Gamma-rays observed at ground level in coincidence with downward lightning leaders

The Telescope Array Surface Detector (TASD) is a cosmic ray observatory in western Utah, consisting of over 500 detectors and covering over 700 km². Between 2014 and 2016, ten bursts of gamma-ray showers were observed in coincidence with downward-propagating negative leaders in lightning flashes by the TASD. For three of the showers, full spatial and temporal structure was recorded using an implementation of the Lightning Mapping Array (LMA). During the other seven events, electric field data was recorded. Information on each of the flashes was also provided by the National Lightning Detection Network (NLDN). Each shower consisted of a burst of 2-5 TASD events during the first 1-2 ms of lightning development. Detector footprints and waveforms suggest the showers were composed primarily of downward-beamed gamma radiation. Every flash began with downward negative leader breakdowns approximately 3-5 km above ground level. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites. From the ground, however, observations are more sensitive and representative of source activity.