Spectral features of ionospheric plasma waves excited by powerful hf radio waves radiated at frequencies near electron gyroharmonics and f2-layer critical frequency

We present the result of the studies into characteristics of high-latitude ionospheric F-region longitudinal plasma waves (Langmuir and ion-acoustic), caused by the impact of powerful HF radio waves of ordinary (O-mode) or extraordinary (X-mode) polarization of the EISCAT/Heating facility, Tromso, Norway. The powerful HF radio waves on October 20, 2012 and February 26, 2013 were emitted in the direction of the magnetic zenith with a step change in the effective radiation power (ERP). The radiation frequency f(H) of the EISCAT/Heating facility on February 26, 2013 was close to the F2-layer critical frequency f(o)F2 (f(H)/f(o)F2 similar to 1) and exceeded the electron gyroresonance frequency f(H)>5f(ce). On October 20, 2012, the conditions f(H)/f(o)F2 similar to 0.85-0.95 and f(H)<6f(ce) were fulfilled. Analysis of EISCAT measurements of an incoherent scatter radar (ISR) at a frequency of 930 MHz, spatially aligned with the heating facility for radiation conditions ERP<200 MW, has shown that parametric decay instabilities are excited at the ionospheric heights where the pump frequency is close to the plasma Langmuir frequency, f(H) approximate to f(PL). We have studied peculiarities of excitation of the decay parametric instabilities as a function of height in the ionosphere and pump wave polarization, the ratios between f(H) and f(o)F2, and also f(H) and nfce.