Bidirectional leader development in a preexisting channel as observed in rocket‐triggered lightning flashes

Abstract

Two cases of bidirectional leader, starting almost immediately below a decaying dart or dart‐stepped leader which terminated before reaching the ground, were identified, for the first time, in rocket‐triggered lightning flashes based on high‐speed video, electric field changes, and channel base current, indicating that a bidirectional leader can be excited by a decayed dart leader and propagate in a preexisting discharge channel. The positive end of the bidirectional leader moved upward along the terminated downward dart leader path, and the negative end extended downward along the decayed initial continuous current or preceding stroke path to the ground and culminated in a return stroke. The positive leader propagated with an average speed of 1.3 × 106 m/s and 2.2 × 106 m/s in the two cases, roughly twice as fast as its negative counterpart with speed of 7.8 × 105 m/s and 1.0 × 106 m/s, respectively. The positive end started earlier than the negative end with a preceding time less than 40.3 µs. The bidirectional leader can be regarded as a recoil leader with the positive end retrogressing along a negative leader channel, whose polarity is contrary to the traditional recoil leader with negative leader end retrogressing along an existing positive leader channel. Weak luminosity was continuously observed during the terminated dart leader and bidirectional leader stage, but the channel current was less than the minimum detection capability of 9.3 A.

Observation Results and Discussion on an Upward Leader Followed by a Long Continuing Current After a Return Stroke

“…Therefore, the propagation speeds of the upward positive leader shown in Figure are ~1–2 orders of magnitude larger than traditional upward positive leaders in triggered lightning (Hill et al, ; Jiang et al, ). The observed speed for the upward positive leader is similar to the upward positive leader in bidirectional leaders reported by Jiang et al () and Qie et al (). The higher speed is due to the fact that all these positive leaders propagated in a preexisting discharge channel.…”

“…However, we noticed some differences between the positive leader and those described in the above mentioned literatures. There were downward propagating dart leaders before bidirectional leaders' initiation in both Jiang et al (2014) and Qie et al (2017), while there was no downward negative leader observed by optical and electrical instruments before the upward discharge reported here.…”

High‐Speed Video Observations of Recoil Leaders Producing and Not Producing Return Strokes in a Canton‐Tower Upward Flash

“…Based on our high‐speed video records of the RLs in this flash and the RL images found in the literatures (e.g., Mazur et al, ; Qie et al, ), we believe that all the RLs were bidirectional, although sometimes only one extending end could be detectable. The negative end of each bidirectional DL extended along one of the decayed branches of the flash‐initiating UPL, attached to the top of the Canton Tower, and initiated a return stroke.…”

“…Zhu et al () argued that RLs were generated in the remnants of either positive or negative leader channel. Qie et al () reported that the positive end of a bidirectional RL retrograded along the previous negative leader channel in a triggered lightning flash. The high‐speed video observations reviewed above confirm that RLs belong to the family of bidirectional leaders.…”

High‐Speed Video and Lightning Mapping Array Observations of In‐Cloud Lightning Leaders and an M Component to Ground

“…Bidirectional leaders have also been observed to occur following termination of failed negative dart leaders in both tower initiated lightning (Jiang, Wu, et al, ) and rocket‐triggered lightning (Qie et al, ). Following termination of the dart leader, the bidirectional leaders in both cases initiated at the termination point within one or two frames (10 μs per frame in the former case and 40 μs per frame in the later).…”