Discharge Processes Associated with Terrestrial Gamma-ray Flashes (TGFs) and Lightning Flashes in General

High resolution spatial and temporal observations of the parent lightning flashes of four downward TGFs detected by the large-area Telescope Array in west-central Utah show that the TGFs are produced during strong, classic initial breakdown pulses (IPBs) in the first few ms of --CG and low-altitude IC flashes. Broadband VHF interferometer (INTF) and fast electric field change (FA) observations show that IBPs in general are produced by streamer-based fast negative breakdown (FNB) containing embedded sub-pulses. By virtue of the fact that several of the TGF events were initiated at the time of one or more of the IBP sub-pulses, we infer that the sub-pulses are produced by highly-impulsive 'spark' discharges that occur several times during the fast (~1--3 x 10⁷ m/s) streamer breakdown. Called transient conducting events (TCEs), the sub-pulse sparks appear to be isolated from each other within the negative streamer system and to initiate relativistic electron avalanches and ensuing TGFs at their negative tips by means of the cold runaway breakdown process. The latter is the basis for the long conducting leader model of TGF initiation, but instead of the sub-pulse sparks being connected to a long negative leader, the breakdown leading up to the IBP appears to also be due to negative streamers of intermediate speed (1--2 x 10⁶ m/s), with the streamer to leader transition occurring during a strong, backward-developing current evidenced by the characteristic opposite-polarity field change in the final half of the IBP sferic.

Upward negative breakdown of intracloud (IC) flashes in the same storm that produced two downward TGFs is shown to be produced by the same basic processes, except the IC development is more intermittent, sometimes generating complex clusters of multiple sub-pulses and IBPs lasting up to several hundred μs, and correspondingly longer step lengths. One of two such clusters during the Utah IC flash was essentially the same as that of the 4 Sept 2015 Florida IC flash studied by Lyu et al. (2018), which produced a 50 μs-duration Fermi-detected TGF. The only difference was the Florida IC initiated relativistic feedback in the middle of the cluster, while the Utah cluster produced a strong central IBP. The comparison indicates that long-duration TGFs are produced by complex IBP clusters that initiate relativistic feedback.