Mechanisms for discrete auroral arc breakup by nonlinear Alfven wave interaction

The breakup of discrete auroral arcs on closed field lines is modeled by nu merical calculations of nonlinear magnetohydrodynamics in a cavity. The bou ndary conditions are shown to be crucially important in determining the ava ilable mechanisms for nonlinear evolution and breakup of the are. Nonlinear evolution is inhibited if the sign of the electric field is reversed in a reflecting Alfven wave. The weaker the shear time of the are compared with the Alfven wave travel time across the cavity, the more strongly the nonlin ear evolution is inhibited. Because different transverse length scales have different reflection properties related to their ability to pass through t he auroral acceleration region, arcs with small horizontal scales are subje ct to the Kelvin-Helmholtz shear flow instability while large horizontal sc ales are not. Parallels are drawn to the observed breakup phenomenology of small-scale and large-scale area.