  Aims: We statistically investigate the plasma and magnetic field
characteristics of the upstream regions of interplanetary coronal mass
ejections (ICMEs) and their evolution as function of distance to the Sun in the
inner heliosphere. We use a sample of 40 well-observed ICMEs from Helios 1/2
(0.3-1au) and 5 from Parker Solar Probe (0.32-0.75au). For each event we
identify four main density structures, namely shock, sheath, leading edge (LE),
and magnetic ejecta (ME) itself. Methods: We derive separately for each
structure averaged plasma and magnetic field parameter values as well as
duration and place the results into comparison with the upstream solar wind
(SW) to investigate the interrelation between the different density structures.
Results: The sheath structure presumably consists of compressed plasma due to
the turbulent SW material following the shock. The sheath lies ahead of a
region of compressed ambient SW, the LE, which is typically found directly in
front of the magnetic driver and seems to match the bright leading edge
commonly observed in remote sensing observations of CMEs. The sheath becomes
denser than the ambient SW at about 0.06au, which we interpret as the average
starting distance for actual sheath formation. Between 0.09-0.28au the sheath
structure density starts to dominate over the density within the ME. The ME
density seems to fall below the ambient SW density over 0.45-1.07au. Besides
the well-known expansion of the ME, the sheath size shows a weak positive
correlation with distance, while the LE seems not to expand with distance from
the Sun. We further find a moderate anti-correlation between sheath density and
local SW plasma speed upstream of the ICME shock. An empirical relation is
derived connecting the ambient SW speed with sheath and LE density that can be
used for modeling of ICME evolution. Constraints to those results are given.
