SHEAR-INDUCED INSTABILITY AND ARCH FILAMENT ERUPTION - A MAGNETOHYDRODYNAMIC (MHD) NUMERICAL-SIMULATION

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a 'Dynamic Shearing Instability', with a growth rate of about square-root 8 VBAR(A)a, where VBAR(A) is the average Alfven speed and a-1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfven speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs). To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.