High rate sampling detectors measuring the potential difference between the main body and the boom antennas of interplanetary spacecraft have been shown to be an efficient means to detect impacts of small dust grains in the nanometer size range (see Meyer-Vernet et al., Solar Physics, 256, 463-474, 2009 and Zaslavsky et al., J. Geophys. Res., 117, 05102, 2012). Rough estimates of the free charge Q in the post impact generated plasma cloud indicate that the cloud's own internal electrostatic field is far too weak to rise the antenna's potential by the observed values. We solve this issue by showing that the cloud's internal field is nevertheless strong enough to transitionally interrupt the photoelectron return current towards the portion of the antenna finding itself within the a critical distance RC ∝ Q~(1/3) from the impact point. The antenna then steadily increases its positive charge (and its electrostatic potential with respect to the satellite's main body) during a time interval of the order of the inverse of the photoelectron plasma frequency. In previous works we interpreted R_C as the final radius of the expanding cloud. Here we propose that RC is the distance beyond which the cloud's field is efficiently screened by the ambient electrons.
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