Using 3-Dimensional lightning interferometry to unravel the mysteries of dart-leader propagation.

It has long been known that the speed of the ionization waves that make up a lightning discharge can vary by three orders of magnitude within the same discharge. With the advent of cloud-penetrating mapping techniques such as the lightning mapping arrays and lightning interferometers we can image and quantify processes that were previously inaccessible. (See for example Jensen et al, J. Geophys. Res. Atmo. https://doi.org/10.1029/2023JD039104). Using BIMAP-3D, a pair of lightning inteferometers located near Los Alamos, we thorougly analyzed 14 in-cloud dart leaders in a single flash. The leader velocities varied from two million to twenty million meters second in a predictable way. In general, dart leaders would begin at two million meters/second and then accelerate to twenty million meters/second in the first 10-30% of their existence. They would trace the remainder for their path decelerating. By combining the speed of the leader tip with measurements of electric field on the ground, we could show that the leader tip speed is directly proportional the tip electric field and that the speed variations measured reflected

variations in electric field along the channel. Further, it could be demonstrated that leader tip fields are of order 15~kV/m, well below the breakdown threshold for

virgin air at seven kilometers altitude. An observable consequence of these low leader tip fields is that dart-leaders are much more spacially localized (sharper, thinner, fewer branches and streamers) than initial stepped leaders. Furthermore, predictable speed variations occurred at leaders approached branch points

from previous lightning channels. These could be traced to localized additional charge concentrations that changed the leader tip electrical environment.

As we learn how to fit ever more lightning phenomena to relatively simple physical models, we are no less amazed by the complexity of behavior of these natural long sparks.