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Sep 1, 2001 - S. Oyama, 1,2 M. Ishii, 1 Y. Murayama, ... OYAMA ET AL. ..... MAS. KIL. MUO. PEL. OUJ. HAN. NUR. Figure 6. For case 2 the Y components, i.e., ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A9, PAGES 18,543-18,554,SEPTEMBER 1, 2001

Generation of atmospheric gravity waves associated with auroral activity in the polar F region S. Oyama,1,2M. Ishii,1 Y. Murayama, 2 R. Fujii,2 and 1 H. Shinagawa, 2 S.C. Buchert, W. Kofman 3

Abstract. Relationsbetweenauroral activities and the generationof neutral-wind oscillationsin the polar F region(150-300kin) wereinvestigatedusingdata from the EuropeanIncoherentScatter (EISCAT) radar, the all-skyauroral camera,and the IMAGE (InternationalMonitor for Auroral GeomagneticEffects)magnetograms. We dealt with two cases:observationson March 1, 1995 (case1), and on March 29, 1995 (case2). For both casesthe field-alignedcomponentof the neutral-wind velocity estimated from EISCAT radar data had dominant oscillation periods of 20-30 min, which are longer than the typical Brunt-V•is/51/5period in the polar F

region(___13 rain). The observedoscillationsshowedthe downwardpropagationof the phase with time. These properties on the oscillation period and the phase are

generalonesof atmosphericgravity waves(AGWs). For caseI the all-sky auroral images obtained at Kilpisj/irvi showed the auroral arc extending in an almost zonal direction near a distanceestimated using wave parametersderived from the equation of the dispersionrelation for AGWs applicable to the observedoscillations. This suggestedthat the auroral arc appeared to be the effective generator of the observedoscillations. The comparisonof observedphase lines with predicted ones

usingmodelsby Francis[1974]and Kato et al. [1977]showedagreementsbetween the two for both cases. The comparisonsuggeststhat effective parameters of the wave source in characterizingneutral-wind oscillationswould be the horizontal distribution of the wave sourceand the distance between the observingpoint and the sourceregion. It was concludedthat geomagneticactivities on March I and 29, 1995, in northern Scandinaviasignificantlyrelated to the generationof the observed oscillations. The conclusionimplies that geomagneticactivities at high latitudes are an important sourceto generate AGWs, as indicated by previous theoretical studies.

1.

Introduction

Theoretical

Thermospheric motions generated at high latitudes significantly affect the global thermosphericstructure, especiallyunder geomagneticallydisturbed conditions.

Atmosphericgravity waves(AGWs) generatedin the auroral region can propagatehorizontally for long distances,both equatorwardand poleward, and are thoug-

calculations

have indicated

that there are

at least three mechanisms associating auroral activi-

ties in the polar thermospherewith the generationof AGWs: (1) perturbationsin the auroralelectrojetcurrent, (2) thermospheric heatingcausedby highly energetic particlesfrom the magnetosphere, and (3) rapid motions of the aurora. Perturbations in the electrojet

currentcausevariationsin the energydissipation(frictional heating)and in the Lorentzforce,and theoretical effectsof geomagneticdisturbances[Hajkowicz,1991; analyseshaveshownthat thesevariationscan generate Millward et al., 1993;Hockeand $chlegel,1996]. ht to be related to atmosphericoscillations due to the

AGWsthroughthermalexpansion and ion drag [Chi-

monas and Hines, 1970; Francis, 1974; Walterscheid 1Communications ResearchLaboratory,Tokyo, Japan. 2Solar-Terrestrial EnvironmentLaboratory,NagoyaUniversity, Nagoya, Japan.

3Laboratoirede Plandtologie de Grenoble,Grenoble,France.

Copyright 2001 by the American GeophysicalUnion.

andLyons,1992;Sun et al., 1995].The relativecontributionsof the Lorentzforceand frictionalheatinghave beeninvestigated[Chimonasand Hines, 1970; Brekke, 1979;Balthazoret al., 1997].The theoreticalinvestigation by Jing and Hunsucker[1993]suggested that the Lorentz force tends to generatemedium-scaleAGWs and that Jouleheatingis moreeffectivein generating

Paper number 2001JA900032.

large-scaleAGWs, but the relative contributionsare not

0148-0227/01/2001JA900032$09.00

fullyunderstood [Williamset al., 1988]. 18,543

18,544

OYAMA ET AL.: GENERATION

OF AGWS IN THE AURORAL

The precipitating energetic particles that enhance ionizationdepositheat in the thermosphere throughion recombination.In the polar thermospherethe rate of heatingby particle precipitationcan be greaterthan that by frictionalenergydissipation[Banks,1977; Weddeet al., 1977].The effectsof particleprecipitation in the F region(150-300km) on AGW generationhave not yet been modeledtheoretically. Acousticwavesare apparentlyproducedwhen auroral arcsexpandpolewardand subsequently moveequa-

F REGION

also describesthe theoretical model for AGWs that was

presented by Francis[1974].Resultsobserved with the EISCAT radarandthe all-skyauroralcameraareshown in section4. We try to identifythe sourceregionthat generated observed neutral-windoscillations usingallsky imagesand the AGW modelsin section5. 2.

Instrumentation

Simultaneous observations with the EISCAT radar, torward[Wilson,1972]. Swift [1973]showedthat su- an all-skyauroralcamera,and the IMAGE magnepersonicmotion of the auroral electrojetcould radiate auroralinfrasonicwaves.The effectsof movingsources on the generationof AGWs havebeeninvestigated theoretically under the assumptionthat auroral arcs move at a subsonic speed[Kato et al., 1977]. The enhancementof the electricfield perpendicular to the geomagneticfield linesin the vicinity of the auroral arc can increasethe frictionalheatingrate there

tograms were conducted in northern Scandinavia on

March I and 29, 1995. The EISCAT radar measured

ionospheric parameters,suchas electrondensity,electron and ion temperatures, and ion velocity,alongthe local geomagneticfield line with an altitude resolution

of 3 km from90 to 140km andof 22 km above140km; the observation modewascommonprogramone,versionK (CP-1-K).TheEISCATCP-1-Kexperiment pro-

[Opgenoorth et al., 1990,andreferences therein].The vides three-dimensional ion velocities at an altitude of enhancementof the frictional heating rate will affect 275km, whichareobserved at Troms0(69.6øN,19.2øE thermospheric dynamicsnot only at high latitudesbut in geographic coordinates), Kiruna (67.9øN,20.4øE), alsoat middleand lowlatitudesthroughthe transfersof and Sodankyl/•(67.4øN,26.6øE).The electricfield is energy and momentum. It may be difficult to estimate derivedfromthe ionvelocityvectorunderthe assumpthe thermosphericresponseto the aboveenhancement tion that the motion of ions and electrons is due to E x B usingresultsfrom previousmodel calculations,because drift alone. The ionospheric valueswereobtainedfrom the electromagnetic energyassumedin the modelstends an incoherentscatterspectrumintegratedover2 min. to havea largehorizontalscaleand stayfor a longtime All-skyauroralcamerarecords obtainedat Kilpisj/•rvi comparedwith typical auroral arcs. (69.0øN,20.8øE)wereused.The CP-1-K antennabeam In this paper we use data obtainedsimultaneously intersected the ionosphere at an altitudeof 110km, fromthe EuropeanIncoherent Scatter(EISCAT)radar, km northwestof Kilpisj/•rvi.

an all-skyauroralcamera,and the IMAGE (International Monitorfor AuroralGeomagnetic Effects)mag- 3. Analysis Method

netograms [Liihr et al., 1998,and references therein]

to investigaterelationshipsbetweenthe thermospheric 3.1. Derivation of the Field-Aligned motions and auroral activities.

The retrieval of AGW

Component of the Neutral-Wind Velocity

informationfromthe waveparameters of travelingionoThe ion and neutral-wind velocities alongthe geospheric disturbances (TIDs) hasbeeninvestigated [e.g., magneticfieldline,V//and U/l, respectively, arerelated Kirchengast et al., 1995;Kirchengast, 1996,1997],al- by the followingequation[Winsetet al., 1988,andref-

though the physicaldescriptionof the AGW-TID rela- erences therein]: tionshipis generallyvery complicated.To obtain AGW parametersfromexperimentsasdirectlyaspossible,the l/in mi Ni I/in OS ' neutral-wind velocity is estimated from data of the EISCAT radar and the Mass Spectrometerand Incoherent whereg is the gravitationalacceleration, ! is the magScatter(MSIS) model[Hedin,1991]. neticdip angle,vi,•is the ion-neutralcollisionfrequency, It is important to consider the effects of local en- k is the Boltzmannconstant,mi is the meanion mass, hancements of geomagnetical energies onthermosphericNi is the ion density,and Ti and Te are the ion and elec-

U//= V// gsin I

k 0[Ni(Ti +Te)](1)

motionsto investigate AGWs at highlatitudes.During tron temperatures, respectively. HereO[Ni(Tiq-Te)]/Os is the partial derivativeof Ni(Ti + To) with respectto oval, wavesthat have oscillationperiodslongerthan the field-aligneddirection. The MS!S modelwasusedto the Brunt-V/iis/il/iperiodare not alwaysrecognized as estimate •/i,•. The ion compositionmodel usedto derive AGWs. If particleprecipitationand the electricfieldare Ti, Tc, Ni, and V// from the EISCAT radar spectrum enhancedperiodically,thermal expansions of the local was used to estimate mi. geomagneticallydisturbed periods, around the auroral

atmospherecan causeperiodicneutral-windoscillations

The above analysismethod has been usedfor deriva-

synchronized with enhancements of the geomagnetic ac- tion of the neutral-windvelocityin the polar F region tivity. usingEISCAT radar data [e.g.,Lilenstenet al., 1992; Section 2 describes the instrumentation. Section 3 Oyamaet al., 2000]. Theseestimatedneutral-windvedescribesthe analysismethod we usedto estimate the locitiesshowedfairly good agreementwith thoseobneutral-wind velocity and the distancebetweenthe observingpoint and the wave-sourceregion. Section3

servedusingoptical instruments,suchas Michelsonin-

terferometers and Fabry-Perotinterferometers (FPI).

OYAMA ET AL.- GENERATION

OF AGWS IN THE AURORAL

F REGION

18,545

where coais the acousticcutoff frequency. We focusedon

3.2. Model of the Atmospheric Gravity Wave

Thermal expansionsdue to frictional and particle

heatingas well as ion drag due to the Lorentzforce

thevelocity perturbationdueto a gravitywave,whichis relatedto P'/Po, shownin (2), with the simpleformula

are known to contributeto the generationof AGWs, but their relative importanceis still not fully under-

u'(x,z,t)=C22•-• p'po(z) (x,z,t) V•

stood. Here we estimate wave characteristicsusing the

two-dimensional, meridional(z) and vertical (z) model

where

of Francis[1974],assuming that theLorentzforcealone

coclt

generatesAGWs. In this model the equations of momentum, energy, and massconservation with respectto neutralsare used to calculate neutral-wind oscillationsgeneratedby the

•--coc2 q-(t2_tL2)l/2 , =

Lorentz force. The Lorentz force is a function of the

auroralelectrojetcurrent,whichhassimpletime variations in this model;it is null at time t 0. The auroral electrojet current in

(6) (7)

•t • (• - O:c2)

X[(• -- COc2) 2 --coc1211/2'

(8)

In the presentwork we assumedC1 = 310 m s-1 C2

=900ms -1 cob1 = 1/600s-1 andcob2 = 1/900s-1 Becausewe did not comparemodeledand observedam-

this model is assumed to extend in the zonal direction or be a line source. The Coriolis force in the momen-

plitudes ofu', J wassimply setto 105A.

tum equationis neglected.The neutral atmosphereis

reflected paths, but we took into account only direct

assumedto havetwo layerswith differenttemperatures;

The ray path of u' can be either direct or Earthwaves

in our

calculation

on neutral-wind

oscillations

the heightz0of the boundarybetweenthem is 150km. becausethe Earth-reflectedwavesdispersetheir energy The altitude zs of the peak auroralcurrentis assumed beforereachingthe F region[Francis,1974]. to be 110 km. The background neutral-windvelocityis 3.3. Distance to the Source set to zero through the calculation. The three equationsin the AGW model are linearized Bertin et aI. [1983]showedthat the distanceDs bethrough Fourier transformation in time, assumingpertweenthe observingpoint and the sourceregionto genturbation of neutral pressure and mass density. This erate neutral-wind oscillationscan be expressedas

linearizationgivesa s•ngleequationfor p/Po (P is the

perturbation term of neutral pressure and P0 is the

Ds = Zøbs -- Zs

equilibriumone) [Chimonasand Hines, 1970]. Frantan6} ' (9) cis [1974]found that when a sourcefunctionthat is where zobsis the altitude at which neutral-wind oscil-

representative of the Lorentz force is convoluted with

Green's function, p'/Posatisfies p'(x,z,t)

lations are observedand O is the upward propagation angle from the horizontal plane. If the wave packet is assumedto propagatestraight from the sourceregion, 6}is definedas the ratio of the meridionalgroupvelocity to the vertical

po()

one.

The phasefor AGWs shiftslinearlywith heightabove •200 km becauseof the small vertical gradient of the neutral temperature. This suggestsa stable vertical phasevelocity Vpz with altitude. The vertical wave

where J is the total auroral current integrated along the line source,• is the ratio of specificheat (_•1.66), cobis the Brunt-Viiisiilii frequency,Bzs is the magnitude of the geomagneticfield at an altitude of z,, t is time, C is the speedof sound,P0 is the equilibrium term of neutral-mass density, and x is the distance from the

numberkz is expressed as 2•rco/Vpz.To derivethe meridional wave number kx, we substituted kz into the equation of the dispersionrelation for AGWs given by

Hines [1960]:

co2

co2coa 2 _ co2

kx2 = -q--(10) cob2 _ co2kz2 C2 cob2 _ co2 '

__

sourcealongthe meridionaldirection.ITI is the magnitude of the transmissioncoefficientfor waves at z0, and

The meridionaland vertical groupvelocities,vgx and • is definedby the equationT = ITI exp (/O). Sub- Vgz,respectively, werederivedfromOco/Okx andOco/Okz,

scripts i and 2 represent the layers below and above, respectively. respectively,the boundary height z0. The terms cocl, coo2,and t L are defined as 4.

cocl= cobl(Z0 -- Zs)/X,

(3)

Results

We investigated relations between characteristicsof neutral-wind

(4) tL = (coal/Clcobl) X,

(5)

oscillations

observed

with

the

EISCAT

radar and auroral activities for two observation peri-

ods: 1800 to 2200 UT on March 1, 1995 (case1), and 1130 to 1600 UT on March 29, 1995 (case2). All-sky auroral imagesobtained at Kilpisj/irvi were availablefor

18,546

OYAMA

ET AL.- GENERATION

OF AGWS

case 1, but becauseof daylight, they were not for case 2.

IN THE AURORAL

poleward edge of the field of view. From 1918 to 1935 UT the all-sky auroral images showedseveral auroral arcs that extended

4.1.

Case 1:1800-2200

UT on March 1, 1995

Electromagneticenergiesin the source region that generated neutral-wind oscillationsdetected with the EISCAT radar cannot be observeddirectly with the radar itself in the CP-1-K mode, becausethe sourceregion is far from the observingpoint. We can, however, investigate relations between characteristicsof the observedoscillationsand the auroral distributions using EISCAT CP-1-K radar data togetherwith all-sky auroral images. Some of the all-sky auroral images for case i are shownin Figure 1. They show that from 1857 to 1907 UT

an auroral

arc extended

in an almost

zonal direc-

tion. At the same time it also moved steadily poleward at a few hundred meters per secondwithout major changesin its form. At 1908 UT the arc began to move poleward rapidly from the equator side of the EISCAT radar site, then diffuse auroras were seenover the field of view from 1914 to 1917 UT associated with

the gradual appearance of a few auroral arcs on the

F REGION

in an almost zonal direction.

EISCAT radar at 1924 UT (not shown). At 1927 UT the pole-sideedge of the auroral arc appeared to be at the observingpoint of the EISCAT radar. At 1936 UT a breakup occurredon the equator side of the EISCAT radar site. The intense aurora drifted poleward rapidly, and

diffuse

auroras

were

seen over

the

field

rora covered almost half the field of view of the all-sky

image. This breakup was not accompaniedby rapid motions

of the auroras.

Figure 2 showstime variations in electric field, electron density, and electron and ion temperatures as well

as the field-alignedcomponentof the neutral-wind velocity derived from (1). We applied the Lomb periodogram[Hernandez,1999]to the field-alignedneutral-

.....

..................... 1909

•.•,,......•,'•" ...........•.,.•>• ..

....:{ ....

. ::•. ,.•;.;•. ½•'• • *•.' '½•'½••• ....... "re:'.'

½:..• •.:: '.2-.. ß,•'••:'; -'•.,.'

•..•..•,:½; ...... .. ..'..,• .;:•:3 $....... ..:.-.•; .•.•,..•, .. "•': •;.-&½:4,};.:::;;' :':•"; %..;";,.".'•..:." '::i."'

..'•',:%'...' •*½•½'•.•(:2, :½½½• .,,•,*•"...•'.::,•-'

..½..

1918 ß

...

.

..

ß

.

t.:.•!•. ß ....... '120

1937

of view

from 1945 to 2040 UT. At 2041 UT another breakup occurred, a little poleward from the region where the breakup at 1936 UT occurred. A relatively intense au-

.ot.............. 1900

......

At the

same time these arcs also moved steadily equatorward at a few hundred meters per secondwithout showing major changesin their forms. The arc closestto the equator appeared to crossthe observingpoint of the

2005

2041

Figure 1. All-sky camera imagesobtained at Kilpisj/irvi at 1900, 1908, 1909, 1918, 1927, 1936, 1937, 2005, and 2041 UT. North is to the top; east is to the right. The white dot indicates the Troms½ field line at an altitude of 110 kin. The three circles shown with solid lines indicate

distances of 50, 100, and 120 km from the zenith at Kilpisj/irvi. The dotted curves indicate distancesof 140, 200, and 290 km from the Troms½field line at an altitude of 110 kin.

OYAMA ET AL.' GENERATION OF AGWS IN THE AURORAL F REGION

18,547

Case I (1800--2200 UT on 1 March 1995) (a)

50

ß Enorth o Eeast

-50 (b) 11.5

_

11.0

_

10.5

_

i

i

i

i

t

i

i

i

i

i

i

i

i

i

i

i

i

2000

(c)

_

_

1500 •

ßTe

ooo (d)

_

_

300

t

\

•

-

•.

-

250

I10 [m/s]

200

I

18

I

I

I

I

19

I

I

I

20

I

I

I

I

21

I

I

22

Universaltime [hr]

Figure 2. For case! (a) electricfield derivedfrom ElSCAT radar d• be•wee• •800 •d 2200 • o• M•ch •, •995. Solidc[m[es[•d[c•e [he toefldiode[compo•e•[; ope• circles[•d[c•[e

of 209 km derived•om •he •[SCA• md•. (c) •tec•o• (sot[dc[•ctes)•d [o• (ope• c[•ctes) •empem•a•es• • •[fi•ade of 209 km derivedfrom [he •[SCA• md•. (d) Sape•pos[•[o•of modeledph•e [[•es o• obse•ed oscfi[•[o•s [• fie]d-•[[[•ed •ea•m]-w[•d ve]oc[W.•he modeled phase [[•es w[•h • m[•[mam v•[ae •e p[o[[ed w[[h •h[ck dash-dolled m•x[mam v•]ae •e p[o•[ed w[•h [hick solid [[•es. •he base [[•e of •he obse•ed

•[fi•de of •65 km corresponds [o • he[•h• of •65 kin, •d •he base co•mspo•d[• •he s•me w•. Pos[[[vew[•d is apw•d •]o•[ [he •eom•[•efic

wind velocities,shownin Figure 2d, in an altitude range from 165 to 253 kin, in which oscillationperiods shorter than 13 rain, the typical Brunt-Viiisiilii period in the F region, were filtered out. The thick solid and dashdotted lines in Figure 2d are discussedin section 5.1. When the auroral arcsmovingsteadilypoleward(from 1857to 1907UT) and equatorward(from 1918to 1935 UT) crossedthe EISCAT radar observingpoint, the electric field had peak values, which increasedthe ion temperature becauseof frictional heating. Around 2000 UT, when diffuseaurora wereobserved, the field-alignedneutral-wind velocitiesoscillatedwith relatively large amplitudes. Downward propagations of the phasewith time were seenin the field-aligned neutral-wind velocities. The velocity at an altitude of

165 km had a peak near 2000 UT, whereasthat at 231 km had a peak •-7 min earlier. To investigatethe wavecharacteristicsin more detail,

we appliedthe Lomb periodogramto the velocitiesobservedfrom 1945to 2040UT (shownin Figure2d with an arrow). Figure3 showsthe spectraobtainedat alti-

tudesof 209(solidline)and231km (dashedline). The spectraat the other altitudes have peaks at the same oscillationperiodswith differentamplitudes.The vertical dottedline showsthe typicalBrunt-V/iis/il/iperiod in the F region. While the spectra for both altitudes are relativelybroad,they havea peak amplitudeat an oscillationperiodof 24 min, obviouslylongerthan the typical Brunt-V/iis/il/i period. The downwardpropagationof the phasewith time

18,548

OYAMA ET AL- GENERATION

OF AGWS IN THE AURORAL F REGION

Period [min] 60

50

40

30

20

13

10

10-

0-

I

2xl 04

4x10'4

I

6x10'4

8x10'•

lx10-3

Frequency [s"] Figure 3. Spectra from the Lomb periodogramappliedto field-alignedneutral-wind velocities at altitudesof 209 km (solidline) and 231 km (dashedline). Opencirclesshowresultsfor case 1, and solid circlesshow results for case2. The vertical dotted line at a period of 13 rain shows the typical Brunt-V/iis/il/i period in the F region.

and the oscillationperiod longerthan the typical BruntV/iis/il/i period are general characteristicsof AGWs, which implies that the observed oscillations from 1945 to 2040 UT

are AGWs.

Figure 4 showsthe altitude profile of the phaseat an oscillationperiod of 24 min. The vertical phasevelocity is almost stable with height becausethe phase shifts approximately proportionally with height. The vertical phase velocity for the oscillation period of 24 rain was

The verticalphasevelocitiesfor oscillationperiodsof 27 and 22 min were•0170and --•190m s-1, respectively. We could derive the distance between the observing

point and the regiongeneratingthe observedoscillations usingthe method describedin section3.3, althoughwe couldnot estimatethe propagatingdirection of the observed oscillations because of the obser-

vationbeingfixedto look alongthe geomagnetic field line. This distanceis listed in Table 1, along with the

--•190m s-1 according to a fittingwith a straightline, valuesof the other wave parameters. The meridional ,Xx(= 2rc/kx)andthephasevelocityvp• (= as shownby the dashedlinesin Figure 4. (The cross- wavelength correlationcoefficientwas 0.973.) Altitude profilesof the phaseat oscillationperiodsof 27 and 22 rain (not shownhere) alsoshowedshiftsproportionalwith height.

2rcco/k•) were•0160km and•0110m s-1, respectively. Distancesfor oscillationperiodsof 27 and 22 rain were •299 and •0140 km, respectively.Comparedwith the distancefor an oscillationperiod of 24 rain, the differences are •090 and •060 km.

From 1800 to 1945 UT the electric field varied consid-

350

erablywith time, as shownin Figure 2a. The neutralwind oscillationsobservedduring this time period may haveresultedfrom not only local heating but alsopropagationof someneutral-windperturbationsfrom farawayplaces.From 1945to 2040 UT, whenthe all-sky

3OO

images showed diffuseauroras,the perturbations in the electric field were smaller than those in the disturbed

period. This suggests that wavepropagation fromfarawayplaceswas moreimportantfor the neutral-wind

250

oscillations observed from 1945 to 2040 UT than local

heating was. 200

Table 1. Resultsof Lomb PeriodogramAnalysis From 1945 to 2040 UT for Case I and From 1230

150

to 1400 UT

I

60

120

180

240

300

0

60

Phase [deg] Figure 4. Altitude profile of the phasefor cases1 (opencircles)and 2 (solidcircles).Dashedlinesshow the altitude profiles used in this paper.

for Case 2. Case 1

Case 2

Dominant period, min Distance, km Meridionalwavelength,km

24 _•200 _•160

32 _•260

•_230

Meridionalphasevelocity,m s-1

•_110

_•120

Vertical wavelength,km

_•270

•_510

Verticalphasevelocity,m s-1

•_190

•_270

OYAMA

ET AL.'

GENERATION

OF AGWS

IN THE

AURORAL

F REGION

18,549

Case 2 (1130--1600 UT on 29 March 1995)

•, 25

ß Enorth o Eeast

m -25

_

i

i

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I

i

i

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i

(b)

11.25

_

11.00

_

10.75

_

t

i

t

i

i

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I

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t

t

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2500 _

2000 -

(c) -

-o

1500

.Te o Ti

ooo 850 ß

•

7k•

800

•75

kin_

ß.•,_,.• . -T. 3kra

.• 250

I10 [m/s]

k•

200 _

kr•

150 --

I

I

I

I

I

I

12

I

I

I

13

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15

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-

16

Universal time [hr]

Figure 5. SameasforFigure2, but forcase2 (between 1130and1600UT onMarch29, 1995). The altitude range for the field-alignedneutral-wind velocity is from 165 to 319 km.

4.2.

Case 2:1130-1600

UT on March

29, 1995

amplitudes, as shown in Figure 5d. The oscillations were observedfrom altitudes of 165 to at least 319 km,

We couldnot identifythe auroraldistributionsaround the sourceregionbecauseof daylight. There is, however, a method that can be usedto investigatethe relationshipbetweenthe characteristics of observedneutralwind oscillationsand geomagneticactivities, as explained in the following. The electric

field increased from

1145 to 1200 UT

and their amplitudeincreasedgraduallywith increases of altitude. When they were observed,the electric field magnitudedecreasedgraduallyand the electrondensity and electrontemperature in the F regiondid not show remarkableenhancementsdue to energeticparticle precipitation. Periodic perturbations in the auroral particle precipitation and the electric field thus contributed

and from 1230 to 1300 UT (Figure 5a). The electric little to the generationof the observedneutral-wind os-

fieldduringthe latter periodreached•40 mV m-i,

cillations. The oscillations appear to have a downward a level which, by frictional energy dissipation,signifi- propagation of the phase with time. Figure 3 shows cantly increasedthe ion temperature in the F region that the Lomb periodogramspectra of the oscillations (Figure5c). Accordingto the IMAGE magnetograms observedfrom 1230 to 1400 UT (shownin Figure 5c (Figure6), the frictionalenergyseemedto dissipateinto with an arrow) at altitudesof 209 (solidline) and 231 the ionosphere not only overthe EISCAT radar site but km (dashedline) had a peak amplitudeat an oscillaalso in the other regions. From 1230 to 1300 UT the tion period of 32 min, which was obviouslylongerthan Y components (east-westcomponents)measuredfrom the typical Brunt-V/iisiilii period. These results show Bj0rn0ya(BJN) (74.5øN,19.2øE)to Ouluj/irvi(OUJ) that observedoscillationshad generalcharacteristicsof (64.5øN,27.2øE) had comparativelylarge amplitudes, AGWs. which seemedto decreasewith goingequatorward. The vertical phase velocity for an oscillation period After 1300 UT the field-alignedneutral-windvelocity of 32 min, which was derived using the altitude proin the F regionbeganto oscillatewith remarkablylarge file shownin Figure4, was•270 m s-1. (The cross-

18,550

OYAMA

ET AL.'

GENERATION

OF AGWS

IN THE AURORAL

F REGION

IMAGE magnetometer network 1995-03-29 1 minute averages

NAL

LYR

HOR

HOP

BJN

SOR

KEV

TRO

MAS

KIL

MUO

PEL

OUJ

HAN

NUR

11

12

13

14

15

16

Universal time [hr]

Figure 6. For case2 the Y components,i.e., east-westcomponents,of magnetogramsfrom the IMAGE

network.

correlation coefficientbetween the observedphase and

sucker[1982]. Theseoscillationswere associatedwith

fitted line was0.986.)

increasesin the plasma temperature and density. To assessthe validity of the hypothesisthat the observed neutral-wind oscillationsweregeneratedin the expected sourceregion, we used three analyses. First, we investigated the auroral activity around the expectedsource region for case I using all-sky auroral images. Second, the distance between the observingpoint and the generation regionwas calculatedfor caseI usingthe model

The estimated distance between the observingpoint and the sourceregion for generatingobservedoscillations was -0260 km (Table 1). The meridionalwavelength was-0230 kin, and the meridionalphasevelocity was -o120m s-1. For oscillationperiodsof-35 and 29 rain, the vertical phase velocities were -0200 and -0370

m s-1 respectively, and the distanceswere-0430and --•150 kin, respectively.The distancesdifferedfrom that for an oscillationperiod of 32 rain by -o170km and -o110 kin, respectively.

of Kato et al. [1977],and it was comparedwith the distance

tabulated

in Table

1.

In the other

we com-

pared, for both cases,the observedphase line with the

onecalculatedusingthe modelof Francis[1974]. 5.

Discussion

5.1. Assessing the Validity of the Hypothesis on the Wave-Source Region The neutral-wind velocity estimated from EISCAT radar data in the F region oscillatedwith periods of 20-30 rain and had wavelengthsof severalhundredkilometers. These values are typical ones for the mediumscale AGWs according to the classificationby Hun-

5.1.1. Using all-sky auroral images. The estimated distancefrom the observingpoint to the source regionfor case1 was 140-290kin. The meridional phase

velocitywas -o110m s-1, thus the front of the oscillations

observed

from

1945 to 2040 UT

should-have

reachedthe observingpoint 21-44rain after leavingthe sourceregion if the phasevelocity remainsconstantdur-

ing the propagation.Hencethe sourceshouldhaveappeared at 1900-1920 UT at a distance of -o140-290 kin.

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18,551

arc just before the rapid motion was --200 km. Thus after 21-44 min, D is ""800-1900km. The Cs for case 1

Accordingto the all-sky auroral imagesobtained between 1900 and 1920 UT, the auroral arc extended in an almost zonal direction from 1900 to 1908 UT appears

was""110m s-1 (seeTable1) thusat c•-- 0ø the dis-

to be the generatorof the neutral-wind oscillationsob-

tance r is 11/69 D or ""160-330km. This rangeincludes

served with

the estimated

the ElSCAT

radar.

The distance

between

the observingpoint and the auroral arc is roughlyconsistent with the estimated distance, as shown in Figure 1. Although it is difficult to estimateeffectsof the thermal expansionsand the momentum transfer between ions and neutrals on motions of neutrals quantitatively usingthe presentdata set, we can presumethat Joule and particle heating and the Lorentz force enhancein the vicinity of the auroral arc and that these mechanisms are candidatesof the generator of the observed oscillations. The rapid motion of the auroral arc at 1908 UT is also the candidate of the generator of the observedoscillations.Although the relative importance in the three mechanisms,Joule and particle heating, the Lorentzforce,and the rapid motion, for the generation of the observed oscillations was not found from the

present observation, it is concluded that the auroral arc that appeared from 1900 to 1908 UT has affected the

distances listed in Table

1.

5.1.3. Using the model of Francis[1974]. In Figures 2d and 5d the neutral-wind oscillationscalculated usingthe modelof Francis[1974]are compared with

the observed ones for cases i and 2.

The

dis-

tances between the observingpoint and the sourceregion,whichwereusedfor calculatingthe modeledphase lines, were 200 km for case i and 260 km for case 2. These values were estimated from the dispersion relation applied to the observedoscillationsduring limited time periods: from 1945 to 2040 UT for case i and from 1230 to 1400 UT

for case 2. The wave source was assumed to be the Lorentz force in the model. This

assumption seemsto be acceptable, because the characteristics of observedoscillations were categorized in medium-scaleAGWs, which can be generated by the Lorentz force more effectively than by Joule heating

[Jing and Hunsucker,1993].

generationof the observedoscillationsstrongly. 5.1.2. Using the model of Karo et al. [1977]. From 1908 UT the arc acceleratedpoleward,crossing the zenith at Kilpisj/irvi rapidly. If this motion is the generatorof the observedoscillations,we can estimate the distancebetweenthe observingpoint and the generation regionusingthe modelof Karo et al. [1977].The position where a line sourcemoving at a speed of Vo can generateAGWs with a phasevelocity of Cs can be expressed by the followingequation[Kato et al., 1977]:

For case 1, from 1945 to 2040 UT, there were approximate peak-to-peak correspondences between the modeled phase line and the one observedat altitudes from 165 to 253 km. For case 2, from 1230 to 1400 UT, the modeledphaseline seemsto agreewith the oneobserved at altitudes from 165 to 319 km. These agreementssuggestthat the line sourcein the model can reproducethe phase line similar to the observedone. For case i the

the direction

next

section.

5.2.

Effective

auroral

arc extended

in an almost

zonal direction

from

1900 to 1907 UT, which could be identified as the line sourcein the model. The agreementsalso suggestthat r r cos c•+ D the generatorof the observedoscillationshas someconCs Vo (11) sistent properties with ones of the wave source in the where r is the distancebetweenthe observingpoint and model. Effective parameters of the wave sourcein charthe generationregionand (•r - c•) is the anglebetween acterizing neutral-wind oscillationswill be discussedin of the source motion

and a line between

the observingpoint and the generationregion. An intense auroral arc extending in an almost zonal direction is the most likely candidate for the line source in the present observation. The motion of the line source in the model agreeswell with the actual motion of the

Parameters

in Characterizing

of the

Neutral-Wind

Wave

Source

Oscillations

An evaluationof the consistencyof the assumptions with the presentobservationwill be directly related to a auroral arc. In this model the line source is assumed prediction of some properties of the wave source. This to drift without any time variationsin the magnitude. suggeststhat if the characteristics of AGWs, period, Although the auroral emissionintensity may not be wavelength,amplitude, etc., are measured,we can estidirectly proportional to the magnitude of the auroral mate the wave source,but it is generallydifficult. We used two models to assessthe validity of the hyelectrojet current and of the Joule heating rate, this assumption may be inconsistent with actual time varipothesison the wave-sourceregion under someassumpations in the magnitudeof the line sourceaccordingto tions. Prediction from thesemodelsagreedwith results from the observations,althoughthe time variation and the all-skycameraimages(seeFigure 1). For case 1, when the rapid motion of the intenseau- the motion of the wave source for both models were conroral arc occurredat 1908UT, the arc movingpoleward siderably simplified under different assumptions. The wason the equatorsideof the EISCAT observingpoint, consistencyof these assumptionsfor the present obserthus an angle c• of 0ø correspondsto equatorwardfrom vation should be discussed. For case i the auroral arc that was estimated as the the observingpoint. When the arc movespoleward at a constant velocity, it arrives at a distance D from the wave source moved steadily poleward at a relatively observingpoint after a time t. Accordingto the all-sky slowvelocity(from 1857to 1907UT), then it beganto auroralimages,the Vofor case1 was""800m s-1. The move poleward rapidly at 1908 UT. If the observedosestimateddistancebetweenthe observingpoint and the cillationswere generatedby the slowmotion, not by the

18,552

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ET AL.: GENERATION

OF AGWS IN THE AURORAL

F REGION

rapid motion,the motionassumedin the Francis[1974] field(Figures2a-2c). This canactivatethe thermalexmodel may be more suitable than that in the Karo et

al. [1977]model. If the observedoscillations weregen-

pansionof the atmosphereand the ion-neutral momentum transfer, thus producingneutral-wind oscillations.

erated by the rapid motion, the motion assumedin the Kato model may be more suitable. The wave source was estimated to have appeared at 1900-1920 UT. The above two motions occurred during this time period, thus we cannot identify which motions of the auroral arc were effectivein generatingthe observedoscillations.

The oscillationshave relatively broadbandspectra in the vicinity of the generationsourcebecausepermitted spectra,as determinedby viscousdamping,can survive becauseof the short distancethey propagatefrom the generationsource.Neutral-wind oscillationsoriginated from the auroralbreakupat 2041 UT might be super-

The Francis [1974]model assumedsimplifiedtime variations in the auroral electrojet current, but the current remains constant after its appearance. The wave

sourceassumedin the Karo et al. [1977]model had no time variations. Electromagneticenergiesassociated with the auroral arc, however, seem to vary with time accordingto the emissionintensity obtained from the all-sky camera images. Both models may not be suitable with respect to the time variations in the wave

imposed on the neutral-wind oscillations observedbe-

forethe auroralbreakup,whichcan modulatethe wave characteristicsof the phaseand the period. Even during relatively quiet periods, from 1945 to 2040 UT for case i and from 1230 to 1400 UT for case

2, the electricfield and the electrondensityshowedperturbations with small amplitudes. These perturbations may causediscrepanciesbetween the modeled and ob-

servedphaselinesthroughlocalthermospheric heating.

5.3.2. Assumptions of the AGW model. As For case2, discussions on time variations in the wave mentionedin section3.2, the Francis[1974]modelmakes sourcemay be possibleusing IMAGE magnetometer various assumptions. While any of them could prodata (Figure 6), althoughany discussions on the mo- duce discrepanciesbetween predictions and observation will be impossible.The magnetometerat Bj0rn0ya tions, here we considersome that are likely to be imshowedat least two negativepeaksduring 1230 to 1300 portant. The modelassumesthat the Lorentz forceis the only UT, which suggeststhat auroral electrojet current had fluctuated with someamplitude and that the time vari- sourceto generateAGWs, though frictional and particle energydissipationhave alsobeen thought to be the ations assumedin both models may not be suitable.

source.

We cannot

discuss the merits

of the model based

on the consistencyof the assumptionsin both models for the present observation. It is concluded,from the agreementsbetweenresultsfrom the predictionand the observation, that the effective parameters of the wave sourcein characterizingthe observedoscillationswill be

sourceof AGWs. If particleheatingsignificantlyaffects motionsof neutralsin the F region,the sourceregion in the model should be shifted upward from the 110km altitude given in the model. An upward shift of the source altitude

in the model decreases the distance to

the sourceaccordingto (9) and tendsto decreasethe

distribution of the wave source and the oscillation period of the modeled AGWs. Background-neutralwindsmay affectwavecharacterdistance between the observingpoint and the source region. This is becauseboth models use identical as- istics,suchas the frequencyand the ray path. For case1 the meanvalueof the field-alignedneutral-windvelocity sumptionsfor theseparametersof the wave source. in the F regionis positivethus upwardalongthe geo5.3. Differences Between Predictions and magneticfield line, whichsuggeststhat the background Observations neutral-wind velocity appearsto be equatorward. This The model did not reproduceall the observedoscil- is consistentwith previous works, from EISCAT radar lations. Discrepanciesbetweenthe phaselinesfrom ob- and FPIs, that equatorwardwinds are generallydomi[Aruservation and those from calculation using the Fran- nant duringnighttimein the upperthermosphere cis [1974]model graduallybecameobviouswith time. liah et al., 1996; Witasseet al., 1998]. The estimated One reasonfor these ambiguitiesis that the modeled wave sourceis located on the equator side of the EISthat the observedoscillaoscillationswere reproduced using the distance esti- CAT radar site. This suggests tions propagated poleward and that the groupvelocity mated from the oscillationsobservedduring limited of the observed oscillations has a componentopposite time periods. For the other candidateswe will discuss three explanationsfor the discrepancies: (1) local iono- to the background-neutralwind. An opposite direction can also be seen for case 2. sphericperturbations associatedwith enhancementsof the plasmatemperatureand density,(2) assumptions The meanneutral-windvelocityis negative,whichsugof the AGW model, and (3) the propagationof waves geststhat the poleward winds appear to be dominant. The magnetogramsin Figure 6 show that, from 1230 from other sourceregions. to 1300 UT, the amplitude of the Y componentseems 5.3.1. Effects of local ionospheric perturbation. For case i the amplitudes of the oscillationsob- to have a peak around Bj0rnoya (BJN), which is the served at altitudes of 165, 187, and 209 km suddenly pole side from the EISCAT radar site. This suggests decreasedafter •2040 UT, as shownin Figure 2d. The that the observedoscillationspropagatedequatorward all-skyauroralimageat 2041UT (Figure1) showedthe in the polewardbackgroundwinds. The opposite direction causesan upward Doppler auroral breakup, which resulted in the enhancementof the plasma density and temperature and the electric shift of the oscillation frequency and an upward shift the horizontal

OYAMA

ET AL.:

GENERATION

OF AGWS

of the direction of the group velocity. The effect on the groupvelocity may reducethe distancebetweenthe observingpoint and the wave-sourceregionin terms of the wavelength. The antiparallel direction is consideredto be one of the reasonsfor discrepanciesbetween the observedand predicted phase lines. The model assumesthat the altitude profile of the neutral temperature has no altitude gradient above150

IN THE

AURORAL

F REGION

18,553

analysisof the observedoscillationsis -•200 km, which is obviously different from that at 1927 and 1937 UT.

6. Summary

Relations between auroral activities and the generation of neutral-wind oscillations in the polar F region have been presented using data from EISCAT radar, the all-sky auroral camera, and IMAGE magkm and thus that the speedof soundand the'Bruntnetograms. When the ionosphericperturbations were Viiisiilii frequency are almost constant, regardlessof height. This assumptionmay causelarge differencesbe- relatively small, the field-alignedneutral-wind velocity tween the assumedand actual temperatures, especially estimated from EISCAT radar data oscillated at peri-

in the lowerF region(150-200km). This couldresultin ods of 24 min for case 1 (from 1945 to 2040 UT on discrepanciesbetween the modeled and observedphase March 1, 1995) and of 32 min for case2 (from 1230 to line becausethe phaseand the oscillationfrequencyare 1400 UT on March 29, 1995). These oscillationperiods were longer than the typical Brunt-Viiisiilii period a function of the neutral temperature. in the polar F region (_•13 min). The observedoscilThe simplified assumptionsused to derive equation lations showedthe downwardpropagationof the phase (10) from Hines [1960]may causediscrepancies.This equation is based on the assumptionthat the horizon- with time. These properties on the oscillation period tal wave number kx is purely real, which indicates that and the phaseare generalonesof AGWs. The valuesof viscousdamping is ignored. This is not held precisely the wave parameters were derived from the equation of in proportion to the distance from the wave source. the dispersionrelation for AGWs applicable to the obNumerical simulationsshow that AGWs generatedby served oscillations. These values were used to estimate Joule energy dissipation propagate in an attenuated the distancebetweenthe observingpoint and the source fashion with a component in the horizontal direction region where the oscillationswere generated: -,•200 km [Fujiwara et al., 1996]. Interactionsamong multiple for case 1 and -,•260 km for case 2. For case 1 the wavesare not considered either. Hines [1960]assumed all-sky auroral images obtained at Kilpisj/irvi showed an idealized atmospheretaken to be stationary in the that the auroral arc extended in an almost zonal direcabsenceof waves. The actual atmosphere,however,al- tion near the estimated distance. The comparison of observedphase lines with predicted ones using models ways has some fluctuations. by Francis[1974]and Karo et al. [1977]showedagree5.3.3. Effects of propagation from other source regions. For case 2 there are notable differences between the modeledphaseline and the observedone after 1400 UT, though significantionosphericheatingwasnot observedfrom 1300 to 1600 UT. Local heating thus is not the reason for the discrepancy. A possiblereason for the differenceis that other sourcesmight be generating neutral-windoscillations.If oscillationspropagating from severalsourcesare superimposedat the observing point, the observedoscillationscannot be reproduced by the model that assumesa singlesource. 5.4. Was it Possible to Generate the Observed AGWs due to the Auroral Activities at 1927 and

1937

UT

for Case

17

FigureI showsthat the auroralarc crossed the EISCAT observingpoint rapidly at 1937 UT and that the auroral arc extended in an almost zonal direction and

wasmovingat a few hundredmetersper secondwhen it crossedthe EISCAT observingpoint at 1927 UT.

ments between the two for both cases. Results from the

comparisonsuggestedthat effectiveparameters of the wave sourcein characterizingneutral-wind oscillations will

be the horizontal

distribution

of the wave source

and the distance between the observingpoint and the sourceregion. It is concludedthat geomagneticactivities on March I and 29, 1995, in northern Scandinavia significantly related to the generation of neutral-wind oscillations observedwith the EISCAT radar in the F region. Although we could not find conclusiveevidencethat the observed oscillations were AGWs, the wave parameters of observed oscillations were typical ones for the

medium-scaleAGWs accordingto Hunsucker[1982]. The conclusionimplies that geomagneticactivities at high latitudes are an important sourceto generate AGWs, as indicated by previoustheoretical studies. Acknowledgments.

We are indebted to the director

These auroral arc motions affectedthe local ionosphere. and staff of the EISCAT for operating the facility and sup-

Figure 2 showssignificantenhancements of the electric field, the ion temperature,and the electrondensity andtemperatureat thosetimes. Theseauroralarc motions,however,are thought not to be significantly related to the generationof neutral-windoscillations observed from 1945 to 2040 UT. This is because wave

characteristics dependon the distancebetweenthe ob-

servingpoint and the sourceregion,as concluded in section 5.2. The distance estimated from the spectrum

plying data. EISCAT is an international associationsup-

portedby Finland (SA), France(CNRS), the FederalRepublic of Germany(MPG), Japan(NIPR), Norway (NFR), Sweden (NFR), and the United Kingdom (PPARC). We thank the Finnish MeteorologicalInstitute for providing data from the all-sky auroral camera and the IMAGE magnetometers. This researchwas supported by grants-in-aid for Scientific ResearchA (12373002),B (11440144),C (11640441),and C (10640428)by the Ministry of Education, Science,Sports, and Culture, Japan. This study was part of the U.S.-Japan

18,554

OYAMA

ET AL.: GENERATION

OF AGWS

International ResearchProject to observethe middle atmosphere, CRL, Japan. Hiroshi Matsumoto thanks S. Kato and K. Schlegelfor their assistancein evaluating this paper.

IN THE

AURORAL

F REGION

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