“Chaotic” dart leaders in triggered lightning: Electric fields, X‐rays, and source locations

Abstract

We present the first published observations of “chaotic” dart leaders in triggered‐lightning discharges. We examine four leaders that exhibited “chaotic” electric field derivative (dE/dt) signatures in their final 10 to 12 μs. The dE/dt signatures were characterized by bursts exhibiting widths of the order of hundreds of nanoseconds on which were superimposed irregular pulses with widths of the order of tens of nanoseconds. These unique signatures were dissimilar from the dE/dt waveforms observed from dart or dart‐stepped leaders in triggered lightning. Three‐dimensional locations for dE/dt pulses that radiated from the bottom 200 m of the leader channels were determined, as were emission times. Vertical leader speeds for the four “chaotic” dart leaders were estimated to range from 2.0 to 4.3 × 107 m/s. A relatively continuous flux of energetic radiation (X‐rays and gamma rays) was observed during the final 10–13 μs of each “chaotic” dart leader. Some individual photons had energies more than 1 MeV. High‐speed video images of three “chaotic” dart leaders were obtained at a frame rate of 300 kilo‐frames per second (exposure time of 3.33 μs). One image, in the frame immediately prior to the return stroke, shows an upward positive leader 11.5 m in length propagating from the launch facility and a downward negative leader above with a streamer zone length of about 25 m. Channel base currents preceding the four “chaotic” dart leaders were of unusually long duration and large charge transfer, and return strokes following them had larger than usual peak currents.