Comparison of Spectral Features of Narrowband Stimulated Electromagnetic Emission Excited by an Extraordinary Pump Wave in the High-latitude Ionospheric F Region at Frequencies below and above the F2 Layer X-component Critical Frequency

Abstract— The paper presents the results of investigating generation conditions and features of the narrowband stimulated electromagnetic emission (NSEE) induced by an extraordinary (X-mode) high-frequency (HF) pumping into the high-latitude ionosphere F region. It was shown that NSEE spectral features were recorded at a distance of more than 1100 km from the heating facility. Distinctive features of NSEE discrete spectral lines at the pump frequencies  \(f_\mathrm{H}\)  below and above the F2 layer X-component critical frequency ( \(f_\mathrm{H} < f_\mathrm{X}\mathrm{F2}\)  and  \(f_\mathrm{H} \ge f_\mathrm{X}\mathrm{F2}\) ) were analyzed. It was found that the NSEE spectrum contains the strongly pronounced discrete lines ordered by the electrostatic ion cyclotron frequency for the atomic oxygen ions (O \(^{ + }\) ). When  \(f_\mathrm{H} <f_\mathrm{X}\mathrm{F2}\) , the multiple downshifted and upshifted spectral components (Stokes and anti-Stokes modes, respectively) were excited in the NSEE spectra. When \(f_\mathrm{H}\ge f_\mathrm{X}\mathrm{F2}\) , the Stokes modes were only generated. Plausible mechanisms for the NSEE excitation by X-mode HF pump waves are discussed