What characterizes the D and E sub-regions of the ionosphere?

The D and E sub-regions of the ionosphere are characterized by their presence primarily during the daytime. The ionosphere is a region of the Earth's atmosphere that is ionized by solar and cosmic radiation, and its layers fluctuate based on the solar cycle and time of day. During the daytime, the intensity of solar radiation is sufficient to build up ionization in these layers, making them conducive to reflecting radio waves.

During nighttime, the ionization levels in the D region diminish significantly due to the lack of solar radiation, which can affect propagation conditions for radio communication. Thus, stating that the D and E sub-regions exist only during the daytime aligns with the established understanding of ionospheric behavior. This is crucial for amateur radio operators, as they must consider these variations in ionospheric conditions for effective signal propagation.