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One of the very common in situ signatures of interplanetary coronal massejections (ICMEs), as well as other interplanetary transients, are Forbushdecreases (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 presumablyowes its specific morphology to the fact that the measuring instrument passedthrough the ICME head-on, encountering first the shock front (if developed),then the sheath and finally the CME magnetic structure. The interaction of GCRsand the shock/sheath region, as well as the CME magnetic structure, occurs allthe way from Sun to Earth, therefore, FDs are expected to reflect theevolutionary properties of CMEs and their sheaths. We apply modelling todifferent ICME regions in order to obtain a generic two-step FD profile, whichqualitatively agrees with our current observation-based understanding of FDs.We next adapt the models for energy dependence to enable comparison withdifferent GCR measurement instruments (as they measure in different particleenergy ranges). We test these modelling efforts against a set ofmulti-spacecraft observations of the same event, using the Forbush decreasemodel for the expanding flux rope (ForbMod). We find a reasonable agreement ofthe ForbMod model for the GCR depression in the CME magnetic structure withmulti-spacecraft measurements, indicating that modelled FDs reflect well theCME evolution.

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