Abstract
We determine the time-variable shape of the outer edge of Saturn’s B ring using the complete set of Cassini radio and stellar occultation data obtained between mid-2005 and the End-of-Mission in late 2017, considerably expanding the range and number of individual ring edge measurements used in previous analyses (Spitale and Porco, 2010; Nicholson et al., 2014a). During this 12-year interval, the dominant m=2 pattern driven by the Mimas 2:1 inner Lindblad resonance completed just over two rotations relative to Mimas, with a circulation period of 5.362 yr, while its radial amplitude varied from a minimum of 4 km to a maximum of 71 km. This circulation pattern has remained essentially unchanged over the full period of the observations. We confirm the existence of four additional perturbations with azimuthal wavenumbers m=1, 3, 4 and 5 and mean amplitudes ranging from 5 to 24 km, which we interpret as normal or edge modes, possibly triggered by viscous overstabilities in the dense B ring (Borderies et al., 1985; Longaretti, 2018). Fits of a simple WKB model to the observed pattern speeds of the edge modes with m≠1 suggest an average surface mass density in the outer 30 km of the B ring of ∼100 g cm−2, somewhat greater than the 50–70 g cm−2 inferred from density and bending waves in most other regions of this ring (Hedman and Nicholson, 2016). The m=1 mode, which extends further into the B ring, yields a more typical value of 60 g cm−2. Surprisingly, all four of these modes exhibit significant librations in their amplitudes and phases, with periods between 2.3 and 8.6 yr and amplitudes of 1.6 to 7.4 km. The origin of these librations is unknown and it is unclear if they are truly periodic and will maintain their amplitudes, periods, and phases over timescales of centuries. Their frequencies do not match those expected for interference between edge modes with varying numbers of radial nodes. Instead, they may represent periodic oscillations in the amplitudes of individual normal modes or nonlinear, non-resonant coupling between normal modes with different values of m, leading to long-term quasi-periodic variations in the mode amplitudes.
•We determine the time-variable shape of the outer edge of Saturn’s B ring.•We identify a dominant m=2 pattern driven by Mimas, stable over the 12 years of observations.•We identify additional perturbations of m=1, 3, 4, and 5 with amplitudes from 5 to 24 km.•These modes exhibit significant variations in their amplitudes and phases.