Abstract
Abstract. Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-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). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was 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 strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation of striations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization.
Highlights
It is well known that intense HF radio waves transmitted from high-power ground-based HF heating facilities strongly modify the ionospheric plasma
Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø, and the Tromsø dynasonde. During the experiment both the HF pump beam and the UHF radar beam were directed towards the magnetic zenith (MZ)
We have presented the results of our analysis of multiinstrument data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway
Summary
It is well known that intense HF radio waves transmitted from high-power ground-based HF heating facilities strongly modify the ionospheric plasma. Observations of airglow at 630 nm (red line) and 557.7 nm (green line) during the HF ionospheric modification experiments at the HAARP heating facility have shown HF-induced green and red lines at the magnetic zenith (MZ) to persist after the critical frequency foF2 dropped below the heater frequency (Pedersen et al, 2003). Mishin et al (2005) proposed a scenario for the HF heater-induced phenomena at magnetic zenith in the underdense ionosphere when foF2 dropped below the heater frequency, but HF-induced airglow at green and red lines remained He explained this feature in terms of strong turbulence at magnetic zenith, with the oscillating two-stream instability (OTSI) of upper hybrid waves as the primary source of Langmuir waves.
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