102, NO. A9, PAGES 20,063-20,067, SEPTEMBER 1, 1997. Equatorial plasma bubble evolution and its role in the generation of irregularities in the lower F ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A9, PAGES 20,063-20,067,SEPTEMBER 1, 1997
Equatorial plasma bubble evolutionand its role in the generationof irregularities in the lower F region R. Sekar,R. Sridharan,andR. Raghavarao PhyscialResearchLaboratory,Alamedabad, India
Abstract. Rocketmeasurements from equatorialregionsduringequatorialspreadF (ESF) consistentlyreveal the presenceof plasmadensityirregularitiesin the lower F region (200-300
kin) wherethe initial conditionsincludingthe verticalelectrondensitygradientare not favorablefor the development of linearinstability.An investigation c,'u'ried out usinga nonlinear numerical simulation model and the results obtained fi'om the Ionization Hole
campaignrevealedthat the altitudevariationof the recombinationcoefficientand the vertical
pol,-u'ization velocitydue to fringefieldsassociated with the plasmabubbleat the baseof the F layer are responsible for changingthe pol,'u'ity of the verticalgradientin the plasmadensity profile in the lowerF region,while the penetrationof the fringe fieldsis mainlyresponsible for the developmentof zonal electrondensitygradientwhichplaysa crucialrole in the secondary
plasmainstabilitiesresultingin the generation of kilometerscalesizeilTegulafities in the vertical direction.
Introduction
Plasmabubbleswith differentzonalscalesizesrangingfrom a few hundred kilometers to a few tens of kilometers have been
observedduringnighttimein the equatorialionosphere by various experi•nental techniques [WoodmanandLa Hoz, 1976;McClure et al., 1977; Weber et al., 1978; Sinha et al., 1996; Patra et al.,
1995]. They are believed to be generatedby the action of generalizedRayleigh-Taylorinstabilitymechanisminvolvingthe drivingagencieslike electricfields [Hansonet al., 1986], zonal winds [Kelley, 1985] and vertical winds [Raglmvcu'ao et al., 1987], apart froin gravity [Haeremtel,1974]. Linere'theories [Haerendel, 1974; Sekar and Raghavarao, 1987] and nonlinear numericalsimulations[Ossakow,1981;Raghavaraoet al., 1992; Sekar et al., 1994] have been developedto understandthe dynamicsof the plasmabubblesand their shapes[Sekar and Raghavarao, 1995;1997]. Earlier work on simulationshad revealedthat an initial plasmadensityperturbationin the zonal directionwith an amplitudeof 5% of the ambientdensitywould be requkedfor the developinentof a plasmabubble[Ossakow, 1981]. However,recentinvestigations revealedthatperturbations of even0.5% in mnplitudecouldgrow intoa plasmabubbleunder suitable thermosphericand ionosphericdynamical conditions [Sekar et al., 1995]. It was also shown by simulationstudies [ZalesakcmdOssakow,1980] that the zonal sizesof the bubbles aredependent on the zonalscalesizesof the initial perturbation. Further,for the generationof a large-scalebubbleat the baseof the F region (~ 350 km), plasma has to be drawn froIn a significantlylower altituderegion[Zalesakand Ossakow,1980].
Thusthe generationprocessof the large-scaleplasmabubblesis likely to alter the plasma density distributionin the lower F region (200-300 km), too. Though the earlier work by Zalesak and Ossakow[1980] demonstratedthe possiblerole of fringe fields in the developmentof a large-scalebubble,the role of these fields in the generationof irregularitiesbelow andaround250 km region, with different backgroundplasma density gradientsas revealed by a few observationsat different phasesof ESF reported in the literature [Narcisi and Szuszcezewicz,1981; Vickreyet al., 1984; Prakashand Pal, 1985; Raghavaraoet al., 1987;Sridharanetal., 1997] havenot beeninvestigated.Two of these measurements
were
obtained
at the onset time
of ESF
[Raghavaraoet al., 1987; Sridharanet al., 1997] alongwith the otherplasmaand neutralparametersto determinethe stabilityof the F region. In the latter measurementsthe initial conditions includingthe verticalplasmadensitygradientare not foundto be favorablefor thedevelopment of linearplasmainstabilityprocess. However,theobservations didrevealthepresence of irregularities in the altituderegion of 220 to 290 km inspiteof unfavorable conditions. Hence an exercise was carried out to examine the role
of large-scale bubblegenerationat the baseof F regionin altering the lowerF region.In this paper,onesuchobservation whichled to the following investigationusing the nonlinear numerical simulationmodel is presentedand the resultsdiscussed.
Observation in the Lower F Region During ESF Events
Figure I is an outcomeof the recentlyconductedIonization Hole campaign[Sridharanet al., 1997] andrepresents the sumof
Copyright1997by the AmericanGeophysical Union.
theioncurrents corresponding to thedominant NO+,O+•,andO+
Paper number97JA01528.
MolecularionsNO+ andO+•whichwerecomparable in densities
0148-0227/97/97 JA-01528509.00
dominatedup to 290 km and were foundto be more than the O
ionsmeasured by the Indo-RussianBennetion massspectrometer.
20,063
20,064
SEKAR ET AL.'
LOWER F REGION IRREGULARITIES
gemnagneticfield lines over the dip equator.In this model, the basic plasmafiuid equationsapplicableto equatorialF region were reducedinto the following two coupledpartial differential equations in cgsunitsdescribingthe generalized Rayleigh-Taylor instability[Sekaret al., 1995].
3OO
25O
2O0
(1)
150
aN 3•Nc30,]+3Nca½,, -3'7 -7J: -v,N 100
(2)
where 200
1000
5000
Total ion current (pA)
N plas•nadensity; q)• pertm-bation potential;
Figure 1. Altitude profilesof total ion currentobtainedin the Ionization Hole campaign [Sridharanet al., 1997].
E•o,Eyo alnbientelecla'ic fieldcomponents in zonal
ions.During this campaign,coordinatedmeasurements on neutral andplasmaparameterswere obtainednearlysimultaneously. The zonal componentof the wind was found to be eastwardbeyond
g accelerationdue to gravity; •)i ion-neutralcollisionfrequency; % recolnbinationrate;
and vertical directions;
W•, Wy neutralwindvelocities in zonaland vertical directions;
217km with itsmagnitudes rangingfrom23 to 110m s-• in the altitude region of 247 to 282 km. The vertical wind was upward
L5 tilt angleof the ionosphere with respect to the zonal direction;
witha magnitude of-10 m s'• at 180km andbecamedownward B strength•f Earth's lnagnelicfield; with a magnitude of-12 m s'• at 217 km. The estimated vertical c velocity of light. velocities of the ambient plasma by means of vapour cloud releaseswere found to be upward with their magnitudesranging
from18to 49 m s-• in thealtituderegionof 180to 290 km.This is indicativeof the presenceof eastwardelectric field which is alsoin conformitywith the movementof the baseof the F region (-340 km) as seen in the ground-basedionosondedata. The detailed results of the campaign are presented elsewhere [Sridharanet al., 1997]. Regardingthe presenceof irregularities duringthis event,the total ion currentprofile in Figure I reveals irregular structuresof a few kilometer scalesizesin the altitude region of 220 to 290 km. The runningaverageof the densities gives rise to a smooth profile. This smooth profile (to be presentedin Figure 3) is combinedwith the profile reducedfrom the iongramat SHAR (Sriharikotarange,5.5ø diplat) taken at 1910 Indian Standard Time (IST) and used in the numerical simulationas the basic backgroundprofile at the onsettime of ESF. The smoothprofile revealsa negativegradient(decreasing ion densitieswith altitude) in the total ion densityin the altitude
regionof 235 to 300 kin, whereinas mentionedabovethe initial conditions arenotfavorablefor thegen, erationof irregularities by the linear instability processes.The present paper offers a plausibleexplanationfor the generationof irregularstructure based on the results obtained
from the numerical
simulation
studies.
Numerical
Simulation
Model
In the present investigation, the nonlinear numerical simulationmodel describedelsewhere[Raghavaraoet al., 1992; Sekar et al., 1994] is used.As the h'regularities associated with the ESF are alignedto the Earth's •nagneticfield lines,the model was developedin a two-dimensionalplane perpendicularto the
While (1) describesthe spatialdistributionof the perturbation potentialgenerated by th'egeneralized Rayleigh-Taylor instability, the temporalevolutionof total plasmadensityis describedby (2). The forcing tel'ins orthogonalto the gravity and the Earth's magneticfield are not includedin (1) as they contributeonly to the secondaryinstabilityon the walls of the bubbleandnot to the primaryRayleigh-Taylormodeasshownby theearliersimulation study[Zah,saket al., 19821. The coupledequationsare solvednmnericallyover a plane orthogonalto E,-u'th'smagneticfield over the dip equator.The ,-egionencmnpasses + 200 km in the zonaldirection(x is positive westward)and from 182 to 532 km in vertical (y is positive upward)directions.In contrastto the earliersimulations[Sekaret al., 1994, 1995], this time a larger region is chosen to accommodatethe lower F region and to choosea proper zonal scaleto matchthe typically observedzonal width of the plasma bubble based on the results of the all-sky imaging camera. However, the numerical methodsand the boundaryconditions were adoptedas describedin the earlier papers[Raghavaraoet al., 1992; 'Sekar et al., 1994] with the same grid size. As mentionedearlier, the smoothedplasmadensityprofilewas used asthe basicinputto the model. It might be notedthatthe effect of the ion compositionis not accountedfor, as it would call for solvingthe continuityequationsfor a multiconstituentmedium.
Theotherinputsfor E,,o,E,.o,W,,,Wy,andiSwerebasedon the coordinatedmeasurements conductedduringthe IonizationHole campaign in order to represent realistic conditions. The contributionsof theseagenciesotherthan gravity, in causingthe instabilitywere combinedto be calledaseffectivevelocity,which in turn was shown to reduce the amplitude of the initial perturbationin the zonal direction[Sekaret al., 1995]. Basedon
SEKAR ET AL.'
LOWER F REGION IRREGULARITIES
io$
the campaignresults,the effectivevelocity can rangefrom 55 to
80 m s-•atthebaseof theF region.Thusaninputof 1%or0.5%
io4 I
20,065
io5
io6
I
1200s
of the ambientdensityas the initial plasmadensityperturbation
alongwith55or 80 m s'• effective velocityrespectively givesrise to similar resultswhich are presentedin the following section. 420
Results /
Figure 2 depicts the normalized contours of constant / I perturbationpotential in zonal and vertical plane at 1200 s after , • •..-• / 2_the initiation of the instability process. The dashed line 340 corresponds to the negativepotential while the continuousline representsthe positive potential. The direction of the Earth's magneticfield is perpendicularandcomingout of the planeof the \ diagram.The contoursare normalizedto the maximumpotential 260 9" (PIMAX) value of 2.083 mV (0.6249 stat V) and is located around340 km where the plasmabubbleis foundto evolve. The ! . interestingfeaturethat can be seenin thisfigure is the penetration / / of the fringe fields to the lower F region. Further, a noticeable / I I , I I 180 kink in the equipotentiallines is seenat around250 km altitude -200 -ioo o ioo 200 correspondingto the negative gradient region in the initial Zonal Distance {km) electrondensityprofile. As the plasmamovesin the Hall direction in the F region of the ionosphere,thesepotential lines represent Figure 3. Isoelectrondensitycontoursin theregionof simulation the streamlines of plasma flow which redistributethe plasma at 1200 s after the onset of ESF on February 19, 1993. The densitiesin the presenceof gradients.The consequences of this result will be discussed later.
Figure3 depictsthe isoelectrondensitycontoursin the region of simulation at 1200 s after the initiation of the instability
process. The contours 1 to 7 correspond to thevaluesof 1055to
contours 1 to 7 correspond to thevaluesof 10s'•to 104 cm'3in steps of 10ø25whilethecontours 8 and9 correspond to 103and 10275,respectively. The dashedcurverepresents the altitude profileof theplasmadensityat theonsetof ESFwhosevaluesare
104 cm-3in stepsof 10ø25,whilethecontours 8 and9 represent given on the top. thevaluesof 103and10275,respectively. Thegeomagnetic field 5OO
1200S
is directedout of the diagram. The dashedcurve representsthe smoothedaltitudeprofile of the plasmadensityat the onsetof the instability processwhose values on the bottomsideof the F region, basedon the measurements,are given at the top of the panel.As mentionedbefore,thisprofileshowsa negativegradient
region from235to 300kmanda steep 'positive gradient of 420
-
/.- ,,•• I//'"'x IllIll
340-
_ I
plasmadensityat the baseof the F regionaround340 km. At the baseof the F region, a plasma bubble is seen in its formative stage.Interestingly,instabilityprocessis alsofoundto growto a level of ashigh as70% in the negativegradientregion,i.e., from 250 to 280 km, where a kink is seen in the corresponding potentialfield (seeFigure 2). The consequences of this resultand its relevance to the observation are discussed below.
/tIN260
-
Discussion
In order to understandthe resultsphysically, intermediate stepsare providedin Figure 4. The electrondensityvaluesat 260
km altitudefor variouszonallocations are depictedin 4a at differenttime levels.This providesthe temporalevolutionof the i 18½ perturbation in electron density at 260 km where the initial !oo 200 o - 2½ •0 conditionsare not favorablefor the developmentof irregularities. Zonal Distance (km) The developmentof depletion at altitudesabove 260 km also givesrise to a setof depletionlevelswith altitudesimilar to that Figure 2. Contoursof constantperturbationpotentialin zonaland of 260 km altitude. Though the perturbationtends to decay by verticalplaneat 1200s aftertheonsetof equatoFial spread F 500 s, after 700 s it startsgrowing nonlinearly.This can be (ESF) on February19, 1993. The potentialvaluesare normalized understoodfrom the altitude profiles of electrondensityat zero to theabsolute maximum value(PIMAX)(0.6249statV). The referencelongitude(0 km zonal distance)at different times dashed curves correspond to thenegative potential whiie'the depictedin Figure 4b. The sequenceof profilesrevealsthat the continuous curvescorrespond to positivepotentials.The locations plasmadensitygradientin the altituderegionof 230 to 300 km of the positive and negative maxima are denotedby plus and gradually changes fromnegativ e'to positiveafter700 s whichin minus signs, respectively. Thedirection of the magnetic field turn acceleratesthe growthof the perturbationin electrondensity as seenin Figure 4a. In orderto understandthe gradualchangeof (QB) is coming towardsthe reader.
20,066
SEKAR ET AL.'
LOWER F REGION IRREGULARITIES
3200
O
IOOs
2400
300
500
1600
--
"
700
900
800
,.,
0
i
- 200
- I00
I
i
0
I00
200
Zonal Distance (km)
320 •
320
'I 200 •
•0
•-
,
,-
,
•02 •03 ElectronDensity
-
•04
200''"
0
'
20 40 60 Vert•colPolc]r•zot•on velocity(m/s)
Figure 4. (a) The evolutionof electrondensityperturbations in the zonaldirectionat differenttimes
(100,300,500,700,900,and1200s) at thealtitudeof 260km.(b) Thealtitudeprofiles of electron density in thelowerF regionat 0, 300,500,700,and900s indicating thegradual change of the polarity of theplasma density gradient. (c) Thealtitude profiles of thevertical polarization velocity at 500, 700, 800, and 900 s.
thepolarityof electrondensity, theverticalpolarization velocities at differenttimesareevaluated fromtheperturbation potential structures. Thealtitudeprofilesof verticalpolarization velocityat 500,700, 800,and900 s aredepictedin Figure4c. In theinitial phaseu.pto 700s,the magnitudes arerathersmallin thealtitude rangeof 230 to 260 km. The slightnegativeshears up to - 260 km andsignificant positiveshears beyondareconsistent withthe presence of the kinksin perturbationstructures similarto the one shownin Figure 2. The significantaltitudevariationsin the
velocities after700 s redistribute the plasmain sucha wayto change thepolarityof electrondensitygradientb.eyond 260 km. However, theelectron density gradient intheregionof 235to 260 km is found(fromFigure4b) to changeevenat 300 s whenthe
larger.Theretbre theplasmatransport associated withthefringe fieldis crucialat higherheights. Thusthelossandthetransport processes togetherchangethe negative(initial) electrondensity gradientintopositivegradientsin the altituderegionof 230 to 300 km. Oncethe positivegradientsare setup, after700 s the transportprocess associated with thefringefieldsredistributes the
plasmafor theformationof thedepletion asshownin Figure4a. Since the loss processreducesthe overall plasmadensity uniformlyin thezonaldirection(in thenumerical regionof +200 km) with time(asthe zonalvariationof ORis negligiblysmall), the relative variationof the electrondensityin that direction whichmanifests asa depletionat 1200s asseenin Figure4a, has to be mainly dueto the transportof the plasma.Oncethe basic
polarization velocitiesareinsignificant. Thisisbelievedto bedue zonalgradients aregenerated in thelowerF regionbytheabove tothealtitudevariationin therecombination effects; theeffective process, few kilometerscalesize irregularities can grow in recombination at 230 and260 km altitudes typicallyvariesfrom verticaldirectionsupported by t.he observed eastward windand 3.63x10 '3to 1.1x10 -3.Withina timeinterval of700s,theabove downwardelectricfield. Thoughin the presentexercise,the variationis capableof reducingthe electrondensitiesin these measured atmospheric andionospheric pqrameters representing altitudes soastocreate apositive gradient fromaninitialnegative realisticco.nditions havebeenused,directcomparison of the gradient. However, theabove mechanism cannot altertheprofile modelresultswiththe experimental datahasnotbeenpossible significantlyhigherabove(3260 km) within the sametime due to want of information on the exact location and orientation interval(i.e., 70.0,s), asthe lifetimeof theionis significantlyof thebubblewithrespect to therockettrajectory. Astherocket ß
,
moved eastward more horizontally near the apogee in the ionizationholecampaign,it appearsthat itstrajectorymighthave cut acrossthe setof depletionlevelsin a slantmannerto give rise to the negativetrend in the electrondensitygradient.Also, the appearanceof a ledgeat ---270km, closerto apogeeof the rocket trajectoryis possiblydue to the passageof the rocketthroughthe enhancementregionswhich usually flank the depletionlevels (Figure 4a). Incidentally, the RTI map obtainedfrom the VHF backscatterradar locatedat Gadanki(13.5øN,79.2øE, 12.5ødip) [Rao et al., 1995] closeto the launchsite, did revealthe presence of 3-m irregularitiesin the lower F region corroboratingthe simulation results. In this context, one of the possible interpretationsby I/ickeryet al. [1984], offering an explanation for the presenceof plasmairregularitieswith a weak gradientin the lower altitude region of 160 to 200 km away from the dip equatorwhich has also been interpretedin terms of "images"of the instabilitiesoccurringelsewherealongthe magneticfield line, is relevant. They had discussedthe the formation of the image striationswhich dependsuponthe compressibilityof the ion gas in those altitude region. The compressibilityof the ion gas dependson the Pedersenmovementof the ion which is somewhat considerable around 180 km. However, the Pedersen movement
Hanson,W. B., B. L. Cragin,andA. Dennis,The effectof verticaldrift on the equatorialF region stability,d. Atrnos.Terr. Phys., 48, 205-212, 1986.
Kelley, M. C., Equatorial spread F: Recent results and outstanding problems,d. Atmos.Terr. Phys.,47, 745-752, 1985. McClure, J.P., W. B. Hanson, and J. H. Hofl•nan, Plasmabubblesand
irregularitiesin the equatorialionosphere, d. Geophys.Res., 82, 2650-2656, 1977.
Narcisi, R. S., and E. P. Szuszczewicz,Direct measurementsof electrondensity,temperature and ion composition in an equatorial spreadF ionosphere, d. Atmos.Terr. Phys.,43, 463-471, 1981. Ossakow,S. L., SpreadF theories- A review,d. Atmos.Terr. Phys.,43, 437-452, ! 981. Patra, P. K., V. K. Ariandan, P. B. Rao, and A. R. Jain, First observation
of equatorialspreadF from Indian MST radar, Radio Sci., 30, ! 159-1165, 1995.
PrakashS., and S. Pal, Studiesof electrondensity irregularitiesduring strongspreadF, Adv. Space.Res., 5(7), 39-42, 1985. Raghavarao,R., S. P. Gupta, R. Sekar, R. Narayanan,J. N. Desai, R. Sridharan,V. V. Babu,and V. Sudhakar,Insitumeasurements of winds,electricfields and electrondensitiesat the onsetof equatorial spreadF, d. Atmos.Terr. Phys.,49, 485-492, 1987.
Raghavarao,R., R. Sekar, and R. Suhasini,Nonlinear numerical simulationof equatorialspreadF - Effectsof windandelectricfields,
Adv. Space.Res., 12(6), 227-230, 1992. is significantlylesscomparedto Hall movementbeyond250 km Rao, P.B., A.R. Jain, P. Kishore, P. Balamuralidhar, S.H. Damle, altitude, due to the exponentialdecreasein ion-neutralcollision and G. Viswanathan,Indian MST radar, l, Systemdescriptionand frequency. Thus it is rather difficult to envisage the image samplevector wind measurements in ST mode, Radio Sci., 30, striationsin the altitude region above250 km to accountfor the 1125-1138, 1995. presenceof the irregularitiesdiscussedin the presentpaper. Sekar, R., and R. Raghavarao, Role of vertical winds on the Further, the simulation results presentedherein, provide the Rayleigh-Taylor mode instabilities of the nighttimeequat9rial possibilityof the generationof the irregularitiesexactlyover the ionosphere, d. Atmos.Terr. Phys.,49, 981-985, 1987. magneticdip equatorin contrastto thatdiscussed by I/ickreyetal. Sekar,R., and R. Raghavarao,Criticalrole of the equatorialtopsideF regionon the evolutionary characteristics of the plasmabubbles, [1984], which is for regionsaway from the dip equator. Geophys. Res.Lett.,22, 3255-3258,1995. Anotheraspectis the growthof irregularitiesin the topsideof the ionosphere wheretypically oneencounters negativegradients. Sekar, R., and R. Raghavarao,A case study on the evolutionof equatorial spreadF by a nonlinear numerical modelusingtheresults The growth in the lower F region with negative gradient is from a set of co-ordinatedmeasurements, d. Atmos. Terr. Phys., 59, different from their appearancein the topsideof the F region. 343-350, 1997. Basically, the bubble is generatedat the baseof F layer which Sekar,R., R. Suhasini,andR. Raghavarao, Effectsof verticalwindsand movesupward nonlinearlydue to the polarizationfield, and it electricfields in the nonlinearevolutionof equatorialspreadF, 3•. eventually penetratesbeyondthe F region peak and therebythe Geophys.Res., 99, 2205-2213, 1994. appearanceof the irregularities in the topside is explained Sekar, R., R. Suhasini, and R. Raghavarao,Evolution of plasma [Ossakow,1981]. Once the irregularitiesappearon the topside bubblesin the equatorialF regionwith differentseedingconditions, Geophys.Res. Lett., 22, 885-888, 1995. their further evolutiondependson the penetrationof the fringe ß
field andalsoon the background ionospheric plasmadensity distributionsin the topside [Sekar and Raghavarao, 1995]. A depletedregion like a bubblegeneratedat the baseof F region, particularlywhen the baseis around350 km and above,cannot descend
downward
under
normal
circumstances
to
cause
irregularitiesin thelowerF region.Thusthepenetrationof fringe fields associatedwith the bubblebecomesimportantin order to explain the generationof irregularitiesin the lower F region. Conclusion
Sinha, H. S.S., R. N. Misra, H. Chandra,S. Raizada,N. Dutt, and G. D. Vyas, Multiwavelengthoptical imaging of ionosphericplasma depletions,Indian d. Radio SpacePhys.,25, 44-52, 1996. Sridharan,R. et al., 'Ionization Hole Campaign' - A co- ordinated
rocketandground-based studyat the onsetof equatorial spreadF: First results,3•. Atmos. Terr. Phys., in press, 1997.
Vickrey, J. F., M. C. Kelley, R. Pfaff, and S. R. Goldman, Low altitude image striationsassociatedwith bottomsideequatorial spreadF: Observations andtheory,d. Geophys.Res.,89, 2955-2961, 1984.
Weber, E. J., J. Buchau, H. Eather, and S. B. Mende, North-South alignedequatorialairglowdepletions,d. Geophys.Res.,83, 712-716,
It hasbeendemonstratedby nonlinearnumericalsimulation usingrealisticthermospheric and F regionparametersas inputs 1978. that the penetrationof fringe fields associatedwith a plasma Woodman, R. F., and C. La Hoz, Radar observationsof F region bubbleat the baseof the F regionis capableof generatingplasma equatorialirregularities,d. Geophys.Res., 81, 5447-5466, 1976. density irregularitiesin the lower F region where the initial Zalesak, S. T. and S. L. Ossakow,Nonlinear equatorialspreadF: conditionsare otherwise unfavorable, thus offering a viable Spatiallylarge bubblesresultingfrom large horizontalscaleinitial perturbation,d. Geophys.Res., 85, 2131-2142, 1980. explanationfor manyof the insituplasmadensitymeasurements. Acknowledgments.We thank R. Suhasinifor her help withoutwhich it wouldhavebeendifficultto developthe numericalmodel.Our thanksare due to S. R. Das for the analysisof massspectrometerdata.The work is supportedby the Departmentof Space,Governmentof India. The Editor thanksM. C. Kelley and C. La Hoz for their assistance in evaluatingthis paper. References
Haerendel, G., Theory of equatorial spread F, report, Max-Planck Inst. fur Phys. Und Astrophys.,Garching,West Germany, 1974.
Zalesak, S. T., S. L. Ossakow, and P. K. Chaturvedi, Nonlinear
equatorialspreadF: The effect of neutral winds and background Pedersenconductivity,d. Geophys.Res., 87, 151-166, 1982.
R. Raghavarao,R. Sekar, and R. Sridharan,Physical Research Laboratory,Navrangpura,Ahmedabad-380009, India (e-mail: rsekar•prl.ernet.in)
(ReceivedMay 31, 1996; revisedApril 28, 1997; acceptedMay 9, 1997).
