Terrestrial Gamma‐Ray Flashes, Relativistic Runaway, and High‐Energy Radiation in the Atmosphere

High energy radiation from tens of keV to tens of MeV in energy, the transition region from x‐rays to gamma rays, has been observed in association with high electric fields in air in many contexts: from the ground, associated with thunderstorms and with individual flashes of natural and rocket‐triggered lightning [1, 2, 3, 4, 5, 6, 7, 8] from sparks in the laboratory [9, 10, 11, 12], from Earth orbit in the form of millisecond Terrestrial Gamma‐ray Flashes (TGFs) [13, 14], and at intermediate altitudes from aircraft and balloons [15, 16, 17, 18, 19]. The gamma radiation in all cases is almost certainly bremsstrahlung generated by the collision of accelerated electrons with nuclei in the atmosphere. The penetrating gamma‐rays can provide a mechanism for coupling between the troposphere, mesosphere, ionosphere and magnetosphere in the form of energetic particles and ionization.