Lightning‐channel morphology revealed by return‐stroke radiation field waveforms

Simultaneous video and wideband electric field recordings of 32 cloud‐to‐ground lightning flashes in Florida were analyzed to show that the formation of new channels to ground can be detected by examination of the return‐stroke radiation fields alone. The return‐stroke E and dE/dt waveforms were subjectively classified according to their fine structure. Then the video images were examined field by field to identify each waveform with a visible channel to ground. Fifty‐five correlated waveforms and channel images were obtained. Of these, all 34 first‐stroke waveforms (multiple jagged E peaks, noisy dE/dt), 8 of which were not radiated by the chronologically first stroke in the flash, came from new channels to ground (not previously seen on video). All 18 subsequent‐stroke waveforms (smoothly rounded E and quiet dE/dt after the initial peak) were radiated by old channels (illuminated by a previous stroke). Two double‐ground waveforms (two distinct first‐return‐stroke pulses separated by tens of microseconds or less) coincided with video fields showing two new channels. One “anomalous‐stroke” waveform (beginning like a first stroke and ending like a subsequent) was produced by a new channel segment to ground branching off an old channel. This waveform classification depends on the presence or absence of high‐frequency fine structure. Fourier analysis shows that first‐stroke waveforms contain about 18 dB more spectral power in the frequency interval from 500 kHz to at least 7 MHz than subsequent‐stroke waveforms for at least 13 μs after the main peak.