Enhanced plasma and ion oscillations on the topside of the ionosphere have been observed during high frequency (HF) ionospheric modification experiments at Troms¿. The topside enhancements occurred when the critical frequency of the F layer was below that of the Z mode reflection. Thus, one can consider Z mode waves as a plausible candidate to produce the enhancements. However, the topside echoes were observed over an angular range of many degrees, apparently not compatible with the prediction of the standard theory of a very narrow coupling window located at the Spitze angle. We propose resonant scatter of the HF pump wave into Z mode waves on small-scale field-aligned irregularities to be the cause of the broadening. The irregularities are generated by the HF wave, likely via the resonant instability. ¿ 2001 American Geophysical Union