Cross helicity of magnetic clouds observed by Parker Solar Probe

Magnetic clouds are large-scale transient structures in the solar wind with low plasma β, low-amplitude magnetic field fluctuations, and twisted field lines with both ends often connected to the Sun. We analyse the normalised cross helicity, σ_c, and residual energy, σ_r, in magnetic clouds observed by Parker Solar Probe (PSP). In the November 2018 cloud observed at 0.25 au, a low value of σ_c was present in the cloud core, indicating that wave power parallel and anti-parallel to the mean field was approximately balanced, while the cloud’s outer layers displayed larger amplitude Alfvénic fluctuations with high σ_c values and σ_r ~ 0. These properties are compared and contrasted to those found in clouds observed by PSP at larger heliocentric distances. We suggest that low σ_c is likely a common feature of magnetic clouds given their typically closed field structure, in contrast to the generally higher σ_c found on the open field lines of the solar wind.