Experimental evidence points toward magnetic-field-aligned small-scale plasma density striations as responsible for the long time features of stimulated electromagnetic emissions (SEE) in ionospheric radio modification experiments. This paper emphasizes that such striations act as resonators for the generation of trapped upper hybrid oscillations. The sidebands observed in long time SEE experiments are associated with nonstationary behavior, such as parametric instabilities, of these trapped oscillations. A theoretical model for this is formulated and studied numerically. Source spectra for SEE generation, consistent with the main properties of observed downshifted maximum and broad continuum features, will be presented. The first is associated with a trapped oscillations decay instability, while the latter is associated with cavitation in the pre-formed striations. ¿ 1998 American Geophysical Union