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

TL;DR

This study examines the turbulent properties of a magnetic cloud observed by the Parker Solar Probe, revealing balanced wave power in the core and Alfvénic fluctuations in outer layers, with implications for magnetic cloud structure.

Contribution

It provides the first detailed analysis of cross helicity and residual energy in a magnetic cloud at 0.25 au, highlighting their spatial variation and connection to solar wind interactions.

Findings

Low cross helicity in the cloud core suggests balanced wave power.

Outer layers exhibit high |cross helicity| and near-zero residual energy.

Upstream fluctuations show strongly negative residual energy.

Abstract

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.