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

TL;DR

This study investigates the spatial distribution and modulation of galactic cosmic rays behind interplanetary shocks using global detector data, revealing east-west asymmetries and gradient behaviors consistent with drift models.

Contribution

It provides the first comprehensive analysis of GCR spatial distribution behind IP-shocks using combined GMDN and neutron monitor data, highlighting asymmetries and gradient patterns.

Findings

GCR density shows distinct modulations in the sheath and CME ejecta.

East-west asymmetry in GCR modulation is more prominent at higher rigidities (~60 GV).

GCR density gradients align with drift model predictions and vary with heliospheric position.

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 $\sim$ 60 GV GCRs than in NM data responding to $\sim$ 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.