Antarctic Radio Mystery: ANITA’s Upward Signals Still Unexplained After Pierre Auger Search

Overview: Radio pulses recorded over Antarctica by NASA's ANITA balloon program between 2016 and 2018 continue to puzzle researchers. The signals appeared to come from an upward direction — as if originating from beneath the ice — which is inconsistent with the usual downward arrival of cosmic-ray air showers.

Background

The Antarctic Impulsive Transient Antenna (ANITA) flew radio detectors on high-altitude balloons scanning the polar skies for radio emissions from cosmic rays interacting with Earth’s atmosphere. While most radio signatures ANITA was designed to detect matched expectations for downward-moving cosmic showers, a small number of detections showed an anomalous geometry consistent with upward-moving sources.

Follow-up with the Pierre Auger Observatory

An international team searched archival data from the Pierre Auger Observatory (2004–2018) to look for events that might match ANITA’s anomalous signals. Auger combines an array of water-Cherenkov detectors on the ground with optical telescopes that observe faint atmospheric fluorescence from particle showers. Because the hypothesized upward-going showers would not trigger Auger's ground array, the search relied primarily on the telescope data.

Simulations and analysis

The researchers ran millions of large-scale simulations modeling both conventional downward cosmic-ray cascades and hypothetical upward-going showers that could produce ANITA-like radio pulses. Those simulated datasets were used to develop selection criteria and to estimate how often an upward-going event should appear in Auger’s telescopes if that mechanism were responsible for ANITA’s detections.

Findings and interpretation

The Auger search identified only a single event with characteristics remotely similar to the ANITA anomalies. That scarcity — despite the extensive simulations and long time span of Auger data — suggests a significant mismatch between the frequency of ANITA detections and what would be expected if ordinary upward-going air showers were the full explanation.

The authors therefore conclude that the handful of ANITA events cannot be straightforwardly explained as routine upward-moving cosmic-ray air showers. Several speculative mechanisms remain under discussion, including unusual particle interactions, unexpected radio propagation effects in or beneath the ice, or instrumental or analysis biases. None are confirmed by the present evidence.

What’s next

To resolve the mystery, teams are planning targeted experiments: new balloon missions with improved instrumentation and a dedicated detector optimized to search for upward-going particle signals. Coordinated observations and improved instrumentation will be crucial for determining whether the ANITA pulses reflect new physics, an environmental propagation effect, or a subtle measurement artifact.

Conclusion: The ANITA radio anomalies remain an open question. The limited overlap between ANITA’s detections and comparable records at other observatories means more targeted data—and better cross-instrument checks—are needed before the source can be confidently identified.