Insights on Space-Leader Characteristics and Evolution in Natural Negative Cloud-to-Ground Lightning

We present sub-microsecond-scale, high-speed video camera observations of three negative stepped leaders in cloud-to-ground flashes with return-stroke peak currents (estimated by the U.S. National Lightning Detection Network) of -17, -104, and -228 kA. The camera frame exposure times for these observations were 1.8, 1.0, and 0.74 mu s, respectively. The 0.74 mu s exposure time is the shortest reported to date. We observed the temporal and spatial evolution of space leaders from their inception to their attachment to the pre-existing leader channel (PELC). For stepped leaders that led to return strokes having higher peak currents, the space leaders appear to have incepted at farther median two-dimensional distances from their respective PELC-attachment points. These median distances were 6.1, 16.6, and 17.6 m, respectively, for the three strokes. Our observations indicate that space leader characteristics are likely influenced by stepped-leader line-charge-density, which is expected to be higher in strokes with higher return-stroke peak currents.