‘sidereal’ cosmic-ray diurnal variations

New data from underground mu-meson telescopes, at depths of 40 MWE, in the northern and southern hemispheres are presented. Analysis in sidereal time of three years of data from the northern hemisphere telescopes and two years of data from the southern hemisphere telescopes confirm that a maximum intensity is observed near 18 hours sidereal time for telescopes pointing into the northern hemisphere and near 06 hours sidereal time for telescopes pointing into the southern hemisphere, in agreement with previous observations by others. A model is proposed in which just such a ‘sidereal’ variation can be produced in the vicinity of the earth as a result of the radial cosmic-ray density gradient and the interplanetary magnetic field; the model is tested by dividing the cosmic-ray data according to the direction of the interplanetary field and performing the same sidereal time analysis on these data. The results confirm the principal features of the model and demonstrate that the ‘sidereal’ variation can be produced in the inner solar system, correctly predicting the observed times of maximum in the northern and southern hemispheres, without recourse to assumptions about anisotropies of the particle flux in galactic space.