Charge moment change and lightning‐driven electric fields associated with negative sprites and halos

Abstract

Sprites are structured high altitude optical emissions produced by lightning‐driven electric fields. Both strong positive and negative cloud to ground flashes (CGs) are capable of initiating sprites. However, reported sprites are almost exclusively produced by +CGs. The very limited number of negative polarity sprites makes it difficult to reveal their morphologies and mechanisms. Since 2008, we have operated low light cameras at 5 locations in the United States to detect lightning‐driven transient luminous events (TLEs). At Duke University, two pairs of magnetic sensors simultaneously record lightning‐radiated magnetic fields. During 4 years of observations, the low light cameras collectively captured 1651 sprite events. Among them, 6 were produced by −CG lightning, which was confirmed by both the National Lightning Detection Network (NLDN) and magnetic field measurements. All of these negative sprites show similar features in their morphology, lightning source current, and lightning‐driven ambient electric fields. They all initiate within a few ms from their parent lightning discharges and always are accompanied by sprite halos. Compared to positive sprites, the downward streamers in negative sprites terminate at higher altitudes, about 55–60 km. The extracted source current of their parent lightning discharges is very impulsive and produces at least 450 C km charge moment change in 0.5 ms or less. Unlike most +CG strokes, essentially no continuing current follows these −CGs. Thus the uniformity of negative sprite morphology appears to reflect the uniformity of the characteristics of high charge transfer negative strokes. Numerical simulation shows these impulsive source currents produce very high (>2 Ek, where Ek is the local air breakdown field) but short‐lived electric fields at halo altitudes between 70 km and 90 km. At streamer termination altitudes, the inferred background electric field is 0.2–0.3 Ek, which is close to but below the critical field (0.4 Ek) for negative streamer propagation. The simulations also show that the timescale of the lightning source current has a significant impact on the high altitude electric fields and thus the sprite initiation and morphology. With the same amount of charge transfer, a more impulsive current source produces a higher electric field of shorter duration at halo and sprite altitudes. The short timescales may explain why a larger threshold of total charge moment change is required to initiate negative sprites than short‐delayed positive sprites, whose parent lightning source current is usually less impulsive.