The nature of the interaction of electromagnetic waves with the ionosphere depends strongly on the frequency of operation. Therefore, the choice of the most convenient model of the resulting propagation phenomena also depends on the frequency.
In the VLF band, for example, the most convenient way to describe the propagation is by means of the so-called “spherical waveguide” mode. This formalism has the ionosphere (modeled as a magnetic conductor) acting as the upper boundary and the Earth surface (modeled as an electric conductor) acting as the bottom boundary of an effective curved “waveguide”. This formalism is particularly convenient at VLF because the transverse dimensions of the effective waveguide are of the same order of magnitude as the wavelength of operation, which means that the propagation can be well described using a few low-order modes. Ionospheric VLF signals can be reasonably stable, but two major obstacles exist in this frequency band. First, such low frequencies require large antennas with high transmit powers. Second, the information (or bit) transmission rates achieved are relatively low because of the low carrier frequency. Despite these disadvantages, ionospheric VLF propagation has been used in applications related to long-distance communications, navigation, and standard frequency dissemination. With the advent of communication satellites, long-distance fiber-optic systems, and global navigation satellite systems ...