Terrestrial lightnings produce high-speed electrons that create bursts of gamma-rays as bremsstrahlung. The energy of these rays is sometimes sufficient to start photonuclear reactions resulting in emitted neutrons. One such reaction, 7 N (γ,n) 7 N, is the only natural process other than those induced by cosmic rays in which 7 N is produced on Earth. The unstable isotopes remaining from the reaction may subsequently emit positrons by β decay.