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.
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.
Aguado, E. and James E. Burt. 2010. Understanding Weather and Climate. Fifth Edition. Prentice Hall, Upper Saddle River, New Jersey.
Lutgens, F.K. and E.J. Tarbuck. 2004. The Atmosphere: An Introduction to Meteorology. Ninth Edition. Prentice Hall, Upper Saddle River, New Jersey.