Activation of solar flares

The physics of the activation of two-ribbon solar flares via the MHD instability of coronal arcades is presented. The destabilization of a preflare magnetic field is necessary in order for a rapid energy release, characteristic of the impulsive phase of the flare, to occur. The authors examine the stability of a number of configurations and discuss the physical consequences and relative importance of varying pressure profiles and different sets of boundary conditions (involving field-line tying). Interchange modes, driven unstable by pressure gradients, are candidates for instability. Shearless vs. sheared equilibria are also discussed.