Progression features of a stepped leader process with four grounded leader branches

The propagation features of stepped leader with four grounded branches during a negative CG flash have been analyzed by using the data from a high-speed digital camera with 1 ms time resolution in correlation with wide-band electric field change signatures. Four branches that initiated from a main leader trunk connected to ground in sequence and induced four separate return strokes. The propagation speed of four grounded leader branches shows an increase tendency when they progressed downward to ground. The average 2-D speed of four grounded branches was estimated to be about 1.1 x 10(5) m/s. The electric field changes caused by the stepped leader were characterized by clustered pulses near the time of return stroke. Each cluster of pulses was composed of 3-4 pulses with an average time interval in a cluster of about 7.7 mu s, and the time interval between two first pulses in two adjacent clusters was 30.1 mu s.