This paper gives a short description and interpretation of some solar magneto-hydrodynamical and plasma phenomena, based mainly on work by the writer and his collaborators (for an extended review see Kaplan et al. 1974).
The magnetic field is considered as the main factor responsible for a number of manifestations of solar activity. At the photospheric level active regions are displayed as plages, i.e., bright areas, seen near the limb. This means that the temperature gradient in plages is smaller than in the undisturbed photosphere. The decrease in gradient is a result of an increase of convective transport of energy.