Needles and Lightning Leader Dynamics Imaged with 100–200 MHz Broadband VHF Interferometry

Abstract

A new short‐baseline VHF interferometer with 200 MHz bandwidth images lightning with significantly higher spatial resolution than our previous system with a 60 MHz maximum frequency. Needles (Hare et al., 2019, https://doi.org/10.1038/s41586-019-1086-6), needle‐associated negative leaders, a nearby bidirectional leader, and their interactions were imaged during an intracloud lightning flash. Six recoil leaders traversing the positive leader channel definitively determined the leader path and verified that needles originate from positive channels. Two negative leaders initiated from the positive channel, either from or close to needles, when another active positive leader passed ~1 km away. While the needles develop continuously with a VHF source density decreasing from the front end to the back end, a bidirectional leader initiated nearby and had strong influence on subsequent needle dynamics. These rarely observed yet perhaps common lightning processes are analyzed comprehensively to give new insight into the origin and impact of needles on positive lightning leaders.

Optical observation of needles in upward lightning flashes

“…The lengthiest needle, 73 m long (Case 2 in Supplementary Table S2 ), lasted 3 frames. The average 2D speed of this needle (2.7 × 10 5 m s −1 ) is very similar to what was reported by Hare et al 1 , Pu and Cummer 2 (3 × 10 5 m s −1 and 1–10 × 10 5 m s −1 respectively) and also exhibit clear outward propagation from the positive leader channel. The average 2D length of these structures is 14 m and the distance between them 80 m (Supplementary Table S2 ).…”

“…4c,d). This is the only optical observation we have obtained of a negative leader initiation from a pulsing needle site but serves to confirm the observations of negative leader initiation from needle sites as reported by Pu and Cummer 2 .…”

“…Although the pulse interval of a needle is shorter than the time interval between the brush corona split and the first needle appearance, the pulse interval of a needle increases with time (Supplementary Fig. S2 ) as also observed by Pu and Cummer 2 . It is unclear why this behavior occurs, but it may be tied to the lengthening of the leader and the increasing number of needles.…”

“…These leader segments first appear some milliseconds after the corona brush splitting and they pulsate as the leader propagates upward (see Supplementary Video ). These structures are identified as needles due to the high degree of similarity with the ones observed with VHF detection by Hare et al 1 , Pu and Cummer 2 and Shao et al 3 .…”

“…The needle-like structures analyzed by Hare et al 1 , Pu and Cummer 2 and Shao et al 3 , revealed some new details of the positive leader propagation to explain why lightning sometimes has multiple discharges. Needles could cause enough instability on the lightning channel current (see also Williams and Montanyá 4 ) leading to a current cut-off that could enable the presence of recoil leaders and thus the formation of further strokes in the lightning flash (more information on current cut-off and recoil leaders in Heckman 5 , Williams 6 , Williams and Heckman 7 , Saba et al 8 , Mazur et al 9 Warner et al 10 , Tran and Rakov 11 ).…”