Shear Alfven Waves Research Articles

Alfvenic instabilities driven by energetic alpha particles

A discussion of linear drive and damping mechanisms on shear Alfven waves in hot, reactor-relevant plasmas is presented. According to whether equilibrium geometry, kinetic effects, or both are important in determining the mode structure, different branches of Alfven waves may exist: TAE, KAW and KTAE, respectively. The existence of open problems, as the necessity of non-perturbative calculations, is also emphasized.

Focusing of Alfvenic wave power in the context of gamma-ray burst emissivity

Highly dynamic magnetospheric perturbations in neutron star environments can naturally account for the features observed in gamma-ray burst spectra. The source distribution, however, appears to be extragalactic. Although noncatastrophic isotropic emission mechanisms may be ruled out on energetic and timing arguments, MHD processes can produce strongly anisotropic gamma rays with an observable flux out to distances of about 1-2 Gpc. Here we show that sheared Alfven waves propagating along open magnetospheric field lines at the poles of magnetized neutron stars transfer their energy dissipationally to the current sustaining the field misalignment and thereby focus their power into a spatial region about 1000 times smaller than that of the crustal disturbance. This produces a strong (observable) flux enhancement along certain directions. We apply this model to a source population of 'turned-off' pulsars that have nonetheless retained their strong magnetic fields and have achieved alignment at a period of approximately greater than 5 sec.

Ion acceleration and abundance enhancements by electron beam instabilities in impulsive solar flares

We show that a nonrelativistic electron beam in a hydrogen-helium solar flare plasma will excite H(+) electromagnetic ion cyclotron, shear Alfven, and R-X waves, in addition to waves resulting from the two-stream instability. The H(+) electromagnetic ion cyclotron and shear Alfven waves are able to selectively accelerate ambient He-3 and Fe, respectively, to MeV energies through first harmonic gyroresonance, and thereby account for the large (He-3)/(He-4) and Fe/C ratios seen in the energetic particles from impulsive solar flares. In this model, separate heating and acceleration mechanisms for either He-3 or Fe are not required, and Fe acceleration is quite efficient since it does not need to occur by second harmonic gyroresonance. The combination of the other two unstable modes is able to accelerate ions to hundreds of MeV if the particles become trapped in an electrostatic potential well of a two-stream wave.

A detailed study of sheared Alfven waves and their possible application to gamma-ray bursts

view Abstract Citations (6) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Detailed Study of Sheared Alfven Waves and Their Possible Application to Gamma-Ray Bursts Fatuzzo, Marco ; Melia, Fulvio Abstract Strong particle acceleration in neutron-star environments may result in the upscattering of soft photons to gamma-ray energies. Sheared Alfven waves, which may be generated by several plausible disturbances in the stellar crust, can energize the charges to Lorentz factors in excess of 10 exp 5. However, a full understanding of the physics of these MHD waves is lacking due to the complications arising from the charge separation within the sheared region. Here, we present a detailed study of this mechanism, with particular attention to a comparison of the nonrelativistic and relativistic domains. We find that the charge separation is consistent with our previous classical MHD solutions and that it provides a natural description of the sheared current via a relativistic flow. These waves can apparently produce the particle distributions that are necessary to produce the gamma-ray fluxes observed in gamma-ray bursts, regardless of whether the sources are Galactic or extragalactic. Publication: The Astrophysical Journal Pub Date: April 1993 DOI: 10.1086/172549 Bibcode: 1993ApJ...407..680F Keywords: Gamma Ray Bursts; Neutron Stars; Plasma Waves; Computational Astrophysics; Particle Acceleration; Plasma Dynamics; Shear; Astrophysics; GAMMA RAYS: BURSTS; ACCELERATION OF PARTICLES; MAGNETOHYDRODYNAMICS: MHD; STARS: NEUTRON full text sources ADS |

High‐latitude distributions of plasma waves and spatial irregularities from DE 2 alternating current electric field observations

An 18‐month data base from the Dynamics Explorer 2 AC electric field spectrometers is used to obtain average high‐latitude magnetic local time (MLT) versus invariant latitude (INL) distributions of signal intensities in 12 frequency bands between 4 Hz and 512 kHz. Three distinctly different distributions are obtained, corresponding to (1) Doppler‐shifted signals from spatial structures in the electric field (i.e., irregularities) and Alfven waves between 4 and 512 Hz, (2) ELF waves between 256 Hz and 4.1 kHz, and (3) VLF waves between 4.1 and 64 kHz with extensions into the 128–512 kHz band. The ELF and VLF distributions closely resemble previously published results based on more limited sampling. Comparable distributions for the seven channels between 4 and 512 Hz, showing a prominent zone of maximum intensities at 72.5°–80° INL between 0500 and 1300 MLT, have not previously been reported. The power law frequency dependence of average power spectral densities (PSDs) between 4 and 512 Hz is also mapped in MLT‐INL coordinates. At all locations, two power law indices (slopes) are required to closely fit the PSDs with an inverted knee joining the two slopes in the 32–64 Hz band. This knee band corresponds to the range of O+ cyclotron frequencies encountered, and it lends credence to Gurnett et al.'s (1984) contention that Alfven waves are an essential ingredient in explaining the low‐frequency in situ satellite signals which were previously attributed to polarization fields accompanying spatial irregularities in plasma densities. However, other aspects of the 4–512 Hz observations, including seasonal variations, favor the earlier spatial irregularity interpretation. As discussed, the difficulties encountered in seeking interpretations exclusively in terms of either spatial irregularities or Alfven waves can be resolved with a synthesis approach requiring both types of signals. It is proposed that the averaged intensities and corresponding spectral characteristics in the 4–512 Hz band represent the consequence of intermittently superimposing shear Alfven waves on a spatially irregular medium. There are then three principal contributions: (1) an omnipresent 4–512 Hz signal from Doppler‐shifted responses to 2000–15 m spatial irregularities having an average power law spectral index near −1.9, (2) intermittent signals from locally generated shear Alfven waves having maximum power at frequencies of <4 Hz and average power law spectral indices of ≤(−2.8) extending only to fc(O+), and (3) spatial irregularity modulations of shear Alfven waves originating both locally and in the distant magnetosphere.

Field-aligned current generation from MHD waves in the terrestrial magnetosphere

It is shown by the quasilinear Fokker-Planck approach that MHD, lower hybrid, whistler, electron- cyclotron and ion-cyclotron waves present in the magnetosphere can generate a noninductive DC current, aligned with the ambient magnetic field. The efficiency of current generation is determined for waves damped via Cherenkov (Landau and TTMP) damping. As a specific application, the current generation by the kinetic shear Alfvén wave is considered for plasma- pause electrons. The efficiency obtained is η ⋍ 1(μ A m −2)(μ W m −2) and the wave-power dissipation necess- ary to generate currents at the Birkeland current level ( J ∥ − 10 ⋍4 A m −2 ) is P d ⋍ 100 μW m −2 . Finally, it is also shown that the ideal (MHD) shear Alfven wave cannot generate a DC field-aligned current via Cher- enkov damping, since E ∥ = 0 and B ∥ = 0, where E ∥ and B ∥ are the field-aligned wave electric and magnetic fields, respectively.

Alpha physics in tokamaks

After briefly reviewing recent experimental and theoretical alpha -particle physics simulation results self-consistent values of the anomalous radial diffusion coefficient Dalpha from two related theories and/or numerical simulations of energetic alpha -excited shear Alfven wave turbulence are derived and used in a reduced drift kinetic equation (DKE) for the alpha distribution function Falpha (r,E,t) which contains a diffusion term 1/r delta rDalpha delta . Since Dalpha depends on delta nalpha / delta r through w* alpha the ensuing nonlinear diffusion equation for nalpha (r,t) is solved iteratively with the DKE for Falpha and the effects of Dalpha on the alpha -power coupling efficiency eta alpha and the ignition margin are analyzed for an ITER-like device ('ignited operation' of the ITER conceptual design activity (CDA)). From a power balance point of view the resulting Dalpha >or approximately=1 m2/s appears tolerable, Dalpha >3 m2/s would yield eta alpha <0.95.

Electric field and current distribution near the ablation cloud of a pellet injected into a tokamak

The high density, low temperature ionized cloud surrounding a pellet moving across a tokamak magnetic field forms a long field aligned 'ablation channel'. This plasma mass shields the pellet from the incident plasma electrons streaming toward the pellet along field lines. Therefore, the shielding capability of the cloud-the effective column ablation density intercepting the incident electrons-depends on the relative cross-field displacement between pellet and ablation channel. This transverse displacement is caused by the J⊥ × B Lorentz force, where the transverse current density passing through the channel J⊥ is powered by the motional E-field as seen in the reference frame co-moving with the pellet. It is shown that the currents inside the ablation channel are closed by background field aligned currents carried by outgoing shear Alfven waves. From ∇.J=0, one can obtain a self-consistent equation which describes the electrostatic field near the channel. The transverse force and deflection of the ablation channel are estimated from these results in order to determine the effective shielding length

Alpha physics in tokamaks

After briefly reviewing recent experimental and theoretical alpha -particle physics simulation results self-consistent values of the anomalous radial diffusion coefficient Dalpha from two related theories and/or numerical simulations of energetic alpha -excited shear Alfven wave turbulence are derived and used in a reduced drift kinetic equation (DKE) for the alpha distribution function Falpha (r,E,t) which contains a diffusion term 1/r delta rDalpha delta . Since Dalpha depends on delta nalpha / delta r through w* alpha the ensuing nonlinear diffusion equation for nalpha (r,t) is solved iteratively with the DKE for Falpha and the effects of Dalpha on the alpha -power coupling efficiency eta alpha and the ignition margin are analyzed for an ITER-like device ('ignited operation' of the ITER conceptual design activity (CDA)). From a power balance point of view the resulting Dalpha >or approximately=1 m2/s appears tolerable, Dalpha >3 m2/s would yield eta alpha <0.95.

Enhanced damping of Alfven waves in the solar corona by a turbulent wave spectrum

The effect of a background spectrum of Alfven waves on the rate of dissipation of a test shear Alfven wave is numerically calculated. The results demonstrate that as the classical resistivity eta and classical viscosity mu become small, the damping rate of the Alfven wave remains large and depends only on the amplitude for the scalar potential of the wave spectrum and the wavenumber of the Alfven wave. The damping rate is virtually independent of eta and mu. The wave spectrum need not be turbulent or stochastic to affect the damping rate. The dissipation rate is nonlinear enhanced by nonstochastic spectra as well as by stochastic spectra if two conditions are met. First, the perpendicular magnetic field associated with Alfven wave spectrum must exceed a certain collision-frequency threshold and second, for nonstochastic spectra only, the magnetic field must exceed a threshold proportional to the parallel wavenumber of the shear Alfven wave. These conditions can be easily satisfied in the solar corona.

Nonresonant resistive dissipation of compressible magnetohydrodynamic waves

view Abstract Citations (25) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonresonant Resistive Dissipation of Compressible Magnetohydrodynamic Waves Califano, Francesco ; Chiuderi, Claudio ; Einaudi, Giorgio Abstract The effect of compressibility on the propagation and resistive dissipation properties of magnetohydrodynamic waves in nonuniform media is studied by using a normal mode analysis. The paper concentrates on wave models for which the resistivity acts over the entire system, thus excluding localized resonant absorption. The wave modes in nonhomogeneous situations are identified in term of their familiar homogeneous counterparts and it is shown that only the shear Alfven and slow magnetosonic modes survive in the resistive regime. The most promising heating agents in the astrophysical context are shown to be the shear Alfven waves that behave almost incompressibly. Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/171306 Bibcode: 1992ApJ...390..560C Keywords: Compressibility; Magnetohydrodynamic Waves; Wave Attenuation; Wave Propagation; Analytic Functions; Magnetosonic Resonance; Mathematical Models; Propagation Modes; Shear; Plasma Physics; MAGNETOHYDRODYNAMICS: MHD full text sources ADS |

Alfven wave transmission through the solar atmosphere

The transmission and reflection coefficients of shear Alfven waves in an open magnetic structure emerging from the Sun are determined. Both two-dimensional slab and axisymmetric static equilibrium configurations are solved numerically on a curvilinear coordinate grid adapted to the magnetic field geometry and to the density stratification. The equilibrium structure obtained numerically extends from the photosphere up to an altitude of 20,000 km, spanning 14 density scale heights and allowing for a magnetic flux tube expansion by a factor 40-400, corresponding to a filling factor of 2.5% to 0.25%. From 20,000 km up to 4 R ⊙ a magnetic monopole model is adopted

Transformation of MHD modes near magnetic separatrix surfaces

The linear propagation of magnetosonic and of shear Alfven waves is considered in a magnetic configuration with an X-line and a parallel (along the X-line) magnetic field B/sup ///. The two modes are decoupled both in the limits of zero and of infinite B/sup ///, whereas for finite B/sup /// they transform one into the other. The spatial structure of the current densities induced by this transformation is examined in the small and in the large B/sup /// limit. In both limits the induced currents exhibit, either at the separatrices or at the X-line, singular structures which can be associated with the formation of current sheets.

Transformation of MHD modes near magnetic separatrix surfaces

The linear propagation of magnetosonic and of shear Alfven waves is considered in a magnetic configuration with an X-line and a parallel (along the X-line) magnetic field B/sup ///. The two modes are decoupled both in the limits of zero and of infinite B/sup ///, whereas for finite B/sup /// they transform one into the other. The spatial structure of the current densities induced by this transformation is examined in the small and in the large B/sup /// limit. In both limits the induced currents exhibit, either at the separatrices or at the X-line, singular structures which can be associated with the formation of current sheets.

Gamma-ray bursts from sheared Alfven waves

The physical process by which sheared Alfven waves can accelerate electrons to a Lorentz factor of 10,000 to 100,000 within 5 km of the stellar surface is applied to a study of gamma-ray bursts, taking both resonant and nonresonant scattering into account. Several very encouraging features of the model are discussed. Although the field is oscillatory, virtually all the charges are ejected from the system, resulting in very little backheating of the stellar surface. The particle number density is accounted for naturally in terms of BA0 and m, which in principle are known from the physical manifestation of the agent causing the crustal disturbance. The resulting gamma-ray spectrum compares very favorably with the observation. The model restricts the geometry of the emission region, in the sense that only the Compton upscattering of soft photons from a warm polar cap can produce the correct spectral shape.

A new semi-implicit method for MHD computations

An efficient semi-implicit method for the solution of the nonlinear, three-dimensional, resistive MHD equations is presented. The method is unconditionally stable with respect to the compressional fast magnetoacoustic and shear Alfven waves. The time step is limited instead by the nonlinear physical plasma phenomena. Furthermore, the method allows a high spatial resolution. Physically relevant test cases are presented and the feasibility of longtime simulations is discussed.

Transient particle acceleration in strongly magnetized neutron stars. II - Effects due to a dipole field geometry

Sheared Alfven waves generated by nonradial crustal disturbances above the polar cap of a strongly magnetized neutron star induce an electric field component parallel to B. An attempt is made to determine the manner in which the strong radial dependence of B affects the propagation of these sheared Alfven waves, and whether this MHD process is still an effective particle accelerator. It is found that although the general field equation is quite complicated, a simple wavelike solution can still be obtained under the conditions of interest for which the Alfven phase velocity decouples from the wave equation. The results may be applicable to gamma-ray burst sources.

Global Wave Solution of Generalized MHD Equations in a Cylindrical Plasma

The linear eigenstate problem of generalized magnetohydrodynamics (MHD) equations in a cylindrical plasma is discussed. The effects of finite frequency and finite pressure perturbation lead to an important result: the resonant layer of the shear Alfven waves is not a singular layer. In this paper, the MHD equations are reduced to four differential equations of first order for perturbed quantities. An analytical dispersion relation for a homogeneous plasma cylinder is obtained. The K. Appert theory is a limiting case of our theory.

Theory of Shear Alfven Vortex in Current-Carrying Plasma Plasma

Nonlinear shear Alfven waves in low P, current-carrying magnetized plasma are studied. Sets of nonlinear equations describing the shear Alfven waves are derived and corresponding dipolar vortex solutions are given for both regions of P, 0<β< me/mi and me/mi<<β<<1, respectively. Result shows that the equilibrium plasma current has remarkable effect on the behaviour of vortex motion. For the 0<β<< me/mi case the current adds a low speed vortex propagating region in the opposite current direction, while for the < me/mi<<β<<1 case the current reduces the region of vortex propagating speed by cutting a low speed region. The implication of this effect is discussed.

Experimental Observations of Surface Electrostatic Wave on KT-5B Tokamak

Shear Alfven waves have been successfully excited in KT-5B small tokamak by means of the one turn longitudinal loop antenna located in the shadow area. The measured antenna loadings show their rich structure, and the loadings are also found to be sensitive to the plasma current. Preliminary evidence of surface electrostatic wave was observed.