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

Abstract. The ESRAD 52-MHz and the EISCAT 224-MHz radars in northern Scandinavia observed thin layers of strongly enhanced radar echoes from the mesosphere (Polar Mesosphere Winter Echoes -PMWE) during a solar proton event in November 2004. Using the interferometric capabilities of ESRAD it was found that the scatterers responsible for PMWE show very high horizontal travel speeds, up to 500 ms −1 or more, and high aspect sensitivity, with echo arrival angles spread over as little as 0.3 • . ESRAD also detected, on some occasions, discrete scattering regions moving across the field of view with periodicities of a few seconds. The very narrow, vertically directed beam of the more powerful EISCAT radar allowed measurements of the spectral widths of the radar echoes both inside the PMWE and from the background plasma above and below the PMWE. Spectral widths inside the PMWE were found to be indistinguishable from those from the background plasma. We propose that scatter from highly-damped ion-acoustic waves generated by partial reflection of infrasonic waves provides a reasonable explanation of the characteristics of the very strong PMWE reported here.

Theoretical models and observations have suggested that the increasing greenhouse gas concentration in the troposphere causes the upper atmosphere to cool and contract. However, our understanding of the long-term trends in the upper atmosphere is still quite incomplete, due to a limited amount of available and well-calibrated data. The European Incoherent Scatter radar has gathered data in the polar ionosphere above Tromsø for over 33 years. Using this long-term data set, we have estimated the first significant trends of ion temperature at altitudes between 200 and 450 km. The estimated trends indicate a cooling of 10-15 K/decade near the F region peak (220-380 km altitude), whereas above 400 km the trend is nearly zero or even warming. The height profiles of the observed trends are close to those predicted by recent atmospheric general circulation models. Our results are the first quantitative confirmation of the simulations and of the qualitative expectations.

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