The ability of the ionosphere to refract high frequency radio signals depends on:

The ionosphere is formed when solar radiation, specifically ultraviolet light and X-rays from the sun, ionizes gas molecules in the upper atmosphere. The denser this ionization, the better the ionosphere can bend (refract) high-frequency (HF) radio signals back to Earth. Weather conditions in the troposphere, receiver sensitivity, and transmit power do not alter the fundamental refractive properties of the ionosphere. While transmit power dictates the strength of the signal, it does not change the physical ability of the ionosphere to refract the radio waves.