Investigation of potential mechanisms responsible for green emissions observed at sprite tops

Sprites are predominantly red color electrical discharges that occur at mesospheric/lower ionospheric altitudes in response to lightning activity in the troposphere [e.g., Stenbaek‐Nielsen et al., GRL, 52, e2024GL112537, 2025, and references therein]. It has been recently discovered that some sprites occasionally exhibit enigmatic green emissions at their tops, referred to as Green emissions from excited Oxygen in Sprite Tops (GhOSTs) [e.g., Lyons, Weatherwise, 75:6, 14-23, 2022, and discussion therein]. Since their discovery by citizen scientists in 2019, green ghosts have received attention from the research community and general public capturing images of the elusive events. This phenomenon has gained interest for its characteristic green color (most likely produced due to the O(1S→1D) transition, that manifests as the 557.7 nm green line emission) [Passas-Varo et al., Nat. Commun., 14, 7810, 2023], long duration [Stenbaek-Nielsen et al., Authorea, 10.1002/essoar.10504953.1, 2020], and reignitions corresponding to secondary sprite occurrences elsewhere [Huang et al., GRL, 51, e2024GL108397, 2024]. Previous studies have examined the possible role of some electron impact excitation processes in the generation of green emissions [Celestin et al., Abstract AE33A-2839, AGU Fall Meeting, San Francisco, CA, 11-15 Dec., 2023]. Electron impact excitation of atomic oxygen (e + O → O + O(1S)), electron impact dissociative excitation of molecular oxygen (e + O2 → e + O + O(1S)), and energy transfer (N2(A3) + O → N2 + O(1S)) all serve as potential viable candidates [Vallance Jones, Aurora, 142-143, 1974; Itikawa and Ichimura, J. Phys. Chem. Ref. Data, 19, 637–651, 1990]. In this work we compare the contribution of the above mechanisms to green emissions from O(1S) under the application of a lightning-induced electric field to the lower ionosphere.