COSMIC-RAY DIURNAL ANISOTROPY, 1936-1988 - IMPLICATIONS FOR DRIFT AND MODULATION THEORIES

Data from five neutron monitor stations and one ionization-chamber station are employed to determine the variation of the cosmic-ray diurnal anisotropy over a time span exceeding 50 years. The amplitude of the diurnal anisotropy varies with a period of one sunspot cycle, while the phase varies with a period of two sunspot cycles. Using minimum-variance analysis, we show that the principal axis of variation of the anisotropy is nearly aligned with the mean magnetic field, in agreement with earlier conclusions of Forbush. These results provide new information on the long-term variation of cosmic-ray gradients. Taking nominal values for the parallel and perpendicular diffusion coefficients, we find that the value of the radial gradient during epochs of negative solar magnetic polarity is persistently larger than during positive polarity epochs, while the bidirectional latitude gradient reverses sign with the solar magnetic polarity reversal. The behavior of both gradients favors the drift theory during solar minimum periods.