A neutrino telescope that makes use of thick Antarctic ice and a giant balloon is unexpectedly detecting mysterious, ultra-high-energy cosmic rays.
The Antarctic Impulsive Transient Antenna (ANITA) is a radio wave detector mounted on a giant balloon at an altitude of 38 kilometres. It is designed to detect the radio waves produced when a high-energy cosmic neutrino smashes into the thick ice below.
But although ANITA has yet to capture this signature, it has found another kind of particle: ultra-high-energy cosmic rays. These are protons and other charged nuclei that zoom in from space with mysteriously high energies.
At first, unexpected patterns of radio waves seen by ANITA were dismissed as noise. Then Eric Grashorn of Ohio State University in Columbus and colleagues noted that some of these radio waves had a similar pattern in their distribution of frequencies.
In what Grashorn calls a "serendipitous discovery", his team has worked out that these signals are generated by speeding electrons produced when cosmic rays collide with molecules in the air. These electrons spiral through Earth's magnetic field, emitting radio waves as they do so.
"They have actually found a new way to detect high-energy cosmic rays," says Francis Halzen of the University of Wisconsin in Madison. Halzen is a collaborator on IceCube, a giant neutrino telescope buried in the Antarctic ice.
So far, ANITA has captured the signature of 16 ultra-high-energy cosmic rays. With the ability to scan a vast area, the balloon telescope could one day rival the Pierre Auger Observatory in Argentina, which was built specifically to detect cosmic rays. Halzen reckons that ANITA's cosmic ray detections could end up being as important as its original neutrino mission.