TC 9-64 _________________________________________________________________________
Four Distinct Layers
2-64. The ionosphere is composed of three layers designated D, E, and F,
from lowest level to highest level as shown in figure 2-14. The F layer is
further divided into two layers designated F1 (the lower layer) and F2 (the
higher layer). The presence or absence of these layers in the ionosphere and
their height above the earth varies with the position of the sun. At high noon,
radiation in the ionosphere directly above a given point is greatest. At night
it is minimum. When the radiation is removed, many of the particles that
were ionized recombine. The time interval between these conditions finds the
position and number of the ionized layers within the ionosphere changing.
Because the position of the sun varies daily, monthly, and yearly, with
respect to a specified point on the earth, the exact position and number of
layers present are extremely difficult to determine. However, the following
general statements can be made:
The D layer ranges from about 30 to 55 miles. Ionization in the D
layer is low because it is the lowest region of the ionosphere. This
layer has the ability to refract signals of low frequencies. High
frequencies pass right through it and are attenuated. After sunset,
the D layer disappears because of the rapid recombination of ions.
The E layer limits are from about 55 to 90 miles. This layer is also
known as the Kennelly-Heaviside layer, because these two men were
the first to propose its existence. The rate of ionic recombination in
this layer is rather rapid after sunset and the layer is almost gone by
midnight. This layer has the ability to refract signals as high as 20
megahertz. For this reason, it is valuable for communications in
ranges up to about 1500 miles.
The F layer exists from about 90 to 240 miles. During the daylight
hours, the F layer separates into two layers, the F1 and F2 layers.
The ionization level in these layers is quite high and varies widely
during the day. At noon, this portion of the atmosphere is closest to
the sun and the degree of ionization is maximum. Because the
atmosphere is rarefied at these heights, recombination occurs slowly
after sunset. Therefore, a fairly constant ionized layer is always
present. The F layers are responsible for high frequency, long
distance transmission.
2-20
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