Cross helicity of interplanetary coronal mass ejections

Abstract

<p>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<sub>c</sub>, quantifying the difference in power between the counter-propagating fluxes. We have determined σ<sub>c</sub> 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<sub>c</sub> 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<sub>c</sub> tends to be larger and anti-sunward fluctuations show a greater predominance. Superposed epoch profiles show σ<sub>c</sub> 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<sub>c</sub> 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<sub>c</sub>. Coronal and interplanetary origins of low σ<sub>c</sub> in ICMEs are discussed.</p>