D region

The D region is a layer of Earth's atmosphere located roughly between 50 and 90 kilometers above the surface. It is the lowest part of the ionosphere and contains a small number of ions and free electrons. During the daytime, solar radiation causes this layer to become ionized, which can weaken radio signals traveling through it, especially at lower frequencies. At night, it largely dissipates, allowing radio waves to travel more effectively. The D region plays a crucial role in radio communication, affecting signal propagation, particularly for shortwave and radio broadcast transmissions.