Collision of downward negative leader branches: Why does this happen?

Abstract:

It is known that streamers can interact with each other in a number of ways, including collisions. Nijdam (2011, pp. 71-74) presented experimental evidence of positive laboratory streamers making connection to the lateral surface of other positive streamers. He referred to this apparent streamer collision as “reconnection” and suggested that it is caused by electrostatic attraction of a later streamer to the remnants of an earlier streamer that already has crossed the gap and changed polarity. Cummer et al. (2006), from video recordings at 5000 and 7200 frames per second, reported on collisions of adjacent downward-moving sprite streamers, with the collision points becoming long-persisting sprite beads. They attributed this phenomenon to electrostatic attraction between the charged streamer tip and the conducting but uncharged nearby streamer channel. Very complex behavior of sprite streamers (recorded at 100,000 frames per second), including collisions, was reported by McHarg et al. (2019).

In this study, we observed a similar behavior in downward-moving, branched negative lightning leaders, which has not been reported before. Examples of negative-leader tips colliding with the lateral surface of adjacent active branches of the same leader in high-speed video recordings (50-µs interframe interval) will be presented. Typically, a lagging branch approaches a leading branch from the side at about 90° angle. The lagging branch may or may not survive collision. In the latter case, it is absorbed by the leading branch, which becomes invigorated. The collision point may exhibit enhanced brightness after the absorbed branch is no longer luminous.

One possible explanation for collision of a negative branch tip with an adjacent branch of the same negative leader is the rapidly occurring step-formation process. This process is known (e.g., Wang et al. 1999) to generate a backward propagating positive-charge wave which serves to momentarily compensate the negative charge behind the leader branch tip (over 100 m or so) and attract the negatively charged tip of another (lagging) branch that happens to be nearby. The resultant collision leads to draining negative charge from the lagging branch and facilitating further extension/stepping of the leading one.