Active regions

The concentrations of strong magnetic field occur in characteristic time-dependent configurations. A prominent configuration is the bipolar active region, which is formed during the emergence of strong magnetic flux. Around the time of its maximum development, a large active region comprises sunspots, pores, and faculae arranged in plages and enhanced network (Fig. 1.2). In this book, the term active region is used to indicate the complete area within a single, smooth contour that just includes all its constituents. We prefer the term active region over the older term center of activity. Classical descriptions of active regions are found in Kiepenheuer (1953, his Section 4.12), De Jager (1959), and Sheeley (1981).

The present chapter is restricted to simple bipolar active regions. Throughout this book we use the term bipolar active region for regions consisting of only two fairly distinct areas of opposite polarity, in contrast with complex active regions in which at least one of the polarities is distributed over two or more areas. Bipolar active regions are building blocks in complex active regions and nests (Section 6.2.1).

At maximum development, directly after all magnetic flux has emerged, bipolar active regions range over some 4 orders of magnitude in magnetic flux, size, and lifetime; see Table 5.1. The division between small active regions and ephemeral active regions is based on an arbitrary historical choice: there is no qualitative difference between the two groups of bipolar features other than gradual trends.