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Structural changes within Saturn’s A ring density waves revealed by Cassini UVIS stellar occultation statistics

Presentation #110.07 in the session Many Planets, More Rings Posters (Poster)

Published onOct 23, 2023
Structural changes within Saturn’s A ring density waves revealed by Cassini UVIS stellar occultation statistics

The Cassini Ultraviolet Imaging Spectrograph (UVIS)’s High-Speed Photometer (HSP) observed numerous occultations of stars by Saturn’s rings over 13 years in orbit. Occultation statistics are uniquely capable of detecting structures as small as meters. The combined analysis of observations from multiple viewing configurations can be used to investigate different facets of the ring structure. Showalter and Nicholson (1990, Icarus, 87, 285) and Colwell et al. (2018, Icarus, 300, 150) interpreted the excess variance above that predicted by Poisson counting statistics in terms of an effective particle size, RE, which depends on the length of shadows cast by the particles. However, the assumption of uncorrelated spherical particles is not valid in Saturn’s A ring, which is dominated by elongated clumps of particles called self-gravity wakes (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). Here we present an analysis of the normalized excess variance, skewness, and excess kurtosis within the Janus 5:4, Mimas 5:3, and Janus 6:5 density waves. We remove secular trends within density wave peaks and troughs and calculate the statistical moments of the data. We model the higher order moments with a simple “granola bar” model of the self-gravity wakes that describes the wakes as rectangular blocks of width W, height H, length L, with an average separation S (Colwell et al. 2006). To determine the best fit of the self-gravity wake parameters, we require that the wake wavelength (S+W) equal the Toomre most-unstable wavelength (Toomre 1964, Astrophys. J. 139, 1217) for each wave. We fit the wave troughs separately from the peaks and combine the best fits for many occultations of the same star to better constrain S and W. We find that regularly spaced wakes cannot match the observed statistical moments of the troughs and peaks of the waves simultaneously, suggesting that S and W vary on short timescales between the wave peaks and troughs. Additionally, we find that the model cannot account for the observed skewness, indicating the presence of gaps and/or ghosts not accounted for in the model. Our results are consistent with Esposito et al. (2012, Icarus, 217, 103)’s predator-prey dynamical model which predicts that wakes grow and erode as they are perturbed by passing density waves, exhibiting a temporal response to forcing by nearby moons.

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