HF ground scatter from the polar cap: Ionospheric propagation and ground surface effects

transcript

HF ground scatter from the polar cap: Ionosphericpropagation and ground surface effects

P. V. Ponomarenko,1 J.‐P. St. Maurice,1 G. C. Hussey,1 and A. V. Koustov1

Received 16 June 2010; accepted 22 July 2010; published 12 October 2010.

[1] In addition to being scattered by the ionospheric field‐aligned irregularities,HF radar signals can be reflected by the ionosphere toward the Earth and then scatteredback to the radar by the rugged ground surface. These ground scatter (GS) echoes areresponsible for a substantial part of the returns observed by HF radars making up theSuper Dual Auroral Radar Network (SuperDARN). While a GS component isconventionally used in studying ionosphere dynamics (e.g., traveling ionosphericdisturbances, ULF waves), its potential in monitoring the state of the scattering surfaceremains largely unexploited. To fill this gap, we investigated diurnal and seasonal variationof the ground echo occurrence and location from a poleward‐looking SuperDARNradar at Rankin Inlet, Canada. Using colocated ionosonde information, we have shown thatseasonal and diurnal changes in the high‐latitude ionosphere periodically modulate theoverall echo occurrence rate and spatial coverage. In addition, characteristics of GSfrom a particular geographic location are strongly affected by the state of theunderlying ground surface. We have shown that (1) ice sheets rarely produce detectablebackscatter, (2) mountain ranges are the major source of GS as they can produce echoes atall seasons of the year, and (3) sea surface becomes a significant source of GS once theArctic sea ice has melted away. Finally, we discuss how the obtained results canexpand SuperDARN abilities in monitoring both the ionosphere and ground surface.

Citation: Ponomarenko, P. V., J.‐P. St. Maurice, G. C. Hussey, and A. V. Koustov (2010), HF ground scatter from the polarcap: Ionospheric propagation and ground surface effects, J. Geophys. Res., 115, A10310, doi:10.1029/2010JA015828.

1. Introduction

[2] The Super Dual Auroral Radar Network (Super-DARN) consists of pairs of HF radars with overlappingfields of view (FOV) whose primary aim is to map hori-zontal plasma convection at high latitudes [Greenwald et al.,1985; Chisham et al., 2007]. The plasma convection is re-constructed based on the Doppler shift of the ionosphericscatter (IS) from decameter irregularities that are stronglyaligned with the geomagnetic field [Ruohoniemi and Baker,1998]. The maximum backscattered power is producedwhen the radar wave propagates orthogonally to the geo-magnetic field (aspect condition). The strong ionosphericrefraction of HF signals (frequencies f0 = 10–20MHz) allowsto achieve this orthogonality over much larger areas com-pared with UHF‐VHF waves propagating along straight linetrajectories. In addition, the ionospheric refraction causesbending of some of the rays to the ground, which provides abasis for the over‐the‐horizon ground‐to‐ground HF com-munication. Upon reaching the ground, a part of the HFenergy is scattered back to the radar by the suitably sized andoriented irregularities of the ground surface. These signals

are regularly observed by HF radars and classified as groundscatter (GS) echoes based on their intrinsically low Dopplershift and spectral width values.[3] While SuperDARN GS echoes, due to their insensi-

tivity to horizontal drifts, cannot be incorporated in moni-toring the ionospheric convection, their regular presence overwell‐defined range bands has proven to be useful in studyinga number of other ionospheric phenomena. In accordancewith the HF propagation theory, GS power is expected tomaximize in the vicinity of the skip zone boundary, where“focusing” of the upper and lower propagation rays takesplace [e.g., Hunsucker, 1991, pp. 98–103]. Characteristicvariations in the skip zone location caused by atmosphericgravity waves are routinely used for their monitoring bySuperDARN radars (for an extended reference list see, e.g.,He et al. [2004]). Importantly, the GSDoppler shifts regularlyshow periodicities closely related to vertical plasma dis-placements generated by ULF waves allowing for GS‐basedULF wave monitoring [e.g., Ponomarenko et al., 2003,2005]. In separate developments, Hughes et al. [2002] dem-onstrated SuperDARN's ability to map maximum usablefrequencies, while André et al. [1998] utilized GS elevationdata from the Saskatoon radar to reconstruct critical fre-quencies for the E and F layers.[4] In contrast to the ionospheric diagnostics, application

of GS to monitoring the underlying surface that actuallyproduces these echoes has been rarely discussed in the lit-

1Institute of Space and Atmospheric Studies, University of Saskatchewan,Saskatoon, Saskatchewan, Canada.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, A10310, doi:10.1029/2010JA015828, 2010

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erature. We have identified only one such publication,which is concerned with studying the Greenland glaciertongue dynamics using GS from the Iceland West Super-DARN radar [Shand et al., 1998]. Furthermore, we believethat in the literature the role of surface conditions is gen-erally underestimated while interpreting the experimentallyobserved GS characteristics.[5] In the present work, we study the temporal and spatial

variations of GS echoes over the polar regions in an attemptto establish both the ionospheric and ground conditions thatcontrol GS echoes. To achieve our goals, we use data fromthe Rankin Inlet SuperDARN radar, which convenientlycovers a broad range of landscapes. The experimentaltechnique and data processing algorithms are described insection 2. In section 3 we systematize the spatiotemporalpatterns that we uncovered in echo occurrence. In section 4,based on ionosonde and satellite data, we show how changesin ionospheric and ground conditions with location, seasonand local time can explain the observations. Section 5offers a synthesis of our results and discusses their implica-tions for HF propagation at high latitudes.

2. Experimental Setup and Data ProcessingDetails

[6] The Rankin Inlet radar (RI, 62°49′N, 92°05′W) wasput in operation early in the second half of 2006. Its 16‐beam FOV covers a large part of the Northern polar cap withbeam 7 pointing toward the magnetic pole (Figure 1). In anormal sounding mode, the radar consecutively scansthrough all 16 beams with integration time ’3 s/beam, i.e.,

the whole FOV scan takes ’1 min. The azimuthal beamwidth is ’3.5° at the half‐power level. The line‐of‐sightspatial extent of the range gate is 45 km. The radar operatingfrequency, f0, depends on the propagation conditions and itis periodically switched between “permitted” frequencybands to maximize the amount of the ionospheric scatterechoes. In this study we used the first two full calendar yearsof Rankin Inlet records, 2007 and 2008. Statistical analysisrevealed that during this time, frequencies of’12 MHz wereused during local daytime, while f0 ’9–10 MHz were usu-ally utilized at night. We limited our study to echoes withsignal‐to‐noise ratio (SNR) ≥ 6dB. Separation of GS and ISsignals was performed using the standard SuperDARNalgorithm based on narrow spectral widths and low Dopplervelocities [Blanchard et al., 2009]. The echo occurrence,PGS,IS, was calculated as the ratio between the measurednumber of valid GS (IS) echoes observed at a given beam‐range cell divided by the maximum possible echo number forthe interval of interest (normally 60 measurements per hour).With respect to this, all necessary precautions were made toaccount for discretionary time modes, when the samplingrate varied from beam to beam, so that the equal time in-tervals contributed equally to the calculated occurrence rate.

3. Experimental Results

3.1. Seasonal Variations

[7] We started with an analysis of the average monthlyecho occurrence during 2007–2008. Figure 2 shows thestatistics for beams 0, 7 and 15 corresponding to the west-ern, central and eastern parts of the RI FOV (Figure 1). PGS

Figure 1. Field of view for Rankin Inlet SuperDARN radar (gray area).

PONOMARENKO ET AL.: HF GROUND SCATTER FROM THE POLAR CAP A10310A10310

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Figure

2.Range‐seasonoccurrence

of(top)GSand(bottom)IS

inbeam

7,and15

for2007–2

whitediagonal

shadingin

thetopmarks

thearea

ISexhibitshigher

medianSNR

GS(see

formoredetail).

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and PIS are shown in the top and bottom panels, respec-tively. Notice the difference in the upper limits of the colorscales for GS (≤0.25) and IS (≤0.4). White filling depictsrange‐time intervals for which no valid GS echoes (SNR ≥6 dB) were recorded. Also note that in order to reflectproportionally the group range of the radar returns, as seenin Figure 2, the range scale is shifted back by four rangegates, which corresponds to the group delay between thebeginning of the radar emission and range gate 0 (180 kmin group range).[8] The seasonal statistics reveal several distinct features:[9] 1. IS occurs more frequently than GS.[10] 2. PGS and PIS are out of phase: there is a pronounced

summer maximum for GS, while the ionospheric componentis more frequently observed during the autumn‐winter‐spring period.[11] 3. GS is observed most frequently beyond range gates

#25–30, while IS is generally confined to the closer ranges.[12] 4. While PIS variations are uniform from beam to

beam, GS returns are mostly coming from the central part ofFOV.[13] 5. Both GS and IS exhibit strips of enhanced occur-

rence at fixed ranges. These enhancements are uniformacross the whole FOV for PIS, but for PGS they show asignificant azimuthal variability.

[14] Ground and ionospheric scatter echoes can alsooverlap in group range, especially when several ionosphericlayers coexist. In this case, strong IS returns can distort theactual GS occurrence pattern. To quantify this effect, wecompared median SNR distributions for both components,RISmed and RGS

med. In Figure 2 (top) the white diagonal shadingrepresents the areas where IS dominates GS, i.e., RIS

med ≥RGSmed. According to Figure 2, IS effectively “blankets” GS at

gates ≤ 20–30). In contrast, at farther ranges GS becomesdominant, and the measured PGS values should accuratelyreflect the actual seasonal dynamics of the GS occurrence.[15] Finally, there is a distinct population of GS‐like

echoes at very close ranges (gates 0–5), which exhibit asharp occurrence maximum during the summer months.These returns cannot be actual GS because their group rangeis too small for them to be reflected from E region to theground and back (more on this below).

3.2. Diurnal Variations

[16] To study diurnal dynamicswe analyzed data from beam7, which is approximately aligned with both magnetic andgeographic meridians (Figure 1). Figure 3 shows the UTdynamics of GS echo occurrence during different seasonsrepresented by March, June, September and December 2007.As with Figure 2, the white diagonal shading corresponds to

Figure 3. Diurnal variation of GS occurrence in beam 7 for March, June, September, and December2007. The vertical dashed line corresponds to 12:00 LT. The white diagonal shading shows area whereIS exhibits higher median SNR than GS (see text for more detail).

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the data dominated by IS. The vertical dashed line indicates anapproximate time of LT‐MLT noon at the radar site (’18UT).[17] At range gates ≥25–30, which are not affected by the

overlap with IS, the winter, spring and autumn data show adominance of daytime echoes. At first sight, this is not asobvious in the winter data (lower right). However, one shouldnotice the detectable change from no data at night (white fill-ing) to at least some data, in spite of the low occurrence (darkblue), during daytime. The range of GS daytime occurrencemaximum increases fromgate 25 in summer to 40–45 in springand autumn. By contrast with other seasons, the summer echooccurrence remains relatively high for most of the day exceptfor a 2–3 h gap centered on 02 UT. It shows a clear diurnaltrend with the maximum occurrence observed at closer rangesduring daytime and farther ranges at local night. The summerenhancement in PGS at the very close ranges mentioned in theprevious section is mostly observed across the morning‐day-time LT sector (’10–20 UT). All these features are alsopresent in the 2008 statistics (not shown).

3.3. Relation to Geographic Signatures

[18] As we mentioned in the Introduction, the spatialdistribution of GS depends on the scattering and reflectingproperties of the underlying surface. For reference, in

Figure 4 we present a relief map of the Arctic Archipelago,where white dashed lines show approximate RI FOVboundaries. To study the surface effects, the monthly PGS

values across the RI FOV were overplotted on the geo-graphic map (fan plots). The four plots in Figure 5 repre-sent GS statistics for March, June, September andDecember 2008 (the respective plots for 2007 are almostidentical). To emphasize the spatial structure of PGS ratherthan its magnitude, in each frame the color scales arenormalized to the maximum monthly occurrence.[19] The spring, summer and autumn intervals all have

similar spatial distributions characterized by an enhancedPGS in the central part of the FOV. The maximum in PGS isobserved at closer ranges in June and moves to fartherranges in March and September. The GS almost disappearsin December except for a relatively “bright” area in thenorthern part of Baffin Island.[20] In general, the most frequent echoes are coming

from the land, while the other surface types produce sig-nificantly lower numbers of returns (open sea) or evenvirtually no echoes at all (Greenland ice sheet). A moredetailed analysis reveals that the GS enhancements arecolocated with Ellesmere, Devon and Baffin Islands(Figure 4) and the coastal areas of Greenland, while the

Figure 4. Relief map of Arctic Archipelago (adaptation of Relief map, Atlas of Canada, 1986, availablefrom http://atlas.nrcan.gc.ca/site/english/maps/archives/5thedition/environment/land/mcr4097). © Depart-ment of Natural Resources Canada. All rights reserved.

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landmasses in the western part of the Arctic Archipelagoproduce a much lower number of returns. Another salientfeature in Figure 5 is the increase in the sea scatteroccurrence from Baffin Bay and the southern part of ArcticArchipelago in September.

4. Discussion and Interpretation

[21] In determining GS characteristics, we need to ana-lyze two physical systems, (1) the ionosphere as a conduitfor HF radiation and (2) the ground surface as a targetproducing radar returns.

4.1. Ionospheric Propagation Effects

[22] The ionospheric effects were analyzed using dataprovided by the Canadian Advanced Digital Ionosondes(CADI) [see e.g., Grant et al., 1995] from RI and ResoluteBay (RB) (geographic coordinates 74°43′N, 94°40′W)(Figure 1). The RB instrument is conveniently located nearthe center of the FOV at ’1400 km from the radar so thatits data are more relevant for analyzing the F modes. TheE modes are affected by the ionosphere at the closer ranges≤500 km and are better represented by the RI ionosonde.Unfortunately, neither instrument covered the whole 2007–

Figure 5. Seasonal variations in GS occurrence (color map) and sea ice cover (gray shading) for March,June, September, and December 2008. The data were averaged for each month over all local times. Noticedifferent color scale limits (see text for comments).

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2008 interval so that we had to resort to analyzes of the RIdata for 2007 and the RB data for 2008.[23] The ionosonde data were processed in the following

way: for each month, all echoes with SNR ≥ 20 dB werebinned by UT hour and frequency, so that median virtualheights, hv, and occurrence rates were calculated for eachbin. Figures 6 and 7 show results for the RB (2008) and RI(2007) ionosondes, respectively. The four plots in Figures 6and 7 correspond to the same months as those in Figure 3except the winter frame for RI (Figure 7 (bottom right)),which is represented by January because there were too fewdays of the ionosonde data available for December 2007.We analyzed separately the data from below and above the150 km virtual height, which we attributed to E and F regionreflections, respectively.[24] In Figures 6 and 7, a monthly average ionogram for

each UT hour is represented by a vertical set of diamonds,whose colors correspond to the ionosonde frequencies asspecified by the color scale at the bottom. The symbol sizesare proportional to the echo occurrence rates with graydiamonds at the bottom left corners corresponding to a100% occurrence rate (unit probability, p = 1.0). Finally, thevertical coordinate describes the median virtual height. Note

that plotting results for all frequencies on the same graphproved to be ineffective because it made the plots very“busy” and incomprehensible. To avoid this, we show onlya representative set of evenly spaced frequencies steppingby 0.5 MHz across a 2–8 MHz range.[25] First, we analyze F layer variations using the RB

ionosonde data. During summer (Figure 6 (top left)), theF region data are observed for the whole day and they show apronounced diurnal variation in the reflection height char-acterized by the minimum at’12 LT (hv ’200–250 km) andmaximum near the LT midnight (hv’300–350 km), which isconsistent with the variation in the solar zenith angle, c. InDecember (Figure 6 (bottom right)) a sufficient number ofF region echoes is detected during local daytime‐evening(∼08–20 LT, 14–04 UT) and from higher altitudes (hv’300–350 km). By contrast to the regular behavior of thesummer F layer, the winter heights and critical frequenciesvary considerably. This is consistent with the fact thatwinter ionization is mainly produced by a combination ofthe soft particle precipitations and/or patches of enhancedionization generated at the lower latitudes on the daysideand propagating antisunward across the polarcap withphotoionization playing a secondary role [Buchau et al.,

Figure 6. Virtual height versus UT maps of monthly echo occurrence at fixed ionosonde frequencies forResolute Bay during March, June, September, and December 2008. The color of the symbols shows theecho frequency (stepping by 0.5 MHz between 2 and 8 MHz). The symbol size is proportional to theoccurrence, and the gray diamond at the left bottom corner of each frame corresponds to a 100% occur-rence rate.

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1983; McEwen et al., 1994]. The spring and autumn data(Figure 6 (left)) reflect the transition between winter andsummer conditions. While the F layer maximum heightvaries significantly with the season, the critical frequenciesstay around fcF ’4.5–5 MHz through the whole year. Quiteunexpectedly, the daytime occurrence in September issignificantly higher than in March. This spring‐autumnasymmetry appears to be the result of relatively highermagnetic activity in March (’28% of time with kp ≥ 3),which led to enhanced ionospheric absorption as comparedto September (’9% of time with kp ≥ 3). The aboveasymmetry was duly accompanied by the noticeably higherGS occurrence in September (Figure 5).[26] The E region echoes are much more frequent at RI

than at RB. Here, the returns from the high‐density ( fcE ≥8 MHz) E region often dominate at night (00–10 UT) as aresult of the intense particle precipitation in the auroral zone.The simultaneous presence of both auroral E and F regionreturns is regularly observed at night which argues for apatchy (semi‐transparent) nighttime E layer. During day-time, the E layer changes to the conventional Chapman type,which is governed by c and characterized by much lowercritical frequencies fcE ≤ 4–4.5 MHz. This layer is easilyobserved during summer but it disappears in winter asexpected from the variations in the solar zenith angle.[27] The general tendency for F mode GS echoes to shift

to closer ranges in summer and to move farther away in

spring and autumn agrees with the seasonal variations in theionospheric propagation conditions. The lower‐altitudesummer F layer refracts radar signals back to the Earthsurface at closer ranges. The low occurrence rate of GSbeyond range gate 25 in winter is conversely due to thehigher reflection heights with a skip zone boundary for theF region shifted to the far ranges that cover the Northernpolar ice cap with sea ice representing a relatively weaktarget [e.g., Shand et al., 1998]. This effect culminates inthe nighttime winter conditions, when the F region densi-ties might be too low even to bend the radar ray path tothe ground anywhere within the FOV.[28] To check if the above interpretation agrees quantita-

tively with the observation results, we modeled HF propa-gation at 12 LT for summer and winter conditions using thenumerical ray‐tracing routine described by Ponomarenkoet al. [2009]. To estimate the input ionospheric parameters,we adjusted a synthetically generated ionogram, based on athree‐layer Chapman model, to the average Rankin Inletionograms from 18 to 19 UT (’12–13 LT) for June andDecember 2007. The resulting midday electron densityprofiles are shown in Figure 8 by black and red curves,respectively. The summer profile is characterized by threedistinct layers, E, F1 and F2, while the winter ionosphere isonly represented by the F2 layer. We ran the ray‐tracingroutine for both profiles using a typical daytime RI Super-DARN frequency f0 = 12 MHz and obtained the number of

Figure 7. Same as in Figure 6, but for Rankin Inlet ionosonde during 2007.

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rays reaching the ground as a function of range gate number(Figure 9). In agreement with the experimental data, the raytracing confirmed that the winter ionosphere illuminates theground surface at relatively large distances beyond therange gate 50 (i.e., F layer skip zone boundary), while inJune the radar signals can reach the ground as close as rangegates 16–17 (E layer skip zone boundary). By contrast toDecember, the June dependence exhibits several ray densityenhancements reflecting the multilayer nature of the day-time summer ionosphere.[29] Finally, in the above context, the winter occurrence

maximum coming from the north of Baffin Island (Figure 5(bottom right)) is located much closer than the predictedvalue for the daytime F layer skip zone boundary. However,a comparison of Figure 3 (right bottom) and Figure 7 (rightbottom) suggests that these echoes are provided by thereflection from the high‐density nighttime E layer generatedby the intensive particle precipitations in the auroral zonenear the radar location.[30] While the experimental data are in a general agree-

ment with the seasonal‐diurnal variations in the propagationconditions, there is an apparent problem related to theexistence of the occurrence enhancements/depletions fixedin range (“preferred” ranges). These observations do not fitthe ionospheric variability paradigm and require furtheranalysis. One of the factors that affects the echo occurrenceat fixed ranges is the transmitter pulse‐overlap interference

(POI) caused by the receiver being blocked during theemission of the transmitted pulse. This leads to a decrease inthe number of valid echoes from the affected ranges. Adetailed analysis (see Appendix A) reveals that for the pulsesequence used in our measurements, POI should causeperiodic depletions in the number of valid lags at range gates5i‐4 (i = 1, 2, 3 …), i.e., gates 1, 6, 11, 16, etc., are expectedto produce a smaller number of valid ACFs. In Figures 2and 3 these gates are indicated by the dashed horizontallines. In Figure 2, while the IS echo occurrence (Figure 2(bottom)) is clearly affected by POI, the GS echoes(Figure 2 (top)) show no such effect. Instead, PGS exhibitssharp peaks at range gates 26 and 31, where POI‐relateddepletions would be expected. To illustrate this effect moreclearly, in Figure 10 we plotted range variations of IS andGS occurrence and SNR in beam 7 for June 2008. Theapparent insensitivity of GS to POI might be attributed to itsmuch larger decorrelation time constant, which exceeds themaximum ACF lag ’40 ms and thus provides a largerquantity of the valid ACF lags for further processing.

4.2. Ground Surface Effects

[31] The observed lack of correspondence between PGS

range variations and POI implies that there should beanother factor modulating GS occurrence at fixed ranges.The obvious candidate is the spatial variability in the groundscattering/reflection properties. While the HF radiation froma ground‐based transmitter illuminates the surface outsidethe skip zone, the maximum GS occurrence/power is con-ventionally expected to come from the skip zone boundary,where the focusing of the upper and lower rays takes place.However, this assumption is accurate only if the backscat-tering properties of the underlying surface are uniform alongthe beam direction. This is definitely not the case for theRankin Inlet FOV. A detailed matching of the occurrencepattern with the geographic map of the Arctic Archipelago(Figure 4) reveals that the most frequent echoes come fromthe mountainous areas of Baffin, Devon and EllesmereIslands. This implies that the echoes result from a directreflection by suitably oriented mountain slopes, therebyproviding higher SNR values (notice the coinciding peaksin GS occurrence and SNR at gates 26 and 31 in Figure 10),in contrast to the relatively “flat” landmasses in the westernpart of the FOV. The nature of the enhancements in PGS

Figure 8. Model electron density profiles at 18 UT forJune (black) and December (red) used in ray‐tracing. Seetext for more detail.

Figure 9. Modeled range gate coverage by GS for the elec-tron density profiles presented in Figure 8.

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observed along the Greenland coast has to be similar due tothe presence of the substantial mountains in these regions.[32] Therefore, we conclude that the observed range dis-

tribution in PGS from Rankin Inlet radar represents asuperposition of (1) ionospheric propagation effects causingdiurnal and seasonal range variations in accordance withsolar zenith angle and (2) ability of the Earth surface toscatter/reflect the HF signals back to the radar, which pro-duce occurrence features at fixed (“preferred”) ranges colo-cated with mountainous areas. By contrast to the pulseoverlap effect always depleting the occurrence rate at certainranges, the surface variability can lead to both increase anddecrease in PGS with respect to the “background” values.[33] With respect to the ground surface effects it is also

important to analyze the influence of the sea ice cover,which can effectively suppress generation of waves pro-viding conditions for Bragg scatter of HF radiation from the“rough” sea surface. For this purpose, in Figure 5 the gray

dotted pattern shows sea regions covered by sea ice for100% of the time during the given month (the sea ice dataused in this study are a 24 km resolution records from “IMSdaily Northern Hemisphere snow and ice analysis at 4 kmand 24 km resolution. Boulder, CO: National Snow and IceData Center. Digital media.” provided by NOAA/NESDIS/OSDPD/SSD at http://nsidc.org/data/g02156.html).[34] Figure 5 (bottom left) representing September 2008

shows increased occurrence of echoes from Baffin Bay.Figure 11 compares annual variations in average occurrencefor range gates 30–35 in beams 7 and 15, which correspondto scatter from land (Ellesmere Island) and sea (Baffin Bay),respectively. By choosing the same range gates for bothbeams we made sure that the ionospheric propagation effectsare essentially the same for both locations. The number ofechoes from the landmass (black) maximizes during sum-mer, while the sea scatter occurrence (red) exhibits anadditional enhancement in August–October. This would beexpected due to the fact that by this time Baffin Bay becomesclear from the sea ice and starts to produce Bragg scatterfrom the waved sea surface, i.e., indicating open water.Another interesting feature of the September 2008 map is thepatchy character of PGS at close ranges (gates 10–25), whichapparently resembles the system of straits in the southernpart of the Arctic Archipelago. However, to validate thishypothesis it would be necessary to carry out a very carefulanalysis of the echoes bearing in mind that they are stronglyaffected by the overlap with the IS returns. Finally, the vir-tual absence of GS from inland Greenland is in agreementwith previous studies [Leonard, 1991] and results from thestrong absorption of HF radio waves by the ice sheet.

4.3. Very Close Range Summer Echoes

[35] The sharp summer enhancement in occurrence of thevery close range GS‐like echoes is rather enigmatic andcannot be readily explained within the conventional HFpropagation paradigm. These echoes were marked as GSdue to their relatively low Doppler velocities and spectral

Figure 10. GS (red line) and IS (black line) (top) occur-rence and (bottom) SNR versus range in beam 7 for June2008. The vertical dashed lines correspond to rangesaffected by the pulse‐overlap interference.

Figure 11. Seasonal GS occurrence variations over BaffinBay (red) and the south part of Ellesmere Island (black).Solid and dashed lines show results for 2007 and 2008,respectively. See text for more detail.

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width values, but the respective group ranges are too short toaccommodate any realistic propagation scenario involvingscattering from the ground surface. Also, these echoescannot result from the conventional Bragg scatter by thefield‐aligned E region irregularities because at the closeranges the aspect conditions are far from being satisfiedunder realistic E region densities. There are three previouslyreported types of the nonaspect radar returns: (1) backscatterfrom meteor trails [e.g., Hall et al., 1997; André et al., 1998;Hussey et al., 2000], (2) so‐called HAIR (high‐aspect angleirregularity region) echoes [Milan et al., 2004], and (3) polarmesosphere summer echoes (PMSE) [Ogawa et al., 2004].While the meteor echoes are most frequently observedbetween 00 and 06 LT [Hall et al., 1997], the very closerange echoes from RI show the summer occurrence maxi-mum close to 12 LT (Figure 3 (top right)) therefore arguingagainst their meteor origin. A further clarification of thenature of these echoes lies outside the scope of the presentpaper and will be addressed in a future publication.

5. Summary and Conclusions

[36] The statistical studies of the ground (surface) scatteroccurrence observed by the Rankin Inlet SuperDARN radarover 2007–2008 allowed us to establish the following:[37] 1. The diurnal‐annual variations in PGS range are in

general agreement with regular variations in the ionosphericpropagation conditions. General occurrence maximizes insummer owing to the increase in illumination of the groundby the HF radiation within the FOV due to the higher iono-spheric densities and lower maximum heights. It explains theobserved shift in themaximum occurrence to farther ranges inspring and autumn. This effect culminates under the night-time winter conditions, when the virtual disappearance of theF mode echoes is caused by the inability of the low‐densityionosphere to reflect the HF radiation back to the groundwithin the FOV boundaries. E layer propagation modes areresponsible for some of the observed GS features, e.g.,nighttime occurrence enhancement in winter.[38] 2. The spatial pattern in GS occurrence is strongly

affected by the type of the underlying surface. The non-uniformity of the scattering/reflecting properties is respon-sible for the persistent enhancements in PGS at fixedlocations, coinciding with mountains and coastal areas. At

the same time there are no returns from the Greenland icesheet. There are only minor contributions to the overall GSstatistics from the “flat” land and permanent sea ice. The seascatter occurrence from Baffin Bay is enhanced in autumn,when there is a minimum sea ice cover.[39] 3. We observed a distinct population of very close

range GS‐like returns, which show a sharp daytime occur-rence maximum in summer months. These echoes seem toresult from a high‐aspect angle E region backscatter ormesospheric echoes (PMSE's), but their exact nature requiresa separate study.[40] 4. Finally, we identified two major artifacts affecting

the observed occurrence of SuperDARN GS echoes. Theseare (1) the masking of the ground scatter returns at closeranges by the more powerful ionospheric scatter componentand (2) the effect of transmitter pulse overlap on the qualityof the data at fixed ranges.[41] In conclusion, we have provided a detailed insight

into the nature of the HF ground scatter returns from thepolar cap areas. This study can have important applicationfor both theory and practice of HF propagation at high lati-tudes. SuperDARN data could be used for near‐real‐timeforecasting of HF propagation conditions over the polar cap.These forecasts become particularly important as the numberof cross‐polar passenger flights increases, and HF is fre-quently the only available communication option to airtraffic control operators in this region. The well‐definedcorrespondence between GS occurrence and geographicalfeatures (mountains, coastal lines) provides an independentway to calibrate the ground range of the radar echoes. Thisinformation is also very important for a correct interpretationof the propagation conditions, especially the location of theskip zone boundary. Finally, sea scatter occurrence exhibitshigh sensitivity to the presence of sea ice so that it couldbe utilized for monitoring the ice cover extent under theSuperDARN FOV.

Appendix A: Pulse Overlap Interference

[42] SuperDARN operation is based on the use of asequence of 7 or 8 nonevenly separated pulses of the sameduration, Dt = 300 ms (spatial resolution 45 km) [e.g.,Ponomarenko and Waters, 2006]. A complex autocorrela-tion function (ACF) for each range gate is calculated usingdifferent pairs of these pulses to produce different ACF lags(Figure A1). The duration of the shortest pulse separationinterval (basic ACF lag), t, is an integer multiplier of Dt,and all the other lags are integer multipliers of t. The pulsesequence is emitted every ’100 ms, while the sampling ofthe echoes continues with the rate equal to Dt. During thepulse emission the receiver is blocked so that the respectivereceiver samples are invalid and removed from the analysis.As a result, the ACF lags utilizing the “blanketed” pulses arediscarded from the analysis, thereby decreasing the qualityof the respective ACFs (pulse overlap interference, POI). Todetermine the receiver samples for each range gate, the pulsesequence “mask” should be shifted along the receiver timeseries stepping one range gate at a time. After spanning thefirst Dn = Dt/t gates, one of the required samples coincideswith the transmitter pulse emission time and has to be dis-carded, eliminating the respective ACF lags at this particularrange. After shifting by further Dn range gates this effect is

Figure A1. Time diagram of (top) the transmitted pulsesequence and (bottom) the receiver samples (adapted fromPonomarenko and Waters [2006]).

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repeated for a different pulse in the sequence, etc. As aresult, the periodic depletion in the quality of the ACFoccurs at range gates separated by Dn. In our measure-ments Dt = 300 ms and t = 1500 ms, so that POI happensevery 5 range gates. In addition, SuperDARN data sam-pling normally starts at 180 km, which is equivalent to fourrange gates. Therefore, the POI effect should occur in rangegates iD n − 4 (i = 1, 2, 3…), so that the first occurrence ofPOI should be observed in range gate 1 and then repeatedwith a five‐gate step (gates 1, 6, 11, 16, etc.).

[43] Acknowledgments. This work was supported in part by thefunding from the province of Saskatchewan and Government of Canadafor a Canada Research Chair (J.P.S.M.) and discovery grants to J.P.S.M.and A.V.K. The Saskatoon SuperDARN radar operations are funded by aMRS grant from Natural Sciences and Engineering Research Council ofCanada (NSERC) and a Canadian Space Agency (CSA) contract. We alsoacknowledge CSA and NSERC for providing CADI ionosonde data as wellas National Snow and Ice Data Center, NOAA, for providing the on‐lineaccess to sea ice data. Finally, we thank Chris Meek for the fruitful discus-sions on close‐range echoes and Raj Choudhary for early discussions onground scatter statistics. Resolute Bay CADI ionosonde as part of theCanadian High Arctic Ionospheric Network (CHAIN) infrastructure isfunded by the Canada Foundation for Innovation and the New BrunswickInnovation Foundation. CHAIN operation is conducted in collaborationwith the Canadian Space Agency. RB ionosonde data were obtainedthrough the Canadian Space Science Data Portal (CSSDP) at http://136.159.94.141:8080/ssdp/app/home.

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G. C. Hussey, A. V. Koustov, P. V. Ponomarenko, and J.‐P. St. Maurice,Institute of Space and Atmospheric Studies, University of Saskatchewan, 116Science Place, Saskatoon, SK S7N 5E2, Canada. (pasha.ponomarenko@usask.ca)

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HF ground scatter from the polar cap: Ionospheric propagation and ground surface effects

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