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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 It is well known that intense HF radio waves transmitted from high-power ground-based HF heating facilities strongly modify the ionospheric plasma. Among the phenomena discovered from ionospheric modification experiments one of the most outstanding is the excitation of small-scale artificial field-aligned irregularities (AFAIs) or striations. Their spatial scale is of the order of a few meters across and several kilometers along the magnetic field line. AFAIs have been excited through a large number of experiments at a variety of HF heating facilities located at mid and high latitudes [e.g., Minkoff et al., 1974; Robinson, 1989; Robinson et al., 1997, 2006; Kelley et al., 1995; Frolov et al., 1997; Rietveld et al., 2003]. They occur at the upper hybrid resonance altitude when ordinary (O-mode) polarized HF pump waves reach the ionospheric reflection height. An O-mode HF pump wave couples through striations into electrostatic (upper hybrid) waves at the upper-hybrid resonance altitude, where the heater frequency is fH2 = fUH2 = fp2 + fce2, here fp is the local plasma frequency and fce is the electron gyrofrequency. Upper hybrid waves propagate in a direction near perpendicular to the magnetic field and their energy dissipation heats the electrons. Through the thermal parametric (resonance) instability [Grach and Trakhtengerts, 1975; Vas'kov and Gurevich, 1976], these waves can excite AFAIs which can trap the excited upper hybrid field. Since the resonance altitude in the ionosphere cannot be reached by an extraordinary polarization (X-mode) HF pump wave transmitted from the ground, only O-mode waves can excite striations. The observations of O- and X-mode heating effects observed simultaneously with the SuperDARN (Super Dual Auroral Radar Network) [Greenwald et al., 1995] CUTLASS (Co-operative UK Twin Located Auroral Sounding System) [Lester et al., 2004] and EISCAT (European Incoherent Scatter) [Rishbeth and Van Eyken, 1993] radars reported by Robinson et al.  clearly demonstrated evidence of heater-induced striations in the F-region ionosphere causing CUTLASS backscatter only during O-mode heating. Similarly, the electron temperature enhancements seen in the Tromsø EISCAT UHF radar data were also observed only during O-mode heating. It is significant that, during the observations, the heater frequency was lower than the ordinary mode critical frequency of the F2 layer, fH < foF2. This indicates that the O-mode HF pump wave was certainly reflected from the ionosphere and the thermal parametric (resonance) instability was excited at the upper hybrid resonance altitude.
 In this paper it is shown for the first time that the X-mode HF pump wave, radiated into the magnetic zenith from the EISCAT/Heating facility, can also generate very strong AFAIs in the high-latitude F-region ionosphere when the heater frequency is above foF2 but comparable to the extraordinary mode critical frequency, fxF2 ≈ fH > foF2.
 This letter will present the first experimental evidence for plasma modification associated with the small-scale AFAIs induced by an X-mode HF pump wave and to discuss the behavior and properties of such irregularities.
2. Experimental Observations
 The experiments reported here were conducted in the course of two Russian EISCAT heating campaigns from October–November 2009 and March 2010. The EISCAT HF heating facility [Rietveld et al., 1993] located near Tromsø in Northern Norway (geographical coordinates 69.6°N, 19.2°E; magnetic dip angle I = 78°) was used to modify the ionosphere in the high-latitude F region. The HF heater antenna beam was tilted 12° to the south of zenith, thus allowing HF pumping in the field–aligned direction (magnetic zenith, MZ). In the course of the experiments the heating facility was operating at 3.95, 4.04, 4.9128, and 5.423 MHz, with X-mode polarization, and with 10 min continuous HF pulses. Table 1 describes the heater and ionosphere parameters for the experiments on November 5 and 6, 2009 and on March 5, 6, and 8, 2010 during which the ionosphere was modified by X-mode HF pump waves. The experiments were carried out during quiet magnetic conditions.
Table 1. List of the EISCAT Heating facility and ionospheric parameters during experiments when the F-region of the ionosphere was modified by X-mode powerful HF radio wave
ERP calculated assuming perfectly conducting ground. Recent calculations suggest that a realistic ground gives ERP values that are 25% lower.
power stepping from 30–157
 The modified ionospheric F region was probed by the CUTLASS Hankasalmi, Finland radar (63°N, 27°E). CUTLASS is a pair of HF coherent backscatter radars located in Finland and Iceland and forms part of the SuperDARN array [Lester et al., 2004]. The CUTLASS (Finland) radar transmitter site is located approximately 1000 km south of the Tromsø heating facility. The CUTLASS radar beam is approximately 3.3° wide and can dwell on 16 independent adjacent positions. In the course of the experiments described below, one beam position centered on the Tromsø heater (beam 5) was utilized. The HF heating facility at Tromsø is located adjacent to the EISCAT UHF incoherent radar which operates at a frequency of 930 MHz. During the experiments the UHF radar measured the ionospheric plasma parameters in the direction of the magnetic zenith. Ionograms were taken every four minutes using the EISCAT dynasonde, which is co-located with both the EISCAT UHF and heater array at Tromsø.
2.1. Excitation of Artificial Small-Scale Field-Aligned Irregularities
 In all of the above cases listed in Table 1 the excitation of very strong AFAIs was observed from the CUTLASS Hankasalmi, Finland radar. The Tromsø ionosonde data indicated a smooth F2 layer with the critical frequency foF2 below the heater frequency, fH, by 0.4–0.6 MHz. At the same time the heater frequency was near or just below the extraordinary mode critical frequency (fxF2). Therefore, we suggest that in all listed experiments the HF pump wave with X-mode polarization was reflected from the ionosphere. In most cases, with the exception of November 5, 2009, sporadic E layers were absent. Figures 1 and 2 show key measurements made on November 5, 2009 and March 6, 2010.
Figure 1 illustrates the CUTLASS Hankasalmi backscatter power for a beam position centered on the Tromsø heater (beam 5) during the experiment on November 5, 2009 from 14:00 to 15:30 UT. The CUTLASS radar operated at a frequency of about 10 MHz and was therefore sensitive to small-scale field-aligned irregularities with the spatial size across the geomagnetic field of the order of l⊥ ≈ 15 m (l⊥ = λ/2, where λ is a wave length of radar). The CUTLASS Hankasalmi radar was operating in a standard mode with 45 km range gates with the first range gate starting at 180 km. Range gate 17 corresponds to the centre of the heated patch above Tromsø. For O-mode heater effects in the first three heater-on periods from 14:05 UT the HF pump wave with O-mode of polarization was radiated towards the magnetic zenith. As can be seen from Figure 1, the intense signals scattered from AFAIs were observed in the first two heater-on periods. Here the electron density in the F region dropped, and the heater frequency fH = 3.95 MHz was near and then slightly above the critical frequency of F2 layer. During the heater-on period from 14:35–14:45 UT, when the values of foF2 become about 3.5 MHz, the signals scattered from the AFAIs disappeared. In the next heater-on cycle from 14:50–15:00 UT, when foF2 decreased to 3.3–3.4 MHz, the polarization of HF pump wave was changed from O- to X-mode. The polarization change corresponds to the appearance of intense scattered signals above Tromsø. Their growth time is about 1 min and their decay time extends up to 30 min (see Figure 1, top), which is in contrast to O-mode effects where the growth and decay times of artificial small-scale irregularities varies from a few seconds to a few tens of seconds.
 The same behaviour of AFAIs from the CUTLASS Hankasalmi radar observations was observed during the experiment on November 6, 2009 (second case of Table 1). However, a power stepping mode, utilizing an orderly sequence of 20%, 50%, 70%, 85%, 100%, 100%, 85%, 70%, 50%, 20% (from 30 to 157 MW and back) was used. The duration of each step is 1 min. AFAIs appeared under 50% effective radiated power (ERP) and reached their maximum at full ERP. The decrease of ERP from 100% to 20% in the second part of the heater-on cycle was not affected by the intensity of signals scattered from AFAIs. Moreover, their intensity remained constant during the first 5 min after the heater was turned off. The decay time extended up to 15 min.
 In the course of the experiments on March 5, 6 and 8, 2010 the CUTLASS Hankasalmi radar was running a non-standard mode optimized for the observations of heating effects over Tromsø with one fixed beam (beam 5) with a high temporal and spatial resolution. It operated almost simultaneously at three frequencies of ∼10, 13 and 17 MHz and was therefore sensitive to AFAIs with the spatial size across the geomagnetic field of the order of l⊥ ≈ 15, 11, and 9 m respectively. A temporal resolution of 1 s and 15 km range gates, with the first gate starting at a range of 480 km, were utilized. Figure 2 shows the CUTLASS Hankasalmi backscatter power at three frequencies of ∼10, 13 and 17 MHz for a beam position centered on the Tromsø heater (beam 5) during the experiment on March 6, 2010 from 15:20 to 17:00 UT. Similar to the experiment on November 5, 2009, before X-mode heating, the HF pump wave with O-mode polarization was radiated for the first two heater-on periods from 15:26 UT. It is seen from Figure 2 that the intense signals scattered from AFAIs, excited by the HF pump wave with O-mode polarization, were observed at three CUTLASS frequencies in the heater-on period from 15:26–15:36 UT, when the electron density in the F region dropped, and the heater frequency, fH = 4.9128 MHz, was near and then slightly above the critical frequency of F2 layer, foF2 = 4.7–4.9 MHz. During the heater-on period from 15:41–15:51 UT the values of foF2 became ∼4.4 MHz and the signals scattered from AFAIs disappeared at 17 MHz. At 10 and 13 MHz the weak scattered signals were observed during the first 3–4 minutes of the 10 min heating cycle and then disappeared. The polarization of the HF pump wave was changed from O- to X mode in the next heater-on period from 15.56–16.06 UT. The polarization change of the heater wave produced scattered signals over Tromsø at three CUTLASS frequencies. On March 6, 2010 X-mode heating was performed during three consecutive heating cycles (see Table 1). The distinctive features of X-mode AFAIs with different spatial size across the geomagnetic field are clearly seen from the behaviour of the backscattered power at every operational frequency averaged over three 15 km gates, corresponding to the central part of the heated beam (see Figure 2, top). The excitation of AFAIs with l⊥ ≈ 11 m occurred in all three heater-on periods. They appeared within from 10 s to 3 minutes after the heater was turned on. Their decay time was approximately 6 and 15 min after the first and second X-mode cycle respectively. The results of the experimental observations on March 5, 2010 (third case of Table 1) are very similar to the case of March 6, 2010 but in this experiment only AFAIs with l⊥ ≈ 15 and 11 m were excited. They appeared within 1–4 min after the heater was turned on and their decay time varied between 5 and 20 min. The experiment on March 8, 2010 (fifth case of Table 1) was the only experiment, from all those listed in Table 1, when O-mode heating at the same frequency was not carried out prior to the X-mode heating. Nonetheless during the experiment on March 8, 2010 the AFAIs with different spatial sizes of l⊥ ≈ 15, 11 and 9 m were excited in four consecutive 10 min heater-on periods. AFAIs appeared within 1–4 min after the heater was turned on. Their decay time varied between 3 and 5 min and during some X-mode cycles was limited by the duration of the heater off, which was 5 min on March 8, 2009.
2.2. Heater-Induced Plasma Parameter Changes
 EISCAT UHF incoherent scatter data from Tromsø site, obtained in the direction of the magnetic zenith, were examined to estimate the changes in the electron temperature (Te) and electron density (Ne) at different altitudes, accompanying the generation of AFAIs induced by the powerful HF radio waves with X-mode polarization. The results obtained for contrasting O/X–mode heating clearly demonstrate that an HF pump wave with X-mode polarization heats the ionosphere through collision processes more effectively than an O-mode HF heater wave. This is in agreement with the recent results of Gustavsson et al.  who showed, both observationally and theoretically, that heating of ionospheric electrons is more efficient for X-mode heating than for O-mode heating. Note, that it is true only for ohmic heating of electrons when the O-mode HF pump wave is not reflected from the ionosphere because fH > foF2. In such conditions the thermal resonance instability can not be realized and the anomalous heating of electrons at the upper-hybrid resonance altitude can not be produced. In our experiments the Te enhancements under X-mode heating reached values of ∼50%, above the background level observed just before the heater-on cycle, over a wide altitude range from 186 to 247 km.
 Temporal variations of Te with 30 s integration time at different altitudes in the direction of the magnetic zenith during the experiment on March 6, 2010 from 15:38–17:00 UT are plotted in Figures 3a and 3b respectively. As shown in Figure 2, weak scattered signals were observed at 10 and 13 MHz during the O-mode heating cycle from 15:41–15:51 UT. Therefore here we observed not only the pure ohmic heating. During the next three consecutive X-mode heating cycles Te enhancements of up to 50% above the background level occurred. The strongest Te enhancements took place at 214 km from 16:16–16:26 UT cycle. The electron density behaviour in the magnetic zenith (Figure 3b) shows some interesting features. It is seen that Ne increased by up to 30% above the background level at an altitude of 282 km during the X-mode heating cycles. Ne enhancements were also observed at 246 km, however no significant enhancements were observed at 214 km where the strongest Te enhancements occurred. The total electron content along magnetic field line was calculated in the altitude range from 185 to 325 km and its behaviour is also shown in Figure 3b (top). The well-defined increases are clearly seen in the total electron content from the 15:56–16.06 and 16:16–16:26 UT heater-on cycles. Such electron density enhancements can be attributed to HF-induced ionization production rather than the change of the density distribution due to the thermal diffusion. Note that the experiment was conducted under quiet magnetic conditions, thus any Ne increases due to soft electron precipitation from the magnetosphere could be excluded. The possible mechanism for the ionization production due to X-mode HF pumping requires clarification.
 We presented the first experimental evidence of the excitation of strong small-scale AFAIs, with spatial scales across the geomagnetic field of the order of l⊥ ≈ 9–15 m, in the high-latitude F region of the ionosphere due to an X-mode HF pump wave, radiated in the direction of magnetic zenith. Their generation requires X-mode F-region heating at frequencies, fH,which are above the ordinary-mode critical frequency, foF2, by 0.4–0.6 MHz. At the same time fH lies in the vicinity of the extraordinary component of critical frequency, fxF2 ≈ fH > foF2.
 From all experiments listed in Table 1 it was found that the X-mode AFAIs appeared 10 s–4 min after the heater is turned on. Their decay time varied in a wide range between 3 and 30 min. In some heater-on sessions (see, for example, Figure 2 (middle) for 13 MHz) the AFAIs spread out from the scattered patch in a southward direction. Furthermore their behavior did not depend on the HF heating.
 The mechanism of the X-mode AFAIs is not clear. Vas'kov and Ryabova  have theoretically shown that the generation process of short wavelength (upper-hybrid and electron cyclotron) plasma oscillations can be produced by induced scattering of a powerful extraordinary HF radio waves by ions. They also note that the high frequency turbulence excited near the reflection level of the powerful extraordinary HF wave leads to the substantial enhancements of low frequency plasma perturbations. We have to mention that the incoherent backscatter spectra obtained with the EISCAT UHF radar during X-mode HF pumping demonstrate strongly enhanced ion line shoulders and even central ion line peak, which are unusual for X-mode heating. The ion line shoulders are due to the electromagnetic parametric decay instability [Stubbe, 1996]. The large growth and decay times observed are typical for large-scale ionospheric irregularities with spatial scales of the order of the heated patch (∼60 km). In such a case one would expect that the behavior of the small-scale AFAIs, with spatial scales of the order l⊥ ≈ 9–15 m, which are responsible for the strong backscattered signals from the CUTLASS measurements, is driven by the large-scale ionospheric irregularities. The electron thermal pressure force, arising from the differential ohmic heating on electrons [Gurevich, 1976] pushes electron to form the large-scale irregularities, which in turn break up the HF pump wave via the filamentation instability [Kuo and Schmidt, 1983]. The large-scale irregularities can be produced by the O- and X-mode HF pump wave. As it was shown in Section 2.2, the Te enhancements were observed over an altitude range of 150 km from 180 to 330 km, with a maximum enhancement of up to 50% above the background level at ∼215 km. The Te enhancements were accompanied by Ne enhancements of up to 30% over the background level.
 We would like to thank the EISCAT Scientific Association for performing the Tromsø heating experiments in conjunction with the EISCAT UHF radar measurements. EISCAT is an International Association supported by Finland (SA), Germany (DFG), Japan (NIPR and STEL), Norway (NFR), Sweden (VR), the United Kingdom (NERC) and China (CRIRP). CUTLASS is supported by the UK Science and Technology Facilities Council grant PP/E007929/1, the Finnish Meteorological Institute, and the Swedish Institute of Space Physics. TKY is supported by Science and Technology Facilities Council grant ST/H002480/1. LJB is supported in this work by Norwegian Research Council grants 191628/V30 and 19617/630. Authors are grateful to both reviewers for useful comments.
 The Editor thanks Bjorn Gustavsson and Spencer Kuo for their assistance in evaluating this paper.