Ballooning For Cosmic Rays

"A supernova blast blows a bubble in the interstellar medium that grows until the shock wave runs out of energy," he explained. "They can accelerate particles up to some point, about 10¹⁴ electron volts (eV) per nucleon, but not beyond that. Below that level all of the different cosmic ray species - protons, helium nuclei, etc; should have the same kind of energy spectrum: a power law with index around -2.7."

A "power law" spectrum is one that looks like a straight line on a piece of log-log graph paper. In the energy range ~10¹⁰ eV to 10¹⁴ eV, the supernova theory of cosmic ray acceleration predicts that the power law spectrum of protons should have the same slope as the power law spectra of heavier nuclei (about -2.7).

The problem is when scientists compare the energy spectra of protons and helium nuclei, the two don't resemble one another as much as they should. Both are power laws, as expected, but "existing data indicate a possible spectral index difference between protons and helium of about 0.1," says Eun-Suk Seo, a cosmic ray researcher at the University of Maryland. "The [slope of the] proton spectrum is close to -2.7, but the energy spectra of helium and heavier nuclei seem to be flatter. The difference is small and it might not be statistically significant." If there is a genuine discrepancy, she added, it could signal trouble for supernova models of cosmic ray acceleration.

To find out if the supernova theory is indeed in peril, a team of scientists led by John Wefel ( Louisiana State University) and Eun-Suk Seo, and aided by personnel from the National Science Balloon Facility, launched a helium-filled balloon from McMurdo, Antarctica on Dec. 28, 2000. The payload, which soared to120,000 feet above Earth's surface, included a NASA-funded cosmic ray spectrometer known by its builders as the Advanced Thin Ionisation Calorimeter or "ATIC" for short.

"ATIC is sensitive to cosmic rays with energies between ~10¹⁰eV and 10¹⁴eV," says Wefel. By covering such a wide range of energies with a single modern spectrometer, the team hopes to measure the proton and helium cosmic ray spectra with better precision than ever before.