A historical account of the subject's development is attempted. Prior to 1940, the most significant papers were those by Larmor (1919) and Cowling (1934) on dynamo theories of solar fields: by Kiepenheuer (1935) on the corona; and by Ferraro (1937) on isorotation. These indicated the importance of electromagnetic forces and were groping towards the idea of frozen-in fields. The latter idea was, however, not clearly stated before Alfvén's 1941–2 papers.Theory since then is divided into sections concerned with mechanical effects of magnetic fields, theories of sunspots, and the nature and origin of solar magnetic fields. The first includes theories of magnetic control of support of coronal filaments and prominences (van de Hulst, Alfvén, Dungey) and theories of magnetic influence on sunspot equilibrium. The second includes Alfvén's and Walén's theories of the solar cycle, and Biermann's explanation of sunspot coolness in terms of magnetic inhibition of convection. Sunspot theories, being discussed more fully by Biermann, are considered only briefly.Electromagnetic heating covers theories of coronal heating and flares, discharge phenomena, particle acceleration and radio emission. Many of the older theories (Alfvén's on coronal heating, Giovanelli's on flares, that of Bagge and Biermann on cosmic rays) are set aside because of their neglect of self-induction effects and inadequacy of the mechanism of conversion. The relative motion of charged particles and neutral atoms (Piddington, Cowling) is described as supplying a powerful heating effect.As regards the magnitude of the general solar magnetic field, it is suggested that the observed value can be discarded only if decisive reasons are given. Other theories having so far proved inadequate, dynamo theories of the origin of solar fields are regarded as the most promising. These can be partial, as when a toroidal field capable of explaining spot fields is supposed to be generated from the general field (Walén and others), or when a turbulent field is supposed to be generated from a smaller regular field (Alfvén and others): or total, when a simultaneous explanation of all fields is attempted (e.g. Parker). A general appraisal is made of the different theories.
12. Solar electrodynamics