Terrestrial Gamma-Ray Flashes (TGFs) Above Thunderstorms

Intense of gamma rays have been observed by five different space-borne detectors. The TGFs have hard spectra, with photons extending to over 50 MeV. Most of these flashes last less than a millisecond. Relativistic electrons and positrons associated with TGFs are also seen by orbiting instruments In a special mode of operation, the Fermi-GBM detectors are now detecting an average of about one TGF every two hours. The Fermi spacecraft has been performing special orientations this year which has allowed the Fermi-LAT instrument also detect TGFs. The most likely origin of these high energy photons is bremsstrahlung radiation from electrons, produced by relativistic runaway electrons in intense electric fields within or above thunderstorm regions; the altitude of origin is uncertain. These TGFs may produce an appreciable radiation dose to passengers and crew in nearby aircraft. The observational aspects of TGFs will be the main focus of this talk; theoretical aspects remain speculative.