CO+ and N2+ in the Venus ionosphere

Abstract

The photochemistry of CO+ and N2+ in the Venus ionosphere is investigated. Analytic expressions for the distribution of O+ (²D), O+ (²P), CO+, and N2+ in the Venus thermosphere are reported. These expressions reproduce the results obtained from the solution of the coupled mass‐continuity equations to within a few percent from 110 km to 195 km. Predicted CO+ and N2+ densities are compared to Pioneer Venus Orbiter ion mass spectrometer (PVOIMS) data. Changes in model atmosphere or solar flux are not sufficient to bring the observations and theory into agreement using the photochemistry of Fox (1982a, b). Use of the proper CO+ dissociative recombination coefficient (Mitchell and Hus, 1985) together with metastable O+ chemistry (Fox, 1982b) removes much of the previous discrepancy (e.g., Nagy et al. 1980) between the mass 28 observations and the calculated CO+ + N2+ density. The Hedin et al. (1983) model atmosphere formulation is supported by this study, though it is apparent that in situ measurements of the mass 28 ion density are not entirely adequate to the task of ruling out specific model atmospheres. Calculated limb and disk intensities for the CO+ (B²Σ+−X²Σ+) first negative, CO+ (AΠ−X²Σ+) comet tail bands, and N2+ (B²Σu+−X²Σg+) first negative bands are reported. Rayleigh and cloud scattering precludes use of a nadir‐viewing geometry at low (Δλ/λ > 10−3) spectral resolution; however, limb imaging of these bright ( > 10 kR) bands from low (500–150 km) orbit would enable future emissions to derive the CO+ and N2+ density profiles as well as CO/CO2 and N2/CO2 mixing ratios in the thermosphere.