  In this paper we report the first close, high-resolution observations of
downward-directed terrestrial gamma-ray flashes (TGFs) detected by the
large-area Telescope Array cosmic ray observatory, obtained in conjunction with
broadband VHF interferometer and fast electric field change measurements of the
parent discharge. The results show that the TGFs occur during strong initial
breakdown pulses (IBPs) in the first few milliseconds of negative
cloud-to-ground and low-altitude intracloud flashes, and that the IBPs are
produced by a newly-identified streamer-based discharge process called fast
negative breakdown. The observations indicate the relativistic runaway electron
avalanches (RREAs) responsible for producing the TGFs are initiated by embedded
spark-like transient conducting events (TCEs) within the fast streamer system,
and potentially also by individual fast streamers themselves. The TCEs are
inferred to be the cause of impulsive sub-pulses that are characteristic
features of classic IBP sferics. Additional development of the avalanches would
be facilitated by the enhanced electric field ahead of the advancing front of
the fast negative breakdown. In addition to showing the nature of IBPs and
their enigmatic sub-pulses, the observations also provide a possible
explanation for the unsolved question of how the streamer to leader transition
occurs during the initial negative breakdown, namely as a result of strong
currents flowing in the final stage of successive IBPs, extending backward
through both the IBP itself and the negative streamer breakdown preceding the
IBP.
