Interplanetary coronal mass ejections (ICMEs) cause decreases, so-called Forbush decreases, in the cosmic ray (CR) intensities. FDs are seen as up to 25% decreases in neutron monitor counts at Earth, lasting up to over a week. An ICME is thought to cause a FD through two mechanisms: by enhancing diffusion in the ICME shock wave sheath; and by preventing the CRs from penetrating the magnetic fluxrope embedded in the ICME. CR propagation during a FD is usually modelled as enhanced diffusion either within the whole ICME or within the embedded fluxrope. However, a question that is so far unanswered is how the CRs can reach the isolated fluxrope fieldlines from the open, external interplanetary fieldlines. We study the propagation of CRs from external field into a fluxrope by employing full-orbit particle simulations with scattering. The interface between the internal and external field lines is modelled analytically. We find that the CRs can access the fluxrope rapidly through x-point region, where the external magnetic field partially cancels the magnetic field of the fluxrope. The access is rapid compared to diffusive radial propagation of CRs within the rope. We find that CR propagation within the fluxrope can be modelled using diffusion models, without need to separately model the access to the isolated field lines, provided that the bounds of the diffusion area are taken as that of the isolated fieldlines instead of the region with smoothly rotating magnetic field. Thus, to evaluate the role of a fluxrope in FDs, the extent of the region where the rope magnetic fields are not connected to the external field must be analysed.
Forbush Decrease; ICME;simulations