Plasmoid formation in a single sheared arcade and application to coronal mass ejections

We address the question whether a plasmoid can be produced and ejected by an isolated x-invariant arcade located in a half-space, when its footpoints are sheared parallel to the x-axis, but no converging y-motions are applied. We use a recently developed MHD numerical code based upon a new efficient semi-implicit method, and well adapted for treating long time evolution problems in which small spatial scales develop spontaneously. Starting from a configuration created by a dipole located under the photosphere, we follow numerically the evolution of the sheared arcade. In the ideal case, and in contrast to previous attempts effected in such a geometry, our simulations show that, after a long phase of slow evolution, the poloidal magnetic configuration strongly inflates, while the electric current concentrates into a thin layer extending vertically in the central region, as predicted by analytical studies. Adding a small amount of resistivity then leads to the formation of a rising plasmoid, with stronger flows appearing along the separatrices.