Spectral Analysis of Flickering Gamma‐Ray Flashes Observed During the ALOFT 2023 Campaign

We examine a newly discovered category of high‐energy radiation events called flickering gamma‐ray flashes (FGF) detected during the airborne lightning observatory for FEGS and TGFs campaign in July 2023. These FGFs are characterized by highly pulsating gamma‐ray emissions that have no detectable concurrent optical or radio signals (Ostgaard et al., 2024, https://doi.org/10.1038/s41586‐024‐07893‐0 ). They are consistently followed by a Narrow Bipolar Event (NBE) occurring a few to 50 milliseconds after the last gamma‐ray pulse. We perform a comprehensive spectral analysis of the FGFs using forward modeling and Monte Carlo simulations. We assume the NBE occurs either at the same location as the gamma‐ray emission or close to it ( kilometer). Of 24 FGF events, we have position constraints for 21 and additional altitude constraints for 3. With sufficient photon counts, we can determine source photon brightness within a few orders of magnitude, down to one order of magnitude for some events. All FGFs are consistent with a fully developed relativistic runaway electron avalanche spectrum with varying atmospheric attenuation, even at low photon source brightnesses ( to , above 100 keV). FGFs are produced in the atmosphere (8–16 km altitude) with a large range of source photon brightness, from to about .We estimate they are on average 3 orders of magnitude below the ASIM‐MXGS space detection threshold, though a small part of the FGF population could be detected from space. FGFs have been detected by CGRO‐BATSE, and recently confirmed in ASIM data with several candidates. We encourage researchers to search for these events in data sets collected by current space, air and ground instruments.