Abstract
It is shown that the sidereal anisotropy (SiA) of cosmic rays (CRs) with energies smaller than 104 GeV consists of three kinds: one (GA) is of galactic origin from the direction ΦG (αG = 0 hr; δG = −20°), and the other two (tail-in TA and nose-in HA) are of solar origin from the respective directions ΦT (αT = 6 hr; δT ∼ −24°) and ΦH (αH = 18 hr; δH > 0°) and supposed to be produced by the acceleration of CRs on the tail and nose boundaries of the heliomagnetosphere (HMS). This conclusion was arrived at in 1995 after a long-term delay since the beginning of CR observations in the early 20-th century. This delay was mainly due to the inconsistency among observations caused by the belief that the sidereal anisotropy must be unidirectional in space. The inconsistency has been solved at least qualitatively by the discovery of GA and TA. These anisotropies, including also HA, are subject, respectively, to their proper solar modulations in the HMS characterized by a polarity reversal every 11 years of the solar polar magnetic field and solar activity dependence with an 11-year periodicity. By using these modulation patterns, the origins of the three anisotropies have been determined. TA and HA thus determined inversely produce the following kinds of evidence and problems in the HMS: (1) the structure of the HMS, (2) acceleration of CRs on the boundary of the HMS, (3) CR Lens Effect of the HMS for the sharp concentration of TA and HA, (4) the proper motion (VHMS) of the HMS relative to neighboring stars, (5) the proper motion of interstellar gaseous matter (including the magnetic field) relative to neighboring stars, and (6) the existence of the Subordinate HMS surrounding the HMS for the explanation of the duality of the motion of the HMS and also of the absence of the Compton-Getting (C-G) effect on the HMS. The present paper not only presents a brief summary of the studies of CR sidereal anisotropy made by many researchers during the 20th century leading to the present understanding, but also presents some problems to open up a new vista of the future.
Highlights
The sidereal anisotropy SiA of cosmic rays (CRs) has been inferred indirectly from observations of the sidereal daily variation (SiD) on the ground ever since the discovery of Cosmic rays (CR) (e.g. Forbush, 1937a, b, 1939)
A low-energy region, SiD, with the frequency 366 cycles/yr must be detected under the following difficult condition that it is always superposed by the solar daily variation (SoD) with the adjacent frequency of 365 cycles/yr produced inside the HMS and disturbed by the daily variations of the Earth’s atmospheric pressure, temperature and wind velocity
We present a brief summary of the study of SiA of CRs made by many researchers with a dedicated attention to the direct and indirect influences in order to reach a better understanding of the nature of SiA and the features associated with it
Summary
The sidereal anisotropy SiA of CRs has been inferred indirectly from observations of the sidereal daily variation (SiD) on the ground ever since the discovery of CRs (e.g. Forbush, 1937a, b, 1939). In spite of such differences in phase in the low- and high-energy regions, the coincidences in phase observed by the neutron monitor (Ex. 4, ∼20 GeV, 1983), ion-chamber (Ex. 3, ∼67 GeV, 1976) and the underground muon telescope (Ex. 2, ∼184 GeV, 1966) seem to suggest the existence of some sidereal anisotropy in the low-energy region in space from α = 6 hours (Nagashima et al, 1983a, 1984) This was not accepted unanimously at that time for the following reasons: (1) the difference of the phases in the low- and high-energy regions mentioned above cannot be explained by the difference between CR deflections produced by the single anisotropy in the HMS, (2) the signal of the anisotropy in the low-energy region (∼20 GeV) would be difficult to maintain its shape until its arrival at the Earth because of disturbance in the HMS and the observed SiD1 might be some residual of SSiD, and (3) the CSSA-method was not unanimously accepted in those days. The anomaly failed to be explained by the unidirectional anisotropy, it is conducive to the determination of GA and TA that follows
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