Which region of the atmosphere can refract or bend HF and VHF radio waves?

The ionosphere is a region of the Earth's upper atmosphere, ranging from about 30 to 600 miles above the surface. Solar radiation ionizes the thin gases in this layer, creating free electrons that can interact with passing radio waves. This ionization makes the ionosphere highly refractive to High Frequency (HF) and occasionally Very High Frequency (VHF) signals. It bends these signals back toward the Earth, enabling long-distance 'skip' or skywave communication well beyond the horizon. The troposphere is the lowest weather layer and affects UHF/VHF ducting, but it is not responsible for typical HF refraction. The stratosphere and mesosphere do not sustain the necessary ionization levels to refract these radio waves effectively.