Cosmic ray anisotropy as signature for the transition from galactic to extragalactic cosmic rays

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/Zgtrsim10^16-17 eV, we propagate individual cosmic rays 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 ≈ 10¹⁸ eV. Therefore, a transition at the ankle or above would require a heavy composition or a rather extreme Galactic magnetic field with strength gtrsim10 μG. Moreover, the fast rising proton contribution suggested by KASCADE-Grande data between 10¹⁷ eV and 10¹⁸ eV should be of extragalactic origin. In case heavy nuclei dominate the flux at Egtrsim10¹⁸ eV, the transition energy can be close to the ankle, if Galactic CRs are produced by sufficiently frequent transients as e.g. magnetars.