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We analyze the galactic cosmic ray (GCR) density and its spatial gradient inForbush Decreases (FDs) observed with the Global Muon Detector Network (GMDN)and neutron monitors (NMs). By superposing the GCR density and density gradientobserved in FDs following 45 interplanetary shocks (IP-shocks), each associatedwith an identified eruption on the sun, we infer the average spatialdistribution of GCRs behind IP-shocks. We find two distinct modulations of GCRdensity in FDs, one in the magnetic sheath and the other in the coronal massejection (CME) behind the sheath. The density modulation in the sheath isdominant in the western flank of the shock, while the modulation in the CMEejecta stands out in the eastern flank. This east-west asymmetry is moreprominent in GMDN data responding to $\sim$ 60 GV GCRs than in NM dataresponding to $\sim$ 10 GV GCRs, because of softer rigidity spectrum of themodulation in the CME ejecta than in the sheath. The GSE-y component of thedensity gradient, $G_y$ shows a negative (positive) enhancement in FDs causedby the eastern (western) eruptions, while $G_z$ shows a negative (positive)enhancement in FDs by the northern (southern) eruptions. This implies the GCRdensity minimum being located behind the central flank of IP-shock andpropagating radially outward from location of the solar eruption. We alsoconfirmed the average $G_z$ changing its sign above and below the heliosphericcurrent sheet, in accord with the prediction of the drift model for thelarge-scale GCR transport in the heliosphere.

Average spatial distribution of cosmic rays behind the interplanetary shock -Global Muon Detector Network observations-