During the day, what two sub-regions appear in the ionosphere?

The ionosphere's F region is highly sensitive to intense solar radiation. During the daylight hours, the heavy bombardment of ultraviolet energy causes the single F region to split into two distinct sub-layers: the lower F1 layer and the higher F2 layer. As evening approaches and solar radiation ceases, the F1 and F2 layers recombine and merge back into a single, unified F layer that persists throughout the night. The D and E layers do not divide into numbered sub-regions regardless of the time of day, making any options suggesting D1/D2 or E1/E2 completely fictitious.