General Astronomy/Solar Activity
Sunspots are caused by magnetic disturbances that erupt through the visible surface of the Sun (photosphere). Deep inside the Sun many free moving subatomic particles produce very strong and complicated magnetic fields. When strong magnetic field lines erupt through the photosphere, they slightly impede the flow of energy from deeper layers outward to the surface, thus causing the area of the sunspot to be slightly cooler than the rest of the Sun, and hence darker.
Sunspots typically have a dark inner zone called an umbra, surrounded by a lighter outer zone called a penumbra. Many are as large as the Earth, and some have been known to be as large as Jupiter. Larger sunspots are usually groups of spots rather than just one. Often, adjacent sunspots have been shown to be of opposite magnetic polarity, with magnetic field lines flowing from one to the other, much like a bar magnet.
The solar activity cycle corresponds to the sunspot cycle, and is on the average about 11 years. That is, the numbers of sunspots, along with other less obvious activity on the Sun, waxes and wanes about every 11 years. In addition, the magnetic polarity scheme of sunspots reverses once every cycle, so we can also say that there is a 22-year cycle in solar activity.