DENSITY WAVES IN SATURNS RINGS PROBED BY RADIO AND OPTICAL OCCULTATION - OBSERVATIONAL TESTS OF THEORY

Voyager radio (RSS) and photopolarimeter (PPS) occultation observations of density waves in Saturn's Ring A are examined for the first time in parallel. Over 40 wave features are studied in each data set, all excited by the satellites Prometheus and Pandora at first-order Lindblad resonances. Theoretical models of density wave dynamics are used to define a generalized standard model for the wave structures that depends on the satellite mass, the disk mass, and an azimuthal phase shift as the model parameters. Least-square fits of the model to the first 1.5 wavelengths of the observed waves quantitatively test the ability of the theory to describe the waves in the wave launch zone near resonance. Recent refinement of the absolute radial scale in Saturn's rings removes resonance location as a free parameter needed in previous fits. Results show that the theory adequately describes the waves observed in the PPS data if satellite masses are decreased from observationally estimated values, but the waves observed at radio wavelengths show significant deviations from the model. The RSS waves exhibit an average ≈ -90° offset from the dynamical phase. The most satisfactory preliminary explanation of this phase residual exploits the exceedingly oblique incidence angle of the radio occultation and requires a small-amplitude bending mode coupled with the horizontal density wave mode. A warping height of about 100 m amplitude can reproduce qualitatively the RSS phase shift yet preserve the overall model wave shape. These results may have important implications for satellite-ring torque calculations in Saturn's rings, since all of the net torque in the standard model is deposited in the first wavelength. Even for the reduced satellite masses inferred here, the dynamical age of Saturn's rings determined from standard theory torques is young. Standard torques are due to small surface density effects which cannot be verified in the observations. The phase shifts of the observations from the standard model predictions therefore indicate that the torque and age problem should be reconsidered. Oblique ring occultation observations by the Cassini spacecraft will be important to corroborate these findings and refine the model comparisons. © 1992.