Dust Impacts Detected by Voyager-2 at Saturn and Uranus: A Post-Halley View

A new approach to the Voyager-2 dust impact observations near the ring plane of Saturn and Uranus is proposed in the paper, based on the experience from analyses of simultaneous dust and electric field observations by Vega-2 at Halley. Taking into account the impact geometry and the ambient plasma parameters, the possible responses of the two instruments, PRA and PWS, utilizing the same Voyager antenna, are evaluated as functions of the impact-induced charge. It is shown that the PRA instrument, which used the antenna elements as monopoles, responded mostly to pulses of the spacecraft potential, while the PWS instrument, working in dipole configuration, responded mostly to charge-separation electric fields. Due to the negative floating potential during both ring plane crossings the effect of charging the antenna was weak. The dust mass spectra near both ring planes are derived from the apparent impact rates and the V_rms voltages, observed simultaneously by the PWS instrument. At Saturn's ring plane at 2.86 R_s the obtained peak number density of particles bigger than 2 × 10_-7 g is 3.7 × 10^-3/m³, while the integral mass spectrum index α is about 1.5 at this mass magnitude and decreases toward lower masses down to values less than 1. In the ring plane region of Uranus at 4.51 R_U the maximum number density for the limiting mass of 3.5 × 10^-10g is found to be 4.4 × 10^-4/m³, while the index α at this mass is about 1.