Which region of the ionosphere separates into two sub-regions in the daytime?

The F region is the highest layer of the ionosphere. During the daytime, intense ultraviolet radiation from the sun heavily ionizes this region, causing it to split into two distinct sub-layers: the lower F1 layer and the higher F2 layer. This splitting is crucial for daytime high-frequency (HF) radio propagation, with the F2 layer being the most responsible for reliable long-distance communication. When the sun goes down, solar radiation stops, and the F1 and F2 layers merge back together into a single F region overnight. The D and E layers do not split into distinct sub-regions, and the 'G region' is a fictitious distractor not used in standard amateur radio propagation theory.