  Interplanetary coronal mass ejections (ICMEs) are the interplanetary
manifestations of solar eruptions. The overtaken solar wind forms a sheath of
compressed plasma at the front of ICMEs. Magnetic clouds (MCs) are a subset of
ICMEs with specific properties (e.g. the presence of a flux rope). When ICMEs
pass near Earth, ground observations indicate that the flux of galactic cosmic
rays (GCRs) decreases. The main aims of this paper are to find: common plasma
and magnetic properties of different ICME sub-structures, and which ICME
properties affect the flux of GCRs near Earth. We use a superposed epoch method
applied to a large set of ICMEs observed \insitu\ by the spacecraft ACE,
between 1998 and 2006. We also apply a superposed epoch analysis on GCRs time
series observed with the McMurdo neutron monitors. We find that slow MCs at 1
AU have on average more massive sheaths. We conclude that it is because they
are more effectively slowed down by drag during their travel from the Sun. Slow
MCs also have a more symmetric magnetic field and sheaths expanding similarly
as their following MC, while in contrast, fast MCs have an asymmetric magnetic
profile and a compressing sheath in compression. In all types of MCs, we find
that the proton density and the temperature, as well as the magnetic
fluctuations can diffuse within the front of the MC due to 3D reconnection.
Finally, we derive a quantitative model which describes the decrease of cosmic
rays as a function of the amount of magnetic fluctuations and field strength.
The obtained typical profiles of sheath/MC/GCR properties corresponding to
slow, mid, and fast ICMEs, can be used for forecasting/modelling these events,
and to better understand the transport of energetic particles in ICMEs. They
are also useful for improving future operative space weather activities.
