Coupling processes in the D- and E-regions at low and very low frequencies—I Frequencies less than critical: a study of the daytime ionosphere

Abstract

Use is made of a generalized numerical method of thin film optics to derive 2 × 2 reflection and transmission coefficient matrices of horizontal ionospheric slabs, the top ends of which become progressively higher. The matrix elements are expressed in terms of characteristic magneto-ionic modes, off-diagonal elements representing inter-mode coupling terms. Propagation processes in the frequency range 10–200 kHz are followed by presenting the various matrix elements graphically as functions of the ionospheric slab height. In the VLF range direct (gradient) reflections of the ordinary wave and the Z-trace reflection processes are competitive, but in the LF range the Z-trace is the main process reflecting ordinary wave energy. The ‘coupling echo’ whereby the upgoing ordinary wave is converted to downgoing extraordinary at the X = 1 level (when Z ≈ Z c ), is almost non-existent, but there is a strong ‘ Z coupling echo’ produced by the reflection at the X = 1 + Y level of an extraordinary wave generated near the X = 1 level by an upgoing ordinary wave. The same process taken backwards, with upgoing extraordinary returning as ordinary is, by reciprocity, of equal magnitude. The ‘inverse Z-transmission’ process, with upgoing extraordinary being coupled into upgoing ordinary at the X = 1 level is very weak. It is shown that a wave of arbitrary polarization, incident on the ionosphere from below, will tend to maintain its initial polarization well into the ionosphere, even though the characteristic local wave-polarization may have changed considerably. In the upper LF range, where the X = 1 + Y level is virtually the only reflection region, a number of simple relations are shown to exist between the various transmission and reflection coefficient matrix elements. The problem of nomenclature of characteristic magneto-ionic modes at frequencies ‘less than critical’ is discussed.