C and C + in the Venusian thermosphere/ionosphere

We have constructed standard low and high solar activity models of the Venus thermosphere, which take into account revised rate coefficients for production and loss processes for C and C + , high‐resolution cross sections for photodissociation of CO, and recent solar fluxes from the Solar 2000 v1.24 and v2.22 models of Tobiska [2004]. Among the most important changes is the inclusion of the branching ratio for the channel of dissociative recombination of CO 2 + that produces C + O 2 , which has been measured recently by Seiersen et al. [2003]. We find that unlike Mars, where the production of C is dominated by dissociative recombination of CO 2 + , photodissociation of CO is the most important source of C in the Venus thermosphere, as previous models have shown. The loss of C is dominated by reaction with O 2 for molecular oxygen mixing ratios greater than 1 × 10 −4 . We also construct here a model that is appropriate to the first year of the Pioneer Venus mission, when the solar activity was moderately high. We vary the O 2 mixing ratio at 90 km from 1 × 10 −4 to 1 × 10 −2 , and we predict the resulting C density profiles. By comparing these profiles to that derived from the Pioneer Venus Orbiter Ultraviolet Spectrometer limb profiles of the 1561 and 1657 Å resonance lines, we derive a “best fit” value of the O 2 abundance, which is determined to be slightly larger than 3 × 10 −4 . We construct model density profiles of C + for four values of the the rate coefficient for the charge transfer reaction O + + C → C + + O from 1 × 10 −11 to 3 × 10 −10 cm 3 s −1 . We then compare the moderately high solar activity model C + profiles for a solar zenith angle of 25° to that obtained by the Pioneer Venus Orbiter Ion Mass Spectrometer for orbit 200, for which the value of F 10.7 ∼ 200. We find that the “best fit” rate coefficient for the charge transfer reaction is in the range (0.9–1.3) × 10 −10 cm 3 s −1 .