Cometary H₂⁺ and solar wind He²⁺ dynamics across the Halley cometopause

Two Mass/Charge (M/Q) ≈ 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 He²⁺. The second, detectable only within ∼6 × 10⁵ 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”.