Venus' ultraviolet airglow and aurora: Monte Carlo simulations and comparison with observations

The Venus airglow has been observed from spectrometers on board rockets probes and satellites such as OUVS on Pioneer Venus Venera Galileo HUT on the Space Shuttle and quite recently SPICAV on Venus Express The spectrum is dominated by emissions from helium hydrogen oxygen and carbon lines and CO bands Localized emissions of OI at 1304 and 1356 A have been sporadically observed on the nightside and are likely caused by precipitation of auroral electrons in the wake of the planet We have developed a Monte Carlo code solving the Boltzmann equation for energetic electrons to calculate the energy distribution function and fluxes of primary and secondary auroral electrons and for photoelectrons The model is used to calculate the vertical distribution of the excitation rate of various excited states For optically thick transitions such as the 3P-3S triplet at 1304 A a radiative transfert code is used to calculate the emergent emission rate We find that the relative intensity of the oxygen and CO Cameron band emissions is a sensitive indicator of the energy of auroral electrons The observed values indicate that the mean energy is on the order of 10-50 eV Dayglow intensity and distributions are also compared with observed characteristics