Nov 1, 2000 - G. D. Earle and R. L. Bishop. William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas. S.C. Collins.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. All, PAGES 24,951-24,961,NOVEMBER 1, 2000
Descendinglayer variability over Arecibo G. D. Earle and R. L. Bishop William B. HansonCenterfor SpaceSciences,Universityof Texasat Dallas,Richardson, Texas
S.C.
Collins
ElectricalEngineeringDepartment,CornellUniversity,Ithaca,New York
S. A. Gonzfilez and M.P.
Sulzer
NationalAstronomyandIonosphere Center,AreciboObservatory, Arecibo,PuertoRico
Abstract. Descending layersof ionizationoverAreciboexhibitvery diversebehaviorfrom nightto nightthat doesnot appearto be stronglycorrelatedto geomagnetic activity,solar forcing,or averagesemidiurnal tidal winds. On somenights,threeor moredistinctlayersare observedto form near 170 km over timescalesof ~2 hours. Rathertha_ndescendingsmoothly overperiodsof severalhours,theselayersstall,abruptlydisappear,or evenreversedirectionin the midstof their descent.The time scalesfor their disappearance are examinedand compared to lossratesarisingfrom diffusionandrecombination.Diffusionaloneis foundto be too slow to accountfor the observations, but recombinationis fast enoughprovidedthat the convergent wind shearthat formsthe layer is relativelyweak coincidentwith their disappearance. The continuityequationis solvedin conjunction with a time sequence of radarprofriesto estimate the vertical drift and horizontal neutral wind consistent with the observed behavior. The
resultant windfieldis northward, hasanaverage speedof ~80m s't, andvariessignificantly nearthe altitudewherethe layersare observed.Theseinferredwindsare consistent with the presence of the observed layers,andtheirmagnitudes as obtainedfromthe classicalcontinuity andmomentumequations are reasonable for thisaltituderange.
1. Introduction
Layersof enhanced ionizationthatform at the bottomof the F regionandgradually descemd throughthe E-F valleyregionof the ionospherehave been studiedwith a wide variety of techniques overthelasthalf cemmry.McNicol and Gipps[1951] utilizedionosonde observations andreportedthe appearance of two distinctlayer typesin the E region,notingthat one type appeared to formin situwhilethesecxmd formedat highaltitudes anddescemded to thelowerE region. Furtherionsonde studiesby MacDougall[1974]investigated E regionplasmalayersandtheir relationshipto zonalwindsat a varietyof stationsaroundthe globe. This studyused observations of sporadicE layers at widelyseparated stations to infercharac•cs of the zonalwind in thelowerE region.In a laterstudy,MacDougall[ 1978]noted theappearance of descending layersin ionograms andlinkedtheir periodicityto long-periodtidal oscillations. These early observationsprovidedgood agreementwith theoretical treatments of E regionlayer formationby Whitehead [1961]andAxford[1963]. Thesetheories showedhow horizontal wind shearsin the lower E region could produceconvergent vea'tical ion driftsvia collisionalcouplingbetweenneutralsand
ions. In theupperE region,meddional windsare moreeffective
Copyright 2000by theAmericanGeophysical Union. Papernumber2000JA000029. 0148-0227/00/2000JA000029 $09.00
at producing suchlayers,but at loweraltitudesthelargercollision frequencymakeszonal windsmore effective. Followinga procexture similar to that of MacDougall[1978], Tong etal. [1988] usedincoherentscatterradar (ISR) data to revealthepresence of long-period variations in descending layers overtheAreciboObscn'va• (18.35' N, 66.75' W). Shenetal. [ 1976] showeda seriesof densityprofiles that illustratedthe steadydownward progression of thelayersoverperi• of several hours. Mortonet aL [1993] and Mathews etal. [1993] usedthe sametechnique duringtheAreciboInitiative in Dynamicsof the Atmosphere(AIDA) campaignto confirmthe regularquarterdiurnalperiodicityof the layers. On the basisof theseresults theypostulated a tidal influenceas the causeof the periodicity. All of theseradarstudieswereperformedat Arecibo,whichhas the ability to view low-densitydescending layersdespitethe presence of lower-altitudesporadicE. High-densitysporadicE layersprecludeobservations of weakerdescending layersby ionosondes, so Arecibois uniquelysuitedto studylow-density layeringphenomena. In contrast, ionosonde observations havethe distinctadvantage of globalcoverage,makingthemable to infer charac•cs that are commonover large scales. $zuszczewicz etal. [1995]exploited thiscapabilityto studydescending layers present in stmlitconditions at severalstationsaroundthe globe. Descemding layershavealsobeenstudied usingin situprobes. Smith [ 1970] studiedthe wind field aroundlayersat Wallops Island(37.95' N, 75.47' W) usinga sequence of five chemical releasepayloadslaunchedat 90-rainintervals. A companion
paperby Bishopetal. [thisissue]describes windmeasurements froman instormented rocketpayloadthat waslaunchedthrough 24,951
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a weak, high-altitudedescendinglayer over Arecibo. Heelis [1999]examined theion composition of descending layersabove 130 km using data from the AtmosphericExplorerC (AE-C) satellite.The AE-C datasuggest thatthe layerscomprisemainly molecular ions when they begin their descent. However, the presenceof significantmetallic ion contentin the E regionhas been confirmedusingboth satellites[Miller etal., 1993] and rockets[Grebowskyetal., 1998]. Numericalstudiesof layeringprocesses in the upperE region by Ostermanet al. [1994, 1995] suggestthat the compositionof descending layersmay be dominatedby molecularions at high altitudes,changingto mainly metallic ions as they descend. Carterand Forbes[1998]performeda complementary numerical studyof E regionmetallic ion layers,with an emphasison the global distributionand transportof metallic ions and their ultimateannihilationthroughchemicalprocesses. Thesepreviousstudiesfocusedon the normalbehaviorof the layers, including their composition,the periodicity of their appearance,or the wind fields associatedwith their gradual descentfrom the bottomsideF regionto altitudesof ~ 110 kin. In thispaperwe contrast thesecharacteristics with ISR evidence of complexlayerbehavior,inclu• short-lived descending layers thatabruptlydisappear midwaythroughtheirdescent.We restrict ourstudyto postsunset periodsbecausethesewere the focusof therocketcampaignthat Arecibo was supportingwhen the data wereobtained.Descending layersalso occurin the daytime,but becauseof the higher backgroundplasmadensityin the E-F valley region,they are lessprominentin radarpowerprofiles. The causesof the abruptdisappearance of layersduringtheir d•t throughthevalleyregionare investigated, includingthe
TheAreciboObservatory operates at 430 Ml4_z,andduringthe campaignit wasoperatedin threedifferent observationalmodes.
The first wasa high-resolution (150 m) mode,utilizingan 88baud codingschemeat 1 [xsper baudandmeasuring electron densities with a resolutionof 150 m [Mathews, 1986]. The
second modeuseda 13-baudBarkercodingschemeat 4 [xsper baud [Ioannidis and Farley, 1972]. In this mode, electron densities were measured with 600-m resolution.
The third mode
of operationmeasuredthe spectralpropertiesof the plasma [$ulzer,1986]. Datafrom thismodeare not usedin our study. Priorto March5 thehigh-resolution modewasusedexclusively. Overthe rest of the campaignall threemodeswere interleaved throughoutthe observational period. When all 3 modeswere interleaved eachwasrunfor 10-s. An interpulseperiod(IPP) of 10 ms was used, and ~1000 profiles were integratedin postprocessing to producea powerprofile of the ionosphere. Densitieswere obtainedby scalingthe resultingprofiles to measurements made at the F2 peak by an on-site ionosonde. Whenthehigh-resolution modewasrunexclusively, the temporal resolution was 10 s. When all three modes were interleaved the
temporalresolution became~30 s for both the 150-m and 600-m resolution measurements. Exceptfor a few daysin early March all radar observations were made at vertical incidence.
With thesemodesof observation theradarprovides reasonably accurate density profiles throughoutthe E region of the ionosphere. In our experienceduring the campaignthe background noiselevel for measuring the ionospheric E region
density wastypicallysomewhat lessthan1000cm-3. For lower
densities thesignal-to-noise ratio in the E regionbecomessmall enoughsothatadditionaltime averagingis necessary to resolve effects of diffusion and recombination. We evaluate the relative geophysicallysignificantfeatures. The advantageof using to investigatedescending layers importance of several mechanismsto explain the abrupt Areciboinsteadof an ionosonde disappearance, and we calculatethe vertical ion drifts and is its ability to observethe layerseven whentheir densityis meridional wind profiles that are consistentwith the observed lowerthanthat of an underlyingsporadicE layer. In addition, layerstendto standoutby virtueof their motion. By behavior. We also investigatethe correlationof enhancedE descending of densityprofilesthat are updatedevery 30 regionion contentwith geomagnetic and solarindices,andwe viewinga sequence s, onecan detecteven low-densitylayers,becausetheir motion find no significantcorrelation. Sinceourdatasetis limitedto Arecibo,it is impossibleto say stands outagainstthebackground noiselevel. whethertheobservedcomplexbehavioris uniqueto this site or Figure1 shows twogray-scale plotsof ISRdatatakenshortly typicalof descending layerbehaviorat all locations.It is quite after sunset.Bothpanelsin Figure1 showlayeraltitudeand
density versus time,withthegray-scale levelindicating difficult toinvestigate thisissue using ionosonde data,since it is plasma Thetopplotshows datafromMarch21, 1998,and often necessary toexamine complete E region density profries and thedensity. plotshows thenightof March 22, 1998.Thevertical theirvariations with timein orderto unambiguously detectthe thebottom striping evident in the figure that occum at ~2230in thetoppanel layers. Ionosondes donotprovide thiscapability inanyroutine panelarecaused byradio way.If complex behavior similar towhat weobserve atAreciboandshortlyafter2300in thebottom interference. In bothpanels a singlelayerbegins to is typical of global descending layermorphology, thenlayer frequency
shortlyaftersunset, andthedescent continues observations mayprovide a useful waytoassess thevariability of formanddescend overa period of several hours.Therelatively smooth descent is tidalwinds andgravity waves atmidlatitudes. Moreobservations punctuated by intervalswhenslightupwelling froma widervariety ofstations arenecessary to investigate this. occasionally
occum.In bothcasesthedescending layergraduallyapproaches
the altitudeof sporadic E layers,with whichtheysometimes merge in theearlymorning hours[$henetal., 1976]. Therate forthese layersis initiallyquitefast,~35-50kmh4. TheE1CoquiDossounding rocketcampaign tookplacein ofdescent slows rather abruptly to -5 kmh4 oncethe PuertoRicoduringFebruary andMarchof 1998. Duringthe Thisrapiddescent layer drops below 135 kin. The descent is continuous; thereare campaign theArecibo ISRprovided nightly ground-based support from view. These for thelaunches. Thedataobtained duringthisperiodprovidean no intervalsin whichthe layerdisappears aregenerally considered thenormfor descending layers excellent resource forstudying descending layersoverArecibo, trends of theirsimilarityto theearlier because thefocusof therocketcampaign wastheE regionand overArecibo,largelybecause by $henetal. [1976]andMathewsetal. [1993]. upper mesosphere. During lateFebruary toearlyApriltheradar observations These general trendsare shownhereto establish a baseline wasoperational andsupporting thecampaign for35nights, and whichto compare datafromothernights, whena wide d•ding layers in theE region wereobserved on33of these, against of variability isobserved in thebehavior of thedescending or over90% of the time. Representative casesfromthese range
2. Data Presentation
observations willbepresented here,butthefulldatasetcanbe layers. Figure2 shows several examples of verydifferent layerevents viewed ontheinternet at http://129•110.7.5/~e•rle/obs_list.html.
Gray-scale range-time-intensity plots foreach nightareavailableoverAreciboin March 1998. onthewebsite simplyby selecting a particular date.
The local time intervalplottedin
Figure 2 isthesame asthatin Figure1, andall datain thisplot
EARLE ET AL.: LAYER VARIABILITY
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Figure 1.Incoherent scatter radar data from theArecibo Observatory showing descending layers ofenhanced plasma density inthepostsunset period. Thelayers initially descend very rapidly, butthedescent slows below altitudes of - 140 kin.
wereobtainedwithin a few weeksof the data shownin Figure 1.
Despitethe similar timesand datesof the observations, the behaviorof the descending layersis quite differentin Figure2.
abruptdisappearance of thefirst layerin section3 of thispaper, emphasizing thetimeperioddelineated by thedottedlinesin the middle panel.
In thebottompanelof Figure2 datafromMarch26, 1998are by theformationof sequence of two or morelayersappearing in rapidsuccession, shown.Thisnightwasalso characterized rather thana singleevent. This succession of layersformsat two distinctlayers,but in thiscasethey appearto form almost2 aboutthesameinitialaltitudeover a periodof a few hours,and hoursapart.The fn•'tlayerfadesawayjustbeforelocalmidnight, initiallytheselayersbehavemuchlike the layersin Figure 1. In at aboutthesametimeas thesecondlayerapproaches its altitude.
The mostobviousdifferenceis that eachnight showncontainsa
fact,theirinitial ratesof descentmatchthoseof the layersshown in Figure 1 very closely. The datafrom March6, 1998 (top panel)showat leastthree
Fifteen minutes later, this secondlayer experiencesa strong upwelling that lasts for over 30 min. The layer disperses somewhatduringthis excursionbut later convergesagain and
distinctlayersbeginning to format -2015, 2040, and2120 local time. Each successive layer descendsa bit farther than the preceding onebeforeitspeakdensityabmpdydrops.The second and third layers have tenuousconnections to the gradually intensifyinglayer at -115 kin. This low-altitudelayer is the remnantof a descending layer that beganits descentbefore sunset. The peak densityof the lower-altitudelayer increases shortly after the fade-outof the higher-altitude layer. This behavior suggeststhat the ions from the higher-altitude descending layermaybe spreadoutovera broaderaltituderange as they descendfrom -140 to -120 kin, where they again converge to produce thehigherdensityof thelower-altitude layer. The middle panelof Figure2 showsdata from March 16,
continues to descend.
1998. In thiscasethereare two distinctlayersformed at -170 km withina timespanof only 30 min. The first abruptlyfades out at 2215 LT near 135 kin, at about the same time as the
secondlayer intensifies15 km above. In this casethereis no obvious connection to any lower-altitudelayer. We discussthe
The abruptdisappearance of severalof the descending layers in Figure 2 was a fairly commonevent over Arecibo during March 1998. In fact, over our 2-monthdatabasespanninglate Februaryto earlyApril, it actuallyoccursmuchmore frequently than the so-calledclassicalbehaviorshownin Figure 1. The numberof byersobserved on anyonenight,the intervalbetween successivelayers,the altitudeat which they fade out, and the concurrent behaviorof higher- and lower-altitudelayersduring thefade-outare all variableduringtheseevents. However,the overall morphologyof the layers themselvesis similar in all cases.
Figure3 usesthesameformatto showtwo more examplesof datafrom a nightjust beforeand 1 week after the observations shownin Figure 1. We includethesedatato demonstrate still more variabilityin the appearance and behaviorof descending layers over Arecibo. The top panel showsthe postsunset ISR data from March 19, 1998. In this examplea layer that began
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descending priorto sunset dropsfrom~125to 120km by 2100 pointed outthatduring periods of enhanced magnetic activity,an LT. It continues to descend andintensify,butthenit bifurcates increased semidiurnal temperature oscillation is expected, which near2200LT. Thirtyminutes laterthetwoseparate layersmerge mightinfluence layerdevelopment. However, theyalsonoted at ~105kmandthenfadeaway. At aboutthe sametimeasthe thatincreased iondragstemming fromhigherplasma densities in bifurcafion occtns a weaklayerformsat 170kin,onlyto descend theE region coincident withincreased magnetic activity might •pidly to 150kmbeforedisappearing. A second layerrepeats partiallyoffsetthiseffect. Theirdatashowed no statistically theprocess but descends to almost140 km coincidentwith tthe significant evidence for a correlation between layeroccurrence mergingof thebifurcatedlow-altitudelayer. probability andthelocalK indexof geomagnetic activity. Thebottompanelof Figure3 presents thedatafromMarch28, Figure4 shows solarandgeomagnetic indexdatafor a period 1998. Thisnightis characterized by a succession of aboutseven including thenightsshownin Figures1-3. Thefirstnighton
weaklayersthatinitiallyappearbetween altitudes of 120 and 140 whicha layerformed anddescended to sporadic E altitudes kin. Eachsuccessive layerfirst appears somewhat abovethose without interruption or complicated motions (day80)coincides preceding it, sothegeneraltrendof thelayersis upwardwith with a spikein Av,butno distinguishing characteristics are time. Onlyoneof the layerson March28 followsthe standard apparentin anyotherparameter compared to therestof the progression of beginning at highaltitudeanddescending rapidly month.Days80 and81 weretheonlynightsin themonthof to themiddleE region. Thesedatahave somesimilaritiesto the ion rain phenomenon describedby Mathewset al. [1997].
March forwhich theavailable ISRdatashow descending layers withrelatively highdensities andcontinuous smooth trajectories
Mathews [1998]disc•es thepossibility thatthisphenomenon is through theE region.However, Figure4 shows thatsimilar
relatedto the Perkinsor gradientdrift instabilities. Thereis somepriorevidence thattheplasmacontentin the EF valleyregionoverAreciboon a givennightis relatedto the intensity of geomagnetic and/orsolaractivity[Rowe,1973]. This early statisticalstudyfound a correlationof 0.73 betweenthe time-averaged integrated ion contentin the 130-160km altitude rangeand the 12-houraveragedmagneticindexmeasured at Fredericksburg (Av,). Only 17 nightsof datawereusedin the
geomagnetic andsolarindices occurred onseveral other nights, whenthelayers eitherdidnotdescend fullyor wereunusual in someregard.
Thepossibility of a correlation between layerbehavior and solar fluxisintriguing, since horizontal winds areresponsible for forming thelayers anddrivingtheirdescent [Constantinides and Bedinger, 1971;Bishopet al., thisissue].Increased solarflux
should strengthen thedifferential atmospheric heating thatdrives
study,andthesewerespreadoveran entirecalendar year. A thetidalwindsystem, whichcouldaffectboththeformation and morestatistically significant studywascarriedoutby Rodgeret downward progression ofthelayers.Thediamonds ontheplot a/. [1981]overSouthGeorgiaIsland(54' S, 37' W). They in Figure4 showthetime-averaged ioncontent between 130and
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LocalTime (AST) :•ure 3. Moreexamples of layervariabilityoverAreciboin March1998. (top)BJ•a'caQon of a low-altitudelayer coincident with theformationof weaklayersat highaltitude.(bottom)A succe•on of layersthat progressupward over time.
160 km for 20 nightsfor whichwe haveradardata. These20 ion contentin the nighttimeE region. Our resultstherefore nightswerechosen because theradaroperated in thesamemode contradict theearlierstudyby Rowe[1973]butarein agreement overtheperiodfromnights65 to 92, sotheintegration yields with the later studyby Rodgeret al. [1981]. Rowewasnot data that can be comparedfairly. The total ion content studying descending layers specifically butwasinstead examining (equivalent to totalelectron content, TEC) wascalculated by general plasma content in theE-F valleyregion.Oneprincipal performing anhourlyaverage dmSng all intervalswhentheradar differencebetweenour studyand that of Roweis that Rowe wasoperating normallyandno interference waspresent.The founda correlation witha specificregionalA index,Aesfrom rangeof timescoveredby thesehourlyaverages coincides with Fredricksburg.A seconddifferenceis that the Rowe useddata thetotaltimeintervals plotted in Figures 1-3,whendescendingfrom17nights distributed randomly overa calendar year,while layers mostcommonly appeared. Theheightintegration hasbeen we havea similarnumberof successive nightsduringa quiet carded outover130-160kmbecause thatregioncontains all the geomagneticperiod. These factors could contributeto the
descending layersweobserved, andto allowcomparison to the disparateconclusions. work of Rowe[1973],who usedthis heightrangefor his To investigate theabrupt disappearance of thelayerson some correlationstudy. nights,we haveextracted a time seriesof individualdensity Osterman et al. [1994,1995]pointoutthatdescending layers profiles fromoneof thenights shown in Figure2. Theparticular mayinvolve a simple redistribution of ionsintoanenhanced layer eventwe areinterested in is theabruptfadingof thelayerthat dueto neutral windaction.In sucha casetheplasma density occurs between 2210 and 2230 LT at -135 kin. Each of the above and/orbelowthelayeris reduced, andtheintegrated ion curvesin Figure5 is theresultof averaging 4-rainof consecutive contentin theE regionis approximately constant despitethe density profilesfromtheradarandthensmoothing theprofile appearance of a layer. If this hypothesis is correct,then a witha rutming average.Theaveraging professreduces thenoise correlation between ioncontent in thisaltitude rangeandsome levelin therawdatabutdoesnotalterthealtitudeor density of
geophysicalindex is not indicative of a correlation with descending layers.
thelayerpeak. Threeprofilestakenover a 20-rainintervalare plottedin Figure5 to allowcomparison of thenumberdensities
Thereis no strongevidence in Figure4 for a correlation asa function of time. Between 2210and2220LT thepeakof between E regionioncontent andanygeophysical index.The thelayerdescends by -5 kmandthepeakdensity dropsby 25%. peaks inA•,neardays69and80 correlate to largerthanaverage Overthenext10-minthealtitudeof thelayerpeakremainsabout integratedion content,but suchenhancedion contentalso occurs
thesame,butthe densitydropsby another37%. Lessthan20-
ondays 65,66,and88,when theApindexis depressed. Neither rainis therefore required for thedensityto decrease by a factor thesunspot numbernor theF•0.7indexcorrelates with enhanced of 2. Thecaseshown is particularly interesting sincethelayer
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