Dayglow and nightglow of the upper atmosphere of Venus
Résumé
Aims. We present the modelling of the production of excited states of O, CO and CO2 in the Venusian upper atmosphere, which allows to compute the nightglow emissions. On the dayside, several emissions are computed, taking advantage of the small influence of resonant scattering for forbidden transitions. Methods. Photoionisation, photodissociation mechanisms, electron impact excitation and ionisation are computed by a multi-stream stationary kinetic transport code called TransVenus [1, 2]. Results. Altitude emission profiles for the transitions from O(1S) and O(1D) states, at the origin of the green and red lines, are computed. They are found to be very comparable to the observations. On the nightside, the origin of the main green line production mechanism remains an important unknown and we show that both the Barth (three body, mesospheric) or the Frederick/Kopp processes (two body, ionospheric) can account for the measured intensities recorded by Slanger [3]. We also calculate the production intensities of O(3S) and O(5S) states, which are needed for radiative transfer models. Concerning the CO molecule, we compute the emissions of the Cameron bands and the fourth positive bands. All these values are successfully compared to the experiment when data are available. Conclusions. A comprehensive model is proposed to compute dayglow and nightglow emissions of the Venusian upper atmosphere. It relies on previous studies with noticeable improvements, both on the transport and on the chemical aspects. Observations and simulations are in good agreement and can prepare for comparisons with SPICAV onboard Venus Express. This study shows also that fundamental uncertainties remain concerning the origin of the green line nightside production. Deciding which process is at its origin will require new lab and satellite experiments.