ANITA in flight, as seen from a telescope on the ground. Fully inflated it is bigger than a detector at the Large Hadron collider (LHC).
They are looking for the answer to one of the great questions in astrophysics. We know that there are really really high energy particles hitting the Earth all the time from outer space. We would really like to know where they are coming from. The aim of ANITA (Antarctic Impulsive Transient Antenna) is to address this by looking for neutrinos.
In their first flight, though, they found something else.
What they actually detect is short radio bursts. As part of what may become (along with coffee seen by neutrons) a series of weird ways to look at the world, here’s the the image ANITA builds up of the continent in radio bursts.
The Antarctic in radio waves.
They can measure the polarization of the radio waves – which direction the electromagnetic field oscillates in as the waves travels. Neutrino interactions produce vertically polarized pulses. They didn’t see any of these. But they did see 16 pulses of horizontally polarized radio waves.
These turned out to be the signature of ultra high energy particles hitting the atmosphere and causing a cascade of particles – “cosmic-ray air-showers”. In these showers, electron-positron pairs are produced and they spiral around the Earth’s magnetic field lines, giving a characteristic radio signal seen by ANITA. As Ryan puts it:
The cosmic-ray air-shower radio signals were really unexpected and we only found them by checking a ‘background’ event sample for the neutrino search. It took us a long time to understand their significance to the point that for the second ANITA we removed the horizontal polarisation from the trigger to maximise neutrino efficiency. Whoops! Needless to say we will be reinstating it for the third flight.
These showers have been seen before, for example by the Auger experiment. But new ways of seeing them, and measuring where they come from, are valuable. ANITA has, serendipitously, demonstrated an important new technique with a lot of potential.