3D MHD Simulation of a Coronal Arcade Eruption by Self-Induced Shearing

We present the results of a three-dimensional time-dependent magnetohydrodynamic(MHD) simulation of the nonlinear development of instabilities of a magnetically-sheared arcade and show how it relates to coronal mass ejections (CMEs). To model the arcade eruption, we capitalize on a family of analytical solutions for initial states, which describe magnetic arcades in uniform gravity that are characterized by magnetic shear. In our simulations we find that such an arcade is unstable when subjected to small velocity perturbations and responds by slowly rising and expanding. Most significantly, we find shearing motions naturally arise in conjunction with the instability. This field line shearing is in response to the Lorentz, force which drives large amplitude Alfvén waves, which transport magnetic shear from the lower to the upper extremities of the arcade. The self-induced shear Alfvén waves, coupled with magnetic buoyancy, provide a powerful feedback mechanism that drives the arcades to a loss of equilibrium and eruption following a long period of expansion. The simulation of the arcade eruption is significant with regard to CME initiation for two major reasons. First, the arcade eruption is the result of undriven MHD instabilities and second, magnetic field line shearing is an intrinsic aspect of the instability that occurs spontaneously.