Earth-Ionosphere waveguide
From Wikipedia, the free encyclopedia
The Earth-Ionosphere waveguide refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere.
Because the ionosphere contains charged particles, it can behave as a conductor. The earth operates as a ground plane, and the resulting cavity behaves as a large waveguide.
ELF and VLF (300 Hz - 30 kHz) frequency signals can propagate efficiently in this waveguide. For instance, lightning strikes launch a signal called radio atmospherics, which can travel many thousands of miles, because they are confined between the Earth and ionosphere.
The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7Hz.
