How does the D layer ionization density compare to that of the F layer?

The D layer ionization density is indeed lower than that of the F layer. The D layer is the lowest part of the ionosphere, starting from about 30 miles above the Earth's surface and extending to about 50 miles. It is primarily influenced by solar radiation but is only ionized during the day because its ionization is quickly lost in the absence of sunlight, resulting in a comparatively low density of free electrons.

In contrast, the F layer, which lies above the D layer and can extend from about 90 miles to over 300 miles above the Earth's surface, maintains a much higher ionization level. The F layer can retain a significant number of ions even during the nighttime due to its greater altitude and the presence of ultraviolet and X-ray radiation from the sun, allowing it to exhibit a higher ionization density.

Therefore, when assessing the ionization densities of these two layers, it is clear that the D layer has a lower ionization density when compared directly to the F layer.