Observation of a Downward Terrestrial Gamma‐Ray Flash Associated With Dart Leaders of a Negative Cloud‐To‐Ground Lightning Flash

We report the observation of a downward terrestrial gamma‐ray flash (TGF) during the dart‐leader stage in a negative cloud‐to‐ground lightning flash. The observation was recorded by the Telescope Array Surface Detector, in conjunction with a Lightning Mapping Array, an interferometer (INTF), a fast antenna, and a high‐speed camera. The return stroke was the fifth stroke of a negative cloud‐to‐ground lightning flash, which consisted of nine return strokes (RS) with three distinct ground‐strike points. The peak current of the return stroke associated with the TGF was the highest among the nine RS. Its dart leader had the highest propagation speed in the flash, measuring at m/s. The TGF was detected during the final stages of the dart leader, lasting 68 s. The TGF emission was detected within a few microseconds of the return stroke, indicating that it persisted until the final stage of leader development. The total deposited energy of 1,352 MeV was recorded by the TASD; the fluence was estimated to be 8 at the source, and the source altitude was estimated at 1.45 km above the ground. The observation of a downward TGF associated with a dart‐leader stage supports the idea that the production of X‐rays and downward TGFs are part of a continuum of energetic radiation related to lightning leaders.