Micron-sized particle impacts detected near Uranus by the Voyager 2 plasma wave instrument

During the Voyager 2 flyby of Uranus the plasma wave and radio astronomy instruments detected a region of impulsive noise near the equatorial plane just inside the orbit of Miranda, at a radial distance of 4.51 R_U. This noise is believed to be caused by micron-sized particles hitting the spacecraft. Analysis of various coupling mechanisms shows that when a dust particle hits the spacecraft at a high velocity, the particle is instantly vaporized and ionized, thereby releasing a cloud of charged particles, some of which are collected by the antenna. The resulting voltage pulse is detected by the plasma wave instrument. Based on reasonable assumptions about the charge yield and collection efficiency of the antenna, the number density and mass of the particles can be estimated from the rate and amplitude of the voltage pulses. The analysis shows that the maximum number density of the particles is about 1.6×10^-3 m^-3, and the thickness of the impact region, based on a Gaussian fit, is 3480 km. The maximum number density occurs slightly after the ring plane crossing at a distance of about 280 km from the equatorial plane. The mass threshold for detecting the particles is estimated to be about 4.5×10^-10 g, and the rms mass of the particle is about 2.6×10^-9 g. For a density of a few grams per cubic centimeter the particles have radii of the order of a few microns. Possible sources for these particles include the rings, the small satellite 1985U1 discovered outside the ring system, or other unseen small bodies that lie between synchronous orbit (3.15 R_U) and 4.51 R_U. If the particles are charged, electromagnetic forces produced by the rotating tilted dipole of Uranus may play a role in their transport and diffusion.