Solar Flares: Current Dissipation or Magnetic Annihilation?

Abstract

Solar flares involve the explosive release of energy, 1022_1025 J in 102-103 s, in the solar corona. A substantial fraction of this energy goes into energetic (~10 keY) electrons, and these produce most of the familiar signatures of flares, such as Ha emission, hard X-ray bursts and type III radio bursts. Solar flares occur in magnetic flux tubes in which the fields are highly stressed with associated large currents, I ~ 1012 A, flowing into the corona. Despite the enormous variety of detailed data on solar flares, there is no wide consensus on the essential theoretical ingredients in an acceptable flare model. Since the first detailed flare models were proposed in the 1940s, there have been two competing types: models based on an electric-current viewpoint and models based on a magnetic-field viewpoint. In principle these are equivalent, but in practice they have led to different and seemingly incompatible models. In this paper the theory of solar flares is reviewed, comparing and contrasting these two viewpoints. It is argued that all models, as presently formulated, contain serious deficiencies. One feature that is unsatisfactory is the treatment of energy propagation into a flare kernel. A specific model for such energy propagation is outlined.