Scientists have finally obtained evidence of a rare cosmic phenomenon first predicted decades ago using a novel detector deep beneath the frozen southern continent of Antarctica.
Initially conceived during the Cold War by a Soviet physicist, researchers say the first experimental confirmation of radio pulses emanating from powerful cosmic rays, which scatter into small “waterfalls” of particles beneath Antarctic ice, has now been detected.
The result is a cascade of secondary particles carrying negative charges that produce radio frequencies as they move, according to astronomers with the Askaryan Radio Array (ARA) Collaboration, whose work aims to uncover the mysteries of some of the most powerful particles in the cosmos.
The team’s work was recently featured in Physical Review Letters.
A Cold War-era Prediction is Confirmed
The phenomenon dates back to the early 1960s, when physicist Gurgen Askaryan first proposed that high-energy particles would generate radio waves as they passed through dense materials. This phenomenon results from particles colliding with atoms, which produces an additional shower of particles that captures nearby electrons. The product of this electron-sweep is a sprinkling of negatively charged particles, which Askaryan predicted would generate detectable radio emissions.
Appropriately named “Askaryan radiation,” the existence of this phenomenon had already been confirmed in the lab, although the focus of ongoing experiments had sought to determine whether dense media found in nature—namely, thick sheets of ice—could create ideal circumstances for its generation.
This is no simple task, as physicists over the years who have failed to detect such evidence can attest. One reason for the difficulty is that background radio “noise” in ideal environments—such as Earth’s poles—is abundant enough to conceal it.
Another challenge in the past involved the limitations of simulations needed to properly model the effect in media such as ice. However, overcoming such issues in a demanding polar environment is exactly why the Askaryan Radio Array (ARA) was developed.
Anomalous Signals Emerge
With five individual stations in Antarctica, the ARA operates radio antennas at depths of up to 200 meters beneath the ice, covering an area of around 2 kilometers.
Observations spanning more than 200 days in 2019 first recorded a series of anomalies involving mysterious radio signals emanating from beneath the ice. At that time, more than a dozen anomalous events initially perplexed scientists. Since then, more advanced simulations have been developed, and the ARA team has been working to determine whether these mysterious radio signals could be the long-sought Askaryan phenomenon.
Other sources of emissions had been considered; in addition to background radio noise, possibilities include radio communications from the nearby Amundsen-Scott South Pole Station and even radio from nearby aircraft.
Fortunately, the ARA team’s analysis hit paydirt, as the key characteristics of the signals they examined possessed the anticipated hallmarks of the elusive Askaryan radiation. Overall, the team concluded that it remains highly unlikely that more than a dozen events could be attributed to common background sources or other known sources of radio emissions generated by human activity.
Cascades of Particle Showers
Fundamentally, the team’s findings resolved that the signals were the byproduct of vertically oriented cascades of cosmic particle showers, which, as they enter the uppermost portions of Antarctica’s ice sheets, propagate downward and generate the expected negative particle showers that produce radio emissions.
“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,” the team writes in their study.
“Considering the arrival angles, timing properties, and 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,” the ARA team adds.
This is significant because it is an important step toward detecting cosmic neutrinos, an elusive variety of high-energy subatomic particles that the ARA detector was designed to study. The successful detection of Askaryan radiation is an important advancement in that broader mission, since the conditions which produce it are remarkably similar to those which produce signals associated with neutrinos, with the primary difference being that neutrinos, unlike cosmic rays, can penetrate far deeper, which generates signals that emanate from a far steeper angle.
In the coming years, the team expects more than a dozen similar events involving neutrinos to be detected using the ARA array, which will help provide meaningful insights into some of the rarest cosmic phenomena.
The team’s recent paper, “Observation of In-Ice Askaryan Radiation from High-Energy Cosmic Rays,” appeared in Physical Review Letters.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.