Determining a Lower Limit of Luminosity for the First Satellite Observation of a Reverse Beam Terrestrial Gamma Ray Flash Associated With a Cloud to Ground Lightning Leader

We provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020, https://doi.org/10.1029/2020GL089427). A TGF produced 3 ms prior to a negative cloud‐to‐ground return stroke was close to simultaneous with an isolated low‐frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In previous observations, this “slow” low‐frequency signal has been strongly correlated with upward‐directed (opposite polarity) TGF events (Pu et al., 2019, https://doi.org/10.1029/2019GL082743; Cummer et al., 2011, https://doi.org/10.1029/2011GL048099), leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward‐directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined that the most likely source altitude of the TGF reverse beam was 7.5 km ± 2.6 km, just below an estimated negative charge center at 8 km. At that altitude, the Monte Carlo simulations indicate a lower luminosity limit of 2 × 1018 photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor. Plain Language Summary Pu et al. (2020, https://doi.org/10.1029/2020GL089427) published an article on a terrestrial gamma ray flash (TGF) observation made by the Fermi satellite with a simultaneous radio wave observation of a lightning event almost directly below Fermi’s position. The radio data unequivocally showed that the lightning event was negative cloud to ground (−CG). This implied that the electric field orientation of the lightning would have produced a downward TGF with a source altitude near 6 km. Fermi must have seen the reverse beam, which is the consequence of antimatter production within the strong electric fields that produce the TGF. Predictions of the reverse beam indicate that it is far less bright than the main forward beam. The scenario seems at first glance very unlikely, as it would require the TGF to be far brighter than any previous observation. It turns out that characteristics of the antimatter‐induced reverse beam, such as it