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Modulation Cycles of Galactic Cosmic Ray Diurnal Anisotropy Variation

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Abstract

The diurnal variation of the galactic cosmic ray (GCR) count rates measured by a ground-based neutron monitor (NM) station represents an anisotropic flow of GCR at 1 AU. The variation of the local time of GCR maximum intensity (we call the phase) is thought in general to have a period of two sunspot cycles (22 years). However, other interpretations are also possible. In order to determine the cyclic behavior of GCR anisotropic variation more precisely, we have carried out a statistical study on the diurnal variation of the phase. We examined 54-year data of Huancayo (Haleakala), 40-year data from Rome, and 43-year data from Oulu NM stations using the ‘pile-up’ method and the F-test. We found that the phase variation has two components: of 22-year and 11-year cycles. All NM stations show mainly the 22-year phase variation controlled by the drift effect due to solar polar magnetic field reversal, regardless of their latitudinal location (cut-off rigidity). However, the lower the NM station latitude is (the higher the cut-off rigidity is), the higher is the contribution from the 11-year phase variation controlled by the diffusion effect due to the change in strength of the interplanetary magnetic fields associated with the sunspot cycle.

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Correspondence to Y. Yi.

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Oh, S.Y., Yi, Y. & Bieber, J.W. Modulation Cycles of Galactic Cosmic Ray Diurnal Anisotropy Variation. Sol Phys 262, 199–212 (2010). https://doi.org/10.1007/s11207-009-9504-9

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Keywords

  • Galactic cosmic rays
  • Diurnal variation
  • Modulation cycle
  • Solar cyclic variation