Electron Landau Resonance Research Articles

Energy Transfer and Electron Heating in Turbulent Flux Pileup Region

AbstractWe report observations of intense energy transfer developed within a flux pileup region (FPR) inside a plasma jet in the Earth's midtail. Strong electromagnetic turbulences, with ( and are wave numbers perpendicular and parallel to background magnetic field respectively), are observed together with intermittent sub‐ion‐scale structures, generating intense fluctuating electric fields and currents inside the FPR. The highly anisotropic turbulence is carried by kinetic Alfvénic waves with ( is ion gyroradius), which are weakly damped due to electron Landau resonance and cause electron heating with a ratio close to 4 eV/s. Concurrent with the ion‐scale turbulences and structures, strong energy conversion occurs inside the FPR (stronger than that at the jet front), with magnetic field energy being dissipated into particle energy, leading to efficient electron acceleration. These results highlight the crucial role played by turbulent FPRs in energy transfer in the magnetosphere.

Ionosphere Feedback to Electron Scattering by Equatorial Whistler Mode Waves

AbstractPlasma sheet electron precipitation, much of it driven by whistler mode waves, is critical for magnetosphere‐ionosphere coupling. This precipitation leads to a secondary electron population at low altitudes, which moves upward along magnetic field lines to the equatorial plasma sheet. We investigate observational evidence for such electron precipitation and the ionospheric feedback provided by the secondary electron outflows. Using the near‐equatorial Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we show that whistler mode wave bursts are accompanied by enhanced field‐aligned electrons. The properties of these electrons are not related to the whistler mode waves, as would have been expected from local wave‐particle interactions. Thus, it is unlikely that the field‐aligned electrons are formed by electron Landau resonance with whistler mode waves. This population can be the secondary electron outflows resulting from plasma sheet electron precipitation to the ionosphere. Combining THEMIS, Cluster, and Fast Auroral Snapshot Explorer (FAST) measurements, we show that this field‐aligned electron population is also observed at low altitudes, where it is associated with high‐frequency electrostatic waves. These low‐altitude waves are likely generated by the secondary electron outflows and, in turn, scatter these electrons outside of the loss cone, allowing them to be observed by the wide field‐of‐view particle instrument on the near‐equatorial THEMIS spacecraft. We discuss how this secondary electron population can subsequently alter whistler mode wave generation and propagation in the magnetosphere.

Electromagnetic particle simulation of the effect of toroidicity on linear mode conversion and absorption of lower hybrid waves

Effects of toroidicity on linear mode conversion and absorption of lower hybrid (LH) waves in fusion plasmas have been studied using electromagnetic particle simulation. The simulation confirms that the toroidicity induces an upshift of parallel refractive index when LH waves propagate from the tokamak edge toward the core, which affects the radial position for the mode conversion between slow and fast LH waves. Furthermore, moving LH antenna launch position from low field side toward high field side leads to a larger upshift of the parallel refractive index, which helps the slow LH wave penetration into the tokamak core. The broadening of the poloidal spectrum of the wave-packet due to wave diffraction is also verified in the simulation. Both the upshift and broadening effects of the parallel spectrum of the wave-packet modify the parallel phase velocity and thus the linear absorption of LH waves by electron Landau resonance.

An assessment of full wave effects on the propagation and absorption of lower hybrid waves

Lower hybrid (LH) waves (Ωci⪡ω⪡Ωce, where Ωi,e≡Zi,eeB/mi,ec) have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons and consequently are well-suited to driving current. Established modeling techniques use Wentzel–Kramers–Brillouin (WKB) expansions with self-consistent non-Maxwellian distributions. Higher order WKB expansions have shown some effects on the parallel wave number evolution and consequently on the damping due to diffraction [G. Pereverzev, Nucl. Fusion 32, 1091 (1991)]. A massively parallel version of the TORIC full wave electromagnetic field solver valid in the LH range of frequencies has been developed [J. C. Wright et al., Comm. Comp. Phys. 4, 545 (2008)] and coupled to an electron Fokker–Planck solver CQL3D [R. W. Harvey and M. G. McCoy, in Proceedings of the IAEA Technical Committee Meeting, Montreal, 1992 (IAEA Institute of Physics Publishing, Vienna, 1993), USDOC/NTIS Document No. DE93002962, pp. 489–526] in order to self-consistently evolve nonthermal electron distributions characteristic of LH current drive (LHCD) experiments in devices such as Alcator C-Mod and ITER (B0≈5 T, ne0≈1×1020 m−3). These simulations represent the first ever self-consistent simulations of LHCD utilizing both a full wave and Fokker–Planck calculation in toroidal geometry.

Electron acceleration in the Van Allen radiation belts by fast magnetosonic waves

Local acceleration is required to explain electron flux increases in the outer Van Allen radiation belt during magnetic storms. Here we show that fast magnetosonic waves, detected by Cluster 3, can accelerate electrons between ∼10 keV and a few MeV inside the outer radiation belt. Acceleration occurs via electron Landau resonance, and not Doppler shifted cyclotron resonance, due to wave propagation almost perpendicular to the ambient magnetic field. Using quasi‐linear theory, pitch angle and energy diffusion rates are comparable to those for whistler mode chorus, suggesting that these waves are very important for local electron acceleration. Since pitch angle diffusion does not extend into the loss cone, these waves, on their own, are not important for loss to the atmosphere. We suggest that magnetosonic waves, which are generated by unstable proton ring distributions, are an important energy transfer process from the ring current to the Van Allen radiation belts.

Kinetic Alfvén waves and electron physics. I. Generation from ion-ion streaming

Short-wavelength kinetic Alfvén waves (KAWs) that propagate at large angles with respect to the magnetic field and interact with both electrons and ions have broad application to laboratory and space plasmas. Using linear Vlasov theory and particle-in-cell (PIC) simulations, the generation of KAWs by ion-ion streaming along the magnetic field at relatively low (2.5× the Alfvén speed) and low plasma beta (ratio of plasma thermal pressure to magnetic pressure ⩽0.1) are investigated. The instability has been examined previously using linear theory and a hybrid simulation method in which the ions are treated kinetically and the electrons as an adiabatic fluid. In this work it is found that when the electron Landau resonance factor or equivalently the ratio of the Alfvén speed to the electron thermal speed is large enough, the interaction of KAWs with the electrons produces strong parallel electron heating. The electron parallel heating in turn increases both the wave growth rates and the range of unstable modes of the instability, leading to enhanced saturated wave levels, increased perpendicular ion heating and larger spatial fluctuations in the drift speeds of the ions that appear as greater reductions of relative ion streaming in localized regions, compared to results from hybrid simulations. The interaction of KAWs with the electrons shifts from the bulk to the tail of the parallel velocity distribution at larger values of the resonant factor, corresponding to situations at lower electron beta or more obliquely propagating waves. Ion-to-electron mass ratio effects are considered together with the scaling to electron beta and the resonant factor in order to apply the results to systems of interest. In particular, KAWs and electron kinetics are discussed in the context of oblique slow shock formation and structure [Yin et al., Phys. Plasmas 14, 062105 (2007)].

Alfvénic Electron Acceleration in Aurora Occurs in Global Alfvén Resonosphere Region

Auroral emission caused by electron precipitation (Hardy et al., 1987, J. Geophys. Res. 92, 12275–12294) is powered by magnetospheric driving processes. It is not yet fully understood how the energy transfer mechanisms are responsible for the electron precipitation. It has been proposed (Hasegawa, 1976, J. Geophys. Res. 81, 5083–5090) that Alfven waves coming from the magnetosphere play some role in powering the aurora (Wygant et al., 2000, J. Geophys. Res. 105, 18675–18692, Keiling et al., 2003, Science 299, 383–386). Alfven-wave-induced electron acceleration is shown to be confined in a rather narrow radial distance range of 4–5 R E (Earth radii) and its importance, relative to other electron acceleration mechanisms, depends strongly on the magnetic disturbance level so that it represents 10% of all electron precipitation power during quiet conditions and increased to 40% during disturbed conditions. Our observations suggest that an electron Landau resonance mechanism operating in the “Alfven resonosphere” is responsible for the energy transfer.

Different Alfvén wave acceleration processes of electrons in substorms at ~4-5 <i>R<sub>E</sub></i> and 2-3 <i>R<sub>E</sub></i> radial distance

Abstract. Recent statistical studies show the existence of an island of cavities and enhanced electric field structures at 4-5RE radial distance in the evening and midnight magnetic local time (MLT) sectors in the auroral region during disturbed conditions, as well as ion beam occurrence frequency changes at the same altitude. We study the possibility that the mechanism involved is electron Landau resonance with incoming Alfvén waves and study the feasibility of the idea further with Polar electric field, magnetic field, spacecraft potential and electron data in an event where Polar maps to a substorm over the CANOPUS magnetometer array. Recently, a new type of auroral kilometric radiation (AKR) emission originating from ~2-3RE radial distance, the so-called dot-AKR emission, has been reported to occur during substorm onsets and suggested to also be an effect of Alfvénic wave acceleration in a pre-existing auroral cavity. We improve the analysis of the dot-AKR, giving it a unified theoretical handling with the high-altitude Landau resonance phenomena. The purpose of the paper is to study the two types of Alfvénic electron acceleration, acknowledging that they have different physical mechanisms, altitudes and roles in substorm-related auroral processes.

Alfvén‐cyclotron fluctuations: Linear Vlasov theory

Linear Vlasov dispersion theory for a homogeneous, collisionless electron‐proton plasma with Maxwellian velocity distributions is used to examine the damping of Alfvén‐cyclotron fluctuations. Fluctuations of sufficiently long wavelength are essentially undamped, but as k∥, the wave vector component parallel to the background magnetic field Bo, reaches a characteristic dissipation value kd, the protons become cyclotron resonant and damping begins abruptly. For proton cyclotron damping, kdc/ωp ∼ 1 for 10−3 ≲ βp ≲ 10−1, where βp ≡ 8πnpkBTp/Bo2 and ωp/c is the proton inertial length. At k∥ < kd, me/mp < βe, and βp ≲ 0.10 the electron Landau resonance becomes the primary contributor to fluctuation dissipation, yielding a damping rate that scales as ωr (k⊥c/ωp)2, where ωr is the real frequency and k⊥ is the wave vector component perpendicular to Bo. As βp increases from 0.10 to 10, the proton Landau resonance makes an increasing contribution to damping of these waves at k∥ < kd and 0° < θ < 30°, where θ = arccos( · o). The maximum damping rate due to the proton Landau resonance scales approximately as βp(kc/ωp)2 over 0.50 ≤ βp ≤ 10. Both magnetic transit time damping and electric Landau damping may contribute to Landau resonant dissipation; in the electron Landau resonance regime the former is important only at propagation almost parallel to Bo, whereas proton transit time damping can be relatively important at both quasi‐parallel and quasi‐perpendicular propagation of Alfvén‐cyclotron fluctuations.

Kinetic Alfvén waves: Linear theory and a particle‐in‐cell simulation

An Alfvén‐cyclotron fluctuation of sufficiently short wavelength has a strong proton cyclotron resonance at propagation parallel to the background magnetic field Bo in a homogeneous, collisionless electron‐proton plasma. As k∥, the wavevector component parallel to Bo, decreases, the proton cyclotron wave‐particle interaction becomes nonresonant, and the electron Landau resonance becomes effective at propagation oblique to Bo. Here linear Vlasov theory is used to determine the dispersion and damping properties of Alfvén‐cyclotron fluctuations associated with the transition from the proton cyclotron resonance regime to the electron Landau resonance regime. Also, a particle‐in‐cell plasma simulation is used to examine the electron response to the initial imposition of an Alfvén‐cyclotron wave in the electron Landau resonance regime. The computation shows heating of the electrons in the direction parallel to Bo and the formation of a beam in the direction of the parallel component of k.

Destabilitization of low mode number Alfvén modes in a tokamak by energetic or alpha particles

With the inclusion of finite Larmor radius effects in the shear Alfvén eigenmode equation, the continuous Alfvén spectrum, which has been extensively discussed in ideal magnetohydrodnamics, is removed. Neutrally stable, discrete radial eignmodes appear in the absence of sources of free energy dand dissipation. Alpha (or energetic) particle toroidal drifts destabilize these modes, provided the particles are faster than the Alfvén speed. Although the electron Landau resonance contributes to damping, a stability study of the parametric variation of the energy and the density scale length of the energetic particles shows that modes with low radial mode numbers remain unstable in most cases. Since the alpha particles are concentrated in the center of the plasma, this drift-type instability suggests anomalous helium ash diffusion. Indeed, it is shown that stochasticity of alpha orbits due to the overlapping of radially neighboring Alfvén resonances is induced at low amplitudes, eiφ/Ti≳0.05, implying a diffusion coefficient Drα≳4.4×103 cm2/sec.

Destabilization of drift-universal eigenmodes by toroidal effects

An eigenvalue equation representing the drift-universal instability in toroidal geometry is investigated numerically. It is demonstrated that the form of passing electron Landau resonance appropriate to toroidal geometry can destabilize the slab drift wave.

Localized drift-Alfvén mode instability due to the current density gradient in plasmas

A new mode produced by coupling of the Alfvén mode and the drift mode is found to become unstable due to the current density gradient which causes a frequency downshift and a resulting electron Landau resonance enhancement. This instability is identified with that found numerically by Ueno et al. [1].

Magnetic reconnection in collisionless plasmas

A class of processes involving magnetic field reconnection, in collisionless plasmas and magnetic configurations where the field undergoes a finite change of direction, is investigated. Reconnecting modes that rely on the effects of electron Landau resonance and density gradient for their excitation are found to require the analytical or numerical treatment of four consecutive asymptotic regions. The influence of finite electron temperature gradient in the region where the effects of electron Landau resonance prevail, and the convection of energy toward the region where ion Landau resonance is dominant tend to dampen these modes. Conversely, significant distortions of the ion distribution can follow their excitation. The relevance of the obtained results to experimental observations on laboratory plasmas and in space physics is discussed. Different processes are involved with magnetic reconnection in magnetic configurations where the field does not have appreciable shear but has a neutral surface on which it vanishes.

Reconnecting Modes in Collisonless Plasmas

Processes involving magnetic-field reconnection in collisionless plasmas and relying on the effects of electron Landau resonance for their excitation are found to require the consideration of the self-consistent electric potentials and the systematic analysis of four consecutive asymptotic regions. The new effects found are relevant to both laboratory and space plasmas.

Turbulent Destabilization and Saturation of the Universal Drift Mode in a Sheared Magnetic Field

In a sheared magnetic field, turbulent diffusion of electrons in the vicinity of a mode rational surface can eliminate the stabilizing influence of nonresonant electrons and lead to an absolute instability at small but nonzero wave amplitudes. As the turbulence grows, the inverse electron Landau resonance is broadened in both velocity and configuration space, and the convective shear damping due to ions is enhanced by turbulent spatial broadening of the mode until saturation occurs.

Stability of drift waves in axisymmetric toroidal plasmas

The two-dimensional eigenvalue equation for electrostatic drift waves in axisymmetric toroidal geometry is investigated. A model version, relevant for the Culham levitron, is constructed, and solutions are obtained when poloidal variations of shear, curvature and magnetic field are included. General criteria for the existence of localized undamped eigenmodes are established, and it is found that for sufficiently strong modulations of various equilibrium quantities, the stabilizing effect of magnetic shear is completely nullified. Equivalent criteria are obtained for the large-aspect-ratio tokamak. Investigation of the electron Landau resonance in strongly modulated magnetic fields indicates that for electron drift waves the growth rate will be only logarithmically weaker than in the equivalent slab-model calculation.

Stabilization of the trapped-electron mode by a collisionally broadened Landau resonance

The non-local theory of the dissipative trapped-electron instability, including magnetic shear, is extended to higher electron collisionality. In regions sufficiently close to a rational magnetic surface the trapped-electron mode becomes a ‘collisionless’ drift mode, which is Landau-damped (where η ≡ ∂ ln Te/∂ ln ne > 0). The collisional broadening of the untrapped electron Landau resonance makes this an effective stabilization mechanism, even when the parallel phase velocity lies in the trapped-particle region of parallel velocities. For the normal temperature gradient (η > 0) and for large aspect ratio (ϵ−1 > 1) a sufficient condition for stability is obtained that is independent of shear: ϵ½ η1/3 < 1. For the inverted gradient case (η < 0) a shear stabilization criterion is derived.