The set of electromagnetic resonances generated by lightning activity in the cavity between Earth's surface and the ionosphere. The fundamental mode occurs at approximately 7.83 Hz, with harmonics at roughly 14.3, 20.8, 27.3, and 33.8 Hz. Some PEMF devices reference the Schumann fundamental as a baseline frequency, on the premise that terrestrial life evolved alongside this natural field. That premise is a design rationale some manufacturers adopt, not an established biological requirement.
PEMF sits at the extremely-low-frequency end of the electromagnetic spectrum, pulsing a handful to a few thousand times per second. That is far below radio, and far, far below visible light. At these low frequencies the body barely absorbs the field, which is why reach is set by geometry, not by tissue.
