PHOTOSPHERIC MAGNETIC-FIELD EVOLUTION AND ERUPTIVE FLARES

Using an eruptive flare model based on a loss of equilibrium in a coro nal flux rope, we show that the average horizontal component of the ph otospheric field does not necessarily become more like a potential (cu rrent-free) field immediately after the eruption begins. Therefore, re cent observations showing that the average horizontal field becomes le ss potential during the impulsive phase of a flare do not necessarily imply that the magnetic energy in the corona has increased as has some times been assumed. The flux-rope model which we use also has relevanc e to coronal mass ejections. It differs from previous models because e ruption is triggered solely by the converging motion of two photospher ic field sources that lie below the coronal flux rope. Because this bo undary condition is much simpler than that used in previous formulatio ns, this particular version of the model is especially well-suited for two-dimensional numerical simulations with translational symmetry.