2013MNRAS.436.2326P

We study the influence of the random part of the galactic magnetic field on the propagation of ultrahigh energy cosmic rays. Within very mild approximations about the properties of the electron density fluctuations in the Galaxy, we are able to derive a clear and direct relation between the observed variance of rotation measures and the predicted cosmic ray deflections. Remarkably, this is obtained bypassing entirely the detailed knowledge of the magnetic properties of the turbulent plasma. Depending on the parameters of the electron density spectrum, we can either directly estimate the expected deflection, or constrain it from above. Thanks to the latest observational data on rotation measures, we build a direction-dependent map of such deflections. We find that over most of the sky the random deflections of 40EeV protons do not exceed 1°-2°, and can be as large as 5° close to the Galactic plane.