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Simulations of the magnetohydrodynamic (MHD) dynamics of a current-vortex sheet for low values of the Alfve´n number (A=V[sub A]/V[sub 0]) in both the subsonic and transonic regimes are described. It is shown that, in agreement with previous linear results, the shear layer is unstable for all values of the sonic Mach number (M). However, the subsonic and transonic disturbances which develop differ significantly in many of their properties. In the subsonic case the evolution of the system is similar to the incompressible one since the growing perturbation is symmetric and evanescent in the cross stream direction. The inclusion of compressible effects mainly permits the study of its thermodynamic behavior. In the transonic case the growing perturbation appears to be overstable, asymmetric, oscillatory, and weakly evanescent in the cross stream direction. In their nonlinear evolution such modes, which we identify as fast MHD waves, lead to the formation of shocks and to a very different dynamics of the current-vortex system. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

It is shown that three-dimensional ideal instabilities can occur for the plane current-vortex sheet in the magnetically dominated regime. For the parameters under consideration, increasing the mean flow strength leads to decreased secondary instability growth rates. A calculation is presented which tracks the evolution of the secondary instability into the fully nonlinear regime, in which it is found that spanwise modes are dominant.© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effects of compressibility on the linear and nonlinear properties of the magnetized wake are examined, with an emphasis on the high speed flow situation. It is found that compressibility can modify properties of this system previously identified for the incompressible case. Of particular interest is an investigation of how the properties of the magnetized wake vary with the sonic Mach number. It is found that, in general, the growth rates of the unstable sinuous and varicose modes decrease with increasing Mach number and with increasing Alfvén number. However, at high sonic Mach numbers the varicose modes can have a growth rate which increases as the spanwise wave number increases, a significant difference from the incompressible case. The linear compressible equations are solved by a Chebyshev collocation technique. Nonlinear computations based on a finite volume method are also presented. Growth rates computed by both codes in the linear regime are in excellent agreement. At long times the system relaminarizes to an overall accelerated and broadened wake channel. It is found that variations in the Mach and Alfvén numbers have a strong affect on the evolution of the magnetized wake, e.g., for high M fast magnetosonic shocks are observed to develop. [ABSTRACT FROM AUTHOR]

The plane current-vortex sheet is a magnetohydrodynamic configuration in which a vortex sheet lies at the location of an electric current sheet. Most previous research on this structure has focused on the incompressible situation. In this paper some effects of compressibility on the linear stability properties of the plane current-vortex sheet are examined. The relevant compressible equations are derived and then solved by a new magnetohydrodynamic extension of the SPEctral Compressible Linear Stability (SPECLS) algorithm, a Chebyshev collocation code. Of particular interest is an investigation of how the properties of the low sonic Mach number (M) analogs of previously investigated incompressible unstable modes vary as M is increased to supersonic values. It is found that, in general, the growth rates of these modes decrease as M increases. However, new unstable modes are found to appear at high M. These new modes, which have a finite phase velocity, are also found to be weakly evanescent and oscillatory in the cross-stream direction. Further data is presented on the influence of the streamwise and spanwise wave numbers, and also the Alfvén number. The morphology of the perturbations is also discussed, with an emphasis on the temperature and mass density structure. A short discussion is also given of the effect of spatial variation of the zeroth-order temperature and mass density fields, a situation that would arise when magnetofluids with different thermodynamic properties are brought into contact with each other. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents theoretical evidence in favor of the hypothesis that coronal dissipation occurs in bursts at very small spatial scales. Total energy, duration and peak luminosity of coronal activity; Simulation of the energy dissipation in a simple forced magnetohydrodynamic system; Power-law statistics of observed energetic events in the Sun.

The properties of resistive instabilities developing in equilibrium configurations in which flows are present are studied. The linearized magnetohydrodynamics (MHD) equations are solved numerically, taking into account the effects of resistivity, adopting equilibria with magnetic and velocity fields in the ( y-z) plane, and varying the x direction with two different scale lengths. The results show that the stability properties of the system are strongly influenced by the profile of the velocity field considered. The static tearing mode is recovered in the limit of small amplitude of the velocity with respect to the Alfvén velocity and/or small values of the ratio between magnetic and velocity scale lengths. In the opposite limit we obtain a purely ideal behavior in which the Kelvin–Helmholtz instability appears to be modified by the presence of a shear magnetic field. In between those limits the effect of the velocity is to change the dependence of the growth rate on the relevant parameters and to distort the spatial profile of the perturbations with respect to the static case. [ABSTRACT FROM AUTHOR]