www.elsevier.com/locate/icarus Lightning on Jupiter observed in the Hα line by the Cassini imaging science subsystem

Night side images of Jupiter taken by the Cassini Imaging Science Subsystem (ISS) camera with the Hα filter reveal four lightning clusters; two of them are repeated observations of the same storm. All of these flashes are associated with storm clouds seen a few hours earlier on the day side of Jupiter. Some of the clouds associated with lightning do not extend to the upper troposphere. The repeated lightning observations taken 20 hr apart show that storm clouds, whose mean lifetime is ∼ 4 days, are electrically active during a large fraction of their lifetime. The optical power of the lightning detected with the Hα filter compared to the clear-filter power of Galileo lightning may indicate that the Hα line in the lightning spectrum is about ten times weaker than expected, consistent with a flat spectrum having no prominent Hα line. This may suggest that lightning is generated in atmospheric layers deeper than 5 bars. This, in turn, may suggest that the water abundance of the jovian interior is more than 1 × solar. Averaged over many flashes, the most powerful Cassini lightning storm emits 0.8 × 109 W in the Hα line, which implies 4 × 1010 W of broadband optical power. This is 10 times more powerful than the most intense jovian lightning observed before by Voyager 2. © 2004 Elsevier Inc. All rights reserved