Deuterium on Venus: Model comparisons with pioneer Venus observations of the predawn bulge ionosphere

A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D⁺ (and not H₂⁺). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (～ 130 km) for the two orbits. The H₂⁺ contribution to the mass-2 ion density is less than 10%, and the H₂ mixing ratio must be <0.1 ppm at 130 km altitude. The He⁺ data require a downward He⁺ flux of ～2 × 10⁷ cm^?2 sec^?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O⁺ ion flow.