Modeling of Lightning Stepped Leader Characteristics

The paper constitutes a brief account of an extensive investigation into the different stages of first stroke negative lightning leader stepping, starting with determination of the length of the negative streamer zone below the leader tip. An already formed space stem/ space leader results in propagation of a positive space leader which bridges the streamer zone. This takes a time too short for leader thermalization. Such thermalization occurs over a much longer time, through corona current injected at the tip of the moving negative space leader. The latter stage is decisive for the time interval between steps. The model has then been applied to leaders with prospective return stroke current of 3-100 kA, leader tip height of 23-3000 m, leader charge linearly decaying to practically vanish at heights of either 2500 m or 4500 m. The dependence of the negative leader stepping characteristics on the above parameters are shown below. Under the conditions investigated, the step length varied between 3.5 m and 39 m, the bridging time of the streamer zone in the range 0.8-3 μs, the time interval between steps in the range 8.5-92 μs corresponding to a mean stepped leader speed in the range 1.0-11.6*10 5 m/s. Whenever possible the model predictions were satisfactorily compared to field measurements. Triggered lightning, dart leaders and the attachment process to ground objects are out of scope of this investigation.