Lower Hybrid Drift Instability Research Articles

Lower hybrid and solitary waves in dusk flank region of the Earth’s magnetosphere

A variety of plasma waves have been detected in the vicinity of the magnetopause by various spacecraft missions. In this paper, utilizing high-resolution data from the Magnetospheric Multiscale (MMS) mission, we present new observations of simultaneous lower hybrid and ion solitary waves in the dusk flank region of Earth’s magnetosphere. All four MMS spacecraft consistently observed this wave activity during their traversal from Earth’s magnetosphere to the magnetosheath. Our analysis suggests that the lower hybrid drift waves, driven by lower-hybrid drift instability, were observed in correlation with density gradients. Furthermore, the analysis indicates that the entire ion bulk population drifts, which drives ion solitary waves. It is found that these waves play a crucial role in particle heating in the dusk flank region of Earth’s magnetosphere.

Complete quasilinear model for the acceleration-driven lower hybrid drift instability and a computational assessment of its validity.

A complete quasilinear model is derived for the electrostatic acceleration-driven lower hybrid drift instability in a uniform two-species low-beta plasma in which current is perpendicular to the background magnetic field. The model consists of coupled nonlinear velocity space diffusion equationsfor the volume-averaged ion and electron distribution functions. Each species' diffusion coefficient depends on a time-evolving spectral density of the electric-field energy per unit volume and a time-evolving dispersion relation. The dispersion relation is expressed analytically in integral form without the use of asymptotic limits and applies to arbitrary distribution functions, so long as they can be expressed as a function of one velocity coordinate, e.g., f(v_{y}) or f(v_{⊥}). The quasilinear model conserves energy and is complete in that it fully describes the evolution of the distribution functions, including resonant and nonresonant particle-wave interactions, while accounting for distribution-function-dependent mixed-complex frequencies. The quasilinear diffusion model is solved numerically and self-consistently using a Crank-Nicolson temporal discretization and a second-order finite-volume velocity-space discretization. Numerical solutions are compared to nonlinear fourth-order accurate continuum kinetic Vlasov-Poisson simulations. Evolution of electric-field energy, growth rates, distribution functions, and diffusion coefficients are shown to be in agreement with Vlasov simulations. The quasilinear model is shown to predict anomalous transport terms, like resistivity and heating, to within a factor of order unity. Discrepancies between the quasilinear model and Vlasov simulations are assessed and attributed primarily to lack of damping in the quasilinear description and to the use of unperturbed-orbit susceptibilities in the linear theory dispersion relation. The results illuminate the predictive accuracy of the quasilinear model, place approximate bounds on its validity, and provide much needed vetting of quasilinear theory's ability to predict the nonlinear state of a microturbulent plasma.

Energy Conversion in the Dip Region Preceding Dipolarization Front

AbstractDipolarization fronts (DFs), characterized by sharp increases in the northward magnetic field and usually preceded by magnetic dips, are suggested to play an important role in energy conversion and transport in the magnetotail. It has been documented that strong energy conversion typically develops right at the fronts. Here we present spacecraft observations of electron‐scale energy conversion (EEC) developed inside the dip region ahead of a DF, by using high‐cadence data from the Magnetospheric Multiscale Mission. The EEC, with magnitude comparable to that at the front, is primarily driven by ion current and electron‐scale electric field. The electric field inside the dip is provided by electrostatic waves fed by lower hybrid drift instability, which experiences temporal decaying. Such decaying leads to nonhomogeneity of EEC along the dawn‐dusk direction. These results, uncovering a new channel for DF‐driven energy conversion, can provide important insights into understanding energy transport in the magnetotail.

The lower-hybrid drift instability during the evolution of Kelvin-Helmholtz instability

Abstract The lower-hybrid drift instability (LHDI) is a pivotal phenomenon in astrophysics, playing a critical role in energy transfer, macroscopic structures, and evolutionary processes between the magnetosheath and magnetosphere. Using 2D two-fluids numerical simulation, we investigate the spatiotemporal distribution of LHDI during the evolution of Kelvin-Helmholtz instability (KHI) at Earth’s dusk-flank magnetopause. The numerical simulation results show that, during the linear phase of KHI, the LHDI, whose duration time is approximately ∆t1∼10tA, appears around the high-density arms. During the nonlinear phase of KHI, the LHDI appears around the KH vortexes, with a duration of about ∆t2∼5tA. The LHDI disappears with the decay of KH vortex.

Successive Energy Conversion at a Stepwise Dipolarization Front

Dipolarization fronts (DFs), ion-scale magnetic structures characterized by sharp enhancement of northward magnetic field developed within plasma jets, have been suggested to play a crucial role in the energy transfer chain in the terrestrial magnetotail. Here we present the first observation of successive energy conversion driven by multiple current layers at a stepwise DF, using high-cadence measurements from NASA’s Magnetospheric Multiscale mission. The multiple current layers are adjacent and cause gradual variations of particles and electromagnetic fields, leading to an intense, successive increase of energy conversion rates at the DF in the satellite frame, with electromagnetic field energy being transformed into particle energy. The energy conversion is contributed by both ion and electron currents. The motional electric field drives the energy conversion, while the wave electric field fed by lower hybrid drift instability modulates the energy partition. These results provide new insights into understanding energy transfer in the terrestrial magnetotail.

3D simulation of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments

The magnetic reconnection experiment has recently seen short wavelength ( k ρ e ∼ 1) lower-hybrid waves near the electron diffusion region in strong guide field reconnection. Based on plasma parameters from the experiment, we perform a three-dimensional fully kinetic simulation in order to investigate the generation of the lower-hybrid waves and their effects on the reconnection process. We find that the low-beta regions around the reconnection site are unstable to the lower-hybrid drift instability propagating in the outflow direction, driven by the difference between the electron and ion outflows. The waves modify the electron distributions, leading to periodic opening and closing of gaps in electron velocity space, and provide a small contribution to the anomalous resistivity. Finally, the simulation results are discussed in the context of space observations and laboratory experiments.

Lower Hybrid Waves in High Altitude Echoes of the Topside Equatorial Ionosphere

AbstractWe investigate the mechanism underlying lower hybrid waves associated with high altitude echoes recently detected in the post‐sunset equatorial topside ionosphere and inner plasmasphere by the Jicamarca VHF radar. These waves are visible as prominent sidebands in the echo Doppler spectra. New experimental results and newly processed incoherent scatter radar (ISR) datasets are presented that provide clues as to the conditions in which the echoes and associated waves occur. Numerical simulations are presented which demonstrate the feasibility of an inverse energy cascade coupled with a short wavelength instability, that is, the lower hybrid drift instability, in explaining the waves. An inverse cascade is required for short wavelength lower hybrid waves to extend to the 3 m wavelengths measured by the Jicamarca radar. The simulations were able to reproduce some features of the measurements including the lower hybrid sidebands at 3 m wavelengths, asymmetry in the sidebands, and the damping effect of higher densities and lower altitudes.

In Situ Observations of Lower Hybrid Drift Waves at the Inner Edge of the Ring Current by the Van Allen Probe‐A

AbstractIn the magnetosphere of the Earth, lower hybrid drift (LHD) waves effectively cause plasma diffusion and couple well to both electrons and ions. LHD waves have the characteristics of linear polarization and highly oblique propagation with frequencies near the lower hybrid resonance frequency, which is similar to the characteristics of high‐frequency magnetosonic (MS) waves. The existence of diamagnetic drift at the inner edge of the ring current provides free energy for the excitation of LHD waves, while the free energy to generate MS waves are provided by proton ring distribution. Here, we report on a typical case of LHD waves which was excited by LHD instability. Different from the global distribution of high‐frequency MS waves, statistical results indicate that most LHD waves have a uniform distribution over all MLTs except the noon sector inside the plasmapause. Besides, the occurrence of LHD waves reaches a maximum in the dusk sector outside the plasmapause.

First Observation of Lower Hybrid Drift Waves at the Edge of the Current Sheet in the Martian Magnetotail

Lower hybrid drift waves are commonly observed at plasma boundaries, playing an important role in plasma dynamics. Such waves have been widely investigated in the terrestrial magnetosphere but have never been reported in other planetary environments. Here, using the measurements from the Mars Atmosphere and Volatile EvolutioN mission, we present the first observation of electromagnetic lower hybrid drift waves at the edge of the current sheet on the dusk side of the Martian magnetotail, which should be locally excited rather than propagated from other regions. These plasma waves are associated with significant density gradients and magnetic field gradients. Based on the measured local plasma parameters and the sufficient condition for lower hybrid drift instability to be excited, we find that the proton density gradient is sharp enough to excite the lower hybrid drift instability. The analysis of the existence condition for lower hybrid drift instability indicates that these lower hybrid drift waves at the edge of the current sheet are generated through lower hybrid drift instability. The above results can improve our understanding of Mars’ magnetospheric dynamics.

Magnetospheric Multiscale Mission Observations of Lower-hybrid Drift Waves in Terrestrial Magnetotail Reconnection with Moderate Guide Field and Asymmetric Plasma Density

Utilizing high-resolution data from the Magnetospheric Multiscale mission, we present new observations of lower-hybrid drift waves (LHDWs) in terrestrial magnetotail reconnection with guide field levels of ∼70% and asymmetric plasma density (N high/N low ∼ 2.5). The LHDWs, driven by lower-hybrid drift instability, were observed in correlation with magnetic field and density gradients at separatrices on both sides of the reconnection current sheet. The properties of the LHDWs at both sides of the separatrices are different: (1) At high-density side separatrices, the LHDWs with wavelength kρ e ∼ 0.41 propagated away from the X-line mainly in the L–M plane; (2) at the low-density side separatrices, the LHDWs with wavelengths kρ e ∼ 0.76 and kρ e ∼ 0.35 propagated mainly along the outflow direction and current sheet normal. It is also found that the perpendicular magnetic field fluctuations were comparable to the parallel component. Wave potential of the LHDWs was 20% ∼ 35% of the electron temperature. The LHDWs contributed to electron demagnetization and energy dissipation. Our study can promote understanding of properties of LHDWs during magnetic reconnection.

Diffusive Plasma Transport by the Magnetopause Kelvin-Helmholtz Instability During Southward IMF

At the Earth’s low-latitude magnetopause, the Kelvin-Helmholtz (KH) waves, which are driven by the super-Alfvénic velocity shear across the magnetopause, have been frequently observed during periods of northward interplanetary-magnetic-field (IMF) and believed to contribute to efficiently transporting the solar wind plasmas into the magnetosphere. On the other hand, during southward IMF periods, the signatures of the KH waves are much less frequently observed and how the KH waves contribute to the solar wind transport has not been well explored. Recently, the Magnetospheric Multiscale (MMS) mission successfully detected signatures of the KH waves near the dusk-flank of the magnetopause during southward IMF. In this study, we analyzed a series of two- and three-dimensional fully kinetic simulations modeling this MMS event. The results show that a turbulent evolution of the lower-hybrid drift instability (LHDI) near the low-density (magnetospheric) side of the edge layer of the KH waves rapidly disturbs the structure of the layer and causes an effective transport of plasmas across the layer. The obtained transport rate is comparable to or even larger than that predicted for the northward IMF. These results indicate that the diffusive solar wind transport induced by the KH waves may be active at the flank-to-tail magnetopause during southward IMF.

Multi-scale evolution of Kelvin–Helmholtz waves at the Earth's magnetopause during southward IMF periods

At the Earth's low-latitude magnetopause, the Kelvin–Helmholtz instability (KHI), driven by the velocity shear between the magnetosheath and magnetosphere, has been frequently observed during northward interplanetary magnetic field (IMF) periods. However, the signatures of the KHI have been much less frequently observed during southward IMF periods, and how the KHI develops under southward IMF has been less explored. Here, we performed a series of realistic 2D and 3D fully kinetic simulations of a KH wave event observed by the Magnetospheric Multiscale (MMS) mission at the dusk-flank magnetopause during southward IMF on September 23, 2017. The simulations demonstrate that the primary KHI bends the magnetopause current layer and excites the Rayleigh–Taylor instability (RTI), leading to penetration of high-density arms into the magnetospheric side. This arm penetration disturbs the structures of the vortex layer and produces intermittent and irregular variations of the surface waves which significantly reduces the observational probability of the periodic KH waves. The simulations further demonstrate that in the non-linear growth phase of the primary KHI, the lower-hybrid drift instability (LHDI) is induced near the edge of the primary vortices and contributes to an efficient plasma mixing across the magnetopause. The signatures of the large-scale surface waves by the KHI/RTI and the small-scale fluctuations by the LHDI are reasonably consistent with the MMS observations. These results indicate that the multi-scale evolution of the magnetopause KH waves and the resulting plasma transport and mixing as seen in the simulations may occur during southward IMF.

Statistical investigation of electric field fluctuations around the lower-hybrid frequency range at dipolarization fronts in the near-earth magnetotail

Dipolarization fronts (DFs) are thin magnetic boundary structures, embedded in short-duration, fast earthward-moving plasma flows, so-called bursty bulk flows. Previous case studies have shown that the density gradient at DFs can excite the lower-hybrid drift instability (LHDI) and resulting kinetic-scale waves. These waves feature strong electric field fluctuations perpendicular to the ambient magnetic field and associated effective particle acceleration/heating. In this study, we statistically examine electric field fluctuations in the lower-hybrid (LH) frequency range of 61 DF events, using data from the Magnetospheric Multiscale (MMS) Mission between 2017 and 2018 when the MMS apogee was in the magnetotail. We observed that all DF events exhibit enhanced power in the electric field fluctuations in the LH frequency range. Among the observed events, the power can vary by several orders of magnitude. The waves are detected for both high and low values of the perpendicular electron density or pressure gradient. In addition, the peak wave power within the DF is often observed at the time of steepest density gradient within the DF. The results also reveal that the wave power correlates with the magnetic flux transport rate of the DFs. These findings suggest that enhanced density and pressure gradients, which can be formed by large-scale flux transport at DFs, lead to LHDI-related kinetic-scale wave signatures at the DFs, and this may modify the original gradient layer profile of the DF in the course of its propagation from the source to the observation point.

Multi-scale observations of the magnetopause Kelvin–Helmholtz waves during southward IMF

In this study, we present the first observations from the Magnetospheric Multiscale (MMS) mission of the Kelvin–Helmholtz instability (KHI) at the dusk-flank magnetopause during southward interplanetary magnetic field conditions on September 23, 2017. The instability criterion for the KHI was fulfilled for the plasma parameters observed throughout the event. An analysis of the boundary normal vectors based on the application of the timing method onto the magnetic field and the electron density data and the minimum directional derivative method onto the magnetic field data shows signatures of surface waves in the plane parallel to the velocity shear. A comparison to 2D fully kinetic simulations demonstrates reasonable consistencies with the formation of surface waves generated by the KH instability, as well as the structures of rolled-up KH waves. The observations further indicated low density faster than sheath plasma as an indicator of rolled-up vortices, which is also consistent with the simulations. All of these results show that the observed waves and vortices are most likely generated by the KH instability. High-time resolution MMS measurements further demonstrate kinetic-scale electric field fluctuations on the low-density side of the edges of surface waves. Detailed comparisons with the simulations suggest that the observed fluctuations are generated by the lower-hybrid drift instability excited by the density gradient at the edges of these surface waves. These secondary effects can lead to a flattening of the edge layers, indicating the connection between kinetic and larger scales within the KH waves and vortices.

The effect of anomalous resistivity on fast electrothermal instability

This manuscript presents a new theoretical contribution toward the growth rate of the “fast” form of electrothermal instability (ETI) in the presence of anomalous resistivity (AR). Current-driven ETI is present in all pulsed-power platforms, and has been shown to seed the disruptive magneto Rayleigh–Taylor instability. Fluid simulations of low-density, current-carrying plasmas are often subject to nonphysical runaway Ohmic heating due to an under predicted resistivity when using purely collisional resistivity models. AR models provide mechanisms to increase the resistivity as the drift speed increases through increased current density. The derivation of a new, generalized growth rate is presented for the ETI, which includes a resistivity that is dependent on current density, and marks the key contributions of this work. This new growth rate is then compared to the growth rate without AR. Although the striation form of the ETI growth rate is unaffected by the inclusion of AR, the filamentation form of the ETI growth rate depends on the AR. Hence, the new growth rate is verified through 1D simulations of the filamentation form of ETI. The impact of AR can be significant: up to twelve orders of magnitude on the temporally varying local growth rate for a certain choice of parameters. For experimentally relevant conditions based on kinetic simulations, the growth rate can be increased by up to four orders of magnitude if the AR is dominated by the lower-hybrid drift instability.

Ion acceleration to 100 keV by the ExB wave mechanism in collision-less shocks

ABSTRACTIt is shown that ions can be accelerated to about 100 keV in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves. The acceleration occurs in discrete steps of duration being a small fraction of the gyroperiod and can explain observations of ion energization to 10 keV at quasi-perpendicular shocks and to hundreds keV at quasi-parallel shocks. A general expression is provided for the maximum energy of ions accelerated in shocks of arbitrary configuration. The waves involved in the acceleration are related to three cross-field current-driven instabilities: the lower hybrid drift (LHD) instability induced by the density gradients in shocks and shocklets, followed by the modified two-stream (MTS) and electron cyclotron drift (ECD) instabilities, induced by the ExB drift of electrons in the strong LHD wave electric field. The ExB wave mechanism accelerates heavy ions to energies proportional to the atomic mass number, which is consistent with satellite observations upstream of the bow shock and also with observations of post-shocks in supernovae remnants. The results are compared with other acceleration mechanisms traditionally discussed in the literature.

Local Acceleration of Protons to 100 keV in a Quasi‐Parallel Bow Shock

AbstractRecent observations in the quasi‐parallel bow shock by the Magnetospheric Multiscale spacecraft show rapid heating and acceleration of ions up to an energy of about 100 keV. It is demonstrated that a prominent acceleration mechanism is the nonlinear interaction with a spectrum of waves produced by gradient‐driven instabilities, including the lower hybrid drift (LHD) instability, modified two‐stream (MTS) instability, and electron cyclotron drift (ECD) instability. Test‐particle simulations show that the observed spectrum of waves can rapidly accelerate protons up to a few hundreds of keV by the ExB mechanism. The ExB wave mechanism is related to the surfatron mechanism at shocks, but through the coupling with the stochastic heating condition, it produces significant acceleration on much shorter temporal and spatial scales by the interaction with bursts of waves within a cyclotron period. The results of this study are built on the heritage of four‐point measurement techniques developed for the Cluster mission and imply that the concepts of Fermi acceleration, diffusive shock acceleration, and shock drift acceleration are not needed to explain proton acceleration to hundreds of keV at the Earth’s bow shock.

High-fidelity kinetic modeling of instabilities and gyromotion physics in nonuniform low-beta plasmas

A fourth-order accurate continuum kinetic Vlasov solver and a systematic method for constructing customizable kinetic equilibria are demonstrated to be powerful tools for the study of nonuniform collisionless low-beta plasmas. The noise-free methodology is applied to investigate two gradient-driven instabilities in 4D (x,y,vx,vy) phase space: the Kelvin–Helmholtz instability and the lower hybrid drift instability. Nonuniform two-species configurations where ion gyroradii are comparable to gradient scale lengths are explored. The approach sheds light on the evolution of the pressure tensor in Kelvin–Helmholtz instabilities and demonstrates that the associated stress tensor deviates significantly from the gyroviscous stress tensor. Even at high magnetization, first-order approximations to finite-gyromotion physics are shown to be inadequate for the Kelvin–Helmholtz instability, as shear scales evolve to become on par with gyromotion scales. The methodology facilitates exploring transport and energy partitioning properties associated with lower hybrid drift instabilities in low-beta plasma configurations. Distribution function features are captured in detail, including the formation of local extrema in the vicinity of particle-wave resonances. The approach enables detailed targeted investigations and advances kinetic simulation capability for plasmas in which gyromotion plays an important role.

Growth of the lower hybrid drift instability in the plume of a magnetically shielded Hall thruster

Analytical and numerical investigations have been carried out that show the lower hybrid drift instability is active in a region of low-β plasma outside the discharge channel of a magnetically shielded Hall thruster. The region is located downstream of the inner front pole magnet and is contained by the thruster ion beam and hollow cathode plume. It is found that large enough E × B and diamagnetic drifts in the out-of-plane (azimuthal) direction are produced in this region to destabilize lower hybrid waves, even in the presence of electron collisions. The results have been obtained by solving an electrostatic dispersion relation in which the density inhomogeneity is accounted for in the velocity distribution function of the strongly magnetized electrons. Ions in the azimuthal direction have been treated as stationary and unmagnetized. The plasma properties in the dispersion have been provided independently by large-scale 2D multi-fluid/particle-in-cell simulations. The growth rates are found to be even greater than those associated with the modified two-stream instability, which was also predicted recently to be active in this region. The implication of the findings is that ions may be heated anomalously by these instabilities, in both the out-of-plane and in-plane directions perpendicular to the applied magnetic field, thereby altering the velocity field and enhancing the energy of those ions that strike nearby thruster surfaces.

Fluid simulations of three-dimensional reconnection that capture the lower-hybrid drift instability

Fluid models that approximate kinetic effects have received attention recently in the modelling of large-scale plasmas such as planetary magnetospheres. In three-dimensional reconnection, both reconnection itself and current sheet instabilities need to be represented appropriately. We show that a heat flux closure based on pressure gradients enables a 10-moment fluid model to capture key properties of the lower-hybrid drift instability (LHDI) within a reconnection simulation. Characteristics of the instability are examined with kinetic and fluid continuum models, and its role in the three-dimensional reconnection simulation is analysed. The saturation level of the electromagnetic LHDI is higher than expected, which leads to strong kinking of the current sheet. Therefore, the magnitude of the initial perturbation has significant impact on the resulting turbulence.