Light, an electromagnetic wave, travels in a straight line outward from the source. If the light strikes a surface, it will reflect and if it encounters a new transparent medium it will refract. When light passes from one medium to another, some of the light passes into the new medium where its new speed (the speed of light in the new medium) causes its direction to change. This is the behavior observed when light passes from air into water or air into glass.

Radio waves have the same behavior.

Since lightning strikes somewhere on earth many times (current estimate is about 100 times per second), sferics are being created constantly in thunderstorms scattered over the earth. Sferics are caused by thousands of amperes of current moving through hundreds of thousands of volts of potential difference, so sferics constitute a wave source of many millions of watts. These are strong signals. You can hear sferics any time you observe natural radio even if there is no thunderstorm nearby. As a matter of fact, if there is lightning nearby, that is not a good time to sit outside with an antenna extending upward and headphones on your head! In that case, the antenna can also be called a lightning rod!

But if radio waves travel in straight lines from the source, how can we explain the very common experience of hearing sferics when there is no lightning within our line of sight? The answer to this problem is the ionosphere and the surface of the earth.

The ionosphere is a layer of the atmosphere extending from about 75 kilometers above the surface of earth to thousands of kilometers. This is the region where the atmospheric gases have been ionized by ultraviolet radiation from the sun. As electrons are removed, positive ions are created. Electrons and these positive ions will attract one another and form neutral atoms, but if there is enough ultraviolet radiation from the sun, the creation of new positive ion free electron pairs will keep the ionosphere charged. At night, when there is no ultraviolet from the sun, the free electrons and positive ions combine and the ionosphere is reduced.

When the radio waves of the sferic, as they move outward from the source, encounter the ionosphere they are partially transmitted into the ionosphere and partially reflected back toward the surface of earth. The reflected radio waves are then reflected from earth, which reflects radio waves, back toward the ionosphere. The reflection at the ionosphere is repeated followed by another reflection from earth. The ionosphere and the earth form what is called a waveguide that keeps the radio wave near earth and guides them around the curve of earth. Since sferics are composed of such high-powered radio waves, they can be detected at a great distance even though the amplitude is declining as described with the inverse square law. Strong sferics can propagate all the way around the earth, so you will always hear sferics.

If you don't hear sferics, it is a sign that your receiver is not working!

3. What can this tell us about the environment of earth?

Analysis of sferics can tell us about the ionosphere. For example, the fact that we can hear sferics that originate around the curve of earth tells us that there is a reflective layer of the atmosphere- the ionosphere.

The operation of the earth-ionosphere tells us that radio waves will partially reflect off a charged layer in the atmosphere and that radio waves reflect from earth.

Listening conditions are very different at night than in the daytime. This is because the ionosphere discharges on the night side of earth resulting in the ionosphere moving to a higher altitude and this changes the earth-ionosphere waveguide. Signals can be detected from even farther away at night.

The ionosphere also plays a key role in other natural radio signals that will be described later.

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