1. Ionosphere. The ionosphere is the region (or layer) of the atmosphere that extends from 31 miles to
about 250 miles above the Earth's surface. Its gets its name because it consists of several layers of
electrically charged atoms called ions. Ions are formed by a process called ionization.
a. When high energy ultraviolet light waves from the sun enter the atmosphere's ionospheric
region, they strike gas atoms, knocking negative electrons free. Normally, atoms are electrically neutral.
When they lose an electron, atoms become positively charged and are called positive ions. This process
of upsetting electrical neutrality is known as ionization. The rate at which ionization occurs depends on
the density of atoms in the atmosphere and the intensity of the ultraviolet light waves, both of which
vary with the activity of the sun. The ultraviolet waves striking the atmosphere are of different
frequencies, causing several ionized layers to be formed at different altitudes. The density of ionized
layers is partially attributed to the elevation angle of the sun, which changes constantly. Consequently,
the altitude and thickness of the ionized layers vary, depending on the time of day and even the season
of the year.
b. When free electrons and positive ions collide with each other, a reverse process called
recombination occurs. This results in positive ions returning to their original neutral state.
Recombination depends on the time of day. Between the hours of early morning and late afternoon, the
rate of ionization exceeds the rate of recombination. It is during this period that the ionized layers reach
their greatest density and exert maximum influence on radio waves. Conversely, during the late
afternoon and early evening hours, the rate of recombination exceeds the rate of ionization, and the
density of the ionized layers begins to decrease. This density decreases throughout the night, reaching a
low point just before sunrise. You can better appreciate this phenomena by listening to a far away
commercial AM radio station at night and at sunrise. As the ionization rate picks up, the reception
grows fainter until you lose the station completely.
c. The ionosphere is composed of three regions (D, E, and F), as shown in Figure 3-1. The F
region is further divided into two layers designated F1 (lower layer) and F2 (higher layer), which change
with the position of the sun. The radiation in the ionosphere directly above a given point is greatest at
noon, while it is least at night. When the radiation is not present, recombination sets in.
(1) The D region ranges to 55 miles above the Earth's surface. This low region of the
atmosphere has low ionization. It refracts low frequency signals, but high frequencies pass through it,
with some attenuation that varies with frequency and region density. The D region disappears after
sunset because of recombination.
(2) The E region ranges from about 55 to 90 miles in altitude. After sunset, recombination
occurs rapidly, and this region is almost gone by midnight. The E region is used during the day for HF
radio transmissions ranging up to about 1500 miles.
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