Multiple ion species fluid modeling of sprite halos and the role of electron detachment of O − in their dynamics

A two‐dimensional fluid model with multiple charged species is developed for simulating the ionospheric responses to the lightning quasi‐static electric (QE) field. In addition to electrons, the model takes into account four symbolic ion species and O − ions that are considered separately to investigate the role of the fast electron detachment process from O − in the ionospheric response. The modeling results of a sprite halo driven by positive cloud‐to‐ground lightning indicate that the halo can descend to lower altitude with much higher electron density behind its front when the O − detachment process is included. Electron density ahead of the halo front is not significantly reduced from the ambient value, so there is no attachment “hole” forming in that region that is commonly observed in previous modeling studies. The fast O − detachment process affects the dynamics of the halo by allowing the growth of electron density in the upper atmosphere under sub‐breakdown condition, i.e., the electric field is smaller than the conventional breakdown threshold field. The implications of the results reported here to sprite streamer initiation include (1) low ambient electron density at sprite initiation altitude may be the only means to avoid avalanche overlapping before the avalanche‐to‐streamer transition; and (2) the large downward extent of the halo may offer an explanation for the initiation of sprites at the altitude as low as 65–70 km, which was observed in previous studies.