The rotation period of an object is the time it takes to spin on its axis once. For example, the rotation period of the Earth is one sidereal day. In order to observe the rotation period of any rotating sphere, we can measure how many degrees of longitude a surface feature moves in a given time interval. This method can be applied to the rotation of the Sun by observing active regions as they are carried across the disk by the Sun's rotation. Active regions of the Sun come in a number of varieties, such as sunspots, flares, and prominences.
All active regions have large magnetic fields.
Sunspots are relatively cool areas of the Sun which stand out from the rest of the surface because they are relatively dark. The temperature of a sunspot is about 4200 K compared to a temperature of about 5500 K for the rest of the Sun. If we could see the spots in isolation, they would appear bright red. Flares occur in regions of the Sun where the magnetic field is twisted very tightly by the differential rotation of the Sun. This magnetic twisting then releases energy.