Saturn’s visible lightning, its radio emissions, and the structure of the 2009–2011 lightning storms

Abstract

Visible lightning on Saturn was first detected by the Cassini camera in 2009 at ∼35° South latitude. We report more lightning observations at ∼35° South later in 2009, and lightning in the 2010–2011 giant lightning storm at ∼35° North. The 2009 lightning is detected on the night side of Saturn in a broadband clear filter. The 2011 lightning is detected on the day side in blue wavelengths only. In other wavelengths the 2011 images lacked sensitivity to detect lightning, which leaves the lightning spectrum unknown.The prominent clouds at the west edge, or the “head” of the 2010–2011 storm periodically spawn large anticyclones, which drift off to the east with a longitude spacing of 10–15° (∼10,000km). The wavy boundary of the storm’s envelope drifts with the anticyclones. The relative vorticity of the anticyclones ranges up to −f/3, where f is the planetary vorticity. The lightning occurs in the diagonal gaps between the large anticyclones. The vorticity of the gaps is cyclonic, and the atmosphere there is clear down to level of the deep clouds. In these respects, the diagonal gaps resemble the jovian belts, which are the principal sites of jovian lightning.The size of the flash-illuminated cloud tops is similar to previous detections, with diameter ∼200km. This suggests that all lightning on Saturn is generated at similar depths, ∼125–250km below the cloud tops, probably in the water clouds. Optical energies of individual flashes for both southern storms and the giant storm range up to 8×109J, which is larger than the previous 2009 equinox estimate of 1.7×109J. Cassini radio measurements at 1–16MHz suggest that, assuming lightning radio emissions range up to 10GHz, lightning radio energies are of the same order of magnitude as the optical energies.Southern storms flash at a rate ∼1–2per minute. The 2011 storm flashes hundreds of times more often, ∼5times per second, and produces ∼1010W of optical power. Based on this power, the storm’s total convective power is of the order 1017W, which is uncertain by at least an order of magnitude, and probably is underestimated. This power is similar to Saturn’s global internal power radiated to space. It suggests that storms like the 2010–2011 giant storm are important players in Saturn’s cooling and thermal evolution.