Cosmic Ray Anisotropy as Signature for the Transition from Galactic to Extragalactic Cosmic Rays

TL;DR

This study models cosmic ray anisotropy to identify the energy at which cosmic rays transition from Galactic to extragalactic origins, suggesting a heavy composition or extreme magnetic fields are necessary for certain transition scenarios.

Contribution

It introduces a detailed propagation model of cosmic rays considering Galactic magnetic fields and turbulent components, providing new constraints on the transition energy based on anisotropy data.

Findings

Dipole anisotropy exceeds observed bounds at 10^18 eV for light/intermediate primaries.

Transition at the ankle requires heavy nuclei or strong magnetic fields.

Extragalactic origin for the rising proton component between 10^17 and 10^18 eV.

Abstract

We constrain the energy at which the transition from Galactic to extragalactic cosmic rays occurs by computing the anisotropy at Earth of cosmic rays emitted by Galactic sources. Since the diffusion approximation starts to loose its validity for E/Z >~ 10^(16-17) eV, we propagate individual cosmic rays (CRs) using Galactic magnetic field models and taking into account both their regular and turbulent components. The turbulent field is generated on a nested grid which allows spatial resolution down to fractions of a parsec. Assuming sufficiently frequent Galactic CR sources, the dipole amplitude computed for a mostly light or intermediate primary composition exceeds the dipole bounds measured by the Auger collaboration around E ~ 10^18 eV. Therefore, a transition at the ankle or above would require a heavy composition or a rather extreme Galactic magnetic field with strength >~ 10 muG. Moreover, the fast rising proton contribution suggested by KASCADE-Grande data between 10^17 eV and 10^18 eV should be of extragalactic origin. In case heavy nuclei dominate the flux at E >~ 10^18 eV, the transition energy can be close to the ankle, if Galactic CRs are produced by sufficiently frequent transients as e.g. magnetars.