The D-layer, also known as the D-region, is a part of the Earth's atmosphere located between 30 to 90 miles above the surface. It is a region of ionization that forms during the day when solar radiation interacts with atmospheric gases, creating free electrons and ions. This layer plays a crucial role in radio wave propagation, as it can absorb and reflect radio signals. During nighttime, the D-layer diminishes significantly due to the lack of solar radiation, leading to improved radio wave transmission. The D-layer is part of the larger ionosphere, which includes other layers like the E-layer and F-layer, each with distinct characteristics and functions.