Long-Term Variation of the Solar Diurnal Anisotropy of Galactic Cosmic Rays Observed With the Nagoya Multi-Directional Muon Detector

Abstract

We analyze the three dimensional anisotropy of the galactic cosmic ray (GCR) intensities observed independently with a muon detector (MD) 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 bidirectional latitudinal density gradient is positive in A > 0 epoch, while it is negative in A < 0 epoch, in accord with the drift model prediction. The radial density gradient of GCRs, on the other hand, varies with ∼11-year cycle with maxima (minima) in solar maximum (minimum) periods, but we find no significant difference between the radial gradients in 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 parallel mean free path (radial gradient) appears to persistently increase (decreasing) 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.

The spatial density gradient of galactic cosmic rays and its solar cycle variation observed with the Global Muon Detector Network

“…Yasue (1980) analyzed the GG-component together with the sidereal diurnal variation observed by MDs and NMs and derived the power law-type rigidity spectrum of the average NS anisotropy with a positive power law index of approximately 0.3. By analyzing long-term variations of the 3D anisotropies observed with Nagoya MD and NMs on yearly basis, Munakata et al (2014) confirmed that the perpendicular component including the NS anisotropy increases with GCR rigidity. If these are the case, the magnitude of the NS anisotropy increases with rigidity and the T/A separation and success rate will also increase if the dispersion remains similarly independent of rigidity.…”

“…By analyzing the solar diurnal variation and the NS anisotropy of the GCR intensity recorded by neutron monitors (NMs), Bieber and Chen (1991) and Chen and Bieber (1993) derived the solar cycle variations of 3D anisotropy and modulation parameters on a yearly basis. On the other hand, Munakata et al (2014) derived the long-term variation of the 3D anisotropy from the longterm record of the GCR intensity observed with a single multidirectional MD at Nagoya in Japan. By comparing the anisotropy derived from the MD data with that from the NM data, they examined the rigidity dependence of the anisotropy and its solar cycle variation.…”

North-south excess of hemispheric sunspot numbers and cosmic ray modulation

“…The largest value for λ || appears to occur during the positive polarity period (A>0) near SSN minimum, hence dependence on solar magnetic polarity. This is opposite to result reported by Munakata et al (2014) and Chen & Bieber (1993). The equation for the coupled parameter λ || G r in terms of the coefficient α and SDA components is shown in eqn (3).…”

An analysis of large Forbush decrease events using phase diagrams of view channels of the Nagoya multidirectional muon telescope

“…One of the ways to try eliminating atmospheric and noise effects is to use the daily averaged data similarly to the mentioned publications. However, since to the first order the atmospheric temperature effect on muon intensity is common for all view directions (Sagisaka, 1986;Braga et al, 2011;Munakata et al, 2014) the slope of the regression line of phase diagrams is insensitive to these effects. This is another advantage of a phase diagram approach used in the paper.…”