HF heater-induced phenomena in the ionospheric F-region under the pump frequency above the maximum plasma frequency

Blagoveshchenskaya, N.1, Borisova, T.1, Kornienko, V.1, Rietveld, M.2 and Brekke, A.3

1 Arctic and Antarctic Research Institute, St. Petersburg, Russia
2 EISCAT Scientific Association, Tromsø, Norway
3 University of Tromsø, Tromsø, Norway

It is known that the excitation of artificial small-scale field-aligned irregularities (AFAI), HF-induced electron heating and optical emissions, anomalous absorption and other phenomena related to upper-hybrid waves occurs in HF heating experiments when the HF pump wave with an O-mode polarisation is reflected from the ionosphere. Nonetheless, experimental results from Tromsø heating experiments have shown that AFAIs were excited in the F-region as well as in the auroral E-region when the pump frequency exceeded the maximum plasma frequency. It is assumed that despite of the pump frequency was larger than the critical frequency, it was comparable to the maximum plasma frequency. We present experimental result from Tromsø heating experiment on 13 October 2006 in the evening hours when the critical frequencies in the F-region was gradually decreased from 4.1 to 3.3 MHz. The HF heating facility at Tromsø was operated at 4040 kHz, using O-mode polarization. The HF transmission scheme of 2 min on, 2 min off was used with the HF antenna beam pointed in the magnetic field-aligned direction. In the course of the experiment bi-static HF Doppler radio scatter observations were carried out on the London – Tromsø – St. Petersburg path in the conjunction with the EISCAT UHF radar observations. It was found that strong signals scattered from AFAIs were observed when the pump frequency was up to 0.5 MHz larger than the critical frequency. In these conditions very strong electron temperature enhancements (in 2-3 times) closely related to the heater-on periods took place. They were accompanied by the increase of the electron density from the EISCAT UHF radar observations.