On the nature of radio-wave radiation from particle cascades

TL;DR

This paper clarifies the physical mechanisms and frequency regimes of radio-wave radiation from particle cascades in the atmosphere and dense media, using simple geometrical arguments to explain emission characteristics.

Contribution

It provides a simplified, geometrical explanation of the radiation mechanisms and identifies frequency regimes where different emission processes dominate.

Findings

Askaryan radiation is bremsstrahlung-like below 1 GHz in air and 100 MHz in ice.

Geomagnetic emission resembles transverse current or synchrotron radiation depending on frequency.

Transitions in emission mechanisms can be observed experimentally.

Abstract

The nature of the radio-wave radiation generated by particle cascades in both the Earth's atmosphere and dense media such as ice has, historically, been much debated. This situation changed in the early 2010's, with the community converging on the common terminology of "geomagnetic" and "Askaryan" radiation to describe the two emission mechanisms. However, this convergence arose from discussions at various conferences and workshops, and was ultimately reached through agreement between simulation codes and experimental measurements. In this article therefore, I use relatively simple geometrical arguments, and a minimum of calculations based on single particle tracks, to explain the nature of radiation from extensive air showers (EAS) and cascades in dense media such as ice. I identify well-determined frequency regimes where the radiation from the Askaryan effect will be bremsstrahlung-like and Cherenkov-like, being respectively below/above 1 GHz in EAS and 100 MHz in dense media; and where geomagnetic emission will be transverse-current-like and where it will resemble synchrotron radiation, respectively below/above a few GHz in EAS, depending on the height of cascade development. I suggest how these transitions in the nature of the emission may be experimentally observed.