The radio signal from extensive air showers

TL;DR

This paper reviews how radio signals emitted by extensive air showers caused by ultra-high energy cosmic rays can help determine their composition and understand their origins, focusing on emission mechanisms across frequencies.

Contribution

It provides a comprehensive review of radio emission mechanisms from air showers and discusses their potential in identifying cosmic ray composition.

Findings

Radio signals reflect the longitudinal profile of air showers

Emission mechanisms vary with frequency

Radio detection can constrain primary particle types

Abstract

The field of ultra-high energy cosmic rays made a lot of progresses last years with large area experiments such as the Pierre Auger Observatory, HiRes and the Telescope Array. A suppression of the cosmic ray flux at energies above $5.5x10^{19}$ eV is observed at a very high level of significance but the origin of this cut-off is not established: it can be due to the Greisen-Zatsepin-Kuzmin suppression but it can also reflect the upper limit of particle acceleration in astrophysical objects. The key characteristics to be measured on cosmic rays is their composition. Upper limits are set above $10^{18}$ eV on primary photons and neutrinos and primary cosmic rays are expected to be hadrons. Identifying the precise composition (light or heavy nuclei) will permit to solve the puzzle. It has been proven that the radio signal emitted by the extensive air showers initiated by ultra-high energy cosmic rays reflects their longitudinal profile and can help in constraining the primary particle. We review in this paper the emission mechanisms as a function of the frequency of the electric field.