Kinetic Alfven Waves Research Articles

Nonlinear Dynamics of Kinetic Alfvén and Whistler Waves in the Solar Wind

In this article we investigate the nonlinear dynamics of 3D kinetic Alfven waves (KAWs) and quasi-transverse weak whistler waves in a magnetized plasma. We have studied the problem numerically to examine the transient evolution of localized structures of 3D KAWs and whistler waves. The nonlinearity arises as a result of ponderomotive effects associated with 3D KAWs; consequently, the background density modifies. The weak whistler waves propagating in this modified density are localized and amplified. To improve our insight into the basic physics behind the formation of these localized structures, we have also solved the system semi-analytically. The power spectra show a Kolmogorov scaling (with a power of \(-5/3\)) in the inertial range that lies above the ion gyroradius. Below this scale, dispersive effects start to appear, and the power spectrum follows a steeper scaling (−2 to −4). Our results show the important role that KAWs and whistler waves play in the energy cascading from larger to smaller scales. The results are consistent with the solar wind observations by the Cluster spacecraft.

Kinetic Alfvén wave explanation of the Hall fields in magnetic reconnection

AbstractMagnetic reconnection is initiated in a small diffusion region but can drive global‐scale dynamics in Earth's magnetosphere, solar flares, and astrophysical systems. Understanding the processes at work in the diffusion region remains a main challenge in space plasma physics. Recent in situ observations from Magnetospheric Multiscale and Time History of Events and Macroscale Interactions during Substorms reveal that the electric field normal to the reconnection current layer, often called the Hall electric field (En), is mainly balanced by the ion pressure gradient. Here we present theoretical explanations indicating that this observation fact is a manifestation of kinetic Alfvén waves (KAWs) physics. The ion pressure gradient represents the finite gyroradius effect of KAW, leading to ion intrusion across the magnetic field lines. Electrons stream along the magnetic field lines to track ions, resulting in field‐aligned currents and the associated pattern of the out‐of‐plane Hall magnetic field (Bm). The ratio ΔEn/ΔBm is on the order of the Alfvén speed, as predicted by the KAW theory. The KAW physics further provides new perspectives on how ion intrusion may trigger electric fields suitable for reconnection to occur.

Nonlinear effects associated with quasi‐electrostatic whistler waves relevant to that in radiation belts

AbstractA model is proposed to study the dynamics of high‐amplitude quasi‐electrostatic whistler waves propagating near resonance cone angle and their interaction with low‐frequency kinetic Alfvén waves (KAWs) in Earth's radiation belts. The wave dynamics clearly indicates the whistlers having quasi‐electrostatic character when propagating close to resonance cone angle. A high‐amplitude whistler wave packet is obtained using the present analysis which has also been observed by S/WAVES (STEREO/WAVES) instrument onboard STEREO (Solar Terrestrial Relations Observatory). A numerical simulation technique has been employed to study the localization of quasi‐electrostatic whistler waves in radiation belts. The ponderomotive force of pump quasi‐electrostatic whistlers (high frequency) is used to excite low‐frequency waves (KAWs). The turbulent spectrum obtained using the analysis suggests the presence of quasi‐electrostatic whistlers and density fluctuations associated with KAW in radiation belts plasma. The wave localization and steeper spectra could be responsible for particle energization or heating in radiation belts.

Nonlinear effects associated with kinetic Alfvén wave in magnetic islands

Observations show that kinetic Alfvén waves (KAWs) play a very important role in turbulence as well as in magnetic reconnection and have also been a topic of debate in the context of particle heating. In this paper, we aim to investigate the effect of magnetic islands on the dynamics of the nonlinear KAW. The formation of current sheet/coherent structures has been explored due to the propagation of the nonlinear KAW through the chain of pre-existing fully developed chain of magnetic islands. It is found that the field localization is more intense when the density perturbation due to nonlinearity and the field perturbation due to background islands are taken into account in comparison to the perturbation created by both the sources separately. Thus, it is expected that both nonlinearity and islands (reconnection generated) will aid the field localization and hence turbulence. Based on these coherent structures, we have evaluated the power spectrum and found the scaling after first breakpoint to be consistent with the observation by Chaston et al. [Phys. Rev. Lett. 102, 015001 (2009)] in the ion diffusion region of the magnetotail.

Modeling RF waves in spatially dispersive inhomogeneus plasma using an iterative wavelet spectral method

The wave equation for a spatially dispersive inhomogeneous magnetized plasma is given by an integro-differential equation. The effects caused by spatial dispersion in the directions perpendicular and parallel to the magnetic field are quite different. In this study, we show how to solve the wave equation using a newly developed iterative wavelet spectral method for two cases. In the first case, the method is applied to a propagating kinetic Alfven wave in the perpendicular direction and solved to all orders in FLR. To conserve the kinetic energy flux, first order corrections in equilibrium gradients are used in the dielectric response tensor. In the second case, we verify the method for a fast wave minority heating scenario and study the up- and downshift in the parallel wave number.

An iterative method to include spatial dispersion for waves in nonuniform plasmas using wavelet decomposition

A novel method for solving wave equations with spatial dispersion is presented, suitable for applications to ion cyclotron resonance heating. The method splits the wave operator into a dispersive and a non-dispersive part. The latter can be inverted with e.g. finite element methods. The spatial dispersion is evaluated using a wavelet representation of the dielectric kernel and added by means of iteration. The method has been successfully tested on a low frequency kinetic Alfven wave with second order Larmor radius effects in a nonuniform plasma slab.

Nonlinear effects associated with oblique whistler waves in space plasmas

In the present work, we have examined the nonlinear interaction of pump whistler wave and low frequency kinetic Alfvén wave (KAW) in three regions viz., solar wind, earth's radiation belt, and magnetopause. The modification in the background density leads to the introduction of nonlinearity. The nonlinear ponderomotive force is responsible for this change in density. Low frequency kinetic Alfvén wave is excited by the nonlinear ponderomotive force of pump whistler wave. A set of dimensionless equations characterizing the dynamics of whistler wave and low frequency KAW perturbed by whistler wave were developed. The coupled equations were then simulated numerically. The nonlinear effects related with the whistler wave were studied. The resulting localized structures and the magnetic turbulent spectra in various regions have been investigated.

Role of nonlinear localized structures and turbulence in magnetized plasma

In the present study, we have analyzed the field localization of kinetic Alfven wave (KAW) due to the presence of background density perturbation, which are assumed to be originated by the three dimensionally propagating low frequency KAW. These localized structures play an important role for energy transportation at smaller scales in the dispersion range of magnetic power spectrum. For the present model, governing dynamic equations of high frequency pump KAW and low frequency KAW has been derived by considering ponderomotive nonlinearity. Further, these coupled equations have been numerically solved to analyze the resulting localized structures of pump KAW and magnetic power spectrum in the magnetopause regime. Numerically calculated spectrum exhibits inertial range having spectral index of \(-3/2\) followed by steeper scaling; this steepening in the turbulent spectrum is a signature of energy transportation from larger to smaller scales. In this way, the proposed mechanism, which is based on nonlinear wave-wave interaction, may be useful for understanding the particle acceleration and turbulence in magnetopause.

Alfvén wave propagation through a moderate-amplitude transverse inhomogeneity in a magnetized plasma

Parallel propagation of a plane Alfvén wave in a moderate-amplitude Gaussian-shaped transverse inhomogeneity is studied numerically using a fluid model retaining low-frequency kinetic effects. It is shown that in such a situation, common in the solar wind where elongated pressure-balanced structures are frequently observed, phase mixing competes with wave focusing, in contrast with coronal loops or auroral regions where sharp gradients present at the edges of the inhomogeneities make phase mixing dominant. Some understanding about this competition is provided by a model based on an envelope formalism. Depending on the magnitude of the Alfvén wavelength and of the inhomogeneity transverse scale relative to the ion inertial length, various regimes can develop, ranging from the formation of localized gradients when phase mixing dominates to the development of an intense magnetic filament when focusing is stronger, with a different efficiency for the generation of magnetosonic and kinetic Alfvén waves. Electron parallel heating and parallel electric field generation are also briefly discussed.

Nature of turbulence, dissipation, and heating in space plasmas: From Alfvén waves to kinetic Alfvén waves

AbstractThe nature of turbulence, dissipation, and heating in plasma media has been an attractive and challenge problem in space physics as well as in basic plasma physics. A wide continuous spectrum of Alfvénic turbulence from large MHD‐scale Alfvén waves (AWs) in the inertial turbulence regime to small kinetic‐scale kinetic AWs (KAWs) in the dissipation turbulence regime is a typical paradigm of plasma turbulence. The incorporation of current remote observations of AWs in the solar atmosphere, in situ satellite measurements of Alfvénic turbulence in the solar wind, and experimental investigations of KAWs on large plasma devices in laboratory provides a chance synthetically to study the physics nature of plasma turbulence, dissipation, and heating. A session entitled “Nature of Turbulence, Dissipation, and Heating in Space Plasmas: From Alfvén Waves to Kinetic Alfvén Waves” was held as a part of the twelfth Asia Oceania Geosciences Society Annual Meeting, which took place in Singapore between 2 and 7 August 2015. This special section is organized based on the session.

Small amplitude Kinetic Alfven waves in a superthermal electron–positron–ion plasma

We are investigating the propagating properties of coupled Kinetic Alfven-acoustic waves in a low beta plasma having superthermal electrons and positrons. Using the standard reductive perturbation method, a nonlinear Korteweg-de Vries (KdV) type equation is derived which describes the evolution of Kinetic Alfven waves. It is found that nonlinearity and Larmor radius effects can compromise and give rise to solitary structures. The parametric role of superthermality and positron content on the characteristics of solitary wave structures is also investigated. It is found that only sub-Alfvenic and compressive solitons are supported in the present model. The present study may find applications in a low β electron–positron–ion plasma having superthermal electrons and positrons.

Kinetic Alfvén waves in three‐dimensional magnetic reconnection

AbstractAlfvénic waves are believed to be fundamentally important in magnetic reconnection. Kinetic dynamics of particles can break the Alfvén speed limit in the evolution and propagation of perturbations during reconnection. In this paper, the generation and signatures of kinetic Alfvén waves (KAWs) associated with magnetic reconnection in a current sheet is investigated using a three‐dimensional (3‐D) hybrid code under a zero or finite guide field. In order to understand the wave structures in the general cases of multiple X line reconnection, cases with a single X line of various lengths are examined. The KAWs are identified using the wave dispersion relation, electromagnetic polarization relations, as well as spectral analysis. In the cases in which the X line is so long to extend through the entire simulation domain in the current direction, quasi 2‐D configurations of reconnection are developed behind a leading flux/plasma bulge. KAWs with perpendicular wave number k⊥ρi∼1 (with ρi being the ion Larmor radius) are found throughout the transient plasma bulge region and propagate outward along magnetic field lines with a slightly super‐Alfvénic velocity. These KAWs are generated from the X line and coexist with the whistler structure of the ion diffusion region under a small guide field. In the cases in which the X line has a finite length 2ξ ∼ 10di, with ξ being the half length of the X line and di the ion inertial length, the KAWs originated from the X line are of 3‐D nature. Under a finite guide field, KAWs propagate along the oblique magnetic field lines into the unperturbed regions in the current direction, carrying parallel electric field and Poynting fluxes. The critical X line length for the generation of 3‐D‐like structures is found to be 2ξc≤30di. The structure, propagation, energy, spectrum, and damping of the KAWs are examined. Dependence of the structure of KAWs on the guide field is also investigated.

Potential role of kinetic Alfvén waves and whistler waves in solar wind plasmas

Spacecraft observations indicate the signatures of highly oblique kinetic Alfven waves (KAWs) and whistler waves in the solar wind plasma. In the present work, we explore the possible role of KAWs and whistler waves in the observed solar wind magnetic turbulent spectrum. The nonlinear spatial evolution of KAW is studied including the effects of the ponderomotive force which results in intense localized structures due to the background density modification. Weak quasi-transverse whistler wave propagating through these localized structures also gets localized in the form of small-scale localized structures. We present numerically calculated magnetic power spectra for both KAW as well as for whistler wave. Our obtained results demonstrate the important role that KAWs and whistler waves play in the energy cascading from larger to smaller scales. The relevance of these results to recent spacecraft observations is also pointed out.

Evidence of kinetic Alfvén eigenmode in the near‐Earth magnetotail during substorm expansion phase

AbstractUnipolar pulses of kinetic Alfvén waves (KAW) are first observed in the near‐Earth plasma sheet (NEPS) associated with dipolarizations during substorm expansion phases. Two similar events are studied with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations during substorms on 3 February 2008 and 7 February 2008. The unipolar pulses were located at a trough‐like Alfvén speed profile in the northern plasma sheet at a distance of 10–11 RE from Earth. The dominant wave components consist of a southward δEz toward the neutral plane and a +δBy toward the dusk. The |δEz|/|δBy| ratio was in the range of a few times the local Alfvén speed, a strong indication of KAW nature. The wave Poynting flux was earthward and nearly parallel to the background magnetic field. The pulse was associated with an earthward field‐aligned current carried by electrons. These observational facts strongly indicate a KAW eigenmode that is confined by the plasma sheet but propagates earthward along the field line. The KAW eigenmode was accompanied by short timescale (1 min) dipolarizations likely generated by transient magnetotail reconnection. The observed polarity of the KAW field/current is consistent with that of the Hall field/current in magnetic reconnection, supporting the scenario that the Hall fields/current propagate out from reconnection site as KAW eigenmodes. Aurora images on the footprint of THEMIS spacecraft suggest that KAW eigenmode may power aurora brightening during substorm expansion phase.

Excitation of dust kinetic Alfven waves by semi-relativistic ion beams

The growth rates for dust kinetic Alfvén wave (DKAW) based on semi-relativistic Maxwellian distribution function are investigated in a hot and magnetized plasma. The dispersion relation of DKAW is obtained on a dust acoustic velocity branch, and the kinetic instability due to cross-field semi-relativistic ion flow is examined by the effect of dust parameters. Analytical expressions are derived for various modes as a natural consequence of the form of the solution, and is shown through graphical representation that the presence of dust particles and the cross-field semi-relativistic ions sensibly modify the dispersion characteristics of low-frequency DKAW. The results are valid for a frequency regime well below the dust cyclotron frequency. We suggest that semi-relativistic particles are an important factor in the growth/damping of DKAWs. It is also found that relativistic effects appear with the dust lower hybrid frequency are more effective for dust kinetic Alfvén waves in the perpendicular component as compared to the parallel one. In particular, the relativistic effects associated with electrons suppress the instability while ions enhance the growth rates. The growth rates are significantly modified with dust parameters and streaming velocity of cross-field ions.

Analytical and numerical treatment of resistive drift instability in a plasma slab

An analytic approach combining the effect of equilibrium diamagnetic flows and the finite ionsound gyroradius associated with electron−ion decoupling and kinetic Alfven wave dispersion is derived to study resistive drift instabilities in a plasma slab. Linear numerical computations using the NIMROD code are performed with cold ions and hot electrons in a plasma slab with a doubly periodic box bounded by two perfectly conducting walls. A linearly unstable resistive drift mode is observed in computations with a growth rate that is consistent with the analytic dispersion relation. The resistive drift mode is expected to be suppressed by magnetic shear in unbounded domains, but the mode is observed in numerical computations with and without magnetic shear. In the slab model, the finite slab thickness and the perfectly conducting boundary conditions are likely to account for the lack of suppression.

SUBPROTON-SCALE CASCADES IN SOLAR WIND TURBULENCE: DRIVEN HYBRID-KINETIC SIMULATIONS

ABSTRACT A long-lasting debate in space plasma physics concerns the nature of subproton-scale fluctuations in solar wind (SW) turbulence. Over the past decade, a series of theoretical and observational studies were presented in favor of either kinetic Alfvén wave (KAW) or whistler turbulence. Here, we investigate numerically the nature of the subproton-scale turbulent cascade for typical SW parameters by means of unprecedented high-resolution simulations of forced hybrid-kinetic turbulence in two real-space and three velocity-space dimensions. Our analysis suggests that small-scale turbulence in this model is dominated by KAWs at β ≳ 1 and by magnetosonic/whistler fluctuations at lower β. The spectral properties of the turbulence appear to be in good agreement with theoretical predictions. A tentative interpretation of this result in terms of relative changes in the damping rates of the different waves is also presented. Overall, the results raise interesting new questions about the properties and variability of subproton-scale turbulence in the SW, including its possible dependence on the plasma β, and call for detailed and extensive parametric explorations of driven kinetic turbulence in three dimensions.

Electron trapping and acceleration by kinetic Alfvén waves in solar flares

Context. Theoretical models and spacecraft observations of solar flares highlight the role of wave-particle interaction for non-local electron acceleration. In one scenario, the acceleration of a large electron population up to high energies is due to the transport of electromagnetic energy from the loop-top region down to the footpoints, which is then followed by the energy being released in dense plasma in the lower atmosphere.Aims. We consider one particular mechanism of non-linear electron acceleration by kinetic Alfven waves. Here, waves are generated by plasma flows in the energy release region near the loop top. We estimate the efficiency of this mechanism and the energies of accelerated electrons.Methods. We use analytical estimates and test-particle modelling to investigate the effects of electron trapping and acceleration by kinetic Alfven waves in the inhomogeneous plasma of the solar corona.Results. We demonstrate that, for realistic wave amplitudes, electrons can be accelerated up to 10−1000 keV during their propagation along magnetic field lines. Here the electric field that is parallel to the direction of the background magnetic field is about 10 to 103 times the amplitude of the Dreicer electric field. The acceleration mechanism strongly depends on electron scattering which is due to collisions that only take place near the loop footpoints.Conclusions. The non-linear wave-particle interaction can play an important role in the generation of relativistic electrons within flare loops. Electron trapping and coherent acceleration by kinetic Alfven waves represent the energy cascade from large-scale plasma flows that originate at the loop-top region down to the electron scale. The non-diffusive character of the non-linear electron acceleration may be responsible for the fast generation of high-energy particles.

Dispersive Alfvén waves in a plasma with anisotropic superthermal particles

The dispersion of dispersive Alfvén wave in a low β plasma with anisotropic superthermal particles modeled by a bi-nonextensive distribution is derived from a kinetic way. The effect of anisotropic temperature on inertial Alfvén wave is so small that it is negligible. However, it will play an important role on the property of kinetic Alfvén wave (KAW). The numerical results reveal that the presence of superthermal electrons in the small wavenumber limit will lead the damping rate of the KAW bigger than the one with Maxwellian distribution. Whereas, the damping rate of KAW in the large wavenumber limit will decrease with the presence of superthermal electrons. When the effect of electron anisotropic temperature overwhelms the effect of finite ion gyroradius in the small wavenumber regime, the damping rate of KAW grows with the presence of electron temperature anisotropy. On the other hand, when the effects of finite ion gyroradius play a dominant role in the large wavenumber regime, the damping rate of KAW increases with the effective perpendicular and parallel electron temperatures.

Trapping and amplification of quasi-longitudinal whistler wave in kinetic Alfvén wave localized structures

In this work, we have studied the trapping of obliquely propagating (with respect to the ambient magnetic field) weak whistler wave due to inhomogeneity created by 3D kinetic Alfven wave (KAW) in a magnetized plasma (magnetotail region). The nonlinearity arises due to ponderomotive effects associated with 3D KAW, consequently, the background density gets modified. The weak whistler wave propagating in this modified density gets either trapped or localized. The study has been carried out numerically and semi-analytically. The semi-analytical analysis show that the typical scale size of localized 3D KAW is of the order of ion gyroradius and that of the trapped whistler is even less than that. The relevance of the results is also pointed out in the context of the recent CLUSTER observations in magnetized plasmas where whistler waves have been detected along with coherent ion-scale magnetic structures.