Solar Activity

The changes in the sun's surface and atmosphere, normally referred to as solar activity, are intimately linked to the production, transport, and dissipation of magnetic fields. The most obvious manifestations of the changes are sunspots, flares, and the changing corona. The complex of solar activity can be divided into two parts: (1) the problem of the generation of magnetic fields and of the solar cycle (the 11‐year period) and (2) the surface manifestations of solar activity. The first problem concerns the solar dynamo, and we discuss in some detail dynamo maintenance. In particular, we show that a combination of differential rotation of the outer parts of the sun and cyclonic turbulence of the solar plasma constitutes a very efficient system for generation of magnetic fields. When these fields are transported to the sun's surface, they interact with the motions of the atmospheric plasma to produce the manifestation of solar activity: plages, sunspots, flares, prominences, and the active corona. Models of plages and several types of prominences are discussed, and the first steps toward theories for solar flares are indicated. The role of the magnetic fields, either as a supporting agent to store energy or as an energy source (field annihilation), cannot be overemphasized. Prominence formation and stability, as well as the occurrence of different types of flares, are treated in some detail. We close with a discussion of the active corona and interaction of the coronal plasma with other types of activity (flares and active prominences).