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Multi-instrument observational data from an experiment on13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway areanalysed. The experiment was carried out in the evening hours when theelectron density in the F-region dropped, and the HF pump frequency <I>f<sub>H</sub></I>was near and then above the critical frequency of the F2 layer. Thedistinctive feature of this experiment is that the pump frequency was justbelow the third electron gyro harmonic frequency, while both the HF pumpbeam and UHF radar beam were directed towards the magnetic zenith (MZ). TheHF pump-induced phenomena were diagnosed with several instruments: thebi-static HF radio scatter on the London-Tromsø-St. Petersburg path,the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter(EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). Theresults show thermal electron excitation of the HF-induced striations seensimultaneously from HF bi-static scatter and CUTLASS radar observations,accompanied by increases of electron temperature when the heater frequencywas near and then above the critical frequency of the F2 layer by up to 0.4 MHz.An increase of the electron density up to 25% accompanied by strongHF-induced electron heating was observed, only when the heater frequency wasnear the critical frequency and just below the third electron gyro harmonicfrequency. It is concluded that the combined effect of upper hybridresonance and gyro resonance at the same altitude gives rise to strongelectron heating, the excitation of striations, HF ray trapping andextension of HF waves to altitudes where they can excite Langmuir turbulenceand fluxes of electrons accelerated to energies that produce ionization

annales geophysicae

Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency