Landau Resonance Research Articles

Magnetic flux generation and wave emissions during coalescence of magnetic islands in pair plasmas.

It is shown by using a two-dimensional fully relativistic electromagnetic particle-in-cell code that the tearing instability in a current sheet of pair plasmas is caused by Landau resonances of both electrons and positrons. Strong magnetic flux can be generated during coalescence of magnetic islands in the nonlinear phase of the tearing instability. The magnetic flux produced in an O-type magnetic island is caused by the counterstreaming instability found by Kazimura et al. [Astrophys. J. Lett. 498, L183 (1998); J. Phys. Soc. Jpn. 67, 1079 (1998)]. It is also shown that charge separation with a quadrupole-like structure is generated from the localized strong magnetic flux. During the decay of the quadrupole-like charge structure as well as the magnetic flux, there appear wave emissions with high-frequency electromagnetic waves and Alfvén waves as well as Langmuir waves.

Solar3He‐rich Events and Electron Acceleration in Two Stages

The heating, acceleration, and enhancement of electrons in solar 3He-rich events are studied according to the two-stage acceleration model proposed for the enhancement of 3He and heavy ions. First, electrostatic ion cyclotron instabilities destabilized by electron currents are theoretically and numerically investigated for a plasma consisting mainly of H, 4He, and electrons. Since the parallel phase velocities of the ion cyclotron waves are smaller than around the electron thermal velocity, electrons can have strong Landau resonance (?r-k?ve, ?=0) with the ion cyclotron waves. Here ?r and k? are, respectively, the real frequency and wavenumber parallel to the external magnetic field and ve, ? is the parallel electron velocity. Second, preferential heating of electrons by the ion cyclotron waves is studied through the quasi-linear theory. It is shown that H cyclotron waves are very efficient at heating electrons through the Landau resonance. Third, these preheated electrons can be further accelerated to high energies in the Fermi acceleration process. The abundance ratio (e/H) of high-energy electrons and protons is estimated. The abundance ratio is very high when electrons are sufficiently heated relative to protons and agrees well with observations. Therefore, the present two-stage model explains all aspects of acceleration (3He, electrons, and heavy ions) in flares in a self-consistent way.

Comparison between the Landau and Cyclotron Resonances in the Electron Beam-Plasma Interactions

It is well-known that a cool field-aligned electron beam streaming in a plasma can destabilize electrostatic waves, in the frequency ranges of 0<ω< m i n (Ω e ,ω p ) and m a x (Ω e ,ω p )<ω<ω uh , where ω p , Ω e , and ω uh are the plasma, the electron cyclotron, and the upper-hybrid frequencies, respectively. Although both the Landau and cyclotron resonances lead to the generation of the waves, not much attention has been paid to the wave generation via the cyclotron resonance. In this paper we compare the two types of interactions for both non-relativistic and relativistic electron beams by considering a simple plasma model which consists of main and beam electrons and background ions. The plasma is charge neutral and current carrying. The linear theory based on this model predicts that, when the electron beams are non-relativistic, the waves are generated predominantly via the Landau interaction for a wide range of plasma parameters, except for a limited parameter regime with cool main electrons, Ω e /...

Limits of applicability of the quasilinear approximation to the lower hybrid wave-plasma interaction

The diffusion of electrons in velocity space induced by a spectrum of electrostatic waves is numerically investigated. From the scatter of particles, the diffusion coefficient is calculated and compared with the quasilinear value. A strong disagreement is found at high electric field amplitudes, narrow spectra and short (spatial) lengths of the interaction region, which are however usual in Lower Hybrid Heating and Current Drive experiments in tokamaks. In particular, the (velocity) interaction region becomes significantly broader than the Landau resonance region for electric field amplitudes above .

Magneto-optics of InAs/Ga1−xInxSb infrared superlattice diodes

Spectrally resolved measurements of the responsivity of infrared photodiodes based on InAs/(GaIn)Sb superlattices (SL) were performed in applied magnetic fields. For the field oriented parallel to the growth axis, interband Landau transitions related to both the center and the edge of the SL Brillouin zone in the growth direction were observed, in accordance with the parity selection rules that are expected for the type II system under consideration. For the field oriented perpendicular to the growth axis, the interband Landau resonances are broadened and the widening of the band gap is smaller.

Landau resonance mechanism for plasma and wind-generated water waves

A profound analogy between the Landau damping of plasma waves and the resonant mechanism of wave generation on a water surface by wind is discussed. A new simple derivation of the former effect is presented which explicitly demonstrates the essential role of the integrals of motion (the vorticity for an inviscid fluid, and the particle distribution function for a collisionless plasma) in these phenomena.

High altitude trapping of beam ions by self‐generated plasma waves in interhemispheric plasma flows

Counterstreaming ion beams originating from the ionosphere are a natural product of interhemispheric plasma flow in empty plasmaspheric flux tubes after geomagnetic storms. Owing to the prevailing collisionless conditions during the early stage of the flow, trapping of the plasma in the flow at relatively high altitudes has remained a mystery. Using 2.5‐D particle‐in‐cell simulations, a study of the ion‐beam driven plasma instabilities indicates that the fastest growing wave mode is an effective source for the pitch angle scattering of the ions transferring part of the parallel energy in the magnetic field‐aligned flows into the ions' perpendicular energy. The pitch angle scattering is facilitated by the anomalous cyclotron resonance of the beam ions with the fastest growing waves driven by the interaction between the Doppler‐shifted cyclotron modes of the two counterstreaming ion beams; the interaction generates purely growing waves at nearly zero frequency. The Landau resonance of the Doppler‐shifted cyclotron mode of one of the beams with the other beam, and vice versa, also generates waves which are relatively slow growing at frequencies ω ≅nΩi /2, where n is an integer and Ωi is the ion cyclotron frequency. Since these waves appear at ω ≅Ωi in the rest frame of the beams, they transversely accelerate the ions. Both the pitch angle scattering and the transverse acceleration contribute to the trapping of the flow in the flux tubes, but the former being a much faster and more effective process than the latter. This collisionless trapping at high altitudes, facilitated by the self‐generated waves, can be a major process in plasmaspheric refilling.

Electron Acceleration from Electron-Ion Plasma Cloud Moving across a Magnetic Field

The dynamics of the electron-ion plasma cloud moving perpendicular to an ambient magnetic field in the background plasma is investigated by means of a three-dimensional, electromagnetic, and relativistic particle simulation code. The simulation results show that the cloud separates into electrons and ions due to their gyromotions with Larmor radius of V 0 /ω c j ( V 0 is the initial cloud velocity and ω c j is the cyclotron frequency of species j ). Consequently, electromagnetic and whistler waves are excited from the cloud due to coherent gyration of cloud electrons. In addition, Langmuir waves are also excited along a magnetic field. It is shown that cloud electrons are accelerated by Langmuir waves along the magnetic field and the population inversion is created in the velocity distribution of cloud electrons. The acceleration mechanism of cloud electrons is due to Landau resonance between Langmuir waves and cloud electrons. We apply our results to the electron acceleration mechanism from plasma cloud ...

Electron energization by lower-hybrid waves as a possible source for x-ray emission from comets

A goal of this presentation is to demonstrate that recently observed x-ray emission from comets can be interpreted as a combination of bremsstrahlung and line k-shell radiation produced by interaction of energetic electrons with the cometary atmosphere. Electron energization inside the cometary bow-shock is due to mass-loading of the shocked solar wind flow by newly born heavy cometary ions created by photoionization of the neutral gas outflow. The new ion component in the crossed E × B fields of the host (solar wind) plasma moves in cycloidal orbits, forming an unstable ring distribution, which drives the modified two stream instability and excites lower hybrid waves. These waves have the phase velocity along the magnetic field much larger than across it. For this reason, they can be in simultaneous Landau resonance with both slow unmagnetized ions and fast, but magnetized (field aligned), electrons efficiently transferring ion energy to the electrons and leading to their energization. We have investigated this process in a quasilinear approximation. By solving the Fokker-Planck equation for the field aligned electron energy diffusion, we have been able to calculate fluxes and energy spectra of electrons accelerated by the lower hybrid waves and to determine spectrum and total luminosity of x-ray radiation. Results of theoretical analysis are compared with ROSAT and Beppo-Sax observations of x-ray emission from the different comets.

The radial electric field dynamics in the neoclassical plasmas

A numerical simulation and analytical theory of the radial electric field dynamics in low collisional tokamak plasmas are presented. An initial value code “ELECTRIC” has been developed to solve the ion drift kinetic equation with a full collisional operator in the Hirshman–Sigmar–Clarke form together with the Maxwell equations. Different scenarios of relaxation of the radial electric field toward the steady-state in response to sudden and adiabatic changes of the equilibrium temperature gradient are presented. It is shown, that while the relaxation is usually accompanied by the geodesic acoustic oscillations, during the adiabatic change these oscillations are suppressed and only the magnetic pumping remains. Both the collisional damping and the Landau resonance interaction are shown to be important relaxation mechanisms. Scalings of the relaxation rates versus basic plasma parameters are presented.

Toward a consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines

Simultanous Freja observations of precipitating keV electrons and electromagnetic emissions around half the local proton gyrofrequency are analyzed. The observed wave fields are used for reconstructing the energy density in wave vector space, the wave distribution function (WDF), and detailed observations of the electron distribution are used to analyze the linear stability of the plasma. We find that a local Landau resonance with precipitating electrons can generate electromagnetic waves having their wave vector (κ) pointing obliquely relative to the background magnetic field. However, the observed parallel electron drift energy suggests that the unstable frequencies are located above the peak in the observed power spectrum and that the κ spectrum predicted from the linear stability analysis does not overlap with the reconstructed WDF. This apparent inconsistency between the proposed instability mechanism and the available Freja data is resolved by placing the source region within 2400 km above Freja. By using a ray‐tracing mapping of an instability at an assumed generation altitude down to Freja altitudes, we show that an electron Landau instability occurring above Freja can be made consistent with the observed wave frequency as well as the reconstructed WDF. The agreement that we obtain between data and theory leads to consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines.

Guided plasmaspheric hiss interactions with superthermal electrons: 1. Resonance curves and timescales

Under the proper conditions, guided plasmaspheric hiss is shown to be more efficient than Coulomb collisions at scattering electrons in the superthermal energy range of 50 to 500 eV. Broadband, whistler mode hiss becomes guided by plasma density gradients, intensifying the wave energy densities and focusing the wave normal angles. These waves are shown to interact through Cherenkov (Landau) resonance with electrons below 500 eV, and the presented equatorial plane timescales for pitch angle, energy, and mixed diffusion are shown to be faster than Coulomb collision timescales for typical values at the inner edge of the plasmapause and in detached plasma regions. In the latter case, energy diffusion timescales of less than 100 s for small pitch angle electrons between 250 and 500 eV indicate that these waves have the potential to dramatically change the distribution function.

Global structure of mirror modes in the magnetosheath

A global stability analysis of mirror modes in the magnetosheath is presented. The analysis is based upon the kinetic‐MHD formulation which includes relevant kinetic effects such as Landau resonance and gradient drift effects related to inhomogeneities in the background density, temperature, pressure and its anisotropy, magnetic field, and plasma flow velocity. Pressure anisotropy provides the free energy for the global mirror mode. The local theory of mirror modes predicts purely growing modes in the frame of the moving plasma confined to the unstable magnetosheath region; however, the nonlocal theory that includes the effects of gradients and plasma flow predicts modes with real frequency which propagate with the flow from the magnetosheath toward the magnetopause boundary. The real frequency is of the order of a combination of the diamagnetic drift frequency and the Doppler shift frequency associated with plasma flow. The real frequency is not simply a Doppler‐shifted frequency obtained from local calculation. Rather, it is determined from a boundary value constraint on an eigenvalue problem and can differ significantly from the local Doppler‐shifted frequency. Without plasma flow, the diamagnetic drift frequency provides a wave phase velocity in the direction of the magnetopause so that wave energy accumulates against the magnetopause boundary, and the amplitude is skewed in that direction. Such a wave structure might be observed under conditions of small magnetosheath flow. For larger plasma flow (Alfvén Mach number ≥ 0.1), the flow also causes a real phase velocity in the direction of the flow, but the phase velocity increases from a small value near the bow shock to nearly the flow velocity as the wave propagates toward the magnetopause. As a result, wave amplitude is large where the phase velocity is small and small where the phase velocity is large resulting in a skew in wave amplitude toward the bow shock.

Perpendicular ion acceleration in field‐aligned density striations

A basic process similar to transit‐time absorption is evaluated as a possible source of fast perpendicular ions under conditions typically encountered during sounding rocket flights in the auroral ionosphere. Electric fields localized within density striations aligned with the geomagnetic field have a nonuniform perpendicular amplitude that results in irreversible energy absorption. The efficiency of the process depends on the ratio of the ion Larmor radius to the perpendicular scale length of the amplitude gradient and on the scaled frequency ν ≅ ω/Ωi. This absorption mechanism does not depend on a Landau resonance and does not require the electric field to be larger than a threshold value, because it is active in the linear regime. Theory, numerics, and particle‐in‐cell simulations demonstrate the process for electrostatic fields with frequencies a few times the ion gyrofrequency, but the process does not depend explicitly on the fluctuations being lower hybrid waves. Calculations of the evolving ion distribution function due to coherent electric fields suggest that the effect of spatial localization should be considered in the analysis of fast ions of geophysical origin.

Perpendicular ion acceleration by localized high frequency electric fields in magnetized plasmas

A basic process capable of explaining observations of fast perpendicular ions in a wide range of plasma environments is described. Spatial symmetry breaking perpendicular to the confining magnetic field is shown to cause irreversible energy gain for ions gyrating through an electric field having a nonuniform amplitude. The efficiency depends on the ratio of the ion Larmor radius to the scale length of the amplitude gradient, and on the scaled frequency ν≡ω/Ωi. A Landau resonance is not required, and there is no lower threshold on the electric field, because the mechanism is active in the linear regime. Theory, numerics, and particle-in-cell simulations are used to illustrate the interaction for electrostatic fields in the lower-hybrid range of frequencies, but the process does not depend on a particular type of mode.

Whistler absorption and electron heating near the plasmapause

Using the HOTRAY code, we demonstrate that lightning‐generated whistlers which enter the magnetosphere over a broad range of latitudes (Δλ ≈ 5°) just inside the plasmapause are strongly focused by the steep plasma density gradient into a narrow range of L shells near the equatorial region. The wave normal angle also remains closely aligned (±20°) with the magnetic field direction along the entire ray path. Under such conditions, Landau resonance is relatively unimportant, and the wave amplitude is controlled by cyclotron resonant interactions with energetic electrons. All waves with frequencies comparable to or larger than one third of the equatorial electron gyrofrequency can be strongly absorbed by resonant electrons, leading to electron heating perpendicular to the ambient magnetic field at energies above 100 eV. Consequently, in the presence of this strongly focused source of wave energy, the electron distribution should evolve toward a marginally stable anisotropic equilibrium distribution with T⊥&gt;T‖. In order to simulate this perpendicular heating, we allow the anisotropy of the electron distribution to evolve so that damping is minimized at a frequency of 5 kHz, corresponding to the peak in the power spectrum of spherics above the ionosphere. When the plasmapause is located at Lp = 4.5, whistlers above 4 kHz experience more than 20 dB attenuation owing mainly to cyclotron resonance with 0.1 to 1 keV electrons near the equator. It is unlikely that these waves would be detectable on the ground. This attenuation will produce an upper cutoff in the whistler frequency considerably below one half the equatorial electron gyrofrequency for waves that are guided along the plasmapause. In contrast, lower‐frequency whistlers (ƒ ≈ 1–3 kHz) should be amplified by the anisotropic electron population; such waves are able to propagate to the conjugate ionosphere and thus be detected on the ground. This energy transfer between whistlers and cyclotron resonant electrons is relatively unimportant when Lp ≤ 3.0, but it should become significant for Lp ≥ 4.5.

Electron acceleration by inertial Alfvén waves

Alfvén waves reflected by the ionosphere and by inhomogeneities in the Alfvén speed can develop an oscillating parallel electric field when electron inertial effects are included. These waves, which have wavelengths of the order of an Earth radius, can develop a coherent structure spanning distances of several Earth radii along geomagnetic field lines. This system has characteristic frequencies in the range of 1 Hz and can exhibit electric fields capable of accelerating electrons to several keV. These electric fields have the potential to accelerate electrons in several senses: via Landau resonance, bounce or transit time resonance as discussed by André and Eliasson [1992] or through the effective potential drop which appears when the transit time of the electrons is much smaller than the wave period, so that the electric fields appear effectively static. A time‐dependent model of wave propagation is developed which represents inertial Alfvén wave propagation along auroral field lines. The disturbance is modeled as it travels earthward, experiences partial reflections in regions of rapid variation, and finally reflects off a conducting ionosphere to continue propagating antiearthward. The wave experiences partial trapping by the ionospheric Alfvén resonator, which is the effective cavity formed between the ionosphere and the Alfvén speed peak discussed earlier by Polyakov and Rapoport [1981] and Trakhtengerts and Feldstein [1981, 1984, 1991] and later by Lysak [1991, 1993]. Results of the wave simulation and an accompanying test particle simulation are presented, which indicate that inertial Alfvén waves are a possible mechanism for generating electron conic distributions and field‐aligned particle precipitation. The model incorporates conservation of energy by allowing electrons to affect the wave via Landau damping, which appears to enhance the effect of the interactions which heat electron populations.

Anomalous transport due to the ion acoustic drift mode in tokamaks

In toroidal geometry, the ion Landau resonance is suppressed by the ion magnetic drift for modes propagating in the electron diamagnetic drift, and long wavelength modes with (k⊥ρ)2 ≃ \U0001d4aa(0.01) remain unstable. Coupling to the ion acoustic transit mode becomes predominant in such long wavelength regime allowing the mode frequency to exceed the diamagnetic drift frequency by a large margin. Finite β has a destabilizing influence on the toroidal ion acoustic mode particularly in the long wavelength regime. The quasilinear electron and ion thermal fluxes both increase with β partly through finite β modification of the mode frequency and/or growth rate, and partly through magnetic fluctuations. Even the trapped electron thermal flux exhibits an increase with β. In strong turbulence regime, the thermal diffusivity scales as Bohm corrected for finite β, χ ∝ (r/R)2(cT/eB)q2β.

Parametric decays of electromagnetic ion cyclotron waves in a H+ ‐He+ ‐O+ magnetosphericlike plasma

Parametric decays of large‐amplitude electromagnetic ion cyclotron waves (EICW) due to a minor O+ ion component in the magnetosphere are studied. It is shown that the presence of O+ ions leads to a number of new wave couplings which in turn lead to new instabilities. Some couplings involve sound waves carried mainly by the O+ ions, and a sideband EICW which has a resonance at the O+ ion gyrofrequency. These are decay instabilities which can lead to O+ heating through Landau damping and/or resonance absorption. There is also a modulational instability involving two sideband EICW, one propagating forward and the other propagating backward relative to the external magnetic field. These waves can also transfer energy to the O+ ions through resonance absorption. The other branches of the dispersion relation, namely, the He+ and proton branch, have additional decay instabilities (Gomberoff et al., 1995) due to the presence of a minor O+ ion component. It is also shown that in the fluid description, the decays to sound waves associated with the minority heavy ion species have growth rates comparable to, or even larger than, the decays to the acoustic branch corresponding to the majority proton species.

Electromagnetic ion acoustic ballooning mode in tokamaks

It is shown that in toroidal geometry, extremely long-wavelength modes with the ion finite Larmor radius parameter (kperpendicular to rho )2 approximately=O(10-3) and mode frequency of the order of the ion acoustic transit frequency can be strongly destabilized by a finite beta (the ratio of plasma to magnetic pressure) well below the MHD ballooning limit. Ion Landau resonance is suppressed by the ion magnetic drift for modes propagating in the electron diamagnetic drift. The quasilinear electron and ion thermal fluxes both increase with beta . In the strong turbulence regime, a Bohm-type thermal diffusivity corrected for finite beta , chi alpha (r/R)2(cT/eB)q2 beta , emerges.