Origin and Development of Solar Flares

Abstract

AbstractA review is given of knowledge on solar flares with particular emphasis on progress made during the international Solar Maximum Year (1979-1981).The pre-flare structure is described by a flux tube model or a circuit model. The instability leading to a flare may occur by a disturbance of the field topology (mostly: field emergence) followed by field-line reconnection (circuit coupling). In the first, impulsive, phase of a flare this causes jets of energetic electrons, originating near the top of the flux tube, to bombard lower chromospheric regions: footpoint heating. In the second (‘gradual’ or ‘diffuse’) phase heated gas from the footpoints ascends convectively upward producing a large cloud of hot gas. Consequent shock wave phenomena cause moving fronts and associated waves in the high parts of the corona; these show up in coronagraphic or radio-observations. In some cases it happens that several hours after a large flare extended loop-like structures appear of fairly high temperature (> 6 MK), emitting a very faint X-ray flux. They are the basic structures of a more extended configuration, visible on metric radio waves, and pointing out from the area where the flare occurred. They may extend to distances of ≈ 106 km or more from the solar surface.