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

Abstract

We derive the long-term variation of the three-dimensional (3D) anisotropy of approximately 60 GV galactic cosmic rays (GCRs) from the data observed with the Global Muon Detector Network (GMDN) on an hourly basis and compare it with the variation deduced from a conventional analysis of the data recorded by a single muon detector at Nagoya in Japan. The conventional analysis uses a north-south (NS) component responsive to slightly higher rigidity (approximately 80 GV) GCRs and an ecliptic component responsive to the same rigidity as the GMDN. In contrast, the GMDN provides all components at the same rigidity simultaneously. It is confirmed that the temporal variations of the 3D anisotropy vectors including the NS component derived from two analyses are fairly consistent with each other as far as the yearly mean value is concerned. We particularly compare the NS anisotropies deduced from two analyses statistically by analyzing the distributions of the NS anisotropy on hourly and daily bases. It is found that the hourly mean NS anisotropy observed by Nagoya shows a larger spread than the daily mean due to the local time-dependent contribution from the ecliptic anisotropy. The NS anisotropy derived from the GMDN, on the other hand, shows similar distribution on both the daily and hourly bases, indicating that the NS anisotropy is successfully observed by the GMDN, free from the contribution of the ecliptic anisotropy. By analyzing the NS anisotropy deduced from neutron monitor (NM) data responding to lower rigidity (approximately 17 GV) GCRs, we qualitatively confirm the rigidity dependence of the NS anisotropy in which the GMDN has an intermediate rigidity response between NMs and Nagoya. From the 3D anisotropy vector (corrected for the solar wind convection and the Compton-Getting effect arising from the Earth's orbital motion around the Sun), we deduce the variation of each modulation parameter, i.e., the radial and latitudinal density gradients and the parallel mean free path for the pitch angle scattering of GCRs in the turbulent interplanetary magnetic field. We show the derived density gradient and mean free path varying with the solar activity and magnetic cycles.

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

“…For example, Kozai et al (2014) found that NS anisotropy averaged over period from 2006 to 2013 for Nagoya MT hourly data is of 0.1324%. NS anisotropy can be characterized by the "GG-component" of Nagoya MT defined for median primary rigidity of approximately 80 GV as Pomerantz, 1971) …”

Estimation of index of power law rigidity spectrum of cosmic rays using effective rigidity of multidirectional muon detector

“…However, the Nagoya muon detector measurements have also been used for studies of anisotropy of CRs, for example, by Kozai et al (2014) who derived the yearly mean values for the threedimensional anisotropy and the modulation parameters and found it to be consistent with those derived from the Global Muon Detector Network data.…”

TeV Cosmic-Ray Anisotropy from the Magnetic Field at the Heliospheric Boundary

“…Galactic CRs in the energy range below about 50 GeV are strongly affected by modulations of the inner heliospheric magnetic field, in correlation with solar cycles (see, e.g., Florinski et al 2013Florinski et al , 2015Potgieter 2013;Manuel et al 2014). Above 50 GeV, the modulation in the CR energy spectrum is negligible; however, the effects of long-term solar cycles on particle distribution are still observed up to an energy of few hundred GeV (Munakata et al 2010;Kozai et al 2014). The gyroradius of 10-to 100-GeV CR particles in the interplanetary magnetic field of <1 μG is typically smaller than the size of the termination shock (about 80-90 au; see .…”