2024-03-29T14:11:25Z
https://soar-ir.repo.nii.ac.jp/oai
oai:soar-ir.repo.nii.ac.jp:00019841
2022-12-14T04:16:21Z
1169:1170
LONG-TERM VARIATION OF THE SOLAR DIURNAL ANISOTROPY OF GALACTIC COSMIC RAYS OBSERVED WITH THE NAGOYA MULTI-DIRECTIONAL MUON DETECTOR
Munakata, K.
Kozai, M.
Kato, C.
Kota, J.
cosmic rays
methods: data analysis
Sun: heliosphere
Sun: magnetic fields
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 theA > 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 similar to 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 the 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.
Article
ASTROPHYSICAL JOURNAL. 791(1):22 (2014)
journal article
IOP PUBLISHING LTD
2014-08-10
application/pdf
ASTROPHYSICAL JOURNAL
1
791
22
0004-637X
AA00553242
https://soar-ir.repo.nii.ac.jp/record/19841/files/Munakata_2014_ApJ_791_22.pdf
eng
10.1088/0004-637X/791/1/22
https://doi.org/10.1088/0004-637X/791/1/22
© 2014 The American Astronomical Society