In the preceding chapters we have learned about convection and rotation, the main causes of solar magnetism. And we have learned about this magnetism itself. We shall now recognize that the Sun¯s magnetic field is the main source of almost all structure and variability which we find in the outermost layers: the chromosphere, the corona, and the solar wind. Moreover, we shall see that the physical state and the extent of these outer layers are most certainly determined by the solar magnetic field.