Presentation #407.06 in the session “Planetary Rings: Theory and Observations”.
Cassini’s Ultraviolet Imaging Spectrograph (UVIS)’s High-Speed Photometer (HSP) observed 276 stellar occultations of Saturn’s rings from a multitude of viewing geometries. Because photon counts follow a Poisson distribution, the variance is approximately equal to the mean for a translucent screen. However, when starlight passed through Saturn’s rings from the point-of view of Cassini, the finite sizes of intervening particles caused excess variance above the mean due to a correlation in the blocking of photons. Showalter and Nicholson (1990, Icarus, 87, 285) (hereafter SN90) and Colwell et al. (2018, Icarus, 300, 150) interpreted this excess variance in terms of an effective particle size, RE, which depends on the length of shadows cast by ring particles. The works well in low optical depth regions like the C ring, but its assumptions are invalidated in Saturn’s A ring, which hosts self-gravity wakes, elongated clumps of ring particles under the competing influence of mutual self-gravity and Keplerian shear (e.g. Colombo et al. 1976, Nature, 264, 344; Colwell et al. 2006, Geophys. Res. Lett. 33, L07201; Hedman et al. 2007, Astron. J., 133, 2624). We focus on density waves in the A ring whose optical depth profiles are characterized by sharp peaks and smooth troughs. We introduce free parameters S(wake separation), and W (wake width) from the granola bar model for self-gravity wakes (Colwell et al. 2006). For each wave we set the wake wavelength (S+W) equal to the Toomre most-unstable wavelength (Toomre 1964, Astrophys. J.139, 1217), fit S/W models compatible with l=S+W separately for the wave peaks and troughs, and select the best fit. We apply this to the Janus 4:3 and Pandora 5:4, Janus 5:4, Mimas 5:3, and Janus 6:5 waves and find an average best fit S/W ratio for each. Although the wave troughs are usually well fit by the moments model, the statistics in the peaks are not captured by either the SN90 or moments model. We find that the troughs are statistically similar to nearby adjacent regions. Our average best-fit S/W ratios indicate that the spacing between wakes is narrower in the inner A ring than in the outer A ring, consistent with Colwell et al. (2006), with the highest S/W values occurring in the outer A ring waves.