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 since it contains both a positive proton in its nucleus and a negative orbiting
electron. When the negative 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 neutrality is known as IONIZATION.
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.
Since 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.
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. Since these collisions are inevitable, the positive ions return to their original
neutral atom state.
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.
Four Distinct Layers
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. Since the position of the sun varies daily,
monthly, and yearly, with respect to a specified point on Earth, the exact position and number of layers
present are extremely difficult to determine. However, the following general statements can be made: