Cross Helicity of the November 2018 Magnetic Cloud Observed by the Parker Solar Probe

Magnetic clouds are large-scale transient structures in the solar wind with low plasma $\beta$, low-amplitude magnetic field fluctuations, and twisted field lines with both ends often connected to the Sun. Their inertial-range turbulent properties have not been examined in detail. In this Letter, we analyze the normalized cross helicity, $\sigma_c$, and residual energy, $\sigma_r$, of plasma fluctuations in the November 2018 magnetic cloud observed at 0.25 au by the Parker Solar Probe. A low value of $|\sigma_c|$ 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 Alfv\'enic fluctuations with high $|\sigma_c|$ values and $\sigma_r\sim0$. These properties are discussed in terms of the cloud's solar connectivity and local interaction with the solar wind. We suggest that low $|\sigma_c|$ is likely a common feature of magnetic clouds given their typically closed field structure. Anti-sunward fluctuations propagating immediately upstream of the cloud had strongly negative $\sigma_r$ values.
Comment: 9 pages, 4 figures; accepted for publication in the Astrophysical Journal Letters 2020 August 17