Terrestrial gamma‐ray flashes (TGFs) are bright, sub‐millisecond bursts of gamma‐rays, originating within the Earth's atmosphere. Most TGFs have been detected by spacecraft in low‐Earth orbit. Only two TGFs have previously been observed from within our atmosphere: one at ground level and one from an aircraft at 14.1 km. We report on a new TGF‐like gamma‐ray flash observed at ground level, detected by the 19‐station Thunderstorm Energetic Radiation Array (TERA) at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT). The gamma‐ray flash, which had a duration of 52.7 μs, occurred on June 30, 2009 during a natural negative cloud‐to‐ground lightning return stroke, 191 μs after the start of the stroke. This event is the first definitive association of a gamma‐ray flash with natural CG lightning and is among the most direct links to a specific lightning process so far. For this event, 19 gamma‐rays were recorded, with the highest energy exceeding 20 MeV. The high‐energy radiation exhibited very different behavior from the typical x‐ray emission from lightning. Specifically, the gamma‐ray flash had a much harder energy spectrum, consistent with relativistic runaway electron avalanche (RREA) multiplication; it did not arrive in sub‐microsecond bursts, typical of leader emission from lightning, and it occurred well after the start of the return stroke, which has not been previously observed for the x‐ray emission from lightning. Nevertheless, we present evidence that the source region for the gamma ray flash was the same as that for the preceding leader x‐ray bursts.
