In the photosphere, we typically observe two latitudinal bands of sunspots where the magnetic field is strong ( G) and the plasma is cool ( K). The number of sunspots observed at any one time varies with an approximate 11 year period, called the Sunspot cycle. A sunspot group is immersed in an area of moderate magnetic field, called an active region. Occasionally, a violent release of energy will occur in an active region as a solar flare.
The corona is observed during eclipses as a beautiful halo of closed and open structures outlining the local magnetic field. The hot corona is heated by the magnetic field. At the base of closed arcades there are prominences (also called filaments), cool, dense sheets of plasma supported by the magnetic field.
Figures 4.1-4.3 show the Sun on 11 July, 1991. The Hydrogen alpha () image (Figure 4.1) shows up magnetic structures at chromospheric temperatures. The X-ray picture (Figure 4.2) shows hotter, denser structures at coronal temperatures. Finally, the magnetogram in Figure 4.3 shows the distribution and polarity of the magnetic field at the photosphere. Comparing the three figures, we see that the hottest coronal features are associted with bi-polar magnetic features in the photosphere.