Lightning on Saturn observed by Cassini ISS and RPWS during 2006-2009

Throughout the Cassini mission thousands of images had been taken on the night side of saturn in search for optical lightning flashes. No flashes were unambiguously detected so far. The reasons for that may be the lightning located too deep and covered by the thick clouds, and thus faint as seen from the orbit, cosmic rays hitting the detector that could be confused with lightning, and the ringshine compromising the observations both by potential saturation of the images and by illuminating small convective clouds whose shape in reflected light can be confused with lightning flash. The only time of nearly zero ringshine in the 30-year-long Saturnian year is during the equinox, which happened on August 11, 2009. Cassini ISS took 211 lightning search images within ten days from the equinox. We will report on possible lightning detections in those images and also in the previous Cassini ISS lightning searches. We also report on Cassini Imaging Science Subsystem (ISS) and Radio and Plasma Wave Science (RPWS) observations that indicate lightning on Saturn. A lightning storm that began in 2007 lasted for 7.5 months. Another storm started in mid-January 2009 and was still active in August of 2009. We will compare these recent storms with those studied by Cassini in 2004 and 2006. In all cases, radio emissions (Saturn Electrostatic Discharges, or SEDs) occur when a rare bright cloud erupts at a unique latitude ∼ 35 degrees South (planetocentric).The cloud typically lasts for several weeks to months, and then both the cloud and the SEDs disappear.The cloud and SED's reappear synchronously after being inactive for several months. The SEDs are periodic with roughly Saturn's rotation rate, and show correlated phase relative to the times when the clouds are seen on the spacecraft-facing side of the planet. The storm clouds erupt to unusually high altitudes and then slowly descend and spread.The eruption lasts for less than a day during which time the SEDs reach their maximum rates.This suggests vigorous atmospheric updrafts accompanied by strong precipitation and lightning. Unlike ubiquitous thunderstorms on Earth or Jupiter, only one latitude on Saturn has produced exceptionally strong thunderstorms during the five years of Cassini observations.