Cometary plasma boundaries

The solar wind starts to interact with comets at distances from the nucleus of several million kilometers. The nature of the interaction changes as a function of cometocentric distance. Several dynamically important boundaries have been observed in the cometary plasma environment by instruments on several spacecraft: (1) the bow shock marks the transition from supersonic to subsonic solar wind flow, (2) the cometopause was observed at a distance of about 100,000 km from Comet Halley where the flow begins to stagnate and where charge exchange with neutrals becomes important, (3) the diamagnetic cavity boundary (i.e., contact surface, ionopause) separates magnetized and unmagnetized cometary plasma, (4) the magnetotail boundary defines the tail lobes, (5) the plasma sheet boundary defines the extent of the plasma sheet, and (6) the density enhancement layer was observed at a distance of 10,000 km from Comet Halley and might be located where the neutrals and plasma thermally decouple.