Geomagnetic effects on atmospheric neutrinos

Geomagnetic effects distort the zenith angle distribution of sub-GeV and few-GeV atmospheric neutrinos, breaking the up-down symmetry that would be present in the absence of neutrino oscillations and without a geomagnetic field. The geomagnetic effects also produce a characteristic azimuthal dependence of the $\nu$ fluxes, related to the well-known east-west effect, that should be detectable in neutrino experiments of sufficiently large mass. We discuss these effects quantitatively. Because the azimuthal dependence is in first-order independent of any oscillation effect, it is a useful diagnostic tool for studying possible systematic effects in the search for neutrino oscillations.