Cross helicity of interplanetary coronal mass ejections

Like the solar wind in general, interplanetary coronal mass ejections (ICMEs) display magnetic field and velocity fluctuations across a wide range of scales. These fluctuations may be interpreted as Alfvénic wave packets propagating parallel or anti-parallel to the local magnetic field direction, with cross helicity, σ_c, quantifying the difference in power between the counter-propagating fluxes. We have determined σ_c at inertial range frequencies in a large sample of ICME flux ropes and sheaths observed by the Wind spacecraft at 1 au. The mean σ_c value was low for both the flux ropes and sheaths, with the balance tipped towards the positive, anti-sunward direction. The low values indicate that Alfvénic fluxes are more balanced in ICMEs than in the solar wind at 1 au, where σ_c tends to be larger and anti-sunward fluctuations show a greater predominance. Superposed epoch profiles show σ_c falling sharply in the upstream sheath and being typically close to balance inside the flux rope near the leading edge. More imbalanced, solar wind-like σ_c values are found towards the trailing edge and further from the rope axis. The presence or absence of an upstream shock also has a significant effect on σ_c. Coronal and interplanetary origins of low σ_c in ICMEs are discussed.