Dependence of spectral width of ionospheric F region HF echoeson electric field

A. Kozlovsky,1 S. Shalimov,2,3 A. V. Koustov,4 R. Lukianova,3,5 and T. Turunen1

Received 3 May 2011; revised 3 June 2011; accepted 8 June 2011; published 7 July 2011.

[1] The EISCAT Svalbard radar (ESR) monitors plasma parameters in the ionosphericregion that is frequently located near the polar cap boundary. The SuperDARN radarat Hankasalmi, Finland, detects coherent echoes from this region, and these echoestypically show increased spectral width. We consider data of joint ESR and SuperDARNobservations to show that the spectral width of HF echoes tends to increase with theionospheric electric field. This relationship is explained in terms of nonlinear evolution ofthe E × B gradient drift instability with energy cascade from hundreds of meterwavelengths to meter wavelengths. We assume that decameter waves (seen by theHankasalmi radar) with relatively large amplitude decay through a three‐wave interactionwith shorter wavelengths and estimate that the decay time of the decameter waves/irregularities is determined by the parameters of the shorter‐wavelength structures. Weassociate the decameter wave decay time with the correlation time, and thus the spectralwidth, of HF echoes.

Citation: Kozlovsky, A., S. Shalimov, A. V. Koustov, R. Lukianova, and T. Turunen (2011), Dependence of spectral width ofionospheric F region HF echoes on electric field, J. Geophys. Res., 116, A07302, doi:10.1029/2011JA016804.

1. Introduction

[2] The Super Dual Auroral Radar Network (SuperDARN)high‐frequency (HF, 8–20 MHz) coherent radars have beendesigned, first of all, for monitoring of ionospheric plasmaflows at the F region heights by analyzing velocity ofreceived coherent echoes [Greenwald et al., 1995]. Theradars, however, routinely provide information on the powerand spectral width of echoes. All three parameters areinferred from autocorrelation function (ACF) of a receivedsignal. Research showed that the spectral width (SpW) ofSuperDARN echoes is a very useful parameter. Baker et al.[1995] noted that SpW of echoes is strongly enhanced in thevicinity of the cusp, so that an increased spectral width ofechoes near noon can be considered as a signature of thecusp. Comparison of SuperDARN SpWs with precipitationdata from low‐altitude satellites showed that the boundaryof increased SpW can be often used as a proxy for the open‐closed field line boundary (OCB) [e.g., Chisham et al.,2004, 2005].[3] Although SuperDARN SpW data are used for identi-

fication of magnetospheric boundaries, the physical reasonsfor increased SpW of HF echoes are not well understood.One can relate broadening of the HF spectra with the tur-

bulent character of ionospheric plasma flows, i.e., withoccurrence of random plasma drifts in microstreams within arelatively large (∼50 km) radar scattering volume. Anotherpossible reason for the spectrum broadening is the limitedlifetime of the ionospheric irregularities responsible forscattering of HF radio waves. In fact, many of SuperDARNradar ACFs are well described in terms of the exponentialdecay (Lorentzian distribution) related to this mechanism[Hanuise et al., 1993; Villain et al., 1996; Ponomarenko andWaters, 2006]. Ponomarenko et al. [2007] investigated thedaytime radar echo properties by considering multifrequencyobservations. They found that the spectral width of F regionechoes, expressed in velocity units, decreases with increasingradar frequency. But the ACF correlation time (10–25 ms)was found to be essentially independent of the radarfrequency/sensed irregularity scale size.[4] Clearly, understanding factors determining the spec-

tral width of HF echoes is an important problem and anyadditional information on the issue is highly desirable. In thepresent paper we study experimentally the relationshipbetween the spectral width of SuperDARN F region HFechoes and the ambient electric field (E field), one of thefactors responsible for the irregularity creation. We thendiscuss this relationship in terms of the most likely mech-anism of the irregularity production, the E × B gradient driftplasma instability.

2. Experimental Data

[5] The EISCAT Svalbard radar (ESR) is convenientlylocated for monitoring plasma parameters in the ionosphericregion from which SuperDARN HF echoes can be receivedat Hankasalmi (HAN) via one and half hop propagation

1Sodankylä Geophysical Observatory, Sodankylä, Finland.2Institute of Physics of the Earth, Moscow, Russia.3Space Research Institute, Moscow, Russia.4Department of Physics and Engineering Physics, University of

Saskatchewan, Saskatoon, Saskatchewan, Canada.5Arctic and Antarctic Research Institute, St. Petersburg, Russia.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2011JA016804

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path, at ranges of ∼1700 km. This region has geomagneticlatitudes (MLAT) of ∼75°; that is, it is in the vicinity of thepolar cap boundary where increased spectral width of HFradar echoes is frequently observed [Villain et al., 2002].[6] During 2000–2007, the ESR radar spent about

1700 h working in the Common Program 2 (CP2) with thebeam being periodically alternated between three positions.Under an assumption that the plasma flow is temporally andspatially uniform over the region of ESR scans, vectors oflarge‐scale plasma flow in the F region were calculated andthe electric field vectors were inferred by assuming that theplasma experiences the E × B drift. These measurementsrefer to the heights of 250–350 km with spatial resolution ofthe order of 100 km in the horizontal plane. The scanningcycle was typically 5 min.[7] In this study, the ESR electric field data were com-

bined with simultaneous HAN SuperDARN radar mea-surements of the echo spectral width in gates 35–37 ofbeam 9. These gates correspond to the area of the ESRmeasurements. In total, about 3% of the ESR measurementswere accompanied by HF data obtained simultaneouslyfrom at least two of the three (35–37) HAN gates. Thecommon ESR‐HAN data that we consider were more or lessuniformly distributed in magnetic local time (MLT), exceptfor the near‐noon (10–15 MLT) sector where the amount ofechoes per hour was about half of that in other local times.[8] We would point out two of the most obvious reasons

for possible errors in this ESR‐HAN data set. First, theelectric field was derived under the assumption of a uniformflow which is true on average, but can be violated in somecases. Second, there might be errors in the location of HFscattering volume as the radar path is fairly long so thationospheric propagation conditions might vary.[9] To exclude such errors we checked whether or not the

line‐of‐sight (LOS) Doppler velocity observed by the HF

radar was consistent with the ion velocity estimated from theESR data. The comparison of these two velocities is pre-sented in Figure 1. There are a number of data points wherethe LOS Doppler velocity is not consistent with the ESRestimate, which suggest that the observed location of the HFechoes is not from the same region as the ESR data or theESR data are not correct. However, for 70% of the data theabsolute difference between the two velocities (DV) is lessthan 260 m/s, which corresponds to the strip indicated by thediagonal dashed lines in Figure 1. Only these data were usedin our study.[10] The HAN spectral widths that we consider here were

obtained under an assumption of the Lorentzian powerspectrum of a received signal so that the measured ACFdecay time, tcorr, and spectral width, W, are related through

W ¼ c

2f0� 1

��corr; ð1Þ

where f0 is the radar frequency and c is the speed of light.However, we decided to use the “normalized” spectralwidth,

F ¼ 1

�corr; ð2Þ

measured in Hz instead of m/s. This parameter relates theecho spectral broadening with the irregularity lifetimedirectly, and this aspect of the observations is the target ofour study.[11] We note that the SpW values depend on the algorithm

used for the radar data processing. This issue has beencomprehensively discussed in the paper by Ponomarenkoand Waters [2006]. In this study, we adopted their newalgorithm and reanalyzed all Hankasalmi data involved sothat the old and relatively recent autocorrelation data aretreated in the same way.[12] Figure 2 is a scatterplot of the spectral width (in Hz)

of Hankasalmi echoes versus the electric field magnitude(ESR). In total, 699 data points are available for those ESRmeasurements that had Hankasalmi data in at least two outof three (35–37) gates and |DV| < 260 m/s. In addition,Figure 2 presents occurrence histograms for the electric field(bottom) and spectral width (right). Importantly, the electricfield distribution is nearly uniform between 5 and 30 mV/m.In this range, ∼60% of the data are contained.[13] The scattering of points in Figure 2 is significant.

From the linear fit line to the data (shown in Figure 2by dashed line) we get the regression coefficient 0.24 ±0.18 Hz/(mV/m). This indicates some tendency for SpW toincrease with the electric field magnitude, however completeinformation on the dependency can be provided only bytwo‐dimensional (2‐D) distributions of the data on theSpW‐E plane. We calculated the occurrence distribution as asum of the two‐dimensional Gaussian density distributionscorresponding to individual measurements:

Y E;Fð Þ /Xi

exp � E � Eið Þ22�2E

� F � Fið Þ22�2F

!: ð3Þ

Here Fi and Ei are measured spectral widths and corre-sponding electric field magnitudes. Parameters sF = 2 Hz

Figure 1. A scatterplot of the line‐of‐sight (LOS) Dopplervelocity observed by the HF radar versus the ion velocity(component along the HF radar beam) estimated from theESR data.

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and sE = 2 mV/m correspond to typical error values of themeasurements. The density distribution was normalized bymaximum and presented in Figure 3 as the 2‐D occurrenceplot of the spectral width versus the electric field magnitude.[14] The dependence presented in Figure 3 is rather

complex. The most pronounced are the following two fea-tures. First, one can see that the spectral width tends toincrease with the electric field. This is presented by theelongated distribution along the dashed white line indicatedby number 1. The SpW increases from 15 Hz to 35 Hz forthe electric field range of 10–20 mV/m. In this range of theelectric field the bulk of data are available and the data areuniformly distributed according to the E field magnitude.Second, large spectral width (above 35 Hz) frequentlyoccurs at small electric field magnitudes of the order of10 mV/m. This is presented by the detached spot‐like dis-tribution at 10 mV/m, above the elongated distribution.[15] Obviously, there may be at least two different

mechanisms responsible for these two distributions. The firstmechanism is consistent with dependence on the electricfield, whereas the second one is associated with weakelectric field. In the present paper we pay the main attentionto the first mechanism, whereas the second mechanism isonly briefly discussed.

3. Possible Explanation of Observations

[16] As has been already mentioned in the Introduction,the HF echo spectrum broadening may be related to therandom plasma drifts within the scattering volume or/and tothe limited lifetime of the irregularities responsible forbackscatter. Ponomarenko et al. [2007] have shown thatnear the polar cap boundary the spectral broadening isdetermined by the irregularity lifetime. However, they alsofound that the spectral width of F region echoes, expressedin velocity units, decreases with increasing radar frequency,

and the ACF correlation time (10–25 ms) was found to beessentially independent of the radar frequency/sensedirregularity scale size. This finding disproved any diffusionmechanism as a candidate to determine the irregularitylifetime. Taking this conclusion into account and motivatedby the relationship reported in Figure 3 between the HFradar spectral width and background electric field, weattempted below to quantitatively introduce a potentialmechanism of plasma irregularity production and decay that

Figure 2. The experimental data in frame of the spectral width and the electric field magnitude. Histo-grams show distributions of electric field and spectral width values.

Figure 3. The occurrence distribution of the spectral widthas a function of the electric field magnitude. Dashed linesshow theoretical dependences for two values of the diffusioncoefficient. The model curves were obtained by using para-meters indicated on the plot.

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determines both the irregularity lifetime and takes intoconsideration the background electric field.[17] It is widely accepted that one of the most likely

mechanisms of decameter irregularity production is the gra-dient drift (GD) plasma instability [Keskinen and Ossakow,1983; Kelley, 2009]. Theory of the GD instability asapplied to the F region plasma has been advanced in anumber of papers [Kelley, 2009, and references therein]. Insection 3 we investigate theoretically the relationshipbetween the lifetime of ionospheric irregularities and E field.We follow an accepted concept [see, e.g., Kelley, 2009, andreferences therein] that the decameter‐scale plasma irregu-larities in the high‐latitude F region are developed throughan energy cascade from long‐ to short‐scale structures. Morespecifically, we use a model that describes the dynamicalbehavior of the E × B gradient drift instability as proposedby Zargham and Seyler [1987] for the kind of instabilityoperational in the equatorial F region at intermediate scales(hundred meters). We apply this formulation to decameterirregularity generation in the high‐latitude F region. In gen-eral, such irregularities are magnetic field aligned, needle‐like structures with kk � k?, where k is the irregularity wavenumber. This allows one to employ a two‐dimensionaldescription of the processes in a plane transverse to thegeomagnetic field.[18] For the GD instability and the cascade process in

progress, we first show that as the amplitudes of the electrondensity perturbations become large enough, their evolutionis determined by the nonlinear three‐wave interactionbetween wave modes, and the mode‐coupling coefficientdoes not depend on the scale of irregularities seen by an HFradar. Second, we show that the characteristic inverse timeof the decameter‐scale mode in the three‐wave interactionprocess increases with a background electric field. Third, weestimate the inverse time using a simplified numerical modeland compare that with the observed values, to check whetherthe suggested mechanism corresponds to the observations.[19] We consider weakly ionized plasma in a configura-

tion relevant to the high latitudes: the magnetic field B0 =−B0ez with upward z axis, the ambient electron densitygradient along the y axis, and the background electric field(in the neutral frame) E0 = E0ex, where x, y and z axes forma right‐handed framework. We take into account the factthat the ions and electrons are magnetized in the F regionand their inertia can be neglected for low‐frequency pro-cesses (w � na � Wa, collisional regime; here na is thecollision frequency of specie a (electron or ion) with neu-trals, Wa is the gyrofrequency). The starting equations arethe equations of motion, continuity equations and quasi‐neutrality condition which can be written in the frame ofreference moving with E0 × B0 velocity as [Zargham andSeyler, 1987]

@N

@tþ 1

B0ez �rF � rN ¼ Dar2N ; ð4Þ

r � NrFð Þ ¼ 0; ð5Þ

where Da is the ambipolar diffusion coefficient and weneglected, in the second equation, a minor term of the order

of the ratio of the ion Larmor radius to the scale L of thebackground density gradient. Following Zargham andSeyler [1987], we linearize these equations by letting forthe density N = Ne(y) + n(x, y, t) with Ne = N0 · (1 + y/L), andfor the potential F = Fe(y) + �(x, y, t) with dFe/dy = −E0 ·[1/(1 + y/L)−1], where index e indicates equilibrium valuesand N0 is the constant background density. Next we sub-stitute these in (4) and (5), and under assumptions thatz/L � 1 and the density perturbations are small in com-parison with background density, for spatial scales muchsmaller than L one obtains after some algebra and Fouriertransformation the wave mode coupling equations [Kintnerand Seyler, 1985; Zargham and Seyler, 1987]

@nk@t

þX

k¼pþq

M p; qð Þnpnq ¼ E0

B0L

� �k2xk2

nk � k2Dank; ð6Þ

where

M p; qð Þ ¼ i

2

E0

B0N0

pxp2

� qxq2

� �p� qð Þ � ez ð7Þ

is the mode‐coupling coefficient characterizing the nonlin-ear interaction between modes p, q, and k such that theirwave numbers satisfy p + q = k. It follows from (7) thatM(p, q) vanishes if p and q are parallel or if p and q aresymmetric with respect to the kx axis. According to Zarghamand Seyler [1987], the wave mode coupling described by (6)and (7) is effective for distributing energy from low ky tohigh.[20] We suggest the following scenario for the evolution

of the GD modes. The primary large‐ or intermediate‐scalewaves decay through the cascade into the smaller‐scalesecondary waves up to the decameter‐scale size. Thedecameter‐scale wave of wave number k (such wave can beseen by an HF radar), in turn, decays into daughter waves,of wave numbers p, q (such that p ∼ q > k) with shorterscales (these cannot be detected by the radar because theirscales are not half of the radar wavelength). Next we assumecomparable amplitudes of the unstable k wave and thedaughter waves. This assumption follows directly fromnumerical simulations [Zargham and Seyler, 1987], whichshows that nonlinear mode conversion is highly anisotropicand occurs predominantly along the direction of the densitygradient through the propagating shock–like structureswhose amplitude is comparable with that of the primarywave. Finally, we estimate the inverse nonlinear oscillationtime of the three‐wave interaction processes, tnl

−1, by bal-ancing the first two terms in (6) for the nonlinear regime.This results in tnl

−1 ≈ E0p∣np∣/2B0N0. We note that tnl doesnot depend on the wave number ∣k∣. We assume that thisnonlinear time can be associated with the irregularity life-time, and thus it gives an estimate of the spectral width ofHF echoes.[21] In order to proceed with the spectral width estimate,

the ratio ∣np∣/N0 needs to be evaluated. Several theoreticaland computational studies have shown that the nonlineardynamics governed by the mode coupling (equations (6) and(7)) produces spectral anisotropy in the plane transverse toB0 [Perkins et al., 1973; Zabusky et al., 1973; Zargham and

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Seyler, 1987, 1989]. To take into account this effect, we usethe two‐dimensional power spectrum in the form

S p; �ð Þ ¼ S0 cos2 � 1þ p=p0ð Þ2� ��3=2

; ð8Þ

suitable for the E × B gradient drift instability in ionosphericF region [Keskinen and Ossakow, 1981]. In (8) the termcos � reflects the spectral anisotropy, p0 is the wave numbercorresponding to the turbulence outer scale. Similar toKeskinen and Ossakow [1981], we take � = 0; this corre-sponds to the linearly most unstable direction. The scalingfor p0 (in the collisional regime) in the inertial limit isp0/pmax ≈ E0/B0LDapmax

2 , where pmax is the wave numberthat, in our case, should be comparable with the inverseLarmor radius and L is the scale of the plasma gradient[Keskinen and Ossakow, 1981]. The expression for theinverse correlation time or spectral width (in Hz) can be thenpresented in the form

F � 1

�c� 1

�nl� E0p

2B0

np�� ��N0

� E0p

2B0S1=20 1þ p

p0

� �2" #�3=4

; ð9Þ

where p0 ≈ E0/B0LDapmax. This formula suggests that thespectral width may increase with E field.[22] Next we compare the model predictions given by (9)

with observations. We note that formula (9) was obtainedwith many simplifications. The inverse correlation time inthe model depends on a number of parameters which mayvary significantly. Thus, this formula may be used only forrough numerical estimations to check whether the depen-dence corresponds to the observations. For comparison withexperiment, we use below some realistic values.[23] The scale of the background plasma density gradient

may extend from the radar wavelength (15–30 m) up tohundreds kilometers. We select a median value of L−1 ≈1 km−1 which seems to be reasonable for the generation ofdecameter‐scale waves. For the relative amplitude of tur-bulence at these scales we take S0

1/2 ≈ 0.2 in agreement withmeasurements by Kelley et al. [1980].[24] In the Earth’s ionosphere with the magnetic field of

B0 ≈ 5 · 10−5 T, the ion gyroradius (for dominating ions O+)is typically between 2 and 6 m for the ion temperatures of300–3000 K. In our calculations we take the ion gyroradiusto be 5 m, which gives pmax = 2p/5 m−1.[25] The HAN radar operates at wavelengths of 15 to 30 m;

that is, it is sensitive to 7.5–15 m irregularities. According tothe proposed scenario, these irregularities decay into irregu-larities of shorter wavelengths, however our considerationsare not applicable for scales shorter than the ion gyroradius(which is ∼5 m). Therefore, we consider the daughter irreg-ularity wavelength lp to be of the order of 7 m.[26] The expected value of the ambipolar diffusion coef-

ficient varies in the nighttime F region from about 0.1 (orless) to 1 m2/s [Hysell and Kelley, 1997; Hysell et al., 1996].To take into account various conditions (different heights ofechoes and/or temperatures) of the HAN radar measure-ments we select two representative values of the diffusioncoefficient Da of 0.1 and 0.5 m2/s.[27] Dashed curves in Figure 3 show the dependence of

the spectral width (9) upon the background electric fieldcalculated for the selected values of the parameters involved

(L = 1 km, S01/2 = 0.2, lp = 7 m) and the two values of

the diffusion coefficient (0.1 and 0.5 m2/s for the curves 1and 2, respectively). Comparing the curves with the experi-mental distribution one can see that the model and experi-mental data are in reasonable agreement. Thus, for somerealistic parameters the simplified model can reproduce theobserved SpW values. This indicates that the spectral widthincrease with E field can be determined by the nonlinearthree wave interaction of GD waves.

4. Discussion

[28] Our experimental finding that the spectral width ofHF radar echoes tends to be larger at larger backgroundelectric field is consistent with the earlier studies byFukumoto et al. [1999] and Nishitani et al. [2004], whonoticed that the HF spectral width increases with the line‐of‐sight velocity measured by the same radar. In those studies,however, the total magnitude of the electric field was notmeasured and no interpretation of the tendency was offered.In this study, we combined HF radar data on spectral widthwith E field measurements by the ISR within the gates of theHF radar and inferred a somewhat similar tendency.[29] In an attempt to understand the reason for the

dependence, we proceeded from the idea that the iono-spheric irregularities are produced through the GD insta-bility development with energy cascading to smaller‐scalewaves. Our data were collected in the high‐latitude regionnear Svalbard (at around 75° MLAT), which is located inthe vicinity of the polar cap boundary. At these latitudes,due to different characteristics of precipitating particles onclosed and open magnetic field lines, the existence of large‐scale density gradients of the ionospheric plasma is verylikely. This is favorable for the GD instability onset. Throughthe cascade, irregularities of various scales (wavelengths) canbe generated [e.g., Kelley, 2009]. We assumed that theirregularities seen by the HAN radar were produced while thewaves were involved in three‐wave interactions. The decaytime of the decameter irregularities has been associated withthe correlation time of received HF signals. The theoreticaldecay time was found to be independent of the wavelength ofthe decaying decameter structure. This implies that the cor-relation time of the received HF signal should not depend onthe radar frequency. This expectation agrees well with theexperimental results by Ponomarenko et al. [2007].[30] Experimental data and the model curves in Figure 3

indicate reasonable agreement for the electric field magni-tudes larger than 10 mV/m. At smaller electric fields, thereis a population of points with large spectral widths of up to40 Hz. Our model cannot explain these points. We feel thatthe spectral width widening in these cases might be causedby random flows within the radar cell rather than by thelimitation of the irregularity lifetime. To support this opin-ion, we note that the observed low ambient electric fields forthese cases could have resulted from occurrence of strongshears and field‐aligned currents in the region of measure-ments. Such conditions are favorable for the onset of theKelvin‐Helmholtz instability associated with flow shearsand the ion cyclotron instability associated with intensefield‐aligned currents. Thus, flow vortices of various scales,could have been formed and they could have led to unusuallystrong spectral broadening of HF radar echoes. In addition,

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structured precipitation may be developed within the regionsof field‐aligned currents and these also can increase thewidth of echoes [Ponomarenko et al., 2007]. Finally, in therange of small electric field the interpretation of the spectralwidth in terms of anomalous (Bohm) diffusion might be inuse.

5. Conclusions

[31] By combining the SuperDARN HF radar data on thespectral width of coherent echoes with the ESR measure-ments of the ambient electric field, we found that thespectral width of echoes tends to increase with the electricfield. To explain these observations, it was assumed that theirregularities observed were excited through the E × Bgradient drift plasma instability with energy cascading toshorter wavelengths. We assumed that once the waves in thedecameter range (detectable by the HAN radar) reach largeamplitudes, their evolution is fully controlled by the non-linear three‐wave interaction with waves of a smaller size.We found that the decay time of these irregularities dependson the intensity of smaller‐scale irregularities but not theones seen by the radar. The quantitative estimates performedshow that the inverse decay time of decameter irregularitiesincreases with the electric field magnitude and the trend wasfound to be similar to the reported experimental data.

[32] Acknowledgments. We are indebted to the director and staff ofEISCAT for operating the facility and supplying the data. EISCAT is aninternational association supported by research organizations in China(CRIRP), Finland (SA), France (CNRS, until end of 2006), Germany(DFG), Japan (NIPR and STEL), Norway (NFR), Sweden (VR), and theUnited Kingdom (STFC).We thank all participants of the SuperDARN proj-ect who collected data used in this study. We thank P. V. Ponomarenko foruseful discussions. The study was supported by the Academy of Finland pro-jects 115920 (A.K. and T.T.), 134283 (S.S.), 132441 and 138289 (R.L.), andan NSERC grant to A.V.K.[33] Robert Lysak thanks the reviewers for their assistance in evaluat-

ing this paper.

ReferencesBaker, K. B., J. R. Dudeney, R. A. Greenwald, M. Pinnock, P. T. Newell,A. S. Rodger, N. Mattin, and C.‐I. Meng (1995), HF radar signaturesof the cusp and low‐latitude boundary layer, J. Geophys. Res., 100,7671–7695, doi:10.1029/94JA01481.

Chisham, G., M. P. Freeman, and T. Sotirelis (2004), A statistical compar-ison of SuperDARN spectral width boundaries and DMSP particle pre-cipitation boundaries in the nightside ionosphere, Geophys. Res. Lett.,31, L02804, doi:10.1029/2003GL019074.

Chisham, G., M. P. Freeman, T. Sotirelis, R. A. Greenwald, M. Lester, andJ.‐P. Villain (2005), A statistical comparison of SuperDARN spectralwidth boundaries and DMSP particle precipitation boundaries in themorning sector ionosphere, Ann. Geophys., 23, 733–743, doi:10.5194/angeo-23-733-2005.

Fukumoto, M., N. Nishitani, T. Ogawa, N. Sato, H. Yamagishi, andA. S. Yukimatu (1999), Statistical analysis of echo power, Dopplervelocity and spectral width obtained with the Syowa South HF radar,Adv. Polar Upper Atmos. Res., 13, 37–47.

Greenwald, R. A., et al. (1995), DARN/SuperDARN: A global view of thedynamics of high‐latitude convection, Space Sci. Rev., 71, 761–796,doi:10.1007/BF00751350.

Hanuise, C., J. P. Villain, D. Gresillon, B. Cabrit, R. A. Greenwald, andK. B. Baker (1993), Interpretation of HF radar ionospheric Dopplerspectra by collective wave scattering theory, Ann. Geophys., 11, 29–39.

Hysell, D. L., and M. C. Kelley (1997), Decaying equatorial F regionplasma depletions, J. Geophys. Res., 102, 20,007–20,017, doi:10.1029/97JA01725.

Hysell, D. L., M. C. Kelley, Y. M. Yampolski, V. S. Beley, A. V. Koloskov,P. V. Ponomarenko, and O. F. Tyrnov (1996), HF radar observations ofdecaying artificial field‐aligned irregularities, J. Geophys. Res., 101,26,981–26,993, doi:10.1029/96JA02647.

Kelley, M. C. (2009), The Earth’s Ionosphere: Plasma Physics and Elec-trodynamics, 2nd ed., 580 pp., Elsevier, Burlington, Mass.

Kelley, M. C., K. D. Baker, J. C. Ulwick, R. L. Rino, and M. J. Baron(1980), Simultaneous rocket probe, scintillation, and incoherent scatterradar observations of irregularities in the auroral zone ionosphere, RadioSci., 15, 491–505, doi:10.1029/RS015i003p00491.

Keskinen, M. J., and S. L. Ossakow (1981), On the spatial power spectrumof the E × B gradient drift instability in ionospheric plasma clouds,J. Geophys. Res., 86, 6947–6950, doi:10.1029/JA086iA08p06947.

Keskinen, M. J., and S. L. Ossakow (1983), Theories of high‐latitude iono-spheric irregularities: A review, Radio Sci., 18, 1077–1091, doi:10.1029/RS018i006p01077.

Kintner, P. M., and C. E. Seyler (1985), The status of observations andtheory of high latitude ionospheric and magnetospheric plasma turbu-lence, Space Sci. Rev., 41, 91–129.

Nishitani, N., M. Lester, S. E. Milan, T. Ogawa, N. Sato, H. Yamagishi,A. S. Yukimatu, and F. J. Rich (2004), Unusual ionospheric echoes withhigh velocity and very low spectral width observed by the SuperDARNradars in the polar cap during high geomagnetic activity, J. Geophys.Res., 109, A02311, doi:10.1029/2003JA010048.

Perkins, F. W., N. J. Zabusky, and J. H. Doles III (1973), Deformation andstriation of plasma clouds in the ionosphere, 1, J. Geophys. Res., 78,697–709, doi:10.1029/JA078i004p00697.

Ponomarenko, P. V., and C. L. Waters (2006), Spectral width ofSuperDARN echoes: Measurements, use and physical interpretation,Ann. Geophys., 24, 115–128, doi:10.5194/angeo-24-115-2006.

Ponomarenko, P. V., C. L. Waters, and F. W. Menk (2007), Factors deter-mining spectral width of HF echoes from high latitudes, Ann. Geophys.,25, 675–687, doi:10.5194/angeo-25-675-2007.

Villain, J.‐P., R. Andre, C. Hanuise, and D. Gresillon (1996), Observationof the high‐latitude ionosphere by HF radars: Interpretation in terms ofcollective wave scattering and characterization of the turbulence, J. Atmos.Sol. Terr. Phys., 58, 943–958, doi:10.1016/0021-9169(95)00125-5.

Villain, J.‐P., R. Andre, M. Pinnock, R. A. Greenwald, and C. Hanuise(2002), A statistical study of the Doppler spectral width of high‐latitudeionospheric F region echoes recorded with SuperDARN coherent HFradars, Ann. Geophys., 20, 1769–1781, doi:10.5194/angeo-20-1769-2002.

Zabusky, N. J., J. H. Doles III, and F. W. Perkins (1973), Deformation andstriation of plasma clouds in the ionosphere: 2. Numerical simulationof a nonlinear two‐dimensional model, J. Geophys. Res., 78, 711–724,doi:10.1029/JA078i004p00711.

Zargham, S., and C. E. Seyler (1987), Collisional interchange instability:1. Numerical simulations of intermediate‐scale irregularities, J. Geophys.Res., 92, 10,073–10,088, doi:10.1029/JA092iA09p10073.

Zargham, S., and C. E. Seyler (1989), Collisional and inertial dynamics ofthe ionospheric interchange instability, J. Geophys. Res., 94, 9009–9027,doi:10.1029/JA094iA07p09009.

A. V. Koustov, Department of Physics and Engineering Physics,University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada.A. Kozlovsky and T. Turunen, Sodankylä Geophysical Observatory,

Sodankylä FI‐99600, Finland. (alexander.kozlovsky@oulu.fi)R. Lukianova, Arctic and Antarctic Research Institute, St. Petersburg

199397, Russia.S. Shalimov, Institute of Physics of the Earth, Moscow 123995, Russia.

KOZLOVSKY ET AL.: BRIEF REPORT A07302A07302

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