Observation of Large-scale Kelvin–Helmholtz Instability Wave Driven by a Coronal Mass Ejection

Abstract The Kelvin–Helmholtz instability (KHI) can occur when there is a relative motion between two adjacent fluids. In the case of magnetized plasma, the shear velocity must exceed the local Alfvén speed for the instability to develop. The KHI produces nonlinear waves that eventually roll up into vortices and contribute to turbulence and dissipation. In the solar atmosphere, KHI has been detected in coronal mass ejections (CMEs), jets, and prominences, mainly in the low corona. Only a few studies have reported the KHI in the upper corona, and its vortex development there has not been previously observed. We report an event with large-scale KHI waves observed from ∼6 to 14 R ⊙ on 2024 February 16 using Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph and STEREO-A coronagraphs. A KHI appeared during the passage of a fast CME and evolved into the nonlinear stage showing evidence of vortices. A closely timed subsequent CME in the same region further developed the fully nonlinear KHI waves along its flank. We find that the radial speed of the CMEs exceeds the estimated local Alfvén speed obtained from in-situ Parker Solar Probe magnetic field data at perihelia. We propose that such events are rare because the fast CME created specific conditions favorable for instability growth in its trailing edge, including radial elongation of magnetic-field lines, reduced plasma density, and enhanced velocity and magnetic-field shear along the developing interface. The observed growth rate of the KHI wave is in qualitative agreement with the theoretical predictions.