Why HF skywave propagation depends on the ionosphere, the upper region of the atmosphere (roughly 60 to 400 km up) where solar ultraviolet and X-ray radiation strips electrons from gas atoms, creating free ions and electrons. These charged layers (D, E, F1, F2) bend HF signals back toward Earth, allowing worldwide contacts far beyond line of sight. The amount of refraction depends on the ionization level, which varies with the sun, time of day, season and solar cycle.
Watch out The troposphere is the weather layer nearest the ground, and it can duct VHF and UHF signals over the horizon, but it does not return HF waves. The stratosphere is not ionized enough to refract radio waves, and 'electrosphere' is not a real atmospheric layer.
Ionized means bent back: 'ion' in ionosphere is the clue for HF skip.