The outer edge of Saturn's A ring, as revealed by Cassini occultation observations.

Voyager observations first suggested that the outer edge of Saturn's A ring is controlled by the 7:6 inner Lindblad resonance with Janus, the larger of the two co-orbital satellites that switch orbits every 4.0 years (Porco et al., 1984, Yoder et al., 1989). Early Cassini imaging observations confirmed this view, showing a 7-lobed pattern with a radial amplitude of 14 km corotating with Janus in 2006-09, while the satellite was on the inner leg of its horseshoe orbit (Spitale & Porco 2009). During this period, the 7:6 ILR fell only 4 km interior to the mean radius of the A ring's outer edge. However, subsequent imaging and occultation data in 2012-13 showed that this pattern disappeared after the co-orbital swap in January 2010, when Janus' orbit moved outward by 21 km (El Moutamid et al., 2016). We have now analyzed the full set of 321 occultation measurements by the Cassini RSS, UVIS and VIMS instruments through mid-2017, spanning 12 years. As expected, the 7-lobed pattern locked to Janus reappeared following the co-orbital swap in January 2014, with an amplitude of 12 km and a pattern speed within 0.005 deg/day of Janus' mean motion. The observations also reveal multiple free normal modes at the edge of the A ring. In 2006-09, the largest modes were m = 5 and m = 9 with amplitudes ae = 5 and 3 km, respectively. These were replaced during 2010-13 by modes with m = 9 and m = 12 and ae = 5 and 6 km, respectively. But after the final co-orbital swap in 2014, the m = 5 mode reappeared with an amplitude of 6 km and a phase similar to that seen prior to 2010. At the same time, the m = 9 amplitude was reduced to 3 km and the m = 12 mode vanished, suggesting a forcing mechanism involving the Janus resonance.