Sunspots on the surface of the Sun. This optical image was taken on 23rd September 2000, during the period of maximum activity in the solar cycle. Sunspots are cooler, darker areas on the Sun's surface thought to be caused by interactions in the Sun's magnetic field. They typically last a few weeks. They occur in two bands on either side of the solar equator, with those appearing later in the cycle being nearer the equator. The Sun has an 11-year period of activity, with its peak marked by an increase in sunspot numbers, flares, prominences and coronal mass ejections. At the end of the cycle the Sun's magnetic field reverses.