Modulation Cycles of GCR Diurnal Anisotropy Variation

The diurnal variations of GCR intensity observed by the ground NM stations represent the anisotropic GCR flow at 1 AU. It is generally believed that the variation of the local time of the GCR maximum intensity (phase) has 22-year period of two sunspot cycles. However, there even exists doubt on such anisotropy variation cycle. In order to determine the cycle of GCR anisotropy variation, we carried out the statistical study on the diurnal variation of phase. We examined the 52 years data of Huancayo (Haleakala), 38-year data from Rome, 42-year data from Oulu NM stations. We applied the F-test to determine the statistically meaningful period of anisotropy phase variation. The phase variation has two components of 22-year and 11-year cycles. The NM station in the high latitude (low cut-off rigidity) shows mainly the 22-year cycle. However, the lower the latitude of NM station is, the higher contribution from 11-year cycle.