Studying the Optical Emission of Lightning at the Telescope Array Detector

Lightning flashes consist of many different stages, two key stages include stepped and dart leaders. Stepped leaders are the downward-propagating ionized channels that establish a new path for the initial return stroke in a flash or stroke group. Dart leaders follow in subsequent strokes, re-energizing the pre-existing channel created by the stepped leader to initiate additional return strokes. We report the first observation of a downward Terrestrial Gamma-ray Flash (TGF) during the dart-leader stage of a negative cloud-to-ground lightning flash. The event was recorded by the Telescope Array Surface Detector (TASD), alongside a Lightning Mapping Array (LMA), interferometer (INTF), fast antenna, and high-speed camera. The fifth stroke was associated with the TGF and had the highest peak current of -67.3 kA. Our findings support the idea that X-ray emissions and TGFs represent a continuous spectrum of energetic radiation produced by lightning leaders.