The two main questions for MHD to try and answer about major eruptive solar flares are: why does the eruption occur and what are the details of the magnetic energy conversion ? Trying to answer the first has had a long history and it has proved very difficult to persuade magnetic arcades to erupt in numerical experiments. However, a viable answer has been proposed by Priest and Forbes (1990, Solar Phys. 126, 319), who developed a catastrophe model for the eruption, in which a slow evolution through a series of equilibria in response to flux cancellation leads at some point to a loss of equilibrium and eruption. The basic model has more recently been developed further, both analytically and numerically, in a series of papers by Forbes and coworkers.
Fig 2.1. A catastrophe model for prominence eruption.
Later Priest (1992) put forward an Interacting Flux Model for flares with the following elements: flux interaction or horizontal motions; different locations of stored energy, either in one flux system or the other or in the mutual interaction between them; a magnetic helicity constraint implying free energy in excess of a linear force-free field; a three-dimensional structure with separators and separatrix surfaces.
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