THE EVOLUTION OF PLANE CURRENT-VORTEX SHEETS

The linear and nonlinear evolution of the plane current-vortex sheet, with a basic magnetic field given by B-0(y)= tanh y (e) over cap(z) an d a basic velocity field given by W-0(y)= V tanh Ry (e) over cap(z), i s examined. The discovery of an ideal instability in a large region of parameter space previously found to be stable is reported. In this pa per numerical evidence is presented that this parameter regime is in f act highly unstable, with growth rates exceeding those of the modes ex isting in the region of parameter space previously found to be unstabl e. An examination of the perturbation energy balance indicates that en hanced energy transfer from the basic velocity field to the perturbed velocity and magnetic fields is responsible for the enhanced growth ra te. This occurs due to processes absent from both the resistive and Ke lvin-Helmholtz instabilities. Nonlinearly it is found that magnetic re connection can occur on an ideal time scale in certain cases. These fa ster instabilities lead to a more violent cascade of excitation in the streamwise direction, as evidenced by the rapid formation of higher h armonics of the initial disturbance. A nonlinear saturation due to inc reased correlation of the perturbed velocity and magnetic field occurs for all cases. (C) 1997 American Institute of Physics.