We learn in Technician Class and General Class studies that the ionosphere’s D layer is created in the daytime by ionizing solar rays and fades away completely at night, and we learn that the D layer absorbs HF frequencies below the 20-meter band. When the D layer dissipates after dark those lower frequencies are no longer absorbed and are free to propagate by skywave or skip from the E and combined F layers. The practical operational impact of this phenomenon is that for long distance communications we use the higher HF bands of 20-meters and above during the daytime, and we can use the lower bands at night.
Why, you may ask, does the D layer absorb lower HF signals but not higher HF signals? One way to answer is that it does absorb those higher frequency signals somewhat, but not nearly as much as it absorbs the lower frequencies. The severity of the absorption is a function of frequency, or more correctly, a function of the wavelength size.
As a radio wave travels through the ionosphere it causes the electrons (ions) in the layer to be displaced, or moved, a distance that is commensurate with the size of the wavelength. The alternating electric field of the wave will pull the electron back and forth, accelerating it first in one direction and then in the other as the waveform travels past the particle. In this process a little energy is temporarily transferred from the RF wave into the accelerated motion of electrons.
If an accelerated electron remains freely moving it will re-radiate RF (secondary emissions) and reinforce the original RF signal in the direction of wave travel (little net RF energy loss). However, if the accelerated electron encounters another electron in collision or, more likely, encounters a positively charged ion with which it combines and neutralizes the electric charges, then it will cease its re-radiating of RF. The remaining energy it received from the passing RF wave is instead wasted as heat in collision or recombination into a neutrally charged atom. In this case, the energy taken from the RF wave is lost, or absorbed, and not returned in the form of secondary emissions from the electron.
In the relatively low altitude D layer, the density of atmospheric particles is much greater than the density in the higher E and F layers. The consequence is that the probability of an accelerated electron encountering other particles in collision or recombination is much greater in the D layer than in the higher altitude ionospheric layers. But particle density is a factor for all wavelengths and frequencies, so why are the lower frequencies absorbed more readily?
For equivalent signal strength or waveform amplitude, a lower frequency signal (and hence a longer wavelength) will displace an electron a greater distance during waveform travel than will a higher frequency signal. This is because the electric field of a longer wave has a longer period of time to act upon the electron and accelerate it as the wave passes by at the constant speed of light.
These longer travel distances dramatically increase the probability that another particle will be encountered as the electron accelerates. Thus, low HF signals lose much energy to colliding electrons and recombining atoms because the electrons must travel a great distance through a high density population of D layer particles, and the probability of surviving freely is very low. Alternatively, higher HF signals cause electrons to move smaller distances and the probability of energy loss to collision heat is commensurately reduced, so free survival and re-radiation is more likely with higher frequency signals.
The upshot of all this is that the D layer RF energy absorption increases as frequency is lowered. While the 20-meter band and higher frequencies are not significantly attenuated by D layer energy absorption due to their smaller wavelengths, absorption at lower frequencies is usually too severe for long distance contacts during the daylight manifestation of the D layer.
However, the Near Vertical Incidence Skywave (NVIS) technique causes lower HF signals to travel through the D layer in the shortest dimension, nearly at a right angle to the layer’s spherical shell overhead. As a result, absorption of low frequency signals is minimized and NVIS regional communications is often viable during daylight hours, particularly on the 40-meter band.
-- Stu WØSTU