Fermi Gamma-ray Space Telescope

Terrestrial Gamma ray Flashes as Seen by the Fermi LAT

J. E. Grove
M. M. Schaal, A. Chekhtman, on behalf of the Fermi LAT team, M. S. Briggs, V. Connaughton, R. H. Holzworth, S. Heckman, S. A. Cummer

Abstract:

The Fermi Large Area Telescope (LAT) regularly detects Terrestrial Gamma-ray Flashes (TGFs) during its nominal astrophysical sky-survey observing program. Because of the LAT's flexible trigger logic, TGF emissions at and above 10 MeV are detected with high sensitivity despite their having arrived from outside the instrument's field of view. We can exploit the LAT's imaging capability toderive geolocations of bright Terrestrial Gamma ray Flashes (TGFs) directly in gamma rays to compare with geolocations given by LF and VLF radio networks intended to study lightning discharges. We show that the Fermi LAT can geolocate bright TGFs in favorable geometries with accuracies <100 km, a factor of a few larger than radio geolocation accuracies. A large fraction of TGFs appear to be temporally coincident with radio pulses detected by lightning geolocation network. Recent work by Cummer et al. (2011), Connaughton et al. (2013), and Dwyer and Cummer (2013) strongly suggests that the pulse of relativistic electrons that generates the gamma rays via bremsstrahlung also generates the radio signal via its secondary ionization electrons. Our analysis confirms this picture by establishing that the radio and gamma ray signals are indeed temporally and spatially coincident. Work at NRL is supported under contract with NASA and by the Chief of Naval Research.