Radio Emission from Cosmic Ray Air Showers: Coherent Geosynchrotron Radiation

TL;DR

This paper models radio emission from cosmic ray air showers as coherent geosynchrotron radiation, explaining spectral and spatial features and aligning with experimental observations, aiding future radio detection efforts.

Contribution

It introduces a new model interpreting radio emission as coherent geosynchrotron radiation, incorporating realistic shower geometries to explain observed spectral and spatial characteristics.

Findings

Model reproduces qualitative emission spectrum trends

Radial dependence governed by synchrotron beaming and shower superposition

Consistent with experimental emission levels within systematic errors

Abstract

Cosmic ray air showers have been known for over 30 years to emit pulsed radio emission in the frequency range from a few to a few hundred MHz, an effect that offers great opportunities for the study of extensive air showers with upcoming fully digital "software radio telescopes" such as LOFAR and the enhancement of particle detector arrays such as KASCADE Grande or the Pierre Auger Observatory. However, there are still a lot of open questions regarding the strength of the emission as well as the underlying emission mechanism. Accompanying the development of a LOFAR prototype station dedicated to the observation of radio emission from extensive air showers, LOPES, we therefore take a new approach to modeling the emission process, interpreting it as "coherent geosynchrotron emission" from electron-positron pairs gyrating in the earth's magnetic field. We develop our model in a step-by-step procedure incorporating increasingly realistic shower geometries in order to disentangle the coherence effects arising from the different scales present in the air shower structure and assess their influence on the spectrum and radial dependence of the emitted radiation. We infer that the air shower "pancake" thickness directly limits the frequency range of the emitted radiation, while the radial dependence of the emission is mainly governed by the intrinsic beaming cone of the synchrotron radiation and the superposition of the emission over the air shower evolution as a whole. Our model succeeds in reproducing the qualitative trends in the emission spectrum and radial dependence that were observed in the past, and is consistent with the absolute level of the emission within the relatively large systematic errors in the experimental data.