A Four-Year Fermi Large Area Telescope Survey of Terrestrial Gamma-ray Flashes

The Fermi 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. Bright TGFs can be imaged with good accuracy for comparison with VLF geolocations. A deep search of the first four years of LAT data reveals about 200 TGFs with hard gamma-ray emission, of which many were independently detected by Fermi GBM. Here we present a summary of the spectral, temporal, diurnal, and geographic characteristics of this sample of high-energy TGFs. This work was performed at NRL under NASA sponsorship.