Simultaneous observations of quasistatic electric fields and large dust particles during the Vega-2 flyby of comet Halley

Abstract

The quasistatic electric field records made in Halley's coma by the APV-V experiment represented a superposition of the overall negative variation with a number of positive spikes. A correlation has been found between the overall variation and the count rates of dust particles in the mass range 10 −11–10 −8 g. For interpretation purposes the impact-induced plasma cloud radius has been specifically defined. Owing to the asymmetry of the double-probe antenna with respect to the impact direction, the particles producing plasma clouds with radii greater than several metres induced impulsive negative potentials of different magnitudes on the two probes, thereby generating an apparent electric field. In the inner coma, the electron emission generated by impacts of cometary gas molecules maintained the sensitivity of the antenna to dust impacts, but implied that the lower mass limit of particles responsible for the apparent electric field varied proportionally with the secondary electron current. The linear dependence of the quasistatic signal on the particle flux breaks down for high fluxes ; hence, the positive spikes are interpreted as manifestations of smallscale, dense clouds of dust. The present study has shown that a long double-probe antenna can be used as a large dust particle detector, whose target area is comparable to the spacecraft projected area.