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Title: Long-term variation of the solar diurnal anisotropy of galactic cosmic rays observed with the Nagoya multi-directional muon detector

We analyze the three-dimensional anisotropy of the galactic cosmic ray (GCR) intensities observed independently with a muon detector at Nagoya in Japan and neutron monitors over four solar activity cycles. We clearly see the phase of the free-space diurnal anisotropy shifting toward earlier hours around solar activity minima in A > 0 epochs, due to the reduced anisotropy component parallel to the mean magnetic field. This component is consistent with a rigidity-independent spectrum, while the perpendicular anisotropy component increases with GCR rigidity. We suggest that this harder spectrum of the perpendicular component is due to contribution from the drift streaming. We find that the bi-directional latitudinal density gradient is positive in the A > 0 epoch, while it is negative in the A < 0 epoch, in agreement with the drift model prediction. The radial density gradient of GCRs, on the other hand, varies with a ∼11 yr cycle with maxima (minima) in solar maximum (minimum) periods, but we find no significant difference between the radial gradients in the A > 0 and A < 0 epochs. The corresponding parallel mean free path is larger in A < 0 than in A > 0. We also find, however, that themore » parallel mean free path (radial gradient) appears to persistently increase (decrease) in the last three cycles of weakening solar activity. We suggest that simple differences between these parameters in A > 0 and A < 0 epochs are seriously biased by these long-term trends.« less
Authors:
; ;  [1] ;  [2]
  1. Physics Department, Shinshu University, Matsumoto, Nagano 390-8621 (Japan)
  2. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 87721 (United States)
Publication Date:
OSTI Identifier:
22365414
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 791; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ANISOTROPY; COSMIC RADIATION; DATA ANALYSIS; DENSITY; FORECASTING; HELIOSPHERE; MAGNETIC FIELDS; MEAN FREE PATH; MUONS; NEUTRON MONITORS; SOLAR ACTIVITY; SPACE; SPECTRA; SUN; THREE-DIMENSIONAL CALCULATIONS; VARIATIONS