Lightning on planets with focus on Saturn

Thunderstorms play an essential role in the atmospheres of various planets. Lightning discharges can produce changes in atmospheric chemistry by providing energy for the formation of various molecules, which might have been essential for the evolution of life. Like on Earth, lightning seems to be a good indicator of vertical convection in the atmospheres of the gas planets, thereby opening a window to study deeper levels of the atmosphere. The study of seasonal changes of Saturn`s atmosphere including observations of the giant thunderstorms has been identified by NASA as one of the main scientific objectives of the Cassini Solstice mission which will last until 2017. This project will focus on the investigation of lightning discharges in Saturnian thunderstorms using mainly the data from the Cassini/RPWS (Radio and Plasma Wave Science) instrument, for which the proposer has recently been appointed as a Co-Investigator. RPWS measures the radio waves from Saturn lightning, which are shortly called SEDs for Saturn Electrostatic Discharges. The broad international cooperation with 11 partners from 7 different institutes in this project should ensure that the SED measurements are put in a broader context. There will be comparisons with imaging observations of storms and their dynamics, with ground-based SED radio observations, and with lightning from other planets. Furthermore, there are models of SED radio wave propagation and models of Saturn`s ionospheric properties, which can be validated with the SED data. In the frame of the proposed 3-year project the Saturn lightning activity will be permanently monitored, and its main physical characteristics will be evaluated. One special point is to investigate the temporal nature of Saturn lightning on time scales from milliseconds to years to determine their short burst duration, the daily/weekly intensity variation of on- going storms, and the influence of the Saturn season. The seasonality of the atmosphere seems to be a main parameter, but other internal and external influences (e.g. solar activity) might play a role as well. Another main point is to study the propagation of SEDs through Saturn`s ionosphere to determine the ionospheric peak electron densities and to measure radio wave absorption. This project should be a successful follow-up investigation of a previous FWF project named "Atmospheric electricity in the Saturnian system", which as a highlight resulted in the publication of a paper in Nature with a Saturn storm image on the cover of the issue.

The project 'Lightning on planets with focus on Saturn' has resulted in new findings about the Great White Spot (GWS), a massive thunderstorm the raged in the northern hemisphere of Saturn from December 2010 until August 2011, and in new findings about the so-called equinox storms, which are smaller thunderstorms in Saturn's southern hemisphere that took place from 2007 until 2010. The GWS had a latitudinal diameter of 10,000 km, and it developed an eastward tail that was wrapped all around Saturn. The storm's birth, evolution and demise were investigated by comparing optical images with the RPWS lightning flash rate. It was found that a large anticyclonic vortex, which had developed in the storm's tail, collided with the storm's head, leading to a significant decrease in lightning activity which ended two months later. Most of the lightning activity took place in the storm's head, but RPWS data as well as the first detection of optical flashes on Saturn's dayside showed that further storm cells were also located in the tail. We detected that the rotation rate of Saturn kilometric radiation (SKR) was temporarily slowed down by ~0.5% during the occurrence of the GWS and published the controversial theory that the SKR modulation period was influenced by the GWS. There are rotating field-aligned current systems rooted in the auroral thermosphere which generate the SKR. The proposed mechanism works via thunderstorm-induced gravity waves that might have caused a global change in Saturn's thermospheric winds, which determine the rotation rate of the current system. We made a statistical analysis of the flashes from the equinox storms (~2000 km in diameter, located in the 'storm alley' at 35S) and determined their intensities, polarization, and flash rates, and the latter were normalized with respect to Cassini distance and RPWS observational modes. The flashes appear in episodes due to Saturn's fast rotation with respect to Cassini, and we analysed the episode repetition periods (~10 h 40 min.) and durations (5-6 h on average). We identified the so-called 'over-horizon effect', when the radio waves of the flashes were already detected before the storm was within the visible horizon. We analysed the dynamics of Saturn's thunderstorms and found that they tend to split similar to terrestrial supercells. Our constant monitoring of Saturn lightning using data from the RPWS (Radio and Plasma Wave Science) instrument on the Cassini spacecraft revealed only marginal and sporadic lightning activity after the GWS in April and July 2012 and from July to October 2013.