Approximate method for determining ELF eigenvalues in the earth-ionosphere waveguide

A simple approximate expression is derived for the TEM eigenvalue for ELF propagation in the earth-ionosphere wave-guide under conditions where anisotropy due to the earth's magnetic field may be neglected. Eigenvalues obtained in this manner are in excellent agreement with full-wave test calculations. It is shown that the eigenvalue is determined primarily by the location of two altitudes at which the conductivity attains certain values, and on the local conductivity scale heights at those altitudes.