Earth–ionosphere waveguide
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.
Extremely low frequency (ELF) (< 3 kHz) and very low frequency (VLF) (3–30 kHz) 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 the ionosphere. The round-the-world nature of the waveguide produces resonances, like a cavity, which are at ~7 Hz.
Radio propagation within the ionosphere depends on frequency, angle of incidence, time of day, season, Earth's magnetic field, and solar activity. At vertical incidence, waves with frequencies larger than the electron plasma frequency (fe) of the F-layer maximum
(1) fe = 9 (Ne)1/2 kHz
(Ne in cm−3 is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than fe are reflected within the ionospheric D-, E-, and F-layers. fe is of the order of 8–15 MHz during day time conditions. For oblique incidence, the critical frequency becomes larger.
Very low frequencies (VLF: 3–30 kHz), and extremely low frequencies (ELF: <3 kHz) are reflected at the ionospheric D- and lower E-layer. An exception is whistler propagation of lightning signals along the geomagnetic field lines.
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