Electromagnetic instability in collisionless anisotropic plasma contrastreams

The electromagnetic instability in collisionless, anisotropic magnetized plasma contrastreams for perturbations propagating normally to the ambient magnetic field is investigated. It is found that the parallel temperature stabilizes the configuration forT _‖≤T _⊥ while it shows destabilizing effect forT _‖>T _⊥. A comparative study of the growth rates associated with various coexisting modes reveals that the electromagnetic instability may, under certain conditions, be associated with larger growth rates than the electrostatic instability.     

Contrastreaming instability in collisionless anisotropic plasmas

The problem of contrastreaming instability in collisionless anisotropic plasmas is investigated both for electrostatic and electromagnetic perturbations propagatingalong the ambient magnetic field. Both electron-electron and ion-ion streams are considered. It is found that the electromagnetic instability may, under certain conditions, be characterised by larger growth rates than the electrostatic instability.     

Jeans instability of collisionless systems

Linear dispersion relations are solved numerically for collisionless self-gravitating systems. The results are compared to those of hydrodynamic approach. Both theories yield similar dispersion relations, with predictions of instability when the system is cold enough. In a collisionless system, however, the perturbation is found to die away without oscillations when the system is stable. A mixed system of hot-component particles and cold-component particles is also studied. The stability of such system is dominated by the temperature of the cold particles. Again, the oscillatory behaviour is not found in this case, regardless of the stability of the system.     

Two-stream instability in the presence of longitudinal magnetic field

A multiple sclaes perturbation theory has been applied to investigate the nonlinear behaviour of beam-plasma system near a marginally stable state in the presence of longitudinal magnetic field. The perturbation method leads to a nonlinear Schrödinger equation for the finite amplitude. The coefficients of this equation show that only if the beam is compressed isothermally can there exist a range of wavenumbers for which stabilization might occur. The stable region increases with the applied magnetic field.     

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.     

Wave coupling at a collisionless plasma discontinuity

The coupling of magnetoacoustic waves at a plane interface that separates two semiinfinite collisionless fluids is studied. The fluids are characterized by different temperatures along and transverse to the ambient magnetic field. Continuum equations obtained by Chewet. al. (1956) are used and expressions for the reflection and transmission coefficients are derived. Extensive numerical computations are done to study the variation of the reflection coefficient, with the angle of incidence, for various temperature anisotropies of the media. Relevance of these investigations to the magnetosphere-solar wind boundary is discussed.     

A shock hodograph in collisionless plasma

In this paper, the shock hodograph (polar) for a collisionless transverse shock has been developed in the plane of the flow deflection angle and total pressure jump. The sonic point on the hodograph lies closer to the characteristic than to the point of maximum flow deflection for an attached shock. This hodograph is particularly useful in the analysis of three shock confluences and refraction of shock waves at gas interfaces. The first analysis is fully described in this paper. It is observed that the third shock wave is forward facing. It is pertinent to note that the limited region of supersonic flow also restricts the occurrences of three shock interactions.     

Current interruption in a collisionless plasma by non-linear electrostatic waves

The existence of current-interrupting non-linear electrostatic waves in the form of negative solitons is demonstrated. Self-consistent, non-linear electrostatic potentials are constructed assuming that a current may be interrupted by trapping current-carrying electrons in such a potential. A significant fraction of the current-carrying electrons is trapped by the potential if the electron thermal velocity is much less than the electron streaming velocity. In one class of solutions, the negative solitons, the current may be reduced to a fourth of its initial value in the limit of high ion-electron temperature ratio.     

Double probe theory for a low-density flowing collisionless plasma

Existing theory for cylindrical and spherical probes are used to derive expressions for double-probe current-voltage characteristics under low density, flowing collisionless conditions. These conditions prevail when the following conditions hold: charged particle mean free path 》 Debye length 》 probe radius, and ion thermal velocity ? probe speed ? electron thermal velocity. Explicit formulae are given for calculating electron temperature and plasma density for both spherical and cylindrical probes.     

Long-wavelength MHD instability in the prefront of collisionless shocks with accelerated particles

Collisionless shocks in turbulent space plasmas accelerate particles by the Fermi mechanism to ultrarelativistic energies. The interaction of accelerated particles with the plasma inflow produces extended supersonic MHD flows of multicomponent plasma. We investigate the instabilities of a flow of three-component turbulent plasma with relativistic particles against long-wavelength perturbations with scales larger than the accelerated particle transport mean free path and the initial turbulence scales. The presence of turbulence allows us to formulate the system of single-fluid equations, the equation of motion for the medium as a whole, and the induction equation for the magnetic field with turbulent magnetic and kinematic viscosities. The current of accelerated particles enters into the induction equation with an effective magnetic diffusion coefficient. We have calculated the local growth rates of the perturbations related to the nonresonant long-wavelength instability of the current of accelerated particles for MHD perturbations in the WKB approximation. The amplification of long-wavelength magnetic field perturbations in the flow upstream of the shock front can affect significantly the maximum energies of the particles accelerated by a collisionless shock and can lead to the observed peculiarities of the synchrotron X-ray radiation in supernova remnants.     

Total current to cylindrical collectors in collisionless plasma flow

A theory is presented for charged-particle collection by a cylindrical conducting object, such as a spacecraft or an electrostatic probe, which is moving transversely through a collisionless plasma, such as those in the upper atmosphere and space. The calculation is approximate, using symmetric potential profiles which are exact for the infinite-cylinder stationary case. Theoretical current predictions are presented for ratios of collector potential to electron thermal energy eφ_c/kT_e from 0 to ?25, for ion-to-electron temperature ratios T_i/T_c = 1 and 0.5, ratio of collector radius to electron Debye length r_c/λ_D from 0 to 100, and ratio of flow speed to ion thermal speed $\text{S}{}_{\mathrm{i}}\text{= U/(2kT}{}_{\mathrm{i}}\text{/m}{}_{\mathrm{i}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}$ from 0 to 10. Comparisons with existing exact calculations by other authors show that none of these fulfil all of the requirements for nontrivial comparison. Appropriate parameter ranges for future exact calculations are thereby suggested. These are as follows: (a) r_c/λ_D should be large enough that the collector not be in or near orbit-limited conditions; (b) the ratio $\text{S}{}_{\mathrm{i}}{}^2\text{/¦χ}{}_{\mathrm{c,\; i}}\text{¦}$ of ion directed energy to potential energy change in the sheath, should be close to unity or if

$\text{S}{}_{\mathrm{i}}{}^2\text{/¦χ}{}_{\mathrm{c,i}}\text{¦? 1,}\text{then}\text{S}{}_{\mathrm{i}}\text{? 1}$

.     

Magnetostatic structures in collisionless plasma and their synchrotron radiation

The properties of the magnetostatic structures in a collisionless relativistic plasma found by exactly solving the nonlinear self-consistent kinetic and Maxwell equations using the method of invariants of particle motion are analyzed. These structures include individual neutral current sheets and cylindrically symmetric filaments as well as their ensembles, admitting a wide variety of types of particle energy distribution functions. Relationships are established between the possible parameters of the current sheets and filaments—their sizes, currents and magnetic fields, and the degree of anisotropy of the particle momentum distribution. The extent to which these parameters are conditioned by the properties of the Weibel instability that can produce these structure is also discussed. The spectral peculiarities of the synchrotron radiation from the particles that form the current sheets and filaments are investigated in the special case of power-law energy distribution functions.     

Thermal instability in a weakly ionized plasma

We revisit the problem of clump formation due to thermal instabilities in a weakly ionized plasma with the help of a linear perturbation analysis, as discussed by Nejad-Asghar & Ghanbari. In the absence of a magnetic field and ambipolar diffusion the characteristic equation reduces to the thermal instability described by Field. We derive the critical wavelengths, which separate the spatial ranges of stability and instability. Contrary to the original analysis of Nejad-Asghar & Ghanbari, perturbations with a wavelength larger than the critical wavelength destabilize the cloud. Moreover, the instability regime of isentropic perturbations is drastically reduced. Isobaric modes with real values of the critical wavelength appear only if the density dependence of the cooling rate is more pronounced than the temperature dependence. Isentropic modes arise only if the power of the density in the cooling rate is smaller than 1/2, which is not fulfilled for CO cooling. We find that ambipolar diffusion is not a dominating heating process in molecular gas.     

Current-driven dust-acoustic instability in a collisional plasma

The excitation of the dust-acoustic instability in a collisional dusty plasma is investigated. For conditions similar to those of recent laboratory experiments with neutral gas pressures of 100mTorr, it is found that zero-order electric fields E₀ 10 Vm⁻¹ are sufficient for the growth of perturbations with centimeter wavelengths. Much larger wavelengths generally require larger values of E₀. Free electrons in the dusty plasma have a stabilizing effect, which can be very pronounced at the longest wavelengths.     

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.     

Theory of instantaneous triple-probe method for direct-display of plasma parameters in a low density flowing collisionless plasma

An exact theory is developed for a triple-probe in an orbit-motion-limited flowing collisionless plasma, i.e. when the charged particle mean free path ? Debye length ? probe radius, and the electron thermal velocity ? probe speed ? ion thermal velocity. Formulae for determining electron temperature and electron density are given for both spherical and cylindrical probes. Analytical results show that the effect of ion temperature on measurements of plasma parameters is small when the probe speed is large.     

Rädler effect in a collisionless plasma and field-aligned currents in the magnetotail

The mean electromotive force perpendicular to the mean current (Rädler effect) by random hydromagnetic waves in a collisionless plasma is derived. The results are applied to the field-aligned currents in the Earth's magnetotail. It is shown that the Rädler-effect electric field is large enough to give the observed value of the field-aligned currents and can be identified as a possible source for the field-aligned currents.     

Thermosolutal instability of a partially-ionized plasma

The thermosolutal instability of a partially-ionized plasma in the presence of a horizontal magnetic field is considered to include the frictional effect of collisions of ionized with neutrals. The sufficient conditions for non-existence of overstability are derived. The solute gradient and magnetic field introduce oscillatory modes in thermosolutal convection which were non-existent in their absence. The magnetic field and stable solute gradient are found to have stabilizing effects whereas collisional effect of ionized with neutrals is found to have destabilizing effect on thermosolutal instability of a partially ionized plasma.