Comment on acp-2022-851

Wind shear has important implications for Kelvin–Helmholtz instability (KHI) and gravity waves (GWs) in the mesosphere–lower thermosphere (MLT) region where its momentum transport process is dominated by short-period (<1 h) GWs. However, the sub-hourly variation in wind shear is still not well quantified. This study aims to improve current understanding of vertical wind shear by analyzing multi-year meteor radar measurements at the Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), Wuhan (WH, 30.5°N, 114.6°E), and Fuke (FK, 19.5°N, 109.1°E) stations in China. The wind field is estimated by a new algorithm, e.g., the damped least squares fitting. Taking the wind shear estimated by normal products as a criterion, the shear produced by the new algorithm has more statistical convergence as compared to the traditional algorithm, e.g., the least squares fitting. Therefore, we argue that the 10 min DLSA wind probably produces a more reasonable vertical shear. Both intensive wind shears and GW kinetic energy can be simultaneously captured during the 0600–1600 UTs of May at MH and during the 1300–2400 UTs of March at FK, possibly implying that the up-propagation of GWs could contribute to the production of large wind shears. The sub-hourly variation in wind shears is potentially valuable for understanding the interrelationship between shear (or KHI) and GWs.

Context. Hard X-ray (HXR) sources are frequently observed near the top of solar flare loops, which are also bright in soft X-ray (SXR) and extreme ultraviolet (EUV) wavebands. We revisit a recent scenario proposed by Fang et al. (2016) to trigger loop-top turbulence in flaring loops, which can help explain variations seen in SXR and EUV brightenings and potentially impact and induce HXR emission. It is conjectured that evaporation flows from flare-impacted chromospheric footpoints interact with each other near the loop top and produce turbulence via the Kelvin–Helmholtz instability (KHI). Aims. By performing a rigorous parameter survey, in which we vary the duration, total amount, and asymmetry of the energy deposition at both footpoints, we assess the relevance of the KHI in triggering and sustaining loop-top turbulence. We synthesize SXR and EUV emission and discuss the possibility of HXR emission through bremsstrahlung or inverse Compton processes, which scatter SXR photons to HXR photons via the inverse Compton mechanism. Methods. We performed 2.5D numerical simulations in which the magnetohydrodynamic model incorporates a realistic photosphere to coronal stratification, parametrized heating, radiative losses, and field-aligned anisotropic thermal conduction. We focus on the trigger of the KHI and the resulting turbulence, as well as identify various oscillatory patterns that appear in the evolutions. Results. We find that a M2.2-class related amount of energy should be deposited in less than four minutes to trigger a KHI interaction. Slower deposition, or lesser energy (< 0.33 × 1029 ergs) rather leads to mere loop-top compression sites bounded by shocks, without KHI development. Asymmetry in the footpoint deposition determines whether the KHI turbulent zone gets produced away from the apex, and asymmetric cases can show a slow-mode mediated, periodic displacement of the turbulent zone. Our reference simulation further demonstrates a clear 25 s periodicity in the declining phase of the SXR light curve, wherein compressional effects dominate. Conclusions. When turbulence is produced in the loop apex, an index of −5/3 can be found in the spectra of velocity and magnetic field fluctuations. Typical values for M-class flares routinely show KHI development. The synthesized SXR light curve shows a clear periodic signal related to the sloshing motion of the vortex pattern created by the KHI.

Kelvin–Helmholtz (KH) instability is simulated using dissipative particle dynamics (DPD) at the mesoscopic particle level. To model two-phase flow, the ‘color’ repulsion model is introduced to describe binary fluids according to the Rothman–Keller method. The simulated results are compared with similar experiment and simulation using other techniques, and the work concludes that DPD is a promising method for simulating these systems. Furthermore, through the model and numerical simulation, the effects of intermiscibility, temperature, magnitude of velocity difference, and density ratio on the KH instability are investigated, respectively. It is shown that the poor intermiscibility reduces the mixing of two fluids and weakens the eddy currents. It is also found that as the temperature increases, the turbulence feature, i.e. plume-like structures, is more obvious. It is also revealed that as the initial horizontal velocity difference increases, the interface rolls up faster and higher. And it is also observed that as the density ratio increases, the KH instability becomes more suppressed.

BackgroundThe northern part of India harbours a great diversity of medicinal plants due to its distinct geography and ecological marginal conditions. The traditional medical systems of northern India are part of a time tested culture and honored still by people today. These traditional systems have been curing complex disease for more than 3,000 years. With rapidly growing demand for these medicinal plants, most of the plant populations have been depleted, indicating a lack of ecological knowledge among communities using the plants. Thus, an attempt was made in this study to focus on the ecological status of ethnomedicinal plants, to determine their availability in the growing sites, and to inform the communities about the sustainable exploitation of medicinal plants in the wild.MethodsThe ecological information regarding ethnomedicinal plants was collected in three different climatic regions (tropical, sub-tropical and temperate) for species composition in different forest layers. The ecological information was assessed using the quadrate sampling method. A total of 25 quadrats, 10 × 10 m were laid out at random in order to sample trees and shrubs, and 40 quadrats of 1 × 1 m for herbaceous plants. In each climatic region, three vegetation sites were selected for ecological information; the mean values of density, basal cover, and the importance value index from all sites of each region were used to interpret the final data. Ethnomedicinal uses were collected from informants of adjacent villages. About 10% of inhabitants (older, experienced men and women) were interviewed about their use of medicinal plants. A consensus analysis of medicinal plant use between the different populations was conducted.ResultsAcross the different climatic regions a total of 57 species of plants were reported: 14 tree species, 10 shrub species, and 33 herb species. In the tropical and sub-tropical regions, Acacia catechu was the dominant tree while Ougeinia oojeinensis in the tropical region and Terminalia belerica in the sub-tropical region were least dominant reported. In the temperate region, Quercus leucotrichophora was the dominant tree and Pyrus pashia the least dominant tree. A total of 10 shrubs were recorded in all three regions: Adhatoda vasica was common species in the tropical and sub-tropical regions however, Rhus parviflora was common species in the sub-tropical and temperate regions. Among the 33 herbs, Sida cordifolia was dominant in the tropical and sub-tropical regions, while Barleria prionitis the least dominant in tropical and Phyllanthus amarus in the sub-tropical region. In temperate region, Vernonia anthelmintica was dominant and Imperata cylindrica least dominant. The consensus survey indicated that the inhabitants have a high level of agreement regarding the usages of single plant. The index value was high (1.0) for warts, vomiting, carminative, pain, boils and antiseptic uses, and lowest index value (0.33) was found for bronchitis.ConclusionThe medicinal plants treated various ailments. These included diarrhea, dysentery, bronchitis, menstrual disorders, gonorrhea, pulmonary affections, migraines, leprosy. The ecological studies showed that the tree density and total basal cover increased from the tropical region to sub-tropical and temperate regions. The species composition changed with climatic conditions. Among the localities used for data collection in each climatic region, many had very poor vegetation cover. The herbaceous layer decreased with increasing altitude, which might be an indication that communities at higher elevations were harvesting more herbaceous medicinal plants, due to the lack of basic health care facilities. Therefore, special attention needs to be given to the conservation of medicinal plants in order to ensure their long-term availability to the local inhabitants. Data on the use of individual species of medicinal plants is needed to provide an in-depth assessment of the plants availability in order to design conservation strategies to protect individual species.

The Kelvin–Helmholtz instability (KHI) is a ubiquitous physical process in ordinary fluids and plasmas, frequently observed also in space environments. In this paper, kinetic effects at proton scales in the nonlinear and turbulent stage of the KHI have been studied in magnetized collisionless plasmas by means of hybrid Vlasov–Maxwell simulations. The main goal of this work is to point out the back-reaction on particles triggered by the evolution of such instability, as energy reaches kinetic scales along the turbulent cascade. Interestingly, turbulence is inhibited when KHI develops over an initial state that is not an exact equilibrium state. On the other hand, when an initial equilibrium condition is considered, energy can be efficiently transferred toward short scales, reaches the typical proton wavelengths, and drives the dynamics of particles. As a consequence of the interaction of particles with the turbulent fluctuating fields, the proton velocity distribution deviates significantly from the local thermodynamic equilibrium, the degree of deviation increasing with the level of turbulence in the system and being located near regions of strong magnetic stresses. These numerical results support recent space observations from the Magnetospheric MultiScale mission of ion kinetic effects driven by the turbulent dynamics at Earth’s magnetosheath and by the KHI in Earth’s magnetosphere.

It is well established that the magnetically structured solar atmosphere supports the propagation of MHD waves along various kind of jets including also the solar wind. It is well-known as well that under some conditions, namely high enough jet speeds, the propagating MHD modes can become unstable against to the most common Kelvin--Helmholtz instability (KHI). In this article, we explore how the propagation and instability characteristics of running along a slow solar wind MHD modes are affected when they are investigated in the framework of the ideal Hall-magnetohydrodynamics. Hall-MHD is applicable if the jet width is shorter than or comparable to the so called Hall parameter $l_\mathrm{Hall} = c/\omega_\mathrm{pi}$ (where $c$ is the speed of light and $\omega_\mathrm{pi}$ is the ion plasma frequency). We model the solar wind as a moving with velocity $\vec{v}_0$ cylindrical flux tube of radius $a$, containing incompressible plasma with density $\rho_\mathrm{i}$ permeated by a constant magnetic field $\vec{B}_\mathrm{i}$. The surrounding plasma is characterized with its density $\rho_\mathrm{e}$ and magnetic field $\vec{B}_\mathrm{e}$. The dispersion relation of MHD waves is derived in the framework of both standard and Hall-MHD and is numerically solved with input parameters: the density contrast $\eta = \rho_\mathrm{e}/\rho_\mathrm{i}$, the magnetic fields ratio $b = {B}_\mathrm{e}/{B}_\mathrm{i}$, and the Hall scale parameter $l_\mathrm{Hall}/a$. It is found that the Hall current, at moderate values of $l_\mathrm{Hall}/a$, stimulates the emerging of KHI of the kink ($m = 1)$ and high-mode ($m \geqslant 2$) MHD waves, while for the sausage wave ($m = 0$) the trend is just the opposite---the KHI is suppressed.

Kelvin–Helmholtz instability (KHI), driven by the velocity inhomogeneity at Earth's magnetopause, has been shown to play a major role in mixing the magnetospheric and the solar wind plasma during northward periods. In fact, when the magnetospheric and interplanetary magnetic fields are mostly perpendicular to the equatorial plane, KHI can develop at a low latitude without being significantly inhibited by the magnetic tension. In contrast, at a high latitude, the more complex magnetic configuration is believed to totally stabilize the instability. This intrinsic 3D dynamics is investigated in a simplified geometry showing that KHI is able to kink the magnetic field lines at a mid-latitude and to create current layers where magnetic reconnection spontaneously develops. It is shown that a mid-latitude reconnection is able to change the global topology of the magnetic field and to connect interplanetary field lines to the Earth's cups, allowing the solar wind to directly enter the magnetosphere.

The Kelvin–Helmholtz instability (KHI) has been considered important in the energy transfer and momentum coupling between the solar wind and planetary magnetospheres. To explore this issue, we employ a two-dimensional magnetohydrodynamic simulation to study the nonlinear evolution of the KHI at Mercury’s magnetopause using the parameters derived from a global hybrid simulation of MESSENGER’s first flyby of Mercury. Due to the absence of comprehensive plasma observations of Mercury’s magnetosphere, two scenarios are considered: one with a heavily loaded magnetosphere and the other with a weakly loaded magnetosphere, to demonstrate the development of the KHI under distinct levels of magnetospheric plasma density. Our results indicate that the KHI with a heavily loaded magnetosphere leads to a significantly more turbulent magnetopause and grows into the nonlinear fast-mode plane waves expanding away from the magnetopause. The momentum and energy flux quantified from our simulations reveal that the KHI with a heavily loaded magnetosphere can efficiently transport momentum and energy away from the magnetopause in the presence of the fast-mode plane waves. In the cases with a heavily loaded magnetosphere, observed in the inner magnetosphere, the momentum flux can reach 10−3 nPa, i.e., about 0.5% of the initial solar-wind dynamic pressure; the energy flux can be 10-2ergcm-2s-1, and the energy density is about 1.5%–3.0% of the initial solar-wind energy. In the cases with a very thin magnetosphere, observed away from the magnetopause, the momentum flux is negligible and the energy flux is smaller without the presence of the fast-mode plane waves in the magnetosphere.

The Kelvin-Helmholtz instability (KHI) is a standard test of hydrodynamic and magnetohydrodynamic (MHD) simulation codes and finds many applications in astrophysics. The classic linear theory considers a discontinuity in density and velocity at the interface of two fluids. However, for numerical simulations of the KHI such initial conditions do not yield converged results even at the linear stage of the instability. Instead, smooth profiles of velocity and density are required for convergence. This renders analytical theory to be only approximately valid and hinders quantitative comparisons between the classical theory and simulations. In this paper we derive a linear theory for the KHI with smooth profiles and illustrate code testing with the MHD code Athena. We provide the linear solution for the KHI with smooth initial conditions in three different limits: inviscid hydrodynamics, ideal MHD and Braginskii-MHD. These linear solutions are obtained numerically with the framework Psecas (Pseudo-Spectral Eigenvalue Calculator with an Automated Solver), which generates and solves numerical eigenvalue problems using an equation-parser and pseudo-spectral methods. The Athena simulations are carried out on a periodic, Cartesian domain which is useful for code testing purposes. Using Psecas and analytic theory, we outline the differences between this artificial numerical setup and the KHI on an infinite Cartesian domain and the KHI in cylindrical geometry. We discuss several astrophysical applications, such as cold flows in galaxy formation and cold fronts in galaxy cluster mergers. Psecas, and the linear solutions used for code testing, are publicly available and can be downloaded from the web.

Elevated stratopauses formed at ~80–90 km altitude during the recovery phase of stratospheric sudden warmings in February 2006 and 2009. These likely occurred in response to changes in the downward circulation due to gravity waves (GWs) and/or planetary waves in the mesosphere and the lower thermosphere (MLT). However, the physical mechanisms are not fully understood, due in part to the lack of global GW observations in the MLT. This study presents global‐scale GW observations in the MLT during elevated stratopause events using Thermosphere, Ionosphere, Mesosphere Energetics Dynamics (TIMED)‐Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature observation, which provide a better insight into the formation of an elevated stratopause. During the downward movement of elevated stratopause events in 2006 and 2009, GWs were suppressed below ~60 km and enhanced above ~60 km at high latitudes compared to non‐elevated stratopause years (2005 and 2007). Global SABER GW observations indicate that the regions of GW enhancement propagate from low‐mid latitudes to high latitudes in association with the equatorward shift of the polar night jet during elevated stratopause events. Ray‐tracing simulations show enhancements of the poleward propagation of GWs during elevated stratopause events as well as continuous propagation of non‐orographic GWs within high latitudes. Therefore, our results suggest that meridional propagation of GWs from lower to higher latitudes, which is typically not included in global‐scale models, plays an important role in determining GW variations and thus the downward movement of an elevated stratopause, in addition to non‐orographic GWs originating at high latitudes.

The evolution of the Kelvin–Helmholtz instability (KHI) and magnetohydrodynamic (MHD) wave emission is investigated at shear-flow boundaries of magnetized plasmas. While MHD wave emission has been suggested to be only possible during the nonlinear stages, we find that there is also significant wave emission during the KHI’s linear stages. These emitted MHD waves may have stronger impacts than KHI surface waves since they can act to transport energy away from the local region of the shear flow. The removal of energy from the shear-flow region, instead of just the local redistribution considered in previous studies, and its propagation away from the interface could have major implications for the evolution of astrophysical objects characterized by fast plasma flow shears.

Prediction of Kelvin–Helmholtz instability (KHI) is crucial across various fields, requiring extensive high-fidelity data. However, experimental data are often sparse and noisy, while simulated data may lack credibility due to discrepancies with real-world configurations and parameters. This underscores the need for field reconstruction and parameter inference from sparse, noisy data, which constitutes inverse problems. Based on the physics-informed neural networks (PINNs), the KH-PINN framework is established in this work to solve the inverse problems of KHI flows. By incorporating the governing physical equations, KH-PINN reconstructs continuous flow fields and infer unknown transport parameters from sparse, noisy observed data. The two-dimensional unsteady incompressible flows with both constant and variable densities are studied. To our knowledge, this is one of the first few applications of PINNs to unsteady incompressible flows with variable densities. To address the spatiotemporal multiscale issue and enhance the reconstruction accuracy of small-scale structures, the multiscale embedding (ME) strategy is adopted. To address the magnitude multiscale issue and enhance the reconstruction accuracy of small-magnitude velocities, which are critical for KHI problems, the small-velocity amplification (SVA) strategy is proposed. The results demonstrate that KH-PINN can accurately reconstruct the fields with complex, evolving vortices and infer unknown parameters across a broad range of Reynolds numbers. Additionally, the energy-decaying and entropy-increasing curves are accurately obtained. The effectiveness of ME and SVA is validated through comparative studies, and the anti-noise and few-shot learning capabilities of KH-PINN are also validated. The code for this work is available at https://github.com/CAME-THU/KH-PINN.

This paper presents a potential flow numerical solution for the Kelvin–Helmholtz Instability (KHI) problem of an incompressible two-phase immiscible fluid in a stratified shear flow. As a problem: the two-fluid model becomes illposed when the slip velocity exceeds a critical value, and computations can be quite unstable before the flow reaches the ill-posed condition. In this work, computational stability of various convection schemes together with the potential equation method for the time derivatives in conjunction with the two-fluid model is analyzed. The normal stress balance (with the normal viscous stress) at the interface for the two-fluid model is carefully implemented to minimize its effect on numerical stability. Von Neumann stability analysis shows that: stability condition for two-fluid with equal kinematic viscosity ratio and inviscid flow, supply numerical stability. Excellent agreement has obtained according to analytical result that existing of imaginary part in solution which specialized this method. The numerical algorithm presented in this work can easily handle two-phase fluid flow with various density and viscosity ratios in rectangular channel. Simulation of this model has implemented by writing a code in FORTRAN programming.

Kelvin–Helmholtz instability (KHI) is investigated in the context of a non-Maxwellian dusty plasma comprising of electrons, ions and dust particles. The generalized dispersion relation is derived, which is of a complex nature due to some of the source terms like density inhomogeneity and sheared flow A that are responsible for instability in the system. To discuss the instability, the dispersion relation is solved numerically to discuss the growth rates. The primary goal is to understand the dynamics of KH modes in non-Maxwellian plasmas defined by the Kappa and Cairns distribution parameters influence the creation and features of KHI. We have found that the development rates and threshold conditions for these instabilities are significantly altered in the presence of non-Maxwellian distributions, influencing the overall dynamics of space plasma environment. The results demonstrate that superthermal particles in the Cairns distribution speed up the KHI's development rate, but the Kappa parameter has a more stable influence in comparison with Cairns. The effects of different parameters are investigated like sheared velocity, magnetic field, and dust mass. Dust particles present there strongly influence the system dynamics by increasing the chances of growing modes. Increasing dust mass can produce more obvious instabilities in celestial environments such as comet tails and planetary rings, aiding our understanding of their production and evolution.

The drift kinetic approximation (DKA) is developed by combining the macro- and microdynamics of guiding center plasmas. It is shown that the DKA provides a better description for collisionless processes in plasmas than the Chew–Goldberger–Low approximation. A new theory of the Kelvin–Helmholtz instability (KHI) in collisionless space plasmas is established. Calculations show that the general pattern of the growth rate of the KHI versus flow velocity in the DKA is similar to that in compressional magnetohydrodynamics. However, the critical velocity decreases if pressure is anisotropic, and the growth rate and energy flux are reduced in the presence of wave–particle interactions.