An O mode wave launched into the ionosphere around harmonics of cyclotron frequency excites a large-amplitude upper hybrid wave via resonance instability near the critical layer. The upper hybrid wave may parametrically decay into a lower hybrid wave and an electromagnetic wave. This process may account for the appearance of downshifted maximum in the spectrum of stimulated electromagnetic emissions. The threshold power density Pth for the process depends inversely on the spatial extent &xgr;o of the localized upper hybrid pump. Near the seventh cyclotron harmonic, (&ohgr;o≈7&ohgr;c), Pth≈1 &mgr;W/m2 at &xgr;o=50 m, k≈0.085 cm-1. However, the O mode frequency &ohgr;o must be significantly away from cyclotron harmonics to avoid heavy cyclotron damping of the upper hybrid wave. At the fourth and seventh harmonics, (&ohgr;o-n&ohgr;c) must exceed 10 and 1 KHz, respectively, to avoid damping.¿ 1997 American Geophysical Union