Measurement of the cosmic-ray sidereal anisotropy near 1500 GV

Abstract

A large underground muon detector has operated at an average rate of 4.47 Hz for most of a 2.7-year interval, starting on Jan. 1, 1978, in a search for periodic variations in the cosmic-ray flux at a median rigidity of 1.5 x 10 to the 12th V. A peak with relative amplitude of 4.1 +1.2, -0.9 x 10 to the -4th is observed in the Fourier transform of data at the sidereal frequency, where the errors are 68% confidence limits based on an off-center Rayleigh distribution. After correction for solar motion, the projected anisotropy is 6.4 +1.2, -1.0 x 10 to the -4th at the right ascension of 3.8 + or - 0.7 h. Also observed are a second sidereal harmonic, with an amplitude of 2.8 +0.9, -0.7 x 10 to the -4th at the ascension 7.8 + or - 1.0 h and a third harmonic of amplitude 2.3 +1.0, -0.7 x 10 to the -4th at 7.8 + or - 1.4 h ascension. There is no discernible peak at the antisidereal frequency, and the small amplitude at the solar frequency implies an upper atmosphere diurnal temperature amplitude of less than about 0.1 C.