Duration of the recovery phase of sporadic Forbush decreases

A model is proposed to describe the recovery of sporadic Forbush decreases, in which the geometry of the large-scale magnetic field plays a significant role. The dependence of the duration of the recovery of the Forbush decrease on the angular width of the disturbance, the gradient of the solar wind flow velocity, and the heliolongitude of the source of the ejection has been established. The relative contribution of different causes to the duration of recovery varies in events and depends on the parameter values. The modeled duration of the Forbush decrease recovery agrees with the measurements. The model explains the east–west duration asymmetry, consistent with the measurements. Analysis of the observed Forbush decreases using the model shows its applicability. The model can be applied to determine the angular width of an interplanetary coronal mass ejection (a hard-to-determine parameter) using the observed parameters: the duration of the Forbush decrease recovery, the solar wind velocity gradient, and the heliolongitude of the coronal mass ejection source.