Observation of the North-South Asymmetry of Sidereal Anisotropy of Cosmic Rays at Matsushiro Underground

Abstract

We present the observed results of the cosmic-ray sidereal anisotropy in the primary rigidity range 550-950 GV from the Matsushiro underground (geogr. 36.5°N, 138.0°E; 220 m.w.e. depth) for the period 1984-1994. The moon telescope used is of multi-directional with 17 channels, which scan simultaneously over the median latitude of viewings from-60° N to -15°S on the celestial sphere. We obtain significant sidereal diurnal and semi-diurnal terms in the daily variations with several s.d., being consistent with other observations. Most notably, the north-south asymmetry in the diurnal variations is confirmed without solar modulation, in which the amplitude increases as the direction of viewings of the telescopes moves southward towards and over the equator.