The Nature and Cause of Initial Breakdown Pulses and the Implications for Streamer to Leader and Negative Stepping Processes

Initial breakdown pulses (IBPs) have long been known to play an important role in the initial development of lightning and negative leader stepping, but their basic nature has remained unknown. VHF interferometer and fast electric field change observations of downward TGFs in the recent study by Belz et al. (2020) have shown that IBPs are produced by streamer-based fast negative breakdown (FNB), similar to FNB recently discovered during high altitude NBEs (Tilles et al. 2019). The TGF observations showed with s or sub-s precision that downward TGFs are often produced by impulsive sub-pulses that are a characteristic feature of classic IBPs. This leads to the conclusion that the sub-pulses are caused by sudden transient conducting events (TCEs) within the rapidly-developing FNB streamer system. Successive sub-pulse TCEs were found to be isolated from each other and from the incoming negative breakdown until one suddenly causes the electric field to revert to a characteristic opposite polarity, relatively slow and long-lasting change indicative of retrograde current developing back into and through the preceding inter-IBP breakdown, completing the streamer to leader transition not only in the IBP itself but back along the incoming negative breakdown, and constituting a step. The results are fully consistent with and explain the optical observations of negative leader stepping by Stolzenburg et al. (2013, 2014). Following the step, the negative breakdown continues as lesser-speed (~106 m/s) streamer activity until initiating the next IBP. The 107 - 108 m/s speeds of the FNB are a phase velocity due new streamers being produced in the strong E field ahead of the advancing streamer front, making the streamer system discontinuous (Attanasio et al., 2021; da Silva et al., this session), and partially explaining the isolated nature of the sub-pulse TCEs. Under the above scenario, the process repeats, producing increasingly strong IBPs until one becomes capable of initiating a self-propagating conducting leader, at which point the IBPs suddenly decrease in amplitude and frequency. Taken together, the observations suggest a modified version of the space-stem space-leader explanation of negative leader development and stepping (Belz et al., 2020).