Sizes of Particles, Clumps, and Holes in Density Waves in the Cassini Division in Saturn’s Rings from Cassini UVIS Stellar Occultation Statistics

Density waves are excited throughout Saturn’s rings at inner Lindblad resonances with several of Saturn’s moons. The resulting density wave profiles are characterized by sharp peaks and smooth troughs in optical depth. Cassini’s Ultraviolet Imaging Spectrograph (UVIS) High-Speed Photometer (HSP) observed a multitude of ring stellar occultations, and the data are described by Poisson counting statistics in the absence of intervening ring particles. We extend our analysis of statistical moments in density waves using the techniques of Showalter and Nicholson (1990, Icarus, 87, 285) and Colwell et al. (2018, Icarus, 300, 150), who interpreted the excess variance of the data in terms of an effective particle or clump size, R_E. Previously, we analyzed both the variance and the skewness of various ring stellar occultations in the troughs and peaks of strong density waves in the A ring and the Janus 2:1 density wave in the inner B ring. Here we present the results of analysis of three weaker density waves in the Cassini Division: Prometheus 9:7, Pan 6:5, and Atlas 5:4. We compare higher order moments of excess variance and skewness across these waves with our previous results for strong density waves in the A ring. Similar to our findings in the A ring and inner B ring, we find no difference between R_E in density wave peaks and troughs for density waves in the Cassini Division. We find a smaller R_E in Cassini Division density waves than A ring density waves, consistent with the largest ring particles there being ~ 5 times smaller than the largest ring particles in the A ring (Colwell et al., 2009, Icarus, 200, 574). Our results are also consistent with Esposito et al. (2012, Icarus, 217, 103)’s predator-prey model of aggregation and fragmentation, which suggests that the formation of large aggregates at strong resonant locations accelerates the ring particles and leads to increased disruptive collisions which in turn produce a population of smaller particles.