Imaging Step Formation in In‐Cloud Lightning Initial Development With VHF Interferometry

• Published in: Geophysical research letters 2024-01, Vol.51 (1), p.n/a
• Main Authors: Pu, Yunjiao, Cummer, Steven A.
• Format: Article
• Language: English
• Subjects: Breakdown; Cloud formation; Connecting; Corona; fast breakdown; Gamma ray flashes; Gamma rays; Imaging techniques; initial breakdown pulse; Interferometers; Interferometry; Leader currents; Lightning; Lightning flashes; space stem; stepped leader; Streamer gas discharge; VHF interferometry
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL107388

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. Plain Language Summary The initial development of a lightning flash inside a cloud has long been a mystery. This study utilizes state‐of‐the‐art lightning imaging techniques with a 30–250 MHz VHF interferometer, providing clear images of the processes involved in the initial development of in‐cloud lightning flashes. New radio features suggest distinct development phases, including what we interpret as space stems, space leaders, connection between the main leader and the space leader, fast breakdown, and corona fan development within an initial breakdown pulse stepping cycle. This provides evidence of the conducting main leader in the initial breakdown stage. These observations showcase the intricate streamer discharge phenomena during initial lightning development, and shed light on other processes known to occur simultaneously, including Terrestrial Gamma ray Flashes. Key Points We observed four distinct VHF processes in the development of 300–1,000 m long initial breakdown pulses (IBPs) in in‐cloud lightning flashes These four processes appear to map to the known processes in a conventional stepped leader, including space stem and space leader formation During an initial breakdown step, fast extension over several hundred meters indicates that fast breakdown may be an essential part of in‐cloud flash