Plasma boundaries at comet Halley

The properties of the multispecies plasma flow consisting of electrons, solar wind protons and cometary ions are studied by a multifluid approach in which both stationary and non-stationary calculations are combined with a dispersion analysis of the multifluid waves which arise due to the electromagnetic coupling between both ion species. In contrast to earlier studies, finite cometary ion temperatures due to pickup heating are taken into account, leading to an additional resonance at the point where the flow velocity becomes equal to the ion sound velocity. This resonance point marks the first plasma boundary where a transition from a stationary to a non-stationary flow regime takes place. For comet Halley conditions this plasma boundary is located approximately at that distance from the nucleus where the cometary bow shock has been identified experimentally. The non-stationary region has an extension of about 3 x 105 km and, according to the dispersion analysis, belongs to that range of parameters for which no stationary multifluid waves exist. The non-stationary region with turbulent properties is followed again by a stationary flow which is characterized by spatially periodic perturbations of all plasma parameters. The transition layer between both regimes is very sharp (≅ 104 km) and it represents the second plasma boundary. Finally, another non-stationary region begins near to the point where the flow velocity becomes equal to the proton Alfven velocity. The corresponding (third) plasma boundary is suggested to be the cometopause, in accordance with own earlier interpretations. The results of the improved multifluid model seem to be in good agreement with the VEGA/GIOTTO plasma measurements in which several plasma boundaries have been detected. Cometary shocks in the traditional sense do not exist