Modification of the high latitude ionosphere F region by X-mode powerful HF radio waves: Experimental results from multi-instrument diagnostics

We present experimental results concentrating on a variety of phenomena in the high latitude ionosphere F2 layer induced by an extraordinary (X-mode) HF pump wave at high heater frequencies (fH=6.2-8.0 MHz), depending on the pump frequency proximity to the ordinary and extraordinary mode critical frequencies, foF2 and fxF2. The experiments were carried out at the EISCAT HF heating facility with an effective radiated power of 450-650 MW in October 2012 and October-November 2013. Their distinctive feature is a wide diapason of critical frequency changes, when the fH/foF2 ratio was varied through a wide range from 0.9 to 1.35. It provides both a proper comparison of X-mode HF-induced phenomena excited under different ratios of fH/foF2 and an estimation of the frequency range above foF2 in which such X-mode phenomena are still possible. It was shown that the HF-enhanced ion and plasma lines are excited above foF2 when the HF pump frequency is lying in range between the foF2 and fxF2, foF2≤fH≤fxF2, whereas small-scale field-aligned irregularities continued to be generated even when fH exceeded fxF2 by up to 1 MHz and an X-polarized pump wave cannot be reflected from the ionosphere. Another parameter of importance is the magnetic zenith effect (HF beam/radar angle direction) which is typical for X-mode phenomena under fH/foF2 >1 as well as fH/foF2 ≤1. We have shown for the first time that an X-mode HF pump wave is able to generate strong narrowband spectral components in the SEE spectra (within 1 kHz of pump frequency) in the ionosphere F region, which were recorded at distance of 1200 km from the HF heating facility. The observed spectral lines can be associated with the ion acoustic, electrostatic ion cyclotron, and electrostatic ion cyclotron harmonic waves (otherwise known as neutralized ion Bernstein waves). The comparison between the O- and X-mode SEE spectra recorded at distance far from HF heating facility clearly demonstrated that variety of the narrowband spectral structures were only observed under X-mode HF pumping. The potential generation mechanisms of the different narrowband spectral components are discussed.