Two Laterally Distant TGFs From Negative Cloud‐To‐Ground Strokes in Uchinada, Japan

We observed two Terrestrial Gamma‐ray Flashes (TGFs) in Uchinada, Japan associated with negative cloud‐to‐ground lightning strokes exactly 1 year apart on 18 December 2020 and 2021. The events were remarkable for their lateral distance from the associated strokes—each about 5 km away from the detector site. Not only was that lateral distance remarkable on its own for a ground based detection, but the low‐altitude profile of winter thunderstorms in Japan would suggest the detections occurred at unprecedented nadir angles—73.3° off axis for the 2020 event with the standard assumption of a vertically oriented TGF. Unsurprisingly, Monte Carlo simulations of the straightforward interpretation of these events yield fluences 2 orders of magnitude lower than observed data. We investigate a variety of ways to attempt to resolve the contradiction between expected and observed behavior. Plain Language Summary We observed two terrestrial gamma‐ray flashes at a distance and angle from which we should have seen nothing given our current understanding of how bright and wide these events can be. We investigate what must have been different about these events to make them visible to our detectors. Key Points We detected two downward TGFs in Uchinada, Japan, approximately 5 km away laterally from the associated lightning flash's ground strike point Estimates of the altitude of the tops of the associated leader channels from radio, optical, and climate data suggest we were, remarkably, up to 73.3° off of the vertical axes of the TGFs Monte Carlo simulations show our observations were 1–2 orders of magnitude brighter than what would be expected from a standard TGF at that geometry