Experimental results of resonant high-frequency (HF) scattering from small-scale geomagnetic field-aligned plasma irregularities excited by electromagnetic HF pumping of the ionospheric F region from the Sura radio facility in Russia are reported. These results include the first observations of a minimum in the scattered signal intensity when the pump frequency was near the fourth electron gyroharmonic and of a significant broadening of the frequency spectrum of the scattered signal for pump frequencies above the gyroharmonic. After pump-off, the flanks of the broad spectrum decayed rapidly. These new observations link together several previous experimental results of gyroharmonic effects in phenomena attributed to the excitation of small-scale field-aligned irregularities and are therefore thought to be significant contributions to our understanding of the self-organization of the space plasma under the action of an external perturbation. ¿ 1999 American Geophysical Union