Two‐point studies of fast Z‐mode waves with dipoles in the ionosphere

An HF transmitter and synchronized receiver, both connected to dipoles, were operated during the tethered rocket experiment OEDIPUS C. On the flight downleg after the tether had been cut, direct bistatic propagation experiments were carried out with the transmitter in one subpayload and the receiver in the other subpayload. Medium‐frequency Z‐mode transmissions were observed to be strong compared with the adjoining wave modes. This fast Z mode is the left‐hand polarized mode propagating between its cutoff frequency f Z and the local plasma frequency f p . The Z mode is of particular theoretical interest because its refractive index surface evolves from a simple ellipsoid to an ellipsoid with inflections as the frequency is swept through the band. The Z‐mode signal was strongest at frequencies just below f p , at relatively low f p values. At higher values of f p , the Z‐mode signals were strongest at frequencies just above f Z . Details of these characteristics and of the measurements are presented. The magnitudes of transmitted signals predicted by a cold‐plasma theory for short dipoles are found generally to be in good agreement with the measurements. The exceptions were at the highest frequencies where the inflected shape of the refractive index surface leads to three separate wave vector directions for a single group direction. A possible explanation for the relatively strong Z‐mode signal is that the transmitting dipole impedance in the f Z − f p frequency range provides a better match to the output of the transmitter than in the bands of other wave modes swept.