Saturns Lightning Storms

At the time of the Voyager fly-bys, the sporadic radio emissions from Saturn were inferred to be due to electrostatic discharges in storms in the equatorial region, which was then in the shadow of the ring system. Cassini's Radio and Plasma Wave Spectrometer first detected the discharges in July 2003, at a distance of 161 million kilometres. On its Earth fly-by, the instrument had detected lightning at a range of 89,200 kilometres, measured from Earth's surface. The mystery was why Saturn's emission was almost 1 million times as intense as that from terrestrial lightning. It tended to occur in episodes lasting several hours, in some cases involving hundreds of bursts. Although the episodes often recurred every 10 to 11 hours, which matched the length of Saturn's day, for some reason the activity would cease for long periods and then resume. It was not until the spacecraft had closed in sufficiently for the narrow-angle camera to resolve atmospheric detail that it became possible to seek

An image taken in September 2004 of the 'dragon storm' in Saturn's mid-southern latitudes, whose lightning produced radio emissions that were detected by Cassini.

evidence to establish that the radio bursts were correlated with an atmospheric structure. In early August, M.D. Desch of the Goddard Space Flight Center told C.C. Porco that the Radio and Plasma Wave Spectrometer had noted an intense episode in mid-July, and asked whether the imaging team had seen anything unusual. As Porco explained later, ''There was a bigger than average 'white storm' in the southern hemisphere at that time, in a latitude range that we'd come to call 'storm alley'. We caught it with the wide-angle camera, but didn't have enough visible observations of the storm to be confident of a correlation.'' When the storm reappeared in September, the radio team was alerted and, in fact, the Radio and Plasma Wave Spectrometer was in the process of monitoring an episode that was even more intense than that in July. This convective storm was so large and bright that the imaging team named it the 'dragon storm'. The fact that over an 18-day interval the largest storm was brightest when the radio emission was strongest - with the storm always in the same position during a burst - indicated that it was the same phenomenon. However, instead of the burst being detected when the storm rose over the horizon from Cassini's point of view and peaking when it was on the meridian facing the spacecraft, the emission started before the storm rose and halted as it crossed the terminator into daylight.

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