New Observations of the Attachment Process in Natural Lightning

Abstract:

We examined the natural-lightning attachment process using high-speed (HS) video records in conjunction with electric field and electric field derivative measurements, all obtained at the Lightning Observatory in Gainesville (LOG), Florida. In different types of strokes (negative first, negative subsequent, and positive first), we observed faintly luminous formations (FLFs) connecting the downward leader to ground. The 2–D lengths of FLFs ranged from 51 to 200 m in negative first strokes (N = 11), from 130 to 908 m in negative subsequent strokes (N=5), and it was 177 m in 1 positive first stroke.

The nature of FLFs in negative first strokes differs from that in negative subsequent strokes. We inferred that in first strokes FLFs are mostly composed of positive streamers that developed from the prospective strike point (without a significant upward connecting leader), tens of microseconds prior to the return-stroke onset, while in subsequent strokes they are manifestations of elevated conduction current (of the order of 1 A) in the weakly-conducting defunct channel, occurring in response to the increasing average electric field of the descending leader. The FLF in the positive stroke appeared to be similar to its counterparts in negative first strokes.

The streamer connection of the first-stroke leader tip to ground constitutes the break-through phase of the attachment process. Establishment of this connection tens of microseconds prior to the lightning return-stroke onset has never been reported before. We found that the streamer connection did not significantly influence the propagation characteristics of descending leader (and the upward connecting leader, if any), except for always guiding it to the origin of streamers on the grounded object. Implications of the new observations for lightning protection will be discussed.

Acknowledgements

This research was supported in part by NSF and DARPA.