We investigate sequential processes underlying the initial development of in‐cloud lightning flashes in the form of initial breakdown pulses (IBPs) between 7.4 and 9.0 km altitudes, using a 30–250 MHz VHF interferometer. When resolved, IBPs exhibit typical stepped leader features but are notably extensive (>500 m) and infrequent (∼1 millisecond intervals). Particularly, we observed four distinct phases within an IBP stepping cycle: the emergence of VHF sources forming edge structures at previous streamer zone edges (interpreted as space stem/leader development), the fast propagation of VHF along the edge structure (interpreted as the main leader connecting the space leader), the fast extension of VHF beyond the edge structure (interpreted as fast breakdown), and a decaying corona fan. These measurements illustrate clearly the processes involved in the initial development of in‐cloud lightning flashes, evidence the conducting main leader forming, and provide insights into other processes known to occur simultaneously, such as terrestrial gamma ray flashes.
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.
more » « less- PAR ID:
- 10459916
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 124
- Issue:
- 6
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- p. 2994-3004
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Using a 30–250 MHz VHF interferometer, we observed a previously unreported mode of initial lightning development inside thunderclouds. This mode is defined by continuous VHF radiation spanning several km within the first few milliseconds of lightning initiation. Following flash initiation through fast positive breakdown at high altitudes above 9 km, the VHF radiation front of upward negative streamers ascended continuously at a speed of ∼1.0 × 106 m/s, forming a continuous initial breakdown burst (CIBB) about 2 km in length. For the two CIBBs analyzed, the long and narrow CIBB channel was traversed by dart leaders that occurred later in the flash, indicating that the CIBB channel belongs to what becomes the main conducting leader channel. In contrast to classic initial breakdown pulses (IBPs) with sub‐pulses superimposed on the rising edge, CIBBs produced a series of discrete, narrow LF pulses (<10 μs) with an average time interval of 0.20 and 0.14 ms, respectively. We speculate that a CIBB is a continuously developing negative streamer system in the high electric field region at high altitudes, with connections of internal plasma channels producing LF pulses. These results have implications for physical conditions conducive to the formation of a long and continuous negative streamer system.
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