How are Forbush decreases related to interplanetary magnetic field enhancements ?

TL;DR

This study investigates the relationship between Forbush decreases and interplanetary magnetic field enhancements caused by CMEs, revealing that FDs are primarily due to proton diffusion into turbulent magnetic regions.

Contribution

It provides new insights into the cause of FDs, linking them to proton diffusion into magnetic field enhancements in the CME sheath region.

Findings

FDs lag IMF enhancements by a few hours.

IMF enhancements occur mainly in the shock-sheath region.

High rigidity FDs are caused by proton diffusion across magnetic fields.

Abstract

Aims. Forbush decrease (FD) is a transient decrease followed by a gradual recovery in the observed galactic cosmic ray intensity. We seek to understand the relationship between the FDs and near-Earth interplanetary magnetic field (IMF) enhancements associated with solar coronal mass ejections (CMEs). Methods. We use muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We select those FD events that have a reasonably clean profile, and magnitude > 0.25%. We use IMF data from ACE/WIND spacecrafts. We look for correlations between the FD profile and that of the one hour averaged IMF. We ask if the diffusion of high energy protons into the large scale magnetic field is the cause of the lag observed between the FD and the IMF. Results. The enhancement of the IMF associated with FDs occurs mainly in the shock-sheath region, and the turbulence level in the magnetic field is also enhanced in this region. The observed FD profiles look remarkably similar to the IMF enhancement profiles. The FDs typically lag the IMF enhancement by a few hours. The lag corresponds to the time taken by high energy protons to diffuse into the magnetic field enhancement via cross-field diffusion. Conclusions. Our findings show that high rigidity FDs associated with CMEs are caused primarily by the cumulative diffusion of protons across the magnetic field enhancement in the turbulent sheath region between the shock and the CME.