The ionosphere is an atmospheric layer that occurs at a height between 60 and 400 km (40 to 250 mi) (Figure 1). In this relatively thick layer there is a concentration of ions. In the ionosphere, ions are positively charged because of the energizing effects of solar radiation on gas atoms and molecules. This same process also creates an abundance of free electrons.
Figure 1. The ionosphere is a zone that contains large numbers of electrically charged particles (atoms and molecules) known as ions. (Image Copyright: Michael Pidwirny.)
We use the electrically charged ionosphere to help transport radio waves. Certain layers of the ionosphere have the ability to reflect radio waves. By bouncing radio waves off these atmospheric regions we are able to extend transmissions over hundreds of kilometers (Figure 2). This process works best at night because of a unique property of the ionosphere. The ionosphere is divided into three sub-layers: the D-, E-, and F-layers. The lower D- and E-layers differ from the higher F-layer in two ways. First, these two sub-layers only exist during daylight hours. Second, they also have the ability to absorb some of the radio transmission. This absorption weakens the radio transmission requiring that radio stations increase signal strength after sunrise.
Figure 2. We use the ionosphere to help broadcast radio transmissions over long distances. By reflecting radio waves off the ionosphere, surface locations normally obscured by the Earth’s curvature can receive transmissions. During the day (A), radio signals must be strengthened because the D- and E-layers can partially absorb radio waves. At night (B), the D- and E-layers dissipate and only the F-layer is used to reflect radio signals. Because very little absorption occurs in the F-layer, radio transmissions received on the Earth at night are less distorted and stronger. (Image Copyright: Michael Pidwirny.)
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