Cross helicity of interplanetary coronal mass ejections at 1 au

Abstract

ABSTRACT Interplanetary coronal mass ejections (ICMEs) contain magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfvénic wave packets propagating parallel or antiparallel to the background magnetic field, with the difference in power between counterpropagating fluxes quantified by the cross helicity. We have determined the cross helicity of inertial range fluctuations at 10−3 to 10−2 Hz in 226 ICME flux ropes and 176 ICME sheaths observed by the Wind spacecraft at 1 au during 1995–2015. The flux ropes and sheaths had mean, normalized cross helicities of 0.18 and 0.24, respectively, with positive values here indicating net antisunward fluxes. While still tipped towards the antisunward direction on average, fluxes in ICMEs tend to be more balanced than in the solar wind at 1 au, where the mean cross helicity is larger. Superposed epoch profiles show cross helicity falling sharply in the sheath and reaching a minimum inside the flux rope near the leading edge. More imbalanced, solar wind-like cross helicity was found towards the trailing edge and laterally further from the rope axis. The dependence of cross helicity on flux rope orientation and the presence of an upstream shock are considered. Potential origins of the low cross helicity in ICMEs at 1 au include balanced driving of the closed-loop flux rope at the Sun and ICME–solar wind interactions in interplanetary space. We propose that low cross helicity of fluctuations is added to the standard list of ICME signatures.