On the coherent emission of radio frequency radiation from high energy particle showers

Extended Air Showers produced by cosmic rays impinging on the earth atmosphere irradiate radio frequency radiation through different mechanisms. Upon certain conditions, the emission has a coherent nature, with the consequence that the emitted power is not proportional to the energy of the primary cosmic rays, but to the energy squared. The effect was predicted in 1962 by Askaryan and it is nowadays experimentally well established and exploited for the detection of ultra high energy cosmic rays. In this paper, we discuss in detail the conditions for coherence, which in literature have been too often taken for granted, and calculate them analytically, finding a formulation which comprehends both the coherent and the incoherent emissions. We apply the result to the Cherenkov effect, obtaining the same conclusions derived by Askaryan, and to the geosynchrotron radiation.