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CME evolution and the corresponding Forbush decrease: modelling vs multi-spacecraft observation

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. FD phenomena are caused by the interaction of GCRs with a magnetic structure, therefore it is expected that different types of interplanetary substructures cause different types of GCR time profiles, allowing us to distinguish between shock/sheath, flux rope and SIR-type of FDs. Moreover, since the interaction of GCRs and CME magnetic structure (i.e. flux rope) occurs all the way from Sun to Earth, FDs reflect the evolutionary properties of CMEs. We apply modelling to different ICME regions in order to obtain a generic FD profile. We model the shock/sheath- related FD using the propagating diffusive barrier (PDB) model, the flux-rope-related FD using the diffusion model for the expanding flux rope (ForbMod), and the exponential time profile approximates the recovery after the event. The modeled generic FD profile qualitatively agrees with our current observation-based understanding of FDs. In addition, we test ForbMod against a set of multi-spacecraft observations of the same ICME. We find a reasonable agreement of the ForbMod model with multi-spacecraft measurements, indicating that modelled FDs reflect well the flux rope evolution.

coronal mass ejections, flux ropes, forbush decreases, multi-spacecraft observations

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