When is ionization at a minimum in the ionosphere?

Ionization in the Earth's upper atmosphere is primarily caused by ultraviolet radiation from the sun. When the sun sets, the source of ionization is removed, and the free electrons and ions gradually begin to recombine throughout the night. Because this recombination process is continuous as long as it is dark, the ionosphere's charge is depleted more and more as the night goes on. Therefore, the lowest level of ionization is reached at the very end of the night cycle, which is shortly before dawn. Distractors like 'Just after dusk' or 'Shortly before midnight' are incorrect because recombination is still actively occurring and has not yet reached its lowest point. 'Just after noon' is incorrect because it is near the peak of daily solar radiation and maximum ionization.