Density Waves in Saturn's Rings: Non-linear Dispersion and Moon Libration Effects

Details Density Waves in Saturn's Rings: Non-linear Dispersion and Moon Libration Effects Density Waves in Saturn's Rings: Non-linear Dispersion and Moon Libration Effects

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.2403s&link_type=abstract

American Astronomical Society, DPS meeting #40, #24.03; Bulletin of the American Astronomical Society, Vol. 40, p.430

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We analyze strong spiral density waves in stellar occultations by Saturn's A ring observed with the Cassini Ultraviolet Imaging Spectrograph (UVIS) and find that the dispersion relation of these waves exhibits a clear deviation from the expected classical linear trend. All waves examined here reveal an intrinsic quadratic radial dependence on the wavenumber k(r). We provide evidence that the deviation from the linear trend is caused by the additional acceleration of ring particles due to the ring's pressure acting against the self-gravity of the ring. From the observed dispersion relation and using the theory of Goldreich and Tremaine (1978, 1979, ApJ) where pressure is parameterized as P=σ c2, we measure the effective sound speed c=3-5 mm/s in the A ring. Our results indicate the thickness of the A ring H=c/n 15-35m and are consistent with previous estimates from the literature. Both c and H increase from the inner to outer A ring.
Additionally, the dispersion relation of all first order Pandora waves exhibits a wiggly structure. Comparing 60 stellar UVIS occultations between 2004 and 2008 we infer that this wavenumber modulation propagates away from the resonance location with a period of about 600 days. This inferred period is consistent with Pandora's libration due to the 3:2 near corotation resonance between Pandora and Mimas (French et al., 2003, Icarus). The observed libration in wavenumber allows us to accurately measure the group velocity VG in the A ring. Moreover, the group velocity slowly decays away from the resonance location and it perfectly follows the theoretically expected dependence VG G π σ - c2 k, which allows us to obtain independent estimates of both surface density σ and sound speed c of the rings.

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