Differences Between Far Electric Field Waveforms of Triggered Return Strokes and Natural Return Strokes From Nine Thunderstorms

The far electric field waveforms of natural first return strokes (FRSs), natural subsequent return strokes (SRSs), and triggered return strokes (TRSs) from nine thunderstorms measured by the same device were compared. The results were as follows: The far electric field peaks of TRSs were 31% smaller than that of SRSs. The geometric means of 10-to-90% risetime and half-peak width of SRS far electric fields are 2.4 μs and 5.4 μs, respectively, while those of TRSs are significantly smaller, i.e., 1.5 μs and 3.2 μs, respectively. Three possible reasons cause the differences between the SRS and TRS far electric field waveforms: TRS and SRS occur in different cloud conditions; triggering experiments are more likely to be successful when the thunderstorm electric field is in a stable period; the rise of the TRS current is steeper. Two possible reasons cause the steeper rise in TRS current: the lower part of the TRS channel is more conductive; the lower part of the TRS channel is shorter than that of the SRS channel.