Observation of Periodic Variation of Cosmic Ray Intensity with the Tibet III Air Shower Array

The periodicities of cosmic ray intensity variation are analyzed using Lomb-Scargle Fourier transformation method with about 37 billion cosmic ray events recorded by Tibet III Air Shower Array from November 1999 to November 2005. To eliminate meteorological effect, we adopt East-West subtraction method. Besides the well known solar diurnal, sidereal diurnal and sidereal semi-diurnal modulations at a level of 10(-3), no other periodicity is found with large enough significance in a range from 1 hour to 1 month.