Secular changes in the upper cut-off rigidity of the solar diurnal anisotropy of cosmic rays

We have calculated the upper cut-off rigidities applicable to the solar diurnal anisotropy of cosmic rays, for the period 1965–1979. Our results are consistent with those reported by others. We note that the mean values of the upper cut-off rigidities during 1975–1977 are less than 50 GV. This explains why no diurnal variation is observed by the muon detectors at Socorro (NM) at a depth of 80 m of water equivalent (MWE) for this period. The threshold (R_o) and the median primary rigidities (R_m) of response applicable to the underground vertical telescope at Socorro are 45 and 300 GV, respectively.