JOURNAL OF GEOPHYSICALRESEARCH,VOL. 98, NO. AI0, PAGES 17,597-17,606, OCTOBER 1, 1993 Spectral Structure of Stimulated Electromagnetic Emissions BetweenElectron Cyclotron Harmonics T. B. LEYSEP. Swedish Institute of Space Physics, Kiruna, Sweden B. THIDI•,M. WALDENVIK, AND S. GOODMAN 1 Swedish Institute of Space Physics, Uppsala Division, Uppsala,Sweden V. L. FP. OLOV, S. M. GRACH, A. N. KAP. ASHTIN, G. P. KOMP. AKOV, AND D. S. KOTIK Radiophysical Research Institute, Nizhniy Novgorod, Russia Experimental results concerning the spectrum ofstimulated electromagnetic emissions (SEE) in the sidebands of a powerful high-frequency electromagnetic ordinarymode pump waveare presented. Thepump wave was vertically injected intotheionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, andbroad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations ofa new wideband emission occurring primarily in the upper sideband. The dependence ofthe SEE spectral structure onthe pump frequency in relation to electron cyclotron harmonics isemphasized. All spectral features in thesidebands of thepump exhibit cyclotronharmonic effects. INTRODUCTION Electromagnetic emissions stimulated by a powerful high- frequency electromagnetic pumpwave injected into the iono- sphere from ground-based transmitters were first investi- gated by Thidd et al. [1982]in experiments performed at the Ramfjordmoenheating facility near Troms• in Norway and have since then become a useful tool for the study of nonlinear processes in ionospheric heating experiments. In addition to the Troms• experiments [e.g., Thidd et al., 1983;Stubbe et al., 1984;Derblom et al., 1989], experiments with stimulated electromagnetic emissions (SEE) have been carried out at Arecibo, Puerto Rico [Thid• et al., 1989], the high-power auroral simulation facility near Fairbanks, Alaska [Armstrong et al., 1990], andseveral facilities in Rus- sia [Boiko et al., 1985; Erukhimov et al., 1987]. SEE exhibit a richspectral structure in both the lower and the upper sidebands of the ionospherically reflected pump wave[Stubbe et al., 1984] as well as at harmonics and sub- harmonics of the pump frequency [Derblom et al., 1989]. The appearance of the differentspectralfeatures depends on ionospheric conditions,pump frequencyf0, and pump power[Leyser et al., 1990]. Particularly, previous exper- iments have shown a great variability in the steady state SEE spectrumfor small variationsof f0 near harmonics n of the electroncyclotronfrequency fc in the bottomsideF region [Leyser et al., 1989, 1990, 1992;Stubbe and Kopka, 1990]. The broadupshifted maximum (BUM), which is a spectral structure upshifted by a few tens of kilohertz from the pump, develops only for f0 slightly above nfc. Widen fo = nf•, the prominent and commonly observeddown- shifted maximum(DM) feature,downshifted approximately 10 kHz from f0, is absent in the SEE spectrum. These spec- tral characteristics of the BUM and DM are used below in relating f0 to nf•. In the present treatment we discuss detailed measure- mentsof the commonlyobserved continuum,DM, and BUM emissions in the sidebands of the pump for f0 ranging from 4f• to above 7f•, where fc typically is 1.3-1.4MHz, and report a new broadband and time varying emission having more energy in the upper than in the lower sideband. The results concern the SEE steady state spectral structure ob- tained a few seconds after pump turn-on. For a description of the technique of data taking, seeDerblom et al. [1989]. The experimental results were obtained in daytime dur- ing two consecutive campaigns in September1990 and 1991 at the Sura ionospheric modification facility near Vasil- sursk, about 100km east of Nizhniy Novgorod in Russia (geographic coordinates, 56øN, 44øE). The latitude of the Sura facility is thus between that of the Ramfjordmoen and Arecibo ionospheric modification facilities. In the F region above the Sura facility, the angle between the geomagnetic field and the downwardvertical is approximately 19 ø at a typical interaction height of 200 km. The effective radiated power of typically 270 MW, corresponding to a pump inten- sity of 0.5 mW/m 2 at 200-km altitude if ionospheric absorp- tion is neglected, was radiated in the ordinary mode in a vertical beam into an overdense ionosphere. Local time at Copyright1993 by the AmericanGeophysical Union. Paper number 93JA01387 0148-0227/93/93J A-01387505.00 • Now at Radio Atmospheric Science Center, Kyoto University, the Sura facility is universaltime plus 3 h. Japan. EXPERIMENTAL RESULTS Lower Sideband Emissions The commonly observed continuum featurein the SEE spectra hasbeen described as an asymmetric skewed wide- 17,$97