>Schumann
The Earth's atmosphere acts as a waveguide for electromagnetic waves, including those generated by lightning strikes. These waves become trapped between the Earth's surface and the ionosphere, an electrically charged layer in the upper atmosphere. This trapped energy creates the Schumann resonances, low-frequency electromagnetic waves that can be detected around the globe.
The frequency of Schumann resonances is directly related to the number of lightning events occurring on Earth. When there are more lightning strikes, the intensity of the electromagnetic waves in the atmosphere increases, resulting in a higher frequency of Schumann resonances. Conversely, a decrease in lightning activity can result in a lower frequency of Schumann resonances.
Reference: https://www.physicsforums.com/threads/connection-schumann-resonance-and-lightning-events.922310/
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