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

TL;DR

This study addresses how large-scale Kelvin-Helmholtz instability can develop in the solar corona during fast CME events. By combining coronagraph observations of two CMEs (≈620 and ≈450 km s$^{-1}$) with in-situ PSP measurements of the ambient Alfvén and solar wind speeds, the authors show that the CME-driven shear exceeds the local Alfvén speed at about 6–14 $R_s$, enabling KHI growth that evolves into nonlinear vortices. They quantify the growth using a linear KHI framework and a Monte-Carlo exploration of magnetic-field orientation, finding a representative growth timescale of $\sim$5 minutes and substantial unstable solid angles under plausible coronal conditions; a second CME further sustains the instability. The results provide rare, direct evidence of large-scale KHI growth and dissipation near the Sun, with implications for energy transfer, coronal heating, and coronal seismology in eruptive events.

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 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.

Figures (5)

• Figure 1: The CME and the associated wave marked with the yellow arrow observed on 2024 February 16. The timing of the various instruments are indicated on the panels. (a) STEREO Cor 2 (the inner regions are obtained from EUVI and Cor 1 images). (b) The SOHO/LASCO C3 observations (the inner regions are obtained from LASCO C2, C3 and SDO AIA 193Å images). The animations of this event produced by JHelioviewer (see, https://www.jhelioviewer.org) are available online. (c) Height-time plot of the main CME observed on 2024 February 16. The CME speed in the plane of the sky obtained from linear least square fit is 620 km s$^{-1}$, with $R^2=0.9959$. The red arrows indicate the approximate onset times of the second CME, initial wave formation, and the observed developed KHI wave. (d) STEREO A view of the Sun (red dot) on 2024 February 16 with $\sim 8^\circ$ separation angle with respect to LASCO.
• Figure 2: (a) Portion of the STEREO COR2-A field of view at 16:38 UT on 2024 February 16, highlighting the KHI wave event. The two vertical dashed and solid lines refer to the two virtual slit positions used to construct the distance–time plots in panels (b) and (c). The position of Slit 1 (dashed line) is fixed, while Slit 2 (solid line) crosses the KHI wave features. Colored dots along Slit 2 mark the locations used to track the wave structure. (b) Distance–time plot of the propagating features along Slit 1, where features #1 (solid), #2 (dashed), and #3 (dotted) are identified. (c) Distance–time plot of the KHI wave features along Slit 2, showing the evolution of features #1 (blue), #2 (green), #3 (yellow), and #4 (red), which correspond to the colored dots with the same color code. Colored curves indicate the fitted propagation tracks used to derive their velocities. The vertical solid line in panels (b) and (c) correspond to the time of the panel (a). An animation of this figure is available online.
• Figure 3: Five snapshots from the STEREO Cor 1 animation of the KHI feature shown Figure \ref{['slit_vr:fig']}a at hourly intervals starting at 13:07 UT on 2024 February 16 showing the development of the KHI wave structure and the developed stage of KHI waves and vortices with their locations marked with yellow arrows.
• Figure 4: (a) The temporal evolution of the wavelength of the KHI wave observed on 2024 February 16 by STEREO COR2 A, determined as shown at Slit 2 in Figure \ref{['slit_vr:fig']}b. The temporal evolution of the velocities: (b) The radial velocities of the features along Slit 1 (#1 solid, #2 dashes, #3 dots) and (c) the evolution of velocities along Slit 2 (#1 solid, #2 dashes, #3 dots, #4 dot-dashes). The line styles are the same as in Figure \ref{['slit_vr:fig']}.
• Figure 5: The statistical distribution vs. heliocentric distance obtained from PSP in the range 10-30 $R_s$ obtained from encounters 4-23. (a) The Alfvén speed with power-law fit; (b) radial solar wind speed with She97 fit. The dashed vertical line marks the mean Alfvén critical surface at 16.5 $R_s$ where $M_A = 1$. The color bar shows the number of counts. The black lines show the fits and the fit parameters are indicated on the panels.