There is a channel between the Earth's surface and the ionosphere through which an electromagnetic wave travels well. Circling our entire planet, it forms a kind of closed waveguide with its own resonant frequency, which was experimentally discovered by the German physicist V.O. Schumann in 1952-1954.

In his works, Schumann proposed a simple formula for determining both the fundamental resonant frequency (first harmonic) and higher harmonics [1]: \[ f_{SH} \approx 6\cdot \sqrt{n (n + 1)}, \quad (Hz) \] where: \(n\) is the harmonic number. The following table shows the values of the first thirty harmonics of the Schumann resonant frequencies, according to this formula.

The source of Schumann resonance waves is global thunderstorm activity, and electromagnetic waves are emitted from lightning channels with some vertical component of charge transfer. Considering that every second on the planet there are from 50 to 100 lightning strikes, the background field of Schumann resonance is constantly present in the atmosphere. The atmosphere for very long waves, which include Schumann frequencies, has very low attenuation (0.1 dB/Mm or 1 dB per 10,000 km). Therefore, extremely low-frequency waves from lightning at any point on the planet are capable of propagating to any other place due to the natural waveguide formed by the ionosphere and the Earth's surface.

The spectral density of oscillations is 0.1 mV/m. The intensity of resonant oscillations and their frequencies depend on:

• time of day – at night the amplitude of resonant waves is 5-10 times smaller (due to the decrease in the rate of water flow in the ocean conveyor);
• time of year – in the summer months (from May to August in the Northern Hemisphere) resonance frequencies increase; in the Southern Hemisphere, resonance frequencies increase from November to February;
• location on the globe – Schumann waves are most clearly expressed near the world's thunderstorm centers: Africa, South America, Indonesia, India. In the polar regions, the amplitude peaks at these frequencies are not so pronounced;
• solar activity – during magnetic storms their intensity increases by 15%. There are cases of excitation of frequencies of 12500 Hz, which corresponds to the movement of the earth's core at a depth of 3.6 km from the center of the earth's core;
• water flow rate in the ocean conveyor;
• moon phases;
• solar activity periods.

There are several stations in the world to determine the current state of Schumann frequencies. One of them operates at Tomsk State University [2].