Dynamics of Saturn’s great storm of 2010–2011 from Cassini ISS and RPWS

Abstract

Saturn’s quasi-periodic planet-encircling storms are the largest convecting cumulus outbursts in the Solar System. The last eruption was in 1990 (Sánchez-Lavega, A. [1994]. Chaos 4, 341–353). A new eruption started in December 2010 and presented the first-ever opportunity to observe such episodic storms from a spacecraft in orbit around Saturn (Fischer, G. et al. [2011]. Nature 475, 75–77; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71–74; Fletcher, L.N. et al. [2011]. Science 332, 1413). Here, we analyze images acquired with the Cassini Imaging Science Subsystem (ISS), which captured the storm’s birth, evolution, and demise. In studying the end of the convective activity, we also analyze the Saturn Electrostatic Discharge (SED) signals detected by the Radio and Plasma Wave Science (RPWS) instrument. The storm’s initial position coincided with that of a previously known feature called the String of Pearls (SoPs) at 33°N planetocentric latitude. Intense cumulus convection at the westernmost point of the storm formed a particularly bright “head” that drifted at −26.9±0.8ms−1 (negative denotes westward motion). On January 11, 2011, the size of the head was 9200km and up to 34,000km in the north–south and east–west dimensions, respectively. RPWS measurements show that the longitudinal extent of the lightning source expanded with the storm’s growth. The storm spawned the largest tropospheric vortex ever seen on Saturn. On January 11, 2011, the anticyclone was sized 11,000kmby12,000km in the north–south and east–west directions, respectively. Between January and September 2011, the vortex drifted at an average speed of −8.4ms−1. We detect anticyclonic circulation in the new vortex. The vortex’s size gradually decreased after its formation, and its central latitude shifted to the north. The storm’s head moved westward and encountered the new anticyclone from the east in June 2011. After the head–vortex collision, the RPWS instrument detected that the SED activities became intermittent and declined over ∼40days until the signals became undetectable in early August. In late August, the SED radio signals resurged for 9days. The storm left a vast dark area between 32°N and 38°N latitudes, surrounded by a highly disturbed region that resembles the mid-latitudes of Jupiter. Using ISS images, we also made cloud-tracking wind measurements that reveal differences in the cloud-level zonal wind profiles before and after the storm.