Ballooning For Cosmic Rays

The higher energy cosmic rays are rare," he continued. "For example, each day ATIC collects no more than ~10 cosmic rays with energies exceeding 10¹³ eV. That's why we have to fly the balloon for such a long time, to gather enough particles for a statistically significant result." By the time ATIC landed in mid-January 2001, the spectrometer had been in the stratosphere counting cosmic rays for nearly two full weeks.

The long flight time, more than any other reason, is why the researchers chose to fly the balloon over Antarctica. "We would be happy to fly this payload over North America," says Adams. "The problem is that we needed the spectrometer to be aloft for a long time. Antarctica has two advantages: It's international territory, so we don't need to apply for lots of overflight permissions, and the Antarctic Vortex (a circulating weather system around the south pole) keeps the balloon confined to airspace over the continent."

"If there is a difference between the proton and the helium spectra - and that's not certain, by the way - it won't necessarily kill the supernova model," continued Wefel. "But a discrepancy would cause problems." Theorists may have to consider the progress of supernova shock fronts in greater detail. "Every supernova explosion is an individual work of art," says Adams. "We use mathematical models that assume the explosions are spherical, but they are not. Within the blast wave itself you can see irregularities. There are bright knots, for example, where shock waves run into interstellar clouds. In crowded groups of massive stars ('OB associations') where supernovae can occur in quick succession, blast waves collide with other blast waves." It can get a little messy! Modelling such details might affect any necessary reconciliation between the theory and the data.

And what if the supernova model can't be rescued? "There are other possibilities," says Wefel, "but not a lot of good ones. We'll really have to look hard to find something other than supernovae that can meet the cosmic ray energy requirement."

The analysis team led by Eun-Suk Seo hope that the new particle count will be the most accurate to date in Alice's energy range, and could shed new light on the decades-old mystery of cosmic rays.