A lightning-based search for nearby observationally dim terrestrial gamma ray flashes

Current space-based observations of terrestrial gamma ray flashes (TGFs) are capable of identifying only TGFs that exceed a lower brightness threshold. Observationally, dim TGFs that fall below this threshold are consequently difficult to find using photon-only search algorithms. Such TGFs are a potentially important part of the overall global TGF rate, and information on their occurrence rate would give important insight into TGF generation mechanisms. We describe and implement a lightning-based search for TGFs that uses the location and time of National Lightning Detection Network reported positive polarity, in-cloud (+IC) discharges of the type known to be directly associated with TGFs. These events identify a 200 mu s search window when any associated TGF photons would have been detected. We show that this approach can detect TGFs without requiring a lower threshold on the detected photon brightness of the event, and thus is capable, in principle, of finding a population of weak TGFs. We find that TGFs occur at a rate between 1 in 40 and 1 in 500 of in-cloud lightning events that meet our study's criteria. The distribution of gamma ray counts in the search windows exhibits a statistically significant lack of nearby dim TGFs below the GBM search threshold. The data favor a brightness distribution in which nearby observationally dim TGFs are rare.