Observation of Terrestrial Gamma-ray Flashes with Fermi LAT

Terrestrial Gamma-Ray Flashes (TGFs) are millisecond bursts of high energy photons, electrons, and positrons originating in Earth's atmosphere and associated with powerful thunderstorms. The Fermi GBM has detected hundreds of TGFs, some with energies up to 40 MeV. Recent AGILE observations of photons up to 100 MeV in TGFs pose a significant challenge to the relativistic runaway electron avalanche mechanism that is generally believed to be responsible for these bremsstrahlung gamma rays. With its large area, high segmentation, and low deadtime, the Fermi Large Area Telescope (LAT) is a powerful instrument for measuring the high energy emission of these short, intense transients. Here we present early results of a program of observing TGFs with the LAT.