How are Forbush decreases related to interplanetary magnetic field enhancements?^⋆

¹ Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, 411 021 Pune, India
e-mail: gupta@grapes.tifr.res.in
² The GRAPES–3 Experiment, Cosmic Ray Laboratory, Raj Bhavan, 643 001 Ooty, India
³ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, Maharashtra, India
⁴ Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
⁵ College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan

Received: 6 October 2014
Accepted: 24 April 2015

Abstract

Aims. A 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 used 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 magnitude >0.25%. We used IMF data from ACE/WIND spacecrafts. We looked for correlations between the FD profile and that of the one-hour averaged IMF. We wanted to find out whether 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 behind 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.