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 x 10(18) photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor.