What is the major cause of cyclical changes in HF propagation?

The sun goes through an approximately 11-year cycle of varying activity, known as the solar cycle. During the peak of this cycle, sunspot numbers increase, leading to higher levels of ultraviolet and X-ray solar radiation bombarding the Earth's atmosphere. Because this solar radiation dictates the electron density of the ionosphere, the 11-year solar cycle is the primary driver of cyclical changes in high-frequency (HF) skywave propagation. Other options like the jet stream or magnetic pole drift do not dictate global HF propagation cycles.