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

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 the event with large-scale KHI waves observed from $\sim 6$ to 14~$R_{\odot}$ on 2024-Feb-16 using SOHO/LASCO and STEREO-A coronagraphs. 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 has further developed the fully nonlinear KHI waves along its flank. We find that the radial speed of the CMEs exceeds the estimated local Alfven speed obtained from in-situ Parker Solar Probe (PSP) 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 KHI wave is in qualitative agreement with the theoretical predictions.

💡 Research Summary

The paper presents a comprehensive observational and theoretical study of a large‑scale Kelvin‑Helmholtz instability (KHI) driven by a fast coronal mass ejection (CME) on 2024 February 16. Using white‑light coronagraph data from SOHO/LASCO C2/C3 and STEREO‑A COR1/COR2, the authors track the evolution of two successive CMEs originating from the same active region (≈S19 W85). The primary CME, associated with an X2.5 flare, propagates with a plane‑of‑sky speed of ≈620 km s⁻¹, while a secondary, slower CME (≈450 km s⁻¹) erupts about five hours later, extending the high‑speed outflow.

Between heliocentric distances of 6 R☉ and 14 R☉, a distinct wave‑like structure appears along the CME flank. Space‑time analysis using two virtual slits reveals that the wave wavelength grows from ~0.6 R☉ to ~2.5 R☉ over ~16 h, with a peak amplitude around 16.5 h after CME onset. The wave initially exhibits linear growth, then transitions to a nonlinear stage characterized by clear vortex rolls—direct visual evidence of KHI in the upper corona, a regime where such signatures have not previously been captured.

Velocity measurements along the background flow (slit 1) give radial speeds of 400–1000 km s⁻¹, whereas the wave itself (slit 2) moves much slower (0–170 km s⁻¹). The resulting shear ΔV ranges from 230 to 830 km s⁻¹. To assess whether the shear exceeds the local Alfvén speed, the authors turn to Parker Solar Probe (PSP) in‑situ data from multiple perihelion encounters (10–30 R☉). Statistical analysis of PSP magnetic field (FIELDS) and proton velocity (SPAN‑I) measurements yields a median Alfvén speed of ≈331 km s⁻¹ at 11.25 R☉ (10th percentile ≈235 km s⁻¹). The solar‑wind radial speed in the same region follows an empirical law V_r² = 2a(r−r₁) with a≈600 km s⁻¹, giving V_r≈600–800 km s⁻¹. Consequently, the CME shear is super‑Alfvénic (M_A > 1), satisfying the classic KHI criterion for magnetized plasmas.

Theoretical growth rates are estimated using the standard incompressible KHI dispersion relation γ≈k ΔV √