We study interplanetary coronal mass ejections (ICMEs) measured by probes at
different heliocentric distances (0.3-1AU) to investigate the propagation of
ICMEs in the inner heliosphere and determine how the generic features of ICMEs
change with heliospheric distance. Using data from the MESSENGER, Venus Express
and ACE spacecraft, we analyze with the superposed epoch technique the profiles
of ICME substructures, namely the sheath and the magnetic ejecta. We determine
that the median magnetic field magnitude in the sheath correlates well with
ICME speeds at 1 AU and we use this proxy to order the ICMEs at all spacecraft.
We then investigate the typical ICME profiles for three categories equivalent
to slow, intermediate and fast ICMEs. Contrary to fast ICMEs, slow ICMEs have a
weaker solar wind field at the front and a more symmetric magnetic field
profile. We find the asymmetry to be less pronounced at Earth than at Mercury,
indicating a relaxation taking place as ICMEs propagate. We also find that the
magnetic field intensities in the wake region of the ICMEs do not go back to
the pre-ICME solar wind intensities, suggesting that the effects of ICMEs on
the ambient solar wind last longer than the duration of the transient event.
Such results provide an indication of physical processes that need to be
reproduced by numerical simulations of ICME propagation. The samples studied
here will be greatly improved by future missions dedicated to the exploration
of the inner heliosphere, such as Parker Solar Probe and Solar Orbiter.
