Voltage Pulses on STEREO/WAVES: Nanoparticles Picked-up by the Solar Wind?

We suggest that the very large number of intense voltage pulses detected by the STEREO/WAVES instrument are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of several hundreds of km/s. Nanoparticles have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce voltage pulses as high as do larger grains of smaller speeds. The flux of nanoparticles inferred in this way is similar to that recently detected on the International Space Station, and it is greater by one order of magnitude than the interplanetary dust flux model but the uncertainties make it marginally compatible. The present results may represent the first detection of fast nanoparticles in interplanetary space near 1 AU.