Characteristics of the optical pulses associated with a downward branched stepped leader

A stepped leader that preceded a natural first return stroke was recorded using a high-speed optical imaging system with a time resolution of 0.1 mu s and a spatial resolution of about 40 m. Within the view of the imaging system, 440 m x 480 m just above ground level, the leader produced one ground termination (main channel for the return stroke) and six ungrounded branches. A total of 153 optical pulses associated with the leader were identified for analysis, with 61 originating from the main channel and 92 from the six branches. The pulses originating from the main channel and from the branches are almost identical in terms of geometric mean (GM) values of 10-90% risetime and half-peak width, near 0.4 mu s and 1.1 mu s, respectively. The pulses from the main channel show smaller interpulse intervals than those from the branches. When the interpulse intervals for pulses from both the main channel and all branches are combined, of a total of 152 intervals, five are essentially 0 and 58 are between 0 and 1 mu s. Thus a significant percentage of optical pulses occurred almost simultaneously in different channel sections separated by distances up to several hundred meters. The light intensity just prior to each pulse exhibited a tendency to increase as the leader approached ground, while the other parameters, such as pulse peak, risetime, half-peak width, and interpulse intervals, showed no systematic changes with time.