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Incoming cosmic ray creates a flash of Askaryan radiation inside the ice. Credit: ARA Collaboration
A detector buried deep in Antarctic ice has captured the first experimental evidence of a predicted but never-before-seen phenomenon: radio pulses generated when high-energy cosmic rays slam into the ice sheet and trigger particle cascades inside it. Through results published in Physical Review Letters, astronomers of the Askaryan Radio Array (ARA) Collaboration have validated a key technique, which they hope will eventually allow them to detect some of the rarest and most energetic particles in the universe.
Flashes in the ice
In 1962, Soviet physicist Gurgen Askaryan predicted that high-energy particles passing through a dense material should produce a distinctive burst of radio waves. When such a particle strikes an atom, it triggers a cascade of secondary particles that sweeps up electrons from the surrounding material, creating a negatively charged shower front that radiates at radio frequencies.
This "Askaryan radiation" was later confirmed in lab experiments and detected in air, but observing it in ice proved far more challenging. This is partly due to the difficulty of distinguishing genuine signals from the many sources of radio noise in polar environments, and partly because the simulations needed to model the effect in ice have only recently become sophisticated enough to make such rigorous analysis possible.
The Askaryan Radio Array (ARA), located near the South Pole, is designed with exactly this challenge in mind. It consists of five stations, each equipped with radio antennas sunk 150 to 200 meters into channels drilled through the ice, spread across an area roughly 2 kilometers wide.
Matching expectations
During a 208-day observation campaign in 2019, the ARA team recorded 13 anomalous events: impulsive radio signals arriving from below the ice surface, whose origins were initially unclear. Using newly available simulation tools, the researchers set about determining whether these events were genuine Askaryan signals or the result of background interference from sources such as aircraft radar, or radio communications from the nearby Amundsen-Scott South Pole Station.
The analysis found that the signals' characteristics (their arrival directions, frequency content, waveform shape, and the orientation of their electric fields) all matched predictions for Askaryan radiation produced by cosmic rays striking the ice. The probability that all 13 events could be explained by background noise alone was less than one in 3.5 million, a statistical significance of 5.1 sigma—well above the threshold conventionally required to claim a discovery.
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