Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700-km 2 cosmic ray observatory located in southwestern Utah, USA. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3-D lightning mapping array, and electric field change measurements were Key Points:• Gamma ray showers have been detected in a surface scintillator array coincident with lightning observed by a lightning mapping array or Delta E antenna • The showers were produced less than 4-5 km above ground in the first 1-2 ms of downward negative breakdown during cloud-to-ground flashes • The source durations are better resolved than for satellite observations and are consistent with being produced by stepping of the initial leader breakdown Supporting Information:• Supporting Information S1 obtained for an additional seven flashes, in both cases colocated with the TASD. National Lightning Detection Network information was also used throughout. The showers arrived in a sequence of 2-5 short-duration (≤10 s) bursts over time intervals of several hundred microseconds and originated at an altitude of ≃3-5 km above ground level during the first 1-2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of ≃10 12 -10 14 photons for 16 ∘ half-angle beams. We conclude that the showers are terrestrial gamma ray flashes, similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful terrestrial gamma ray flashes.

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.

for the Telescope Array Collaboration Terrestrial Gamma-ray flashes (TGFs) are bursts of gamma-rays initiated in the Earth's atmosphere by atmospheric lightning. The Terrestrial Gamma-ray flashes discussed in this work were detected at ground level between 2014 and 2018, by the Telescope Array Surface Detector (TASD), a lightning mapping array installed in 2013, and a broadband interferometer and fast sferics sensor installed in 2018. The TASD is a 700 km 2 ultra high energy cosmic ray detector in the southwestern desert of Utah, U.S.A. It is composed of 507 (3 m 2) plastic scintillator detectors on a 1.2 km square grid. In 2013, a Lightning Mapping Array (LMA) detector and a Slow antenna (SA) were installed at the TASD site. The LMA is a three-dimensional total lightning location system, comprised of nine stations located within and around the TASD array. The SA records the electric field change in lightning discharges. The TASD has become one of the world leading instruments for detection of TGFs from the ground. The downward Gamma-ray showers observed by the TASD detector were all confined to the first 1-2 ms of intracloud and cloud to ground discharges, spanning an overall duration of several hundreds of microseconds. We hypothesize that the observed TGFs are similar to those detected by satellites, but that the TASD ground-based observations are able to detect both the temporal distribution at the source and the full footprint of the gamma shower on the ground. More importantly, the gamma ray bursts observations suggest that the TGFs were produced by one or two particularly energetic leader steps at the initial breakdown pulse (IBP) stage. To determine such correspondence, an interferometer (INTF) and a fast antenna were installed a few kilometers east of the TASD detector on July 2018. With this suite of lightning detection instruments together with the TASD cosmic ray observatory, LMA, and SA, we are able, to present, for the first time, observations of the TGFs clearly associated with the IBPs of downward cloud-to-ground flashes and intracloud flashes. This result sheds new light on the origins of Terrestrial Gamma-ray Flashes.

Simultaneous recordings of a downward-directed terrestrial gammaray flash (TGF), high-speed video images, and radio emissions • TGF events occurred while the leader was already branching below cloud base and even when it was halfway in its propagation to ground • Energetic downward-directed TGFs were associated with fast downward leaders that produced high return stroke peak currents

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