The longest lightning storm ever seen has been raging in the atmosphere of Saturn since January 2009, it has been revealed today at the European Planetary Science Congress (EPSC) in Potsdam, Germany.

Saturn is well-reputed for its giant thunderstorms, which appear to manifest in a region nicknamed ‘storm alley’, 35 degrees south of the equator. “The reason why we see lightning in this peculiar location is not completely clear,” says Dr Georg Fischer of the Austrian Academy of Sciences, who is in Potsdam to present the new results from NASA’s Cassini spacecraft. “It could be that this latitude is one of the few places in Saturn’s atmosphere that allows large-scale vertical convection of water clouds, which are necessary for thunderstorms to develop.”

The lightning emits radio waves that are 10,000 times stronger than the radio waves released by lightning on Earth, and the storms themselves are gargantuan, with diameters of up to 3,000 kilometres. Cassini’s Radio and Plasma Wave Science (RPWS) experiment is able to not only to detect the radio waves, but also study how they pass through Saturn’s ionosphere, which is a layer in the outer atmosphere of the planet filled with charged particles. “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 on ionisation in different regions,” says Fischer.

Saturn isn’t the only gas giant planet that has thunderstorms. Jupiter, too, has been seen to be plagued by lightning storms, but these should not be confused with the Great Red Spot and Red Spot Junior, which are atmospheric vortices rather than thunderstorms. The thunderstorms on both Jupiter and Saturn are giant columns of convection that drag water vapour up from deep within the atmosphere to near the top of the atmosphere, where it condenses into clouds.

The storms on Saturn may also be affected by the change in seasons. “Voyager observed lightning storms near the equator [in 1980 and 1981], so now that Saturn has passed its equinox on 11 August (see our report here) we may see the storms move back to equatorial latitudes.”