Penetration (Critical) Frequency

As mentioned, for the conditions depicted in the left portion of Fig. 5 at noon a wave of frequency greater than about 4.2 megacycles would not be reflected appreciably by the E layer. Some energy would be absorbed by the E layer, and the remainder of the wave would pass on into the F₁ layer, which is more dense (Fig. 6) and accordingly would reflect the wave. If, however, the frequency is too high for the F₁ layer, the wave might pass through to the F₂ layer, which is more dense (Fig. 6) and which might reflect the wave. Waves of frequencies greater than the penetration frequency for the F₂ layer are absorbed in part, and then transmitted out into space.

Because of the presence of the earth's magnetic field double refraction occurs in the ionosphere.⁸ Hence, when a radio wave penetrates the ionosphere, two components emerge; these have different velocities, different polarization, and are attenuated differently. These two components are the ordinary wave¹ (or ordinary ray) and the extraordinary wave¹ (or extraordinary ray). The ordinary wave is designated by an "o" and the extraordinary wave by an "x". The ordinary wave is shown in Fig. 5 for the E layer because it is the more important. For the F layer the extraordinary wave is shown because, for many purposes, it alone is of importance.⁸ A wave emerging from the ionosphere has both horizontally and vertically polarized components.

The penetration frequency is a measure of the ionization density, because, the higher the frequency, the greater must be the density of the ions to be able to reflect the wave back toward the earth. The relation applying is

N = 0.0124f²,

where N is the number of ions per cubic centimeter and f is the penetration, or critical, frequency in kilocycles.⁸