A detector from the Askaryan Radio Array (ARA) near the South Pole captured 13 distinct impulsive radio bursts during a 208-day observation period in 2019 (using just one station). These signals originated from below the ice surface and have now been reanalyzed and confirmed as Askaryan radiation—coherent radio waves emitted by particle cascades in the ice.

The Askaryan effect (also called Askaryan radiation) is a powerful, coherent radio-frequency emission produced when an ultra-high-energy particle creates a dense electromagnetic shower in a transparent dielectric medium such as Antarctic ice, rock salt, or silica.

Physicist Gurgen Askaryan predicted it in 1962. It is now the cornerstone of radio-detection techniques for ultra-high-energy neutrinos and cosmic rays.

In 1962, physicist Gurgen Askaryan predicted that when a high-energy particle (like a cosmic-ray proton or nucleus) slams into dense matter such as ice, it triggers a shower of secondary particles. In this shower, a slight excess of negative charge builds up (electrons outnumber positrons slightly due to scattering and annihilation effects). This moving charge excess produces a brief, powerful pulse of radio waves (in the ~100 MHz to GHz range) via the Askaryan effect—essentially Cherenkov-like radiation but in radio frequencies, coherent and thus amplified at certain wavelengths.

This effect was previously observed in labs and in the atmosphere, but detecting it in situ within Antarctic ice proved technically challenging due to the need for buried antennas, low noise, and precise triggering.

The Askaryan Radio Array (ARA) embeds radio antennas deep in the ice at the South Pole. Its primary goal is hunting ultra-high-energy (UHE) cosmic neutrinos—ghostly particles that could reveal the origins of the universe’s most energetic cosmic rays. Neutrinos interact rarely, so radio detection in vast volumes of transparent ice (where radio waves travel kilometers with little attenuation) offers a scalable way to monitor huge effective volumes cheaply compared to optical detectors like IceCube.

Cosmic rays hitting the atmosphere create air showers whose cores can penetrate and interact in the upper ice layers, producing similar radio pulses. These serve as a background for neutrino searches but also a perfect calibration source. The 13 events matched predictions in:

• Arrival directions
• Signal shape and polarization
• Spectral content
• Overall rate

The statistical significance is high (~5.1 sigma; the chance of them being random background is less than 1 in 3.5 million).

This is the first experimental evidence of in-ice Askaryan radiation from cosmic rays. It validates the radio-detection technique and the detectors’ performance. Future arrays (like expansions of ARA, IceCube-Gen2’s radio component, or the Radio Neutrino Observatory in Greenland) can now confidently use this method to sift through data for the much rarer neutrino signals.

It doesn’t solve lingering mysteries from the ANITA balloon experiment (which detected anomalous upward-pointing radio pulses in 2016–2018 that didn’t fit standard models and sparked speculation about beyond-Standard-Model physics or exotic particles). Those ANITA events remain unexplained and are likely unrelated—the ARA signals here are downward/expected cosmic-ray induced and come from a buried in-ice setup.

In short, it’s a solid, exciting step forward in multi-messenger astrophysics and neutrino astronomy: the “cosmic whisper” is Askaryan radio pulses finally heard clearly in ice, opening the door wider for detecting the universe’s most elusive high-energy messengers.

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Observation of In-Ice Askaryan Radiation from High-Energy Cosmic Rays

“Observation of In-Ice Askaryan Radiation from High-Energy Cosmic Rays”
by the ARA Collaboration (lead author N. Alden et al.), published in Physical Review Letters 136, 151003 (2026).
arXiv: 2510.21104 (submitted October 2025, with an updated HTML version in April 2026).

This is the first experimental detection of in-ice Askaryan radiation produced by high-energy cosmic ray air showers interacting directly with the Antarctic ice sheet.

• Data: From a 208-day observation period in 2019 using the phased-array instrument at one station of the Askaryan Radio Array (ARA) at the South Pole.
• Events: 13 impulsive radio-frequency bursts originating from below the ice surface (i.e., not from the atmosphere or surface noise).
• Significance: The rate, directions, signal shapes, spectra, and polarization strongly match predictions for Askaryan emission from the dense cores of cosmic-ray-induced air showers penetrating and cascading in the ice. Background (thermal noise + surface events) is ruled out at 5.1σ significance (when including impulsivity; 3.5σ from geometry/timing alone). The observed rate is consistent with expectations (22.9 +8.2/−6.2 events per year for this setup).

For the brightest events, the angular radiation pattern even favors an extended cascade-like emitter (as expected for a shower) over a point-like source.

ARA Collaboration (N. Alden et al.)
 Physical Review Letters 136, 151003 (2026)
arXiv:2510.21104 [astro-ph.HE] (v2, 17 April 2026)
DOI: 10.1103/xwqy-yzrk. (PRL) | arXiv DOI: 10.48550/arxiv.2510.21104

Full Abstract (verbatim):

“We present the first experimental evidence for in-ice Askaryan radiation — coherent charge-excess radio emission — from high-energy particle cascades developing in the Antarctic ice sheet. In 208 days of data recorded with the phased-array instrument of the Askaryan Radio Array, a previous analysis has incidentally identified 13 events with impulsive radiofrequency signals originating from below the ice surface. We here present a detailed reanalysis of these events. The observed event rate, radiation arrival directions, signal shape, spectral content, and electric field polarization are consistent with in-ice Askaryan radiation from cosmic ray air shower cores impacting the ice sheet. For the brightest events, the angular radiation pattern favors an extended cascade-like emitter over a pointlike source. An origin from the geomagnetic separation of charges in cosmic ray air showers is disfavored by the arrival directions and polarization. Considering the arrival angles, timing properties, and the impulsive nature of the passing events, the event rate is inconsistent with the estimation of the combined background from thermal noise events and on-surface events at the level of 5.1σ.”

This Letter reports the first in-situ detection of Askaryan radio emission produced inside Antarctic ice by high-energy cosmic-ray air showers. It uses data from a single station (A5) of the Askaryan Radio Array (ARA) at the South Pole, collected March–November 2019.

Background & Motivation

• The Askaryan effect (predicted 1962) generates coherent radio pulses (~100 MHz–1 GHz) from the net negative charge excess in particle cascades in dense dielectrics like ice.
• ARA’s main goal is ultra-high-energy neutrino detection via the same mechanism, but cosmic-ray air showers provide a natural, abundant calibration source/background.
• When a cosmic-ray shower core penetrates the ice surface, a compact cascade develops in the top ~5–10 m, producing downward-going Askaryan radiation detectable by buried antennas.

Data & Analysis

• Instrument: Phased-array trigger on vertical-polarization (VPol) antennas at Station 5 (low threshold).
• Event selection: 13 impulsive RF events in the “target zenith” window (38° ≤ θ ≤ 57°), where surface/atmospheric signals are suppressed by total internal reflection.
• Backgrounds rigorously estimated:
  • Thermal noise: ~0.14 events/yr
  • Near-horizon sources: conservatively ~0.15 events/yr
  • On-surface events: upper limit ~0.12 events/yr (95% CL)
• Events are tested for impulsivity (peak-to-average power ratio), time clustering, azimuthal uniformity, and wind-speed independence (rules out triboelectric/atmospheric noise).

Key Results

• Observed rate: 22.9⁺⁸.²₋₆.² events per year — consistent with Monte Carlo predictions for ~10¹⁷ eV cosmic rays.
• Signal properties perfectly match simulations of in-ice Askaryan emission:
  • Broadband impulsive pulses
  • Radial polarization
  • Arrival directions (Kolmogorov–Smirnov p = 0.88 with CR-shower simulations)
• Brightest events: Angular radiation pattern statistically favors an extended cascade over a point source (ΔlogL > 1.5 for three events).
• Geomagnetic emission (from charge separation in air showers) is strongly disfavored (KS p = 2.6 × 10⁻¹⁵).
• Significance: 5.1σ (including impulsivity) or 3.5σ (geometry/timing alone) against combined background.

Implications

• Validates radio detection of cosmic rays via their in-ice component — a new, complementary channel.
• Critical calibration benchmark for ARA and future radio neutrino telescopes (IceCube-Gen2 radio, RNO-G, etc.).
• Demonstrates that the technique works in real Antarctic ice, including signal propagation and reconstruction.
• The full A5 dataset likely contains >100 similar events, enabling refined simulations and background studies ahead of first neutrino detections.