A two-fluid model of the parametric decay of an upper hybrid wave into an electromagnetic wave and a lower hybrid wave [Murtaza and Shukla, 1984> is applied to conditions in the ionospheric F region. It is found that the instability threshold can be exceeded in ionospheric high frequency heating experiments. Features of the growth rate can explain hitherto unexplained properties of a commonly observed prominent spectral structure of stimulated electromagnetic emission. The stimulated radiation can then be used to determine the magnitude of the geomagnetic field where the upper hybrid frequency matches electron cyclotron harmonics in the ionosphere. ¿ American Geophysical Union 1991