The nature of the 'knee' in the cosmic ray energy spectrum

In view of recent developments, attention is directed again at two aspects of the well-known 'knee' in the cosmic ray energy spectrum at 3 PeV: the mass of the predominant particles at this energy and their source. It is inevitable in a subject such as this that ideas-and conclusions-evolve. Earlier, we had used a particular acceleration model and the nature of the ISM in the local ISM to infer that the particles are mainly oxygen nuclei; direct measurements, when extrapolated (by at least a decade in energy), gave a similar result. Initially, no specific source was identified. More recently, however, we have specified the Monogem Ring supernova remnant as the likely source; this is at just the right distance and age and the energies are reasonable. Concerning the mass composition at the knee, a quantity more difficult to determine, recent direct measurements, which extend to higher energies than hitherto, show a likely flattening in the spectrum above similar to 10(4) GeV/nucleon for He-nuclei, a flattening which, if extrapolated to higher energies, would meet the measured spectrum in the knee region. The other nuclei do not show this feature. He-nuclei in the knee region would also be marginally more consistent with KASCADE extensive air shower data, although there are serious problems with EAS mass estimates in that experiment. Concerning the acceleration, recent models applied to the Monogem Ring SNR allow a satisfactory explanation in terms of either oxygen or helium, but with the latter being a distinct possibility and perhaps more likely.