Needle-like structures discovered on positively charged lightning branches

Lightningis a dangerous yet poorly understood natural phenomenon. Lightningforms a network of plasma channelspropagating away from the initiation point with both positively and negativelycharged ends—called positive and negativeleaders1. Negativeleaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band2–8 that can be remotely sensed and imaged with high spatial and temporal resolution9–11. Positive leaders propagate more continuously and thus emit very little high-frequency radiation12. Radio emission from positive leaders has nevertheless been mapped13–15, and exhibits a pattern that is different from that of negativeleaders11–13,16,17. Furthermore, it has been inferred that positive leaders can become transiently disconnectedfrom negativeleaders9,12,16,18–20, which may lead to current pulses that both reconnect positive leaders to negativeleaders11,16,17,20–22 and cause multiple cloud-to-ground lightningevents1. The disconnectionprocess is thought to be due to negativedifferential resistance18, but this does not explain why the disconnectionsform primarily on positive leaders22, or why the current in cloud-to-ground lightningnever goes to zero23. Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negativeleaders. Here we report three-dimensional radio interferometric observations of lightningover the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures—which we call ‘needles’—that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnectfrom negativeones, and why cloud-to-ground lightningconnects to the ground multiple times.