__________________________________________________________ Radio Wave Propagation
STRUCTURE OF THE IONOSPHERE
2-58. As stated previously, the ionosphere is the region of the atmosphere
that extends from about 31.1 miles (50 km) above the surface of the earth to
about 250 miles (402 km) above the surface. It is appropriately named the
ionosphere because it consists of several layers of electrically charged gas
atoms called ions. A process called ionization forms the ions.
2-59. Ionization occurs when high-energy ultraviolet light waves from the
sun enter the ionospheric region of the atmosphere, strike a gas atom, and
literally knock an electron free from its parent atom. A normal atom is
electrically neutral because it contains equal numbers of positively charged
protons in its nucleus and negatively charged electrons that orbit the
nucleus. When an electron is knocked free from the atom, the atom becomes
positively charged (called a positive ion) and remains in space along with the
free electron, which is negatively charged. This process of upsetting electrical
2-60. The free negative electrons subsequently absorb part of the ultraviolet
energy, which initially freed them from their atoms. As the ultraviolet light
wave continues to produce positive ions and negative electrons, its intensity
decreases because of the absorption of energy by the free electrons, and an
ionized layer is formed. The rate at which ionization occurs depends on the
density of atoms in the atmosphere and the intensity of the ultraviolet light
wave, which varies with the activity of the sun.
2-61. Because the atmosphere is bombarded by ultraviolet light waves of
different frequencies, several ionized layers are formed at different altitudes.
Lower frequency ultraviolet waves penetrate the atmosphere the least;
therefore, they produce ionized layers at the higher altitudes. Conversely,
ultraviolet waves of higher frequencies penetrate deeper and produce layers
at the lower altitudes. An important factor in determining the density of
ionized layers is the elevation angle of the sun, which changes frequently. For
this reason, the height and thickness of the ionized layers vary, depending on
the time of day and even the season of the year.
2-62. Recall that the process of ionization involves ultraviolet light waves
knocking electrons free from their atoms. A reverse process called
recombination occurs when the free electrons and positive ions collide with
each other. Because these collisions are inevitable, the positive ions return to
their original neutral atom state.
2-63. The recombination process also depends on the time of day. Between
the hours of 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. During the
late afternoon and early evening hours, however, the rate of recombination
exceeds the rate of ionization, and the density of the ionized layers begins to
decrease. Throughout the night, density continues to decrease, reaching a low
point just before sunrise.