Initial Breakdown (IB) pulses in lightning flashes have been observed since 1937. Using two sensor arrays to determine the location and amplitude of IB pulses, this study examines the largest IB pulse in 55 flashes with 23 pulses from intracloud (IC) flashes and 32 pulses from cloud-to-ground (CG) flashes. The two goals of this project were (1) to compare the range-normalized amplitude of the largest IB pulse to its altitude, duration, and timing within each flash, and (2) to characterize the shapes of the IB pulses. Since all lightning flashes seem to begin with IB pulses, this study is aimed at providing new information about how lightning flashes initiate. We looked for differences between the initiation of CG and IC flashes, and the results had conflicting indications. The largest IB pulse in IC flashes had an altitude dependence, with pulses at higher altitudes having a smaller amplitude, while CG pulses did not have an altitude dependence. The durations of the pulses in this study were also different, with IC pulses almost twice as long on average as CG pulses. Although these parameters differed between flash types, histograms of range-normalized pulse-amplitudes were very similar for CG and IC flashes. The time from the beginning of the flash to the largest IB pulse (Δt) indicated that 76% of IB pulses occurred within the first 2 ms, that all large pulses (>2.5 V/m) occurred within 2 ms, and that pulses occurring after 3 ms were exclusively smaller pulses (<1.5 V/m). In the analysis of IB pulse shapes, five IB pulse types are described, indicating that pulse shape is more variable than previously appreciated. The shape data suggest that the largest IB pulses are not the same as the events known as Narrow Bipolar Pulses
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