Analytical formulas describing the standing wave pattern in the ionospheric reflection region of a vertically propagating HF electromagnetic wave in the O mode are used to estimate density depletions due to the ponderomotive force of the successive wave maxima. The frequencies of Langmuir waves enhanced through the parametric decay instability in the created density wells are determined. It is suggested that a spectral feature of the stimulated electromagnetic Emissions, downshifted by 1--2 kHz from the pump frequency, is an evidence of parametrically excited Langmuir waves which have been mode converted to electromagnetic waves. Stimulated radiation then provides a means for remote sensing of the peak electric field strength in the reflection region.