Initial breakdown pulses in intracloud lightning flashes and their relation to terrestrial gamma ray flashes

[1] The initial breakdown stage of 10 intracloud lightning flashes that may have produced terrestrial gamma ray flashes (TGFs) is studied with wideband E-change, multiband B-change, and VHF lightning mapping data; these flashes fit published criteria known to be associated with TGFs. The (x, y, z, t) locations of fast initial breakdown pulses (IBPs) were determined with E-change data using a time-of-arrival (TOA) technique. Each IBP includes one or more fast-rising subpulses. Previous research has shown that a typical intracloud flash initiates just above the main negative cloud charge (MNCC), then an initial negative leader propagates upward in 1–20 ms to the bottom of the upper positive cloud charge (UPCC), thereby establishing a conducting path between the MNCC and UPCC. TOA locations indicate that IBPs are directly related to the initial negative leader. The IBPs primarily occur in short (<750 µs) bursts of two to five pulses, and each burst produces a slow, monotonic E-change. Typically, one to three IBP bursts are needed to span the vertical gap from the MNCC to the UPCC, with successive bursts separated by 1–5 ms. In the B-change data, each IBP burst has an associated ULF pulse and several LF pulses, and these are caused by the same physical events that produce the slow, monotonic E-change and fast-rising IBP subpulses, respectively. Based on similarities with known TGF-associated signals, we speculate that a relativistic electron avalanche causes each LF pulse/IBP subpulse pair; thus, each pair has the potential to cause a TGF.