Table 1-1.–Daily Ionospheric Communications
D LAYER: reflects vlf waves for long-range
communications; refracts lf and mf for
short-range communications; has little
effect on vhf and above; gone at night.
E LAYER: depends on the angle of the sun:
refracts hf waves during the day up to 20
MHz to distances of 1200 miles: greatly
reduced at night.
F LAYER: structure and density depend on
the time of day and the angle of the sun:
consists of one layer at night and splits
into two layers during daylight hours.
F1 LAYER: density depends on the angle of
the sun; its main effect is to absorb hf
waves passing through to the F2 layer.
F2 LAYER: provides long-range hf communica-
tions; very variable; height and density
change with time of day, season, and sun-
spot activity.
Figure 1-12.—Ionospheric
layers.
OTHER PHENOMENA THAT AFFECT
of these layers is greatest during the summer. The
COMMUNICATIONS
F2 layer is just the opposite. Its ionization is greatest
during the winter,
Therefore, operating frequencies
Although daily changes in the ionosphere have
for F2 layer propagation are higher in the winter than
the greatest effect on communications, other phenom-
ena also affect communications, both positively and
negatively. Those phenomena are discussed briefly
in the following paragraphs.
SEASONAL VARIATIONS IN THE
IONOSPHERE
Seasonal variations are the result of the earth’s
revolving around the sun, because the relative position
of the sun moves from one hemisphere to the other
with the changes in seasons. Seasonal variations of
the D, E, and F1 layers are directly related to the
highest angle of the sun, meaning the ionization density
in the summer.
SUNSPOTS
One of the most notable occurrences on the surface
of the sun is the appearance and disappearance of dark,
irregularly shaped areas known as SUNSPOTS.
Sunspots are believed to be caused by violent eruptions
on the sun and are characterized by strong magnetic
fields.
These sunspots cause variations in the
ionization level of the ionosphere.
Sunspots tend to appear in two cycles, every 27
days and every 11 years.
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