3. Multifrequency Polarization Measurements of the Radio Wave Absorption by the A1 Method

Figure 1

[10]  The Beynon idea was used in the early 1970s for systematic studies of the effective collision frequency in the ionosphere. Till then, experience had been accumulated in measurements of the absorption by the A1 method first at two and then at five frequencies simultaneously [Svechnikov et al., 1972]. To study the radio wave absorption and, respectively, n_e(h) profiles in the entire thickness of the ionosphere, measurements of the absorption at many frequencies of the F region were required. Among them, there should have been frequencies at which the group delays of signals of the ordinary and extraordinary rays were the same, that is, a polarization fading took place. For getting rid of these signals it was necessary to separate signals by the polarization. A polarization ionosonde was created which registered separately the signals of the ordinary ( o ) and extraordinary ( x ) polarizations [Danilkin et al., 1974]. An example of the first publication of a polarization ionogram is shown in Figure 1.

[11]  To determine the frequency dependencies of the radio wave absorption, the amplitudes of the reflected signals were measured simultaneously at 10 frequencies. Two regimes of operation were used. In the first regime all measurements used only "O" signals. In the second regime measurements were conducted using signals of both polarizations at five frequencies each. All frequencies for the "x" signals were chosen to be by 0.7 MHz higher than for the "O" signals. In the geographical conditions of Rostov-on-Don station this provided approximate equality of the reflection height.

Figure 2

[12]  The calibration of the measurements, i.e., determination of the equipment constant, was conducted for all frequencies below 3 MHz using multiple reflections. For higher frequencies the calibration was performed by the polarization method [Danilkin and Faer, 1972]. To exclude rapid signal fading, an averaging over 15- or 30-min intervals of measurements was used. The influence of focusing or defocusing was excluded by running averaging over a 90-min period of observation [Berezin, 1964; Berezin and Gusev, 1961; Givishvili and Shaulin, 1975]. An example of measurements of the absorption of "o" waves at 10 frequencies at Rostov on Don at the 30-min interval of averaging is shown in Figure 2.