The effect of magnetic field line topology on ICME-related GCR modulation

TL;DR

This study investigates how the magnetic field line topology of ICMEs influences GCR flux modulation, finding that topology has limited impact compared to other dynamic features like speed fluctuations and magnetic field variations.

Contribution

It provides a comparative analysis of open versus closed magnetic topologies in ICMEs and their effects on GCRs, highlighting the importance of internal magnetic and dynamic features over topology.

Findings

GCR response is largely independent of magnetic topology.

Speed fluctuations and magnetic field variations influence Fd profiles.

Magnetic topology plays a minor role compared to other ICME features.

Abstract

The large-scale magnetic structure of interplanetary coronal mass ejections (ICMEs) has been shown to affect the galactic cosmic ray (GCR) flux measured in situ by spacecraft, causing temporary decreases known as Forbush decreases (Fds). In some ICMEs, the magnetic ejecta exhibits a magnetic flux rope (FR) structure; the strong magnetic field strength and closed field line geometry of such ICME FRs has been proposed to act as a shield to GCR transport. In this study, we identify four ICMEs near Earth that drove Fds with similar mean magnetic field strengths (20 - 25 nT); two ICMEs with more typical mean speeds (~400 km/s), and two fast (~750 km/s) ICMEs. Within each speed pairing, we identify an ICME that exhibited an open magnetic field line topology and compare its effect on the GCR flux to that which exhibited a mostly closed topology. We investigate the different mechanisms that contribute to the resulting ICME-related Fds and their recovery, and determine which properties, if any, play a more important role than others in driving Fds. We find that much of the GCR response to the ICME events in this study is independent of the open or closed magnetic field line topology of the flux rope, and that features such as the fluctuations in speed, magnetic field structure, and expansion within the FR may play more of a role in determining the smaller-scale structure of the Fd profile.