Kinetic Alfven Waves Research Articles

Kinetic Alfven double layer formed by electron viscosity

The effect of the electron viscosity on the kinetic Alfven solitary wave is investigated. It is found that small electron viscosity changes the electron motion along the magnetic field producing a boundary layer, and thus that in a low beta electron-ion plasma(β≪me/mi), an obliquely propagating kinetic solitary Alfven wave can become a double layer. This double layer can exist in the sub-Alfvenic and super-Alfvenic regimes. The length scale of density drop for this double layer is on the order of that of the conventional kinetic solitary Alfven wave, and thus this double layer can accelerate electrons on a very short length scale.

THE SLOW-MODE NATURE OF COMPRESSIBLE WAVE POWER IN SOLAR WIND TURBULENCE

We use a large, statistical set of measurements from the Wind spacecraft at 1 AU, and supporting synthetic spacecraft data based on kinetic plasma theory, to show that the compressible component of inertial range solar wind turbulence is primarily in the kinetic slow mode. The zero-lag cross correlation C(delta n, delta B_parallel) between proton density fluctuations delta n and the field-aligned (compressible) component of the magnetic field delta B_parallel is negative and close to -1. The typical dependence of C(delta n,delta B_parallel) on the ion plasma beta_i is consistent with a spectrum of compressible wave energy that is almost entirely in the kinetic slow mode. This has important implications for both the nature of the density fluctuation spectrum and for the cascade of kinetic turbulence to short wavelengths, favoring evolution to the kinetic Alfven wave mode rather than the (fast) whistler mode.

Convective cell generation by kinetic Alfvén wave turbulence in the auroral ionosphere

Modulation of convective cells by kinetic Alfvén wave (KAW) turbulence is investigated. The interaction is governed by a nonlinear dispersion relation for the convective cells. It is shown that KAW turbulence is disrupted by excitation of the large-scale convective motion through a resonant instability. Application of the results to the auroral ionosphere shows that cross-scale coupling of the KAW turbulence and convective cells plays an important role in the evolution of ionospheric plasma turbulence.

Quantum treatment of kinetic Alfvén wave

Dispersion properties of kinetic Alfven wave in quantum magnetoplasma are derived. The quantum contribution to the Landau damping of kinetic Alfven wave is also derived by using linearized Vlasov equation which contains the Bohm quantum potential. Classical Landau damped kinetic Alfven waves play an important role in turbulence of astrophysical plasmas. The quantum modification in Landau damping of kinetic Alfven wave can also play a significant role in changing the scaling law of turbulent spectra as well as the formation of damped localized Alfvenic structures in dense astrophysical plasmas.

Transformations of kinetic Alfvén waves in toroidal plasmas

It is demonstrated that toroidicity (as well as ellipticity and other kinds of deviations of the magnetic configuration from the cylindrical geometry) can result in linear transformations of propagating kinetic Alfvén waves (KAWs) into other KAW branches, which differ by their mode numbers from the initial waves. Analytical expressions for the amplitudes of the transformed and non-transformed waves are derived. The transformation is found to be strong for waves with low poloidal mode numbers in typical present-day tokamaks. It may affect the mode numbers observed by external magnetic measurements and weaken the Landau damping of the waves, thus extending the region of the wave propagation.

Simulation of mode conversion at the magnetopause

Two-dimensional (2-D) and three-dimensional (3-D) hybrid simulations are carried out for mode conversion from fast mode compressional wave to kinetic Alfvén waves (KAWs) at the inhomogeneous magnetopause boundary. For cases in which the incident fast wave propagates in the xz plane, with the magnetopause normal along x and the background magnetic field pointing along z, the 2-D (xz) simulation shows that KAWs with large wave number k x ρ i ∼ 1 are generated near the Alfvén resonance surface, where ρ i is the ion Larmor radius. Several nonlinear wave properties are manifest in the mode conversion process. Harmonics of the driver frequency are generated. As a result of nonlinear wave interaction, the mode conversion region and its spectral width are broadened. In the 3-D simulation, after this first stage of the mode conversion to KAWs with large k x , a subsequent generation of KAW modes of finite k y is observed in the later stage, through a nonlinear parametric decay process. Since the nonlinear cascade to k y can lead to massive transport at the magnetopause, the simulation results provide an effective transport mechanism at the plasma boundaries in space as well as laboratory plasmas.

Quasilinear evolution of plasma distribution functions and consequences on wave spectrum and perpendicular ion heating in the turbulent solar wind

The measured spectrum of kinetic Alfven wave fluctuations in the turbulent solar wind plasma is used to calculate the quasi-linear evolution of the initially stable electron and ion distribution functions. The resulting ion distribution function is found to be unstable to electromagnetic left hand polarized ion cyclotron-Alfven waves as well as right hand polarized magnetosonic-whistler waves. These waves can pitch angle scatter the ion super-thermal velocity component to provide perpendicular ion heating. Additionally, right hand polarized waves transfer some part of kinetic Alfven wave flux to whistler waves.

Nonlinear dispersive scale Alfvén waves inmagnetosphere-ionosphere coupling: Physical processes and simulation results

Dispersive Alfven waves (DAWs) have been demonstrated to play a significant role in auroral generation of the magnetosphere-ionosphere coupling system. Starting from a two fluid reduced MHD model, we summarize the frequency, temporal and spatial characteristics of magnetospheric DAWs. Then, the nonlinear kinetic and inertial scale Alfven waves are studied, and we review some theoretical aspects and simulation results of dispersive Alfven waves in Earth’s magnetosphere. It is shown that dispersive standing Alfven waves can generate the field-aligned currents which transport energy into the auroral ionosphere, where it is dissipated by Joule heating and energy lost due to electron precipitation. The Joule dissipation can heat the ionospheric electron and produce changes in the ionospheric Pedersen conductivity. As a feedback, the conducting ionosphere can also strongly affect the magnetospheric currents. The ponderomotive force can cause the plasma to move along the field line, and generate ionospheric density cavity. The nonlinear structuring can lead to a dispersive scale to accelerate auroral particle, and the Alfven waves can be trapped within the density cavity. Finally, we show the nonlinear decay of dispersive Alfven waves related to two anti-propagating electron fluxes observed in the auroral zone.

Nonlinear interaction of kinetic Alfvén waves with slow Alfvén waves and application to solar wind

This paper presents the nonlinear coupling between kinetic Alfvén waves (KAWs) and slow Alfvén waves (SWs) in high-β plasmas (β ≫me /mi), which is applicable to solar wind plasma. The pump KAW is perturbed by a low-frequency SW. When the ponderomotive nonlinearities are incorporated into the KAW and SW dynamics, the model equations of KAW and SW turn out to be the modified Zakharov system of equations. The growth rate of the instability has been calculated and its dependence on the perturbation wave number has also been presented. The relevance of these investigations for solar wind plasma has been discussed.

Kinetic Alfven wave in the presence of parallel electric field with general loss-cone distribution function: A kinetic approach

In past, kinetic Alfven waves are widely investigated regarding the transfer of energy from the magnetosphere to the ionosphere. These waves are supposed to be responsible for the acceleration of auroral electrons to exhibit auroral phenomena. The existence of parallel electric fields is always reported in association with the kinetic Alfven waves. In the present investigation, assuming parallel electric fields developed by different mechanism, the dispersion relation and growth/damping rate of the kinetic Alfven Wave (KAW) in the presence of parallel electric field are evaluated with general loss-cone distribution using the kinetic approach. The wave frequency w and growth/damping rate gL are obtained for two regimes of propagation k^ri 1, where k^ is perpendicular wave number andri is the ion-gyro radius. The results of the present study show that the frequency w and growth/damping rate of wave depend upon the distribution indices, parallel electric field and k^ri. The generation of kinetic Alfven wave by the combined effect of parallel electric field and loss-cone distribution indices (J), at a particular range of k^ri is noticed. Thus the propagation of kinetic Alfven wave and loss of the Poynting flux from plasma sheet boundary layer (PSBL) to the ionosphere can be explained more effectively on the basis of present investigation. The results of the present study show that the parallel electric field in the presence of loss-cone distribution index is an important parameter to study KAW in the PSBL. The finding of the present investigation is to report that auroral phenomena are suitably described by kinetic Alfven wave in the presence of parallel electric field and the general loss-cone distribution function. Key words: Kinetic Alfven wave, Plasma-sheet-boundary-layer, Parallel electric field, Kinetic theory, Loss-cone distribution function.

The influence of negatively charged heavy ions on the kinetic Alfven wave in a cometary environment

Kinetic Alfven waves are important in a wide variety of areas like astrophysical, space and laboratory plasmas. In cometary environments, waves in the hydromagnetic range of frequencies are excited predominantly by heavy ions. We, therefore, study the stability of the kinetic Alfven wave in a plasma of hydrogen ions, positively and negatively charged oxygen ions and electrons. Each species was modeled by drifting ring distributions in the direction parallel to the magnetic field; in the perpendicular direction the distribution was simulated with a loss cone type distribution obtained through the subtraction of two Maxwellian distributions with different temperatures. We find that for frequencies \(\omega^{*} < \omega_{c\mathrm{H}^{ +}}\) (ω∗ and \(\omega_{c\mathrm{H}^{ +}}\) being respectively the Doppler shifted and hydrogen ion gyro-frequencies), the growth rate increases with increasing negatively charged oxygen ion densities while decreasing with increasing propagation angles, negative ion temperatures and negative ion mass.

IDENTIFICATION OF KINETIC ALFVÉN WAVE TURBULENCE IN THE SOLAR WIND

The nature of small-scale turbulent fluctuations in the solar wind is investigated using a comparison of Cluster magnetic and electric field measurements to predictions arising from models consisting of either kinetic Alfven waves or whistler waves. The electric and magnetic field properties of these waves from linear theory are used to construct spacecraft-frame frequency spectra of (|δE|/|δB|) s/c and (|δB ∥|/|δB|) s/c , allowing for a direct comparison to spacecraft data. The measured properties of the small-scale turbulent fluctuations, found to be inconsistent with the whistler wave model, agree well with the prediction of a spectrum of kinetic Alfven waves with nearly perpendicular wavevectors.

OBSERVATIONAL CONSTRAINTS ON THE ROLE OF CYCLOTRON DAMPING AND KINETIC ALFVÉN WAVES IN THE SOLAR WIND

Certain few intervals with high-β plasma (ratio of gas pressure to magnetic pressure) have been interpreted as containing turbulent fluctuations with wave vectors that are confined to very oblique angles with respect to the mean magnetic field. The fluctuations are theorized to be Kinetic Alfven Waves (KAWs) engaged in an energy cascade that dissipates primarily at electron scales. Dissipation by ions, and by cyclotron damping in particular, is argued to be minimal to non-existent. This interpretation is not supported, generally, by the analysis of larger data sets using other data analysis methods. These prior studies, however, were not conducted for specific β ranges. In this study, we reconsider the analysis for a moderately large set of high-β intervals. The analysis includes magnetic variance, the Bieber ratio test, the cross-helicity versus magnetic helicity correlation, and the implied break frequency versus angle relationship. In our analysis, the results do not support the exclusive KAW interpretation as applied generally to solar wind intervals of high-β while the results do support the presence of cyclotron damping at a significant level.

A weakened cascade model for turbulence in astrophysical plasmas

A refined cascade model for kinetic turbulence in weakly collisional astrophysical plasmas is presented that includes both the transition between weak and strong turbulence and the effect of nonlocal interactions on the nonlinear transfer of energy. The model describes the transition between weak and strong MHD turbulence and the complementary transition from strong kinetic Alfvén wave (KAW) turbulence to weak dissipating KAW turbulence, a new regime of weak turbulence in which the effects of shearing by large scale motions and kinetic dissipation play an important role. The inclusion of the effect of nonlocal motions on the nonlinear energy cascade rate in the dissipation range, specifically the shearing by large-scale motions, is proposed to explain the nearly power-law energy spectra observed in the dissipation range of both kinetic numerical simulations and solar wind observations.

Turbulent spectra and spectral kinks in the transition range from MHD to kinetic Alfvén turbulence

Abstract. A weakly dispersive range (WDR) of kinetic Alfvén turbulence is identified and investigated for the first time in the context of the MHD/kinetic turbulence transition. We find perpendicular wavenumber spectra &amp;amp;propto; k&amp;amp;bot;−3 and &amp;amp;propto; k&amp;amp;bot;−4 formed in WDR by strong and weak turbulence of kinetic Alfvén waves (KAWs), respectively. These steep WDR spectra connect shallower spectra in the MHD and strongly dispersive KAW ranges, which results in a specific double-kink (2-k) pattern often seen in observed turbulent spectra. The first kink occurs where MHD turbulence transforms into weakly dispersive KAW turbulence; the second one is between weakly and strongly dispersive KAW ranges. Our analysis suggests that partial turbulence dissipation due to amplitude-dependent non-adiabatic ion heating may occur in the vicinity of the first spectral kink. The threshold-like nature of this process results in a conditional selective dissipation that affects only the largest over-threshold amplitudes and that decreases the intermittency in the range below the first spectral kink. Several recent counter-intuitive observational findings can be explained by the coupling between such a selective dissipation and the nonlinear interaction among weakly dispersive KAWs.

Arbitrary amplitude kinetic Alfvén solitons in a plasma with a q-nonextensive electron velocity distribution

Arbitrary amplitude solitary kinetic Alfvén waves (KAWs) in a plasma with q–nonextensive electrons are investigated by the conventional Sagdeev pseudopotential method, through which the existence of solitary KAWs is analyzed theoretically and numerically. It is shown only solitons with density hump can exist, the amplitude of which depends sensitively on the parameter q and the plasma β. There is an upper limit for the amplitude of solitary wave which decreases with the increase of q and β. The results obtained in the framework of Maxwellian distribution are reproduced when q → 1.

Formation of density cavities with a nonstationary electric field in the zone of auroral field-aligned currents

The formation of small-scale density cavities with a nonstationary electric field, which are registered in the Earth’s auroral magnetosphere, has been analyzed. It has been indicated that cavities are probably initially caused by quasi-static field-aligned electric currents and currents of kinetic Alfven waves exceeding threshold values. Urgent variants of the linear and nonlinear stages of density disturbance instabilities have been considered. The determined properties of the parameters of small-scale density cavities and nonstationary electric fields are in agreement with the known experimental data.

Polarizations of coupling kinetic Alfvén and slow waves

Kinetic Alfvén waves (KAWs) are dispersive Alfvén waves with short perpendicular wavelengths and have been extensively applied to various energization phenomena of plasma particles. KAWs are coupled to slow magnetosonic waves in the case of a finite-β plasma. In this paper, the electromagnetic polarization states of the coupling KAWs and slow waves are investigated. The results show that the polarization states of these waves depend sensitively on the local plasma parameters such as the ion-electron temperature ratio (α=Ti/Te) and the plasma kinetic-magnetic pressure ratio (β=2μ0n(Ti+Te)/B2) as well as their perpendicular wavenumber (k⊥ρi). The polarization states of waves play an important and key role in wave-particle interactions and hence have a great interest of understanding the physics of particle energization phenomena by these waves.

Electron-ion Coulomb scattering and the electron Landau damping of Alfvén waves in the solar wind

[1] All Alfven waves in the solar wind have parallel electric fields, which enable Landau damping. The Alfven waves' Landau resonate with very low energy electrons; low-energy electrons are easily trapped in the Alfven waves, and at low energies electron-ion Coulomb scattering is very rapid. Analytic fluid theory and numerical solutions to the linear Vlasov equation are used to determine the properties of Alfven waves (and kinetic Alfven waves) in the solar wind. Electrostatic potentials associated with the waves' parallel electric fields are found to be relatively large. Owing to the large potentials, electrons over a broad range of velocities interact with the wave to produce Landau damping. Because of this broad range, linear Vlasov theory is invalid and the Landau-damping rates computed via linear Vlasov theory may not be accurate. Electron velocity diffusion owed to electron-ion Coulomb scattering is analyzed. Electron diffusion times in the Alfven wave Landau resonance are calculated and are found to be faster than wave periods and much faster than Landau-damping time scales. Coulomb collisions should prevent the electron distribution function from evolving away from a Maxwellian form, and thereby Coulomb collisions act to maintain Landau damping (although at a rate different than that given by linear Vlasov theory). Some complications of this Landau-damping picture arise from the interplanetary electric field competing with the Alfven wave parallel electric fields and from ion beams near the Landau resonance.

KINETIC ALFVÉN WAVES EXCITED BY OBLIQUE MAGNETOHYDRODYNAMIC ALFVÉN WAVES IN CORONAL HOLES

Kinetic Alfven waves (KAWs) are small-scale dispersive AWs that can play an important role in particle heating and acceleration of space and solar plasmas. An excitation mechanism for KAWs created by the coupling between large-scale oblique AWs and small-scale KAWs is presented in this paper. Taking into account both the collisional and Landau damping dissipations, the results show that the net growth rate of the excited KAWs increases with their perpendicular wavenumber k{sub perpendicular} and reaches maximum at {lambda}{sub e} k{sub perpendicular} {approx} 0.3, where {lambda}{sub e} is the electron inertial length. However, for KAWs with shorter perpendicular wavelengths, the net growth rate decreases rapidly due to dissipative effects. The evaluation of the threshold amplitude of the AW implies that for KAWs with {lambda}{sub e} k{sub perpendicular} < 0.3, the relative threshold amplitude is well below 10%, which is easy to satisfy. In particular, when applying this mechanism to the case of a solar coronal hole containing a dense plume structure, our results show that KAWs with {lambda}{sub e} k{sub perpendicular} < 0.3 can be not only efficiently excited in the interplume region but also strongly dissipated in the dense plume due to the Landau damping.