Multiple pulses in dE/dt and the fine-structure of E during the onset of first return strokes in cloud-to-ocean lightning

We have analyzed the fine-structure of 131 electric field (E) waveforms that were radiated during the onset of first return strokes in cloud-to-ocean lightning. The dE/dt waveforms were recorded using an 8-bit waveform digitizer sampling at 100 MHz, and the E waveforms were sampled at 10 MHz using a 10-bit digitizer. 49 (or 37%) of the dE/dt waveforms contain one or more large pulses within ± 1 μs of the largest (or dominant) peak in dE/dt, i.e. within an interval from − 1 μs to + 1 μs, where t = 0 μs is the time of the dominant peak, and 37 (or 28%) have one or more large pulses in the interval from 4 μs before to 1 μs before the dominant peak, i.e. − 4 μs to − 1 μs, and only the dominant peak within ± 1 μs . We give statistics on the amplitude and timing of dE/dt pulses that are near the dominant peak, and we show how the presence of these pulses adds considerable fine-structure to the shape of Eint, the integrated dE/dt waveform, on a time-scale of tens to hundreds of nanoseconds. This fine-structure includes fast pulses near the beginning of the slow front, large pulses and shoulders within the slow front and during the fast-transition, and very narrow peaks in Eint. Our overall conclusion is that the electromagnetic environment near the point(s) where lightning leaders attach to the surface is often more complicated than what would be produced by a single current pulse propagating up a single channel at the time of onset.