Guided Z mode propagation observed in the OEDIPUS A Tethered Rocket Experiment


  • H. G. James


The tethered sounding rocket payload OEDIPUS A conducted bistatic propagation experiments on plasma waves in the auroral ionosphere. Synchronized sweeps of the frequency range 0–5 MHz by the 2-W transmitter high-frequency exciter (HEX) on the upper end of the tether and its associated receiver for exciter (REX) on the lower end have produced signatures of quasi-electrostatic waves guided along field-aligned depletions of ambient density. The propagation is in the slow Z mode, between the plasma frequency ƒp and the upper hybrid resonance frequency ƒuhr when ƒp is greater than the cyclotron frequency. The mode identification is based on payload measurements of ƒp. These waves have signal delays of about 1 ms. The delays are much greater than expected for free-space propagation over the transmitter-receiver separation distance which varies up to 960 m during the flight. The transmitted pulses typically appear inside a frequency bandwidth of about 100 kHz just above the plasma frequency, but occasionally occupy most of the available bandwidth, ≃300 kHz, between ƒp and ƒuhr. The observed delays and the stretching by a factor of 3 of the transmitted 300-μs pulses are accounted for with two-dimensional ray tracing using a complete electromagnetic solution of the hot plasma dispersion relation. Delayed Z mode pulses appear in about 20% of the ionograms. Given the weakness of the HEX transmitter and the abundance of examples obtained during the flight, guiding of natural Z mode emissions in the auroral ionosphere may be efficient and widespread.