Cosmic rays in the earth's magnetic field

Abstract

Abstract It is shown that the values of cosmic ray cut-off moments in the earth's magnetic field, observed at many different places, are generally close to the values calculated from Störmer's theory for the motion of charged particles in a dipole field, if the usual centre dipole of the earth is replaced in the Störmer equation by a dipole whose magnitude and direction are determined by the surface field at the place considered. An empirical expression for the actual cut-off moments (in terms of the centre-dipole field, and the ‘surface field’ cut-off moments) is deduced from the variation in sea-level nucleon intensity between London and Cape Town, and gives good agreement with experimental results over a wide range of latitidues and longitudes. It is concluded that discrepancies between centre dipole predictions and experimental observations of cosmic ray intensities and cut-off moments are due to differences between the earth's real field and the dipole approximation to it, rather than to distortion of the earth's outer magnetic field by ionized interplanetary matter.