Proton Temperature Anisotropy within the Interplanetary Coronal Mass Ejections Sheath at 1 au

Abstract

The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, turbulent, and magnetically intense relative to the ambient solar wind. In this Letter, we perform a detailed study of proton temperature anisotropy within the 333 ICME sheath regions observed on board the Wind spacecraft spanning the years 1995–2015. Our observations show that marginal stability thresholds of mirror mode and firehose instabilities predominantly constrain the proton temperature anisotropy within these sheath regions regardless of ICME sheath plasma speed. This is true even when the plasma beta values are less than 2, a parameter space that should have favored the prevalence of parallel firehose and proton cyclotron instabilities according to linear stability analysis. This investigation demonstrates the critical role played by distinct plasma instabilities in shaping the evolution of ICME sheath plasma compared to the broader solar wind environment.