Cosmic-Ray Modulation Parameters Derived from the Solar Diurnal Variation

From an analysis of the solar diurnal variation in Galactic cosmic rays with rigidities between 17 and 195 GV from 1957 to 1990, we have derived the product of the parallel mean free path and the radial density gradient (λ∥ Gr) and the bidirectional latitudinal gradient indicator G|z|. We find the rigidity spectrum of the solar diurnal anisotropy averaged over the entire time span to have a spectral index of -0.1 ± 0.2 and an upper limiting rigidity of 100 ± 25 GV. The yearly averaged spectrum may depend on the heliospheric polarity state, having a slightly positive spectral index during the A > 0 epoch. We find that the parameter λ∥ Gr depends inversely on rigidity, more so during the A > 0 polarity state. Around solar minimum of this epoch, λ∥ Gr is also much lower than for corresponding years in the A < 0 polarity state. Combining the derived values of λ∥ Gr with results from previous studies of the radial gradient, we find evidence for the values of AH (at all the rigidities examined) having a variation related to the magnetic polarity state of the heliosphere. G|z| indicates that at rigidities up to 185 GV a bidirectional latitudinal gradient exists and changes direction following solar magnetic polarity reversals, in agreement with drift theories. This gradient attains its largest magnitudes around times of solar minimum and also appears to be larger during A < 0 polarity states. © 1997. The American Astronomical Society. All rights reserved.