ORIGIN OF LIGHTNING ELECTRIC-FIELD SIGNATURES SHOWING 2 RETURN-STROKE WAVE-FORMS SEPARATED IN TIME BY A MILLISECOND OR LESS

Based on simultaneous single-station electric field and multiple-station TV records of 76 negative cloud-to-ground flashes in Florida, we have examined the relation between (1) the electric field waveforms characteristic of return strokes but separated by the relatively short time interval of typically some tens to some hundreds of microseconds (the extreme values were 15 mus and 3.3 ms) and (2) the number of TV-observed channel terminations on ground. Double field waveforms (15 total) were observed in about 20 % of all flashes analyzed. Nine of the 15 double field waveforms were associated with lightning channels having double terminations on ground. The remaining six double field waveforms were associated with channels showing single ground attachments. The latter observation suggests that double field waveforms can be due to two return strokes, each initiated by its own leader process, occurring in the same channel within a millisecond or less of each other. Such short interstroke intervals imply that the minimum time for the lightning channel to decay to the point that a new leader-return stroke sequence can occur is significantly shorter than previously thought.