| 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. |
 Figure 1-12.—Ionospheric layers. |
| 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 communications; very variable; height and density change with time of day, season, and sun-spot activity. |
OTHER PHENOMENA THAT AFFECT COMMUNICATIONS
Although daily changes in the ionosphere have the greatest effect on communications, other phenomena 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 of these layers is greatest during the summer. The F2 layer is just the opposite. Its ionization is greatest during the winter, Therefore, operating frequencies for F2 layer propagation are higher in the winter than 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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