LIGHTNING-CHANNEL MORPHOLOGY REVEALED BY RETURN-STROKE RADIATION-FIELD WAVE-FORMS

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/d t waveforms were subjectively classified according to their fine struc ture. Then the video images were examined field by field to identify e ach waveform with a visible channel to ground. Fifty-five correlated w aveforms and channel images were obtained. Of these, all 34 first-stro ke waveforms (multiple jagged E peaks, noisy dE/dt), 8 of which were n ot radiated by the chronologically first stroke in the hash, came from new channels to ground (not previously seen on video). Ah 18 subseque nt-stroke waveforms (smoothly rounded E and quiet dE/dt after the init ial peak) were radiated by old channels (illuminated by a previous str oke). Two double-ground waveforms (two distinct first-return-stroke pu lses separated by tens of microseconds or less) coincided with video f ields showing two new channels. One ''anomalous-stroke'' waveform (beg inning 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-fre quency fine structure. Fourier analysis shows that first-stroke wavefo rms 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 mu s after the main peak.