Cross helicity and electron strahl signatures in interplanetary coronal mass ejections

The solar wind is permeated by Alfvénic fluctuations across a broad range of scales, with fluctuations that have an anti-sunward sense of propagation typically dominating.  Recent studies have shown that cross helicity, which can be used to measure the difference between the sunward and anti-sunward fluctuation power, is more balanced inside interplanetary coronal mass ejections (ICMEs) than in the solar wind generally.  A possible cause of balanced cross helicity in ICMEs is their closed magnetic field structure, with a closed loop connected at both ends to the Sun being able to support a more balanced population of Alfvénic fluctuations.  To test this hypothesis, we have performed a statistical study that compares cross helicity with electron strahl signatures for a large number of ICMEs at 1 au.  Bidirectional electrons are a well-established signature of closed magnetic field structures in ICMEs.  A moderate correlation between bidirectional electrons and balanced cross helicity has been identified, supporting the closed-field hypothesis as an origin of the balanced cross helicity found in ICMEs.