MMS Insights into Coronal Mass Ejection–driven Sub-Alfvénic Solar Wind at 1 au

We report the properties of electron distributions and turbulence during a coronal mass ejection (CME) in 2023 April observed by Magnetospheric Multiscale Mission. The CME exhibits a clear sheath and magnetic cloud (MC), and within the MC, the solar wind becomes sub-Alfvénic for 2 hr. We investigate plasma and turbulence properties of the sub-Alfvénic CME wind and compare them with those in the super-Alfvénic solar wind in the MC and CME sheath. Electrons within the sub-Alfvénic MC show significantly higher temperatures than those in the CME sheath and the super-Alfvénic MC, with their 1D distributions revealing superthermal tail and a depletion in electron populations between 15 and 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-Alfvénic 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 turbulence within the sub-Alfvénic MC, resembling conditions commonly observed in planetary magnetospheres such as Jupiter’s.