A century after Victor Hess discovered cosmic rays, the question of their sources remains unsettled. Leaving aside the rare CRs with energies E above 1010 GeV, which probably originate in some sort of monstrous extragalactic accelerators, astrophysicists still don’t know with confidence where most of the nuclei that dominate the intragalactic CR flux are accelerated to their more modest energies. Figure 1 shows those fluxes as functions of the kinetic energy K = E – mc 2 with which a nucleus of mass m arrives at the top of the atmosphere. About 95% of them are simply protons, and 90% of the rest are α particles—nuclei of helium-4, the second most abundant species in the cosmos.
The most extensively studied and cited model for the creation of intragalactic CRs is diffuse shock acceleration (DSA) in the expanding outer shock fronts of supernova remnants. In DSA, repeated scattering off magnetic...
