Terrestrial gamma ray flash production by active lightning leader channels

Abstract

The production of terrestrial gamma ray flashes (TGFs) requires a seed energetic electron source and a strong electric field. Lightning leaders naturally provide seed electrons by cold runaway and strong electric fields by charge accumulation on the channel. We model possible TGF production in such fields by simulating the charges and currents on the channel. The resulting electric fields then drive simulations of runaway relativistic electron avalanche and photon emission. Photon spectra and directional distributions produced by the model agree qualitatively with observations. Simulations with a variety of initial conditions indicate sufficient electric fields are produced if an unbranched channel supports a current pulse of at least 100 kA such as occurs if the channel is at least 1 km long and embedded in an ambient electric field of at least 100 kV m(-1). The mechanism does not strongly depend on altitude as friction and characteristic electric field strengths scale similarly. Seed particle production is not directly simulated, but estimates of seed production rates suggest current pulse activity of similar to 1 ms duration can account for TGF-scale emission.