What happens daily when the solar UV radiation increases?

Solar ultraviolet (UV) radiation is the primary energy source that ionizes the Earth's upper atmosphere, creating the ionosphere. As the sun rises and UV radiation increases throughout the day, the ionization levels in the D, E, and F layers also increase. Higher levels of ionization allow the ionosphere to refract higher frequency radio waves back to Earth instead of letting them pass straight through into space. Consequently, the Maximum Usable Frequency (MUF) for skywave propagation rises daily as UV radiation increases. Other options are incorrect because ground wave propagation is largely determined by terrain and ground conductivity rather than solar UV, and atmospheric noise (often from thunderstorms) does not decrease in response to rising UV levels.