The satellite-borne spectrometer PAMELA, launched in 2006, is particularly good at detecting charged antimatter particles: positrons (e+) and antiprotons (p‾). Last year the PAMELA collaboration, led by Piergiorgio Picozza (University of Rome, Tor Vergata), reported a precision measurement of the minuscule p‾ component of the spectrum of cosmic rays arriving at the top of the atmosphere. The measured p‾ flux, the team found, could be explained by p‾ production in high-energy collisions of cosmic-ray nuclei with ordinary interstellar matter.1 So there was as yet no evidence of exotic sources of cosmic-ray antiprotons.
A similarly prosaic explanation holds for PAMELA’s latest discovery: a significant p‾ population magnetically trapped in Earth’s inner Van Allen radiation belt.2 The flux of trapped p‾s spiraling around the belt’s geomagnetic field lines turns out to be a thousand times greater than the flux of cosmic-ray p‾s entering the atmosphere. That makes PAMELA’s...