Outshifted Plasma Lines Observed in Heating Experiments in the High-Latitude Ionosphere at Pump Frequencies Near Electron Gyroharmonics

We present experimental results concerning the features and origin of the outshifted plasma lines in the high-latitude ionospheric F region, which are excited by high-power HF O-mode radio waves radiated by the EISCAT/Heating facility at pump-wave frequencies near the fourth and fifth gyroharmonics. HF pump waves were transmitted towards the magnetic zenith during long-lasting (2–20 min) heater-on cycles. The studies were carried out using the measurement data obtained by the EISCAT incoherent scatter radar in Tromsø (the radar operating frequency 930 MHz). The analysis of the radar data has demonstrated the simultaneous excitation of several plasma lines at frequencies close to the pump frequency and at frequencies upshifted by 0.15–0.45 MHz on long-lasting (longer than 30 s) time intervals. The spectral width of the outshifted plasma lines was 0.10–0.15 MHz. The features of their occurrence were analyzed as functions of the relation between the pump-wave frequency and gyroharmonics and the proximity of the pump-wave frequency to the cutoff frequency of the ionospheric F 2 layer. Plausible excitation mechanisms of the outshifted plasma lines are discussed. Comparison between the experimental data and calculation results concerning the features of the outshifted plasma lines for different pump-wave frequencies was made.