Interference fringes detected by OEDIPUS C

Abstract

An HF transmitter was operated at one end of the tethered sounding rocket payload OEDIPUS C, and a synchronized receiver was operated at the other end. Both the transmitter and the receiver were connected to dipoles. On the flight downleg after the tether had been cut, direct bistatic propagation experiments were carried out with the transmitter‐receiver pair. During the flight, sharp minima which can be attributed to interference fringes were detected in the directly transmitted signal. Fringe frequencies observed on OEDIPUS C ionograms for four different interference schemes have been examined using the cold‐plasma theory. First, fringes were observed which can be attributed to the Faraday rotation of the plane of linear electric field polarization for the combined O‐ and X‐mode or O‐ and Z‐mode waves. The electric field rotation was calculated using independent measurements of the plasma parameters and the attitudes of the transmitting and receiving dipoles. Another kind of fringe occurred for the whistler (W) and Z‐mode waves as a result of their triple‐valued refractive index below half the cyclotron frequency for the W mode and just below the plasma frequency for the Z mode. The hypothesis has been tested that these “single‐mode” fringes are caused by the cancellation of the wave field of two circularly polarized Z‐mode or W‐mode waves excited by the linearly polarized transmitting antenna, at the point where the two waves' electric fields are shifted in phase by 180°. The theory for the single‐mode nulls correctly predicts the frequency range and the spacing of the fringes but fails to explain absolute frequencies in all cases. This failure points to the need for a complete computation of the dipole radiated field which is expected to have considerable structure on account of the inflection points on the W‐ and Z‐mode refractive index surfaces. The OEDIPUS C bistatic propagation experiment has demonstrated Faraday rotation in situ. The agreement between observations and cold‐plasma theory suggests that future bistatic propagation facilities can exploit the Faraday effect to measure density, density gradient, and parameters dependent upon wave polarization.