Which sub-regions of the ionosphere are only present during the daytime?

The D and E layers of the ionosphere are the sub-regions that are primarily present during the daytime. This is because these layers are formed due to the ionization effect of solar radiation.

During daylight hours, the sun's ultraviolet and X-ray radiation ionizes the gases in the ionosphere, creating the D and E layers. The D layer, in particular, is quite low in the atmosphere and absorbs radio waves at lower frequencies, which is why it's most prominent during the day. The E layer can also reflect certain radio frequencies and is more prominent during daytime due to the increased ionization.

At night, when solar radiation is absent, these layers start to dissipate. The F layer, on the other hand, remains present both day and night as it is higher in the ionosphere and can be seen as resulting from the recombination of ionized particles after sunset.

Thus, stating that the D and E layers are exclusive to the daytime is accurate and reflects the behavior of these ionospheric layers under the influence of solar activity.