Magnetohydrodynamic (MHD) theory for the initiation and development of solar flares has developed considerably over the past 3 years and represents one of the liveliest areas of solar physics (Hood & Priest, 1981a, Priest 1983a, b, Schindler 1982, Van Hoven 1982, Syrovatskii et al. 1983). This has been stimulated by a thorough analysis of the Skylab observations and also by the startling new observations from the Solar Maximum Mission (SMM). In addition, the realization that flares appear to form two basic types, namely, small simple-loop flares and large two-ribbon flares, has focussed the imagination of theorists (e.g.. Priest 1981, 1982), even though reality may be somewhat more complex. In the former type, a single-loop structure brightens up and decays without moving; whereas in the latter, an active region filament erupts and then two ribbons of chromospheric emission form and separate, with an arcade of hot and cool loops joining them.
VIII. Theory of Flares