Long-term variation in the cosmic-ray diurnal anisotropy

The long-term modulation of the diurnal variation of the cosmic-ray intensity, observed in quiet times during the ascending phase of solar cycle number 20, is fully explained by the change in the interplanetary sector pattern. The sector structure seems to generate an upper rigidity cut-off for the diurnal variation which increases from ∼50 GV during 1965 to ∼150 GV during 1968. Moreover, the amplitude of the diurnal variation is reduced by a factor ∼2 in periods of mixed polarities of the interplanetary magnetic field. As a consequence the North-South anisotropy seems to be completely generated by the cosmic-ray radial gradient. Estimates of the time change of this gradient from 1965 to 1968 are in agreement with those calculated from changes in the interplanetary magnetic structure.