Local galactic cosmic ray increases within the sheaths of interplanetary coronal mass ejections

Interplanetary coronal mass ejections (ICMEs) can cause reductions, known as Forbush decreases (Fds), in galactic cosmic ray (GCR) intensities. Two-step Fds occur when both an ICME and its shock encounter a GCR detector. We find 80 ICMEs from 1998 to 2006 that, according to this model, should have created two-step Fds. In 27 of the events, however, a local increase in the GCRs was superposed on the overall decrease of the Fd within the ICME sheath. We discover that these increases generally occur more than a correlation length away from the shock. This implies that the GCR features do not result from turbulence in the interplanetary magnetic field. Therefore we hypothesize that magnetic structure in the sheath may create conduits for the GCRs, thus creating the increases.