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

T Laitinen,S Dalla

doi:10.3847/1538-4357/abc622

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.

Highlights

Propagation of cosmic rays (CRs) in the heliosphere is controlled by the electromagnetic fields within the solar wind plasma
The CR propagation is affected by large-scale stable and transient features in the heliosphere such as corotating interaction regions and interplanetary coronal mass ejections (ICMEs)
The large-scale heliospheric structures are known to cause sudden decreases in the intensities of high-energy galactic cosmic rays (GCRs), which have their sources outside the heliosphere

Summary

Introduction

Propagation of cosmic rays (CRs) in the heliosphere is controlled by the electromagnetic fields within the solar wind plasma. The large-scale heliospheric structures are known to cause sudden decreases in the intensities of high-energy galactic cosmic rays (GCRs), which have their sources outside the heliosphere. These Forbush decreases (Forbush 1937; Lockwood 1971; Cane 2000; Richardson & Cane 2011) are usually attributed to ICMEs, corotating interaction regions and heliospheric current sheet crosses can affect the GCR intensities (e.g., Richardson et al 1999; Thomas et al 2014). The magnitude of GCR intensity decrease varies from event to event, reaching up to 25% as measured by neutron monitors, and has a dependency on particle rigidity R of R− γ with γ ranging between 0.4 and 1.2 (e.g., Cane 2000)

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