Final answer:
The primary periodicity related to solar activity is the sunspot cycle, which lasts for 11 years. Sunspots are dark regions on the Sun's surface that vary in number and polarity during this cycle.
Step-by-step explanation:
The primary periodicity related to solar activity is the sunspot cycle. Sunspots are dark regions on the Sun's surface where the temperature is cooler than the surrounding photosphere. The number of visible sunspots varies according to an 11-year sunspot cycle. During a given cycle, the leading spots in the Northern Hemisphere have the same magnetic polarity, while the leading spots in the Southern Hemisphere have the opposite polarity. This magnetic activity cycle of the Sun lasts for 22 years.