Lightning Initiation Processes Imaged With Very High Frequency Broadband Interferometry

Abstract

Recent measurements of narrow bipolar lightning events (NBEs) by very high frequency (VHF) radio interferometer have resolved the dynamic development of this special lightning process with submicrosecond time resolution, and showed that the fast positive breakdown (FPB) process is responsible for initiating at least some lightning flashes. In this study, with a newly built and deployed VHF interferometer system, we analyzed 31 intracloud lightning flash initiation events during three thunderstorms at short range from the interferometer. These events separate into two distinct classes that can be identified based on the time scale and the occurrence contexts of the first detectable VHF emissions from the flash. One class has features completely consistent with previously reported FPB events and is associated with continuous VHF emissions of 10–20 μs duration. Downward motion of the FPB region centroid merged continuously into the development of the subsequent upward negative leaders. But the majority of the lightning flashes analyzed began with ultrashort, submicrosecond duration, isolated pulses of VHF emission with no identifiable FPB signatures between these pulses and the leader development. These short VHF pulses begin typically a few hundred microseconds before the upward leader developes and are located at the same position where the leader eventually begins. We suggest that the FPB process is responsible for initiating some but not all lightning flashes, and the extremely short pulse‐like VHF emissions play a role in initiating those flashes without any FPB process.

Formation of decimeter-scale, long-lived elevated ionic conductivity regions in thunderclouds

“…On the other hand, not in support of the Rison et al's inference on the universal nature of fast positive breakdown, lightning initiation by fast negative breakdown was reported by Tilles et al 10 The latter observation was unexpected, because the critical electric field for propagation of negative streamers is about a factor of 2 higher than that for positive streamers. Finally, recent interferometric observations 11 demonstrated the possibility of lightning initiation without any form of fast breakdown of the type observed by Rison et al 9 and Tilles et al 10 Clearly, the mechanism of lightning initiation remains the subject of intense debate.…”

Needles and Lightning Leader Dynamics Imaged with 100–200 MHz Broadband VHF Interferometry

“…In this work, we report new observations of needles on positive leaders using a previously described shortbaseline broadband VHF interferometer (Lyu et al, 2019) with its bandwidth extended to above 200 MHz. By directly imaging dart leaders that define the positive leader conducting channel, our observations confirm that needles originate directly from this thin conducting channel and occur in all of the positive leader channels in this flash.…”

On the Transition From Initial Leader to Stepped Leader in Negative Cloud‐to‐Ground Lightning

“…Recent findings (e.g., Marshall et al, ) suggest that the earliest events in a flash seem to occur in the following order: an ionizing initiation event, followed by a slower charge motion (detected as an initial electric field change [called IEC]), followed by a series of initial breakdown (IB) pulses (collectively the IB stage of the flash), and followed by a stepped leader. The initiation event can be a narrow bipolar event with power > 1,000 W in the VHF band (e.g., Rison et al, ) or a much weaker event with VHF power < 1 W (e.g., Marshall et al, ); the weaker events are apparently more usual for initiations of both IC flashes (Lyu et al, ) and CG flashes (Bandara et al, ). Although the mechanism producing IB pulses is unknown, they are understood and modeled as short current surges and have a characteristic bipolar waveform when detected with electric field change (E‐change) sensors (e.g., Weidman & Krider, ).…”