Evolution of coronal mass ejections and the corresponding Forbush decreases: modelling vs multi-spacecraft observations

TL;DR

This paper models the evolution of coronal mass ejections and their associated Forbush decreases, comparing the models with multi-spacecraft observations to understand how CMEs influence cosmic ray flux reductions.

Contribution

The study develops and adapts a model for Forbush decreases that accounts for energy dependence and validates it against multi-spacecraft data, enhancing understanding of CME evolution effects.

Findings

Modelled FDs qualitatively match observations.

The ForbMod model accurately reflects GCR depression in CME structures.

Modelled FDs align with multi-spacecraft measurements, indicating good CME evolution representation.

Abstract

One of the very common in situ signatures of interplanetary coronal mass ejections (ICMEs), as well as other interplanetary transients, are Forbush decreases (FDs), i.e. short-term reductions in the galactic cosmic ray (GCR) flux. A two-step FD is often regarded as a textbook example, which presumably owes its specific morphology to the fact that the measuring instrument passed through the ICME head-on, encountering first the shock front (if developed), then the sheath and finally the CME magnetic structure. The interaction of GCRs and the shock/sheath region, as well as the CME magnetic structure, occurs all the way from Sun to Earth, therefore, FDs are expected to reflect the evolutionary properties of CMEs and their sheaths. We apply modelling to different ICME regions in order to obtain a generic two-step FD profile, which qualitatively agrees with our current observation-based understanding of FDs. We next adapt the models for energy dependence to enable comparison with different GCR measurement instruments (as they measure in different particle energy ranges). We test these modelling efforts against a set of multi-spacecraft observations of the same event, using the Forbush decrease model for the expanding flux rope (ForbMod). We find a reasonable agreement of the ForbMod model for the GCR depression in the CME magnetic structure with multi-spacecraft measurements, indicating that modelled FDs reflect well the CME evolution.