Abstract. We present experimental results concentrating on a variety of phenomena in 1 the high latitude ionosphere F2 layer induced by an extraordinary (X-mode) HF pump wave at 2 high heater frequencies (fH = 6.2 -8.0 MHz), depending on the pump frequency proximity to the 3 ordinary and extraordinary mode critical frequencies, foF2 and fxF2. The experiments were 4 carried out at the EISCAT HF heating facility with an effective radiated power of in October 2012 and October -November 2013. Their distinctive feature is a wide diapason of 6 critical frequency changes, when the fH /foF2 ratio was varied through a wide range from 0.9 to 7 1.35. It provides both a proper comparison of X-mode HF-induced phenomena excited under 8 different ratios of fH /foF2 and an estimation of the frequency range above foF2 in which such X-9 mode phenomena are still possible. It was shown that the HF-enhanced ion and plasma lines are 10 excited above foF2 when the HF pump frequency is lying in a range between the foF2 and fxF2, 11 foF2 ≤ fH ≤ fxF2, whereas small-scale field-aligned irregularities continued to be generated even 12 when fH exceeded fxF2 by up to 1 MHz and an X-polarized pump wave cannot be reflected from 13 the ionosphere. Another parameter of importance is the magnetic zenith effect (HF beam/radar 14 angle direction) which is typical for X-mode phenomena under fH /foF2 >1 as well as fH / foF2≤ 15 1. We have shown for the first time that an X-mode HF pump wave is able to generate strong 16 narrow band spectral components in the SEE spectra (within 1 kHz of pump frequency) in the 17 ionosphere F region, which were recorded far away from the HF heating facility. The observed 18 spectral lines can be associated with the ion acoustic, electrostatic ion cyclotron, and electrostatic 19 ion cyclotron harmonic waves (otherwise known as neutralized ion Bernstein waves). It is 20 suggested that these spectral components can be attributed to the stimulated Brillion scatter 21 (SBS) process. The comparison between the O-and X-mode narrow band spectra clearly 22 demonstrated that only an X-polarized pump wave scattered by SBS can propagate more than 23 one thousand km without significant deterioration. 24 25

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Abstract. We provide the first experimental evidence of the sensitivity of phenomena induced 26 by extraordinary (X-mode) polarized HF high power radio waves to pump frequency stepping 27 across the fifth electron gyro-harmonic (5fce) from below to above. The results were obtained at 28 the EISCAT (European Incoherent Scatter Scientific Association) HF heater facility near Tromsø 29 under effective radiated powers of 456 -715 MW, when the HF pump wave was transmitted into 30 the magnetic zenith. We have analyzed the behavior and intensities of various spectral lines in 31 the narrowband stimulated electromagnetic emission (SEE) spectra observed far from the heater, 32 HF-enhanced plasma and ion lines (HFPL and HFIL) from EISCAT UHF incoherent scatter 33 radar spectra, and artificial field-aligned irregularities from CUTLASS (Co-operative UK Twin 34 Located Auroral Sounding System) observations, depending on the frequency offset of the pump 35 field relative to the 5fce. At pump frequencies below 5fce the narrowband SEE spectra exhibited 36 very intense so-called stimulated ion Bernstein scatter (SIBS), accompanied by other spectral 37 components, associated with stimulated Brillouin scatter (SBS), which are greatly suppressed 38 and disappeared in the vicinity of 5fce and did not reappear at fH > 5fce. As the pump frequency 39 reached 5fce, the abrupt enhancements of the HFPL and HFIL power, the appearance of cascade 40 lines in the plasma line spectra, and the onset of increasing CUTLASS backscatter power 41 occurred. That is opposite to the ordinary mode (O-mode) effects in the vicinity of 5fce. The X-42 mode pumping at frequencies below and in the vicinity of the fifth electron gyro-harmonic 43 clearly demonstrated an ascending altitude of generation of induced plasma and ion lines from 44 the initial interaction height, whereas for O-mode heating the region of interaction descended. 45The observations are consistent with the coexistence of the electron acceleration along and 46 across the geomagnetic field at fH < 5fce, while only very strong electron acceleration along the 47 magnetic field was observed at fH ≥ 5fce. 48 49

We report experimental results regarding the distinctive features and behaviors of high‐frequency (HF)‐induced Langmuir and ion‐acoustic turbulences induced by ordinary (O‐mode) and extraordinary (X‐mode) polarized powerful HF radio waves injected into the high‐latitude ionosphere F region toward the magnetic zenith at EISCAT (European Incoherent SCATter). Alternating O‐/X‐mode pumping was produced at high heater frequencies in the range fH = 6–8 MHz during magnetically quiet background geophysical conditions. An effective radiated power of 450–600 MW was utilized. The radical distinction between the temporal evolution of the O‐ and X‐mode Langmuir and ion‐acoustic plasma waves after the HF heater is switched on in conjunction with the development of artificial field‐aligned irregularities (FAIs) was analyzed. It was found that the excitation thresholds of the Langmuir and ion‐acoustic turbulences significantly differed for the O‐ and X‐mode HF pumping. We have revealed that the X‐mode excitation thresholds for HF‐induced plasma and ion line backscatter are 0.47 and 0.61 V/m, respectively. The persistent O‐mode plasma and ion line backscatters that coexisted with FAIs showed excitation thresholds of 0.62 and 0.73 V/m, respectively, which exceeded the thresholds for the X‐mode plasma and ion lines, while their intensity was 2 orders of magnitude less. For the same background conditions, the “classic” resonance parametric decay instability and modulation instability, excited as the momentary response to the O‐mode HF pump wave switching on (the so‐called overshoot), have thresholds of 0.17 and 0.25 V/m, respectively, which are much below the excitation thresholds of persistent O‐mode plasma and ion lines.

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