We present experimental results from multiinstrument observations in the high-latitude ionospheric F 2 layer at the EISCAT (European Incoherent Scatter Scientific Association) heating facility. The results come from a set of experiments, when an X-polarized HF pump wave at high heater frequencies (f H > 6.0 MHz) was injected into the F region of the ionosphere toward the magnetic zenith. Experiments were carried out under quiet magnetic conditions with an effective radiated power of 458-548 MW. HF pumping was produced at different heater frequencies, away from electron gyroharmonic frequencies, and different durations of heater pulses. We show the first experimental evidence of the excitation of artificial optical emissions at red (630 nm) and green (557.7 nm) lines in the high-latitude ionospheric F 2 layer induced by an X-polarized HF pump wave. Intensities at red and green lines varied in the range 110-950 R and 50-350 R, respectively, with a ratio of green to red line of 0.35-0.5. The results of optical observations are compared with behaviors of the HF-enhanced ion and plasma lines from EISCAT UHF incoherent scatter radar data and small-scale field-aligned artificial irregularities from Cooperative UK Twin Located Auroral Sounding System observations. It was found that the X-mode radio-induced optical emissions coexisted with HF-enhanced ion and plasma lines and strong artificial field-aligned irregularities throughout the whole heater pulse. It is indicative that parametric decay or oscillating two-stream instabilities were not quenched by fully established small-scale field-aligned artificial irregularities excited by an X-mode HF pump wave.

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

[1] The effects on the high-latitude F region of the ionosphere by X-mode powerful HF radio waves injected towards the magnetic zenith (MZ) are analysed. The experiments were conducted using the EISCAT/Heating facility and UHF radar at Tromsø, Norway, the CUTLASS (SuperDARN) radar and the EISCAT ionosonde (dynasonde). The results show that the X-mode HF pump wave, radiated into the magnetic zenith from the HF heater, can generate very strong small-scale artificial field aligned irregularities (AFAIs) in the F-region of the high-latitude ionosphere. These irregularities, with spatial scales across the geomagnetic field of the order of 8-15 m, are generated when the heater frequency is above the ordinary-mode critical frequency but comparable with the extraordinary-mode critical frequency. The generation of the X-mode AFAIs was accompanied by electron temperature (Te) enhancements up to 50% above the background level and an increase in the electron density (Ne) by up to 30%.

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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

Abstract. Experimental results from ionospheric HF pumping experiments in the nightside auroral E region above Troms0 are reported. We found intriguing evidence that a modification of the ionosphere-magnetosphere coupling, due to the effects of powerful HF waves beamed into an auroral sporadic E layer, can lead to a local intensification of the auroral activity. Summarizing multi-instrument observations during two consecutive nights, one can distinguish the following peculiarities related to this auroral activation: a modification of the auroral arc and its breakup above Troms0, local changes in the horizontal currents above Troms0, a burst-like increase of the electron density and temperature, a large increase in the ion temperature in a wide altitude range and in the north-south component of the electric field, distinctive features in dynamic HF radio scatter Doppler spectra, pumpinduced electron precipitation, and substorm activation exactly above Troms0. We discuss the modification of the ionosphere-magnetosphere coupling in terms of the excitation of a turbulent Alfv•n boundary layer between the base of the ionosphere (• 100 km) and the level of sharp increase of the Alfv•n velocity (at heights up to one Earth radius), and the formation of a local magnetospheric current system. The results suggest that a possible triggering of local auroral activation requires specific geophysical conditions.

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