The D & E layers

 The D and E layers of the ionosphere are the two lowest layers and are located at altitudes of around 60-90 km and 90-150 km, respectively. They play a significant role in the propagation of radio waves at high frequency (HF) because they are where most of the absorption of radio waves occurs.

The D layer is the densest of the ionosphere's layers and is primarily composed of neutral atoms and molecules. It absorbs radio waves at frequencies between about 2-10 MHz, making it difficult for signals in this frequency range to be propagated over long distances. The D layer is also affected by solar activity, and its density increases during periods of high solar activity, which leads to increased absorption of radio waves.

The E layer is less dense than the D layer and is composed of ionized atoms and molecules. It absorbs radio waves at frequencies between about 10-50 MHz, and its density also increases during periods of high solar activity. However, the E layer can also reflect radio waves, which can be used to propagate signals over long distances. The E layer is also responsible for the phenomenon of "skip" propagation, in which radio waves are reflected back to the Earth at a much higher altitude than the D layer, allowing them to travel much further than they would be able to otherwise.

In summary, the D and E layers of the ionosphere play a significant role in the propagation of radio waves at high frequencies. They are the layers where most of the absorption of radio waves occurs and are affected by solar activity. Understanding the behavior of these layers and how they are affected by solar activity is essential for predicting and planning radio communications at high frequencies.