In-ice Radio Signatures of Cosmic Ray Particle Cascades

TL;DR

This work uses the FAERIE Monte-Carlo framework to comprehensively characterize in-ice radio signatures from cosmic-ray-induced particle cascades, including both in-air and in-ice emissions. It demonstrates that in-ice emission dominates for vertical showers while in-air emission dominates for inclined showers, and it reveals contrasting frequency content ($<100$ MHz in air vs. $\sim$400 MHz in ice) and polarization patterns (geometric/azimuthal in air versus radial in ice). The study provides scaling laws linking radiation energy to primary energy and ground-part energy, exposes the presence of potentially distinctive double-pulse signatures, and discusses observational strategies for cosmic-ray identification and neutrino discrimination in future in-ice detectors. Collectively, these results supply practical guidelines for event identification, background rejection, and data-driven template generation in large-scale in-ice radio experiments such as RNO-G and IceCube-Gen2 radio.

Abstract

To detect ultra-high-energy neutrinos, experiments such as the Askaryan Radio Array and the Radio Neutrino Observatory in Greenland target the radio emission induced by these particles as they cascade in the ice, using deep in-ice antennas at the South Pole or in Greenland. A crucial step toward this goal is the characterization of the in-ice radio emission from cosmic-ray-induced particle showers. These showers form a primary background for neutrino searches, but can also be used to validate the detection principle and provide calibration signals for in-ice radio detectors. In this work, we use the Monte-Carlo framework FAERIE to perform the first characterization of cosmic ray signals with simulations that incorporate both their in-air and in-ice emissions. We investigate cosmic ray signatures such as their radiation energy, timing, polarization and frequency spectrum and quantify how they depend on shower properties. These results provide key guidelines for cosmic-ray identification and cosmic-ray neutrino discrimination in future in-ice radio experiments.

Figures (20)

• Figure 1: Sketch of a cosmic ray particle cascade and associated emissions. A first emission comes from the in-air cascade (black lines). This emission propagates in the atmosphere and is transmitted to the ice. Another emission is generated directly by the in-ice cascade (red lines) and can also reach deep in-ice antennas. For comparison a typical neutrino event is also shown.
• Figure 2: East-West component of the electric field simulated with FAERIE for the in-air ( left) and in-ice ( right) component of cosmic ray radio emission for a proton-induced shower with primary energy $E=10^{17.5}\, \rm eV$ and zenith angle $\theta=34^{\circ}$. Antennas are located at a depth of 100 m and placed at various radial distances from the shower core.
• Figure 3: ( Left) Sketch of antenna layers at various depths below the ice surface. The red arrow represents the shower axis, the black dotted curved line shows the in-ice continuation of the shower axis using Snell-Descartes law. Each antenna layer is centered on its intersection with the dotted line. The layer width increases with depth. ( Right) Typical antenna layout for a given layer. The central region is a dense core with 10-m spacing out to 150 m, followed by 15-m spacing up to 250 m. Beyond this, the array extends into 24 radial arms separated by $15^{\circ}$, with antennas placed along each arm at logarithmically increasing radial distances. Each given layer has 1568 antennas, the full grid corresponds to 4704 simulated antennas.
• Figure 4: ( Left) Height above sea level of the in-air shower maximum $X_{\rm max}$ from FAERIE simulations, as a function of the shower zenith angle, for different primary energies (colors). The blue shaded area indicates the ice surface. ( Right) Slant depth (along the shower axis), of the in-ice shower maximum, as a function of the shower zenith angle, for the same simulation set.
• Figure 5: Distributions of the in-air shower maximum grammage $X_{\rm max}$, for different primary energies.