Abstract
Why lightning sometimes has multiple discharges to ground is an unanswered question. Recently, the observation of small plasma structures on positive leaders re-ignited the search. These small plasma structures were observed as pulsing radio sources along the positive leader length and were named “needles”. Needles may be the missing link in explaining why lightning flickers with multiple discharges, but this requires further confirmation. In this work we present the first optical observations of these intriguing plasma structures. Our high-speed videos show needles blinking in slow motion in a sequential mode. We show that they are formed at unsuccessful leader branches, are as bright as the lightning leaders, and report several other optical characteristics.
Highlights
Why lightning sometimes has multiple discharges to ground is an unanswered question
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 leading to a current cut-off that could enable the presence of recoil leaders and the formation of further strokes in the lightning flash
Summary
Note that they appear in a sequential way as the positive leader propagates upward at an average 2D speed of 4.2 × 104 m s−1 (see Supplementary Video). Considering that observations were made in the USA and in Brazil in approximately the same conditions (similar high-speed cameras and distance from the upward flashes) we do not think that this is a reasonable hypothesis If this were true, we would expect to see a broad range of needle luminosity, some dim and some bright ones. The negative leader exhibited an average speed of 1 × 105 m s−1, an erratic directional change and stepping which are defining behaviors for negative leaders when observed by high-speed c ameras[10] (see video of Case 2 in Supplementary Information) This negative leader branched once before decaying 4.0 ms after its initiation (Fig. 4c,d). Their relationship with positive leader propagation and the presence of recoil leaders may help to understand why negative cloud-to-ground lightning have multiple strokes
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