The importance of monopole antennas for dust observations: why Wind/WAVES does not detect nanodust

The charge released by impact ionization of fast dust grains impinging on spacecraft is at the basis of a well-known technique for dust detection by wave instruments. Since most of the impact charges are recollected by the spacecraft, monopole antennas generally detect a much greater signal than dipoles. This is illustrated by comparing dust signals in monopole and dipole mode on different spacecraft and environments. It explains the weak sensitivity of Wind/WAVES dipole antennas for dust detection, so that it is not surprising that this instrument did not detect the interplanetary nanodust discovered by STEREO/WAVES. We propose an interpretation of the Wind dust data, elsewhere discussed by Malaspina et al. (2014), which explains the observed pulse amplitude and polarity for interstellar dust impacts, as well as the non-detection of nanodust. This proposed mechanism might be the dominant dust detection mechanism by some wave instruments using dipole antennas.