MMS Insights into CME Driven Sub-Alfv\'enic Solar Wind at 1 AU

TL;DR

This study analyzes electron distributions and turbulence in a sub-Alfvénic CME at 1 AU, revealing weak MHD turbulence and distinct plasma properties compared to super-Alfvénic solar wind.

Contribution

It provides the first detailed characterization of plasma and turbulence properties in a sub-Alfvénic CME wind at 1 AU, highlighting weak turbulence conditions.

Findings

Electrons in the sub-Alfvénic MC have higher temperatures and super-thermal tails.

Magnetic fluctuations show steep spectra, reduced intermittency, and weak compressibility.

The turbulence resembles planetary magnetosphere conditions, such as Jupiter's.

Abstract

We report the properties of electron distributions and turbulence during a Coronal Mass Ejection (CME) in April 2023 observed by Magnetospheric Multiscale (MMS). The CME exhibits a clear sheath and magnetic cloud (MC), and within the MC, the solar wind becomes sub-Alfv\'enic for two hours. We investigate plasma and turbulence properties of the sub-Alfv\'enic CME wind and compare them with those in the super-Alfv\'enic solar wind in the MC and CME sheath. Electrons within the sub-Alfv\'enic MC show significantly higher temperatures than those in the CME sheath and the super-Alfv\'enic MC, with their one-dimensional distributions revealing super-thermal tail and a depletion in electron populations between 15-50 eV. Within the CME sheath, isolated regions of electron heating are observed, where parallel energy flux is enhanced up to ~1 keV. Magnetic field fluctuations within the sub-Alfv\'enic MC interval exhibit negligible cross helicity and steeper-than-Kolmogorov scaling in the inertial range, with no clear spectral break. These fluctuations also show reduced intermittency at ion and sub-ion scales, emerging intermittency at electron scales, and weak magnetic compressibility. Together, these observations point to the presence of weak magnetohydrodynamic (MHD) turbulence within the sub-Alfv\'enic MC, resembling conditions commonly observed in planetary magnetospheres such as Jupiter's.