Proton heating via nonresonant scattering off intrinsic Alfvénic turbulence

A novel yet unsophisticated theory is proposed to show that low-beta protons can be efficiently heated by enhanced Alfvén waves. The present research is motivated by a plasma physics issue relevant to the explanation of hot stellar coronas observed with x-ray telescopes. The efficient heating is attributed to nonresonant wave-particle scattering that tends to randomize proton motion in directions transverse to the ambient magnetic field.     

Amplitude modulation of kinetic Alfvén waves and the formation of nonlinear structures

The theory of the amplitude modulation of finite amplitude kinetic Alfvén waves in a medium beta plasma is re-examined and extended. The modulational and filamentational instabilities as well as associated localized structures in magnetic field and plasma density are investigated. The relevance of our investigation to coherent nonlinear structures in the ionospheric plasma is pointed out.     

Heating of ions by Alfvén waves via nonresonant interactions

Finite-amplitude intrinsic Alfvén waves exist pervasively in astrophysical and solar-terrestrial environment. It is generally believed that linear wave-particle resonant interaction between thermal protons and Alfvén waves is ineffective when the proton beta is low. However, this Letter demonstrates that the ions can be heated by Alfvén waves via nonresonant nonlinear interaction. Contrary to the customary expectation, it is found that the lower the plasma beta value, the more effective is the heating process. It is also shown that the ion temperature increase is more prominent along perpendicular direction.     

High-n ideal and resistive shear Alfvén waves in tokamaks

Ideal and resistive MHD equations for the shear Alfvén waves are studied in a low-β toroidal model by employing the high-n ballooning formalism. The ion sound effects are neglected. For an infinite shear slab, the ideal MHD model gives rise to acontinuous spectrum of real frequencies and discrete eigenmodes (Alfvén-Landau modes) with complex frequencies. With toroidal coupling effects due to nonuniform toroidal magnetic field, the continuum is broken up into small continuum bands and new discrete toroidal eigenmodes can exist inside the continuum gaps. Unstable ballooning eigenmodes are also introduced by the bad curvature when β > β_c. The resistivity (η) can be considered perturbatively for the ideal modes. In addition, four branches of resistive modes are induced by the resistivity: (1) resistive entropy modes which are stable with frequencies going to zero with resistivity as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$; (2) tearing modes which are stable (Δ′ < 0) with frequencies approaching zero as $\text{η}{}^{\mathrm{<mtext>3</mtext><mtext>5</mtext>}}$; (3) resistive periodic shear Alfvén waves which approach the finite frequency end points of the continuum bands as $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$; and (4) resistive ballooning modes which are purely growing with growth rate proportional to $\text{η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{β}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ as η → 0 and β → 0.     

Cross-scale nonlinear coupling and plasma energization by Alfvén waves

We present a new channel for the nonlocal transport of wave energy from the large (MHD) scales to the small (kinetic) scales generated by the resonant decay of MHD Alfvén waves into kinetic Alfvén waves. This process does not impose any restriction on the wave numbers or frequencies of initial MHD waves, which makes it superior compared to the mechanisms of spectral transport studied before. Because of dissipative properties of the nonlinearly driven kinetic Alfvén waves, the decay leads to plasma heating and particle acceleration, which is observed in a variety of space and astrophysical plasmas. Two examples in the solar corona and the terrestrial magnetosphere are briefly discussed.     

Topological constraints on solar loop plasma instabilities and Alfvén waves

Inhomogeneous plasmas-solar instabilities-are investigated by using the techniques of classical differential geometry for curves, where the Frenet torsion and curvature describe completely the motion of a curve. In our case, the Frenet frame changes in time and also depends upon the other coordinates, taking into account the inhomogeneity of the plasma. The exponential perturbation method, so commonly used to describe cosmological perturbations, is applied to the magnetohydrodynamic (MHD) plasma equations to find modes describing Alfvén wave propagation in the medium of planar loops. Stability is investigated in the imaginary axis of the spectra of complex frequencies ω, i.e. m (ω) ≠ 0. A pratical guide for experimental solar physicists is given by computing the twist of force-free solar loops, which generalizes the Parker formula relating the twist to the Frenet torsion. In our expression the twist of the solar loops also depends on the abnormality of the normal vector of the frame.      

Energy spectra stemming from interactions of Alfvén waves and turbulent eddies

We present a numerical analysis of an incompressible decaying magnetohydrodynamic turbulence run on a grid of 1536{3} points. The Taylor Reynolds number at the maximum of dissipation is approximately 1100, and the initial condition is a superposition of large-scale Arn'old-Beltrami-Childress flows and random noise at small scales, with no uniform magnetic field. The initial kinetic and magnetic energies are equal, with negligible correlation. The resulting energy spectrum is a combination of two components, each moderately resolved. Isotropy obtains in the large scales, with a spectral law compatible with the Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear interactions due to Alfvén waves; scaling of structure functions confirms the non-Kolmogorovian nature of the flow in this range. At small scales, weak turbulence emerges with a k{perpendicular}{-2} spectrum, the perpendicular direction referring to the local quasiuniform magnetic field.     

Induced scattering and two-photon absorption of Alfvén waves with arbitrary propagation angles

An equation for the spectral energy density of collisionless Alfvén waves, propagating at arbitrary angles to the average magnetic field, is derived on the basis of the theory of weak turbulence. The main nonlinear processes for the case studied are induced scattering and two-photon absorption of Alfvén waves by thermal ions. An equation is derived for thermal particles which describes particle diffusion, accompanying these processes, in momentum space. The results are qualitatively different from previous results obtained by other authors for Alfvén waves propagating along the average magnetic field.     

Alfvén waves in temperature anisotropic Cairns distributed plasma

The dispersion relations and Landau damping of Alfven waves in kinetic and inertial limits are studied in temperature anisotropic Cairns distributed plasma.In the case of kinetic Alfven waves(KAWs),it is found that the real frequency is enhanced when either the electron perpendicular temperature or the non-thermal parameter A increases.For inertial Alfven waves(IAWs),the real frequency is slightly affected by the electron temperature anisotropy and A.Besides the real frequency,the damping rate of KAWs is reduced when the electron perpendicular temperature or A increases.In the case of IAWs,the temperature anisotropy and A either enhance or reduce the damping rate depending upon the perpendicular wavelength.These results may be helpful to understand the dynamics of KAWs and IAWs in space plasmas where the non-Maxwellian distribution of particles are routinely observed.     

On kinetic Alfvén waves in a dusty plasma

The dispersion and damping of a dust-modified kinetic Alfvén wave, and a lower frequency dust kinetic Alfvén wave, is investigated in a dusty plasma comprising electrons, ions, and negative dust. It is found that, when dust dynamics is neglected, the presence of cold dust modifies the usual kinetic Alfvén wave dispersion and damping owing to the inequality of the electron and ion densities. The dispersion relation of the dust kinetic Alfvén wave, with frequency below the dust cyclotron frequency, depends on the density and temperature parameters of all three species, and the wave damping is due to both electrons and ions.     

Helicon and Alfvén wave propagation in non-magnetic semiconductors and semi-metals: Active and passive waves

It was pointed out in 1960 that metals and semiconductors can support low frequency electromagnetic excitations in the presence of a magnetic field. We now feel that it is an appropriate time to discuss some of the progress made, over the last decade, in understanding and using this novel phenomenon. Naturally the field has grown quite rapidly and it would clearly be a Herculean task to review every aspect of it in other than a superficial manner. We have therefore chosen to discuss only semiconductors and semi-metals. This choice is dictated to us partly by the fact that magneto-plasma effects in metals have been reviewed from time to time but mainly by the fact that magneto-plasma effects in semiconductors have never been previously reviewed.

Of course the term ‘magneto-plasma’ covers a great deal of activity so we have decided to choose a theme which links the beginnings of the subject to the present day. This theme is that of helicon and Alfvén wave propagation.

We have produced a background of theory against which the nature of helicon and Alfvén waves can be readily understood. This background theory can also be used as a starting point for investigations of other plasma effects beyond the scope of this review.

Some considerable attention is paid to waves in active systems, i.e. systems possessing a pool of energy arising from the application of an external electric field. Such systems, while of basic physical interest, are also of technical interest from a solid-state device viewpoint. The possibility of transverse wave instabilities occurring in active systems is discussed and a review of the criteria for labelling the types of instability is presented. As an example of the use of these techniques we have attempted to correlate the high electric field microwave emission from indium antimonide with a helicon-based instability.

The theoretical work is set in perspective by the inclusion of discussions of the experimental work in the appropriate areas. We have also included a brief review of experimental observations of microwave emission from indium antimonide and the proposed mechanisms, other than helicon instability, which may account for it.     

Control of gradient-driven instabilities using shear Alfvén beat waves

A new technique for manipulation and control of gradient-driven instabilities through nonlinear interaction with Alfvén waves in a laboratory plasma is presented. A narrow, field-aligned density depletion is created in the Large Plasma Device, resulting in coherent, unstable fluctuations on the periphery of the depletion. Two independent shear Alfvén waves are launched along the depletion at separate frequencies, creating a nonlinear beat-wave response at or near the frequency of the original instability. When the beat wave has sufficient amplitude, the original unstable mode is suppressed, leaving only the beat-wave response, generally at lower amplitude.     

On the properties of Alfvén waves in relativistic magnetohydrodynamics

Simple Alfvén waves and Alfven shocks are considered in the framework of relativistic magnetohydrodynamics. It is found that the tangential components of vector fields trace ellipses instead of the circles of Newtonian MHD. Their properties are studied in the general wave frame, Hoffmann-Teller wave frame, and the general laboratory frame.     

Mode conversion and anomalous transport in Kelvin-Helmholtz vortices and kinetic Alfvén waves at the Earth's magnetopause

Observations at the Earth's magnetopause identify mode conversion from surface to kinetic Alfvén waves at the Alfvén resonance. Kinetic Alfvén waves radiate into the magnetosphere from the resonance with parallel scales up to the order of the geomagnetic field-line length and spectral energy densities obeying a k(perpendicular)(-2.4) power law. Amplitudes at the Alfvén resonance are sufficient to both demagnetize ions across the magnetopause and provide field-aligned electron bursts. These waves provide diffusive transport across the magnetopause sufficient for boundary layer formation.