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Galactic Magnetic Field Bias on Inferences from UHECR Data

A consequence of Liouville's theorem indicates that the recently observed large scale anisotropy in the arrival direction of Ultra-High-Energy Cosmic Rays (UHECRs) cannot be produced by the Galactic magnetic field, thus this anisotropy already needs to be present outside our Galaxy. But in this case, the observed energy spectrum and composition of UHECRs differs from the one outside of the Milky Way, due to the suppression or the amplification of the UHECR flux from certain directions by the Galactic magnetic field. In this work, we investigate this effect for the case of a dipole and a quadrupole anisotropy, respectively, for the widely-used JF12 magnetic field model. We investigate boundaries on the maximal amplitude of the observed anisotropy and the maximal charge number of UHECRs. Furthermore, the flux modification is discussed in the light of the Auger data on the recent dipole and also the chemical composition. We find that this modification effect yields a modification of the observed flux of up to $\sim 10\%$ for the investigated magnetic field model and the observed dipole, in particular for a heavy chemical composition of UHECRs as suggested by the 'EPOS-LHC' model.