Kelvin-Helmholtz instability in a compressible dust fluid flow

We report the first experimental observations of a single-mode Kelvin-Helmholtz instability in a flowing dusty plasma in which the flow is compressible in nature. The experiments are performed in an inverted [Formula: see text]-shaped dusty plasma experimental device in a DC glow discharge Argon plasma environment. A gas pulse valve is installed in the experimental chamber to initiate directional motion to a particular dust layer. The shear generated at the interface of the moving and stationary layers leads to the excitation of the Kelvin-Helmholtz instability giving rise to a vortex structure at the interface. The growth rate of the instability is seen to decrease with an increase in the gas flow velocity in the valve and the concomitant increase in the compressibility of the dust flow. The shear velocity is further increased by making the stationary layer to flow in an opposite direction. The magnitude of the vorticity is seen to become stronger while the vortex becomes smaller with such an increase of the shear velocity. A molecular dynamics simulation provides good theoretical support to the experimental findings.

Dayside Transient Phenomena and Their Impact on the Magnetosphere and Ionosphere

Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth's radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions.

Kelvin–Helmholtz Waves on the Magnetopause at the Lunar Distances under Southward IMF: ARTEMIS Observations

The Kelvin–Helmholtz (KH) instability, a common phenomenon widely observed at the magnetopause, plays an important role in plasma transport while reconnection at low latitude is less efficient during the northward interplanetary magnetic field (IMF). In this study, we analyze the magnetic field and plasma observations obtained by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft located near the lunar orbit and find KH waves under the southward IMF at the lunar-orbit magnetopause. We also calculate the dominant period, phase velocity, and wavelength of the KH waves and further compare this event with the KH waves seen at the flank magnetopause under the southward IMF, which indicates that the wavelength increases as the distance from the subsolar point increases. The observations also show that the KH waves at lunar distance under the southward IMF are characterized by irregularity and intermittence.

How a Realistic Magnetosphere Alters the Polarizations of Surface, Fast Magnetosonic, and Alfvén Waves

System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods that might be applied to spacecraft observations.

Magnetic Reconnection Inside a Flux Rope Induced by Kelvin-Helmholtz Vortices

On 5 May 2017, MMS observed a crater-type flux rope on the dawnside tailward magnetopause with fluctuations. The boundary-normal analysis shows that the fluctuations can be attributed to nonlinear Kelvin-Helmholtz (KH) waves. Reconnection signatures such as flow reversals and Joule dissipation were identified at the leading and trailing edges of the flux rope. In particular, strong northward electron jets observed at the trailing edge indicated midlatitude reconnection associated with the 3-D structure of the KH vortex. The scale size of the flux rope, together with reconnection signatures, strongly supports the interpretation that the flux rope was generated locally by KH vortex-induced reconnection. The center of the flux rope also displayed signatures of guide-field reconnection (out-of-plane electron jets, parallel electron heating, and Joule dissipation). These signatures indicate that an interface between two interlinked flux tubes was undergoing interaction, causing a local magnetic depression, resulting in an M-shaped crater flux rope, as supported by reconstruction.

Energy Transport by Kelvin-Helmholtz Instability at the Magnetopause

By means of the formation of vortices in the nonlinear phase, the Kelvin Helmholtz instability is able to redistribute the flux of energy of the solar wind that flows parallel to the magnetopause. The energy transport associated with the Kelvin Helmholtz instability contributes significantly to the magnetosphere and magnetosheath dynamics, in particular at the flanks of the magnetopause where the presence of a magnetic field perpendicular to the velocity flow does not inhibit the instability development. By means of a 2D two-fluid simulation code, the behavior of the Kelvin Helmholtz instability is investigated in the presence of typical conditions observed at the magnetopause. In particular, the energy penetration in the magnetosphere is studied as a function of an important parameter such as the solar wind velocity. The influence of the density jump at the magnetopause is also discussed.

Evolution of Turbulence in the Kelvin–Helmholtz Instability in the Terrestrial Magnetopause

The dynamics occurring at the terrestrial magnetopause are investigated by using Geotail and THEMIS spacecraft data of magnetopause crossings during ongoing Kelvin–Helmholtz instability. Properties of plasma turbulence and intermittency are presented, with the aim of understanding the evolution of the turbulence as a result of the development of Kelvin–Helmholtz instability. The data have been tested against standard diagnostics for intermittent turbulence, such as the autocorrelation function, the spectral analysis and the scale-dependent statistics of the magnetic field increments. A quasi-periodic modulation of different scaling exponents may exist along the direction of propagation of the Kelvin–Helmholtz waves along the Geocentric Solar Magnetosphere coordinate system (GSM), and it is visible as a quasi-periodic modulation of the scaling exponents we have studied. The wave period associated with such oscillation was estimated to be approximately 6.4 Earth Radii ( R E ). Furthermore, the amplitude of such modulation seems to decrease as the measurements are taken further away from the Earth along the magnetopause, in particular after X ( G S M ) ≲ − 15 R E . The observed modulation seems to persist for most of the parameters considered in this analysis. This suggests that a kind of signature related to the development of the Kelvin–Helmholtz instabilities could be present in the statistical properties of the magnetic turbulence.

Solar Wind Ion Entry Into the Magnetosphere During Northward IMF

Extended periods of northward interplanetary magnetic field (IMF) lead to the formation of a cold, dense plasma sheet due to the entry of solar wind plasma into the magnetosphere. Identifying the paths that the solar wind takes to enter the magnetosphere, and their relative importance has remained elusive. Any theoretical model of entry must satisfy observational constraints, such as the overall entry rate and the dawn-dusk asymmetry observed in the cold, dense plasma sheet. We model, using a combination of global magnetohydrodynamic and test particle simulations, solar wind ion entry into the magnetosphere during northward IMF and compare entry facilitated by the Kelvin-Helmholtz instability to cusp reconnection. For Kelvin-Helmholtz entry we reproduce transport rates inferred from observation and kinetic modeling and find that intravortex reconnection creates buoyant flux tubes, which provides, through interchange instability, a mechanism of filling the central plasma sheet with cold magnetosheath plasma. For cusp entry we show that an intrinsic dawn-dusk asymmetry is created during entry that is the result of alignment of the westward ion drift with the dawnward electric field typically observed during northward IMF. We show that both entry mechanisms provide comparable mass but affect entering plasma differently. The flank-entering plasma is cold and dawn-dusk symmetric, whereas the cusp-entering plasma is accelerated and preferentially deflected toward dawn. The combined effect of these entry mechanisms results in a plasma sheet population that exhibits dawn-dusk asymmetry in the manner that is seen in nature: a two-component (hot and cold) dusk flank and hotter, broadly peaked dawn population.

Strongly localized magnetic reconnection by the super-Alfvénic shear flow

We demonstrate that the dragging of the magnetic field by the super-Alfvénic shear flows out of the reconnection plane can strongly localize the reconnection x-line in collisionless pair plasmas, reversing the current direction at the x-line. Reconnection with this new morphology, which is impossible in resistive-magnetohydrodynamics, is enabled by the particle inertia. Surprisingly, the quasi-steady reconnection rate remains of order 0.1 even though the aspect ratio of the local x-line geometry is larger than unity, which completely excludes the role of tearing physics. We explain this by examining the transport of the reconnected magnetic flux and the opening angle ma de by the upstream magnetic field, concluding that the reconnection rate is still limited by the constraint imposed at the inflow region. Based on these findings, we propose that this often observed fast rate value of order 0.1 itself, in general, is an upper bound value determined by the upstream constraint, independent of the localization mechanism and dissipation therein.

Ubiquity of Kelvin-Helmholtz waves at Earth's magnetopause

Magnetic reconnection is believed to be the dominant process by which solar wind plasma enters the magnetosphere. However, for periods of northward interplanetary magnetic field (IMF) reconnection is less likely at the dayside magnetopause, and Kelvin-Helmholtz waves (KHWs) may be important agents for plasma entry and for the excitation of ultra-low-frequency (ULF) waves. The relative importance of KHWs is controversial because no statistical data on their occurrence frequency exist. Here we survey 7 years of in situ data from the NASA THEMIS (Time History of Events and Macro scale Interactions during Substorms) mission and find that KHWs occur at the magnetopause ∼19% of the time. The rate increases with solar wind speed, Alfven Mach number and number density, but is mostly independent of IMF magnitude. KHWs may thus be more important for plasma transport across the magnetopause than previously thought, and frequently drive magnetospheric ULF waves.

Role of the Kelvin-Helmholtz instability in the evolution of magnetized relativistic sheared plasma flows

We explore, via analytical and numerical methods, the Kelvin-Helmholtz (KH) instability in relativistic magnetized plasmas, with applications to astrophysical jets. We solve the single-fluid relativistic magnetohydrodynamic (RMHD) equations in conservative form using a scheme which is fourth order in space and time. To recover the primitive RMHD variables, we use a highly accurate, rapidly convergent algorithm which improves upon such schemes as the Newton-Raphson method. Although the exact RMHD equations are marginally stable, numerical discretization renders them unstable. We include numerical viscosity to restore numerical stability. In relativistic flows, diffusion can lead to a mathematical anomaly associated with frame transformations. However, in our KH studies, we remain in the rest frame of the system, and therefore do not encounter this anomaly. We use a two-dimensional slab geometry with periodic boundary conditions in both directions. The initial unperturbed velocity peaks along the central axis and vanishes asymptotically at the transverse boundaries. Remaining unperturbed quantities are uniform, with a flow-aligned unperturbed magnetic field. The early evolution in the nonlinear regime corresponds to the formation of counter-rotating vortices, connected by filaments, which persist in the absence of a magnetic field. A magnetic field inhibits the vortices through a series of stages, namely, field amplification, vortex disruption, turbulent breakdown, and an approach to a flow-aligned equilibrium configuration. Similar stages have been discussed in MHD literature. We examine how and to what extent these stages manifest in RMHD for a set of representative field strengths. To characterize field strength, we define a relativistic extension of the Alfvénic Mach number M(A). We observe close complementarity between flow and magnetic field behavior. Weaker fields exhibit more vortex rotation, magnetic reconnection, jet broadening, and intermediate turbulence. Sufficiently strong fields (M(A)<6) completely suppress vortex formation. Maximum jet deceleration, and viscous dissipation, occur for intermediate vortex-disruptive fields, while electromagnetic energy is maximized for the strongest fields which allow vortex formation. Highly relativistic flows destabilize the system, supporting modes with near-maximum growth at smaller wavelengths than the shear width of the velocity. This helps to explain early numerical breakdown of highly relativistic simulations using numerical viscosity, a long-standing problem. While magnetic fields generally stabilize the system, we have identified many features of the complex and turbulent reorganization that occur for sufficiently weak fields in RMHD flows, and have described the transition from disruptive to stabilizing fields at M(A)≈6. Our results are qualitatively similar to observations of numerous jets, including M87, whose knots may exhibit vortex-like behavior. Furthermore, in both the linear and nonlinear analyses, we have successfully unified the HD, MHD, RHD, and RMHD regimes.

Kinetic effects on the Kelvin-Helmholtz instability in ion-to-magnetohydrodynamic scale transverse velocity shear layers: Particle simulations

Ion-to-magnetohydrodynamic scale physics of the transverse velocity shear layer and associated Kelvin-Helmholtz instability (KHI) in a homogeneous, collisionless plasma are investigated by means of full particle simulations. The shear layer is broadened to reach a kinetic equilibrium when its initial thickness is close to the gyrodiameter of ions crossing the layer, namely, of ion-kinetic scale. The broadened thickness is larger in B⋅Ω<0 case than in B⋅Ω>0 case, where Ω is the vorticity at the layer. This is because the convective electric field, which points out of (into) the layer for B⋅Ω<0 (B⋅Ω>0), extends (reduces) the gyrodiameters. Since the kinetic equilibrium is established before the KHI onset, the KHI growth rate depends on the broadened thickness. In the saturation phase of the KHI, the ion vortex flow is strengthened (weakened) for B⋅Ω<0 (B⋅Ω>0), due to ion centrifugal drift along the rotational plasma flow. In ion inertial scale vortices, this drift effect is crucial in altering the ion vortex size. These results indicate that the KHI at Mercury-like ion-scale magnetospheric boundaries could show clear dawn-dusk asymmetries in both its linear and nonlinear growth.

Time window for magnetic reconnection in plasma configurations with velocity shear

It is shown that the rate of magnetic field line reconnection can be clocked by the evolution of the large-scale processes that are responsible for the formation of the current layers where reconnection can take place. In unsteady plasma configurations, such as those produced by the onset of the Kelvin-Helmholtz instability in a plasma with a velocity shear, qualitatively different magnetic structures are produced depending on how fast the reconnection process develops on the external clock set by the evolving large-scale configuration.

Numerical evidence of undriven, fast reconnection in the solar-wind interaction with earth's magnetosphere: formation of electromagnetic coherent structures

We give evidence for the first time of the onset of undriven fast, collisionless magnetic reconnection during the evolution of an initially homogeneous magnetic field advected in a sheared velocity field. We consider the interaction of the solar wind with the magnetospheric plasma at low latitude and show that reconnection takes place in the layer between adjacent vortices generated by the Kelvin-Helmholtz instability. This process generates coherent magnetic structures with a size comparable to the ion inertial scale, much smaller than the system dimensions but much larger than the electron inertial scale. These magnetic structures are further advected in the plasma in a complex pattern but remain stable over a time interval much longer than their formation time. These results can be crucial for the interpretation of satellite data showing coherent magnetic structures in the Earth's magnetosheath or the magnetotail.

Competing mechanisms of plasma transport in inhomogeneous configurations with velocity shear: the solar-wind interaction with earth's magnetosphere

Two-dimensional simulations of the Kelvin-Helmholtz instability in an inhomogeneous compressible plasma with a density gradient show that, in a transverse magnetic field configuration, the vortex pairing process and the Rayleigh-Taylor secondary instability compete during the nonlinear evolution of the vortices. Two different regimes exist depending on the value of the density jump across the velocity shear layer. These regimes have different physical signatures that can be crucial for the interpretation of satellite data of the interaction of the solar wind with the magnetospheric plasma.