COSMIC-RAY UNIDIRECTIONAL LATITUDE GRADIENT - EVIDENCE FOR NORTH-SOUTH ASYMMETRIC SOLAR MODULATION

The cosmic ray diurnal anisotropy is determined separately in toward and away sectors of the interplanetary magnetic field through analysis of neutron monitor records spanning the period 1953-1988. We suggest that the dominant cause of differences (between sectors) of the anisotropy is a unidirectional latitude gradient, which is indicative of a north-south asymmetry in the solar modulation of cosmic rays. The magnitude of the latitude gradient deduced from the observations is typically of the order of 0.7 %/AU, and its direction varies in a manner not obviously related to either the 11-year sunspot cycle or the 22-year solar magnetic cycle. Further support for the reality of a unidirectional gradient is provided by an analysis of the cosmic ray density segregated according to sector polarity. The density is found to differ significantly in the two sector types, and the amount of difference is correlated with the concurrently measured unidirectional latitude gradient in the expected manner. We examine two hypotheses to account for a north-south asymmetry of solar modulation: (1) that the asymmetric modulation results from a north-south asymmetry in the winding angle of the interplanetary magnetic field spiral and (2) that the asymmetric modulation is related to a corresponding north-south asymmetry of solar activity as indicated by sunspot number. Both hypotheses am consistent with the data, suggesting that multiple factors may give rise to north-south asymmetric solar modulation.