Electric field pulses in close lightning cloud flashes

Electric field pulses, their times of occurrence in the flash, and their relationship to slower electric field variations were studied for lightning cloud flashes at a distance of about 10 km in Florida. Large pulses with a width of a few microseconds tended to occur more frequently early in the flash. If only pulses with amplitudes greater than 50% above the system noise and widths greater than 1.6 μs were counted, 58% of 332 pulses in 89 flashes occurred in the first third of the field records, while only 14% occurred in the final third. The mean total flash duration was 660 ms. The 31 largest pulse peaks (those with amplitudes more than 3 times the system noise) in six flashes that were analyzed in detail had a median 30–90% risetime of 1.0 μs. The 25 pulses in which these peaks occurred had a median half width of 2.7 μs and had mean full widths, including the positive overshoot, of 74 μs. All these pulses were of negative polarity and were superimposed on negative-going field changes. Most occurred in the first third of the overall field change. In the latter part of these six records, microsecond-scale field variations were more than 50% above the tape noise (effectively 4 V/m) in only nine of 49 K changes. Not all pulses associated with K changes had the same polarity as the K change, and their pulse structure was relatively complex in contrast to the negative pulses that occurred early in the records. We infer that the process that produces the K change pulses is different from that which produces the large pulses that occur earlier in the discharges.