C and C+ in the Venusian thermosphere/ionosphere -: art. no. A01311

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 + 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 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 O-2 for molecular oxygen mixing ratios greater than 1 x 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 x 10(-4) to 1 x 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 Angstrom resonance lines, we derive a "best fit" value of the O-2 abundance, which is determined to be slightly larger than 3 x 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 x 10(-1)1 to 3 x 10(-1)0 cm(3) s(-1). We then compare the moderately high solar activity model C+ profiles for a solar zenith angle of 25degrees to that obtained by the Pioneer Venus Orbiter Ion Mass Spectrometer for orbit 200, for which the value of F-10.7 similar to 200. We find that the "best fit" rate coefficient for the charge transfer reaction is in the range (0.9-1.3) x 10(-10) cm(3) s(-1).