Gravitational instability of partially-ionized plasma in an oblique magnetic field

The effects of Hall currents, finite conductivity, and collision with neutrals have been studied on the gravitational instability of a partially-ionized plasma. It is assumed that plasma is permeated by an oblique magnetic field. The dispersion relation has been obtained and numerical calculations have been performed to obtain the dependence of the growth rate of the gravitationally unstable mode on the various physical parameters involved. It is found that Jeans's criterion remains unchanged in the presence of Hall currents, finite conductivity, and collisions. The Hall currents, finite conductivity, and collisions have destabilizing influence on the unstable mode of wave propagation of a gravitational instability of partially-ionized plasma.     

Sunspots and physics of magnetic flux tubes overstability and Kelvin-Helmholtz instability in a magnetic field

We have studied the dynamical properties of convective overstability and Kelvin-Helmholtz instability in a vertical magnetic field with a downdraft. The Kelvin-Helmholtz instability and overstability produce the upward downward propagating Alfvén waves depending upon the magnitudes of the kinematic viscosity coefficient (eddy viscosity) ν, and thermometric conductivity κ. It is found that the instability may be driven by the density stratification and the effect of the eddy viscosity is to make the system stable. We discuss also the interaction of the overstability inx<0 and the downdraft inx>0, and the overstability at a vertical boundary of the field.     

Thermal-convective instability of a composite plasma in a stellar atmosphere with rotation and magnetic field

Thermal-convective instability of a composite plasma in a stellar atmosphere, when the effects of a uniform rotation and a uniform magnetic field are included simultaneously, is discussed. It is found that the criterion for monotonic instability is the same as in the absence or presence (separately) of these two effects.     

Kelvin-Helmholtz instability of superimposed magnetized,rotating fluids with neutral particles and resistivity

Kelvin-Helmholtz instability of two superimposed fluids has been studied. One of the fluids is non-conducting and the other is conducting with finite resistivity. The fluids are assumed rotating and slipping past each other with a relative velocity. The neutral particles are also incorporated in one of the fluids of the system. A general dispersion relation has been derived and discussed under different conditions. The effect of rotation and the neutral particles, considering an infinitely-conducting system has also been analyzed and it is observed that small rotation and the presence of neutral particles destabilize the system. The effect of neutral particles is to decrease the effect of magnetic field, angular velocity, and the gravitational field. It has been found that an otherwise stable mode becomes overstable and grows exponentially in the presence of finite resistivity.     

Nonlinear Kelvin-Helmholtz magnetohydrodynamic instability of a rotating plasma

Nonlinear analysis for Kelvin-Helmholtz instability of an incompressible, inviscid, rotating fluid with infinite conductivity in the presence of gravity and surface tension has been discussed. The unperturbed magnetic field on two sides of the interface is taken to be uniform. The nonlinear Schrödinger equation for the time variation of amplitude of small perturbations with wave number around the neutral stability is derived. It is found that stability of a magnetised K-H rotating configuration depends on the density ratio, surface tension, and discontinuity of velocity and magnetic field. The effect of an aligned magnetic field and rotation on the non-linear instability of a rotating conducting plasma has been discussed in certain important limiting cases.     

Magnetized Kelvin-Helmholtz instability in the presence of a radiation field

The purpose of this study is to analyze the dynamical role of a radiation field on the growth rate of the unstable Kelvin-Helmholtz (KH) perturbations. As a first step toward this purpose, the analyze is done in a general way, irrespective of applying the model to a specific astronomical system. The transition zone between the two layers of the fluid is ignored. Then, we perform a linear analysis and by imposing suitable boundary conditions and considering a radiation field, we obtain appropriate dispersion relation. Unstable modes are studied by solving the dispersion equation numerically, and then growth rates of them are obtained. By analyzing our dispersion relation, we show that for a wide range of the input parameters, the radiation field has a destabilizing effect on KH instability. In eruptions of the galaxies or supermassive stars, the radiation field is dynamically important and because of the enhanced KH growth rates in the presence of the radiation; these eruptions can inject more momentum and energy into their environment and excite more turbulent motions.     

Alfvénic drift Kelvin-Helmholtz instability in the presence of an equilibrium electric field

Alfvénic drift Kelvin-Helmholtz instability is discussed in the presence of equilibrium electric and magnetic fields which are perpendicular to each other. A dispersion relation is achieved with the help of WKB approximation and the instability criterion is established. A comparison is made with the previous studies and it is shown that the electric field has a stabilizing or destabilizing effect on the system according to the conditions discussed in the text.     

Rayleigh instability of compressible plasma in a vertical magnetic field

A study of the Rayleigh instability of a compressible plasma of density stratified in horizontal planes and subjected to a vertical magnetic field is made. The special case of a plane interface separating two superposed uniform plasmas of different densities and speeds of sound is treated as an example to illustrate the compressibility effects on the hydromagnetic Rayleigh instability. It is found that the hydromagneticcompressibility effects act toward reducing the growth rate in a hydrodynamically unstable situation.     

On the Kelvin-Helmholtz instability of structured plasma layers in the magnetosphere

The hydromagnetic Kelvin-Helmholtz (K-H) instability problem is studied for a three-layered system analytically by arriving at the marginal instability condition. As the magnetic field directions are taken to vary in the three regions, both the angle and finite thickness effects are seen on the instability criterion. When the relative flow speed of the plasmas on the two sides of the interfaces separating the inner and the surrounding layers is U < U_c, where U_c is the critical speed, the system is stable both for symmetric and asymmetric perturbations. However, unlike the case of the interface bounded by two semiinfinite media, U_c is no longer the minimum critical speed above which the system will be unstable for all wavenumbers; another critical speed U^* > U_c is introduced due to the finiteness of the system. When U_c < U < U^*, the instability can set in either through the symmetric or asymmetric mode, depending on the ratio of the plasma parameters and angle between the magnetic field directions across the boundaries. The instability arises for a finite range of wavenumbers, thus giving rise to the upper and lower cut-off frequencies for the spectra of hydromagnetic surface waves generated by the K-H instability mechanism. When U > U^*, both the modes are unstable for short wavelengths. The results are finally used to explain some observational features of the dependence of hydromagnetic energy spectra in the magnetosphere on the interplanetary parameters.     

Nonlinear Kelvin-Helmholtz instability in hydromagnetics

By taking into account the temporal as well as the spatial effects, a weakly nonlinear theory of the propagation of wave packets in the Kelvin-Helmholtz instability problem in the presence of uniform magnetic fields, acting along the surface of separation of two moving superposed fluids, is presented. With the use of the method of multiple scaling, the evolution of the amplitude of the two-dimensional wave packets, which is governed by a nonlinear Klein-Gordon equation, is derived. The various stability criteria arising out of this equation are examined. The nonlinear cut-off wavenumber, which separates the region of stability from that of instability, is determined.     

Thermosolutal-convective instability of a composite plasma in a stellar atmosphere including the effects of variable magnetic field and rotation

Thermosolutal-convective instability of a composite plasma in a stellar atmosphere is considered. The effect of a variable horizontal magnetic field and the simultaneous effect of a uniform rotation and a variable horizontal magnetic field have been considered on the thermosolutal-convective instability. We have derived the sufficient conditions for the existence of monotonic instability. It is found that the criteria for monotonic instability hold good in the presence of a variable horizontal magnetic field as well as in the presence of a uniform rotation and a variable horizontal magnetic field.     

The Kelvin-Helmholtz instability in dusty plasmas

The Kelvin-Helmholtz instability in magnetized, dusty plasmas is examined, for both negatively and positively charged dust. The critical shear in the ion velocity along the magnetic field is computed as a function of the charge residing on dust grains.     

On the effect of oblique disturbances on Kelvin-Helmholtz instability at magnetospheric boundary layers and in solar wind

This paper is concerned with the Kelvin-Helmholtz instability in the indissipative plasma with an external magnetic field. A detailed analysis is made of the results known from the approximation of a tangential discontinuity. The finiteness of the interface thickness effect is considered numerically at the arbitrary distribution of the density, velocity and magnetic field vectors inside this shear layer. The influence of plasma compressibility with an arbitrarily varying magnetic field is investigated. The main role of oblique disturbances with respect to the flow rate direction is shown under conditions of a large plasma compressibility. As such perturbations move away from the interface, their amplitude is damped much more slowly than in the case of weak compressibility. However, their wavelength remains, approximately, the same as that of longitudinal waves in the case of incompressibility. The linear approximation suggests the importance of oblique waves in the energetics of the interaction between the shear layer and the outward medium. A comparison is made of the instability period on discontinuities in the solar wind, and at magnetospheric and plasmaspheric boundaries, with the range of geomagnetic pulsations.     

Gravitational instability of a composite and rotating plasma in the presence of a variable magnetic field through a porous medium

The gravitational instability of an infinite homogeneous self-gravitating plasma through porous medium is considered to include, separately, the effects due to rotation and collisions between ionized and neutral components. The dispersion relations are obtained in both cases. It is found that the gravitational instability of a composite and rotating plasma in the presence of a variable horizontal magnetic field through porous medium is determined by the Jeans's criterion.     

Gravitational instability of a rotating partially-ionized plasma carrying a uniform magnetic field with hall effect

The problem of gravitational instability of an infinite homogeneous self-gravitating medium carrying a uniform magnetic field in the presence of Hall effect has been investigated to include the effect due to rotation. The dispersion relation has been obtained. It has been found that the Jeans's criterion for the instability remains unaffected even when the effect due to rotation is considered in the presence of Hall effect carrying a uniform magnetic.     

Thermosolutal-convective instability of a composite stellar atmosphere in the presence of a variable magnetic field

The thermosolutal-convective instability of a composite stellar atmosphere is considered in the presence of variable horizontal magnetic field and collisional effects. The criteria for monotonic instability are obtained which generalize the criterion derived for thermal-convective instability in the absence of above effects.     

Two-stream instability in plasmas with arbitrary orientation of magnetic field

The problem of two-stream instability in plasmas where electrons move through ions with arbitrary orientation of the magnetic field is discussed. Electrostatic and electromagnetic instabilities have both been discussed. It is found that the strength and orientation of the magnetic field both affect the electrostatic waves propagating along the streaming direction to a considerable extent. The electromagnetic instability with a cross-field orientation is associated with a larger range of unstable wavenumber and larger growth rates compared to any other coexisting electrostatic instability.     

Gnetic wave instability in a plasma ith relativistic particles in a random magnetic field

A new mechanism, which accounts for the direct excitation of electromagnetic waves by relativistic particles moving in random magnetic fields, is considered. We are then able to study maser-type wave excitations by slightly anisotropic particle distribution functions as well as electromagnetic wave emissions in beam-plasma interactions     

The Kelvin-Helmholtz instability in the low-latitude boundary layer

The Kelvin-Helmholtz instability on the magnetopause has frequently been invoked as a mechanism for driving geomagnetic pulsations in the Pc3–Pc5 range, as well as to explain the occurrence of surface waves on the magnetopause observed by satellites. Most theories of the instability represent the magnetopause by a sharp boundary with velocity shear. In this paper a linear theory is developed which takes into account the finite thickness of the low-latitude boundary layer on the magnetopause. The theory is in a form suitable for numerical computation and can take into account the effect of gradients in the plasma pressure, magnetic field magnitude and direction, and density. Computations show that the instability is suppressed at wavelengths short compared with the scale width of the boundary. There is thus a wavelength for which the growth rate is maximum. Extensive computations have been carried out and they show that growth can take place for a very wide range of conditions. The computations confirm earlier results snowing that maximum growth occurs for a wave vector which is perpendicular to the magnetic field. For typical solar wind conditions the theory predicts wavelengths on the magnetopause of the order of 10 times the thickness of the low-latitude boundary layer and periods in the Pc3–Pc5 range. The possible non-linear development of the instability is discussed qualitatively. The predicted results are consistent with satellite observations of pulsations.