Green Emissions of Atomic Oxygen at Sprite Tops Explained

Green emissions from excited Oxygen in Sprite Tops (GhOSTs) are due to the 557 nm photons emitted from atomic oxygen (O) excited to the 1S state. In this work we compare the possible contribution of various mechanisms for excitation of O(1S) under application of a lightning-induced electric field to the lower ionosphere. The mechanisms considered are informed by O(1S) studies in the context of airglow and electrical gas discharges. We find that energy transfer from molecular nitrogen (N2) in the A3Σu+ state to ambient atomic oxygen is the dominant source of O(1S) above ~84 km altitude, while photolysis of molecular oxygen (O2) due to extreme ultraviolet photons emitted from N2 singlet states dominates at altitudes below 84 km. The most favorable conditions for the observability of green ghosts are summarized.