Mystery from the Ice: ANITA’s Upward Radio Pulses Still Unexplained After Auger Search

Between 2016 and 2018, balloon-borne instruments from the Antarctic Impulsive Transient Antenna (ANITA) recorded a set of unusual radio pulses over Antarctica that resist a clear explanation. The signals appeared to propagate upward, as if originating beneath the ice, and did not match the radio signatures expected from ordinary cosmic-ray air showers.

Background: ANITA and the unusual pulses

ANITA flew radio antennas on high-altitude balloons to scan the Antarctic skies for brief radio flashes produced when ultra-high-energy particles interact with Earth’s atmosphere. While most detected signals fit known patterns, a handful of events stood out because their waveforms and arrival directions suggested an upward-moving source — an anomaly that sparked international interest and debate.

Follow-up with the Pierre Auger Observatory

An international team revisited the puzzle using data from the Pierre Auger Observatory, the world’s largest cosmic-ray detector. Auger combines a surface array of water tanks, which register secondary particles that reach the ground, with optical telescopes that observe faint fluorescence produced as particle showers traverse the atmosphere. Because genuinely upward-moving showers would not necessarily trigger Auger’s surface array, the team relied primarily on the observatory’s telescopic records to search for ANITA-like events in the 2004–2018 archive.

Simulations, analysis, and results

To discriminate real upward-going events from misidentified or spurious signals, the researchers ran millions of large-scale simulations of both ordinary downward cosmic-ray air showers and hypothetical upward-going showers that could reproduce ANITA-like radio signatures. They varied parameters, tested analysis chains, and checked for possible instrumental or environmental artifacts.

After exhaustive trials, the team found only a single event in the Auger archive that resembled the ANITA anomalies. This strong mismatch — ANITA’s multiple anomalous detections versus just one comparable Auger event over many years — led the authors to conclude that the ANITA pulses cannot be straightforwardly explained as a population of upward-going cosmic-ray showers detectable by Auger. The results were published in Physical Review Letters in 2025 under the title "Search for the Anomalous Events Detected by ANITA Using the Pierre Auger Observatory."

What this means and next steps

The Auger analysis does not solve the ANITA mystery. Possible explanations remain open: a rare background or atmospheric propagation effect, an unfamiliar radio phenomenon associated with particle interactions, or an instrumental artifact specific to balloon-borne measurements. To resolve the question, researchers are developing follow-up efforts, including a new balloon program similar to ANITA and a purpose-built ground detector optimized for the relevant particle and radio signatures.

These investigations highlight how much remains to be learned about ultra-high-energy particles, radio propagation in polar environments, and the complex interactions between cosmic phenomena and Earth’s surface. With improved instrumentation and targeted observations, scientists expect to pin down whether the ANITA pulses represent new physics, a rare natural effect, or an experimental artifact.