Magnetic aspect angle dependence of spectra ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 94, NO. A4, PAGES 3663-3669, APRIL 1, 1989

Magnetic Aspect Angle Dependence of Spectra from Coherent J.

Radio

WATERMANN

Aurora

AND D.

R.

Backscatter

MCDIARMID

HerzbergInstitute o! Astrophysics, National ResearchCouncil o! Canada, Ottawa

J.

A.

KOEHLER

AND

G.

J.

SOFKO

Institute of Spaceand AtmosphericStudies,Universityof Saskatchewan,$askatoon A.

G.

MCNAMARA

HerzberõInstitute o! Astrophysics, National ResearchCouncil of Canada, Ottawa

In 1981, a multiple path radar system was operated on the Canadian prairies with three 50MHz transmitters and two receiversall separately located such as to establish six backscatter links. The radar beams illuminated the same scatter volume in the ionosphericE layer. Four of these links produced coherent backscatter signals from 3-m wavelength irregularities under

differentmagneticaspectanglesrangingfrom 1.5ø to 5.9ø (reference height110km), but under almost identical drift flow angles. The radars were operated in the CW mode which yielded high time and spectral resolution but only moderate spatial resolution. In order to avoid confusion between spectral peaks from different 1ocations•withinthe backscatter volume, only single peaked spectra have been analyzed and compared. The Doppler shifts of both type I and type II spectra exhibit an aspect angle dependencewhich is much weaker than that predicted by existing linear and nonlinear

theories.

INTRODUCTION

backscatter intensity[McDiarrnid1972;Ecklundet al. 1975;

Under disturbed conditions, the electric fields in the auro- KoeMerel al. 1985],otherswith the mean Dopplervelocral ionospherecan generate unstable E region electrostatic ity [Ogawael al. 1980;Haldoupiset al. 1986, 1987;Nielsen et al. 1986,1987]. waves whose electron density variations can be detected by 1986]and the spectralwidth IHaldoupis The present work addresses the magnetic aspect angle desuitable ground-based radar facilities. Large differencesin pendence of Doppler velocity and spectral width and is inthe excitation and damping mechanismsseemto exist between irregularities of different wavelengths. For a compre- tended to improve the data base which will be needed to discriminate between different theoretical explanations of hensive review of well-established results from observations the underlying processes. It confirms qualitatively previof ionospheric irregularities, seeFejerandKelley[1980].

of Haldoupisel al. [1986,1987]aboutthe Sincethe pioneeringtheoreticalworkof Farley[1963]and ousconclusions Buneman[1963],at least four typesof radar Spectrahave aspect angle dependenceof type ! and II phase velocitiesbut

been characterized, of which the spectra commonly named

differs quantitatively, due to the different analysis methods

are believed to be backscattered from primary waves ini-

criteria different from ours, and they derived their relation-

instability, while type II spectra are thought to be associated with echoes from secondary waves which originate in large-scale gradient drift waves.

at differentgeomagneticlatitudes. We have tried to improve

by Keskinenand Ossakow [1983])are thoseof the excitation

Our studycannotbe compared directlyto Nielsen's[1986] workbecausehe useddatafromSTARE (140-MHz radarfrequency),while we used50-MHz transmitters,and he com-

"typeI" and "typeII" are bestestablished. TypeI echoes employed.Haldoupiset al. [1986,1987]useddata selection

tiated by the modifiedtwo-stream(i.e., Farley-Buneman) ship by comparing observationsmade in different years and

the reliability of our results by analyzing such data with a new technique involving simultaneous interlink comparison Amongthe still outstandingquestio.ns (e.g., summarized of mean Doppler shift and spectral width.

mechanisms of type I and type II waves at larger magnetic aspect angles. We note here that we have defined the magnetic aspect angle as the angle between the radar scattering vector and a plane perpendicular to the geomagneticfield. The aspect angle dependence of some of the parameters of the radar echoes,such as their intensity, mean Doppler shift, and spectral width, are theoretically not yet understood. Several papers investigating aspect angle effects of radio aurora have already been published. Some deal with the Copyright1989 by the American GeophysicalUnion.

pared data from different scatter locations to obtain aspect angleeffectsat low aspectanglesup to 1.5ø, while we used radar links with aspectanglesbetween1.5ø and 6ø and compared backscattersignalsobtained from a commonvolume. The conditions in our study are also different from those of

the workof Ogawaet al. [1980].They used2-hourspectral averagesand only one radar beam and convertedrangevariations into aspect angle variations to derive the aspectangle sensitivity, while we used 10-s integration time and several beams simultaneously.

Thus the methodsemployedin previousstudies(partly

Paper number 88JA03986.

dictated by experimental and technical restrictions which

0148-0227/89/88JA-03986505.00

3663

3664

WATERMANN

ET AL.:

MAGNETIC

ASPECT DEPENDENCE

OF RADIO

AURORA

ferentfromthoseusedby Haldoupis et al. [1986]and Koehler et al. [1985],sinceoursare basedon the GSFC 9/80 geomagnetic field model. Table 1 contains the numerical values of the geometric parameters used in our analysis. The radar network was particularly suitable for the study of magnetic aspect angle dependence, since four of the

six transmitter-receiver links,namely,Minot-Havre (M-H), Minot-Kernen(M-K), SwanRiver-Havre(S-H), and Swan River-Kernen(S-K), had radar k vectorsalmost identical in magnitude and codirectionalwithin 6ø in the horizontal plane. Therefore these links were subject to almost identical

Havre

Fig. 1.

Minot

The geographic locations of the antennas used during

the 1981 field campaign. Transmitters:Minor (symbolM in the followingfigures),Swan River (S), and Camrose(C); receivers: Havre (H) and Kernen farm (K), near Saskatoon.Common E layer backscatter region above Reindeer Lake.

flow angles(anglesbetweenthe directionsof main electron drift and backscatterk vector), sincethe radar link k vectors werevirtually coaligned.Their magneticaspectangles, however, varied between 1.5ø and 6ø, and this allowed us to identify the magnetic aspect angle dependenceof mean Dopplershift and spectralwidth of the radar spectra. METHODS

OF DATA SELECTION AND ANALYSIS

Radar spectra of 10-s integration time with 4.9-Hz frequency resolution were used in this study, and 9 hours of

did not allowa moreaccurateanalysis)are not suf•cientto

simultaneous observations on the six links from three differ-

meet our objectiveof quantitavely deriving the dependence of the 50-MHz coherentbackscatter mean Doppler shift on the magnetic aspect angle. Since the matching betweenobservation and current theories is still in the processof evolution, it is important to establishas reliably as possiblethe magneticaspectdependenceof the backscatterDopplershift

ent days in 1981 were evaluated. The intervals were 0700 UT to 1000 UT on day 218, 0400 UT to 0600 UT and 0830 UT

so as to make predictions of electron drift and electric fields

noise ratio but were otherwise

basedupontheCanadian BARS[McNamara etal.,1983]observations as reliableas thoseof STARE [Greenwald ½tal., 1978].

The spectrawere correctedfor receiverbandpasscharacteristics which were determinedfrom quiet periods before and after the active time intervals,i.e., periodswith very low and unstructuredbackscatteractivity. A linear trend was then subtractedin order to correctfor signal overlap from links with adjacentfrequencybands,and spectrawith

THE

EXPERIMENT

In 1981, a multiple path radar system was operated on the Canadian prairies with three 50-MHz transmitters and

to 0930 UT on day 229 and 0030 UT to 0330 UT on day 235. Local magneticmidnightat the backscatterregioncorresponded to 0745 UT. The intervals were selected because

of their high backscatteractivity and their high signal to random.

a high noise level were removed from the data set. The

remainingspectrawere categorizedin the followingway. A categorization parameter based on the number, freter stationswereMinot (symbolM), SwanRiver (S), and Camrose(C), the receiverstationsHavre (H) andKernen quencyshifts, and relative amplitudesof local extrema and spectrawas calcufarm (K) near Saskatoon.The radar signalswerecoher- pointsof inflection of Papoulis-smoothed ently backscatteredfrom 3-m irregularities in a commonvollated to discriminatebetweensingle,multiple, and composume of the ionosphericE region above Reindeer Lake. Thus ite spectra. Singlespectracontainonly onesignificantpeak, six radar links monitoring the same area in the E region, multiple spectraat least two, and compositespectracontain namely, M-H, M-K, S-H, S-K, C-H, and C-K, were avail- at least two different components,for instance,a broad one able. For geographic details see Figure 1, and for a list of and a superimposed narrowone,withoutshowingmorethan two receivers located at five different

sites.

The transmit-

experimental parameters with a more extended description

one significant peak.

of the networkseeTable 1 of Haldoupiset al. [1986].Fur-

The first four moments of the upper 75% of the power

ther details of the experimental setup and a descriptionof

density of each spectrum were calculated. These were used

theroutinedataprocessing aregivenby Koehleret al. [1985].

instead of the 100% moments in order to avoid bias from

Our calculations of magnetic aspect angles and radar beam directionswith respect to geomagneticnorth are slightly dif-

order of the moments

TABLE

1.

Geometric

Parameters

extended spectrum tails, which becomesmore seriousas the

of the Backscatter

increases.

The zeroth moment

k vectors

Based on the G SFC 9/80 GeomagneticField Model

Radar Link

Wavelength

MagneticAspect

C-H C-K M-H M-K S-H S-K

3.12m 3.15m 3.17m 3.13m 3.25m 3.20m

3.35ø 5.27ø 1.54ø 3.40ø 4.08ø 5.93ø

Angle from Geomagnetic North

148.4ø{31.6øE) 150.4ø{29.6øE) 184.2ø (4.2øW) 186.5ø (6.5øW) 187.8ø (7.8øW) 190.2ø(10.2øW)

is sim-

WATERMANN ET AL.'

MAGNETIC ASPECT DEPENDENCE OF RADIO AURORA

3665

to be strongly dependent on the link chosenwhen links with similar horizontal k vector components are compared. Although linear theories predict that the ion acoustic waves can propagate only near perpendicularity to the geo-

magneticfield, the group of Doppler shifts near 400 m/s for all radio links suggests that other modes related to the ion acoustic instability exist. A systematic decrease of the

mean of this Doppler shift groupfrom M-H (smallestaspect angle)to S-K {largestaspectangle)can be seenin the four bottom panels. This suggests that the mean Doppler shift of these waves decreasesslightly with aspect angle. We will look into this in more

detail

in the next

section.

Figure 3 shows the entire data set of single symmetric spectra plotted as spectral width versus mean Doppler velocity. The data do not fill the plot uniformly but rather show some obvious groupings. We will define the regime of the type I spectra as being delimited by Doppler shifts of

morethan about 320 m/s but lessthan 480 m/s andwith a spectralwidth of lessthan 160m/s. This regimeis indicated by a dashed line in the lower right part of Figure 3. Type II spectra are considered to be caused by secondary waves from a variety of primary sources such as turbulent

interaction(involving mode-coupling andcascading) of twostream waves and of gradient drift waves. Thus one would expect To observe Doppler shifts smaller than those associo

IOO

•oo

•oo

4oo

•oo

Ooppte• vetooLt•l Fig. 2.

Distribution of the mean Doppler shift of narrow single

ated with the primary cause. In particular, type II spectra without a superimposednarrow component should be observedonly when the electrondrift velocity along the radar

spectraover the entire range of Doppler velocities(absolutevalues}. The histogramreflectsthe numberof spectrafound in each

line of sight is smaller than the excitation threshold of the

of the Doppler shift bins. The links are, from top to bottom, Camrose-Havre, Camrose-Kernen, Minor-Havre, Minot-Kernen, Swan River-Havre, Swan River-Kernen. The group of spectra

broadspectrawith meanDopplershiftsof lessthan 260m/s and a spectralwidth greaterthan 190m/s (chosenarbitrar-

with a Doppler shift around 400 m/s is regardedas the "type group.

ply the total power of the spectrum; the first moment is the mean Doppler shift; the square root of the secondmoment is the standard deviation of the spectrum and is a measure of the spectral half width; the third moment is a measure of the

skewness (spectralasymmetry). In the following,all terms are used in this sense. The skewhessparameter is useful in detecting asymmetric spectra which would bias the estimation

of the other

moments

and therefore

were removed

Farley-Bunemaninstability. Accordingly,only thosesingle

ily to be somewhatgreaterthan the upper limit of the type I spectralwidth) were consideredto be of type II. This area is also indicated by dashedlines in Figure 3. We assumethe magnetic aspect angle dependenceto be separable, i.e., the Doppler velocity reads

VD-- e•({P))- •(a)

(1)

where{P} is a setof ionospheric andexperimental parameters other than the aspect angle and c• denotes the aspect angle. If observationsfrom two links were obtained at the

from the data set. In a forthcomingpaper (J. Watermann same time and the scatter regions and wave vectors were et al., manusciptin preparation,1989)a correlationbetween identical(as is almosttrue for the M-H, M-K, S-H, and S-K linkson onehandand the C-H and C-K linkson the other}, skewhessand mean Doppler shift of narrow and very broad

weassume {P1) - {P2} in thefollowing andfind

spectra will be reported. The

entire

data

set contains

observations

under

a wide

range of geophysical and plasma conditions. We wished to compare observationsof the classicaltype I and type II spec-

-

(2)

tra (whichhave receivedsometheoreticalattention)to the predictions of the theories for these phenomena. Therefore we removed from the data set all those spectra which were multiple or composite because they might be the result of multiple, discrete scatterers inside the scattering volume. This left some 5200 spectra for the subsequent analysis. A histogram giving the occurrence frequency of the narrow spectra versus mean Doppler shift for the various links is shown in Figure 2. It shows that the relative occurrence of

spectrawith meanDopplershiftsnear400m/s (the velocity range of waves excited by the classical Farley-Buneman in-

stability under near-thresholdconditions)doesnot appear

Forming this ratio eliminates the variability between data

points thatmight arise from temporal variations in•(•P}). Consequently, all simultaneousobservationsfrom pairs of

links that fall into the same class(i.e., either type I or type II) were extractedfrom the data. For each of these paired spectra the ratio of their Doppler shifts was calculated. From the complete set of interlink ratios, consistent and optimal estimatesof their mean ratios and standard deviations were obtained using a least squarestechniquebased on the solution of a system of coupled normal equations.

The first-orderTaylorexpansionof (2) reads

3666

WATERMANNET AL.' MAGNETIC ASPECT DEPENDENCEOF HADIO AURORA '

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