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 d E /d t 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 d E /d t waveforms contain one or more large pulses within ± 1 μs of the largest (or dominant) peak in d E /d t , 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 d E /d t pulses that are near the dominant peak, and we show how the presence of these pulses adds considerable fine-structure to the shape of E int , the integrated d E /d t 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 E int . 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.
