The Onset of Magnetic Reconnection

A fundamental question concerning the release of the magnetic energy on the Sun is why it occurs only after substantial stresses have built up in the field. If reconnection were to occur readily, the released energy would be insufficient to explain coronal heating, CMEs, flares, jets, spicules, etc. How can we explain this switch-on property? What is the physical nature of the onset conditions? One idea involves the ``secondary instability'' of current sheets, which switches on when the rotation of the magnetic field across a current sheet reaches a critical angle. Such conditions would occur, for example, at the boundaries of flux tubes that become tangled and twisted by turbulent photospheric convection. Other ideas involve a critical thickness for the current sheet. Our ultimate goal is to simulate the evolution of a current sheet that is driven by shear flows at the photopheric boundary and extends upward through the chromosphere and transition region and into the corona. We report here on the status of our investigation into this fundamental problem.