Solar modulation origin of ‘sidereal’ cosmic ray anisotropies

On the basis of earlier work that suggested that the sidereal diurnal variation in cosmic ray intensity could be due to the cooperative effects of the interplanetary magnetic field and a radial heliocentric cosmic ray density gradient, a more comprehensive data analysis has been made to test the model. Data from underground μ meson telescopes in the northern and southern hemispheres have been analyzed in sidereal time as a function of the direction of the interplanetary magnetic field. The observed anisotropy is shown to be dependent on the magnetic field direction. Sidereal cosmic ray data from several underground detectors are displayed as a function of the threshold rigidity of the detectors, and the results appear to indicate that the mechanism responsible for the ‘sidereal’ anisotropy has an upper cutoff rigidity below 100 Gv. Other observed sidereal cosmic ray anisotropies are reviewed in the light of these results, and the likelihood of observing a true sidereal anisotropy above and below 100 Gv is discussed.