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A. P. Ingersoll, A. R. Vasavada, S. M. Milkovich (Caltech), B. Little (ITRES Research, Calgary), P. J. Gierasch, D. Banfield (Cornell), Galileo SSI Team
The Galileo Solid State Imager (SSI) detected lightning at six different latitudes between ±50o, each latitude corresponding either to a cyclonic shear zone or its poleward boundary - the center of a westward jet. In day side images the cyclonic shear zones are the most active regions on the planet, which suggests an association between visual activity and moist convection. The zone at 47o - 49o planetocentric still has the highest density of lightning storms, as it had during the Voyager flyby in 1979. Occasionally light from Io allowed us to image the faint moonlit clouds on which the bright lightning flashes are superposed. One set of images over a several-hour period observed a bright, high cloud several hundred km in diameter on the day side and also observed lightning from this cloud on the night side. The largest storm spanned 1500 km in the east-west direction. A typical storm had 20 flashes in one minute, ranging in intensity from 0.2 to 3.5 x 109 J per flash across the visible spectrum. Averaged over the surface, the flash rate in this energy range is 0.01 km-2 yr-1. Although the flash rate is several hundred times less than on Earth, the energy is several hundred times greater than on Earth, so the total optical power may be comparable. The color of the lightning is roughly consistent with Balmer emission plus continuum. Most of the flashes appear wider than 25 km, which is the projected width of a pixel in the image plane, suggesting that the lightning occurs at least 35 km below cloud tops. One flash at 47o latitude is 80 km wide at the half-power point. If this is a single event, it must be at least 110 km below the cloud tops.