Characteristics of Ring Particles and Clumping in Peaks and Troughs of Density Waves from Cassini UVIS Ring Occultation Statistics

Abstract:

Cassini’s Ultraviolet Imaging Spectrograph (UVIS) High-Speed Photometer Cassini (HSP) observed a multitude of ring stellar occultations at high spatial resolution. Photon counts per integration period (1-2 ms) follow a Poisson distribution in the absence of intervening ring particles. Previous analyses of the variance of the data have been interpreted in terms of an effective particle or clump size, R_E (Showalter and Nicholson 1990, Icarus, 87, 285; Colwell et al. 2018, Icarus, 300, 150). Here we analyze both the variance and the skewness of the occultation data in the troughs and peaks of strong density waves in the A and B rings. The presence of small gaps in the rings leads to a larger skewness than a uniform ring distribution, while clumps in a low-optical-depth region leads to a smaller value of skewness. We present a survey of excess variance (E), skewness (S), and R_E within density waves excited by the Janus 4:3, Janus 5:4, and Janus 6:5 resonances in the A ring and the Janus 2:1 resonance in the inner B ring over a range of occultation geometries. Clumps, nicknamed ‘straw’, have been directly imaged in the troughs of strong density waves (Porco et al. 2005, Science, 307, 1226). We find no difference between R_E in density wave peaks and troughs or the immediately adjacent ring regions. Additionally, we report inconsistent values between the best-fit R_E and the best-fit R_S, an analytic solution for effective particle size as a function of skewness. Because S is proportional to τ for 0.5, we compare S/τ to look for systematic differences in the behavior of S between density wave peaks and troughs and the surrounding ring regions. We find S/τ in the peaks of the strong A ring density waves to be consistently high compared to the troughs of those waves. Because the presence of rare, but relatively clear, gaps increases S, this suggests that the peaks have more gaps than the troughs or that the troughs have rare but opaque clumps leading to lower values of S. The discrepancy between S for peaks and troughs is greater in the A ring waves than in the Janus 2:1 density wave in the inner B ring. We also find S is inconsistent between within the peaks and troughs of the density and adjacent ring regions.