A Giotto/cometary dust interaction event near the diamagnetic cavity boundary

Abstract

A new scenario is suggested in order to explain the burst of particles observed by Giotto near the Halley ionopause. According to this scenario, an impact of a cometary dust particle against the Giotto spacecraft (identified by Neubauer et al. (1990)) creates a gas/plasma cloud. Cold cometary ions with a velocity of 70 km/s that impinge on the cloud around the spacecraft are scattered by the secondary ions/neutral species in the cloud. This scattering is shown to allow the cometary ions to enter the angular detectors. This mechanism casts some doubt on alternative explanations of the observed phenomenon and thereby may call into question the very existence of suprathermal particles near the Halley ionopause.

Magnetic effect due to hypervelocity dust particle impact on the Giotto spacecraft

“…They are generally consistent with dust impact events derived from spacecraft attitude perturbations by the Giotto camera [Curdt and Keller, 1990]. Whereas the plasma effects of one of these dust impacts were discussed in a previous paper/Israelerich and Ershkovich, 1994], this extended analysis focuses on the magnetic effects of the same event.…”

“…So the burst of ion fluxes may be explained either by a presence of suprathermal ions during the event, as was suggested by Goldstein et al [1989], or as a result of dust particle impact. We proposed that cometary ions, observed by the angle analyzers, were scattered by secondary ions and neutral atoms of the cloud [Israelevich and Ershkovich, 1994]. The angular distribution of the ions corresponded to the Rutherford formula for scattering, and it was shown that for scattering angles under consideration (and hence for corresponding impact parameters) there was no substantial difference between scattering by ions and by neutral atoms.…”

“…This fact enabled us to determine the quantity NL, where N is the number density of the particles in the cloud and L is the scattering length (i.e., the characteristic size of the cloud). This quantity equalled 3.1015 cm -2 at the moment of maximum scattered flux [Israelerich and Ershkovich, 1994]. Temporal dependence of NL is shown in Figure I [1992] have developed a theoretical model for such a scenario.…”