Galactic cosmic-ray anisotropy and its heliospheric modulation, inferred from the sidereal semidiurnal variations observed in the rigidity range 300–600 GV with multidirectional muon telescope at Sakashita underground station

Abstract

The existence of sidereal semidiurnal variation of cosmic-ray intensity in a rigidity region 10 2-10 3 GV has been reported by many researchers, but there is no consensus of opinion on its origin. In this paper, using the observed semidiurnal variations in a rigidity range (300–600 GV) with 10 directional muon telescopes at Sakashita underground station (geog. lat. = 36°, long. = 138°E, depth = 80 m.w.e.), the authors determine the magnitudes (η 1, η 2) and directions (a 1, a 2) of the first- and second-order anisotropies in the following galactic cosmic-ray intensity distribution ( j) j dp = j 0{1 + η 1P 1( cos χ 1) + η 2P 2( cos χ 2)} dp , where P n is the nth order spherical function and χ n is the pitch angle of cosmic rays with respect to a n . For the determination, the influence of cosmic-ray's heliomagnetospheric modulation, geomagnetic deflection and nuclear interaction with the terrestrial material and also of the geometric configuration of the telescopes are taken into account. Usually, the semidiurnal variation is produced by the second-order anisotropy. The present observation, however, requires also the first-order anisotropy which usually produces only the diurnal variation, but can produce also the semidiurnal variation as a result of the heliospheric modulation. The first- and second-order anisotropies are characterized with η 1) > 0 and η 2 < 0 have almost the same direction (a 1 ∼ a 2) specified by the right ascension ( α ∼ 0.75 h) and declination ( δ ∼ 50°S) and, therefore, they can be expressed, as a whole, by an axis-symmetric anisotropy of loss-cone type (i.e. deficit intensities in a cone). It is noteworthy that this anisotropy approximately coincides with that inferred from the air shower observation at Mt Norikura in the rigidity region ∼ 10 4 GV.