The Venus ultraviolet oxygen dayglow and aurora: Model comparison with observations

Abstract

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with the ultraviolet spectrometer (OUVS) on board the PV. For this purpose, we use a new electron transport model based on a Monte Carlo implementation of the Boltzmann equation and a multi-stream radiative transfer model to calculate the effects of multiple scattering on the intensity field of the 130.4-nm triplet. We show that the consideration of the enhancement of the emergent 130.4-nm to the 135.6-nm intensity by multiple scattering in the optically thick Venus atmosphere increases the auroral 130.4/135.6 ratio by a factor of about 3. We find agreement with the mean 130.4/135.6 ratio observed with PV-OUVS using the typical suprathermal electron energy spectrum reported from PV in situ measurements showing a characteristic energy of about 14 eV. To account for the average OI auroral emissions, the required precipitated energy flux is 2×10 −3 mW m −2, that is about 30% of the measured suprathermal night-side soft electron spectrum used as a reference. The calculated brightness of the CO Cameron bands is about twice as large as the weak observed emission, but within the error bars of the observations and the uncertainties of the dissociative excitation cross-section of CO 2. The electron transport model, coupled with calculations of excitation processes is also applied to an analysis of the FUV oxygen day airglow observations made with PV-OUVS and the Hopkins Ultraviolet Telescope (HUT) spectrograph. Comparisons indicate that the model accounts for both the disc-averaged intensities observed with the HUT spectrograph, the limb scans and the 130.4-nm images obtained with PV-OUVS. The relative contribution of resonance scattering of the solar line and photoelectron impact to the excitation of the 130.4-nm triplet depends on the altitude, but is globally dominated by resonance scattering. The intensity of the 130.4-nm dayglow emission does not vary proportionally with the O density in the lower thermosphere, but provides nevertheless a useful tool to remotely probe the atomic oxygen density and its variations.