Which region of the atmosphere can reflect HF radio waves?

High Frequency (HF) radio waves are uniquely capable of refracting or bouncing back to Earth from the ionosphere, which is an upper region of the atmosphere heavily ionized by extreme ultraviolet solar radiation. This phenomenon allows HF signals to travel globally, bouncing far beyond the visual horizon. The troposphere is the lowest atmospheric layer where our daily weather occurs; while it can affect VHF/UHF signals through temperature inversions, it does not reflect HF waves. The stratosphere sits above the troposphere but lacks the ionization needed for skywave propagation, and the 'electrosphere' is a completely fictitious term used as a distractor.