Accelerator Measurements of Magnetically Induced Radio Emission from Particle Cascades with Applications to Cosmic-Ray Air Showers

For 50 years, cosmic-ray air showers have been detected by their radio emission. We present the first laboratory measurements that validate electrodynamics simulations used in air shower modeling. An experiment at SLAC provides a beam test of radio-frequency (rf) radiation from charged particle cascades in the presence of a magnetic field, a model system of a cosmic-ray air shower. This experiment provides a suite of controlled laboratory measurements to compare to particle-level simulations of rf emission, which are relied upon in ultrahigh-energy cosmic-ray air shower detection. We compare simulations to data for intensity, linearity with magnetic field, angular distribution, polarization, and spectral content. In particular, we confirm modern predictions that the magnetically induced emission in a dielectric forms a cone that peaks at the Cherenkov angle and show that the simulations reproduce the data within systematic uncertainties.

Figure 1

(a) Schematic of the experiment, not to scale. (b) Simulated (Endpoints) electric field $|E|$ in the $x-z$ plane with full magnetic field, 131 pC, and 4.35 GeV.

Figure 2

(a) Simulated and measured voltages 6.52 m above the beam in the horizontal polarization channel at full magnetic field. (b) Corresponding power spectral densities.

Figure 3

(a) Horizontally polarized signal normalized by vertical showing the expected linear behavior versus magnetic field. (b) The oscilloscope traces (solid lines) show the polarity flip. Models (dashed lines) are shown for opposite polarities.

Figure 4

Beam patterns for three frequency bands in the horizontal polarization at 970 G.