HF signals need to bounce off the ionosphere to travel any distance. The more ionized the ionosphere is, the more effectively hf radio waves refract off it. Hig

HF signals need to bounce off the ionosphere to travel any distance. The more ionized the ionosphere is, the more effectively hf radio waves refract off it. High solar activity leads to higher ionization. In ham terms, this is know as “band openings”, eg when ionospheric conditions allow hf at a given frequency to refract off the ionosphere.