PUBLICATIONS. Radio Science

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1 PUBLICATIONS RESEARCH ARTICLE Special Section: Ionospheric Effects Symposium 2015 Key Points: Study of density profile modification in HF-pumped ionosphere by multifrequency Doppler sounding Broad Downshifted Emission: a novel feature of SEE near-electron gyroharmonics Exploring HF-induced ionospheric turbulence by Doppler sounding and stimulated electromagnetic emissions at the High Frequency Active Auroral Research Program heating facility Evgeny N. Sergeev 1,2, Alexey V. Shindin 1, Savely M. Grach 1, Gennady M. Milikh 3, Evgeny V. Mishin 4, Paul A. Bernhardt 5, Carl L. Siefring 5, Stanley J. Briczinski 5, and Michael J. McCarrick 5 1 Faculty of Radiophysics, Lobachevsky State University of Nizhni Novgorod, Russia, 2 Radiophysical Research Institute, Lobachevsky State University of Nizhni Novgorod, Russia, 3 Department of Astronomy, University of Maryland, College Park, Maryland, USA, 4 Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, New Mexico, USA, 5 Plasma Physics and Information Technology Divisions, Naval Research Laboratory, Washington, District of Columbia, USA Correspondence to: S. M. Grach, sgrach@rf.unn.ru Citation: Sergeev, E. N., A. V. Shindin, S. M. Grach, G. M. Milikh, E. V. Mishin, P. A. Bernhardt, C. L. Siefring, S. J. Briczinski, and M. J. McCarrick (2016), Exploring HF-induced ionospheric turbulence by Doppler sounding and stimulated electromagnetic emissions at the High Frequency Active Auroral Research Program heating facility, Radio Sci., 51, , doi:. Received 31 DEC 2015 Accepted 17 APR 2016 Published online 25 JUL American Geophysical Union. All Rights Reserved. Abstract We report on the features of the F region plasma perturbations during HF heating experiments at the High Frequency Active Auroral Research Program facility in March April 2011 and May June The diagnostics included multifrequency Doppler (phase) sounding (MDS) and stimulated electromagnetic emission (SEE). The results concern modification of the electron density profile near the reflection and upper hybrid heights, as well as correlation of the density modification with temporal behavior of narrow continuum, downshifted maximum, and broad continuum SEE spectral features. We reveal also a new SEE spectral feature which appears in the SEE spectra for the pump frequency f 0 near the third and fourth electron gyroharmonics. It is located in the SEE spectrum well below the pump wave frequency, f f 0 ~ (40 220) khz, occupies a wide frequency range till khz, and is termed the broad downshifted emission. 1. Introduction High-power O-mode radio waves transmitted into the F region ionosphere from the ground excite a wide range of plasma processes leading to artificial ionospheric turbulence (AIT). AIT comprises various HF and LF plasma modes, plasma density irregularities with scale lengths from tens of centimeters to kilometers, and leads to heating and acceleration of plasma electrons, generation of ionospheric airglow, and ionization, etc. [Gurevich, 2007]. Diverse diagnostic methods and tools are used to explore AIT and its effects, in particular, sounding of the heated volume by diagnostic waves and registration of secondary, or stimulated, emission in a broad frequency ranges. The pump-plasma interaction is known to be strongest near the pump reflection height z r at which f p (z r ) matches the pump frequency f 0 and near the upper hybrid (UH) resonance height z UH where f p (z UH )= (f 2 0 f 2 ce ) 1/2. Here f p =(e 2 N/πm) 1/2, and f ce = eb/2πmc are the electron plasma frequency and the electron cyclotron frequency, respectively, e and m are the electron charge and mass, N is the electron density, c is the speed of light, and B is the geomagnetic field strength. This corresponds to existing theoretical concepts confirmed by investigations of the HF-pumped ionospheric volume by multifrequency Doppler sounding (MDS) at Sura and European Incoherent Scatter (EISCAT) heating facilities [Vas kov et al., 1986; Berezin et al., 1991; Lobachevsky et al., 1992; Shindin et al., 2012], which have revealed plasma expulsion from the resonance regions. Secondary or stimulated electromagnetic emission (SEE) with frequencies f close to the pump wave frequency f 0 points to nonlinear interactions of high-power HF pump radio waves in the ionosphere. SEE occurs due to conversion of HF pump-driven electrostatic plasma modes, most notably Langmuir (L) and upper hybrid (UH) waves, into electromagnetic waves with amplitudes smaller by db than the reflected pump wave [Thidé et al., 1982; Stubbe et al., 1984] thereby helping to identify nonlinear processes. The prominent SEE spectral features have long been used as indicators of specific nonlinear mode interactions in the altitude region between the reflection z r of the O-mode pump and slightly below the upper hybrid resonance z UH.A number of prominent SEE spectral features have been established from numerous studies of stationary and dynamic SEE spectra performed at the European Incoherent Scatter (EISCAT), Sura, High Frequency Active SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1118

2 Auroral Research Program (HAARP), Arecibo, and Space Plasma Exploration by Active Radar (SPEAR) heating facilities for 2.8 < f 0 < 10 MHz [Stubbe et al., 1984; Leyser et al., 1993, 1994; Frolov et al., 2001; Leyser, 2001; Carozzi et al., 2002; Sergeev et al., 2004; Thidé et al., 2005; Sergeev et al., 2006; Kotov et al., 2008; Grach et al., 2008; Norin et al., 2009; Bernhardt et al., 2010, 2011; Bordikar et al., 2013; Mahmoudian et al., 2013, 2014; Grach et al., 2015]. The red-shifted (Δf = f f 0 < 0) SEE features include the narrow continuum (NC) with frequency shifts Δf 0 7 khz, the downshifted maximum (DM) with Δf 7 20 khz and its family (2DM, 3DM, etc.), new and intermediate downshifted maxima (NDM and IDM), and the broad continuum (BC) with Δf khz. The blue-shifted (Δf > 0) spectrum consists of the upshifted maximum (UM) with Δf 5 18 khz, the broad upshifted maximum (BUM) with Δf khz, and the broad upshifted structure (BUS) with Δf khz. Each feature is coined according to the spectral shape and position relative to f 0. A strongly developed BUM is often accompanied by a much weaker broad downshifted maximum (BDM), which occurs approximately at the mirror frequency of the BUM [Stubbe et al., 1984; Leyser, 2001]. Recent advances in high-power HF excitation of the AIT were made with the help of the HAARP facility (Gakona, Alaska, USA, N, W). Particularly, it was discovered that HAARP facility provides enough power to produce noticeable (up to 100%) artificial ionospheric ionization. This was found for various pump frequencies regardless of their proximity to harmonics of electron cyclotron frequency, sf ce, s =2,3,4,6 [Pedersen et al., 2009, 2010; Sergeev et al., 2013; Grach et al., 2014; Bernhardt et al., 2016]. In Sergeev et al. [2013] and Grach et al. [2014] the SEE measurements and sounding of the perturbed ionosphere by short pulses with frequency f DW = f 0 were utilized and the additional evidences of excitation of the artificial ionospheric layers was presented. Also, narrowband SEE features related to stimulated Brillouin scattering and to excitation of the ion Bernstein waves were revealed and studied [Norin et al., 2009; Bernhardt et al., 2010, 2011; Bordikar et al., 2013; Mahmoudian et al., 2013, 2014]. In this paper we present new results from experiments performed at HAARP. The results were obtained in March April 2011 and in May June 2014 by multifrequency Doppler (phase) sounding (MDS) of the ionosphere and with observation of the stimulated electromagnetic emission (SEE). The observational sites were located along the meridian to the south of the HAARP facility at (A) Riverview Lodge (about 11 km distant, both MDS and SEE), (B) Tonsina River Lodge (83 km, only SEE), and (C) Tiekel River Lodge (113 km, only SEE during 2011 campaign). Site A (B) was nearly under the heated region during injections at vertical (magnetic zenith, MZ). A 30 m folded-dipole BWDS antenna was used in site A, an AS-2259/GR inverted-v antenna was used at site B, and a 10 m 2 diamond magnetic loop was used at site C. The receivers at A, B (C) digitized a band till 850 (300) khz around the pump frequency. The dynamic range of the instruments after spectral processing is estimated to be better than 90 db. In section 2 we show results of the first MDS implementation at the HAARP facility. Concurrent SEE measurement clearly demonstrated close correlation of certain SEE spectral features with the nonlinear processes near the pump wave reflection and UH altitudes. Section 3 describes newly revealed SEE feature, the broad downshifted emission (BDE). The results are discussed and summarized in sections 4 and Phase Sounding of the Heated Volume Experiments on multifrequency Doppler (phase) sounding of the ionosphere heated volume was performed on 4 June 2014 at 23:00 24:00 UT (15:00 16:00 AST). The HAARP transmitter radiation was operated in the following regime. During the first 30 s of a long quasi-continuous pumping (QCW), high-duty cycle pump wave (pulse width τ = 70 ms, interpulse period T = 100 ms, the off-duty ratio Q = T/τ 1.4) was radiated vertically at the frequency f 0. Simultaneously, short (20 μs) pulses (low-duty cycle diagnostic waves) with the same interpulse period (Q = 5000) and same effective radiated power (ERP) P ef ~ 400 MW at two carrier frequencies f DW = f 0 and f DW = f khz were used for sounding perturbed ionospheric region. The low-duty cycle was used during 2 min including 30 s of the quasi-continuous pumping. After that, the whole 2 min session with 30 s of the QCW was repeated twice at the same f 0. Then the pump wave f 0 was changed. During the QCW radiation, the short pulses were radiated within 30 ms pauses. The power of the 20 μs diagnostic pulses was sufficient to create a wide spectrum of diagnostic waves (up to 300 khz near each carrier frequency). The average power of diagnostic waves <P> = P/Q = 80 kw was below SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1119

3 Figure 1. Frequency-time spectrogram of Doppler frequency shifts taken at Riverview Lodge during three successive 30 s quasi-continuous pumping sessions at f 0 = 5500 khz starting at 23:40 UT (t = 0) on 4 June the thresholds of the generation and maintenance of the pump-induced thermal plasma instabilities in the ionosphere [Grach et al., 1981]. Also, the diagnostic pulse duration was too short to excite ponderomotive parametric instability in the ionosphere [Frolov et al., 1997, 2007; Sergeev et al., 2004, 2007]. Therefore, under the combined radiation mode, the quasi-continuous pump perturbed the ionosphere near the plasma resonance (pump wave reflection level) and at the UH resonance, and the pulse sounding provided a diagnostics of the perturbed volume [Berezin et al., 1991; Grach et al., 1997; Sergeev et al., 2007; Shindin et al., 2012]. The size of pump-induced plasma perturbations along the geomagnetic field is determined by the electron thermal conductivity and plasma diffusion. It reaches several tens of kilometers and is comparable to the characteristic scale of the ionospheric layer. Changing the frequency mismatch Δf = f DW f 0 makes possible to investigate the properties of the plasma turbulence near the center (where f DW = f 0 ) and at the periphery of the pumped volume by measuring the amplitude and phase characteristics of the test waves. The use of broadband radio receiver and specially developed signal processing algorithms has allowed the study of the evolution of amplitude and phase (φ) of various spectral components of the reflected probing signals, which passed the perturbed region twice, in a wide (totally ~500 khz) band. The frequency resolution in the analysis was 1 khz, and the temporal resolution was determined by the interpulse period T = 100 ms. The results of measurements of the temporal evolution of Doppler frequency shifts f di (t) = (1/2π)dφ i /dt for different spectral components (f i ) of the reflected signal (Figure 1) provided data for further reconstruction of the electron density profile and its temporal evolution N(z, t) by solving the inverse problem of the phase sounding. Under the geometric optics approximation, each sounding wave at the angular frequency ω i =2πf i, propagating from the ground up to the reflection points z r (f i ), and back, suffers the phase incursion [Ginzburg, 1964] φω; ð tþ ¼ 2ω ref c z 0 n ω; ω π pðz; tþ dz (1) 2 where ω p (z, t)=2πf p (z, t) is the angular plasma frequency and n(ω, ω p (z, t)) is a wave refractive index. The reflection altitude z r is determined by the condition n = 0, and t is the time. This can be translated in the following expression for the phase change Δφ(ω)=φ(ω, t 0 ) φ(ω, t) in the time interval (t 0, t) associated with perturbation of the profile N(z, t) due to ionosphere pumping or natural reasons: yðωþ ¼ c 2ω Δφω ð Þ ¼ ðωþ g ω 1 K ω; ω p Δz ωp dωp (2) Here K (ω, ω p )=dn(ω, ω p )/dω p is a kernel of the integral equation (2), g(ω) is the angular plasma frequency at the reflection point, which is g(ω)=ω for an ordinary wave, and t 0 is initial time, Δz(ω p, t)=z(ω p, t) z(ω p, t 0 )is the altitude shift, i.e., the difference between the sounding radio wave reflection heights at the current (t) and initial (t 0 ) times. In equation (2) the integration variables are changed in comparison with (1): integration over SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1120

4 Figure 2. Consecutive reflection height variations of different spectral components of sounding pulses with added 150 m offsets. The three successive 30 s quasi-continuous pumping sessions started at (a) 23:40:00 UT, (b) 23:42:00 UT, and (c) 23:44:00 UT. The frequency step between spectral components is Δf i = 20 khz, the initial diagnostic frequency f start = 5050 khz, the final frequency is f end = 5650 khz, f p (z UH ) = 5320 khz, and z r (f i = f start, t = 0) is 205 km. Running averaging over the frequency range with a 35 khz window is applied. the altitude z is replaced by integration over the plasma frequency ω p. It is taken into account that at the reflection point n(ω, g(ω)) = 0 and that Δz = 0 at the entrance to the plasma layer. The left-hand side of (2) y(ω) is to be determined from the experimental data (Figure 1). The initial (reference) density profiles z (ω p, t 0 ) or, equivalently, N(z, t 0 ) were taken from the ionograms collected prior to the quasi-continuous pumping session. We use the Tikhonov regularization method in order to solve (2) for each time sample and calculate Δz i (ω, t), thereby following the variation of the electron density N(z, t). It has been done in the broad altitude range using the initial density profile N 0 (z) as the reference. For this purpose, we have transformed Δz(ω, t) toδz (N, t) using the univocal relation between the plasma frequency at the radio wave reflection point and the electron density for ordinary waves ω = ω p =(4πe 2 N/m) 1/2. Therefore, zn; ð N 0 ; tþ ¼ zn ð 0 ÞþΔzðN; tþ (3) is the dependence of the reflection height of radio wave on the density. Then we find the required distribution N(z, t) by calculating the inverse of (3). More details of the reconstruction algorithm and results of similar experiments at the Sura facility in 2008 and 2010 at f 0 = and f 0 = 4.74 MHz are given in Shindin et al. [2012]. Note that the ERP of the pump and diagnostic waves (f DW = f 0 and f DW = f khz) in the Sura experiments were, respectively, P ef0 60 MW and P efdw = 20 MW which is much smaller than at HAARP. The results of the 4 June 2014 experiment for three successive 2 min sessions at f 0 = 5500 khz are presented in Figures 1 4. Doppler frequency shifts f di (f i,t) are shown in Figure 1. The dynamics of the restored plasma density profile is shown in Figure 2. It demonstrates variations of the reflection heights at different spectral components of the diagnostic pulses Δz r (f i, t). Here black lines correspond to the reflection heights of the pump (z = z r (f 0 )) and diagnostic waves at f = f p (z UH )=(f 2 0 f 2 ce ) 1/2. In Figure 2, Δf = 20 khz is the frequency step between successive spectral components of probing pulses. For clarity we introduce an additional height shift 150 m between reflection heights of these components at t = 0. Temporal variations of the reflection heights Δz r (f i, t) allow to calculate velocities of the vertical motion of plasma density at a certain level N i = πf 2 i m/e 2 as V v = Δz r / t at different f i. Figure 3 (top) shows the velocities of the reflection height displacement of sounding waves versus time and sounding wave frequency (pulse spectral component) for the first 10 s of the 2 min pumping started at 23:40 UT. Positive and negative velocities correspond to the downward and upward motion of the plasma density level, respectively. Figure 3 (bottom) displays an instantaneous spectrogram of stimulated electromagnetic emission (SEE). Notice that analysis of the SEE spectra in the MDS experiments was performed for the first time by Grach et al. [1997] to SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1121

5 Figure 3. (top) Velocity of the sounding waves reflection height displacement V v = Δz r (f i, t)/ t versus time and frequency for the pumping session started at 23:40 UT (t = 0). Running averaging over frequency with 35 khz window is applied. (bottom) Spectrogram of the SEE for the same pumping session. The narrow continuum, downshifted maximum, and broad continuum spectral features are labeled. determine the interrelation between the pump frequency f 0 and gyroharmonics nf ce (n = 4, 6). Figure 4 shows the relative changes in the plasma density N versus height reconstructed at 2.4 s after the start of heating and at the end of 30 s long quasi-continuous pumping. It is seen that immediately after the quasi-continuous pump wave is turned on, the Doppler frequency shifts are mainly positive (f di > 0) for almost all spectral components except the ones close to the pump frequency (f i f 0 ) where f di < 0. This behavior of Doppler frequency shifts indicates increasing reflection altitudes z r of the sounding waves with the frequencies close to the pump frequency. The speeds of the reflection altitude motion V v achieve 100 m/s. In contrast, the altitudes of reflection z r descend for both f i > f 0 and f i < f 0,at velocities V v up to 60 m/s. This corresponds to electron density decrease in the vicinity of the pump wave reflection point and the increase at other heights. That is, as clearly seen in zoom panels of Figure 4, plasma is forced out in the vicinity of the reflection point so that the relative depletion reaches up to 0.1% at 2.4 s in the heating. According to Figures 2 and 3 (top), the uplifting Δz r (f i f 0 ) grows during 2 5 s. It reaches m, remains approximately constant during s, and later disappears. Figure 4. Relative variations of electron density [N(t) N(0)]/N(0) versus height after 2.4 s (in upper right zoom panels) and 30 s (main panels) of the quasi-continuous pumping for the same sessions. The zoom panels and main panels have the same height range. The expanded range of the density variations is shown on top of the panels. SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1122

6 Figure 5. (a) Spectrograms and (b and c) individual SEE spectra from Tonsina River Lodge on 27 March 2011 during CW pumping at f 0 = 5700 khz < 4f c into the magnetic zenith starting at 03:48:00 UT. The SEE spectra obtained at 53 rd (Figure 5b) and 113 rd (Figure 5c) sec. The DM, 2DM, UM, and BDE features are marked. According to Figures 1 3, plasma expulsion from the upper hybrid layer begins in 2 3 s after the heater is turned on. This is accompanied by the increase (up to 500 m) of the reflection heights of diagnostic waves with frequencies f i f p (z UH ) and decrease (up to m) of the reflection heights of diagnostic waves with frequencies f i < f p (z UH ) with vertical velocities V v of the same order. The total drop of the electron density at the upper hybrid height reaches % during 30 s (Figure 4). So the plasma depletion near the upper hybrid height is several times deeper than one near the reflection height. As is seen from Figures 1 3, the sign of the Doppler frequency shifts changes after the QCW to the opposite one. This leads to the reduction of the plasma density depletion around the UH height. The depletion relaxes and disappears in ~15 30 s after the pump wave is turned off. The SEE spectrogram (Figure 3, bottom) displays the following spectral features: the NC p (the ponderomotive narrow continuum at frequency shifts Δf NC = f 0 f NC, 10 < Δf NC < 0 khz), DM (the downshifted maximum at Δf DM ( ) khz), and BC (the broad continuum at Δf BC (3 40) khz). Peculiarities of the NC p, DM, and BC are well described in numerous papers [see, e.g., Leyser et al., 1993, 1994; Leyser, 2001; Frolov et al., 2004; Thidé et al., 2005; Sergeev et al., 2006; Grach et al., 2015]. The NC p is called also FNC (fast narrow continuum) in accordance with characteristic time of its development comprising only few milliseconds. Unlike the FNC, slow or thermal narrow continuum (SNC or NC t ) develops with much longer timescale 0.5 5s [Leyser, 2001; Frolov et al., 2004; Thidé et al., 2005]. The NC p develops much faster than the temporal resolution in Figure 3 (bottom). Under such temporal resolution, the NC p feature appears immediately after pump wave is turned on, simultaneous with the start of the plasma expulsion from the vicinity of the pump wave reflection point z r (f 0 ). Then NC p exhibits strong overshoot effect; i.e., its spectral width and intensity noticeably drop during the rise of z r (f 0 ) and achieve stationary values during the growth of the DM and BC features. The DM and BC features develop concurrently with the plasma expulsion from the UH altitude and exhibit noticeable overshoot effect. This demonstrates that the NC p feature shall be attributed to processes near the reflection height, while DM and BC are linked to the UH resonance. 3. Broad Downshifted Emission in the SEE Spectrum 3.1. Experiments at f 0 ~4f ce During the HAARP campaign on of March 2011 we observed a new SEE spectral feature at high powers and call it a broad downshifted emission (BDE). Notice that the term BDE has already been introduced for broad dynamic emission sporadically observed in SEE spectra in September 1991 at Sura for frequencies around (mostly above) f 0 < 5f ce [Leyser et al., 1993]. Since then, no further observations of broad dynamic emission have been reported. Henceforth, the term BDE designates the broad downshifted emission. Figure 5 shows an example of the BDE obtained for f 0 ~4f ce during continuous (CW) pumping. According SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1123

7 Figure 6. (a) SEE spectrogram obtained on 5 June 2014 at Riverview Lodge, 00:00 00:05 AST (08:00 08:05 UT). Pump frequency varied in the interval khz. The time t = 20 s corresponds to 00:01 AST. The double-resonance condition f DM f UH 4f ce is met at t 64.4 ± 2 s (upsweep, f ce 1393 khz) and t 99.0 ± 1.5 s (downsweep, f ce 1389 khz) and marked by horizontal dashed line. (b and c) Individual SEE spectra obtained at 50 th (f 0 =5509kHz< 4f ce )and70 th (f 0 =5609kHz> 4f ce ) seconds. Black and gray lines correspond to O-polarized and X-polarized SEE, respectively. The DM, 2DM, BDE, and BUM features are marked. to Sergeev et al. s [2006] classification [see also Leyser, 2001; Frolov et al., 2001], the SEE spectrum in the frequency range 25 khz < Δf = f f 0 < 15 khz is typical for pump frequencies range below gyroharmonic (f 0 < sf ce, s = 4) as the prominent DM, 2DM, and UM are present. It is seen from Figure 5 that after s in the heating, the BDE in the frequency range Δf ~ (40 80) khz develops with the peak at Δf ~ (55 60) khz (Figure 5b) and drifts to lower frequencies, expanding and amplifying with time. After approximately 3.5 min of pumping, the BDE range expanded to Δf ~ (50 160) khz and peaked at Δf ~ ( ) khz while the peak amplitude was amplified by 4 5 db (Figure 5c). At the same time, the DM and 2DM are also noticeably amplified. During a set of 4 min long pumping sessions with 30 s breaks performed at 03:30 03:50 UT on 27 March 2011 the intensity of traditional SEE features (NC, DM, 2DM, and UM) exhibited strong irregular variations (up to 5 db) with quasiperiod ~2 min. During the minima of traditional SEE, BDE disappeared. Then, concurrently with the amplification of the traditional features, BDE appeared again and demonstrated the temporal behavior similar to that shown in Figure 5 (reappearance at Δf ~ (40 80) khz then a drift to lower frequencies and amplification). The SEE intensity variations were probably related to large-scale plasma irregularities drifting across the pumped volume or, in other words, to changing conditions of the pump-ionosphere interaction. Overall, the BDE was found during several CW runs for pumping near the fourth gyroharmonic. Although it was stronger and observed more frequently for f 0 < 4f ce, it also appeared several times for f 0 > 4f ce during magnetic zenith injections [Grach et al., 2012]. These experiments were performed during the night when the pump frequency f 0 was close to the F region critical frequency, f OF2 f 0 ~ MHz, and the HAARP antenna pattern was directed to the magnetic zenith. In the 2014 campaign (25 May to 10 June), the BDE near the fourth electron gyroharmonic was registered several times in the midnight experiments with the pump frequency sweep. The HAARP antenna pattern was again directed into the magnetic zenith, while the critical frequency was close to the pump frequency, f OF2 f 0. Figure 6 shows the results obtained on 5 June Here the pump frequency f 0 stepped by 1 khz each 0.2 s (the rate r = 5 khz/s) in the range khz. Prior to the end of each step, the pump was turned off for 30 ms. Prior 10 ms to the end of the pause, a 20 μs pulse was radiated for diagnostics of possible generation of artificial ionization layers (for details see Sergeev et al. [2013]). Prior to the frequency sweep, CW pumping at f 0 = 5460 khz < 4f ce was used for 40 s. As in Figure 5, under CW pumping the BDE appears in s in the heating, drifts away from the pump frequency, and amplifies. However, the magnitude of the SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1124

8 frequency shift Δf exceeds that in Figure 5, reaching khz. During the sweep, the pump frequency f 0 passes 4f ce. This is indicated by the disappearance of the DM at f ± 10 khz and f DM 5573 ± 10 khz (upsweep), implying that the double-resonance condition f DM f UH (z) 4f ce (z) [Leyser et al., 1994; Leyser, 2001; Carozzi et al., 2002; Sergeev et al., 2006; Grach et al., 2008] is met at khz and f ce (z) 1393 ± 2.5 khz. During the downsweep this occurs for f ± 5 khz and f DM 5556 ± 5 khz. The electron gyrofrequency can be estimated as f ce (z) 1389 ± 1 khz. This corresponds, according to the International Geomagnetic Reference Field model, to the heights z 265 km (upsweep) and z 270 km (downsweep). According to Leyser et al. [1994], Carozzi et al. [2002], and Kotov et al. [2008], the pump wave frequency range of the DM quenching at f 0 ~4f ce is 2 10 khz. In the experiments presented, the DM quenching was only partial such that during the upsweep the range of the strongest DM weakening was within ~20 khz. Such a wide pump frequency range as well as the absence of total DM quenching can be related to the presence of horizontal density inhomogeneities and, therefore, variations of the double-resonance conditions within the pump beam [Leyser et al., 1994]. During the upsweep (increasing f 0 ) the widest (till 120 khz) and strongest BDE was observed for 5520 < f 0 < 5570 khz 4f ce khz. It is seen that BDE peak position approaches (moves away from) increasing (decreasing) f 0 linearly according to Δf BDE ¼ f BDE f 0 ¼ αðf 0 4f ce Þ df: (4) During the upsweep, the BDE generation continues after f 0 crossing 4f ce until the upper border of the sweep range, f 0 = 5660 khz, but the intensity and width decrease. The BDE generation continues after the sweep direction is changed. However, during the downsweep the intensity and width are smaller than for the upsweep for the same f 0. Furthermore, the BDE peak during the downsweep moves away from f 0 faster (larger α) than it approaches f 0 for the upsweep, and the BDE generation stops when f 0 approaches 4f ce. However, in a few seconds, after f 0 crosses 4f ce, the BDE reappears at different Δf BDE, closer to the pump wave frequency by ~40 50 khz, and goes on to move away from the pump frequency with approximately the same rate α ~ 0.9 (right part of panel a). The polarization SEE characteristics are also shown in Figures 6b and 6c. Here the black (gray) line corresponds to O (X) polarization of the received SEE spectrum. It is seen that in contrast with O-polarized traditional SEE spectral features (DM, UM, and BUM), BDE polarization can change dramatically with frequency shift Δf. Namely, it is ordinary at the high-frequency flank but almost linear at the low-frequency flank of the BDE (Figure 6b). Sometimes, it is linear along with Δf across whole BDE feature (Figure 6c). Overall, four successive sessions with the same pump frequency range of the sweep and different rates r =5, 2.5, 1.25, and 0.77 khz/s (0.2 s, 0.4 s, 0.8 s, and 1.3 s duration of the steps, respectively) were performed. During each successive session the BDE weakened relative to the previous one, while the BDE peak frequency still can be approximated by equation (4). During the third downsweep the BDE has not appeared. During the fourth sweep, f OF2 decreased below f 0, so no SEE was received. At the same time, the double-resonance frequency decreased till 5550 khz (for r = 2.5 khz/s) and 5525 khz (r = 1.25 khz/s), which corresponds to a raise of resonance altitudes by 2 and 10 km, respectively. Thus, the decay of the midnight ionosphere seems to be a reason of the BDE weakening. In addition, the so-called twisted mode [Leyser et al., 2009] midnight experiment was performed on 4 June. During the upsweep, a weak BDE approaching the pump frequency was observed for f 0 < 4f ce [Sergeev et al., 2015]. The BDE intensity was small in this, most probably, due to the considerably lower ERP for the twisted mode due the suppressed side lobe [Leyser et al., 2009]. It should be pointed out that so far, all successful observations of the BDE occur in darkness. It has not been observed under sunlight. During all pump frequency sweeps, the BDE position in the SEE spectra obeys equation (4), but the rate α and the frequency mismatch df vary essentially for different sweeps (0.3 < α <1.2, 5 < df < 100 khz). Namely, α is larger for downsweeps (decreasing f 0 ) than that for upsweeps. This means that the mismatch between the pump f 0 and 4f ce is not the sole parameter which determines the BDE properties. Notice that the artificial ionization layers (dynamic BUM and additional reflection/scattering of the diagnostic pulses) were not observed during the experiment at f 0 ~4f ce and f 0 ~3f ce (the latter experiment is described in section 3.2). On the other hand, the reflected signals of diagnostic pulses provided strong evidence of the intensive F spread. SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1125

9 Figure 7. (a) SEE spectrogram obtained on 6 June 2014 at Riverview Lodge, 00:00 00:05 AST (08:00 08:05 UT). Pump frequency varied in the interval khz. The time t = 20 s corresponds to 00:01 AST. The double-resonance condition f DM f UH 3f ce, f ce 1417 khz is met at t 22.5 ± 0.5 s (upsweep) and t 58.0 ± 0.5 s (downsweep). (b and c) Individual SEE spectra obtained at t =19s(f 0 = 4243 khz, upsweep) and t =61s(f 0 = 4246 khz, downsweep). Black and gray lines correspond to O-polarized and X-polarized SEE, respectively. The DM, 2DM, 3DM, BDE, and BUM features are marked. In addition, in the pump wave frequency range 4275 < f 0 < 4295 khz, f 0 > 3f ce above the third gyroharmonic, at the frequency shifts 20 < Δf < 30 khz, another SEE spectral feature, most probably the broad downshifted maximum (BDM) [Leyser, 2001], can be distinguished in Figure 7a. In the present paper we do not focus on the BDM properties Experiments at f 0 ~3f ce During the HAARP campaigns in March April 2011 and May June 2014 several experiments were performed with the pump wave frequency sweep across the third electron gyroharmonic 3f ce. Experiment during the midnight of 6 June 2014 was organized along the same way as on 5 June (see above). The pump frequency f 0 stepped by 1 khz in the range khz. Prior to the first sweep, ionosphere was irradiated by CW pump at f 0 =4150kHz< 3f ce during 40 s. Four up and down sweeps were performed. The durations of each step in the successive sweeps were 0.2 s (r =5kHz/s),0.4s(r = 2.5 khz/s), 0.8 s (r = 1.25 khz/s), and 1.3 s (r = 0.77 khz/s). Between the second and third sweeps CW pumping at 4150 khz during 40 s was applied. The HAARP antenna pattern was pointed to the magnetic zenith. The critical frequency during the experiment was stable, f OF2 ~ MHz and essentially larger than the pump frequency throughout the experiment. The SEE properties of the in this experiment remained practically the same for different sweep sessions. Therefore, we discuss here SEE spectrogram and individual spectra only for the first sweep (Figure 7). As follows from Figure 7, the minimum of the DM intensity occurs at f DM ± 3 khz, f ± 3 khz. Therefore, from the double-resonance condition (f DM f UH (z) 3f ce (z) the electron gyrofrequency can be estimated as f ce (z) 1417 khz, which corresponds to the height z = 226 km. During the upsweep, the BDE appears in a narrow pump frequency range 4237 < f 0 < 4245 khz, i.e., for f 0 3f ce ~5 15kHz. The BDE occupies a range 90 < Δf < 220 khz with a peak at Δf BDE 150 khz. During the downsweeps, the BDE appears approximately in the same pump frequency range, a little closer to gyroharmonic, f 0 3f ce ~2 10 khz. It is weaker by ~10 db and situated farther from the pump at ( ) khz than for the upsweep. In all runs, unlike pumping at f 0 ~4f ce, the BDE appears just below 3f ce at the same pump frequency range δf 0 ~8 10 khz. The latter is observed also in the 1 April 2011 experiments during 04:50 05:00 UT, with pump frequency stepping in the range khz, 1 khz step, 0.2 s per step. Two sweeps, one for the vertical pumping and the other for magnetic zenith pumping, were performed. The double-resonance conditions were obtained for f 0 3f ce 4225 khz corresponding to the height ~250 km. The BDE was seen in the spectra both for vertical and MZ pumping for 4200 < f 0 < 4225 khz at 60 < Δf < 80 khz. The lower boundary of the BDE here was determined by the receiver bandwidth. The polarization characteristic of the SEE in Figures 7b and 7c are marked by black line (O polarization) line and gray line (X polarization). Here contrary to f 0 ~4f ce, all SEE features DM, 2DM, 3DM, UM, BUM, and BDE are ordinary polarized. SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1126

10 Notice that in all experiments both for f 0 ~3f ce and f 0 ~4f ce BDE was registered in receiving sites situated at River View (11 km from the heating facility to the south) and Tonsina River Lodge (82 km) in 2011 and 2014 and also at Tiekel River Lodge (113 km to the south, 2011). The behavior of the BDE at all the three receiving sites was virtually identical. 4. Discussion 4.1. Multifrequency Doppler (Phase) Sounding of the Heated Volume and the SEE Behavior We present some preliminary results of the multifrequency Doppler sounding of the HF-pumped ionosphere at HAARP for June Under vertical pumping the pump-plasma interaction develops most quickly in the vicinity of the pump reflection height z r (f 0 ) and leads to the plasma expulsion from the interaction region. This is accompanied by the NC p appearance. Plasma expulsion from the upper hybrid height begins in 2 3 s along with the BC and DM development. At the same time, plasma expulsion from the reflection height stops, and intensity of the NC p notably decreases. The expulsion from the upper hybrid height continues until the end of 30 s pumping, while the DM and BC exhibit noticeable overshoot effect. The depth of plasma depletion near the upper hybrid resonance achieves %, while the upper hybrid height rises by 500 m. This behavior indicates that the UH-related processes lead to the essential shielding of the reflection point from the pump wave energy. A sequence of the described effects is consistent with a general scenario of the phenomena developing in the HF-pumped ionosphere. Namely, the fast (in a few milliseconds) excitation of Langmuir waves near the pump wave reflection point z r (f 0 ) is followed by a slow (in a few seconds) excitation of the UH waves near upper hybrid height z UH and small-scale magnetic field-aligned irregularities (FAI) or striations [Frolov et al., 1997; Thidé et al., 2005; Belikovich et al., 2007; Frolov et al., 2007; Grach et al., 2008]. The intimate relation of plasma expulsion from different heights (z r and z UH ) and different SEE features (NC p and DM/BC, respectively) is clearly demonstrated in this paper for the first time. The results obtained are quite similar, even quantitatively, to that obtained earlier at the SURA heating facility with much smaller ERP [Shindin et al., 2012]. One of the possible reasons is that the value of the pump wave frequency f 0 = 5500 khz ~ 4f ce 200 khz. This frequency belongs to the so-called weak emission range where the stationary SEE intensity (DM and BC features) is quite low [Sergeev et al., 2006; Leyser, 2001; Frolov et al., 2001]. The pump frequency at the similar SURA experiments f 0 = 4740 khz, (f 0 ~3f ce +700kHz~4f ce 650 khz), lies in the strong emission range where UH-related SEE features are stronger. Such dependence on the pump frequency has not yet been understood. Possibly, the pump-plasma interaction in the weak emission range is less efficient. Further development of theory is necessary to address this long-standing problem. During the 4 June 2014 experiment we used also different pump frequency in the range 5500 < f 0 < 5850 khz. This range includes the weak emission range, below harmonic range (f 0 ~4f ce (10 100) khz), resonance range (f 0 4f ce ), and above harmonic range (f 0 ~4f ce +( khz). Since the diagnostic pulses suffered strong anomalous absorption during these sessions, the algorithm used for reconstruction of electron density profile is not valid. Therefore, data obtained should be analyzed by different methods. Preliminarily, it is possible to state that the plasma behavior depends noticeably on relation between f 0 and 4f ce Broad Downshifted Emission Feature in Stimulated Electromagnetic Emission Spectra The broad downshifted emission (BDE) in the SEE spectra was identified for the first time during the 2011 HAARP campaign. In the 2014 campaign the BDE was registered several times in the midnight experiments with the pump wave frequency sweep near fourth and third electron gyroharmonics. Let us summarize and briefly discuss the salient features of the BDE obtained during the two campaigns. 1. BDE occupies the frequency range f f 0 = (40 220) khz for f 0 near fourth and third electron gyroharmonics. The time of the BDE development is close, but greater, than the ones for downshifted maximum (DM). 2. BDE appears in the SEE spectra in the nighttime, when the absorption of the pump wave is small. BDE is not observed for moderate pump powers suggesting that its threshold is close to the maximum power of the HAARP heater. Small BDE intensities under pumping with the twisted mode may be attributed to a smaller power. When the amplitude of the regular SEE features (e.g., DM) varies, the BDE follows the trend. That is, it disappears when these features weaken and reappears when they amplify. This seems to be a result of focusing (and amplification) of the pump wave by natural large-scale irregularities crossing the HF beam. SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1127

11 3. Near the fourth gyroharmonic, the BDE is usually observed near the critical F region frequency, f 0 ~ f OF2, for magnetic zenith HF beam pointings and mostly for f 0 < 4f ce. However, during the upsweep through the gyroharmonic, the BDE appears also in the frequency range >4f ce and even is present (with much weaker amplitudes) after switching to the downsweep. The BDE intensity and spectral width are greater at f 0 < 4f ce. It is seen in a wide pump frequency range 100 khz < f 0 4f ce < +100 khz. At the same time, the BDE frequency is always smaller than 4f ce, even at the high-frequency flank and even for f 0 above 4f ce. The BDE peak approaches (moves away from) increasing (decreasing) f 0 according to (4) Δf BDE = f BDE f 0 = α (f 0 4f ce ) df; the rate α and the frequency mismatch df are essentially different for different experimental sessions. 4. For f 0 ~3f ce the BDE is generated in a narrow pump frequency range 3f ce (15 25) khz < f 0 < 3f ce (0 5) khz both for pumping at vertical and magnetic zenith. As for 4f ce, during the downsweeps the BDE is weaker (if visible at all) than for upsweeps. 5. BDE characteristics were similar in all three receiving sites situated at 11, 83, and 113 km from the HAARP facility to the south along the magnetic meridian. 6. For the pump frequency close to f OF2, the BDE can be either ordinary or linearly polarized; the polarization can be different in the different parts of the SEE spectrum. For f OF2 essentially larger than f 0, the BDE is O polarized similar to the regular SEE features. The former condition occurred in our experiments for f 0 ~4f ce, while the latter occurred for f 0 ~3f ce. At present, we have not yet arrived at a universal physical mechanism capable of explaining the above BDE features. It is feasible that the pump energy is introduced to ionospheric plasma via scattering on FAI to the UH waves, but the mechanism of excitation of electromagnetic or electrostatic plasma waves at frequencies below f 0 by khz remains mysterious. It is possible to involve the energy transfer from UH (with wave vectors almost across the geomagnetic field B) to oblique Langmuir waves and subsequent formation of the Langmuir condensate via parametric processes [e.g., Kaplan and Tsytovich, 1973; Thomhill and ter Haar, 1978]. Alternatively, three-wave interaction between UH, electron Bernstein (propagating across B), and lowfrequency plasma waves (f f ce cos θ, θ is the angle between the low-frequency wave vector k and B) may be involved. However, this discussion is well beyond the scope of this observation-based study. 5. Summary In the experiments performed in June 2014 at the HAARP heating facility, multifrequency Doppler (phase) sounding of the HF-pumped ionosphere was implemented to explore the vertical structure (electron density profile modification and vertical velocities) with high altitude and temporal resolution. The method is composed of an alternation of quasi-continuous pumping of the ionosphere and sounding by short (wideband) pulses. We used a broadband radio receiver and specially developed signal processing algorithms to explore the evolution of the amplitude and phase of the various spectral components of the reflected probing signals, in a wide (~500 khz) band. It was found that after the pump wave turns on, the plasma is pushed out from the reflection level of the pump wave accompanied by the development of the NC p.in2 3s a relative depletion depth achieves ~0.1%. Then, much stronger expulsion occurs near the upper hybrid height of the pump wave concurrently with DM and BC development. The latter processes are accompanied with essential decrease of the NC p as well with stabilization and disappearance of the depletion near the reflection point. The resulted reduction of the plasma density at the upper hybrid height reaches % and disappears in ~15 s after the pump wave turns off. In the 2011 (March April) and 2014 (May June) HAARP campaigns we revealed and explored a novel prominent feature in the SEE spectrum, the broad downshifted emission (BDE). The BDE feature was first detected in 2011 and found to be in the frequency range f BDE f 0 = (50 180) khz. In the 2014 campaign BDE appeared only during nighttime (near 00:00 AST) with the pump frequency sweep in the range MHz (near fourth electron gyroharmonic 4f ce )and khz (near 3f ce ). It was found that near the fourth gyroharmonic the BDE appeared only at the ERP maximum, mainly for the pump frequency below the gyroharmonic. The position of the BDE peak in the SEE spectrum approaches (moves away from) f 0 increasing (decreasing) frequency according to Δf BDE = f BDE f 0 = α(f 0 4f ce ) df. The coefficient α and frequency mismatch df vary for different sweeps, and α is larger for decreasing f 0. Near the third gyroharmonic, the BDE shows up in the SEE spectra only in narrow pump frequency range (a few khz) slightly below 3f ce. SERGEEV ET AL. DOPPLER SOUNDING AND STIMULATED EMISSION 1128

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