Cosmic ray sidereal diurnal variation of galactic origin observed by neutron monitors

The sidereal diurnal variation of cosmic rays has been analyzed using 620 station-years of neutron monitor data during the period 1958-1979. The sidereal variation averaged over the period for all the stations in the Northern Hemisphere is different from the corresponding variation in the Southern Hemisphere. The difference is statistically significant and can be identified with the spurious sidereal variation produced from the stationary anisotropy of solar origin, responsible for the solar semi-diurnal variation. The variation common to both hemispheres is also exceptionally significant from the statistical point of view and could be regarded as being due to a uni-directional galactic anisotropy. This variation has an amplitude of 0.0204 +- 0.0015% and a phase of 6.8 +- 0.3 h and is clearly different from that (approx. 0.05%, 0 approx. 3 h) observed in the high rigidity region (500 approx. 104 GV). The physical meaning of the variation is discussed from the standpoint of the heliomagnetospheric modulation of galactic anisotropy. (author)