Formation and characteristics of negative stepped leaders in 4–10m long air gap discharges

Natural lightning flashes are stochastic and uncontrollable, and thus it is difficult to observe the formation process of a downward negative stepped leader (NSL) directly and in detail. In this paper, the formation processes of NSLs in controllable laboratorial long-air-gap discharge experiments were studies. First, a series of negative long-air-gap discharge experiments with scales of 4~10 m were designed and carried out. According to the observation results of these experiments, two kinds of formation processes for NSLs with different scales were presented, and several of the characteristic parameters, including the scale, the propagation velocity and the dark period, were obtained. By comparing these characteristics with that in natural flashes, the similarity between the NSLs in the simulation experiments and those in natural flashes was proved, and the actual formation of NSLs in natural flashes was deduced.