On the Transition From Initial Leader to Stepped Leader in Negative Cloud-to-Ground Lightning

High-speed video and electric field change data are used to describe the first 5 ms of a negative cloud-to-ground flash. These observations reveal an evolution in character of the luminosity and electric field change pulses as two branches of the leader separately transition from initial leader to propagating as a negative stepped leader (SL). For the first time reported, there is evidence of weak luminosity coincident with the initiating event, a weak bipolar pulse 60 mu s prior to the first initial breakdown (IB) pulse. During the IB stage, the initial leader advances intermittently at intervals of 100-280 mu s, in separate light bursts that are bright for a few 20-mu s frames and are time coincident with IB pulses. In the intervals between IB pulses, the initial leader is dim or invisible during the earliest 1.8 ms. Within 2 ms, the leader propagation begins transitioning to an early SL phase, in which the leader tip advances at more regular intervals of 40-80 mu s during relatively dim and brief steps which are coincident with SL pulses having short duration, small amplitude, and typically unipolar waveform. These data indicate that when the entire initial leader length behind the lower end begins to remain illuminated between bursts, the propagation mode changes from IB bursts to SL steps, and the IB stage ends. The results support a hypothesis that the early initial leader development occurs in the absence of a continuously hot channel, thus the initial leader propagation is physically unlike the self-propagating SL advance.