Electron-driven excitation of O2 under night-time auroral conditions: Excited state densities and band emissions

Electron impact excitation of vibrational levels in the ground electronic state and seven excited electronic states in O₂ have been simulated for an International Brightness Coefficient-Category 2+ (IBC II+) night-time aurora, in order to predict O₂ excited state number densities and volume emission rates (VERs). These number densities and VERs are determined as a function of altitude (in the range 80-350 km) in the present study. Recent electron impact excitation cross-sections for O₂ were combined with appropriate altitude dependent IBC II+ auroral secondary electron distributions and the vibrational populations of the eight O₂ electronic states were determined under conditions of statistical equilibrium. Pre-dissociation, atmospheric chemistry involving atomic and molecular oxygen, radiative decay and quenching of excited states were included in this study. This model predicts relatively high number densities for the metastable electronic states and could represent a significant source of stored energy in O₂* for subsequent thermospheric chemical reactions. Particular attention is directed towards the emission intensities of the infrared (IR) atmospheric (1.27 μm), Atmospheric (0.76 μm) and the atomic oxygen ¹S→¹D transition (5577 Å) lines and the role of electron-driven processes in their origin. Aircraft, rocket and satellite observations have shown both the IR atmospheric and Atmospheric lines are dramatically enhanced under auroral conditions and, where possible, we compare our results to these measurements. Our calculated 5577 Å intensity is found to be in good agreement with values independently measured for a medium strength IBC II+ aurora.