What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?

The Sun takes approximately 26 to 28 days to complete one full rotation on its axis. As it rotates, active regions such as sunspots and coronal holes are carried across the solar disk, facing the Earth and then eventually rotating out of view. Because the solar radiation emitted from these active regions directly governs the ionization levels of Earth's ionosphere, HF propagation conditions rise and fall in tandem with this 26- to 28-day solar rotation cycle. While the Moon's orbital period is also about 28 days, it does not emit the ultraviolet radiation or solar wind necessary to alter the ionosphere, making that distractor incorrect.