Terrestrial Gamma Flashes (TGFs) have been observed in satellite-borne gamma ray detectors for several decades, starting with the BATSE instrument on the Compton Gamma-Ray observatory in 1994. Subsequent observation and simulation efforts have led to a model in which TGFs are produced in relativistic runaway electron avalanches (RREA), during upward negative breakdown at the beginning of intracloud lightning discharges. This model suggests that TGFs should also be produced by the downward negative breakdown that occurs at the beginning of negative cloud-to-ground lightning flashes. Here, we present the first clear evidence for this effect.

The Telescope Array Surface Detector (TASD) is a 700 square kilometer cosmic ray detector, an array of scintillators on a 1.2 km grid. Following the observation of bursts of anomalous TASD triggers correlated with local lightning activity, a Lightning Mapping Array (LMA) and slow electric field antenna were installed at the TASD site in order to study the effect. In data collected with this suite of detectors between 2014 and 2016, we find that the anomalous triggers are clearly produced during the initial breakdown phase of fast, downward propagating, negative lightning leaders. The anomalous bursts are a few hundred microseconds in duration, similar to that seen in satellite observations of TGFs. While the TASD is not optimized for gamma ray detection, we present the results of simulations demonstrating that the fluxes and forward-beaming observed are consistent with production in RREA. We conclude that the anomalous triggers observed by TA are due to high energy radiation produced by the fast downward propagating negative leaders, and are most likely downward-directed Terrestrial Gamma Flashes.