dense main rings where collisions and collective processes play an important role, the particle concentration is so
We have reanalyzed the data acquired by the planetary radioastronomy (PRA) experiment during the passage of Voyager small in these faint rings that the grains behave indepen-1 through Saturn's E ring. Depending on the distance from the dently of each other, being mainly subjected to electromagring plane, the instrument detected (i) dust grain impacts on netic forces, light pressure, and other nongravitational perthe spacecraft and/or (ii) plasma waves or noise. The signal turbations, in addition to the usual gravitational forces produced by the dust can be recognized by its power spectrum. (Burns et al. 1984). This makes these etheral rings good
It is dominant in a region
of ศ12,000 km vertical extent around places to study the fundamental processes governing trajecthe ring plane, and has a maximum at roughly 5000 km southtories of individual particles under these conditions. On ward of equator (at 6.1 R S from Saturn). Assuming that the the practical side, studying the E ring is crucial to determine grain concentration is given by the model of Showalter et al. a safe trajectory for the Cassini orbiter, which will spend (Showalter, M. R., J. N. Cuzzi, and S. M. Larson 1991. Icarus 94, 451-473) derived from optical observations, we infer from a long time there.
the mean PRA voltage and from the histogram of the data that Its faintness makes this ring difficult to observe, and a the particles have a mean radius r ศ 1 m and a narrow size large bulk of data is necessary in order to build a consistent distribution of fractional dispersion between 10 and 30%. These picture. Three main kinds of measurements are available: values agree with the above model. We have also investigated (i) optical observations, which have the interest of being the ring thickness. The PRA signal has a full vertical width at also feasible from Earth, but can only reveal properties half-maximum of ศ8000 km, which is 2.3 times less than that integrated along the line of sight, (ii) in situ observations given by the optical model. Since the signal produced by the of dust impacts, which have the interest of being local, but dust varies strongly with the grain size (as r 6 ), our measurements have a limited space coverage since they can only be made can be made compatible with the optical observations if the along the trajectory of available space probes, and (iii)
particle mean size decreases slightly with vertical distance, by absorption signatures on spacecraft-based charged particle about 10% over 4000 km.