Ionospheric duct parameters from a dual‐payload radio‐frequency sounder

Observed characteristics of extraordinary (X) mode reflection pulses in the Observations of Electric‐field Distributions in the Ionospheric Plasma‐A Unique Strategy C (OEDIPUS‐C) topside sounding ionograms have been analyzed to yield the parameters of depletion irregularities that trap and guide (duct) high‐frequency waves. The amplitude of successive ionospheric reflection pulses fluctuates by 10 dB or more during frequency sweeps with 50 kHz steps. The pulse period is 3 ms. At the fixed frequency of 4.5 MHz the reflected amplitudes fluctuate much more slowly. The direct pulse from the OEDIPUS‐C transmitter to the receiver, consisting of an interference signal between the ordinary and extraordinary modes, is very sensitive to local density changes but shows fluctuations of 10 dB over even longer timescales of ∼1 s. An analysis based on the radiation properties of the transmitter dipoles and on ray optics in cylindrical field‐aligned density depletion ducts shows that very shallow depletions of <1% of the ambient density can explain the magnitudes and fluctuations observed in the reflected pulses. Duct density models with steep isodensity contours produce more realistic predictions of signal levels and their fluctuations with frequency than models with gently sloping contours.