High-speed video observations of the fine structure of a natural negative stepped leader at close distance

We present new high-speed video observations of a natural downward negative lightning flash that occurred at a close distance of 350m. The stepped leader of this flash was imaged by three high-speed video cameras operating at framing rates of 1000, 10,000 and 50,000 frames per second, respectively. Synchronized electromagnetic field records were also obtained. Nine pronounced field pulses which we attributed to individual leader steps were recorded. The time intervals between the step pulses ranged from 13.9 to 23.9μs, with a mean value of 17.4μs. Further, for the first time, smaller pulses were observed between the pronounced step pulses in the magnetic field derivative records. Time intervals between the smaller pulses (indicative of intermittent processes between steps) ranged from 0.9 to 5.5μs, with a mean of 2.2μs and a standard deviation of 0.82μs. A total of 23 luminous segments, commonly attributed to space stems/leaders, were captured. Their two-dimensional lengths varied from 1 to 13m, with a mean of 5m. The distances between the luminous segments and the existing leader channels ranged from 1 to 8m, with a mean value of 4m. Three possible scenarios of the evolution of space stems/leaders located beside the leader channel have been inferred: (A) the space stem/leader fails to make connection to the leader channel; (B) the space stem/leader connects to the existing leader channel, but may die off and be followed, tens of microseconds later, by a low luminosity streamer; (C) the space stem/leader connects to the existing channel and launches an upward-propagating luminosity wave. Weakly luminous filamentary structures, which we interpreted as corona streamers, were observed emanating from the leader tip. The stepped leader branches extended downward with speeds ranging from 4.1×105 to 14.6×105ms−1.