Photospheric magnetic field evolution and eruptive flares

Using an eruptive flare model based on a loss of equilibrium in a coronal flux rope, we show that the average horizontal component of the photospheric field does not necessarily become more like a potential (current-free) field immediately after the eruption begins. Therefore, recent observations showing that the average horizontal field becomes less potential during the impulsive phase of a flare do not necessarily imply that the magnetic energy in the corona has increased as has sometimes been assumed. The flux-rope model which we use also has relevance to coronal mass ejections. It differs from previous models because eruption is triggered solely by the converging motion of two photospheric field sources that lie below the coronal flux rope. Because this boundary condition is much simpler than that used in previous formulations, this particular version of the model is especially well-suited for two-dimensional numerical simulations with translational symmetry.