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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 CO2+ that produces C + O2, 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 CO2+, 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 O2 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 O2 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 O2 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 cm3 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 F10.7 ~ 200. We find that the ``best fit'' rate coefficient for the charge transfer reaction is in the range (0.9-1.3) × 10-10 cm3 s-1.


An edited version of this paper was published by AGU. Copyright © 2005, American Geophysical Union.



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