Comparison between Upward Positive and Negative Leaders of lightning in their Initial Stage

The initial development of upward positive and negative leader is studied and compared based on the simultaneous data from high-speed video camera, channel base current, and electromagnetic field changes documented in the rocket-triggered lightning and tower-initiated lightning. The impulsive current, electric field and corresponding optical images suggest stepping-like propagation of positive leader at the initial stage. However, the individual positive leader step, if any, is weak in terms of peak current, charge transfer and step length than the negative one. The negative charge of an individual step was nearly an order of magnitude larger than that of the positive leader step in triggered lightning. The intermittent brush-like corona zone and the intermittent re-illumination of the channel body are the most distinct features for the positive leader stepping, which is obviously different from the stepping processes of the negative leader which is characterized by space leader/stem.