Bidirectional propagation of lightning leader

The characteristics of initial leader of triggered lightning flashes are studied by use of a high-speed digital camera system with the rate of 1 000 frame per second and simultaneous measurements of current and electric field change with 0.1 μs time resolution. The results show that the altitude-triggered lightning was initiated by bidirectional propagation leader because of “current interruption” by air gap, whereas the grounded-triggered one was not. The onset of downward-moving positive leader was prior to that of the upward-going negative leader for 843 ms in an altitude-triggered lightning under the positive ambient electric field. The downward-moving leader, regardless of its polarity, would produce inhomogeneous distribution of luminosity (current) along the channel on the moment of connection to the ground.