Categorization of Coronal Mass Ejection Driven Sheath Regions: Characteristics of STEREO Events

We present a comprehensive statistical analysis of 106 sheath regions driven by coronal mass ejections (CMEs) and measured near 1 AU. Using data from the STEREO probes, this extended analysis focuses on two discrete categorizations. In the first categorization, we investigate how the generic features of sheaths change with their potential formation mechanisms (propagation and expansion sheaths), namely, their associations with magnetic ejectas (MEs) which are primarily expanding or propagating in the solar wind. We find propagation sheaths to be denser and driven by stronger MEs, whereas expansion sheaths are faster. Exploring the temporal profiles of these sheaths with a superposed epoch technique, we observe that most of the magnetic field and plasma signatures are more elevated in propagation sheaths relative to expansion sheaths. The second categorization is based on speed variations across sheaths. Employing linear least squares regression, we categorize four distinct speed profiles of the sheath plasma. We find that the associated shock properties and solar cycle phase do not impact the occurrence of such variations. Our results also highlight that the properties of the driving MEs are a major source of variability in the sheath properties. Through logistic regression, we conclude that the magnetic field strength and the ME speed in the frame of the solar wind are likely drivers of these speed variations.