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Radio Detection of ultra-high-energy Cosmic-Ray Air Showers

Frank G. Schröder

TL;DR

The paper addresses the challenge of characterizing ultra-high-energy cosmic rays with energy above $10^{16}$ eV using radio detection as a high-uptime, scalable alternative to optical techniques. It surveys the physical mechanisms behind radio emission from air showers—predominantly geomagnetic emission with a smaller Askaryan contribution—along with how coherent emission yields signals whose amplitude scales as $P \propto E^2$ and whose footprint depends on shower geometry and the Cherenkov angle (about $1^\circ$). It reviews the progression of radio experiments from first-generation arrays to current and planned facilities, detailing methods such as interferometry and template-fitting validated by CoREAS and ZHAireS simulations, achieving energy and $X_\mathrm{max}$ precision competitive with traditional techniques. The article highlights the role of stand-alone and hybrid (radio plus muon or scintillator) detectors in enabling per-event mass sensitivity and massive exposure, underscoring the technique's significance for next-generation cosmic-ray and multi-messenger astrophysics.

Abstract

Radio antennas have become a standard tool for the detection of cosmic-ray air showers in the energy range above $10^{16}\,$eV. The radio signal of these air showers is generated mostly due to the deflection of electrons and positrons in the geomagnetic field, and contains information about the energy and the depth of the maximum of the air showers. Unlike the traditional air-Cherenkov and air-fluorescence techniques for the electromagnetic shower component, radio detection is not restricted to clear nights, and recent experiments have demonstrated that the measurement accuracy can compete with these traditional techniques. Numerous particle detector arrays for air showers have thus been or will be complemented by radio antennas. In particular when combined with muon detectors, the complementary information provided by the radio antennas can enhance the total accuracy for the arrival direction, energy and mass of the primary cosmic rays. Digitization and computational techniques have been crucial for this recent progress, and radio detection will play an important role in next-generation experiments for ultra-high-energy cosmic rays. Moreover, stand-alone radio experiments are under development and will search for ultra-high-energy photons and neutrinos in addition to cosmic rays. This article provides a brief introduction to the physics of the radio emission of air showers, an overview of air-shower observatories using radio antennas, and highlights some of their recent results.

Radio Detection of ultra-high-energy Cosmic-Ray Air Showers

TL;DR

The paper addresses the challenge of characterizing ultra-high-energy cosmic rays with energy above eV using radio detection as a high-uptime, scalable alternative to optical techniques. It surveys the physical mechanisms behind radio emission from air showers—predominantly geomagnetic emission with a smaller Askaryan contribution—along with how coherent emission yields signals whose amplitude scales as and whose footprint depends on shower geometry and the Cherenkov angle (about ). It reviews the progression of radio experiments from first-generation arrays to current and planned facilities, detailing methods such as interferometry and template-fitting validated by CoREAS and ZHAireS simulations, achieving energy and precision competitive with traditional techniques. The article highlights the role of stand-alone and hybrid (radio plus muon or scintillator) detectors in enabling per-event mass sensitivity and massive exposure, underscoring the technique's significance for next-generation cosmic-ray and multi-messenger astrophysics.

Abstract

Radio antennas have become a standard tool for the detection of cosmic-ray air showers in the energy range above eV. The radio signal of these air showers is generated mostly due to the deflection of electrons and positrons in the geomagnetic field, and contains information about the energy and the depth of the maximum of the air showers. Unlike the traditional air-Cherenkov and air-fluorescence techniques for the electromagnetic shower component, radio detection is not restricted to clear nights, and recent experiments have demonstrated that the measurement accuracy can compete with these traditional techniques. Numerous particle detector arrays for air showers have thus been or will be complemented by radio antennas. In particular when combined with muon detectors, the complementary information provided by the radio antennas can enhance the total accuracy for the arrival direction, energy and mass of the primary cosmic rays. Digitization and computational techniques have been crucial for this recent progress, and radio detection will play an important role in next-generation experiments for ultra-high-energy cosmic rays. Moreover, stand-alone radio experiments are under development and will search for ultra-high-energy photons and neutrinos in addition to cosmic rays. This article provides a brief introduction to the physics of the radio emission of air showers, an overview of air-shower observatories using radio antennas, and highlights some of their recent results.

Paper Structure

This paper contains 5 sections, 1 equation, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Radio Emission of Air Showers
  3. Radio Experiments for Air Showers
  4. Future Developments
  5. Conclusion

Figures (5)

  • Figure 1: Air showers with different depth of shower maximum (A to C, where A is closest to ground) and the lateral distribution of the energy fluence of their radio emission illustrated in two ways: through the darkness of the color of the-two dimensional footprint and through the one-dimensional lateral distribution. The asymmetry is caused by interference of geomagnetic and Askaryan emission. The dip in the center is visible only for distant shower maxima and results from the emission being enhanced at the Cherenkov angle. Reprinted from Glaser:2018byo with permission from Elsevier.
  • Figure 2: Amplitude of the Askaryan emission $A$ relative to the geomagnetic $G$ emission for the ground level at the geographic South Pole for two different zenith angles as function of the distance from the shower axis and the distance to the shower maximum (determined with CoREAS simulations of the radio emission of air showers). The geomagnetic amplitude is corrected for the size of the geomagnetic Lorentz force with the plotted Askaryan fraction defined as $a_\mathrm{rel} = \frac{A}{G} \sin \alpha$, with $\alpha$ the angle between the Earth's magnetic field and the shower axis (figure modified from Ref. Paudel:2022tbe).
  • Figure 3: Location of radio experiments for air-shower detection and the local strength of the geomagnetic field [see reference in the figure and in the text].
  • Figure 4: Figure of merit (FOM) for the event-by-event sensitivity of various air-shower observables and their combination to separate air showers initiated by protons and iron nuclei. The results are for CORSIKA simulations with Sibyll 2.3c and 2.3d in the zenith angle range of $40^\circ - 60^\circ$ for the sites of the Pierre Auger Observatory and IceCube, respectively. $X_\mathrm{max}$ and the muon number $N_\text{\textmu}$ provide the highest separation power, where muons for the Auger site are muons at $800\,$m axis distance at the surface and for IceCube high-energy muons as detectable by the deep detector. $R_{\text{e}/\text{\textmu}}$ is the electron-muon ratio at ground. $X_\mathrm{max}$, $R$, and $L$ are parameters of the Gaisser-Hillas function describing the longitudinal shower profile. No detector simulation has been done, but some measurement uncertainties have been included as stated in the legend (figure modified from Ref. Flaggs:2023exc).
  • Figure 5: Photos of SKALA v2 antennas used in prototype stations of the IceCube-Gen2 surface array at IceCube at the South Pole (left) and at the Pierre Auger Observatory in Argentina (right).