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We examine 188 coronal mass ejections (CMEs) measured by the twin STEREOspacecraft during 2007-2016 to investigate the generic features of the CMEsheath and the magnetic ejecta (ME) and dependencies of average physicalparameters of the sheath on the ME. We classify the MEs into three categories,focusing on whether a ME drives both a shock and sheath, or only a sheath, orneither, near 1 AU. We also reevaluate our initial classification through anautomated algorithm and visual inspection. We observe that even for leadingedge speeds greater than 500 km/s, 1 out of 4 MEs do not drive shocks near 1AU. MEs driving both shocks and sheaths are the fastest and propagate in highmagnetosonic solar wind, whereas MEs driving only sheaths are the slowest andpropagate in low magnetosonic solar wind. Our statistical and superposed epochanalyses indicate that all physical parameters are more enhanced in the sheathregions following shocks than in sheaths without shocks. However, differenceswithin sheaths become statistically less significant for similar driving MEs.We also find that the radial thickness of ME-driven sheaths apparently has noclear linear correlation with the speed profile and associated Mach numbers ofthe driver.

Properties of the Sheath Regions of Coronal Mass Ejections with or without Shocks from STEREO in situ Observations near 1 AU