A detection technique for ultra-high-energy cosmic rays, complementary to the fluorescence technique, would be the use of the molecular bremsstrahlung radiation emitted by low-energy ionization electrons left after the passage of the showers in the atmosphere. In this article, a detailed estimate of the spectral intensity of photons at ground level originating from this radiation is presented. The spectral intensity expected from the passage of the high-energy electrons of the cascade is also estimated. The absorption of the photons in the plasma of electrons and neutral molecules is shown to be negligible. The obtained spectral intensity is shown to be $2\times 10^{-21}\mathrm{W}{\mathrm{cm}}^{-2}{\mathrm{GHz}}^{-1}$ at 10 km from the shower core for a vertical shower induced by a proton of $10^{17.5}\mathrm{eV}$. In addition, a recent measurement of bremsstrahlung radiation in air at gigahertz frequencies from a beam of electrons produced at 95 keV by an electron gun is also discussed and reasonably reproduced by the model.

• Received 5 January 2016

DOI:https://doi.org/10.1103/PhysRevD.93.052004