Coexistence of cosmic-ray sidereal anisotropies originating in galactic space and at the heliomagnetospheric nose and tail boundaries, observed with muon detectors in the energy region of 60∼100 GeV
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2006
Received: 30 March 2005
Accepted: 24 June 2006
Published: 25 December 2006
The coexistence of two kinds of cosmic-ray sidereal anisotropy was found by observations with underground muon telescopes in the energy region (> ∼200 GeV) in 1995: one is the galactic anisotropy with a deficit flux in the direction with right ascension α G = 12 hr and declination δ G = 20°. The other is the excess flux from the heliomagnetospheric tail direction (α T ≃ 6 hr) and would be produced on the heliotail boundary where it is considered that the interaction between the galactic and solar magnetic fields could produce the cosmic-ray acceleration. On the other hand, another anisotropy of helioboundary origin from the helionose direction (α ≃ 18 hr), being accompanied by the heliotail-in anisotropy, was found through the observations with neutron monitors in the low energy region (∼20 GeV) in 2005. These observations, however, lack information in the mid-energy region (20∼200 GeV). In order to bridge the absence of information, the cosmic-ray sidereal daily variations in the energy regions (60∼100 GeV) have been derived from the observations with muon telescopes and ion chambers on the ground in the period 1936–2003. It is shown that all the three anisotropies coexist in this energy region and are subject to their respective solar modulations. On the basis of these modulations, the characteristics of the anisotropies are determined through intercomparison with the observations in the high and low energy regions.