Drift Effects and the Average Features of Cosmic Ray Density Gradient in Cirs During Successive Two Solar Minimum Periods

Abstract

We deduce on hourly basis the spatial gradient of the cosmic ray density in three dimensions from the directional anisotropy of high-energy (∼50 GeV) galactic cosmic ray (GCR) intensity observed with a global network of muon detectors on the Earth's surface. By analyzing the average features of the gradient in the corotational interaction regions (CIRs) recorded in successive two solar activity minimum periods, we find that the observed latitudinal gradient (Gz) changes its sign from negative to positive on the Earth's heliospheric current sheet (HCS) crossing from the northern to the southern hemisphere in A<0 epoch, while it changes from positive to negative in A>0 epoch. This is in accordance with the drift prediction. We also find a negative enhancement in Gx after the HCS crossing in both A<0 and A>0 epochs, but not in Gy. This asymmetrical feature of Gx and Gy indicates significant contributions from the parallel and perpendicular diffusions to the the gradient in CIRs in addition to the contribution from the drift effect.

Average Spatial Distribution of Cosmic Rays Behind the Interplanetary Shock—global Muon Detector Network Observations

“…This drift model prediction of G z has been actually confirmed by previous analyses of the GMDN and NM data (Chen & Bieber 1993;Okazaki et al 2008;Fushishita et al 2010a;Munakata et al 2014;Kozai et al 2014).…”

“…Following theoretical calculations by Bieber et al (2004), we assume in this paper constant α and α ⊥ at α = 7.2 and α ⊥ = 0.05α . This assumption is also used by Okazaki et al (2008) and Fushishita et al (2010a) and proved to result in a reasonable GCR density distribution in the vicinity of the interplanetary disturbance. Moreover, Fushishita et al (2010b) deduced the parallel mean free path λ from the observed "decay length" of the loss-cone precursor of an IP-shock event and obtained λ comparable to our assumption of λ = 7.2R L .…”

“…The drift model (Kóta & Jokipii 1982, 1983) predicts a spatial distribution of the GCR density having a local maximum close to the heliospheric current sheet (HCS) (Wilcox & Ness 1965) in the "negative" polarity period of the solar polar magnetic field (also referred as the A < 0 epoch) when the IMF directs toward (away from) the sun above (below) the HCS. All SSC events before 2012 in Table 1 (Chen & Bieber 1993;Okazaki et al 2008;Fushishita et al 2010a;Munakata et al 2014;Kozai et al 2014). Figure 8 shows the superposed density gradient of 45 central events in away and toward IMF sectors.…”

“…The Global Muon Detector Network (GMDN), which is capable of measuring the isotropic intensity and three-dimensional anisotropy of ∼ 60 GV GCRs on an hourly basis, was completed with four multi-directional muon detectors at Nagoya (Japan), Hobart (Australia), São Martinho da Serra (Brazil), and Kuwait University (Kuwait) in 2006. An analysis method of deducing the GCR density and anisotropy from the GMDN data has been developed (Okazaki et al 2008;Fushishita et al 2010a).…”

A Peculiar ICME Event in August 2018 Observed with the Global Muon Detector Network

“…the simplest adiabatic heating of cosmic rays, is a possible cause of the cosmic-ray increase observed near the trailing edge of MFR in this paper, as discussed above, although we do not know any literatures reporting the compressive heating of plasma or energetic ions/electrons inside an MFR near the trailing edge. On the other hand, there are other possibilities to observe the cosmic-ray increase, because steady structures such as the heliospheric current sheet and corotating interaction region affect the drift of cosmic rays and can also modulate the large-scale spatial distribution of cosmic-ray density and anisotropy (Fushishita et al, 2010;Okazaki et al, 2008). This study therefore provides an important clue to examine cosmic-ray diffusions parallel and perpendicular to the IMF, and the dependence on the IMF magnitude in great detail, via close collaborations with the drift-model simulations of cosmic-ray transport (Miyake et al, 2017) and the cutting-edge MHD simulations (Matsumoto et al, 2019;Shiota and Kataoka, 2016).…”