Imbalanced Turbulence Modified by Large-scale Velocity Shears in the Solar Wind

Abstract

We have investigated how the degree of imbalance in solar wind turbulence is modified by large-scale velocity shears in the solar wind plasma. The balance between counterpropagating Alfvénic fluctuations, which interact nonlinearly to generate the turbulence, has been quantified by the cross helicity and Elsasser ratio. Velocity shears at a 30-min timescale were identified, with the shear amplitude defined in terms of the linear Kelvin-Helmholtz (KH) instability threshold. The shears were associated with 74 interplanetary coronal mass ejection (ICME) sheaths observed by the Wind spacecraft at 1 au between 1997 and 2018. Typically weaker shears upstream of the sheaths and downstream in the ICME ejecta were also analyzed. In shears below the KH threshold, imbalance was approximately invariant or weakly rising with shear amplitude. Above the KH threshold, fluctuations tended toward a balanced state with increasing shear amplitude. Magnetic compressibility was also found to increase above the KH threshold. These findings are consistent with velocity shears being local sources of sunward fluctuations that act to reduce net imbalances in the antisunward direction, and suggest that the KH instability plays a role in this process.

arXiv comment9 pages, 5 figures, accepted for publication in the Astrophysical Journal Letters 2023 March 2

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Cited by in corpus (3)

• Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure
• Modelling the interaction of Alfvénic fluctuations with coronal mass ejections in the low solar corona
• Intermittency in Interplanetary Coronal Mass Ejections Observed by Parker Solar Probe and Solar Orbiter