Voyager Radio Occultation Data And Their Interpretation

In the view of the general public, the radio observations of Saturn's rings generally came in a distant second to imaging observations. Yet in Voyager's observations, it was the radio occultation observations which provided the highest radial resolution data on the rings, which gave the most direct information on ring particle sizes, and which seemed to confirm the suspected nature of the A-ring azimuthal asymmetry. Details of the Voyager 1 observation technique and theory are given by Eshleman et al. [62] and Marouf et al. [63].

Radio occultation observations of the C ring and the Cassini Division revealed differences between S-band and X-band transparencies that are much smaller than those in the A ring. This is consistent with the observation that optical opacity and (adjusted) radio opacity are essentially identical in these regions. The adjustment is a reduction (by a factor of 2) in the calculated radio depths (opacities), related to the coherency [64] of the radio beam; starlight used in the optical occultation measurements is non-coherent. A detailed discussion of ring particle size distributions from Voyager 1's radio occultation experiment is given by Marouf et al. [65].

Within the rings, the radial resolution of the radio occultation measurements is approximately 15 km, but for those areas where the Earth-received signal is strong enough, phase matching can be used to remove the diffracted part of the signal (which originates from portions of the ring away from the center of the beam), thereby improving the radial resolution to better than 1 km [66]. This higher radial resolution was not possible in the denser inner B ring, where the opacity was significantly larger and the signal strength correspondingly weaker, but resolutions on the order of a few kilometers were achieved.

The rings were sampled by the Voyager 1 radio occultation experiment over a radial range that spanned from inside the C ring to beyond the F ring. The ring tilt angle (as seen from the radio receivers on Earth) was 5.9° at the time of the experiment. Practically, this small tilt angle limited the sensitivity of the experiment to radio depths from about 0.005 to about 1.0. In the outer half of the B ring, derived X-band radio depths exceeded in several places a value of 1.0, very near the upper limit of sensitivity. For that reason, the Voyager 1 outer B-ring radio opacities are not as reliable as those for the inner B ring and for the other rings; the S-band sensitivity in the outer B ring was much worse.

The radio occultation data have also been processed in such a way as to provide some information on sub-centimeter-sized particles in the A and C rings and Cassini Division and the distribution in particle sizes in those same rings for particles in the 1-to 10-meter radius size range [67]. The sharp definition of the density waves from radio science data, especially in the A ring, have provided the best values for ring density per unit area and for the masses of the perturbing satellites which give rise to the density waves. In short, the radio occultation experiment produced a wealth of data on Saturn's rings that not only helped to define the rings but provided a strong basis for the follow-on studies of Saturn's ring system by the Cassini Orbiter.

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