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A panoply of waves in Saturn’s C ring driven by internal planetary oscillations

Presentation #513.06 in the session “Planetary Rings: Observational Insights”.

Published onOct 26, 2020
A panoply of waves in Saturn’s C ring driven by internal planetary oscillations

We report on the identification of ten new density waves in Saturn’s C ring driven by the planet’s internal acoustic oscillations. Saturn’s rings act as a system of innumerable test particles that are remarkably sensitive to periodic disturbances in the planet’s gravitational field. The collective response of the rings to Saturn’s oscillations results in a host of inward-propagating density waves at outer Lindblad resonances (OLR) and outward propagating bending waves at outer vertical resonances (OVR). In the emerging field of Kronoseismology, nearly two-dozen OLRs and OVRs have previously been identified in high-resolution radial profiles of the rings obtained from Voyager and Cassini occultation observations (see Hedman et al. (2019) Astron. J. 157:18 and references cited therein for a recent summary). Here, we apply similar wavelet techniques to extract and co-add phase-corrected waveforms from multiple Cassini VIMS stellar occultations. Taking advantage of a highly accurate absolute radius scale for the rings (French et al. (2017) Icarus 290, 14), we are able to detect weak, high-wavenumber (up to m=14) waves with sub-km wavelengths. A systematic scan of the entire C ring has revealed a host of new candidate OLR-type density waves whose wavenumbers and pattern speeds are consistent with their observed radial locations in the rings. Here, we report on ten of these detections and their likely association with waves produced by saturnian f-mode oscillations, as predicted by models of Saturn’s interior (Mankovich et al. (2019) Ap. J. 871:1). Such oscillations are classified by their spherical harmonic shapes, described by Plm(cos θ) cos(mϕ). The modes in question range in azimuthal wavenumber from m=8 to 14, and the associated resonance order (l-m) ranges from 0 to 8, where l is the overall angular wavenumber. Our suite of l-m=4 detections now spans the complete range from m=8 near the inner edge of the C ring to m=14 near 81,300 km. Curiously, l-m=2 detections are less common. The newly-identified non-sectoral (l>m) waves sample a range of latitudinal structure within the planet and may thus provide valuable constraints on Saturn’s differential rotation.

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