Analyzing the Properties and Formation of Diverse CME-driven Sheath Regions Following Fast-Forward Shocks with STEREO in-situ Measurements.

Abstract:

We investigate the propagation of sheath plasma in 106 CME-driven sheath regions near 1 AU from 2007-2016, using STEREO in-situ measurements. We classify their speed profiles into four categories, based on how close to linearly increasing or decreasing the speed in the sheath is found to be. Initial statistics show that sheaths with similar speeds as the upstream solar wind are driven by the slowest and weakest CMEs. In contrast, sheaths for which the front part is slower than the back part are driven by the fastest and much stronger CMEs and they are also the shortest in duration. The propagation speed of CMEs in the solar wind frame is the primary parameter found to be responsible for the formation of this type of sheaths. However, comparable contributions from the propagation and expansion speeds of CMEs give rise to complex and longer than typical sheaths. We observe that sheaths with a faster front than back are denser and more magnetized than the other three types of sheaths, probably resulting from the strong compression associated with the CME-driven shocks. Also, sheaths with significant speed gradient between the front and back exhibit the largest magnetic field fluctuations and are statistically similar in terms of many plasma and interplanetary magnetic field parameters.