Initiation of coronal mass ejections

This paper is a synopsis of the initiation of the strong-field magnetic explosions that produce large, fast coronal mass ejections. The presentation outlines our current view of the eruption onset, based on results from our own observational work and from the observational and modeling work of others. From these results and from physical reasoning, we and others have inferred the basic processes that trigger and drive the explosion. We describe and illustrate these processes using cartoons. The magnetic field that explodes is a sheared-core bipole that may or may not be embedded in surrounding strong magnetic field, and may or may not contain a flux rope before it starts to explode. We describe three different mechanisms that singly or in combination can trigger the explosion: (1) runaway internal tether-cutting reconnection, (2) runaway external tether-cutting reconnection, and (3) ideal MHD instability or loss or equilibrium. For most eruptions, high-resolution, high-cadence magnetograms and chromospheric and coronal movies (such as from TRACE or Solar-B) of the pre-eruption region and of the onset of the eruption and flare are needed to tell which one or which combination of these mechanisms is the trigger. Whatever the trigger, it leads to the production of an erupting flux rope. Using a simple model flux rope, we demonstrate that the explosion can be driven by the magnetic pressure of the expanding flux rope, provided the shape of the expansion is "fat" enough.