Auroral excitation of optical emissions of atomic and molecular oxygen

Abstract

Rocket measurements of the O I ‘green line’ (¹S‐¹D) at 5577 Å and the O2 (0, 0) atmospheric band at 7620 Å were made in a steady IBC II+ aurora, simultaneously with measurements of N2 emissions and the auroral electron flux. An empirical model based on these data demonstrates that the principal excitation source of O2(b¹Σg+) is energy transfer from O(¹D), with direct electron impact of O2 contributing less than 5%. The altitude profile of the green line emission, taking into account quenching of O(¹S) by O(³P) near the lower border of the aurora, resembles what would be produced by electron impact excitation of O2 but would require a dissociative excitation cross section of 10−16 cm². None of the other known O(¹S) excitation mechanisms are capable of producing the observed emission rate.