Micron-size particles detected near Neptune by the Voyager 2 plasma wave instrument

During the Voyager 2 flyby of Neptune the plasma wave and radio astronomy instruments detected numerous impulsive signals that were interpreted as micron-sized particles striking the spacecraft. This paper presents an analysis of the particle impacts observed by the plasma wave instrument. From analysis of wideband antenna voltage waveforms a peak impact rate of 443 s^-1 was observed near the inbound equator crossing, which occurred at a radial distance of 3.45 R_N, and a peak impact rate of 151 s^-1 was observed near the outbound ring plane crossing, which occurred at a radial distance of 4.20 R_N. The inbound peak is offset by 146+/-4 km north of the equatorial plane, and the outbound peak is offset by 948+/-65 km south of the equatorial plane. A Gaussian fit of the impact rate profile has a width (to the e^-1 points) of 537+/-22 km for the inbound ``core'' component and 2,073+/-392 km for the outbound component. These impact rates corresponds to maximum number densities of a few times 10^-3 m^-3.

Analysis of the voltage amplitudes induced on the antenna indicates that the particles have masses ranging from 10^-10 g to a few times 10^-9 g, with an uncertainty of up to a factor of 10. Assuming a mass density of 1 g/cm³, these particles would have radii in the ranges 5-10 μm, with an uncertainity of about a factor of 2 to 3. In addition to the high impact rates observed near the two equator crossings, an impact rate of at least 0.6 s^-1 was observed over the entire region inside about 8 R_N, including the northern polar region. Likely sources for these particles include (1) several of the small satellites (1989N1, 1989N2, 1989N3, and 1989N4) discovered near Neptune by Voyager and (2) the rings. Electromagnetic forces may play an important role in diffusing the particles away from the equatorial plane.