Large-scale anisotropies of extragalactic cosmic rays below the ankle

Abstract

We study the anisotropies on large angular scales which can be present in the flux of cosmic rays reaching the Earth from a population of extragalactic sources, focusing on the energy range between the second knee and the ankle. In this energy range the particles are significantly affected by the Galactic magnetic field, which then plays a relevant role in shaping the expected anisotropies. The Galactic magnetic field deflects the cosmic-ray trajectories and thus modifies the anisotropies present outside the halo of the Galaxy, in particular the dipolar one associated with the translational motion of the observer (Compton-Getting effect). Also, due to the Galactic rotation, in the reference frame of an observer at Earth there is an electric component of the Galactic field that produces a small change in the particles' momentum. This acceleration depends on the cosmic-ray arrival direction and it hence induces anisotropies in the flux observed in a given energy range. We analyse the expected amplitude and phase of the resulting dipolar component of the flux and discuss the possibility to explain via these effects the change in the phase of the right-ascension distribution which is observed at energies around 1 EeV.