Access of Energetic Particles to a Magnetic Flux Rope from External Magnetic Field Lines

Abstract

Abstract Cosmic-ray (CR) fluxes in the heliosphere are affected by the transient interplanetary coronal mass ejections (ICMEs), causing so-called Forbush decreases (FDs), characterized by a decline of up to 25% in the neutron monitor counts at the Earth’s surface, lasting up to over a week. FDs are thought to be caused by the ICME shock wave or the magnetic flux rope embedded in the ICME inhibiting CR propagation through the ICME structure. FDs are typically modeled as enhanced diffusion within the ICME structure. However, so far modeling has not considered the access of the CRs from the interplanetary field lines into the isolated magnetic field lines of the ICME flux rope. We study the effect of an ICME flux rope on particle propagation by using full-orbit particle simulations, with the interface between the external interplanetary magnetic field and the isolated flux rope field lines modeled analytically. We find that the particles can access the flux rope through the x-point region, where the external magnetic fields cancel the azimuthal component of the rope field. The transport through this region is fast compared to diffusive radial propagation within the rope. As a result, the propagation of CRs into the flux rope can be modeled as diffusion into a cylinder. The density cavity within the rope is asymmetric, and limited to the magnetic field lines isolated from the external field. Thus, in order to evaluate the role of the flux rope in FDs, one must analyze the extent of the region where the flux rope magnetic field lines are separated from the interplanetary magnetic fields.