Coronal energy release via ideal three-dimensional instability

It is widely believed that most coronal phenomena involve the release of free energy that is stored within stressed magnetic field configurations. The availability of sufficient free energy to explain everything from coronal heating to flares and coronal mass ejections is well established. How this energy is released remains a major puzzle. Observations reveal that an important property of the energy release mechanism is its "switch on" character. The mechanism must remain dormant for long periods of time to allow the magnetic stresses to build, then it must operate very vigorously once it finally turns on. We discuss a mechanism called the "secondary instability" which exhibits this behavior. It is essentially an ideal instability of the thin twisted magnetic flux tubes that form from the resistive tearing of current sheets. We relate the mechanism to the coronal heating idea of Parker in which the coronal magnetic field becomes tangled by random motions of the photospheric footpoints. Global energy balance considerations imply that magnetic energy dissipation occurs at a particular angle in the field, and the secondary instability offers the first quantitative explanation for why this should be. It thus places Parker's popular idea on a much firmer physical footing. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.