Spatial and temporal properties of optical radiation produced by stepped leaders

The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 x 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 m from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5-11.2 x 105m/s, a step interval of 5-50 μs, and a step length of 7.9-19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14-1.7 x 108m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9-5.8 x 105m/s, 18-21 μs, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.