Modeling of Forbush decreases in cosmic rays

Abstract

An explosion at the Sun that releases a large amount of mass with high velocity, known as a coronal mass ejection (CME), causes a discontinuity in the solar wind known as a shock. The shock and associated energetic cosmic rays could cause various problems for human activities known as "space weather effects." This work studies the particle transport in the region just before the shock (loss cone precursor) and just behind the shock (the first step of Forbush decrease) by modeling and performing time-dependent numerical simulations of GCRs in the vicinity of the shock. We found that the width of the precursor loss cone provides a prediction of whether the approaching shock is quasi-parallel or quasi-perpendicular. We also showed that quasi-perpendicular shocks produce loss cones with opening angles ~25 ํ, whereas quasi-parallel shocks have loss cones with opening angles ~ 50 ํ. This implies that loss cone measurements can in principle provide a quantitative indication of the time when an interplanetary shock will arrive at Earth. We also found that reflection is a possible mechanism for the first step of the Forbush decrease.