Observations of a shock and a recombination layer at the contact surface of comet Halley

Abstract

Observations in the vicinity of the contact surface (ionopause) of comet Halley obtained by the Giotto ion mass spectrometer (IMS) are reported. Two specific events in this region were observed on the inbound pass. Two seconds before the contact surface was encountered, a burst of energized ions (about 20 eV, much greater than thermal energies) was detected by the angle analyzer; the flux of these ions decreased as the contact surface was approached. The burst of energized ions coincided with a pulse in magnetic field strength interpreted by Neubauer (1988) as a fast mode shock traveling away from the contact surface. At the contact surface, a sharp spike in ion densities was observed by the mass analyzer. This pileup region was at least 0.75 s in duration (about 47 km in width); there may also have been a region of less enhanced densities extending inward another 47 km. The spike in densities was centered approximately at the inner edge of the magnetically determined contact surface. The exact magnitude of the density increase is uncertain due to instrumental limitations, but the increase above the ambient external density appears to have been at least a factor of 3.5, and appears to have been more than an order of magnitude for some species. The sharp spike in ion density is interpreted as a boundary layer into which the radial ionospheric flow enters, piles up, and in which the density increase is limited by recombination.