Galactic and heliotail‐in anisotropies of cosmic rays as the origin of sidereal daily variation in the energy region < 104 GeV

Abstract

It is shown that the cosmic ray sidereal daily variation in the energy region <EU ∼ 104 GeV is due to two kinds of anisotropy, one is a galactic anisotropy from the direction with right ascension αG = 0 hours and declination δG = −20° and can be observed even in the energy region as low as ∼60 GeV with the same form and almost the same phase as observed by small air showers with ∼EU [Nagashima et al., 1989]. The other is a newly discovered directional excess flux confined in a narrow cone with a half opening angle of ∼68° from the direction (αT ∼ 6.0 hours; δT ∼ −24°) and observed only in the energy region less than EU with its maximum near 103 GeV. It is suggested that the excess flux is of solar origin and the direction toward it seems to coincide with the expected heliomagnetotail direction (αTP = 6.0 hours; δTP = −29.2°) opposite the proper motion of the solar system but does not coincide with the expected tail direction (αTN = 4.8 hours; δTN = 15° ∼ 17°) opposite to the relative motion of the system to the neutral gas. The flux (called the tail‐in anisotropy hereafter) shows maximum at the December solstice when the Earth is closest to the magnetotail and almost disappears at the remote side of the Earth's orbit from the tail at the June solstice. Owing to the discovery of the tail‐in anisotropy, the observed phase shift of the sidereal diurnal (24 hours) variation from 6 to 0 hours with the increase of energy, which has been one of the unsolved problems, can be explained by the distinctive contributions from the two anisotropies. Finally, it appears that the observed sidereal variations deny the existence of the Compton‐Getting effect due to the motion of the solar system at least in the energy region less than ∼EU. This implies that the solar system drags with it in its motion the surrounding interstellar magnetic field within which the cosmic rays with low energy (less than ∼EU) are isotropically confined.