What is the ionization density of the D layer compared to the layers above it?

The ionization density of the D layer is considered relatively low compared to the layers above it, specifically the E and F layers. The D layer, which is located in the lower part of the ionosphere, experiences a significant amount of absorption from solar radiation, particularly during the daytime. This absorption leads to a lower concentration of free electrons and ions. In contrast, the E and F layers have higher levels of ionization due to more intense solar radiation at those altitudes, resulting in greater electron density. Thus, when discussing the ionization density of the D layer, it is accurately characterized as relatively low when making comparisons with the upper layers of the ionosphere. This understanding is essential for grasping how different layers of the ionosphere influence radio wave propagation.