COMPARATIVE STUDY OF POWER SPECTRA OF GROUND AND SHALLOW UNDERGROUND MUON DATA.

Cosmic muon flux is recorded by two plastic scintillator detectors: one located at ground level and the other located in the Belgrade underground laboratory (25 m w.e.). Power spectral analysis of the raw detector data for the period from January 1^st 2002 to December 31^st 2002 is performed. Similar periodicities are found in both data sets (ground and underground), but with different amplitudes. Main periodicity in the underground detector data has a 26.5 days period, while the highest amplitude signal in the ground detector data has a 243 ± 45 days period. Other signals present in the time series correspond to higher harmonics of the solar rotation period. There is also a wave with a 34.4 days period, whose significance is confirmed by different methods of spectral analysis. The same periodicity is recently found in some processes on the Sun. [ABSTRACT FROM AUTHOR]