Twenty-Seven Day Cycle
The number of sunspots present at any one time is constantly changing as some disappear and new ones emerge. As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, which is the approximate period for the sun to make one complete revolution. During this time period, the fluctuations in ionization are greatest in the F2 layer. For this reason, calculating critical frequencies for long-distance communications for the F2 layer is not possible and allowances for fluctuations must be made.
Eleven-Year Cycle
Sunspots can occur unexpectedly, and the life span of individual sunspots is variable. The ELEVEN-YEAR SUN SPOT CYCLE is a regular cycle of sunspot activity that has a minimum and maximum level of activity that occurs every 11 years. During periods of maximum activity, the ionization density of all the layers increases. Because of this, the absorption in the D layer increases and the critical frequencies for the E, F1, and F2 layers are higher. During these times, higher operating frequencies must be used for long-range communications.
IRREGULAR VARIATIONS
Irregular variations are just that, unpredictable changes in the ionosphere that can drastically affect our ability to communicate. The more common variations are sporadic E, ionospheric disturbances, and ionospheric storms.
Sporadic E
Irregular cloud-like patches of unusually high ionization, called the sporadic E, often format heights near the normal E layer. Their exact cause is not known and their occurrence cannot be predicted. However, sporadic E is known to vary significantly with latitude. In the northern latitudes, it appears to be closely related to the aurora borealis or northern lights.
The sporadic E layer can be so thin that radio waves penetrate it easily and are returned to earth by the upper layers, or it can be heavily ionized and extend up to several hundred miles into the ionosphere. This condition may be either harmful or helpful to radio-wave propagation.
On the harmful side, sporadic E may blank out the use of higher more favorable layers or cause additional absorption of radio waves at some frequencies. It can also cause additional multipath problems and delay the arrival times of the rays of RF energy.
On the helpful side, the critical frequency of the sporadic E can be greater than double the critical frequency of the normal ionospheric layers. This may permit long-distance communications with unusually high frequencies. It may also permit short-distance communications to locations that would normally be in the skip zone.
Sporadic E can appear and disappear in a short time during the day or night and usually does not occur at same time for all transmitting or receiving stations.
Sudden Ionospheric Disturbances
Commonly known as SID, these disturbances may occur without warning and may last for a few minutes to several hours. When SID occurs, long-range hf communications are almost totally blanked out. The radio operator listening during this time will believe his or her receiver has gone dead.
The occurrence of SID is caused by a bright solar eruption producing an unusually intense burst of ultraviolet light that is not absorbed by the F1, F2, or E layers. Instead, it causes the D-layer ionization density to greatly increase. As a result, frequencies above 1 or 2 megahertz are unable to penetrate the D layer and are completely absorbed.
Ionospheric Storms
Ionospheric storms are caused by disturbances in the earth's magnetic field. They are associated with both solar eruptions and the 27-day cycle, meaning they are related to the rotation of the sun. The effects of ionospheric storms are a turbulent ionosphere and very erratic sky-wave propagation. The storms affect mostly the F2 layer, reducing its ion density and causing the critical frequencies to be lower than
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