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In-ice Radio Signatures of Cosmic Ray Particle Cascades

This paper utilizes the FAERIE Monte-Carlo framework to characterize the in-ice radio signatures of cosmic-ray-induced particle showers, providing essential guidelines for distinguishing them from neutrino signals in future ultra-high-energy neutrino detection experiments.

Original authors: Simon Chiche, Simona Toscano, Krijn D. de Vries

Published 2026-03-20

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CC BY 4.0

Original authors: Simon Chiche, Simona Toscano, Krijn D. de Vries

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is sending us secret messages in the form of invisible, ultra-powerful particles called neutrinos. These particles are like ghosts; they can pass through entire planets without stopping. To catch them, scientists have built giant listening posts deep inside the ice sheets of Greenland and Antarctica. They use radio antennas buried hundreds of meters underground to listen for the tiny "radio whispers" these neutrinos make when they finally crash into the ice.

However, there's a problem. The universe is also full of cosmic rays (like cosmic hailstones) that crash into the atmosphere above the ice. When they hit, they create a massive shower of particles that also makes radio noise. This is the "static" on the radio that drowns out the "music" of the neutrinos. In fact, there are a thousand times more cosmic rays than neutrinos!

This paper is like a field guide for a detective. The authors used a super-computer simulation (a digital universe called FAERIE) to figure out exactly how the radio noise from cosmic rays looks when it reaches the deep-underground antennas. Their goal? To teach the detectors how to tell the difference between a "cosmic ray intruder" and a "neutrino guest."

Here is how they break it down using simple analogies:

1. The Two-Stage Fireworks Show

When a cosmic ray hits the atmosphere, it's like setting off a two-part fireworks display:

Part A (In-Air): The explosion happens high in the sky. The radio waves from this part travel down through the air and hit the ice.
Part B (In-Ice): Some of the debris from the sky explosion actually punches through the ice surface and starts a second explosion deep underground. This creates a second radio signal right next to the antennas.

The Neutrino is different. It skips the sky entirely and only creates the "Part B" explosion deep in the ice.

2. The "Fingerprint" Differences

The authors found that the "In-Air" and "In-Ice" signals have very different fingerprints, which helps the detectors tell them apart.

The Pitch (Frequency):
- The In-Air signal is like a deep bass drum. It has a low pitch (low frequency) because the explosion in the sky is spread out over a large area (about a meter thick).
- The In-Ice signal is like a high-pitched whistle. Because the explosion in the ice is tiny and compact (just a few centimeters), it creates a much higher pitch (high frequency).
- Analogy: If you hear a deep rumble, it's likely from the sky. If you hear a sharp squeak, it's likely from deep underground.
The Direction (Polarization):
- The In-Air signal is like a flat pancake. The radio waves vibrate mostly horizontally (side-to-side) because they are pushed by the Earth's magnetic field.
- The In-Ice signal is like a spinning top. The waves vibrate radially (outward in all directions) because of how the ice compresses the charge.
- Analogy: If the radio wave is shaking side-to-side, it's probably a cosmic ray from the sky. If it's spinning outward, it might be a neutrino or a deep cosmic ray crash.
The Depth Sensitivity:
- The In-Air signal is like a distant lighthouse. No matter how deep you dig your antenna, the signal strength stays roughly the same because the source is so far away.
- The In-Ice signal is like a campfire. If you are standing right next to it (shallow depth), it's blazing hot. If you move just a few meters away (deeper depth), the heat drops off quickly.
- Analogy: If the signal gets much weaker as you go deeper, it's coming from right below you (In-Ice). If it stays steady, it's coming from far away (In-Air).

3. The "Double-Clap" (Double Pulses)

Sometimes, the antennas hear both signals at once. It's like hearing a double clap:

First, you hear the "sky boom" (In-Air).
A split second later, you hear the "underground thud" (In-Ice).

The paper found that for very energetic cosmic rays, about 40% of the time, the antennas will catch this "double clap." This is a huge clue! If a detector hears a double clap, it knows for sure: "This is a cosmic ray, not a neutrino!"

Why Does This Matter?

Think of the neutrino detectors as trying to find a single specific bird singing in a forest full of crickets.

Before this paper, scientists knew the crickets were there, but they didn't have a perfect guide on how to distinguish their chirps from the bird's song.
This paper provides the rulebook. It tells scientists: "If the sound is high-pitched and radial, ignore it. If it's a double clap, ignore it. If it's a deep, horizontal rumble, ignore it."

By using these rules, future experiments (like the ones in Greenland and at the South Pole) can filter out the "cricket noise" (cosmic rays) much more effectively. This clears the way to finally hear the "bird song" (neutrinos) and unlock the secrets of the most powerful explosions in the universe.

1. Problem Statement

Ultra-high-energy (UHE) neutrinos ( $E > 10^{16}$ eV) are unique messengers for probing the high-energy universe. Detecting them via radio emission in ice (Askaryan effect) is a promising technique due to the long attenuation length of radio waves in polar ice. However, a major challenge is the overwhelming background of cosmic rays (CRs).

The Background Problem: The flux of cosmic rays is at least 1,000 times higher than that of neutrinos. CRs interacting in the atmosphere can produce radio signals that reach deep in-ice detectors, mimicking neutrino-induced cascades.
The Gap: While neutrino signatures are well-characterized (pure Askaryan emission in ice), the radio signatures of cosmic rays are complex. CRs generate a dual emission:
1. In-air emission: From the atmospheric shower (Geomagnetic + Askaryan mechanisms).
2. In-ice emission: From secondary particles penetrating the ice and triggering a cascade (Askaryan mechanism only).
Objective: The paper aims to perform the first comprehensive characterization of these dual cosmic-ray radio signatures as seen by deep in-ice detectors to enable effective discrimination between CRs and neutrinos.

2. Methodology

The authors utilized FAERIE, a numerical framework combining CORSIKA (for air showers), GEANT4 (for particle transport in matter), and CoREAS (for radio emission), coupled with a ray-tracing algorithm.

Simulation Setup:
- Primary Particles: Protons (chosen for their large shower-to-shower fluctuations, which encompass heavier nuclei like iron).
- Energy Range: $10^{16.5}$ to $10^{17.5}$ eV.
- Geometry: Zenith angles from $0^\circ$ (vertical) to $50^\circ$ . Azimuth fixed to magnetic North.
- Environment: Modeled after Summit Station, Greenland (ice density profile, magnetic field).
- Detector Grid: A 3D grid of 4,704 antennas at depths of 0 m, 60 m, and 100 m, designed to capture both the dense Cherenkov ring of in-ice emission and the sparse footprint of in-air emission.
Analysis: The study analyzed electric field time traces to derive observables including radiation energy, frequency spectra, polarization ratios ( $H_{pol}/V_{pol}$ ), and timing (double pulses).

3. Key Contributions

This work provides the first exhaustive characterization of cosmic-ray radio signatures in a dense, inhomogeneous medium (ice) from the perspective of deep in-ice observers. Key contributions include:

Dual-Component Modeling: Explicitly separating and characterizing the "in-air" (geomagnetic/Askaryan) and "in-ice" (Askaryan) components of the same cosmic-ray event.
Scaling Laws: Establishing robust scaling relations for radiation energy with primary energy and shower geometry, allowing for the rescaling of simulation datasets.
Discrimination Observables: Identifying specific physical signatures (frequency, polarization, depth dependence, double pulses) that distinguish CRs from neutrinos.

4. Key Results

A. Emission Components and Geometry

Vertical Showers ( $\theta \approx 0^\circ$ ): The in-ice component dominates. The in-air shower maximum ( $X_{max}$ ) is often below or near the ice surface, leading to strong in-ice Askaryan emission.
Inclined Showers ( $\theta \gtrsim 20^\circ$ ): The in-air component dominates. The distance between $X_{max}$ and the ice increases, attenuating the particle flux reaching the ice. The in-ice signal becomes negligible for very inclined showers.
Footprints: In-air footprints are "bean-shaped" (interference between geomagnetic and charge-excess) and elongated for inclined showers. In-ice footprints are rotationally symmetric Cherenkov rings (for vertical showers) that deform with depth due to refraction.

B. Radiation Energy and Scaling

Energy Scaling:
- In-air: Radiation energy ( $E_{rad}$ ) scales linearly with primary energy ( $E_{rad} \propto E_{primary}$ ).
- In-ice: Radiation energy scales super-linearly ( $E_{rad} \propto E_{primary}^{>1}$ ) because it depends on the number of particles reaching the ice surface, which is exponentially sensitive to the distance between $X_{max}$ and the ground.
Zenith Dependence:
- In-air: Radiation energy increases with zenith angle (due to lower air density enhancing geomagnetic emission and larger geomagnetic angles).
- In-ice: Radiation energy decreases rapidly with zenith angle (due to attenuation of particles before reaching the ice).

C. Signal Observables for Discrimination

Frequency Spectrum:
- In-air: Peaks at low frequencies (< 100 MHz) due to the meter-scale shower front thickness.
- In-ice: Peaks at high frequencies (~400 MHz) due to the centimeter-scale shower front in dense ice.
Polarization ( $H_{pol}/V_{pol}$ ):
- In-air: Dominated by Horizontal Polarization ( $H_{pol}$ ) due to the geomagnetic mechanism. The ratio $H_{pol}/V_{pol}$ is very high (>10).
- In-ice: Shows Radial Polarization (Askaryan), resulting in a much lower, roughly constant $H_{pol}/V_{pol}$ ratio (~2).
- Significance: Neutrinos (pure Askaryan) resemble the in-ice CR component in polarization, but the in-air CR component is distinct.
Depth Dependence:
- In-air: Signal amplitude is relatively constant with antenna depth.
- In-ice: Signal amplitude decreases rapidly (factor of 2-3) as depth increases from 60m to 100m. This is a powerful tool to reject distant anthropogenic backgrounds (e.g., airplanes) which would show smooth depth profiles.
Double Pulses:
- When both components reach an antenna, they create a double pulse (in-air pulse arrives first, followed by the in-ice pulse).
- Occurrence rate peaks at intermediate zenith angles ( $\sim 34^\circ$ ) and can reach ~40% of triggered events for high-energy showers.
- More likely to be detected in horizontally polarized channels.

5. Significance

Neutrino Discovery: The identified signatures provide critical tools for cosmic-ray/neutrino discrimination. Since neutrino events are expected to arrive at high zenith angles (horizontal trajectories) and produce pure Askaryan signals, the dominance of the in-air component (low frequency, high $H_{pol}$ ) in inclined cosmic rays offers a clear rejection handle.
Detector Calibration: Cosmic rays serve as a natural calibration source. The distinct depth dependence and double-pulse features allow for in-situ validation of detector response and timing.
Future Experiments: These findings are directly applicable to current and next-generation experiments like the Askaryan Radio Array (ARA), Radio Neutrino Observatory in Greenland (RNO-G), and IceCube-Gen2. The results suggest that surface veto antennas (to catch the in-air component) combined with deep in-ice analysis of polarization and frequency can significantly reduce background rates.
Simulation Efficiency: The established scaling laws allow researchers to generate large datasets of simulated events by rescaling a limited set of high-fidelity simulations, drastically reducing computational costs.

In conclusion, the paper establishes that while cosmic rays are a formidable background, their radio signatures possess unique, multi-dimensional features (spectral, polarization, spatial, and temporal) that can be exploited to identify them and isolate the rare neutrino signals.