Cosmic ray north-south anisotropy: The role of the interplanetary magnetic field

Nucleonic intensity data from the north and south polar stations Thule and McMurdo are analyzed to determine the nature of periodicities in the north-south asymmetry of relativistic galactic cosmic rays. The 24-hour average intensity differences on individual days range from 0 to ±1.5%, and the most prominent time variation is a recurrence tendency with a period consistent with the 27-day solar rotation period. A recently developed procedure for evaluating the statistical significance of quasi-persistent data has been utilized to establish the reality of the 27-day signal disclosed by superposed epoch analysis of the north-south asymmetry data. Analyses of various components of the interplanetary magnetic field reveal that the IMF component (parallel to the nominal garden hose direction) also displays a 27-day variation with maximum amplitude. Furthermore, a significant relationship between and the north-south asymmetry is established by Chree analysis. On the basis of this relationship, it is suggested that the 27-day variation in the north-south asymmetry arises from the drift, where represents the radial gradient of cosmic rays.