Discharge Processes During Downward TGFs at the Telescope Array in Utah

High-speed observations of lightning-produced downward terrestrial gamma-ray flashes (TGFs) by the Telescope Array Surface Detector (TASD) network in Utah have been obtained in conjunction with broadband VHF lightning interferometer (INTF), fast electric field change antenna (FA), and 3-D VHF lightning mapping array (LMA) measurements of the parent lightning flashes. The measurements confirm in detail that TGFs are produced during energetic initial breakdown pulses (IBPs) that occur during downward breakdown at the beginning of negative cloud-to-ground (-CG) flashes. The IBPs produce episodes of fast negative breakdown (FNB) that are embedded in slower activity leading up to and between successive IBPs. FNB is the negative-polarity analog of streamer-based fast positive breakdown (FPB) that occurs as the initiating event of intracloud (IC) and -CG flashes. The FNB is also expected to be streamer-based, and progresses 100-150 m or so before dying out. The gamma bursts typically last less than 10 μ

The observations also provide an explanation for how the requisite spatially-extended runaway electron avalanches develop, and why FNB is favored over FPB for TGF production. Energy-based modeling of such streamer systems shows that the electric field is enhanced over a relatively large cross-sectional area ahead of the advancing system, independent of its polarity. Whereas avalanching that develops ahead of FPB will propagate into the advancing positive charge and be quenched, for FNB the avalanching would develop rapidly away from the advancing front and continue through the full extent and significantly greater available potential difference of a localized strong ambient E-field region.

The basic processes are expected to apply to the IBPs of upward negative breakdown at the beginning of IC flashes, and to be involved in producing upward TGFs detected by satellites.