Open Magnetic Flux and Magnetic Reconnection During Steady Magnetic Convection Intervals

The Cassini spacecraft was launched on 15 October 1997. On its long journey to Saturn, the spacecraft took a gravitational assist to gain energy from Venus on 24 June 1999. The UVIS instrument on board Cassini obtained a series of FUV spectra during this flyby, while the solar F10.7 index was larger than 200 at Earth orbit. The UVIS instrument obtained ultraviolet spectra of Venus with a still unmatched spectral resolution of ~0.37 nm. We developed a least squares fit method able to extract the intensity of most of the emissions present in the instrument passband, that has already been used to quantify the CO Fourth Positive spectral system and oxygen emissions, among others.
We identify several emissions of the carbon atom in the UVIS spectra, and estimate their intensity. In particular, we extract the intensity of the CI – 126.1, 156.1 and 165.7 nm multiplets, for every dayglow spectra recorded by UVIS along the Cassini track. These intensities are compared with a full radiative transfer modeling of these emissions, that includes the known photochemical sources of photons and resonant scattering of sunlight. The carbon density profile of the Venus thermosphere has never been directly measured and is taken from a photochemical model. We find a serious disagreement between observation and modeling that can be accounted for by applying a correction factor to the carbon column. This needed correction factor is found to increase monotonically as the solar zenith angle decreases, suggesting a possible photochemical origin to the disagreement, possibly involving the photochemistry of molecular oxygen to which the carbon density is highly sensitive.