<P> Although the details of sunspot generation are still a matter of research, it appears that sunspots are the visible counterparts of magnetic flux tubes in the Sun's convective zone that get "wound up" by differential rotation . If the stress on the tubes reaches a certain limit, they curl up and puncture the Sun's surface . Convection is inhibited at the puncture points; the energy flux from the Sun's interior decreases, and with it, surface temperature . </P> <P> The Wilson effect implies that sunspots are depressions on the Sun's surface . Observations using the Zeeman effect show that prototypical sunspots come in pairs with opposite magnetic polarity . From cycle to cycle, the polarities of leading and trailing (with respect to the solar rotation) sunspots change from north / south to south / north and back . Sunspots usually appear in groups . </P> <P> Magnetic pressure should tend to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days to weeks . In 2001, observations from the Solar and Heliospheric Observatory (SOHO) using sound waves traveling below the photosphere (local helioseismology) were used to develop a three - dimensional image of the internal structure below sunspots; these observations show that a powerful downdraft underneath each sunspot, forms a rotating vortex that sustains the concentrated magnetic field . </P> <P> Sunspot activity cycles are about every eleven years, with some variation in length . Over the solar cycle, sunspot populations rise quickly and then fall more slowly . The point of highest sunspot activity during a cycle is known as solar maximum, and the point of lowest activity as solar minimum . This period is also observed in most other solar activity and is linked to a variation in the solar magnetic field that changes polarity with this period . </P>

Reduced surface temperature creates areas that are darker than the surrounding material