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.

One hundred high‐speed video recordings (72 cases in Brazil and 28 cases in USA) of negative upward lightning flashes were analyzed. All upward flashes were triggered by another discharge, most of them positive CG flashes. A negative leader passing over the tower(s) was frequently seen in the high‐speed video recordings before the initiation of the upward leader. One triggering component can sometimes initiate upward leader in several towers. Characteristics of leader branching, ICC pulses, recoil leader incidence, and interpulse interval are presented in this work. A comparison of the results is done for data obtained in Brazil and USA. The duration of ICC and the total flash duration are on average longer in Brazil than in USA. Only one fourth of all upward leaders are followed by any return strokes both in Brazil and USA, and the average number of return strokes following each upward leader is very low. The presence and duration of CC following return strokes in Brazil is more than two times larger than in USA. Several parameters of upward flashes were compared with similar ones from cloud‐to‐ground flashes.

The physical mechanism of lightning attachment to grounded structures is one of the most important issues in lightning physics research, and it is the basis for the design of the lightning protection systems. Most of what is known about the attachment process comes from leader propagation models that are mostly based on laboratory observations of long electrical discharges or from observations of lightning attachment to tall structures. In this paper we use high‐speed videos to analyze the attachment process of downward lightning flashes to an ordinary residential building. For the first time, we present characteristics of the attachment process to common structures that are present in almost every city (in this case, two buildings under 60 m in São Paulo City, Brazil). Parameters like striking distance and connecting leaders speed, largely used in lightning attachment models and in lightning protection standards, are revealed in this work.

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This work presents the first simultaneous X-ray measurement and high-speed video observation of the propagation of a lightning leader producing X-rays. As a result, the three-dimensional leader distance from the X-ray measurement and, for the first time, the conditions of the preexisting channel during the leader propagation were observed. Although four leaders in this seven-stroke flash followed the same path to ground, X-rays were only observed during the leader before the return stroke with the highest peak current. The fact that the other three leaders following the same path to ground did not produce detectable X-rays confirms the hypothesis that leader line charge density is an important factor that determines X-ray production. The fact that X-rays was recorded only when the leader tip was at a certain portion of the lightning channel confirms that the orientation of the leader plays an important role in the detection of X-rays. Plain Language SummaryIt was known that lightning can produce X-rays. However, in this study, thanks to the use of a high-speed video camera it was possible to determine when lightning produces X-rays, how far it was, how it was oriented when the detection of X-rays, and what the conditions of the preexisting channel were during the leader propagation. The observations of the present work allow for new insights, confirmation of some hypotheses, and comparison with past studies. The results presented help to understand why X-rays are sometimes detected and sometimes not. It is shown that the amount of charge transferred by the discharge plays a crucial role. This study also confirms that the orientation of the descending leader plays an important role in the detection of X-rays.