Long Term Variation of the Solar Diurnal Anisotropy of Galactic Cosmic Rays Over Four Solar Activity Cycles
Abstract
We analyze the three dimensional (3D) anisotropy of the galactic cosmic ray (GCR) intensity observed with a multi-directional muon detector over four solar activity cycles between 197 1 and 201 1 and compare them with the anisotropy seen by neutron monitors (NMs) to examine the rigidity dependence of the anisotropy. We clearly see the phase of the free-space diurnal anisotropy shifting toward earlier hours around solar activity minima in A>0 epochs from 18 hr local solar time in A<0 epochs. The magnitude of the phase-shift is much larger in the muon data than in the NM data. In particular, we find that the anisotropy component perpendicular to the magnetic field, (after correction for the solar wind convection and the Compton-Getting effect due to Earth's orbital motion around the Sun) is significantly larger in the muon data than that in NM data, while the parallel components in two data sets are quite similar to each other. This energy dependence of the perpendicular anisotropy, which is seemingly due to the harder energy spectrum of the drift streaming than the diffusion in this energy region, naturally explains the larger phase-shift observed in muon data. We also find a clear correlation of the magnitude of the parallel anisotropy with the solar wind velocity which varies without any clear 11-year or 22-year periodicity.
- Publication:
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International Cosmic Ray Conference
- Pub Date:
- 2013
- Bibcode:
- 2013ICRC...33.1370M
- Keywords:
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- Diurnal anisotropy;
- Heliospheric modulation of galactic cosmic rays;
- Solar cycle variation of the modulation parameters.