  Interplanetary Coronal Mass Ejections (ICMEs) are known to modify the
structure of the solar wind as well as interact with the space environment of
planetary systems. Their large magnetic structures have been shown to interact
with galactic cosmic rays, leading to the Forbush decrease (FD) phenomenon. We
revisit in the present article the 17 years of Advanced Composition Explorer
spacecraft ICME detection along with two neutron monitors (McMurdo and Oulu)
with a superposed epoch analysis to further analyze the role of the magnetic
ejecta in driving FDs. We investigate in the following the role of the sheath
and the magnetic ejecta in driving FDs, and we further show that for ICMEs
without a sheath, a magnetic ejecta only is able to drive significant FDs of
comparable intensities. Furthermore, a comparison of samples with and without a
sheath with similar speed profiles enable us to show that the magnetic field
intensity, rather than its fluctuations, is the main driver for the FD.
Finally, the recovery phase of the FD for isolated magnetic ejecta shows an
anisotropy in the level of the GCRs. We relate this finding at 1 au to the
gradient of the GCR flux found at different heliospheric distances from several
interplanetary missions.
