Intermediate Fluence Downward Terrestrial Gamma Ray Flashes as Observed by the Telescope Array Surface Detector

On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one-hour timeframe. The TGFs occurred during the initial stage of negative cloud-to-ground flashes whose return strokes had increasingly large peak currents up to (Formula presented.) 223 kA, 147 GeV energy deposit in up to 25 1.2 km-spaced surface detectors, and intermittent bursts of gamma-rays with total durations up to 717 (Formula presented.) s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high-speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of (Formula presented.) and 8 (Formula presented.), logarithmically bridging the gap between previous TASD and satellite-based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production.