Brad D. Wallis

Voyager photopolarimeter observations of Uranian ring occultations

Abstract The Voyager 2 photopolarimeter (PPS) observed the stars σ Sagitarii and§b Persei as they were occulted by the Uranian ring system on January 26, 1986 (A. L. Lane et al. 1986, Science 233, 65–70). Both occultations passed through the ring system so that orbital radii were sampled at two different longitudes in the ring plane for each occultation. The σ Sagitarii occultation was a grazing occultation and only ϵ, Δ, and 1986U1R occulted the star. β Persei was occulted by each of the nine classical rings twice, and each of these rings was identified on ingress and egress, although the 6 ring in β Per ingress is a marginal detection, and the η ring in β Per ingress was not detected. The orbits of the nine classical rings have been successfully modeled by numerous Earth-based stellar occulations (R. G. French et al. 1988, Icarus 73, 349–378), and Voyager occultation profiles of the rings from the radio science occultation (RSS, D. L. Gresh et al. 1989, Icarus 78, 131–168) and the ultraviolet spectrometer occultations (UVS, J. B. Holberg et al. 1987, Astron. J. 94, 178–188) have been published. The PPS occultations provide high resolution profiles of the rings (∼ 10 m/point for σ Sgr, ∼100 m/point for β Per) which contain information on waves (L. J. Horn et al. 1988, Icarus 76, 485–492), edge sharpness, vertical thickness, and azimuthal variations in radial structure within the rings. The PPS profiles of the e ring show structure similar to that seen in the radio, UVS, and Earth-based occultations, although the magnitude of peaks and valleys in optical depth changes with azimuth. 1986U1R, γ, η, α, and 6 all show varying degrees of azimuthal inhomogeneity. The η ring is not present in the β Per ingress data although it shows up clearly in egress, and the γ ring is ony 0.6 km wide at β Per ingress and is opaque to the starlight while on the egress cut it is 2.6 km wide. We also present the results of a rigorous statistical search of the data for previously undetected rings. Several features were identified in the β Per data that were more statistically significant than some of the known ring profiles (6 and η in β Per ingress), but the low signal-to-noise ratio for this occultation prevents the identification of any new rings.