摘要:

Low frequency plane waves supported by a medium containing a thermal plasma of isotropic pressure and a suprathermal collisionless plasma having anisotropic pressure are investigated. The usual Alfve´n, slow and fast modes of isotropic pressure magnetohydrodynamics persist. In addition, a suprathermal mode appears which displays a rich variety of behavior due to an additional degree of freedom compared to the analogous mode when both the plasma components are described by collision‐dominated magnetohydrodynamics. Since these modes are significant in a number of situations, they are extensively investigated by computing their phase speeds for wide‐ranging numerical parameters. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871942

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Classical mechanics provides the intuitive and unified description of spontaneous emission, Landau growth and damping of Langmuir waves, the cold beam–plasma instability, and van Kampen modes. This is done by studying the interaction betweenMweak modes of a plasma without resonant particles andNquasiresonant particles, which leads to an exactly solvable high‐dimensional Floquet problem. Growth corresponds to an eigenmode of the system, whereas damping requires statistical averaging. Both imply synchronization of near‐resonant particles with waves, and the corresponding force on individual particles is computed explicitly. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871943

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In the paper we consider theoretically electromagnetic modes in a homogeneous cold plasma sphere. We find, in addition to the surface transverse magnetic modes, known from the electrostatic analysis of the same structure, two new families of weakly leaky modes (one of them consisting of transverse electric modes). A classification of the modes is proposed. An approximate analytic expression for the wave damping rate of the surface modes due to energy leakage is given, making it possible to draw conclusions on the behavior of more complex three‐dimensional plasmas. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871944

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Linear magnetohydrodynamic (MHD) modes in a cold, nonrelativistic electron–positron plasma shear flow are considered. The general set of differential equations, describing the evolution of perturbations in the framework of the nonmodal approach is derived. It is found, that under certain circumstances, the compressional and shear Alfve´n perturbations may exhibit large transient growth fueled by the mean kinetic energy of the shear flow. The velocity shear also induces mode coupling, allowing the exchange of energy as well as the possibility of a strong mutual transformation of these modes into each other. The compressional Alfve´n mode may extract the energy of the mean flow and transfer it to the shear Alfve´n mode via this coupling. The relevance of these new physical effects to provide a better understanding of the laboratorye+e−plasmas is emphasized. It is speculated that the shear‐induced effects in the electron–positron plasmas could also help solve some astrophysical puzzles (e.g., the generation of pulsar radio emission). Since most astrophysical plasmas are relativistic, it is shown that the major results of the study remain valid for weakly sheared relativistic plasmas. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871945

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Particle‐in‐cell (PIC) simulations are used to study the penetration of magnetic field into plasmas in the electron‐magnetohydrodynamic (EMHD) regime. These simulations represent the first definitive verification of EMHD with a PIC code. When ions are immobile, the PIC results reproduce many aspects of fluid treatments of the problem. However, the PIC results show a speed of penetration that is between 10% and 50% slower than predicted by one‐dimensional fluid treatments. In addition, the PIC simulations show the formation of vortices in the electron flow behind the EMHD shock front. The size of these vortices is on the order of the collisionless electron skin depth and is closely coupled to the effects of electron inertia. An energy analysis shows that one‐half the energy entering the plasma is stored as magnetic field energy while the other half is shared between internal plasma energy (thermal motion and electron vortices) and electron kinetic energy loss from the volume to the boundaries. The amount of internal plasma energy saturates after an initial transient phase so that late in time the rate that magnetic energy increases in the plasma is the same as the rate at which kinetic energy flows out through the boundaries. When ions are mobile it is observed that axial magnetic field penetration is followed by localized thinning in the ion density. The density thinning is produced by the large electrostatic fields that exist inside the electron vortices which act to reduce the space‐charge imbalance necessary to support the vortices. This mechanism may play a role during the opening process of a plasma opening switch. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871946

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A comprehensive investigation of electromagnetic wave and instability phenomena in partially ionized, magnetized dusty plasmas has been carried out. By employing the multi‐fluid balance equations along with the Maxwell equations, a compact set of coupled field equations for the cases in which the dust grains are either robust (vd=0) or dynamic is derived. These systems of partial differential equations are used to study wave phenomena and resistive tearing mode instabilities analytically as well as by means of numerical simulations. For robust dust grains, it is shown that coupled sound‐Alfve´n waves can appear even in the absence of ion‐neutral collisions. The unstable tearing modes are coupled to convective drift modes, if the dust number density is inhomogeneous. In the induction equation two new source terms for self‐generation of magnetic fields can be identified. In parameter regimes that are characterized by dynamic dust grains, the low‐frequency phenomena develop on timescales that are governed by the dust particle inertia rather than the ion inertia, as it is the case in dust‐free plasmas. The results of this investigation should be useful in understanding the properties of low‐frequency electromagnetic wave phenomena and the formation of coherent structures in dusty magnetoplasmas whose main constituents are negatively charged dust grains, singly charged positive ions, and neutrals. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871947

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The relation between the Lyapunov stability of a Hamiltonian system and the thermal stability of a fluid whose temperature is controlled from outside is explored: The free energy as a functional of the correct variables (specific volume, local entropy, and some Clebsch potentials of the velocity) may serve as a Lyapunov functional, depending on the ‘‘Casimirs’’ as exchanged quantities. For a multi‐species plasma one obtains a sufficient condition for stability: &ggr;(v2/c2s)−1 <d ln T/d ln n<&ggr;−1 for each species, where &ggr; is the adiabatic index andcsthe sound speed. Some features of partially relaxed (T=const) cylindrical plasmas are also discussed. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871948

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The eigenvalue problem for linear stability of concentric radial profiles of current and vorticity in reduced forms of three‐dimensional magnetohydrodynamics is solved numerically. Arbitrary relative amplitudes of the velocity and magnetic fields are considered. Vorticity profiles are unstable if nonmonotonic, but are stabilized by a poloidal magnetic field when the on‐axis vertical current is at least as large as the on‐axis vertical vorticity. Nonmonotonic current profiles are less efficient at stabilization. When the neutral modes have vertical structure, an added poloidal magnetic field does not stabilize the mode unless the vertical field is also moderately strong. Current profiles in which the integrated current changes sign, although spectrally stable, are shown to be nonlinearly unstable via both numerical solution and Lyapunov techniques. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871949

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Magnetic annihilation occurs when two oppositely directed magnetic fields are brought together by a plasma flow. Several exact nonlinear solutions exist which typically depend on the ratios of plasma pressure to magnetic pressure (the plasma beta), inflow speed to global Alfve´n speed (the Alfve´n Mach number) and of the advective to diffusive terms of the induction equation (the Lundquist number). Ensuring that the plasma pressure is everywhere positive restricts the freedom of choice of these parameters, however. Restrictions on the plasma beta are derived for the cases of two‐ and three‐dimensional annihilation and two‐dimensional reconnective annihilation. At the inflow speeds typically required for fast reconnection in diffuse astrophysical plasmas the minimum plasma beta is several orders of magnitude larger than the observed values of unity or less. In other words, at the observed plasma beta the models are only valid for extremely small annihilation rates. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871950

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this paper, the effect of velocity shear on Rayleigh–Taylor vortices has been demonstrated. An inhomogeneous plasma is considered with a density profile such that the diamagnetic drift velocityVn=(cTe/eB)dn0/dxis a constant and includes the effect of an ambient poloidal shear flowVeq(x)=V⊥0′(x−x0)y. The final equation describing the stationary Rayleigh–Taylor vortex is shown to have the structure of a nonlinear Poisson equation, where the nonlinearity arises essentially because of the velocity shear term. This equation has been solved numerically and it has been shown that qualitatively new two‐dimensional monopole vortex solutions may be obtained in the appropriate parameter space. Therefore, a new important nonlinear effect related to equilibrium shear flow has been identified in the calculations of Rayleigh–Taylor vortices which results in monopole‐like solutions in plasmas. ©1996 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.871951

出版商:AIP    

年代:1996

数据来源: AIP