's in the right place and it appeared at the right time to match the radio emissions, so it has to be the right storm."

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