Properties of lightning discharges from multiple-station wideband electric field measurements

There has been no detailed analysis of wideband electric fields recorded at a sufficient number of sites, with adequate design constraints, such as baseline length and timing resolution, to give three-dimensional locations of lightning sources. We describe a system to locate and give times of occurrence of the sources of wideband dE/dt pulses. The major focus of this study was sequences of regularly occurring dE/dt pulses, or pulse trains. We identify pulse trains giving rise to three lightning processes: (1) leaders preceding new terminations to ground, (2) dart-stepped leaders along a previously formed channel propagating all the way to ground, and (3) intracloud discharge processes.; Leaders preceding new terminations were dart-stepped in nature prior to their transition to stepped leaders. Speeds over the dart-stepped portion of leaders were 1.0 -- 12 x 106 m/s. Fine structure was present in the dE/dt waveform of both first strokes to ground and subsequent strokes in new channels to ground. The duration of the fine structure was shorter (37 mus median) for strokes in new terminations to ground than for first strokes in a flash (141 mus median). Sources of fine structure were related to the preceding stepped leader.; We provide the first interpretation of the complete electric field waveform of dart-stepped leaders. Near ground, dart-stepped leaders exhibited pulses at regular intervals of several microseconds. Interpulse intervals were much smaller than several microseconds early in the leaders, at which time their speed was greater. Seven dart-stepped leaders decreased in speed towards ground, while two were found to increase, but only when joining an earlier section of the channel. For dart-stepped leaders we found an inverse relationship between average leader speed and interpulse interval.; We provide the first locations of the wideband sources from intracloud pulse trains. Speeds of these trains were similar to those during the dart-stepped portion of leaders preceding new terminations to ground and dart-stepped leaders near ground. A comparison of average speed, interpulse interval and step length is undertaken for the three types of pulse trains identified. Similarities include a constant or decreasing speed and a tendency for interpulse intervals to increase with time. Several important differences were found. Dart-stepped leader waveforms were comprised of pulses of irregular amplitude, making the interpulse interval dependent on the amplitude threshold used in pulse selection. Pulses from intracloud trains were more uniform in amplitude and interpulse intervals were nearly constant with respect to an amplitude threshold used to select pulses. Average step lengths of intracloud trains were several tens of meters, while those of dart-stepped leaders and leaders preceding new terminations were more difficult to determine. Average speeds near the beginning of dart-stepped leaders were higher than speeds near the beginning of intracloud pulse trains. Pulse polarities associated with intracloud pulse trains were dependent on channel orientation.