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

Abstract

We analyze the galactic cosmic ray (GCR) density and its spatial gradient in Forbush Decreases (FDs) observed with the Global Muon Detector Network (GMDN) and neutron monitors (NMs). By superposing the GCR density and density gradient observed in FDs following 45 interplanetary shocks (IP-shocks), each associated with an identified eruption on the sun, we infer the average spatial distribution of GCRs behind IP-shocks. We find two distinct modulations of GCR density in FDs, one in the magnetic sheath and the other in the coronal mass ejection (CME) behind the sheath. The density modulation in the sheath is dominant in the western flank of the shock, while the modulation in the CME ejecta stands out in the eastern flank. This east-west asymmetry is more prominent in GMDN data responding to ∼ 60 GV GCRs than in NM data responding to ∼ 10 GV GCRs, because of softer rigidity spectrum of the modulation in the CME ejecta than in the sheath. The GSE-y component of the density gradient, G y shows a negative (positive) enhancement in FDs caused by the eastern (western) eruptions, while G z shows a negative (positive) enhancement in FDs by the northern (southern) eruptions. This implies the GCR density minimum being located behind the central flank of IP-shock and propagating radially outward from location of the solar eruption. We also confirmed the average G z changing its sign above and below the heliospheric current sheet, in accord with the prediction of the drift model for the large-scale GCR transport in the heliosphere.

An Analytical Diffusion–Expansion Model for Forbush Decreases Caused by Flux Ropes

“…Therefore, the diffusion approach should be valid to explain FR associated FDs detected in a variety of instruments corresponding to different mean energies: muon telescopes at ∼ 50 GV (e.g. Kozai et al 2016), neutron monitors at ∼ 10 GV (e.g. Clem and Dorman 2000), and spacecraft such as the Electron Proton Helium Instrument (EPHIN, Müller-Mellin et al 1995) onboard Solar and Heliospheric Observatory (SOHO, Domingo et al 1995) at ∼ 1 GV (e.g.…”

Effects of ICMEs on High Energetic Particles as Observed by the Global Muon Detector Network (GMDN)

“…As these shocks propagate, they sweep out galactic cosmic rays forming a depleted region behind them. Kozai et al (2016) studied the cosmic ray density and gradient derived by GMDN observations during Forbush decreases of a number of CME-ICME shock events, originating in several positions at the solar disk. These authors found a distinct modulation of galactic cosmic rays, one related to the shock-sheath region and another due to the ICME flux rope.…”

High-energy cosmic ray modulation associated with interplanetary shocks observed by the GMDN

“…A previous study related the cosmic ray decreases associated with shocks were already performed using observations from the GMDN up to 2014 (see, e.g., [13]) and the events were selected using the geomagnetic storm sudden commencement (SSC) listed by the German Research Centre for Geosciences (GFZ) as the identification of the shock arrival at Earth. Although the SSCs are considered to be caused by the interplanetary shocks [14], cosmic rays are not directly modulated by the geomagnetic field conditions but primary by the conditions in the interplanetary magnetic…”