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  • 1
    Electronic Resource
    Electronic Resource
    The collisionless coalescence instability with two-species and in-plane-current effects (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2664-2673 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The investigation by means of particle simulations of the process whereby magnetic island structures tend to merge into larger units is extended to include the effects of electron dynamics and the in-plane components of the currents. With the ions marginally unmagnetized (ρi/λ∼1, where ρi is the asymptotic ion gyroradius and λ is the current sheet half-thickness), the electrons reduce the linear growth rate by some 20%, but they do not alter the basic coalescence physics. During the nonlinear stage of the pairwise merging process, a quadrupole out-of-the-plane magnetic field structure with peak amplitude ∼0.1B0 is formed, but this field collapses after the merging is completed. In the parameter regime accessible to the simulations [flux bundle size L/(c/ωpi)(approximately-greater-than)3 and mass ratio Mi/me≤36], there is no indication of any significant change in the reconnection rate that could be associated with whistler-mediated reconnection. The sequential merging of 4 islands into one large island leads to the formation of a vacuum X point and substantial particle energization. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871230
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  • 2
    Electronic Resource
    Electronic Resource
    Kinetic effects on the velocity-shear-driven instability (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1092-1101 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A comparison is made between the properties of the low-frequency, long-wavelength velocity-shear-driven instability in kinetic theory and magnetohydrodynamics (MHD). The results show that the removal of adiabaticity along the magnetic field line in kinetic theory leads to modifications in the nature of the instability. Although the threshold for the instability in the two formalisms is the same, i.e., VA(parallel)〈V0〈Cs, the kinetic growth rate and the unstable range in wave-number space can be larger or smaller than the MHD values depending on the ratio between the thermal speed, Alfvén speed, and flow speed. When the thermal speed is much larger than the flow speed and the flow speed is larger than the Alfvén speed, the kinetic formalism gives a larger maximum growth rate and broader unstable range in wave-number space. In this regime, the normalized wave number (kL) for instability can be larger than unity, while in MHD it is always less than unity. The normal mode profile in the kinetic case has a wider spatial extent across the shear layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860117
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  • 3
    Electronic Resource
    Electronic Resource
    The coalescence instability in collisionless plasmas (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3371-3381 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The process whereby magnetic island structures tend to merge into larger units is known as magnetic island coalescence. This coalescence process is investigated for the case of a collisionless plasma by means of two-dimensional particle simulations. The pairwise merging of a preformed island chain leads to a gentle redistribution of the density and current density profiles. The linear growth rates substantially exceed the magnetohydrodynamic (MHD) values for ρ/λ≥0.1, where ρ is the asymptotic ion gyroradius and λ is the current sheet half-thickness. The nonlinear evolution of the collisionless tearing instability in a long system is found to lead to a much more violent coalescence process resulting in the formation of a vacuum X region. This stage is characterized by enhanced growth rates for the long-wavelength modes that exceed the maximum linear tearing growth rate, super-Alfvénic flows away from the reconnection X line, substantial bulk heating in the parallel direction, and the formation of energetic particle tails in the direction of the inductive electric field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860392
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  • 4
    Electronic Resource
    Electronic Resource
    Two-dimensional simulations of a charge-neutral plasma beam injected into a transverse magnetic field (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 914-922 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two-dimensional (three-velocity component) electrostatic simulations are performed to study charge-neutral beam injection across a uniform vacuum magnetic field. Parameters are chosen that allow the beam to penetrate across the magnetic field by the polarization drift mechanism. Upon injection, the beam polarizes by virtue of the Lorentz force, forming space-charge boundary layers, and continues to propagate across the field at the injection velocity. The beam path curves in the direction opposite to that of ion gyration. Ions gyrating out of the positive space-charge layer allow a net electron current to flow from the head of the beam to its source. The resulting j×B force is of the correct direction and magnitude to account for the observed beam deflection. The presence of a tenuous (np/nb =1/100) ambient plasma enhances the shielding of the ions in the positive space-charge layer and permits their escape in greater quantities. The j×B force exceeds that of the vacuum case, and a more pronounced beam curvature is observed. In the presence of a marginally dense (np/nb =1/10) ambient plasma, the beam deflects sharply and partially separates into ion and electron streams. The streams then recombine, and the reconstituted beam deflects in the opposite direction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859015
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  • 5
    Electronic Resource
    Electronic Resource
    Propagation of charge-neutral beams in space: Modifications when negative ions are present (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 3587-3599 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two-dimensional (three velocity component) electrostatic simulations are used to investigate the properties of a charge-neutral beam consisting of H+, H−, and electrons which will be used in the Beams on R(underbar)ockets (BEAR) experiment to be launched in late 1987 or early 1988. For cross-field injection and beam densities much greater than the ambient plasma density, the beam splits into two approximately charge-neutral beams: a H+–e− beam that propagates down the field lines and a H+–H− beam that propagates at nearly the initial beam velocity on time scales less than the ion gyroperiod. Because of this splitting, space-charge oscillations are induced in the H+–H− component, which lead to its breakup. At lower beam denisties, particularly when the beam electron density is less than about the density of the ambient plasma, the ambient plasma response reduces the space-charge fields as the beam splits and the space-charge oscillations are suppressed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866440
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  • 6
    Electronic Resource
    Electronic Resource
    The stability of a compressible stratified shear layer (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1767-1775 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stability of a stratified shear layer is studied using the compressible magnetohydrodynamic (MHD) equations, including an effective gravity term to represent curvature effects of the flow and magnetic field line geometry. A general eigenvalue equation is derived for a two-dimensional MHD fluid. For the case of a hyperbolic tangent shear flow and exponential density profile, it is found that in the Boussinesq approximation the compressibility raises the critical Richardson number from (1)/(4) to as much as (1)/(2) , with the exact value depending on the value of the magnetic field at infinity. Under the approximation of a strong asymptotic magnetic field, but without invoking the Boussinesq approximation, it is shown both analytically and numerically that the density gradient terms cause the shear instability to be dispersive. In particular, the long-wavelength stability boundary for the case of Richardson number J=0 is characterized by a normalized phase velocity c=−1. This result implies that the solution for the stability boundary found by Satyanarayana, Lee, and Huba [Phys. Fluids 30, 81 (1987)], which required that c=−β/2 (where β is the ratio of the velocity and density gradient scale lengths), is not valid, in general.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858906
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  • 7
    Electronic Resource
    Electronic Resource
    Simulation of collisionless electrostatic velocity-shear-driven instabilities (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3770-3778 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The properties of electrostatic instabilities in velocity shear layers in collisionless plasmas are investigated by means of two-dimensional particle simulations for the case where the ion gyroradius is comparable to the scale length of the velocity shear. For modes exactly perpendicular to the magnetic field, the Kelvin–Helmholtz instability dominates the evolution of the system producing eφ/Te(approximately-greater-than)1; the observed growth rates show no reduction compared to the hydromagnetic limit. To obtain this result for the case of negative shear (v0y'/Ωi〈0), it is necessary to include the kinetic modifications to the structure of the shear layer equilibrium. For finite k(parallel) in the range 0〈k(parallel)/k〈0.04, the shorter-wavelength inhomogeneous-energy-density-driven instability cannot be identified in the simulations, and the upper limit on its excitation is eφ/Te(approximately-less-than)2×10−3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860847
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  • 8
    Electronic Resource
    Electronic Resource
    Electrostatic Kelvin–Helmholtz instability produced by a localized electric field perpendicular to an external magnetic field (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 272-275 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The evolution of a plasma with a localized electric field perpendicular to an external magnetic field is shown to be dominated by the Kelvin–Helmholtz instability. For small ion gyroradius, the instability is similar to the fluid mode. When the ion gyroradius is an appreciable fraction of the spatial extent of the electric field, the plasma is not in equilibrium, and the initial drift profile relaxes. Subsequent evolution still leads to vortex flows.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866187
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  • 9
    Electronic Resource
    Electronic Resource
    Electron-cyclotron maser instability in relativistic plasmas (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 2919-2930 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electron-cyclotron maser instability is studied for the case of an anisotropic electron velocity distribution in the regime where the relativistic corrections to the wave dispersion are significant. Solution of the linear dispersion relation reveals that when the plasma frequency–gyrofrequency ratio ωpe/Ωe〈vte/c, the instability is localized just below k⊥c/Ωe=1. The growth rate is then strongly peaked for emission at 90° to the magnetic field and is considerably larger than would be the case if the cold-plasma dispersion theory were valid. These features are confirmed by electromagnetic particle simulations. The simulations also show that saturation results from perpendicular diffusion in velocity space and that the saturation level increases as ωpe/Ωe is decreased. A quasilinear analysis predicts that the saturation level scales as (Ωe/ωpe)2 ωmaxI, where ωmaxI is the maximum linear growth rate. Applications of the maser instability to the generation of the Earth's auroral kilometric radiation are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865492
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  • 10
    Electronic Resource
    Electronic Resource
    Electromagnetic particle simulation codes (1985)
    Springer
    Space science reviews 42 (1985), S. 17-27 
    Details
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Electromagnetic particle simulations solve the full set of Maxwell's equations. They thus include the effects of self-consistent electric and magnetic fields, magnetic induction, and electromagnetic radiation. The algorithms for an electromagnetic code which works directly with the electric and magnetic fields are described. The fields and current are separated into transverse and longitudinal components. The transverse $$\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{E} $$ and $$\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{B} $$ fields are integrated in time using a leapfrog scheme applied to the Fourier components. The particle pushing is performed via the relativistic Lorentz force equation for the particle momentum. As an example, simulation results are presented for the electron cyclotron maser instability which illustrate the importance of relativistic effects on the wave-particle resonance condition and on wave dispersion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00218220
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