Voyager Observations of Saturn's Rings. 1. The Eccentric Rings at 1.29, 1.45, 1.95 and 2.27 Rₛ. 2. The Periodic Variation of Spokes

Citation

Porco, Carolyn C. (1983) Voyager Observations of Saturn's Rings. 1. The Eccentric Rings at 1.29, 1.45, 1.95 and 2.27 Rₛ. 2. The Periodic Variation of Spokes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/q3z6-ca90. https://resolver.caltech.edu/CaltechETD:etd-09212004-110904

Abstract

Part 1:

Five major eccentric features in the rings of Saturn are studied. These are the outer A and B ring edges at 1.95 and 2.27 Rₛ and three narrow ringlets at 1.29, 1.45, and 1.95 Rₛ. Data acquired by four Voyager experiments - Imaging Science (ISS), Radio Science (RSS), Ultraviolet Spectrometer (UVS), and Photopolarimeter(PPS) - were used in this investigation.

The shapes and kinematics of the A and B ring outer edges are determined by their proximity to strong low-order Lindblad resonances. The data for the A ring edge are consistent with a 7-lobed distortion rotating with the mass-weighted mean angular velocity of the co-orbital satellite system. The B ring edge has a double-lobed figure which rotates with the mean motion of Mimas.

The Saturnian ringlets are narrow (mean widths vary from ~ 10-60 km) and have eccentricities of order 10^-4. All have sharp edges, normal optical depths τ ~ 1-2, and are embedded in essentially empty gaps (τ < 0.05). The Titan ring at 1.29 Rₛ and the Huygens ring at 1.45 Rₛ exhibit positive linear width-radius relations; the Maxwell ring at 1.95 Rₛ does not. The kinematics of the Huygens ring are determined solely by Saturn's non-spherical gravity field. The kinematics of the Titan ring are apparently completely determined by its interaction with Titan. At present, the most plausible model for the Maxwell ring involves the superposition of two components: one which is freely precessing and the other which is forced by Mimas and the elliptical B ring. Masses, mean surface mass densities, and specific opacities have been calculated for the Titan and Huygens rings.

Part 2:

The discovery of a periodic variation in spoke activity in Saturn's rings from the analysis of Voyager images is reported. A Fourier power spectrum was computed using a data set generated by quantifying spoke activity observed on the morning (western) half of the rings in Voyager images spanning ~ 12 Saturn rotations and in Voyager 2 images spanning ~ 90 Saturn rotations. The period from Voyager 1 data is 631 ± 22 min; from Voyager 2, 640.6 ± 3.5 min. The latter result suggests that the fundamental modulation in spoke activity is due to the rotation of Saturn's magnetic field, the period of which is 639.4 min. Maximum spoke activity observed anywhere on the rings is most likely to be associated with the region of the magnetic field responsible for the most intense emission of the Saturn Kilometric Radiation (SKR). Passage of this region through Saturn's shadow may play a significant role in the creation and/or rejuvenation of spokes.

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