Solar modulation of galactic and heliotail-in anisotropies of cosmic rays at Sakashita underground station (320 ∼ 650 GeV)

Abstract

The sidereal daily variation of cosmic rays with energies less than 10⁴GeV is produced by two kinds of anisotropy, one is the galactic anisotropy with deficient intensity in the direction with right ascension α _G = 12 hours and declination δ _G = 20° and the other is the heliotail-in anisotropy with excess intensity in the direction of α _T ∼ 6 hours and δ _T ∼ −24° (Nagashima et al., 1998). It will be shown that the variation of the galactic origin with energy less than ∼500 GeV is greater in the negative polarity state of solar magnetic field at the north pole than in the positive state owing to the different motion of cosmic rays in the two polarity states and shows a considerably good agreement with its simulation. On the contrary, the variation of the tail-in origin does not show such a polarity dependence. Instead, it becomes greater in the active period of solar cycle than in the quiet period, suggesting the existence of cosmic-ray acceleration due to the interaction between galactic and solar magnetic fields in the heliotail region.