Saturn’s furious five month storm
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: April 30, 2008

Saturn’s longest ever observed electrical storm remains persistent after five months, with lightning bolts 10,000 times more powerful than lightning on Earth penetrating right through the planet’s troposphere.

Saturn’s electrical storms resemble terrestrial thunderstorms, but with diameters several thousand kilometers times larger, which produce radio signals thousands of times more powerful than those on Earth. NASA’s Cassini spacecraft has been monitoring one unrelenting electrical storm for the last five months, having first detected ‘Saturn electrostatic discharges’, that is, radio waves, late November last year. Within a week of the initial observation, the Cassini imaging team had pinpointed the storm to a region in the southern hemisphere nicknamed ‘Storm Alley’, so called because many other lightning storms had already been observed there.

Two side by side views of the longest lived electrical storm yet seen on Saturn. The left hand image shows roughly what the human eye would see, but the storm stands out with greater clarity in the right hand image, which has been sharpened and enhanced. Cassini scientists say that the storm is a vertically extended disturbance that extends from Saturn's lower to upper troposphere. Image: NASA/JPL/Space Science Institute.

"The electrostatic radio outbursts have waxed and waned in intensity for five months now," says Georg Fischer, an associate with the radio and plasma wave science team at the University of Iowa. "We saw similar storms in 2004 and 2006 that each lasted for nearly a month, but this storm is longer-lived by far. And it appeared after nearly two years during which we did not detect any electrical storm activity from Saturn."

Cassini’s radio plasma wave instrument detects the storm every time it rotates into view, every 10 hours and 40 minutes, the length of a Saturn day. Every few seconds the storm gives off a radio pulse lasting for about a tenth of a second, typical behaviour of lightning bolts and other electrical discharges. These radio waves are detected even when the storm is over the horizon as viewed from Cassini, a result of the bending of radio waves by the planet's atmosphere.

Amateur astronomers from around the world are also keeping tabs on the storm, filling in the gaps when Cassini cannot make any observations. As the seasons change, bringing the onset of autumn to the planet's southern hemisphere, Cassini scientists will continue to monitor activity in Storm Alley in order to learn more about the processes powering Saturn's intense lightning activity.