Molecular Bremsstrahlung Radiation at GHz Frequencies in Air

TL;DR

This paper estimates the spectral intensity of molecular Bremsstrahlung radiation in air at GHz frequencies as a potential method for detecting ultra-high energy cosmic rays, supported by experimental measurements.

Contribution

It provides a detailed theoretical estimate of GHz radiation from air showers and compares it with recent experimental data, advancing detection techniques for cosmic rays.

Findings

Spectral intensity at 10 km from shower core is approximately 2×10⁻²¹ W cm⁻² GHz⁻¹.

Photon absorption in plasma is negligible, simplifying detection considerations.

Experimental measurements align reasonably with the theoretical model.

Abstract

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/neutral molecules is shown to be negligible. The obtained spectral intensity is shown to be $2\times10^{-21} $W cm$^{-2}$ GHz$^{-1}$ at 10 km from the shower core for a vertical shower induced by a proton of $10^{17.5}$ 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.