Magnetic Cycle Dependence of the Cosmic-Ray Diurnal Anisotropy

Abstract

The two components of the solar diurnal variation observed with two detectors characterized by linearly independent coupling functions have been used to estimate the free space anisotropy vector during the period 1968–1995 using the least-squares method (LSM). The values of Rcshow ∼20-year magnetic cycle with the lowest values at solar activity minima for positive polarity (qA>0). A good correlation is obtained between Rcand the IMF magnitude. The amplitude of the radial anisotropy (AR) shows ∼20-year magnetic cycle with the highest values around solar activity minima for qA>0 (1975–1976 and 1995), whereas that of the east-west (Aφ) is minimum. This results in shifting the anisotropy vector to the earliest hours. The amplitude of the anisotropy is high around solar maxima and low around solar minima. It is also enhanced during the declining phase of solar activity (1971, 1984–1985, and 1991). Our results of the anisotropy have been used to calculate the cosmic-ray radial and transverse gradients. The value of the radial gradient exhibits a magnetic polarity dependence as well, with larger value during qA 0.