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

TL;DR

This study analyzes 188 CMEs observed by STEREO near 1 AU, revealing how their sheath regions and magnetic ejecta vary with shock presence, speed, and solar wind conditions, providing insights into CME propagation and structure.

Contribution

It offers a detailed classification and statistical analysis of CME sheaths and magnetic ejecta, highlighting differences based on shock driving and solar wind environment.

Findings

CMEs without shocks constitute 25% even at speeds >500 km/s.

Sheaths with shocks show higher physical parameter enhancements.

No clear correlation between sheath thickness and CME speed or Mach number.

Abstract

We examine 188 coronal mass ejections (CMEs) measured by the twin STEREO spacecraft during 2007-2016 to investigate the generic features of the CME sheath and the magnetic ejecta (ME) and dependencies of average physical parameters 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, or neither, near 1 AU. We also reevaluate our initial classification through an automated algorithm and visual inspection. We observe that even for leading edge speeds greater than 500 km/s, 1 out of 4 MEs do not drive shocks near 1 AU. MEs driving both shocks and sheaths are the fastest and propagate in high magnetosonic solar wind, whereas MEs driving only sheaths are the slowest and propagate in low magnetosonic solar wind. Our statistical and superposed epoch analyses indicate that all physical parameters are more enhanced in the sheath regions following shocks than in sheaths without shocks. However, differences within sheaths become statistically less significant for similar driving MEs. We also find that the radial thickness of ME-driven sheaths apparently has no clear linear correlation with the speed profile and associated Mach numbers of the driver.