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Natural intermediate frequency fields

In our natural environment electromagnetic fields with intermediate frequencies are generated mainly in association with the so-called sferics. Sferics are broadband electromagnetic impulses that occur in the Earth's atmosphere as a result of natural lightning discharges. Tornados can also be a source of sferics. Sferics may extend from a few kHz to several tens of kHz (3-100 kHz).
A cloud to cloud lightning
photo: André Karwath aka Aka, license: CC BY-SA 2.5, via Wikimedia Commons