Some background. Take a few minutes to read.
https://www.bibliotecapleyades.net/esp_ondas_shumman_12.htm
more at above
The 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 the Earth and the inner edge of the ionosphere 55 kilometers 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 of 1 - 3 x 10-12 Amperes per square meter.
The resistance of the atmosphere is 200 Ohms. The voltage potential is 200,000 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 the Earth's 'electromagnetic' cavity.
The Schumann Resonances are quasi standing wave 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 the 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, 39 and 45 Hertz, with a daily variation of about +/- 0.5 Hertz.
So long as the properties of Earth's electromagnetic cavity remains about the same, these frequencies remain the same.
Presumably there is some change due to the solar sunspot cycle as the Earth's ionosphere changes in response to the 11-year cycle of solar activity. Schumann resonances are most easily seen between 2000 and 2200 UT.
When we consider that the ionosphere surrounding our planet is electrically positive charged whilst the earth's surface carries a negative charge, we must conclude that this amounts to a prevailing electrical tension within the earth/ionosphere cavity. This tension is discharged when thunderstorms develop in this cavity. In physics two concentric electrically charged balls, one placed inside the other, are called ball condensers, or capacitors.
The inside of the ionosphere layer is used in wireless information transfer to bounce off radio waves emitted by transmitters on the earth's surface. In this way the information can be transferred over large distances.
The physicist and inventor Nikola Tesla was the first to carry out wireless energy experiments at Colorado Springs, USA, which produced such powerful electrical tensions that they resulted in the creation of artificial lightning.
These lightning flashes also produced radio waves. Due to their extremely low frequency these waves could penetrate the earth without resistance and thereby Tesla discovered the resonance frequency of the earth. Unfortunately Tesla was way ahead of his time and his discoveries were not taken seriously because it suggested free energy for all and was not good for 'business'.
It wasn't until more than half a century later in 1952, when the German physicist Professor W.O. Schumann of the Technical University of Munich predicted that there are electromagnetic standing waves in the atmosphere, within the cavity formed by the surface of the earth and the ionosphere.
This came about by Schumann teaching his students about the physics of electricity: