The effect of the geomagnetic field on cosmic ray energy estimates and   large scale anisotropy searches on data from the Pierre Auger Observatory
  We present a comprehensive study of the influence of the geomagnetic field on the energy estimation of extensive air showers with a zenith angle smaller than $60^\circ$, detected at the Pierre Auger Observatory. The geomagnetic field induces an azimuthal modulation of the estimated energy of cosmic rays up to the ~2% level at large zenith angles. We present a method to account for this modulation of the reconstructed energy. We analyse the effect of the modulation on large scale anisotropy searches in the arrival direction distributions of cosmic rays. At a given energy, the geomagnetic effect is shown to induce a pseudo-dipolar pattern at the percent level in the declination distribution that needs to be accounted for. 
P. Abreu, M. Aglietta, E. J. Ahn, I. F. M. Albuquerque, D. Allard, I.   Allekotte, J. Allen, P. Allison, J. Alvarez Castillo...
astro-ph.IM