Features of the broad upshifted structure in stimulated electromagnetic emission spectra

We present experimental results concerning features of the broad upshifted structure (BUS) in stimulated electromagnetic emission spectra, generated i n the ionospheric F region plasma by a powerful electromagnetic wave. The B US is observed for pump frequencies far from electron cyclotron harmonics. Modification experiments, using O-mode HF pump waves, were conducted at the SURA heating facility (56 degrees N, 44 degrees E, Nizhny Novgorod, Russia ). We investigated the BUS properties for pump frequencies ranging from the third to the sixth electron cyclotron harmonic, concentrating on the depen dence of (1) the BUS spectral features on pump frequency and power and (2) the BUS temporal evolution under different ionospheric conditions with and without additional heating of the ionosphere by another pump wave. We find (1) that the frequency range of BUS generation depends on the gyroharmonic mode number, becoming narrower with increasing mode number, and (2) that th e BUS is produced in the vicinity of the upper hybrid resonance layer, loca ted typically a few kilometers below the pump wave reflection height. An im portant result is that small-scale striations, being produced concurrently with the upper hybrid plasma waves due to the thermal parametric instabilit y, do not play a crucial role for the BUS evolution. A possible relationshi p of the BUS to gyroharmonic features is discussed.