Gamma Rays Made on Earth

In 1994, Fishman et al. observed brief (>1 ms) bursts of intense gamma rays, called terrestrial gamma-ray flashes, with the Compton Gamma-Ray Observatory (CGRO) (1). Prior to this observation, intense transient bursts of gamma rays were only known to occur in an astrophysical context. The observed photon energies of more than 1 MeV suggested that the gamma rays were produced by “bremsstrahlung” radiation from high-energy electrons. (Such radiation is emitted when energetic electrons are scattered by nuclei.) The flashes—the most energetic natural photon phenomena on Earth—are caused by upward beams of electrons (“runaways”) that are accelerated by thundercloud fields (2, 3).