Earth-ionosphere waveguide
From Glossary of Meteorology
earth–ionosphere waveguide
The physical structure formed by the conductive earth and the conductive lower ionosphere (D region) that together sandwich the highly resistive atmosphere between them.
This waveguide sustains global electromagnetic wave propagation from the lower end of the Schumann resonance band (5 Hz) to the waveguide cut-off frequency near 1500 Hz, beyond which the radiation will not be retained within the waveguide.
