Cosmic ray anisotropy in three-dimensional space

Abstract

A systematic cosmic ray intensity gradient along the Earth's rotational axis, arising from a spatial anisotropy almost perpendicular to the plane of the ecliptic, has been observed for the first time. Analysis of data recorded by a network of high counting rate neutron monitors during the recovery phase of a Forbush decrease in March, 1966, revealed the occurrence of a significant north-south asymmetry. A theoretical model that accounts for all the observed effects, including the latitude dependence, is developed. The anisotropy is envisaged as arising from the diffusion of cosmic rays through disordered magnetic fields in the vicinity of the Earth, associated with a profusion of solar disturbances that were conspicuous during this period of resurgent solar activity. The two components of the vector in three-dimensional space that defines the direction of anisotropy are determined by combining the information deduced from the study of the north-south asymmetry with the associated diurnal variation. During one of the two epochs that has been studied in detail, the greatest flux of cosmic ray particles was incident from the direction in space −83.5° ± 3.5° declination and 13.3 ± 0.7 h right ascension. Thus, the configuration of magnetic fields of solar origin that enveloped the Earth at this time was not symmetrical about the equatorial plane. Alternative interpretations of this result are: (1) that the lines of force were temporarily directed almost perpendicularly to the solar equatorial plane permitting preferential flow of particles along this direction, or (2) that the lines of force were parallel to the equatorial plane, as usual, and the density gradient in the north-south direction was a consequence of the unsymmetrical spatial distribution of the cosmic ray “shield”.