Schumann resonances are extremely low-frequency (ELF) global electromagnetic resonances, excited by lightning discharges in the cavity formed between the Earth’s surface and the ionosphere.
The base frequency is around 7.83 Hz but there are several higher harmonics. As the range of frequencies is largely congruent with human brain waves, it is thought that disturbances of the Schumann Resonances may be a component in Geopathic Stress. Several commercial GS-reducing devices are based upon Schumann wave generation.