Upward Leaders Initiated From Instrumented Lightning Rods During the Approach of a Downward Leader in a Cloud‐To‐Ground Flash.

In this paper we analyze electric‐field and current measurements of upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was recorded by two high‐speed cameras running at 37,800 and 70,000 images per second and the current measured in two lightning rods. Differently from previous works, here we show, for the first time, current measurements of multiple upward leaders that after initiation propagate to connect the negative downward moving leader. At the beginning of the propagation of the leaders that initiate on the instrumented lightning rods, current pulses appear superimposed to a steadily increasing DC current. The upward leader current pulses increase with the approach of the downward leader and are not synchronized but present an alternating pattern. All 2D leader speeds are approximately constant. The upward leaders are slower than the downward leader speed. The average time interval between current pulses in upward leaders is close to the interstep time interval found by optical or electric field sensors for negative cloud‐to‐ground stepped leaders. The upward leaders respond to different downward propagating branches and, as the branches alternate in propagation and intensity, so do the leaders accordingly. Right before the attachment process the alternating pattern of the leaders ceases, all downward leader branches intensify, and consequently upward leaders synchronize and pulse together. The average linear densities for upward leaders (49 and 82 μC/m) were obtained for the first time for natural lightning. Plain Language Summary: The effectiveness of a lightning protection system depends on its efficiency to intercept the down coming leader of a cloud‐to‐ground lightning flash. The interception is usually done by an upward connecting leader that initiates from grounded structures, humans, or living beings that protrude from nearby ground. The understanding of the upward connecting leader and of the attachment process with the downward leader plays an important role in the determination of the zone of protection and therefore in the improvement of a lightning protection system. Unconnected upward leaders, that is, upward leaders that fail to connect the downward leader, are also of great importance in lightning protection. They can be large enough to cause damage to equipment vulnerable to sparks or induced currents, and enough to injure people from who it initiates. In this paper we analyze electric‐field, speed, and current measurements of upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was simultaneously recorded by two high‐speed cameras, an electric‐field sensor, and current sensors installed on two lightning rods. Differently from previous works we show, for the first time, current measurements of multiple upward leaders induced by the negative downward moving leader. Key Points: Current and charge density measurements of two upward leaders induced by the same downward leaderUpward leaders alternate their progression during initial propagationCurrent pulses of upward leaders increase intensity and synchronize right before attachment [ABSTRACT FROM AUTHOR]