Abstract
We analyze the sidereal daily variation of galactic cosmic-ray intensity observed by the Tibet air-shower (AS) array in one year from January to December 1992. The Tibet AS array can detect the incident direction of each AS and allows us to examine the daily variations in different declination bands on the celestial sphere. The average hourly AS count is 26,372 and the total observation period is 5,651 hours. We analyze the hourly count rates of AS classified into nine cells according to the incident direction of each AS. The hourly count rates are corrected for the atmospheric pressure effect and also for the spurious variation due to the change in the AS size-distribution. We find that the resultant daily variations in sidereal, solar and antisidereal time are influenced by a significant variation (∼0.5%) which is common for the viewing directions. It is also found that such common variation is well eliminated in the “difference” variations between the eastern and western cells. From the harmonic analysis of the “difference” variations, it is concluded that there is no significant variation observed in one year of data.
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