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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

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.