2-30
In the E layer, ionization depends on the angle of the sun. The E layer refracts hf waves during
the day up to 20 megahertz to distances of about 1200 miles. Ionization is greatly reduced at
night.
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 very variable
layer and its height and density change with time of day, season, and sunspot activity.
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 summer.
ELEVEN-YEAR SUN SPOT 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 occur
approximately every 11 years.
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 communications.
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 complete
rotation.
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, allowances for the fluctuations of the F2 layer must be
made.
Irregular Variations
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.
