THE EVOLUTION OF A PLANE JET IN A NEUTRAL SHEET

The linear and nonlinear evolution of the plane magnetized jet, a magn etohydrodynamic configuration consisting of a plane fluid jet embedded in a neutral sheet, is examined. At low Alfven number (A = ratio of t he characteristic Alfven speed to the characteristic flow speed), two ideally unstable modes are found that correspond to the sinuous and va ricose modes of the fluid plane jet. Increasing A leads to a stabiliza tion of both of these modes. For large A there is a separate resistive ly unstable mode. The ideal varicose mode and the resistive varicose m ode are distinct modes with similar properties in a given range of A. A magnetohydrodynamic generalization of the Howard semicircle theorem indicates that a strong enough magnetic field will damp out all ideal modes. The stability properties of the two modes are studied in terms of their perturbation energy balances. The nonlinear evolution is quit e different for the three modes in terms of time scales and of the pro perties and spatial location of the small-scale structures that strong ly modify the initial configuration. The two ideal modes have in commo n the capability of accelerating the fluid initially at rest in a much more efficient way than the resistive one. (C) 1998 American Institut e of Physics.