Finite Ion Cyclotron Frequency Research Articles

Linear instabilities driven by differential rotation in very weakly magnetized plasmas

We study the linear stability of weakly magnetized differentially rotating plasmas in both collisionless kinetic theory and Braginskii's theory of collisional, magnetized plasmas. We focus on the very weakly magnetized limit that is important for understanding how astrophysical magnetic fields originate and are amplified at high redshift. We show that the single instability of fluid theory - the magnetorotational instability mediated by magnetic tension - is replaced by two distinct instabilities, one associated with ions and one with electrons. Each of these has a different way of tapping into the free energy of differential rotation. The ion instability is driven by viscous transport of momentum across magnetic field lines due to a finite ion cyclotron frequency (gyroviscosity); the fastest growing modes have wavelengths significantly longer than MHD and Hall MHD predictions. The electron instability is a whistler mode driven unstable by the temperature anisotropy generated by differential rotation; the growth time can be orders of magnitude shorter than the rotation period. The electron instability is an example of a broader class of instabilities that tap into the free energy of differential rotation or shear via the temperature anisotropy they generate. We briefly discuss the application of our results to the stability of planar shear flows and show that such flows are linearly overstable in the presence of fluid gyroviscosity. We also briefly describe the implications of our results for magnetic field amplification in the virialized halos of high redshift galaxies.

The Alfvén resonance in pair plasmas

The absorption of wave energy in a magnetized cold pair plasma at the analog of the Alfvén resonance is considered. Splitting of the parallel propagating wave modes due to imbalance of pair species densities is discussed. For oblique propagation of the waves, it is shown that, if the wave frequency is much less than the plasma frequency, there is an approximate resonance in the wave vector component perpendicular to the magnetic field, which may be identified as the analog of the Alfvén resonance in normal electron-ion plasmas. The wave differential equations for a nonuniform plasma also exhibit the resonance. The pair species charge imbalance plays a similar role in the Alfvén resonance process to the Hall term (or finite ion cyclotron frequency effect) in electron-ion plasmas. Wave absorption at the resonance can take place via mode conversion to the analog of the short wavelength inertial Alfvén wave, whose properties are discussed.

Magnetohydrodynamic Modes in a Current-Carrying Inhomogeneous Plasma

Magnetohydrodynamic (MHD) modes with m = ±1 ( m : azimuthal mode number) in a current-carrying inhomegeneous plasma are numerically investigated on the basis of wave equations derived from MHD equations, which include effects of finite ion-cyclotron frequency and ion-neutral collisions. In addition to MHD surface waves, kink instability, global Alfven eigenmode (GAE) and shear Alfven wave appear when plasma current flows in a plasma column with inhomogeneous density.

Field line resonances in a cylindrical plasma

An experimental study of the response to an impulsive driver in a low beta helium plasma is presented. Field line resonance (FLR) spectra are recorded and compared to an ideal magnetohydrodynamic (MHD) theory with finite ion cyclotron frequency corrections. The agreement between observed and predicted values is generally within the frequency resolution of the measurements except for the lowest frequency harmonic. The spectrum of the lowest harmonic is compared to various ultra low-frequency (ULF) FLR dispersion relations. Dispersion effects due to nonzero perpendicular wave number are found to be important for the lowest frequency harmonic. Quality factors are measured and compared to theoretical estimates from a two-fluid theory with finite parallel electron temperature. Reflection coefficient values are obtained using measured and estimated Q values.

The Alfvén resonance in a magnetized dusty plasma

Wave propagation in a dusty magnetized plasma at frequencies below and of the order of the ion-cyclotron frequency, and with a non-zero electron temperature is considered. The general dispersion relation for waves propagating at an arbitrary angle with respect to the external magnetic field and for finite ion-cyclotron frequency is derived and discussed. Modification of the Alfvén resonance absorption mechanism due to the presence of a finite proportion of the negative charge of the plasma on stationary dust grains is investigated.

MAGNETOHYDRODYNAMICS WAVES IN A NONHOMEG-ENEOUS CURRENT-CARRYING CYLINDRICAL PLASMA

Considering the effects of both the finite ion-cyclotron frequency and the finite presure, We solved numerically the magnetohydrodynamics (MHD) equations describing a nonhomegeneous current-carrying cylindrical plasma. It is found that the resonant layer of the shear Alfven wave in an ideal plasma impelled by rf wave of antenna is not a singular layer and that Alfven wave can go across the resonant layer. The couple of an antenna with plasma is researched carefully and the results is given by some figures.

Extragalactic jet collimation due to Cherenkov damping of discrete Alfvén waves

We have shown that discrete Alfvén waves can drive noninductive electric current at the level necessary to explain the observation, via radiotelescope, of extragalactic jet (EJ) collimation. The EJ is modeled as a cylindrical current carrying plasma column surrounded by vacuum [external EJ medium], with L >> a where L and a are the EJ length and radius, respectively. Discrete Alfvén waves (DAW) are global Alfvén modes which propagate disturbing all the plasma column. They can be described by the inclusion of finite ion cyclotron frequency and/or the equilibrium plasma current.

The theory of Alfvén surface waves

The effects of non-ideal terms on the dispersion relation of magnetohydrodynamic surface waves in nonuniform, magnetised plasmas are examined. These wave modes play an important role in wave heating processes in laboratory and astrophysical plasmas. Recent results for surface waves in cold and warm plasmas at frequencies less than and equal to the ion-cyclotron frequency are presented. The plasma is modelled by fluid equations and kinetic theory, and the non-ideal effects enter via finite ion cyclotron frequency, multiple ion species, dust, the finite pressure of the ions and electrons, and collisional processes such as resistivity. The cases of low and high plasma β (gas pressure/magnetic pressure) are considered, and it is shown that the thermal and other non-ideal effects give rise to additional short-wavelength modes which can lead to damping of modes localized on a sharp surface, via leakage of energy into the short-wavelength modes. The process of resonance absorption of surface waves at the Alfvén resonance in a smoothed-out surface is examined, and a number of ways of calculating the absorption are summarised. The status of the theory of nonlinear Alfvén surface waves is also reviewed.

Finite-frequency surface waves on current sheets

The dispersion relation for low-frequency surface waves at a current sheet between two magnetized plasmas is derived using the cold-plasma dielectric tensor with finite ion-cyclotron frequency. The magnetic field direction is allowed to change discontinuously across the sheet, but the plasma density remains constant. The cyclotron frequency causes a splitting of the dispersion relation into a number of mode branches with frequencies both less than and greater than the ion-cyclotron frequency. The existence of these modes depends in particular upon the degree of magnetic field discontinuity and the direction of wave propagation in the sheet relative to the magnetic field directions. Sometimes two modes can exist for the same direction of propagation. The existence of modes undamped by Alfvén resonance absorption is predicted. Analytical solutions are obtained in the low-frequency and magnetic-field-reversal limits. The solutions are obtained numerically in the general case.

Discrete Alfvén waves in solar loop prominences

It is shown that a discrete Alfven wave can explain the natural oscillations of solar loop prominences by considering the existence of a current flow. Discrete Alfven waves are a new class of Alfven waves which is described by the inclusion of the finite ion cyclotron frequency (ω/ωcl≠0) and/or the equilibrium plasma current. In this paper we consider only the effect of the current since in solar prominences (ω/ωcl≈0). We have modeled the solar prominences as a cylindrical plasma, surrounded by vacuum (corona), with L ≫ a where L and a are the plasma column, length, and radius, respectively. We have calculated the spectrum of the discrete Alfven waves as function of the magnitude and shape of the plasma current.

Higher Frequency Spectrum of a Diffuse Linear Pinch with Multiple Ion Species

A cold plasma model which takes into account finite ion cyclotron frequency effects and multiple ion species has been developed for wave propagation in arbitrary magnetic field geometries. This model has been used to derive an elegant system of normal mode equations for a 10w-fJ diffuse linear pinch. From a soluble model, general features of the spectrum are discussed up to and including the ion cyclotron range of frequencies. It is indicated that in the vicinity of the ion-ion hybrid cutoff frequency, there could exist global eigenmodes which might be useful for supplementary heating of diffuse linear pinches.

Effects of finite ion cyclotron frequency on the absorption of magnetohydrodynamic waves at the spatial Alfven resonance

The absorption of magnetoacoustic modes at the spatial Alfven resonance of Tokamak plasmas is investigated within the context of an ideal MHD model which includes finite omega / omega ci effects. It is found that these effects can dramatically modify the pure MHD picture of the resonant absorptiometry and the spectrum of global eigenmodes of the Alfven wave. Implications for the interpretation of recent experimental results are discussed.