A variation, with a period of two solar cycles, in the cosmic-ray diurnal anisotropy

Annual means, 1937-1965, of the cosmic-ray diurnal anisotropy component, in the asymptotic direction 128°E of the sun, are well-fitted by a wave with a period of 20 years, which is twice the solar cycle period of 10 years for the interval 1937–1965. This variation is independent of magnetic activity, and when removed the residual variations combine with those in the asymptotic component 387deg;E of the sun (in which there is no 20-year wave) to give resultant variations, which are principally in the asymptotic component 90°E of the sun. Yearly means of this resultant component are well correlated (r = +0.75) with magnetic activity, U0, and, on the average, vanish for U0 = 0. U0 is the absolute value of the southward geomagnetic component of the so-called equatorial ring current [Forbush, 1966].