Summary
The long-term variation of the solar diurnal anisotropy of the galactic cosmic ray intensity in the sub-TeV region is studied by using data recorded over 20 years with the deep underground muon detector at Matsushiro in Japan since 1984. The harmonic dial of the diurnal variation shows a clear 11-year change. At around 1987 and 1998, the variation has a maximum at 06:00 local solar time (LST) being fairly consistent with the Compton–Getting anisotropy due to the Earth’s orbital motion around the Sun, while in 1992 and 2003, it contains an additional variation with the maximum around 15:00 LST. The observed amplitude of this additional variation varies with 11-year cycle and the amplitude reaches a maximum of about 0.04% a few years after sunspot maximum. We discuss these results in the relation to the upper limiting energy of solar modulation in the heliosphere.