Abstract

Interplanetary structures such as shock, sheath, coronal mass ejections (CMEs) and their interplanetary counterpart (ICMEs), magnetic clouds and corotating interaction regions (CIRs) are of special interest for the study of the transient modulation of galactic cosmic rays (GCRs). These structures modulate the GCR intensity with varying amplitude an recovery time. It is known that ICMEs are mainly responsible for Forbush decreases in GCR intensity. However, all the ICMEs do not produce such decreases in GCR intensity. We utilize GCR intensity data as recorded by neutron monitors and solar wind plasma/field data during the passage of ICMEs with different features and structures. We perform superposed epoch analysis of the data and study their relative GCR-effectiveness. We also adopt the best fit approach to search for interplanetary parameter that relates to transient decreases in GCR intensity.