Impacts of dust particles m > 10 −9 g in Halley's coma as seen in the electric field waveforms of Vega 2

Abstract

In this paper we draw some inferences on the response of the APV-N antenna of Vega 2 to plasma clouds induced by dust particles. We show that: (1) electric impulses were mostly generated by impacts of particles bigger than ∼10 −9 g on the whole spacecraft, (2) in the case of large particle impacts the impulse amplitude was weakly dependent on the particle mass, (3) the antenna response to the impact plasma depends on the ambient plasma parameters, and (4) the sensitivity to impacts decreases when the impact rate becomes quasicontinuous. The conclusions are based on an analysis of the wide impulses in the outbound pass of Vega 2 through the outer coma and on a comparison of the electric field spectral densities with the count rates of dust particles in various mass channels. The lower limit, down to which the amplitude vs mass dependence is weak, has been defined as the “least effective mass”; its value has been deduced to be ∼3 × 10 −9 g in the outer coma, and ∼10 −7 g in the inner coma, while near the point of closest approach to the comet it was determined to be more than ∼10 −6 g due to the continuous impact of small particles. The spectral density of impact-generated impulses should be roughly proportional to the integral flux of particles bigger than the least effective mass. Some new peculiarities of the large particle distribution in the coma are derived. The interpretation of the impulse generation involves the response of the antenna to a negative pulse of the spacecraft potential and to electric fields associated with the electron density gradients in the expanding plasma cloud. The correlation between the count rates of the light ( m < 10 −14 g) particles and of the impulses (found in a previous investigation) is reinterpreted in terms of the hypothesis that the light particles were originated by disintegration of the big ones.