Sferics, short for "atmospherics", are lightning-generated electromagnetic waves. The electromagnetic waves are
radio waves, the longest type of electromagnetic waves, with a variety of frequencies present. When these radio waves
are fed directly into a speaker or headphones, an audible sound is heard that has been described variously as "popping",
"clicking" and "sizzling". The common name for this type of radio signal is static. While static may be annoying as it
interferes with radio and television transmissions, sferics are interesting to study because they can tell us a lot about
the radio environment of earth.
To understand what sferics are and what we can learn from them, several questions must be considered:
- What is the mechanism for creating the
radio waves?
- How do they travel?
- What can this tell us about the environment
of earth?
1. What is the mechanism for creating the radio waves?
A common example used to illustrate the creation of waves is ripples created on a pond by disturbing the surface. If you
toss a pebble into a pond, there is a splash followed by expanding circles of ripples. Three things can be observed about
these ripples:
- They move on their own outward in all
directions.
- They travel at a speed that is constant
and depends on the depth of the water.
- The height of the wave decreases as
the wave moves away from the point of the disturbance.
With a radio transmitter, the disturbance is created by charges being forced to move up and down (oscillate) in the transmitting antenna. This movement of charge causes a changing electric and magnetic field that, once created, can then move on their own away from the antenna. This process of a wave moving on its own once created is called propagation. Water waves have this property and so do electromagnetic waves.
Radio waves travel at the speed of light. This speed is constant but depends on the medium through which the radio waves propagate. In outer space, the medium is a vacuum (or nearly so) and near earth, the medium is air. The frequency of the radio waves is the same as the frequency of oscillation of the charge in the transmitting antenna.
As electromagnetic waves propagate outward from the disturbance, the amplitude of the wave decreases. This decrease in the strength of the radio wave follows the familiar inverse-square with distance law.
Sferics are radio waves created by lightning. As a lightning bolt moves from cloud to ground, ground to cloud or cloud to cloud, charge moves rapidly (without a solid antenna!) and emits radio waves. In lightning, the charges oscillate at many frequencies at the same time. This results in the simultaneous emission of radio waves of many frequencies. The frequencies emitted range from very low (less than 1000 hertz) to very high (several million hertz). The strongest signal is at about 10 kilohertz and that is a frequency that can be heard by the human ear if it is fed directly into a speaker or headphones. It is the audible sounds that make up natural radio.
The many frequencies contained in the sferic all travel outward from the lightning at the speed of light. All of the frequencies arrive at the receiver at the same time and produce the "pop" called static.
