Which ionosphere layer is considered useless at night?

• A. F2 layer
• B. D layer
• C. E layer
• D. F1 layer

Answer: (C)

The D layer of the ionosphere is considered less effective at night for radio wave propagation. This is primarily because the D layer is mostly responsible for absorbing high-frequency radio waves during the daytime when it is ionized by solar radiation. As sunlight diminishes at night, the D layer's ionization decreases significantly, which results in a drop in its ability to reflect or refract radio waves effectively.

Therefore, when discussing the D layer, it is characterized by its temporary formation during daylight hours and its diminished presence at night, making it practically useless for radio communication in the dark. The other layers, such as the F2, F1, and E layers, retain some ionization effects during nighttime conditions and can still facilitate radio communication, unlike the D layer.