Desch, M., Kaiser, M., Farrell, W., Kurth, W., Gurnett, D., Zarka, P., Lecacheux, A., Fischer, G., Porco, C., and Ingersoll, A. (2004). "Cassini RPWS Observations of Saturn Lightning." Eos Trans. AGU 85 (47), Fall Meet. Suppl., Abstract P44A-06.


Abstract

The Radio and Plasma Wave (RPWS) instrument on Cassini began observing Saturn Electrostatic Discharges (SED) on a routine basis on 13 July, shortly after closest approach to the planet. SED, first discovered by the Planetary Radio Astronomy instrument on Voyager, are widely believed to be the radio signature of lightning discharges in the atmosphere of Saturn. The radio signatures appear very similar to the dayside signatures observed over 20 years ago by Voyager in that the frequency range extends from about 1 MHz to the highest receiver frequency (~16 MHz), and the individual bursts are of short (10's of msec) duration. However, the Cassini observations differ in important ways: (1) the lightning is generally weaker in intensity, (2) the period of reoccurrence of the storms (~10.24 - 10.75 hr) is decidedly longer than that observed by Voyager (~10.1 hr), and (3) the lightning storms, lasting from minutes to hours, now occur episodically, rather than repeatedly on every rotation. This last point is underscored by the fact that a possible SED storm of great intensity was observed over a year before closest approach in July, 2003. As of this writing, RPWS continues to detect SED. Since 13 July, 40 well-defined episodes, or storms, have been observed, corresponding to a storm occurrence rate of about 40 percent. Careful tracking of the storm phase relative to the planet's rotation has revealed that two probably separate and distinct storm systems have developed since 13 July. The first, lasting from 13 July (or before) until 27 July, reappeared with a period of 10 hr 45 min with highly predictable start times, apparently corresponding to the emergence of the storm over the night side limb of the planet. The storm waxed and waned in intensity, sometimes disappearing for many rotations, but always reappearing in phase. The second system appeared on 30 July, and has been observed on and off to date. This second system has reoccurred with a period of about 10hr 15min. Presumably the storm periodicity can be related toa particular latitude on Saturn where the measured wind speed in the atmosphere gives the appropriate period of rotation. Attempts to associate SED storms with particular atmospheric features are continuing.