First results on terrestrial gamma ray flashes from the Fermi Gamma-ray Burst Monitor

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope detected 12 intense terrestrial gamma ray flashes (TGFs) during its first year of observation. Typical maximum energies for most of the TGFs are similar to 30 MeV, with one TGF having a 38 MeV photon; two of the TGFs are softer and longer than the others. After correcting for instrumental effects, a representative bright TGF is found to have a fluence of similar to 0.7 photons cm (2). Pulses are either symmetrical or have faster risetimes than fall times; they are well fit with Gaussian or lognormal functions. The fastest risetime observed was 7 mu s, constraining the source radius to be less than about 2 km from the velocity of light. TGFs with multiple pulses separated in time have been known since their discovery; the GBM sample also includes clear cases of partially overlapping pulses. Four TGFs are associated with lightning locations from the World Wide Lightning Location Network. With the several mu s absolute time accuracy of GBM, the time order can be confidently identified: one TGF occurred before the lightning, two were simultaneous, and one TGF occurred after the lightning.