Triggered Lightning Return Stroke Luminosity up to 1 km in Two Optical Bands

Abstract

AbstractLuminosity waveforms measured using two types of avalanche photodiodes (APDs) are presented as a function of time and channel height for 15 triggered‐lightning return strokes. Short vertical sections of 20 channel heights ranging from 0 to 1 km were observed by both types of APDs. For APD type I, the return stroke luminosity waveforms decay faster following a single initial peak (IP) than the waveforms recorded by APD type II, which often exhibit a second maximum (SM) following the IP, though the risetimes of the initial luminosity wavefront preceding the IP are similar for both types of APDs. APD type II responds better to longer wavelengths than APD type I, and since the SM occurs about 10 μs after the IP at the channel‐bottom and about 20 μs after the IP at 1‐km height, the SM is likely a consequence of spectral lines excited during the cooling of the channel, following the initial high‐temperature/pressure stage. The initial optical radiation during the return stroke is likely dominated by ionized atomic species radiated at higher temperatures (NII lines between 450 and 600 nm) better captured by APD type I, while the later optical radiation is likely due to neutral atomic species radiated at lower temperatures (e.g., H‐alpha at 656.3 nm, OI at 715.7 nm and 777.4 nm, and the NI at 744.4 nm) better captured by APD type II. The average IP upward speed is 1.3e8 m/s, while the average SM upward speed is 3.1e7 m/s.