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Phase dependence of Cassini UVIS stellar occultation statistics in Saturn’s A Ring density waves

Presentation #112.06 in the session Many Planets, More Rings (Oral Presentation)

Published onOct 23, 2023
Phase dependence of Cassini UVIS stellar occultation statistics in Saturn’s A Ring density waves

Cassini’s Ultraviolet Imaging Spectrograph (UVIS) High-Speed Photometer (HSP) observed Saturn’s rings at a typical radial resolution of ~10 meters from a multitude of viewing geometries. Because the HSP signal uncertainty is only limited by Poisson counting statistics, the statistical moments of stellar occultation data can be directly compared to the expected moments of a Poisson distribution. Deviations from Poisson statistics of these moments contain a wealth of information about unresolved small-scale phenomena such as particle sizes, azimuthally-limited gaps (aka ghosts, Baillie et al., 2012), and larger clumps of particles. Assuming uncorrelated spherical particles, Showalter and Nicholson (1990, Icarus, 87, 285) and Colwell et al. (2018, Icarus, 300, 150) interpreted the excess variance beyond that contributed by Poisson statistics in terms of an effective particle or clump size, RE, which depends on the length scale of shadows cast by particles and clumps. In the A ring however, where ring particles aggregate into trailing spiral structures called self-gravity wakes (e.g. Colwell et al., 2006, Colwell et al., 2007, Hedman et al., 2007, and Nicholson and Hedman, 2010), these assumptions are no longer valid. Density waves are an area of particular ambiguity as perturbations by local satellites induce cyclical clumping, and disaggregation occurs with each passing wave front. The corresponding radial locations of density wave peaks and troughs can be converted to phases of the forcing cycle in these resonant structures. We present an analysis of the phase dependence of the moments of normalized excess variance, skewness, and kurtosis within the Janus 5:4, Mimas 5:3, and Janus 6:5 density waves in multiple stellar occultations. Following the granola bar model for self-gravity wakes, we assume that the wakes are incompressible in the horizontal dimension such that W remains constant during the passage of a density wave. The ring particles in the gaps do respond to the passage of the wave and are conserved so that the optical depth of each gap changes as S changes while H and W do not. We find that the no-wake-growth model is symmetric about phase 180 while the observed moments are not, indicating that the wake structure just before the wave crest passes over is different from just after the crest passes over. We conclude that the normalized excess variance observed in the first quarter of the forcing cycle is consistent with wake growth and the lags are consistent with the lag predicted by the Esposito et al. (2012, Icarus, 217, 103) predator-prey cycle in which density waves trigger aggregate growth.

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