| Summary: | A physical model based on the mechanism observed in experimental investigations is introduced to describe the formation of negative leader steps. Starting with a small length of a space leader located at the periphery of the negative streamer system of the stepped leader, the model simulates the growth and the subsequent formation of the leader step. Based on the model, the step length, the step forming time, and the propagation speed of stepped leaders as a function of the prospective return stroke peak current are estimated. The results show that the step length and the leader speed increase with increasing prospective return stroke current. The results also show that the speed of the stepped leader increases as it approaches the ground. For prospective return stroke currents in the range of 15 kA–60 kA, the step lengths lie within the range 5 m–100 m, the step forming times lie within the range 10 μs–250 μs, and the leader speed lies within the range 105 m/s −1.5 × 106 m/s. The results obtained are in reasonable agreement with the experimental observations.
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