High-rigidity Forbush decreases: due to CMEs or shocks?

Abstract

Aims. We seek to identify the primary agents causing Forbush decreases (FDs) in high-rigidity cosmic rays observed from the Earth. In particular, we ask if these FDs are caused mainly by coronal mass ejections (CMEs) from the Sun that are directed towards the Earth, or by their associated shocks.Methods. We used the muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We selected those FD events that have a reasonably clean profile, and can be reasonably well associated with an Earth-directed CME and its associated shock. We employed two models: one that considers the CME as the sole cause of the FD (the CME-only model) and one that considers the shock as the only agent causing the FD (the shock-only model). We used an extensive set of observationally determined parameters for both models. The only free parameter in these models is the level of MHD turbulence in the sheath region, which mediates cosmic ray diffusion (into the CME for the CME-only model, and across the shock sheath for the shock-only model).Results. We find that good fits to the GRAPES-3 multi-rigidity data using the CME-only model require turbulence levels in the CME sheath region that are only slightly higher than those estimated for the quiescent solar wind. On the other hand, reasonable model fits with the shock-only model require turbulence levels in the sheath region that are an order of magnitude higher than those in the quiet solar wind.Conclusions. This observation naturally leads to the conclusion that the Earth-directed CMEs are the primary contributors to FDs observed in high-rigidity cosmic rays.