Luminosity of initial breakdown in lightning

Abstract

Time correlated high‐speed video and electromagnetic data for 15 cloud‐to‐ground and intracloud lightning flashes reveal bursts of light, bright enough to be seen through intervening cloud, during the initial breakdown (IB) stage and within the first 3 ms after flash initiation. Each sudden increase in luminosity is coincident with a CG type (12 cases) or an IC type (3 cases) IB pulse in fast electric field change records. The E‐change data for 217 flashes indicate that all CG and IC flashes have IB pulses. The luminosity bursts of 14 negative CG flashes occur 11–340 ms before the first return stroke, at altitudes of 4–8 km, and at 4–41 km range from the camera. In seven cases, linear segments visibly advance away from the first light burst for 55–200 µs, then the entire length dims, then the luminosity sequence repeats along the same path. These visible initial leaders or streamers lengthen intermittently to about 300–1500 m. Their estimated 2‐D speeds are 4–18 × 105 m s−1 over the first few hundred microseconds and decrease by about 50% over the first 2 ms. In other cases, only a bright spot or a broad area of diffuse light, presumably scattered by intervening cloud, is visible. The bright area grows larger over 20–60 µs before the luminosity fades in about 100 µs, then this sequence may repeat several times. In several flashes, a 1–2 ms period of little or no luminosity and small E‐change is observed following the IB stage prior to stepped leader development.