Constraints on Saturn's E Ring from the Voyager 1 Radio Astronomy Instrument

Abstract

We have reanalyzed the data acquired by the planetary radioastronomy (PRA) experiment during the passage of Voyager 1 through Saturn's E ring. Depending on the distance from the ring plane, the instrument detected (i) dust grain impacts on the spacecraft and/or (ii) plasma waves or noise. The signal produced by the dust can be recognized by its power spectrum. It is dominant in a region of ≈12,000 km vertical extent around the ring plane, and has a maximum at roughly 5000 km southward of equator (at 6.1RSfrom Saturn). Assuming that the grain concentration is given by the model of Showalteret al.(Showalter, M. R., J. N. Cuzzi, and S. M. Larson 1991.Icarus94, 451–473) derived from optical observations, we infer from the mean PRA voltage and from the histogram of the data that the particles have a mean radiusr≈ 1 μm and a narrow size distribution of fractional dispersion between 10 and 30%. These values agree with the above model. We have also investigated the ring thickness. The PRA signal has a full vertical width at half-maximum of ≈8000 km, which is 2.3 times less than that given by the optical model. Since the signal produced by the dust varies strongly with the grain size (asr6), our measurements can be made compatible with the optical observations if the particle mean size decreases slightly with vertical distance, by about 10% over 4000 km.