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Aims: We statistically investigate the plasma and magnetic fieldcharacteristics of the upstream regions of interplanetary coronal massejections (ICMEs) and their evolution as function of distance to the Sun in theinner 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 weidentify four main density structures, namely shock, sheath, leading edge (LE),and magnetic ejecta (ME) itself. Methods: We derive separately for eachstructure averaged plasma and magnetic field parameter values as well asduration 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 tothe turbulent SW material following the shock. The sheath lies ahead of aregion of compressed ambient SW, the LE, which is typically found directly infront of the magnetic driver and seems to match the bright leading edgecommonly observed in remote sensing observations of CMEs. The sheath becomesdenser than the ambient SW at about 0.06au, which we interpret as the averagestarting distance for actual sheath formation. Between 0.09-0.28au the sheathstructure density starts to dominate over the density within the ME. The MEdensity seems to fall below the ambient SW density over 0.45-1.07au. Besidesthe well-known expansion of the ME, the sheath size shows a weak positivecorrelation with distance, while the LE seems not to expand with distance fromthe Sun. We further find a moderate anti-correlation between sheath density andlocal SW plasma speed upstream of the ICME shock. An empirical relation isderived connecting the ambient SW speed with sheath and LE density that can beused for modeling of ICME evolution. Constraints to those results are given.

astronomy and astrophysics

Characteristics and evolution of sheath and leading edge structures of  interplanetary coronal mass ejections in the inner heliosphere based on  Helios and Parker Solar Probe observations