Back
to the home page
Results are presented from hybrid 2-D quasineutral Darwin simulations of the interchange instability in the presence of a large rf wave in the ion-cyclotron frequency range. The simulation models the plane perpendicular to the background magnetic field using cold particle ions and a cold ExB electron fluid. Related theory is also discussed. Fluid equations appropriate to the simulation model are derived and their properties demonstrated and compared to simulation. A method for solving for the rf-modified growth rates from the fluid equations is described. It is generally expected that the current component associated with the mean, rf-induced ion drift is capable of influencing the stability of the interchange mode; however, no modification of the mean ion drift is observed in simulations in which rf is present. Instead, in both the theory and simulation, an electron rf-field oscillation current dominates the modification to the gravitational current. As a result, even in the presence of large rf fields (B_{rf}/B0=15%), only modest corrections to the interchange growth rates are observed. The effect is stabilizing for kL_n<0.8-0.9, apparently for both signs of the square-electric-field gradient, and is destabilizing for larger values of kL_n, although the credibility of the simulation begins to become suspect here. Fractional reduction of the interchange growth rate is observed to be quadratically dependent on the rf wave amplitude, independent of ion-cyclotron resonant effects, and proportional to grad(B^2_{rf})/grad(B0^2), consistent with an eikonal theory developed for the study of stabilizing effects on perpendicularly propagating Alfv;én waves. The results also suggest that additional gradient-independent stabilizing effects may be operative when kL_n is order 1. Finally, it is also observed that, while the rf wave has little effect on the interchange instability, the interchange mode strongly affects the rf wave, damping it significantly as the mode saturates.
Dynamical invariants are derived for particles moving in a single, circularly polarized electromagnetic wave of arbitrary time dependence propagating parallel to a uniform background magnetic field. The invariant associated with helical symmetry is shown to restrict the particle motion to a very narrow region of velocity space. Features of the slow time-scale motion of fixed points associated with the existence of a fourth adiabatic invariant are described for the case of a slowly varying wave. Characteristics of the particle motion thus derived are applied to the analysis of 1d-3v simulations of the saturation of the Alfvén-ion-cyclotron (AIC) instability for a single wave. In particular, an explanation is offered for the appearance of a sharp edge in the velocity distribution functions observed in the simulation.
See also an abstract on the simulation method used.
A particle-in-cell simulation method is shown effective in modeling strongly coupled plasmas, exhibiting good energy conservation properties and good resolution of the dust-particle interaction. For coupling parameters of order unity, the simulation dust particles exhibit Debye shielding on the spatial scale of the dust Debye length. When initialized with a large coupling parameter, the dust particles do not organize themselves into a crystalline structure as expected, but instead are scattered by the presence of substantial electrostatic wave activity. Liquid-like or crystal-like correlations among the dust particles occur only when annealing is imposed.
Early version of the manuscript and figures.
An exact dispersion relation is obtained for linear dust-compressional waves in a one-dimensional Bravais lattice consisting of cold dust particles located at lattice points uniformly spaced in equilibrium. This general dispersion relation reduces in asymptotic limits to the dust-acoustic and dust-lattice wave dispersion relations, and thus offers a unified perspective on both types of waves. Analysis of the dust-compressional wave dispersion relation in the presence of damping reveals interesting trends that a pose a challenge for future plasma crystal experiments.
A. Bhattacharjee, C. A. Kletzing, Z.W. Ma, C. S. Ng, N. F. Otani, X. Wang, Four-Field Model for Dispersive Field-Line Resonances: Effects of Coupling Between Shear-Alfvén and Slow Modes, Geophysical Research Letters 26, 3281-3284 (1999).
A new theoretical model is proposed far dispersive field-line resonances in collisionless magnetospheric plasmas on the basis of reduced four-field equations. The model improves upon the predictive capabilities of earlier two-field models. In particular, due to the coupling of the shear-Alfven mode to the slow mode in the four-field system, it is now possible to account for the observed low frequencies of field-line resonances. Furthermore, parallel electric fields can be large without requiring the field-aligned current density to be unrealistically large. Qualitative implications for recent FAST and ground-based observations are discussed.
More basic plasma physics: See also the abstracts on MHD dynamos.
Ionospheric, magnetospheric, and solar abstracts.
Computational plasma physics abstracts.
Computational biology abstracts.
Back to the publication list.
Back
to the home page