Geolocation of Terrestrial Gamma Ray Flashes in Gamma Rays Using the Fermi Large Area Telescope

We derive geolocations of bright Terrestrial Gamma ray Flashes (TGFs) directly in gamma rays using the Fermi Large Area Telescope (LAT) and compare with geolocations derived from LF and VLF (radio) networks. Imaging of the gamma ray direction is made possible by the fine spatial resolution of the LAT instrument, which is intended to make maps of the high-energy gamma ray astrophysical sky. A more realistic Monte Carlo simulation and improved direction reconstruction algorithm allowed us to geolocate very bright TGFs in favorable geometries with accuracies of several tens of km. Recent work by Cummer et al. (2011), Connaughton et al. (2013), and Dwyer and Cummer (2013) strongly suggests that the broadband radio signal is produced by large electric currents (generated by ionization of relativistic electrons) that create the gamma ray signal through bremsstrahlung interactions in the atmosphere. Our analysis confirms this picture by establishing that the radio and gamma ray signals are both temporally and spatially coincident. This work was performed at NRL and sponsored by NASA DPR S-15633-Y.