Physics
Scientific paper
Jan 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005jgra..11001311f&link_type=abstract
Journal of Geophysical Research, Volume 110, Issue A1, CiteID A01311
Physics
4
Planetary Sciences: Solid Surface Planets: Ionospheres (2459), Planetary Sciences: Solid Surface Planets: Atmospheres (0343, 1060), Planetary Sciences: Solid Surface Planets: Remote Sensing, Atmospheric Composition And Structure: Thermosphere: Composition And Chemistry
Scientific paper
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.
Fox Lewis J.
Paxton Larry J.
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