Vibrational population of theA3Σu+andB3Πgstates of N2in normal auroras

Abstract

New electron impact excitation cross sections for the 6 lowest triplet states (A, B, W, C, E, D) of N2 have been used and the coupled equations of statistical equilibrium solved to obtain the vibrational population of each electronic state. The results show that cascade from high levels of the A3Σu+ state and from the W3Δu state is significant in populating the lower vibrational levels of the B state and hence the character of its ‘apparent’ excitation cross section. For the B state excited under auroral conditions, the fraction of the total population due to cascade processes exceeds 25% for all levels lower than 7 and is greater than 80% for B(υ′ = 0). For the A state under similar conditions, cascade from the B state contributes 50% or more of the total vibrational population for levels lower than 7, and 80% or more for levels below 4. For levels of the A state greater than 7, the A → B transitions depopulate the levels rapidly and indicate that the Vegard-Kaplan emissions from these higher levels will be weak or totally absent in normal auroras.