__________________________________________________________ Radio Wave Propagation
The structure and density of the F region depend on the time of day
and the angle of the sun. This region consists of one layer during the
night and splits into two layers during daylight hours.
Ionization density of the F1 layer depends on the angle of the sun. Its
main effect is to absorb HF waves passing through to the F2 layer.
The F2 layer is the most important layer for long distance HF
communications. It is a particularly variable layer and its height and
density change with time of day, season, and sunspot activity.
2-98. Seasonal. Seasonal variations are the result of the earth revolving
around the sun; the relative position of the sun moves from one hemisphere
to the other with changes in seasons. Seasonal variations of the D, E, and F1
layers correspond to the highest angle of the sun; thus the ionization density
of these layers is greatest during the summer. The F2 layer, however, does
not follow this pattern; its ionization is greatest in winter and least in
summer, the reverse of what might be expected. As a result, operating
frequencies for F2 layer propagation are higher in the winter than in the
2-99. Eleven-year sunspot cycle. One of the most notable phenomena on
the surface of the sun is the appearance and disappearance of dark,
irregularly shaped areas known as sunspots. The exact nature of sunspots is
not known, but scientists believe they are caused by violent eruptions on the
sun and are characterized by unusually strong magnetic fields. These
sunspots are responsible for variations in the ionization level of the
ionosphere. Sunspots can, of course, occur unexpectedly, and the life span of
individual sunspots is variable; however, a regular cycle of sunspot activity
has also been observed. This cycle has both a minimum and maximum level
of sunspot activity that occurs approximately every 11 years.
2-100. During periods of maximum sunspot activity, the ionization density of
all layers increases. Because of this, absorption in the D layer increases and
the critical frequencies for the E, F1, and F2 layers are higher. At these
times, higher operating frequencies must be used for long distance
2-101. 27-Day sunspot cycle. The number of sunspots in existence at any
one time is continually subject to change as some disappear and new ones
emerge. As the sun rotates on its own axis, these sunspots are visible at 27-
day intervals, the approximate period required for the sun to make one
2-102. The 27-day sunspot cycle causes variations in the ionization density
of the layers on a day-to-day basis. The fluctuations in the F2 layer are
greater than for any other layer. For this reason, precise predictions on a day-
to-day basis of the critical frequency of the F2 layer are not possible. In
calculating frequencies for long-distance communications, one must make
allowances for the fluctuations of the F2 layer.
2-103. Irregular variations in ionospheric conditions also have an important
effect on radio wave propagation. Because these variations are irregular and
unpredictable, they can drastically affect communications capabilities
without any warning. The more common irregular variations are sporadic E,
sudden ionospheric disturbances, and ionospheric storms.