Gamma Rays Made on Earth

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Abstract
About 10 years ago, researchers first observed terrestrial gamma-ray flashes in Earth's atmosphere. In his Perspective, Inan highlights the report by Smith et al., who find that such flashes are much more common than previously suggested and that they occur at even higher energies, up to 20 MeV. The flashes are associated with lightning and are probably caused by intense electron beams with energies of up to 20 MeV that are accelerated upward in the upper atmosphere, but the detailed mechanisms of their generation remain to be elucidated.