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X-ray and gamma-ray emissions observed in lightning and long sparks areusually connected with the bremsstrahlung of high-energy runaway electrons.Here, an alternative physical mechanism for producing X-ray and gamma-rayemissions caused by the polarization current and associated electromagneticfield moving with relativistic velocity along a curved discharge channel hasbeen proposed. It is pointed out that lightning and spark discharges shouldalso produce a coherent radio-frequency radiation. The influence of theconductivity and the radius of the lightning channel on the propagationvelocity of electromagnetic waves, taking into account the absorption, havebeen investigated. The existence of fast electromagnetic surface wavespropagating along the lightning discharge channel at a speed close to the speedof light in vacuum is shown. The possibility of the production of microwave,X-ray and gamma-ray emissions by a polarization current pulse moving along acurved path via synchrotron radiation mechanism during the lightning leadersteps formation and the very beginning of the return stroke stage is pointedout. The existence of long tails in the power spectrum is shown, which explainsobservations of photon energies in the range of 10-100 MeV in the TGF, as wellas measured power spectrum of laboratory spark discharge.

Mechanisms of X-ray and gamma-ray emissions in lightning and spark discharges