Micron-sized particles detected near Saturn by the Voyager plasma wave instrument

During the Voyager 2 Saturn encounter the plasma wave instrument detected a region of intense impulsive noise centered on the ring plane at a distance of 2.88 Saturn radii, slightly outside of the G ring. The noise has been attributed to small micron-sized particles hitting the spacecraft. Investigation of various coupling mechanisms suggested that the noise was produced by impact ionization of particles striking the spacecraft body, thereby releasing a cloud of charged particles, some of which were collected by the plasma wave antenna. Reasonably reliable estimates of the charge yield per unit mass are available from laboratory impact ionization measurements. Based on the assumption that the voltage induced on the antenna is proportional to the mass of the colliding particle, a method was developed to determine the mass and size distribution of the particles from the rms voltage of the induced noise and the impulse rate. The results obtained show that the mass distribution varies as m^-3, and that most of the particles detected had radii in the range from 0.3 to 3 μm. The effective north-south thickness of the particle distribution is 106 km. The mass distribution function derived from these data is shown to be in reasonable agreement with optical depth estimates obtained from imaging measurements and absorption effects detected by energetic charged particle measurements.