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[1] The three-dimensional mathematical field model of HF sky waves reflected by a spatially nonuniform nonstationary magnetoactive ionosphere is described. The model is based on the structural physical approach, which leads to complete understanding of the field structure in space and time associated with specific geomagnetic conditions.

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Experimental data on radio sounding of the ionosphere from satellites at heights below its main maximum are presented. In these data, additional retarded lower trace (RLT) is observed in the ionograms. It is interpreted as a trace of a probing radio signal reflected obliquely from the Earth and coming back to the satellite after refraction by sharp horizontal ionospheric irregularities. Horizontal and vertical cuts of these irregularities as well as the electron-density gradients at their side walls are determined using quantitative methods. Two sequences of ionograms for two adjacent orbits of a satellite are presented and processed for which the critical-frequency maps corrected according to the experiment are constructed. The method of weighted interpolation of approximated values at the adjacent points is used. The horizontal size of the retrieved macroirregularity and the velocity of its motion above the Earth's surface are obtained. A conclusion is drawn that RLT ionograms allow one to diagnose additional parameters of the ionosphere.