(The following material is taken from a really great little book I have titled "The Astronomy Cafe" 365 Questions and Answers From "Ask the Astronomer" by author Sten Odenwald. Thanks are given to the author for the information reproduced here.)
Believe it or not, Earth behaves like an enormous electric circuit. The atmosphere is actually a weak conductor, and if there were no sources of charge, its existing electric charge would diffuse away in about 10 minutes. There is a cavity defined by the surface of Earth and the inner edge of the ionosphere, 55 km up. At any moment, the total charge residing in this cavity is 500,000 coulombs. There is a vertical current flow between the ground and the ionosphere between 1 to 3 X 10
-12 amperes per square meter. The resistance of the atmosphere is 200 ohms. The voltage potential is 20,000 volts. There are about 1000 lightning storms at any given moment worldwide. Each produces 0.5 to 1 ampere, and these collectively account for the measured current flow in Earth's electromagnetic cavity.
The Schumann resonances were predicted to exist in 1952 and were first detected in 1954. They are resonant electromagnetic waves that exist in this cavity. Like waves on a spring, they are not present all the time but have to be excited to be observed. They are not caused by anything internal to Earth, its crust, or its core. They seem to be related to electrical activity in the atmosphere, particularly during times of intense lightning activity. They occur at several frequencies between 6 and 50 cycles per second, specifically, 7.8, 14, 20, 26, 33, and 45 hertz, with a daily variation of about +/- 0.5 hertz. As long as the properties of Earth's electromagnetic cavity remain about the same, these frequencies remain the same. Presumably there is some change due to the solar sunspot cycle as Earth's ionosphere changes in response to the 11-year cycle of solar activity. Schumann resonances are most easily seen between 20:00 and 22:00 universal time (UT).
Given that Earth's atmosphere carries a charge, a current, and a voltage, it is not surprising to find such electromagnetic waves. Much of the research in the past 20 years has been conducted by the Department of the Navy, which investigates extremely low frequency (ELF) communication with submarines. For more information, see Hans Volland, ed., Handbook of Atmospheric Electrodynamics (CRC Press, 1995). Chapter 11 is on Schumann resonances and was written by Davis Campbell of the Geophysical Institute, University of Alaska. There is also a history of this research and an extensive bibliography.
