Data

Figure 1: Representative LASA waveforms of A) cloud-to-ground, B) negative CID, C) postive CID, D) and E) intracloud, and F) CG with fast/intense leader. See description in text.

Figure 1 shows six events recorded by LASA stations. Figure 1 A is a typical -CG LF/VLF waveform. There is some low amplitude leader activity preceding the large negative excursion which is the return stroke. The fall-time, or return from the minimum to zero, is nearly 100 $\mu$s. Figure 1 B is a typical -CID event recorded by a LASA station. The two pulses at approximately 0.1 ms are the ionospheric and ground-ionospheric reflections of the signal (see [Smith et al.(2003)]). Figure 1 C is a typical +CID waveform with two reflections appearing at almost 0.2 ms after the initial burst of radiation. The middle and right columns of Figure 1 are the same events. The middle column is a 600 $\mu$s view of the waveform at the trigger point and the right column shows the full 8 ms record of the event. Figure 1 D is an intracloud lightning event observed by a LASA station. Note the positive polarity of the pulses in this case. Figure 1 E is a second example of an intracloud lightning waveform. Note that the event in E has less high frequency content than the event in D. Event E was 420 km from the sensor, compared to 180 km for event D. The earth-ionosphere waveguide does act as a low-pass filter, but the distance of 420 km is not enough to explain the lack of high frequency content (i.e.other leader events at a distance of 420 km or greater show high frequency content similar to event D). Figure 1 F is an example of a fast/intense stepped leader preceding an initial negative cloud-to-ground return stroke as discussed in [Heavner et al.(2002)]. Figure 1 F1 and D1 are similar waveforms with opposite polarity. Because of the fast/intense nature of the leader, the return stroke is part of the long record presented in Figure 1 F2 at approximately 5 ms. Without the return stroke in the full waveform, it is difficult to distinguish IC and CGs with fast/intense leader activity. Because negative leaders radiate more effectively than leaders associated with positive CGs, the polarity of the pulses provide an indication of the nature of the radiation (IC vs CG).

While typical strong leader pulses occur approximately 20 ms before the -CG return stroke, the literature does provide some examples of the fast/intense leader activity illustreated in Figure 1 F. [Uman et al.(1978)] report an `unusual lightning flash' which included a return strokes with ``a stepped leader of relatively short duration.'' [Brook(1992)] and [Ogawa(1995)] present electric field records of intense lightning leader features within several milliseconds of the return stroke, similar to Figure 1 F2. [Heavner et al.(2002)] reports on fast/intense leader activity within 4 ms of the initial return stroke.