Low-frequency waves in asymmetric magnetized relativistic pair plasma

The properties of waves in a pulsar magnetosphere are considered in the most general case of a non-neutral, current-carrying pair plasma with arbitrary distribution functions for electrons and positrons. General dispersion relations are derived for a strong but finite magnetic field, including gyrotropic terms caused by the deviations from quasi-neutrality and the relative streaming of electrons and positrons. It is shown how the ellipticity of the wave polarization depends on the plasma parameters and angle of propagation. Two examples of plasma distributions are analysed numerically: a waterbag distribution and a piecewise distribution that models the numerical result for pair cascades. A possible application to the interpretation of the observed circular polarization of some pulsars is discussed.     

The instability of the electromagnetic ordinary modes in counterstreaming and counterrotating plasmas

The instability of a linearly-polarised electromagnetic ordinary mode in counterrotating plasmas and propagating perpendicular to a uniform magnetic field caused by a counterstreaming of electrons along the latter is studied using a cold-plasma model. It is found that: (i) In the presence of either a streaming or a rotation or both, the ordinary-wave propagation is possible even for frequencies less than the plasma frequency; (ii) the Coriolis forces like the applied magnetic field stabilise the ordinary modes.     

Propagation of Electromagnetic Waves in Pulsar Magnetospheres

We have developed a numerical code for the propagation of different electromagnetic modes in a pulsar magnetosphere filled by a relativistic, streaming electron-positron plasma in a strong, curved magnetic field. We determine the trajectories, limiting polarization and damping of the waves leaving the magnetosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wave propagation in two-fluids self-gravitating plasma

Wave propagation is considered in self-gravitating collisionless magnetized plasma, when the Larmor frequency exceeds the plasma frequency. The external magnetic field is assumed to be strong and a modified two-fluids theory is used to describe the plasma. We find that there are three modes of wave propagation parallel to the magnetic field. The condition of hose instability is affected. The change in the dispersion relation due to the two-fluids theory is also discussed.     

Production of pulses and interpulses in pulsars

Pulsars accelerate the charged particles moving along their magnetic field lines due to their rapidly spinning motion. Particles gain maximum energy from pulsars within the light cylinder when they are moving along the field lines perpendicular to the rotation velocity. In pulsars with non-aligned rotation and magnetic axes, the production of two intense and sharp pulses (main pulse and interpulse) separated by 180° longitude occur at the two regions near the light cylinder where the rotation velocity is perpendicular to the magnetic field. Since the radiating particles move radially along the relativistically compressed magnetic field lines, the observer in the stationary frame receives beamed and transversely compressed radiation pulse. Near the light cylinder position angle varies smoothly during pulsar rotation in a way as Radhakrishnan and Cook (1969) expect its variation near the magnetic pole, as the field lines experience relativistic compression in the direction of rotation. The motion of two charge species along the field lines produce orthogonal modes at each pulse longitude.     

On the Characteristics of Wave Propagation in a Magnetized Dusty Plasma with Streaming of Electrons and Ions

Propagation of waves in a magnetized dusty plasma are studied for all the range of values of ion-cyclotron frequency, and having streams of electrons and ions. The dispersion relation is obtained for the waves propagating through the dusty plasma and analysed for different modes of propagation for relative abundance of dust in the plasma. It is observed that abundance of dust, streaming motions of electrons and ions have an important influence on the propagation of waves in the dusty plasma particularly when the phase velocity of the wave is in the low frequency region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Numerical computer simulation of cross-field propagation of a plasma stream in a magnetic field

A one-dimensional electrostatic particle simulation of plasma streaming perpendicular to a magnetic field with nonperiodic boundary condition has been carried out. When a bulk of plasma in injected across an ambient magnetic field, a stream of neutral plasma, consisting of equal numbers of ions and electrons, polarizes, and the resulting polarization electric field gives rise to the penetration of plasma across the magnetic field so that the integrity of plasma maintains. Computer simulation demonstrates the properties of cross-field propagation of plasma stream in a magnetic field with different plasma parameters.     

Radiative modes of a weakly ionized,collision-dominated,turbulent plasma

We obtain, and discuss, the roots of the dispersion relation describing normal mode propagation in a weakly ionized, collision dominated turbulent plasma with an isotropically distributed turbulent magnetic field. We demonstrate that, depending on the level of the turbulent field relative to the collision frequency, there may, or may not, be propagating, but decaying, modes present in the system. The structure and properties of the modes depend on both the precise level of the turbulent magnetic field and its spatial and temporal correlation.For simple correlation functions we estimate the decay rate of the radiative modes. Using galactic parameters as an illustration we show that only the long-wavelength (low-frequency) modes last for a significant length of time ( a collision time).We have done this calculation to demonstrate that the response of turbulent plasma systems is more complicated than simple order of magnitude calculations indicate; and to illustrate a method which may, perhaps, be gainfully employed in more complex turbulent astrophysical plasma situations than considered here.     

Magnetogravitational instability of a finitely-conducting medium with variable streaming motion

A discussion of gravitational instability of a finitely conducting medium with streams of variable velocity distribution is made in the presence of a uniform magnetic field. It is found that the variable streaming motion shows a destabilizing effect and affects the instability criterion only in the case of general wave propagation. For purely parallel propagation to the direction of the magnetic field and the streaming motion, the criterion is independent of the variation in the streaming motion and further the Jeans's criterion is found to remain unaffected in this case. For purely transverse propagation, the criterion is independent of any streaming motion and the Jeans's criterion remains unaffected. The criterion is further independent of the magnetic field and the finite conductivity except in the case of transverse propagation where the magnetic field exhibits a stabilizing influence in case of an infinitely conducting medium.     

Saturated magnetic field amplification at supernova shocks

Cosmic ray streaming instabilities at supernova shocks are discussed in the quasi-linear diffusion formalism which takes into account the feedback effect of wave growth on the cosmic ray streaming motion. In particular, the non-resonant instability that leads to magnetic field amplification in the short wavelength regime is considered. The linear growth rate is calculated using kinetic theory for a streaming distribution. We show that the non-resonant instability is actually driven by a compensating current in the background plasma. The non-resonant instability can develop into a non-linear regime generating turbulence. The saturation of the amplified magnetic fields due to particle diffusion in the turbulence is derived analytically. It is shown that the evolution of parallel and perpendicular cosmic ray pressures is predominantly determined by non-resonant diffusion. However, the saturation is determined by resonant diffusion which tends to reduce the streaming motion through pitch angle scattering. The saturated level can exceed the mean background magnetic field.     

On the propagation of hydromagnetic waves in a plasma of thermal and suprathermal components

The propagation of MHD waves is studied when two ideal fluids, thermal and suprathermal gases, coupled by magnetic field are moving with the steady flow velocity. The fluids move independently in a direction perpendicular to the magnetic field but gets coupled along the field. Due to the presence of flow in suprathermal and thermal fluids there appears forward and backward waves. All the forward and backward modes propagate in such a way that their rate of change of phase speed with the thermal Mach number is same. It is also found that besides the usual hydromagnetic modes there appears a suprathermal mode which propagates with faster speed. Surface waves are also examined on an interface formed with composite plasma (suprathermal and thermal gases) on one side and the other is a non-magnetized plasma. In this case, the modes obtained are two or three depending on whether the sound velocity in thermal gas is equal to or greater than the sound velocity in suprathermal gas. The results lead to the conclusion that the interaction of thermal and suprathermal components may lead to the occurrence of an additional mode called suprathermal mode whose phase velocity is higher than all the other modes.     

Model of polarization state of compressional surface MHD waves in the low-altitude cusp

Examination of thermal plasma data obtained by low-altitude satellite measurements indicates that the intersection of the cusp in the dayside magnetosphere with the topside ionosphere creates a distinct plasma geometry at low altitudes. This region consists of one or two plasma discontinuities with steep boundaries. As a result of the plasma structuring in the cusp which commonly takes place in the winter hemisphere, the propagation of compressional surface MHD waves is supported. This point is illustrated by an analysis of the polarization state of compressional surface MHD waves propagating along a plasma layer with thickness a and ambient magnetic field B₀ parallel to the interfaces. The results obtained are applicable to the case of a single interface, which is derived in the limit a → ∞. In the general case the polarization of the compressional surface MHD waves in the plane transverse to the magnetic field B₀ is elliptical. This feature of the polarization state of the compressional surface modes does not follow from the former analysis by Edwin and Roberts (1982, Solar Phys. 76, 239) for a magnetic slab, because the disturbance components parallel to the interfaces and perpendicular to the magnetic field B₀ have not been examined. Although the absence of these components does not prove to be essential for deriving the exact dispersion equation for arbitrary wave directions of the surface modes, they must be included when considering polarization states. The surface mode polarization in the plasma layer changes its sense three times: at interfaces X = 0 and X = a and in the middle plane X = a/2. For the symmetrical (sausage) mode the wave disturbance component b_n transverse (normal) to the interfaces becomes zero in the middle plane; for the asymmetrical (kink) mode, the component b_t parallel to the interfaces and transverse to the ambient magnetic field is zeroed in the same plane. For a moving observer such as a satellite the polarization patterns which might be recorded change, depending on the velocity of the observer and the angles at which the layered cusp is traversed. An essential feature in the polarization of the compressional surface MHD modes is the presence of jumps in the magnetic disturbance component b_t at the interfaces. These jumps disappear only for propagation along the ambient magnetic field. In this particular case the component b_t vanishes and then the surface modes are undistinguishable from the body modes.     

Fluid description of a collisionless plasma and electric current systems in cosmic plasmas

The large-scale structure of a collisonless, two-component plasma with a typical Larmor radius of ions ? and scale-lengthL is discussed using Maxwell transport equations. Special attention is paid to the situations in which the usual one-fluid model of plasma based on the expansion of the transport equations in the powers of the ratio ?/L is not a satisfactory approximation. The one-fluid model fails if the magnetic-field-aligned component of the mass velocity or the magnetic-field-aligned component of the typical random velocity of particles is much larger than the other components of the mass and random velocities. The model also fails if the component of the typical random velocity of particles, which is perpendicular to the field lines, substantially exceeds the mass velocity of particles across the field lines. A quasi-static plasma is discussed as an example of plasmas on which the expansion in the powers of ?/L is not applicable. The relation between the electric current flowing in a quasi-static plasma (or in a hot plasma streaming along the field lines) and the topology of the magnetic lines of force is analysed. There are two distinguishable currents of different origin in such a plasma. Magnetic field generated by the currents acquires a geometry in which one current flows in the surfaces perpendicular to the binormals to the field lines while the other current flows along the binormals.     

MHD studies on the free convection and hall effects on waves in a semi-infinite plasma

In this paper, analytical MHD studies are made on the propagation of oscillatory waves in a semi-infinite plasma which is exposed to an applied magnetic field. The oscillatory wave is introduced into the plasma environment by temperature perturbation.The phase speeds of the resultant disturbance are discussed in terms Grashof, Prandtl, and Eckert numbers (free-convection parameters), Hall parameter, Alfvén parameter, and frequences. Expansion about small Eckert number is made to solve the very nonlinear coupled partial differential equations for the field variables. The appearance of steady streaming at all times in the first order expansion is mentioned.     

Propagation of Alfven waves in a non-uniform plasma stream

The propagation of the Alfven waves in a plasma stream with a non-uniform density distribution is considered. It is shown that the density inhomogeneity will cause self-scattering of the wave. A longitudinal magnetic field component will be generated and part of the energy of the wave will propagate in directions deviating from the given mean magnetic field. Thus, to explain certain observed features of the solar wind, it is not necessary to appeal to a mixture of Alfvenic and magnetosonic modes.     

On the structure of azimuthally small-scale ULF oscillations of hot space plasma in a curved magnetic field. Modes with continuous spectrum

The problem of propagation of azimuthally small-scale ULF modes in plasma with 1D inhomogeneity and variable curvature of magnetic lines of force is analyzed. The propagation regions and the transverse structure of stable Alfven and cusp modes, as well as unstable ballooning modes, are determined. It is shown that long-living ballooning and cusp modes can exist. Our results qualitatively describe the behavior of ULF modes with continuous spectrum in the geomagnetosphere and can be used for interpretation of spacecraft and SuperDARN radar measurement data.     

Effect of rotation and self-gravity on the propagation of MHD waves

Magnetohydrodynamics waves and instabilities in rotating, self-gravitating, anisotropic and collision-less plasma were investigated. The general dispersion relation was obtained using standard mode analysis by constructing the linearized set of equations. The wave mode solutions and stability properties of the dispersion relations are discussed in the propagations transverse and parallel to the magnetic field. These special cases are discussed considering the axis of rotation to be in transverse and along the magnetic field. In the case of propagation transverse to the magnetic field with axis of rotation parallel to the magnetic field, we derived the dispersion relation modified by rotation and self-gravitation. In the case of propagation parallel to the magnetic field with axis of rotation perpendicular to the magnetic field, we obtained two separate modes affected by rotation and self-gravitation. This indicates that the Slow mode and fire hose instability are not affected by rotation. Numerical analysis was performed for oblique propagation to show the effect of rotation and self-gravitation. It is found that rotation has an effect of reducing the value of the phase speeds on the fast and Alfven wave modes, but self-gravitation affect only on the Slow modes, thereby reducing the phase speed compare to the ideal magneto hydrodynamic (MHD) case.     

Dust acoustic shock waves in suprathermal dusty plasma in the presence of ion streaming with dust charge fluctuations

Dust acoustic (DA) shock waves are investigated in a dusty plasma having a high-energy-tail electron distribution. The effects of ion streaming, charge variation and electron deviation from the Maxwellian distribution on the DA shock wave are then considered. It is shown that as the suprathermal character of the plasma is increased, the potential amplitude enhances. It is also found that the ion temperature may be destructive for the formation of DA shock waves. Their strength decreases with increasing ion streaming speed. Our results may be useful in understanding the basic nonlinear features of the DA wave propagation that may occur in space dusty plasmas, especially those including a relative motion between species (comet tails, solar wind streams, etc.).     

Waves in a magnetically structured slab configuration

The problem of wave-propagation in a magnetically structured compressible slab configuration is investigated, allowing for different magnetic field strengths inside and outside the slab and also a general orientation of the field vectors relative to each other and to the propagation vector. Several magnetic field geometries such as equal parallel, and equal orthogonal fields are considered. Properties of body and surface waves both for symmetric and asymmetric modes of perturbation propagating along and normal to the slab field are investigated idealising the slab to be incompressible, or considering the limiting case of wide and slender compressible slab. Numerical results are also obtained for a compressible slab of finite thickness for a specific choice of sound and Alfvén speeds involved.