Kinetic Alfven Waves Research Articles

Wave Activities Throughout a Low‐Mach Number Quasi‐Parallel Shock: 2‐D Hybrid Simulations

AbstractIn this paper, two‐dimensional hybrid simulations are used to study wave excitation and evolution throughout a low‐Mach number quasi‐parallel shock. Simulation results show that quasi‐parallel fast magnetosonic waves, ion Bernstein waves with harmonics and possible Alfven/ion cyclotron waves can be excited in the upstream region, and their small phase velocities compared to injected flow velocity results in the convection to the shock front where they are mode converted into several groups of downstream waves, including Alfven waves along the directions parallel to downstream average magnetic fields and perpendicular to the shock normal, the quasi‐perpendicular kinetic slow waves and possible kinetic Alfven waves. We suggest that downstream Alfven waves originate from the mode conversion of upstream quasi‐parallel fast magnetosonic waves with left‐hand polarization in the downstream rest frame under helicity conservation, while the downstream left‐hand polarized kinetic slow waves and right‐hand polarized kinetic Alfven waves can be from the upstream quasi‐perpendicular ion Bernstein waves.

Particle-sounding of the spatial structure of kinetic Alfvén waves

Kinetic Alfvén waves (KAWs) are ubiquitous throughout the plasma universe. Although they are broadly believed to provide a potential approach for energy exchange between electromagnetic fields and plasma particles, neither the detail nor the efficiency of the interactions has been well-determined yet. The primary difficulty has been the paucity of knowledge of KAWs’ spatial structure in observation. Here, we apply a particle-sounding technique to Magnetospheric Multiscale mission data to quantitatively determine the perpendicular wavelength of KAWs from ion gyrophase-distribution observations. Our results show that KAWs’ perpendicular wavelength is statistically 2.4pm 0.7 times proton thermal gyro-radius. This observation yields an upper bound of the energy the majority proton population can reach in coherent interactions with KAWs, that is, roughly 5.76 times proton perpendicular thermal energy. Therefore, the method and results shown here provide a basis for unraveling the effects of KAWs in dissipating energy and accelerating particles in a number of astrophysical systems, e.g., planetary magnetosphere, astrophysical shocks, stellar corona and wind, and the interstellar medium.

Contribution of Kinetic Alfvén Waves to Energetic Electron Precipitation From the Plasma Sheet During a Substorm

AbstractEnergetic (≳50 keV) electron precipitation from the magnetosphere to the ionosphere during substorms can be important for magnetosphere‐ionosphere coupling. Using conjugate observations between the THEMIS, ELFIN, and DMSP spacecraft during a substorm, we have analyzed the energetic electron precipitation, the magnetospheric injection, and the associated plasma waves to examine the role of waves in pitch‐angle scattering plasma sheet electrons into the loss cone. During the substorm expansion phase, ELFIN‐A observed 50–300 keV electron precipitation from the plasma sheet that was likely driven by wave‐particle interactions. The identification of the low‐altitude extent of the plasma sheet from ELFIN is aided by DMSP global auroral images. Combining quasi‐linear theory, numerical test particle simulations, and equatorial THEMIS measurements of particles and fields, we have evaluated the relative importance of kinetic Alfvén waves (KAWs) and whistler‐mode waves in driving the observed precipitation. We find that the KAW‐driven bounce‐averaged pitch‐angle diffusion coefficients near the edge of the loss cone are ∼10−6–10−5 s−1 for these energetic electrons. The due to parallel whistler‐mode waves, observed at THEMIS ∼10‐min after the ELFIN observations, are ∼10−8–10−6 s−1. Thus, at least in this case, the observed KAWs dominate over the observed whistler‐mode waves in the scattering and precipitation of energetic plasma sheet electrons during the substorm injection.

Nonlinear Interaction of a 3D Kinetic Alfvén Wave with a Null Point and Turbulence Generation in the Solar Corona

In the present paper, we have studied nonlinear kinetic Alfvén waves (KAWs) in the vicinity of a null point. We have considered the nonlinearity due to ponderomotive effects associated with KAWs in the solar corona. A 3D model equation representing the dynamics of KAWs is developed in this null point scenario. Using numerical methods, we have solved the model equation for solar coronal parameters. The pseudospectral method and the finite difference method have been applied to tackle spatial integration and temporal evaluation, respectively. The outcome of the simulation demonstrates the formation of localized structures. With the evolution of time, these localized structures become more chaotic. Chaotic (turbulent) structures can efficiently transfer energy. The power spectrum of these turbulent structures shows the Kolmogorov spectral index of nearly $-5/3$ in the inertial range followed by a steeper spectrum of nearly −3.3 (in the range of −2 to −4). These structures also lead to the generation of a current sheet. To understand the physics of our model, we have also done a semi-analytical study for our model equation. Semi-analytical calculations reveal that the current sheet structures have scale sizes of the order of the ion gyro-radius. The relevance of this investigation to the current observations by Parker Solar Probe has also been discussed.

Emission of whistler mode radiation with kinetic Alfven wave in burning plasma

Emission of whistler mode radiation with kinetic Alfven wave in burning plasma

Electron Temperature Anisotropy Effects on Alpha/Proton Instability in the Solar Wind

In situ recordings by the solar Wind spacecraft reveal the ubiquitousness of alpha particles, whose drift velocities to the background proton vα are generally less than or equal to the local Alfvén velocity vA. The alpha beam instability plays a significant role in the alpha beam deceleration in the solar wind; nonetheless, the detailed mechanism of deceleration remains unclear. By using the linear Vlasov equation of the PDRK/B0 solver, the present work investigates the kinetic instability caused by both the alpha beam and the electron temperature anisotropy in the solar wind and assesses the effects of the electron temperature anisotropy on such instability. The results show that both anisotropic electrons and alpha beams lead to the excitation of several plasma waves, and the wave frequency, growth rate, and polarization properties are sensitive to the electron temperature anisotropy (Te⊥/Te‖), the parallel electron beta (βe‖), and the alpha beam drift velocity (vα/vA). With an excess parallel temperature Te⊥/Te‖<1, the parallel magnetosonic/whistler (PM/W), parallel Alfvén wave (PAW), and oblique Alfvén/ion cyclotron (OA/IC) instabilities could be generated, while for an excess perpendicular temperature Te⊥/Te‖>1, the PM/W, OA/IC, parallel whistler (PW), and kinetic Alfvén wave (KAW) instabilities could grow. In the region of Te⊥/Te‖<1, the thresholds of the PM/W, PAW, and OA/IC instabilities extend to lower drift velocity vα/vA. In the region of Te⊥/Te‖>1, the thresholds of the PM/W and OA/IC instabilities increase, while those of the PW and KAW instabilities are shifted to lower vα/vA. The current study presents a comprehensive overview for alpha beam instabilities that limit the alpha beam drift velocity in the solar wind.

Mode Conversion of Kinetic Alfvén Waves in the Presence of Proton Beams and Its Role in Plasma Heating in the Solar Corona

The density inhomogeneity is a typical feature in various magnetoplasmas in the corona, where kinetic Alfvén waves (KAWs) are effectively generated and contribute greatly to the inhomogeneous heating of coronal plasmas. Proton beams exist widely in various space and solar plasma environments. In this paper, based on the kinetic Vlasov theory, we investigate the resonant mode conversion of Alfvén waves to KAWs in the presence of proton beams in an inhomogeneous plasma and the plasma heating of KAWs due to wave−particle interactions. It is found that the wave properties of excited KAWs are highly sensitive to the density of proton beams n bi /n 0, the drift velocity of proton beams v bi /v A, the proton-to-electron temperature ratio T i /T e , and the proton-beam-to-proton-temperature ratio T bi /T i . In addition, the maximum heating rate of KAWs Q m /Q 0 obviously increases with increasing n bi /n 0 and/or v bi /v A. As the electron beta β e increases, Q m /Q 0 decreases sharply in the region of 6 × 10−4 < β e < 10−2 and decreases slowly in the region of β e > 10−2. In addition, the applications of KAW dissipation to plasma heating are discussed in the corona and coronal loops. The KAWs associated with perturbed electric field E x ∼ a few V · m−1 are enough to supply the energy loss in the corona and coronal loops. The present results are of significant importance for comprehending the KAW excitation and the particle energization in the corona.

Inhomogeneous Kinetic Alfvén Waves in the Near-Sun Solar Wind

Intervals of intense electromagnetic, broadband plasma waves are reported in the near-Sun solar wind. These waves are identified as kinetic Alfvén waves (KAWs), based on comparison between data and theory for their observed electric- to magnetic-field ratio, and magnetic compressibility, as a function of frequency. In contrast to KAW observations at 1 au, KAWs in the near-Sun solar wind are found to be spatially inhomogeneous, preferentially occurring where the ambient magnetic field experiences strong deviations from the Parker spiral direction, including during magnetic switchbacks and clusters of switchbacks. To the extent that turbulent fluctuations in the solar wind at small scales are characterized as KAWs, the observations reported here create an expectation that KAW-driven particle heating should also be inhomogeneous in the near-Sun solar wind, being strongest in regions of intense KAWs. Finally, a discrepancy between KAW observation and theory is identified at frequencies two decades above the frequency where kinetic effects begin to manifest, and possible origins of this discrepancy are considered.

Electron Energization Signatures in Traveling Kinetic Alfvén Waves at Storm Time Injection Fronts

AbstractThe properties of traveling kinetic Alfvén waves (KAWs) and their role in energizing electrons in the inner magnetosphere during a geomagnetic storm are examined using measurements from the Van Allen Probes and Gyrofluid‐Kinetic Electron (GKE) model simulations. Traveling KAWs occur in the vicinity of energetic plasma injection fronts in association with magnetic field dipolarizations. The KAWs coincide with energized field‐aligned electrons at energies ≲1 keV. By using observational constraints and incorporating hot and cold electron populations, the GKE simulations are able to reproduce the observed energized electron distribution signatures. The modeling results demonstrate the crucial importance of cold electrons for best observational agreement. The results show that the electron response to KAWs can be substantially different for opposing current regions and are a sensitive function of the cold electron relative density.

Energetic electron scattering by kinetic Alfvén waves at strong magnetic field gradients of dipolarization front

Energetic electron scattering and precipitation from the Earth's plasma sheet to the ionosphere is an important contributor to magnetosphere–ionosphere coupling. In this study, we investigate the role of one of the most intense wave emissions, kinetic Alfvén waves (KAWs), in energetic electron scattering. We have evaluated the effect of KAWs on energetic electrons within a curved magnetic field configuration exhibiting sharp cross field gradients. The magnetic field in Earth's magnetotail plasma sheet with an embedded dipolarization front is used as a working example. Taking into account electron bounce motion and perpendicular guiding-center drifts, we have shown that electrons with energies of tens to hundreds of keV can be scattered by KAWs in pitch angle and momentum through Doppler-shifted Landau resonance near the magnetic equator. The bounce-averaged pitch-angle diffusion coefficients for near-loss-cone (∼2°) electrons are on the order of 10–7–10–6 rad2/s for a characteristic KAW amplitude of 1 mV/m and approach the strong diffusion limit of ∼10–4 rad2/s for amplitudes of greater than 10 mV/m. These results suggest that under such ambient conditions, KAWs can pitch-angle scatter energetic electron population into the loss cone. In Earth's plasma sheet, this scattering is, thus, very likely to cause significant precipitation during active times. The diffusion coefficients of energetic electrons at large pitch angles (∼45°–∼80°) are more than two orders of magnitude larger than those of electrons near the loss cone, suggesting that KAWs contribute to isotropization of anisotropic electrons due to adiabatic heating should they drift into the vicinity of the magnetic field gradient.

Tens to Hundreds of keV Electron Precipitation Driven by Kinetic Alfvén Waves During an Electron Injection

AbstractElectron injections are critical processes associated with magnetospheric substorms, which deposit significant electron energy into the ionosphere. Although wave scattering of &lt;10 keV electrons during injections has been well studied, the link between magnetotail electron injections and energetic (≥100 keV) electron precipitation remains elusive. Using conjugate observations between the Electron Loss and Fields Investigation (ELFIN) and Magnetospheric Multiscale (MMS) missions, we present evidence of tens to hundreds of keV electron precipitation to the ionosphere potentially driven by kinetic Alfvén waves (KAWs) associated with magnetotail electron injections and magnetic field gradients. Test particle simulations adapted to observations show that dipolarization‐front magnetic field gradients and associated ∇B drifts allow Doppler‐shifted Landau resonances between the injected electrons and KAWs, producing electron spatial scattering across the front which results in pitch‐angle decreases and subsequent precipitation. Test particle results show that such KAW‐driven precipitation can account for ELFIN observations below ∼300 keV.

A nonlinear wave-based model to study the magnetic turbulence generation associated with the nonlinear evolution of Kinetic Alfven Wave in laser-produced plasma relevant to laboratory and astrophysical plasmas

A nonlinear wave-based model is introduced to understand the behaviour of the ion-scaled magnetic turbulence in the laser-produced plasmas generated during the high-intensity laser-plasma interaction. The coupled model equations are developed for the pump Kinetic Alfven Wave and the low-frequency Magnetosonic Wave by taking into account the ponderomotive nonlinearity. Numerical simulation is performed using the pseudo-spectral and the finite difference method to solve the model equations. The simulation results illustrate the nonlinear evolution of the pump wave at the early stage, which subsequently becomes turbulent. From the simulation results, the time-averaged turbulent power spectrum has been studied. The observed MHD type and KAW type plasma turbulence in different spatial scale lengths are nearly close to the magnetic turbulence reported in Chatterjee et al.12 experimental results. A semi-analytical method is also given to better understand the nonlinear evolution of the pump wave inside the plasma medium. Such studies of nonlinear wave-based models could be crucial in imitating the astrophysical phenomena by laboratory simulations and understanding the inherent essential physics of magnetic turbulence.

Numerical simulations to study nonlinear wave-based interaction and turbulent magnetic field amplification in the laboratory and astrophysical plasmas

A theoretical model is proposed to study the interaction of high-frequency oblique whistler wave (OWW) and low-frequency kinetic Alfvén waves (KAWs). In our previous investigation (Dewan et al 2020 Phys. Plasmas 27 032111), we established the existence of these two plasma modes (OWW and KAW) by virtue of parametric decay instability. For the present investigation, a numerical simulation technique is employed to solve the coupled system of equations. The ponderomotive force exerted by OWW excites the low-frequency KAW. This quasi-static force induces the genesis of density cavitation (regions of accumulation and depletion) in low-frequency KAW in the magnetized plasma. The simulation results give the localization of the OWW, which amplifies with time. At a later time, the structures become chaotic. The energy cascade is presented in terms of the ensemble-averaged power spectrum. We have also developed a semi-analytical model for this wave–wave interaction mechanism to understand the underlying physics of the field localization process.

The Role of O+ and He+ in the Propagation of Kinetic Alfvén Waves in the Earth’s Inner Magnetosphere

Interactions between plasma particles and electromagnetic waves play a crucial role in the dynamics and regulation of the state of space environments. From plasma physics theory, the characteristics of the waves and their interactions with the plasma strongly depend on the composition of the plasma, among other factors. In the case of the Earth’s magnetosphere, the plasma is usually composed of electrons, protons, O+ ions, and He+ ions, all with their particular properties and characteristics. Here, using plasma parameters relevant for the inner magnetosphere, we study the dispersion properties of kinetic Alfvén waves (KAWs) in a plasma composed of electrons, protons, He+ ions, and O+ ions. We show that heavy ions induce significant changes to the dispersion properties of KAWs, such as polarization, compressibility, and the electric-to-magnetic amplitude ratio, and therefore the propagation of KAWs is highly determined by the relative abundance of He+ and O+ in the plasma. These results, when discussed in the context of observations in the Earth’s magnetosphere, suggest that for many types of studies based on theory and numerical simulations, the inclusion of heavy ions should be customary for the realistic modeling of plasma phenomena in the inner magnetosphere or other space environments in which heavy ions can contribute a substantial portion of the plasma, such as planetary magnetospheres and comet plasma tails.

Solar Wind Turbulence Outlined Through Magnetic Islands and Nonlinear Waves

Various space missions and observations over the past decades have provided unexampled details about the nature of solar wind, the acceleration mechanism, and different nonlinear phenomena responsible for energy transfer and turbulence in the interplanetary space. This review focuses on the role of Alfvénic fluctuations—both kinetic Alfvén wave (KAW) and dispersive Alfvén wave (DAW)—in driving solar wind turbulence and magnetic reconnection at 1 AU. The process of filamentation has been studied through a nonlinear coupling system of KAW/IAW (ion acoustic wave) and relatively high-frequency pump KAW (HKAW, i.e., frequency less than ion cyclotron frequency) in the presence of LKAW (low-frequency KAW, i.e., frequency very much less then ion cyclotron frequency) perturbation by formulating their dynamical equations in the presence of ponderomotive force and using the numerical results for the same. A simplified model is presented to have a deeper insight into the evolution pattern using the results of simulation. The formation of coherent structures and current sheets using a numerical and semi-analytical approach is elaborated near the magnetic reconnection sites. In addition to this, the relevance of the generated turbulence is also depicted through the energy spectrum by examining the spectral index which is noticeable in determining the energy cascade down to smaller scales.

Auroral Beads in Conjunction With Kinetic Alfvén Waves in the Equatorial Inner‐Magnetosphere

AbstractAuroral beads are spatially wavy forms routinely seen before the onset of auroral substorms and are closely related to the onset‐related instabilities. To date, the acceleration mechanism of electrons that create auroral beads is not fully determined. Here, we present a fortuitous event when the Van Allen Probe A (RBSP‐A) was in magnetic conjunction with auroral beads. RBSP‐A observed Alfvén waves, locally generated kinetic Alfvén waves (KAWs) and Alfvénic accelerated electrons at several 100 eV. The Alfvén waves and KAWs carried sufficient Poynting flux to power visible aurora and may control the beads' motion. These observations and previous simulations support that the Alfvénic acceleration is the acceleration mechanism of the auroral beads. Specifically, KAWs are generated around the equator and accelerate local cold electrons to several 100 eV. The waves are suggested to propagate to both hemispheres and accelerate electrons to several keV, which directly account for the auroral beads.

Kinetic Heating by Alfvén Waves in Magnetic Shears.

With first-principles kinetic simulations, we show that a large-scale Alfvén wave (AW) propagating in an inhomogeneous background decays into kinetic Alfvén waves (KAWs), triggering ion and electron energization. We demonstrate that the two species can access unequal amounts of the initial AW energy, experiencing differential heating. During the decay process, the electric field carried by KAWs produces non-Maxwellian features in the particle velocity distribution functions, in accordance with space observations. The process we present solely requires the interaction of a large-scale AW with a magnetic shear and may be relevant for several astrophysical and laboratory plasmas.

Observations of kinetic Alfvén waves and associated electron acceleration in the plasma sheet boundary layer

Kinetic Alfvén waves (KAWs), with a strong parallel disturbed electric field, play an important role in the energy transport and particle acceleration in the magnetotail. Based on the high-resolution observations of the Magnetospheric Multiscale (MMS) mission, we present the detailed acceleration process of electrons by KAWs in the plasma sheet boundary layer (PSBL). MMS observed strong electromagnetic disturbances carrying parallel disturbed electric field with an amplitude up to 8 mV/m. The measured ratio of the electric to magnetic field perturbations is larger than the local Alfvén speed and enhances as the frequency increases, in consistent with the theoretical predictions for KAWs. These evidences indicate that the electromagnetic disturbances should be identified as KAWs. During the KAWs, the energy flux of electrons at energies above 1 keV in the parallel and anti-parallel direction significantly enhance, implying occurrences of electron beams at higher energies. Meanwhile, the KAWs become more electrostatic-like and filled with high frequency ion acoustic waves. The energy enhancement of electron beams accords to the derived work done by the observed parallel disturbed electric field of KAWs, indicating electron acceleration caused by KAWs. Therefore, the paper provides a direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.

On Mechanisms of Proton Perpendicular Heating in the Solar Wind: Test Results Based on Wind Observations

The solar wind protons undergo significant perpendicular heating when they propagate in the interplanetary space. Stochastic heating and cyclotron resonance heating due to kinetic Alfvén waves (KAWs) are two proposed mechanisms. Which mechanism accounts for the perpendicular heating is still an open question. This paper performs tests for the two mechanisms based on Wind observations during 2004 June and 2019 May. Results show that heating rates in terms of stochastic heating theory considerably depend on the parameter of plasma β. For the solar wind with moderately high β, the theoretical heating rates are comparable to or larger than empirical heating rates, suggesting that the stochastic heating could be a powerful mechanism. For the solar wind with low β, on the contrary, the majority of data have theoretical heating rates much lower than empirical heating rates, showing that the stochastic heating seems to be weak in this case. On the other hand, it is found that, when the propagation angles of KAWs are around 70°, theoretically predicted damping wavenumbers of KAWs are equal to the observed wavenumbers at which magnetic energy spectra become significantly steep. This may imply that resonance heating due to cyclotron damping of KAWs could be another mechanism if KAWs have propagation angles around 70°.

Periodic kinetic Alfvén waves and overtaking between multi-solitons in nonthermal electron–positron–ion plasma

In this paper, the formation of periodic (cnoidal) and solitary kinetic Alfvén waves (KAWs) in nonthermal electron–positron–ion plasma has been illustrated. The KdV equation has been derived by using the reductive perturbation technique. The Sagdeev potential method is used to obtain the solution of KdV equation for examining the dynamics of periodic KAWs. Further, Hirota bilinear method is applied to examine the overtaking process and the expressions of phase shifts of multi-solitons have been determined. Only positive potential kinetic Alfvénic (KA) solitary and periodic waves are evolved in the given plasma environment. The influence of plasma beta, positron density, angle of propagation, nonthermality of electrons and positrons has been investigated on the behaviour of periodic (cnoidal) KAWs. The combined impact of density, nonthermality of electrons as well as positrons has vastly modified the phase shifts of double-as well as triple-solitons. This investigation may have applications for particle acceleration and energy transportation in galactic nucleus and interstellar medium where nonthermal electron and positron pairs exist.