PARIS ? A tempest that erupted on Saturn in January has become the Solar System's longest continuously observed lightning storm, astronomers reported on Tuesday.

The storm broke out in "Storm Alley," a region 35 degrees south of the ringed giant's equator, researchers told the European Planetary Science Congress in Potsdam, near Berlin.

Thunderstorms there can be as big as 3,000 kilometers (nearly 2,000 miles) across.

The powerful event was spotted by the US space probe Cassini, using an instrument that can detect radiowaves emitted by lightning discharge.

"The reason why we see lightning in this peculiar location is not completely clear," said Georg Fischer of the Austrian Academy of Sciences in a press release.

"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."

But another possibility for the southerly location of "Storm Alley" could be seasonal, said Fischer.

In 1980 and 1981, the Voyager spacecraft flew by Saturn and observed lightning storms near the equator.

It could be that the mega-storms will now shift back to equatorial latitudes as Saturn continues its crawl around the Sun. A "year" in Saturn is equivalent to more than 29 Earth years.

The previous record-breaker for a Solar System thunderstorm was an event that lasted seven and a half months, running from November 2007 to July 2008, also spotted by Cassini.