The effects of the GMF on the transition from Galactic to extragalactic cosmic rays

Abstract

Understanding the nature of the transition from Galactic to extragalactic cosmic rays (GCRs and EGCRs) has become especially challenging in light of recent spectral and composition data. Galactic contributions appear to be disfavoured at energies beyond the “second knee” where the composition becomes lighter. At the same time, a dominant EGCR contribution at the “2nd knee” appears unlikely. As a result, the measured flux in the transition region cannot easily be accounted for. With the model-dependence of proposed extensions to both the Galactic and extragalactic contributions, a deeper understanding of CR propagation, particularlywithin theGalacticmagnetic field (GMF), is in order. This is because propagation in this energy range shifts from diffusive to ballistic, which is expected to lead to a number of observable effects on CRs. Using CRPropa3, we study these effects for rigidities between 1016 and 1020 V. We identify various features at rigidities where the gyroradius is comparable to typical length scales of the Galaxy, suggesting causes related to changes in the propagation regime. We further quantify general modifications in the spectrum, composition and arrival direction of GCRs and EGCRs. We find that the GMF naturally induces a flux suppression of GCRs towards higher rigidities due to their increased leakage from the Galaxy. This, in consequence, would lead to an increase in the mean mass of GCR primaries up to energies around the “ankle” in the cosmic ray spectrum. It is also shown that the distribution of GCR arrival directions would be correlated with the Galactic plane for rigidities above 1017 V if sources are distributed evenly within the plane. EGCRs experience no flux modification in the GMF if injected isotropically. Injection of pure dipoles, as well as single source scenarios indicate that the GMF isotropises injected anisotropies below 1018 V, but can still cause flux modifications which depend on the direction of the anisotropy.