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Originally, Kelvin–Helmholtz instability (KHI) describes the growth of perturbations at the interface separating counterpropagating streams of Newtonian fluids of different densities with heavier fluid at the bottom. Generalized KHI is also used to describe instability of free shear layers with continuous variations of velocity and density. KHI is one of the most studied shear flow instabilities. It is widespread in nature in laminar as well as turbulent flows and acts on different spatial scales from galactic down to Saturn’s bands, oceanographic and meteorological flows, and down to laboratory and industrial scales. Here, we report the observation of elastically driven KH-like instability in straight viscoelastic channel flow, observed in elastic turbulence (ET). The present findings contradict the established opinion that interface perturbations are stable at negligible inertia. The flow reveals weakly unstable coherent structures (CSs) of velocity fluctuations, namely, streaks self-organized into a self-sustained cycling process of CSs, which is synchronized by accompanied elastic waves. During each cycle in ET, counter propagating streaks are destroyed by the elastic KH-like instability. Its dynamics remarkably recall Newtonian KHI, but despite the similarity, the instability mechanism is distinctly different. Velocity difference across the perturbed streak interface destabilizes the flow, and curvature at interface perturbation generates stabilizing hoop stress. The latter is the main stabilizing factor overcoming the destabilization by velocity difference. The suggested destabilizing mechanism is the interaction of elastic waves with wall-normal vorticity leading to interface perturbation amplification. Elastic wave energy is drawn from the main flow and pumped into wall-normal vorticity growth, which destroys the streaks.

The Kelvin‐Helmholtz instability (KHI) at the Kronian magnetopause is an important process resulting from the interaction between the magnetosphere and the solar wind. The Kronian magnetosphere is rotationally driven, thus dawn‐dusk asymmetry has been observed in the KHI structures. However, analyses based on the Cassini data have yielded a discrepancy, with some studies indicating more KHI features on the duskside and others on the dawnside. To investigate the physical origin of the dawn‐dusk asymmetry of the Kronian KHI, three‐dimensional high‐resolution global simulations of the Kronian magnetosphere have been developed. We found that KHI structures continuously develop in the prenoon sector and propagate toward both the dawn and dusk sides, exhibiting clear dawn‐dusk asymmetry during their growth and propagation. Our study reveals the dawn‐dusk asymmetric characteristics of KHI along Kronian magnetopause, including its growth and propagation, providing important insights into the interaction between rotation‐driven magnetosphere and solar wind. Plain Language Summary The Kelvin‐Helmholtz instability (KHI) vortex at Saturn's magnetopause, driven by the interaction between Saturn's magnetosphere and the solar wind, represent a significant process. Observational evidence confirms the existence of these KHI structures at Saturn's magnetopause. With rapid rotation, Saturn is expected to exhibit dawn‐dusk asymmetry in KHI characteristics or spatial distribution. Statistical analyses of Cassini mission data have yielded conflicting results, with some studies indicating more KHI features, such as bipolar signatures in the magnetic field data, on the duskside and others on the dawnside. To investigate the dawn‐dusk asymmetry of Saturn's KHI, we conducted global three‐dimensional magnetohydrodynamic simulations of Saturn's magnetosphere. We found that KHI structures continuously develop in the prenoon sector of Saturn's magnetosphere and propagate toward both the dawn and dusk sides, exhibiting clear dawn‐dusk asymmetry in their growth and propagation. At a fixed eigenfrequency, KHI structures propagate more stably toward the duskside with higher eigenfrequencies. Conversely, on the dawnside, KHI structures exhibit lower eigenfrequencies and unstable growth and propagation due to the presence of more complex physical processes. Our work elucidates the characteristics of KHI at Saturn's magnetopause, which is important for understanding the interaction between rotation‐driven planetary magnetosphere and solar wind. Key Points In the global MHD model, Kelvin‐Helmholtz vortices start to form near the noon sector and propagate toward the dawn and dusk directions The Kelvin‐Helmholtz Instabilities along the Kronian magnetopause shows low frequency, high amplitude on dawnside, opposite on duskside The Kelvin‐Helmholtz vortices are comparatively stable during propagation toward the duskside, but are less stable on the dawnside

The Kelvin‐Helmholtz waves have been observed along the Earth's low‐latitude magnetopause and have been suggested to play a certain role in the entry of solar wind plasma into Earth's magnetosphere. In situ observations of the KH waves (KHW) and, in particular, a nonlinear stage of the KH instability, i.e., rolled‐up KH vortices (KHVs), have been reported to occur preferentially for northward interplanetary magnetic field (IMF). Using Cluster data, we present the first in situ observation of nonlinearly developed KHW during southward IMF. The analysis reveals that there is a mixture of less‐developed and more‐developed KHW that shows inconsistent variations in scale size and the magnetic perturbations in the context of the expected evolution of KH structures. A coherence analysis implies that the observed KHW under southward IMF appear to be irregular and intermittent. These irregular and turbulent characteristics are more noticeable than previously reported KHW events that have been detected preferentially during northward IMF. This suggests that under southward IMF KHVs become easily irregular and temporally intermittent, which might explain the preferential in situ detection of KHVs when the IMF is northward. MHD simulation of the present event shows that during southward IMF dynamically active subsolar environments can cause KHV that evolve with considerable intermittency. The MHD simulations appear to reproduce well the qualitative features of the Cluster observations. Key Points First in‐situ observations of well‐developed KHW under southward IMF Solar‐wind IMF effects on the development and evoluction of KHW Turbulent/irregular characteristics of southward‐IMF KHW explains rare detection

We analyzed six Kelvin‐Helmholtz (K‐H) vortex events from Mars Atmosphere and Volatile EvolutioN (MAVEN) measurements. We found that fully developed vortices can occur at Mars' equatorial flanks and in the southern hemisphere, while they were previously observed only in the northern hemisphere. This implies that they do not exhibit a hemispheric asymmetry, and may occur globally as long as onset conditions are satisfied. We also estimated growth rates of 10−3$1{0}^{-3}$ –10−2$1{0}^{-2}$s−1${\\mathrm{s}}^{-1}$ , and found that the inclusion of heavy planetary ions reduces growth rates while increasing the directions over which K‐H instability occurs. We calculated instantaneous ion loss rates due to detachment of K‐H vortices of 1025$1{0}^{25}$ –1027$1{0}^{27}$s−1${\\mathrm{s}}^{-1}$ , rivaling other loss mechanisms in contributing to Mars' global atmospheric escape. The inferred higher occurrence rate of K‐H instability at Mars over a larger spatial domain strongly suggests a more significant contribution to overall atmospheric loss than previously thought.

A boundary layer of finite thickness separates the magnetosheath and the magnetosphere. Within this boundary, we have the outer low‐latitude boundary layer (LLBL), the magnetopause current layer, and the inner LLBL. Earlier works point out that the interface on which Kelvin‐Helmholtz Instability (KHI) develops has implications for how effective this mechanism is in transporting solar wind mass, momentum, and energy into the magnetosphere. We provide evidence that the KHI begins developing between the inner LLBL and the magnetosphere and can continue to grow to form rolled‐up vortices. The KH waves developing in the inner edge can grow in amplitude and become as large as the boundary layer itself, thereby enabling the transport of the magnetosheath particles into the magnetosphere. We outline possible scenarios for KH wave growth in this multilayered boundary layer and discuss the observational evidence for the presented scenarios.

The Buoyancy-driven flow of two immiscible liquids having varying density and viscosity is studied in a three-dimensional inclined confined channel. Initially, the heavier/lighter liquids occupy the upper/lower parts of the channel, respectively, which is an unstable configuration. The numerical simulations are performed using a multiphase lattice Boltzmann method (LBM) that is further implemented on the graphics processing unit (GPU). The three-dimensional flow dynamics and the associated physics are studied based on various parameters such as viscosity ratios (m), Atwood numbers (At) and Reynolds numbers (Re). The results were presented in the form of iso-surface/contour plots, average density profiles, and lengths of interpenetration. It is observed that larger interpenetration occurs with iso-viscous liquids having higher density gradients (higher At). The Reynolds number had a non-monotonic effect on the axial lengths of interpenetration (Lp∗); Lp∗ increases till Re = 500 and then decreases for Re = 1000. At larger Re, due to the development of Kelvin-Helmholtz instabilities higher transverse interpenetration is observed.

Starting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm in ∼${\\sim} $ 20 years. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event. The IMF bz${b}_{z}$ranged from −60 to +40 nT both with ∼${\\sim} $ hour to minutes periodicity with plasma jets propagating in ±zGSE$\\pm {z}_{\\mathit{GSE}}$ ‐direction within multi‐scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi‐scale density and magnetic field fluctuations including reconnection jets. The wavelengths varied from ∼${\\sim} $ 60 to 270 RE${R}_{E}$ , suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading. Plain Language Summary Coronal mass ejections (CMEs) are giant explosions of plasma and magnetic field from the Sun which can produce beautiful Auroras, but also destroy our satellites, power delivery networks, and avionics systems. The seriousness of the storm depends on the pre‐existing conditions of the magnetosphere, the strength and orientation of the magnetic field within the CME structure, its size and speed which drives its duration, as well as the properties of the plasma within it. In this paper, I discuss and analyze the CME observations during the recent Mother's day storm (started on 10 May 2024) using the ARTEMIS spacecraft data at the lunar orbit. During the CME encounter, the SC were exposed to periodic north‐south‐variations of the magnetic field orientation and plasma density which allowed the Earth's magnetosphere to be successively filled with plasma, and magnetic flux likely enabling its effective acceleration in large‐quantities. I show that these periodic variations were produced by giant ∼${\\sim} $60–270 RE${R}_{E}$waves at the boundary of the CMEs, such that the entire Earth's magnetosphere was surfing the crests and troughs of these waves, and absorbing the kinetic and magnetic energy from the solar wind. Key Points Velocity shear at the boundary of a coronal mass ejection (CME) ejecta created Kelvin‐Helmholtz Instability (KHI) ∼${\\sim} $ 7 million km upstream the Earth‐Sun L‐1 point KHI generated multi‐scale density and IMF bz${b}_{z}$fluctuations from −60 to +40 nT at the boundary layers of the CME ejecta encounters KH wavelengths varied from ∼${\\sim} $ 60 to ∼${\\sim} $ 270 RE${R}_{E}$implying periodic and successive mass and magnetic flux loading of the magnetosphere system

The asymmetric distribution of Kelvin‐Helmholtz (KH) instability at Venusian ionopause is investigated using a multifluid model. Results show that KH instability distributes asymmetrically, preferentially developing in the −E (anti‐parallel to the interplanetary electric field) hemisphere. In the magnetosheath, solar wind H+ ions deflect toward both ±E hemispheres. Within the ionosphere, however, ionospheric ions are accelerated in the +E direction by the convective electric field. In addition, the ionopause is elevated in the −E hemisphere. These effects lead to a stronger velocity shear at the ionopause of the −E hemispheric, thereby promoting the KH growth there. The magnetic field variations in the KH region correlate with the H+ density but anti‐correlate with ionospheric ion density. The period of KH instability gradually decreases as it propagates downstream. During its evolution, plasma clouds form and take away ionospheric (especially O+) ions effectively, resulting in considerable ion escape.

Fritts, Wang, Lund, and Thorpe (2022, https://doi.org/10.1017/jfm.2021.1085) and Fritts, Wang, Thorpe, and Lund (2022, https://doi.org/10.1017/jfm.2021.1086) described a 3‐dimensional direct numerical simulation of interacting Kelvin‐Helmholtz instability (KHI) billows and resulting tube and knot (T&K) dynamics that arise at a stratified shear layer defined by an idealized, large‐amplitude inertia‐gravity wave. Using similar initial conditions, we performed a high‐resolution compressible simulation to explore the emission of GWs by these dynamics. The simulation confirms that such shear can induce strong KHI with large horizontal scales and billow depths that readily emit GWs having high frequencies, small horizontal wavelengths, and large vertical group velocities. The density‐weighted amplitudes of GWs reveal “fishbone” structures in vertical cross sections above and below the KHI source. Our results reveal that KHI, and their associated T&K dynamics, may be an important additional source of high‐frequency, small‐scale GWs at higher altitudes. Plain Language Summary A high‐resolution compressible atmosphere model is applied to explore gravity wave emissions from a shear with Kelvin‐Helmholtz Instability initiated by a three‐dimensional, small‐amplitude initial noise field in velocity, such as must always occur in the atmosphere. Simulations reveal that a wind shear with an amplitude of 65 m/s and a half‐width of 0.8 km can induce strong Kelvin‐Helmholtz Instability dynamics, which can further emit gravity waves having periods of ∼10–20 min and horizontal wavelengths of ∼20 km. These gravity waves have high frequencies and small horizontal scales. The density‐weighted amplitudes of gravity waves created a “fishbone” structure in z‐t plots due to upward‐ and downward‐propagating gravity waves arising at the layer of Kelvin‐Helmholtz Instability. Our results demonstrate that Kelvin‐Helmholtz Instability and the resulting instability dynamics may be a prevalent source of gravity waves impacting higher altitudes. Key Points Kelvin‐Helmholtz instabilities (KHI) generated by a stratified shear layer induce gravity waves (GWs) that penetrate to high altitudes KHI‐radiated GWs may be a major influence of near‐stationary shears at high altitudes to which they cannot readily propagate directly GWs generated by KHI can account for “fishbone” structures seen in vertical profiling

We show that the dynamical system of an inviscid fluid with velocity shear admits parity-time (PT) symmetry, which provides a physical explanation of the well-known observation that the spectrum of the perturbation eigenmodes of the system is symmetric with respect to the real axis. It is found that the Kelvin-Helmholtz instability is triggered when and only when the PT symmetry is spontaneously broken. The analysis of PT symmetry also reveals that the relative phase between parallel velocity and pressure perturbations needs to be locked at π/2 when the instability is suppressed.