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 on the time of day and the season of the year. Another important factor in determining layer density is known as recombination.

Recombination

Recombination is the reverse process of ionization. It occurs when free electrons and positive ions collide, combine, and return the positive ions to their original neutral state.

Like ionization, the recombination process depends on the time of day. Between early morning and late afternoon, the rate of ionization exceeds the rate of recombination. During this period the ionized layers reach their greatest density and exert maximum influence on radio waves. However, during the late afternoon and early evening, the rate of recombination exceeds the rate of ionization, causing the densities of the ionized layers to decrease. Throughout the night, density continues to decrease, reaching its lowest point just before sunrise. It is important to understand that this ionization and recombination process varies, depending on the ionospheric layer and the time of day. The following paragraphs provide an explanation of the four ionospheric layers.

Ionospheric Layers

The ionosphere is composed of three distinct layers, designated from lowest level to highest level (D, E, and F) as shown in figure 1-2. In addition, the F layer is divided into two layers, designated F1 (the lower level) and F2 (the higher level).

The presence or absence of these layers in the ionosphere and their height above the earth vary with the position of the sun. At high noon, radiation in the ionosphere above a given point is greatest, while at night it is minimum. When the radiation is removed, many of the particles that were ionized recombine. During the time between these two conditions, the position and number of ionized layers within the ionosphere change.

Since the position of the sun varies daily, monthly, and yearly with respect to a specific point on earth, the exact number of layers present is extremely difficult to determine. However, the following general statements about these layers can be made.

D LAYER.—The D layer ranges from about 30 to 55 miles above the earth. Ionization in the D layer is low because less ultraviolet light penetrates to this level. At very low frequencies, the D layer and the ground act as a huge waveguide, making communication possible only with large antennas and high-power transmitters. At low and medium frequencies, the D layer becomes highly absorptive, which limits the effective daytime communication range to about 200 miles. At frequencies above about 3 MHz, the D layer begins to lose its absorptive qualities.

Figure 1.2 — Layers of ionosphere. 1-3