Cross helicity of interplanetary coronal mass ejections at 1 au

TL;DR

This study analyzes the cross helicity of magnetic and velocity fluctuations in interplanetary coronal mass ejections at 1 au, revealing their mostly anti-sunward fluxes and variations across different regions, which may serve as new ICME signatures.

Contribution

It provides the first comprehensive analysis of cross helicity in ICMEs at 1 au, highlighting its potential as a diagnostic tool for ICME characterization.

Findings

ICMEs have mean normalized cross helicities of 0.18 (flux ropes) and 0.24 (sheaths).

Cross helicity drops sharply inside flux ropes near the leading edge.

More balanced cross helicity is observed towards the trailing edge and away from the flux rope axis.

Abstract

Interplanetary coronal mass ejections (ICMEs) contain magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfv\'enic wave packets propagating parallel or anti-parallel to the background magnetic field, with the difference in power between counter-propagating fluxes quantified by the cross helicity. We have determined the cross helicity of inertial range fluctuations at $10^{-3}-10^{-2}$ 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, normalised cross helicities of 0.18 and 0.24, respectively, with positive values here indicating net anti-sunward fluxes. While still tipped towards the anti-sunward 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.