Cometary H2(+) and solar wind He(2+) dynamics across the Halley cometopause

Two Mass/Charge (M/Q) of about 2 distributions were observed by the Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) during the encounter with comet Halley. The first, present throughout the encounter, is identified as solar wind He(2+). The second, detectable only within about 600,000 km of the nucleus, is identified as H2(+) (produced primarily by dissociation and ionization of cometary H2O). When these two distributions are separated, distinct differences in their dynamics are apparent, particularly in the density and velocity profiles near the 'cometopause'.