Kinetic Alfven Waves Research Articles

Solitary kinetic Alfvén waves in dense plasmas with relativistic degenerate electrons and positrons

Propagation of solitary kinetic Alfvén waves (KAWs) is investigated in small but finite (particle-to-magnetic pressure ratio) collisionless dense plasma whose constituents are nondegenerate warm ions, and relativistic degenerate electrons and positrons. Through the use of reductive perturbation technique, Kortweg–de Vries equation is derived to obtain small amplitude localized wave solution of KAWs. The effects of plasma positron concentration, electron relativistic degeneracy parameter, ion thermal temperature and obliqueness parameter on solitary KAWs are studied. The results of this theoretical investigation are aimed at elucidating characteristics of kinetic Alfvén solitary waves in relativistic degenerate e–p–i plasmas found in dense astrophysical objects specifically neutron stars and white dwarfs.

Charged particle nonlinear resonance with localized electrostatic wave-packets

A resonant wave-particle interaction, in particular a nonlinear resonance characterized by particle phase trapping, is an important process determining charged particle energization in many space and laboratory plasma systems. Although an individual charged particle motion in the nonlinear resonance is well described theoretically, the kinetic equation modeling the long-term evolution of the resonant particle ensemble has been developed only recently. This study is devoted to generalization of this equation for systems with localized wave packets propagating with the wave group velocity different from the wave phase velocity. We limit our consideration to the Landau resonance of electrons and waves propagating in an inhomogeneous magnetic field. Electrons resonate with the wave field-aligned electric fields associated with gradients of wave electrostatic potential or variations of the field-aligned component of the wave vector potential. We demonstrate how wave-packet properties determine the efficiency of resonant particle acceleration and derive the nonlocal integral operator acting on the resonant particle distribution. This operator describes particle distribution variations due to interaction with one wave-packet. We solve kinetic equation with this operator for many wave-packets and show that solutions coincide with the results of the numerical integration of test particle trajectories. To demonstrate the range of possible applications of the proposed approach, we consider the electron evolution induced by the Landau resonances with packets of kinetic Alfven waves, electron acoustic waves, and very oblique whistler waves in the near-Earth space plasma.

Stochastic proton heating by kinetic-Alfvén-wave turbulence in moderately high-β plasmas.

Stochastic heating refers to an increase in the average magnetic moment of charged particles interacting with electromagnetic fluctuations whose frequencies are much smaller than the particles' cyclotron frequencies. This type of heating arises when the amplitude of the gyroscale fluctuations exceeds a certain threshold, causing particle orbits in the plane perpendicular to the magnetic field to become stochastic rather than nearly periodic. We consider the stochastic heating of protons by Alfvén-wave (AW) and kinetic-Alfvén-wave (KAW) turbulence, which may make an important contribution to the heating of the solar wind. Using phenomenological arguments, we derive the stochastic-proton-heating rate in plasmas in which β p ∼ 1 - 30, where β p is the ratio of the proton pressure to the magnetic pressure. (We do not consider the β p ≳ 30 regime, in which KAWs at the proton gyroscale become non-propagating.) We test our formula for the stochastic-heating rate by numerically tracking test-particle protons interacting with a spectrum of randomly phased AWs and KAWs. Previous studies have demonstrated that at β p ≲1, particles are energized primarily by time variations in the electrostatic potential and thermal-proton gyro-orbits are stochasticized primarily by gyroscale fluctuations in the electrostatic potential. In contrast, at β p ≳ 1, particles are energized primarily by the solenoidal component of the electric field and thermal-proton gyro-orbits are stochasticized primarily by gyroscale fluctuations in the magnetic field.

A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth’s nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.

Effect of Kinetic Alfvén Waves on Electron Transport in an Ion-Scale Flux Rope**Supported by the National Natural Science Foundation of China under Grant Nos 41474145, 41574159, 41731070 and 41504114, the Frontier Science Foundation of the Chinese Academy of Sciences under Grant No QYZDJ-SSW-JSC028, the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDA15052500, and the

At the Earth’s magnetopause, the electron transport due to kinetic Alfvén waves (KAWs) is investigated in an ion-scale flux rope by the Magnetospheric Multiscale mission. Clear electron dropout around 90° pitch angle is observed throughout the flux rope, where intense KAWs are identified. The KAWs can effectively trap electrons by the wave parallel electric field and the magnetic mirror force, allowing electrons to undergo Landau resonance and be transported into more field-aligned directions. The pitch angle range for the trapped electrons is estimated from the wave analysis, which is in good agreement with direct pitch angle measurements of the electron distributions. The newly formed beam-like electron distribution is unstable and excites whistler waves, as revealed in the observations. We suggest that KAWs could be responsible for the plasma depletion inside a flux rope by this transport process, and thus be responsible for the formation of a typical flux rope.

Effects of He+ and O+ ions on kinetic Alfvén waves: application to PSBL region

Kinetic Alfven waves (KAWs) are investigated considering existence of multi-ions (H+, He+ and O+) in plasma sheet boundary layer (PSBL) region. The dispersion relation and damping rate of wave are derived by kinetic approach. The loss-cone index (for $J=1$ and $J = 2$ ) and densities of multi-ions are varied to study the frequency and damping rate of wave over wide range of $k_{\bot} \rho_{\mathrm{H}^{+}}$ (where $k_{\bot}$ is perpendicular wave vector and $\rho_{\mathrm{H}^{+}}$ is Larmor radius of H+ ion). The presence of multi-ions in plasma is assumed for four cases: (a) H+ only, (b) H+ and He+, (c) H+ and O+, (d) H+, He+ and O+ ions. The results of the cases (b), (c) and (d) are compared with (a) to understand the effects of He+ and O+ ions on KAW. It is observed that the frequency of the wave lies in range 0.1–4 Hz for each case. He+ enhances wave frequency with increase in steepness of loss-cone indices. O+ is more effective in Maxwellian plasma resulting maximum frequency for $J=0$ . Increasing densities of He+ and O+ result in reduction of wave frequency at $k_{\bot} \rho_{\mathrm{H}^{+}} <1$ and enhancement in frequency at higher $k_{\bot} \rho_{\mathrm{H}^{+}}$ . Presence of He+ and O+ induce fluctuations in wave frequency. Reduction in damping rate due to He+ and O+ ions in loss-cone distribution signifies propagation of wave over long distances from PSBL towards auroral ionosphere. The parameters relevant to PSBL region are used in calculation of theoretical results. The results predict that the multi-ions possessing loss-cone distribution with varying densities significantly affect nature of KAW propagation.

Generation of THz radiation by parametric coupling of laser and Trivelpiece–Gould mode

A scheme to generate terahertz radiation is proposed, wherein the high power, perpendicular-extraordinary laser pump (X-mode) (ω0) parametrically decays into the Trivelpiece-Gould (TG) (ω1) mode and kinetic Alfvén wave (ω2) at an oblique angle with the external magnetic field. The nonlinear interaction amongst the three interacting waves leads to the existence of quasi-static ponderomotive force. Furthermore, under the influence of this force, electrons acquire nonlinear oscillatory velocity. A strong transient current is generated by the nonlinear coupling of laser velocity and TG density perturbation. This nonlinear current drives a wave, whose frequency is obtained in the terahertz range (ω2=ω0−ω1) after optimizing various laser-plasma parameters. For the resonant excitation of the wave, the requisite phase matching condition is shown by modelling the parallelogram, which follows the principle of conservation of energy and momentum simultaneously. The coupling coefficients for the three-wave interaction process along with the growth rate of decay instability are investigated.

Nonlinear kinetic Alfven waves in space plasmas with generalized ( r , q $r,q$ ) distribution

In this paper, we have investigated linear and nonlinear propagation of kinetic Alfven waves in which the electrons have been assumed to follow generalized ( $r,q$ ) distribution. We have shown that ( $r,q$ ) distribution gives us most of the distributions observed in space plasmas. We have varied the flatness parameter $r$ and the tail parameter $q$ to explore the linear and nonlinear propagation characteristics of kinetic Alfven waves. We have also discussed the limiting cases. It has been shown that our results agree well with Fast and Freja observations of the nonlinear kinetic Alfven waves. An important feature of our study is the formation of rarefactive solitary structures. It has been shown that this result cannot be obtained with Maxwellian distribution and that it agrees well with the observations of Fast and Freja satellites.

Effect of general loss-cone distribution function on kinetic Alfvén wave in multi-ions plasma by kinetic approach

Kinetic Alfven waves with general loss-cone distribution function are investigated in multi-ions (H+, He+ and O+) plasma. Dispersion relation and damping rate for the wave are derived using Vlasov equation. Variations in frequency and damping rate versus $$ k_{ \bot } \rho_{i} $$ (where k⊥ is wave vector perpendicular to ambient magnetic field, ρi is ion Larmor radius and i denotes multi-ions) are investigated. Parameters relevant to plasma sheet boundary layer are used for graphical analysis. It is observed that wave frequency fluctuates with loss-cone distribution indices of ions. In comparison with Maxwellian plasma (J = 0), the wave frequency is enhanced for He+ and O+ and reduced for H+ with the increase in J indices and the wave existence limit shift towards lower $$ k_{ \bot } \rho_{i} $$ for lighter ions. This may be due to difference in penetration of multi-ions through the cone. H+ and He+ ions show damping at lower k⊥ρi only, whereas O+ ions exhibit damping over wide range of $$ k_{ \bot } \rho_{{O^{ + } }} $$ . The damping is reduced with the increase in loss-cone indices for all the ions which signify propagation of wave over long distances towards auroral ionosphere. The applications of these results are in understanding the effect of gyrating multi-ions in transfer of energy and in describing the generation of aurora.

A two-fluid modeling of kinetic Alfvén wave turbulence

We present an analytical model to explore the magnetic field turbulent spectrum by coupled high-frequency kinetic Alfven wave (KAW) and slow mode of Alfven wave (AW). The spectrum is computed as a realization of energy cascades from larger to smaller scales for a specific case of solar wind plasma at 1 AU. A two-fluid technique is implemented for the derivation of model equations leading two wave modes. These coupled, nonlinear equations are solved numerically. The nonlinearity in the system arises due to nonlinear ponderomotive force, which is believed to be responsible for the wave localization and magnetic islands formation. The numerical results show that the magnetic islands grow with time and attain a quasi-steady state after the modulation instability is saturated. The magnetic field spectrum and associated spectral indices are computed near the time of saturation of instability. The simulated spectrum in dispersion region follows a power-law with an index of −2.5. The steeper spectrum could be attributed as energy transfer from larger to smaller scales and helps to study turbulence in solar wind. The magnetic field spectrum and spectral index show a good agreement with the observation of solar wind turbulent spectra.

Spectral Approach to Plasma Kinetic Simulations Based on Hermite Decomposition in the Velocity Space

Spectral (transform) methods for solution of Vlasov-Maxwell system have shown significant promise as numerical methods capable of efficiently treating fluid-kinetic coupling in magnetized plasmas. We discuss SpectralPlasmaSolver (SPS), an implementation of three-dimensional, fully electromagnetic algorithm based on a decomposition of the plasma distribution function in Hermite modes in velocity space and Fourier modes in physical space. A fully-implicit time discretization is adopted for numerical stability and to ensure exact conservation laws for total mass, momentum and energy. The SPS code is parallelized using Message Passing Interface for distributed memory architectures. Application of the method to analysis of kinetic range of scales in plasma turbulence under conditions typical of the solar wind is demonstrated. With only 4 Hermite modes per velocity dimension, the algorithm yields damping rates of kinetic Alfven waves with accuracy of 50% or better, which is sufficient to obtain a model of kinetic scales capable of reproducing many of the expected statistical properties of turbulent fluctuations. With increasing number of Hermite modes, progressively more accurate values for collisionless damping rates are obtained. Fully nonlinear simulations of decaying turbulence are presented and successfully compared with similar simulations performed using Particle-In-Cell method.

Generation of Electron Acoustic Waves in the Topside Ionosphere From Coupling With Kinetic Alfven Waves: A New Electron Energization Mechanism

AbstractResults from a new drift kinetic model in the topside ionosphere capture the mode conversion from kinetic Alfven waves to electron acoustic waves. When the kinetic Alfven waves propagate into the transition region, where the electron density of ionospheric origin becomes comparable to that of magnetospheric origin, the steep temperature gradient leads to the mode conversion. The electron acoustic waves are short‐lived by dissipating their energy into the electron energization, thus revealing a new type of electron acceleration in the topside ionosphere.

Linear and nonlinear analysis of kinetic Alfven waves in quantum magneto-plasmas with arbitrary temperature degeneracy

Linear and nonlinear kinetic Alfven waves (KAWs) are studied in collisionless, non-relativistic two fluid quantum magneto-plasmas by considering arbitrary temperature degeneracy. A general coupling parameter is applied to discuss the range of validity of the proposed model in nearly degenerate and nearly non-degenerate plasma limits. Linear analysis of KAWs shows an increase (decrease) in frequency with the increase in parameter ζ(δ) for the nearly non-degenerate (nearly degenerate) plasma limit. The energy integral equation in the form of Sagdeev potential is obtained by using the approach of the Lorentz transformation. The analysis reveals that the amplitude of the Sagdeev potential curves and soliton structures remains the same, but the potential depth and width of soliton structure change for both the limiting cases. It is further observed that only density hump structures are formed in the sub-alfvenic region for value Kz2&amp;gt;1. The effects of parameters ζ, δ on the nonlinear properties of KAWs are shown in graphical plots. New results for comparison with earlier work have also been highlighted. The significance of this work to astrophysical plasmas is also emphasized.

Modulation of kinetic Alfvén waves in an intermediate low-beta magnetoplasma

We study the amplitude modulation of nonlinear kinetic Alfvén waves (KAWs) in an intermediate low-beta magnetoplasma. Starting from a set of fluid equations coupled to the Maxwell's equations, we derive a coupled set of nonlinear partial differential equations (PDEs) which govern the evolution of KAW envelopes in the plasma. The modulational instability (MI) of such KAW envelopes is then studied by a nonlinear Schrödinger equation derived from the coupled PDEs. It is shown that the KAWs can evolve into bright envelope solitons or can undergo damping depending on whether the characteristic ratio (α) of the Alfvén to ion-acoustic speeds remains above or below a critical value. The parameter α is also found to shift the MI domains around the kxkz plane, where kx (kz) is the KAW number perpendicular (parallel) to the external magnetic field. The growth rate of MI, as well as the frequency shift and the energy transfer rate, are obtained and analyzed. The results can be useful for understanding the existence and formation of bright and dark envelope solitons, or damping of KAW envelopes in space plasmas, e.g., interplanetary space, solar winds, etc.

Comment on “Pulsating Auroras Produced by Interactions of Electrons and Time Domain Structures” by Mozer Et Al.

AbstractMozer et al. (2017, https://doi.org/10.1002/2017JA024223) suggested that time domain structures (TDSs) drive pulsating aurora (with additional contributions by kinetic Alfvén waves (KAWs)) and that chorus waves have negligible effects. In this comment, we point out that electrons scattered by TDS or KAW (dominantly at ~0.1–3 keV, &lt;1 s modulation) cannot explain key features of pulsating aurora, which require precipitation above a few keV with a couple of tens of second modulation. Their study did not conduct quantitative evaluations of wave‐aurora correlation. The use of short burst mode data (~&lt;10 s) may only cover a single pulse of pulsating aurora and is not suitable for examining connections to pulsating aurora. “Field‐aligned” electrons do not necessarily represent loss cone population, and their characteristic energy (hundreds of eV) is much lower than typical precipitation over pulsating aurora. By reexamining the events studied by Mozer et al., we quantitatively demonstrate that TDS and KAW are uncorrelated with pulsating aurora and that only chorus waves showed high correlations with pulsating aurora. Occasional simultaneous occurrence of TDS/KAW and pulsating aurora is found to be coincidental, because the correlation over a time scale of minutes is poor. Several auroral features analyzed in that paper are not pulsating aurora but other types of aurora. We also show that the chorus‐pulsating aurora correlation can last for 2 h or longer and can be used to highlight dynamic changes in magnetic field mapping. Chorus waves can resonate with electrons above a few keV and are in agreement with pulsating auroral properties.

Effect of ion-neutral collisions on the evolution of kinetic Alfvén waves in plasmas

This paper studies the effect of ion-neutral collisions on the propagation of kinetic Alfvén waves (KAWs) in inhomogeneous magnetized plasma. The inhomogeneity in the plasma imposed by background density in a direction transverse as well as parallel to the ambient magnetic field plays a vital role in the localization process. The mass loading of ions takes place due to their collisions with neutral fluid leading to the damping of the KAWs. Numerical analysis of linear KAWs in inhomogeneous magnetized plasma is done for a fixed finite frequency taking into consideration the ion-neutral collisions. There is a prominent effect of collisional damping on the wave localization, wave magnetic field, and frequency spectrum. A semi-analytical technique has been employed to study the magnetic field amplitude decay process and the effect of wave frequency in the range of ion cyclotron frequency on the propagation of waves leading to damping.

Super‐Alfvénic Propagation and Damping of Reconnection Onset Signatures

AbstractThe quadrupolar out‐of‐plane Hall magnetic field generated during collisionless reconnection propagates away from the x line as a kinetic Alfvén wave (KAW). While it has been shown that this KAW carries substantial Poynting flux and propagates super‐Alfvenically, how this KAW damps as it propagates away from the x line is not well understood. In this study, this damping is examined using kinetic particle‐in‐cell simulations of antiparallel symmetric magnetic reconnection in a one‐dimensional current sheet equilibrium. In the reconnection simulations, the KAW wave vector has a typical magnitude comparable to an inverse fluid Larmor radius (effectively an inverse ion Larmor radius) and a direction of 85–89° relative to the local magnetic field. We find that the damping of the reconnection KAW is consistent with linear Landau damping results from a numerical Vlasov dispersion solver. This knowledge allows us to generalize our damping predictions to regions in the magnetotail and solar corona where the magnetic geometry can be approximated as a current sheet. For the magnetotail, the KAW from reconnection will not damp away before propagating the approximately 20 Earth radii associated with global magnetotail distances. For the solar corona, on the other hand, these KAWs will completely damp before reaching the distances comparable to the flare loop length.

Density variation effect on multi-ions with kinetic Alfven wave around cusp region—a kinetic approach

The kinetic Alfven waves in the presence of homogeneous magnetic field plasma with multi-ions effect are investigated. The dispersion relation and normalised damping rate are derived for low- $\beta$ plasma using kinetic theory. The effect of density variation of $\text{H}^{+}$ , $\text{He}^{+}$ and $\text{O}^{+}$ ions is observed on frequency and damping rate of the wave. The variation of frequency ( $\omega$ ) and normalised damping rate ( $\gamma / \varOmega_{H^{ +}} $ ) of the wave are studied with respect to $k_{ \bot} \rho_{j}$ , where $k_{ \bot} $ is the perpendicular wave number, $\rho_{j}$ is the ion gyroradius and $j $ denotes $\text{H}^{+}$ , $\text{He}^{+}$ and $\text{O}^{+}$ ions. The variation with $k_{ \bot} \rho_{j}$ is considered over wide range. The parameters appropriate to cusp region are used for the explanation of results. It is found that with hydrogen and helium ions gyration, the frequency of wave is influenced by the density variation of $\text{H}^{+}$ and $\text{He}^{+}$ ions but remains insensitive to the change in density of $\text{O}^{+}$ ions. For oxygen ion gyration, the frequency of wave varies over a short range only for $\text{O}^{+}$ ion density variation. The wave shows damping at lower altitude due to variation in density of lighter $\text{H}^{+}$ and $\text{He}^{+}$ ions whereas at higher altitude only heavy $\text{O}^{+}$ ions contribute in wave damping. The damping of wave may be due to landau damping or energy transfer from wave to particles. The present study signifies that the both lighter and heavier ions dominate differently to change the characteristics of kinetic Alfven wave and density variation is also an important parameter to understand wave phenomena in cusp region.

Dispersion of kinetic Alfvén wave in a rotating plasma with an inhomogeneous magnetic field

The effects of inhomogeneity of a magnetic field on the dispersion of kinetic Alfvén waves (KAWs) in a rotating plasma is investigated under the framework of magnetohydrodynamic theory. The magnetic field should be in a non‐uniform state if the centrifugal force is balanced only by the magnetic pressure. The inhomogeneity of the magnetic field increases the frequency of KAW and drives it into an unstable state. The growth rate of KAW varies non‐monotonously with respect to the distance. The KAW will be excited in a certain region with maximum growth rate. And the growth rate of KAW in the region near and far from the centre of rotation approaches zero. These results will be helpful in understanding the properties of KAWs in rotating astrophysical and laboratory plasmas.

Solitary kinetic Alfvén waves in a dense electron–positron–ion plasma with degenerate electrons and positrons

Through the use of a reductive perturbation technique, solitary kinetic Alfvén waves (KAWs) are investigated in a low but finite (particle-to-magnetic pressure ratio) dense electron–positron–ion plasma where electrons and positrons are degenerate. The degenerate plasma model considered here permits the existence of sub-Alfvénic compressive solitary KAWs. The influence of r (equilibrium positron-to-ion density ratio), (electron-to-positron Fermi temperature ratio), and obliqueness parameter on various characteristics of solitary KAWs are examined through numerical plots. We have shown that there exists a critical value of at which a soliton width attains its maximum value which decreases with an increase in r and It is also found that solitons with a higher energy propagate more obliquely in the direction of an ambient magnetic field. The results of the present investigation may be useful for understanding low frequency nonlinear electromagnetic wave propagation in magnetized electron–positron–ion plasmas in dense stars. Specifically, the relevance of our investigation to a pulsar magnetosphere is emphasized.