Sep 1, 1998 - Regions) program, involved many instnanents and technique devel- opments .... the Mass Spectrometer/Incoherent Scatter (MSIS-86) model ..... Altitude profiles for seasonally averaged northward (top) and eastward (bottom) ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 103, NO. A9, PAGES 20,715-20,726, SEPTEMBER
1, 1998
Climatology and variability of the semidiurnal tide in the lower thermosphereover Millstone Hill L.P. Goncharenko
and J.E. Salah
HaystackObservatory, Massachusetts Instituteof Technology, Westford
Abstract.Data obtainedby the MillstoneHill incoherentscatterradarduringLowerThermosphere CouplingStudycampaignsin 1987-1997were analyzedin orderto determinethe characteristics of the semidiurnal variationsin horizontalneutralwindsandtemperatures at midlatitudes in the altituderange of 95-130 km. We presentaltitudeprofilesof amplitudeandphaseof the semidiurnalvariationsfor climatological meandata,seasonally averageddata,aswell asindividualmultidaycampaigns. The climatologicallymeansemidiurnalamplitudeattains~50m/sfor eastwardwind, ~35m/sfor the northwardwind, and ~27 K for thetemperature. At mostaltitudes,the standarddeviationis 50-70% of themagnitudeof the climatological meanvalue,anda largepartof the observedvariabilityis dueto seasonal differences.The strongest semidiurnaloscillations arefoundfor equinoxandsummerwinds, reaching80-90 m/s,whilewinteramplitudes aregenerallysmaller.Summerwindsarecharacterized by a consistent patternin amplitudeandphase;however,winterwindsaremorevariablefrom campaignto campaign.Similarly,temperatures reveallow variabilityin phasefor the summerseasonandhigh variabilityin winter,buttheamplitudes of the 12-houroscillations in temperature arevariablefor all seasons. Comparison with otherexperimental datasuggests goodagreement in generalbut larger variability.We discuss thepossiblesources of the observed variabilityandlink it to processes in the mesosphere.
paper,we address the issueof semidiurnal tidal structure of horizontal neutral winds and temperature.Section 2 containsa Understandingthe characterictics of the neutralwind andtem- descriptionof the LTCS datasetandthe wind calculationmethod. peraturein the ionospheric E regionis a crucialelement in the In section3, we identifythe dominanceof the semidiurnaltide in investigation ofthermosphere-ionosphere couplingandin thestudy our data.In section4, we presenttidal characteristics of the horiof interactionsof the lowerthermosphere with the upperthermozontalneutralwind for threedifferentaverages: a climatological spherethroughwavepropagation.Thoughsignificant progress was meanutilizingdatafrom all campaigns, seasonal means,and indiachievedbefore1980(seereviewby Evans[1978]),therewasstill vidualmultidaycampaign averages. We alsopresenta quantitative a lack of experimentaldata,especiallyin the 90-130 km altitude measureof tidal variabilityand discussthe comparison of our intervaldueto technicaldifficultiesfacedby variousinsmnr•ents to resultswith a previousMillstoneHill studyby Wand[1983]. Secprobethisregion.The formationof theLowerThermosphere Coution5 contains theresultsof temperature dataanalysis conducted in pling Study (LTCS) campaigns,organizedin 1987 by the NSF thesamemannerasthewinddataanalysis. Finally,in section6, we CEDAR (Coupling,Energeticsand Dynamicsof Atmospheric compareourresultswith experimental datafrom Sondrestrom and Regions)program,involvedmanyinstnanents andtechnique develEISCATandwith a climatological studyfrommediumfrequency opmentsand was a major stepin the coordinatedstudyof the andmeteorradars.We alsodiscuss theobserved variabilityin our dynamicsof the mesosphere and lowerthermosphere. The Milldatacomparedto thevariabilityreportedfromothermeasurements 1. Introduction
stoneHill incoherentscatterradarlocatedat 42.6øN,71.5øWis one
and indicate the connection of the semidiurnal tide in the lower
of the instruments that has participated in the LTCS studysince thermosphere at midlatitudes withprocesses in themesosphere. 1987.Previouslypresented resultsfor someof thetechniques and locations include incoherent scatter radar data from Millstone Hill
[Salah,1994],Sondrestrom [AzeemandJohnson, 1997],Europian 2. Observations and Wind Derivation IncoherentScatter(EISCAT) [Kunitakeand Schlegel,1991;KofThe MillstoneHill observations reportedherecover15 multiman et al., 1996] and Arecibo [Zhou et al., 1997], medium day campaigns conducted between1987and1997asa partof the frequencyandmeteorradardata[Mansonet al., 1989;Namboothiri LTCS initiative.The datesof the campaigns andthe geophysical et al., 1993], as well as a numberof paperson multi-instrument conditions pertainingto themare listedin Table 1. This dataset measurements and comparison with models[Johnson and I4'rdi, resultsin a totalof 93 daysof measurements andrepresents large 1991;Denget al., 1997;Salahet al., 1994,1997].Thispaperfolvariety of geophysical conditions including periods with low, modlows this initial work and utilizes Millstone Hill results from all erate, and high solar activity,low and moderategeomagnetic LTCScampaigns conducted to datecovering1987to 1997.In this activity,aswell asdifferentseasons. Thissetisalsolargeenough to allowusto studythevariabilityof tidaloscillations in thewindand Copyright1998by theAmericanGeophysical Union. temperature of the lowerthermosphere. Thetechnique usedto collectandanalyzetheE region(90-150 Papernumber98JA01435. 0148-0227/98/98JA-01435509.00 km) dataat MillstoneHill hasbeendescribed in detailby Teten20,715
20,716
GONCHARENKOAND SALAH:CLIMATOLOGYOF SEMIDIURNALTIDE Table 1. Geophysical Conditions DuringLTCS Campaigns LTCS #
Date
F10.7
Ap
LTCS-1
Sept.21-25, 1987
77.5 - 82.4
10 - 46
0.7 - 6.3
LTCS-2 LTCS-3 LTCS-4 LTCS-5 LTCS-6
Dec. 6-10, 1988 June2-05, 1989 Feb. 12-17, 1990 March 14-22, 1991 Dec. 4-11, 1991
157.6 - 170.3 197.5 - 215.0 143.7- 155.2 244.4 - 277.4 190.6 - 260.3
3-8 7 - 19 6- 50 6 - 26 5 - 15
0.0- 3.3
LTCS-7 LTCS-8 LTCS-9 LTCS-10 LTCS-11
March30-Apr.3, 1992 July30-Aug.7, 1992 Jan.20-30, 1993 Aug. 9-16, 1994 May 1-5, 1995
159.7- 191.4 96.9 - 138.0 104.2 - 114.0 75.3 - 88.9 68.6 - 72.9
6 - 32 4- 35 4 - 25 4 - 27 3 -49
0.7 -6.0
LTCS-12 LTCS-13 LTCS-14 LTCS- 15
Oct. 23-27, 1995 March 19-22, 1996 Oct. 8-12, 1996 Jan.6-10, 1997
73.9 - 76.7 70.4- 73.8 66.7 - 68.5 73.1 - 75.0
3 - 19 8 - 38 8 - 20 2 - 31
0.0- 5.0
1.0- 5.0 0.3 - 6.7 0.0-6.7 0.3 - 4.3 0.7- 5.7 0.0 - 5.3 0.3 - 5.0
0.0 - 6.3
0.7- 5.7 1.0 -4.7 0.0-6.0
baurnet al. [ 1990]andSalahet al. [ 1991], and we will onlypresent to derivetheneutralwindvelocityin therangebetween130km and 300 km are currentlybeingundertaken, and the resultshavenot measureselectrondensity,temperature, andion drifts with ~4.2 km beenincludedin thispaper. The temperature resultsdescribed in thispaperaretypically resolution for analternating codewith a baudlengthof 28 gstransmittedin the zenithdirection.An averageion massof 31 amu is obtained from measurementsmade with the zenith-pointing assumed at altitudesbelow130km, andan ioncomposition model antenna and have an ~4 km altitude resolution. For some camis usedabovethisaltitude.Becauseof thelow electrondensityat paigns, fitting the measuredautocorrelationfunctionswith night,it isonly possible tomakeusefulobservations in theE region theoreticalfunctionswas not reliable below 106 km, and this altiduringdaytime. tudewas chosenas a lower limit for temperaturedata.The upper During the LTCS campaigns,the MillstoneHill radarwas limit of temperature datarepresented hereis the sameas for the operatedin a multipositionmode.Measurements weremadewith wind results,namely 130 km. a brief outline here. The incoherent scatter radar at Millstone Hill
both the zenith and steerable antennas. The steerable antenna was
pointedat 45o-60' elevation,generallynorthwardand eithereastward or westward dependingon clutter conditions.In later campaigns (LTCS-10to LTCS-15),theantennawaspointedto the north,west,south,andeast.The cycle time for theseexperiments was30-40min depending on thenumberof positions. The eastward andnorthwardcomponents of thehorizontalneutral windin theE regionaredetermined fromthesemultiposition
3. Harmonic Analysis In order to determine the
dominant oscillations in the
observedwind andtemperature, we usedthe Lomb-Scargle periodogrammethodof spectralanalysis,which was developedto process unevenlysampleddata [Presset al, 1992].Figure1 shows themeasured eastward windfor theLTCS-14campaign (October812, 1996)in the97-121km altituderange,togetherwiththespectra. Dashedlineson the spectrarepresent50%, 90%, and99% significance levels for different harmonics. A strong semidiurnal harmonicin theeastwardwind is seentogetherwith a small8-hour wave.Althougha 12-hourharmonicdominatesat altitudes97-130 km, the spectrareveala strongdiurnalcomponent at altitudes106 km andabove.However,sincethemeasurements areobtainedonly in daytime,the 24-hourharmonicis not considered to be a reliable
ion driftmeasurements assuming spatial(~100-200km) andtemporal (30-40 min) homogeneityand negligiblevertical wind velocity.Drifts causedby electricfieldsarecalculatedfrom simultaneous F regionmeasurements obtainedusinga longpulseof 410 gs duration(21 km altituderesolution),andthe ion-neutralcollisionfrequency usedin thecalculations isbasedondensitygivenby the Mass Spectrometer/Incoherent Scatter (MSIS-86) model [Hedin, 1987]. The horizontalneutralwindresultspresented heretypically determination of a true 24-hour wave but rather a result of the modcover94-130km altituderangeandareobtainedwith 3 km altitude ulationin the dataacquisition. resolution. Someof theearlierobservations andwintercampaigns Anotherfeaturepresentin Figure 1 is the decrease in thenorhave a lower altitude limit of 100 km. On occasion contamination malizedspectraldensityfor the 12-hourharmonicat 121km. For thisdecrease wasobservedat altitudesof 124-127 by interference at low altitudesresultedin reliableobservations somecampaigns onlyabove106-109km. Therefore,for the climatological averages km, but for mostexperiments, the semidiurnalcomponent is domiof the data,the lowestaltitudewaschosenas 100 km. The upper nantup to 130km. This decrease representsthedissipation of the limit of 130 km represents the altitudewheretheanalysisassump- 12-hourwavein thevicinityof 130 km. tionschangeowingto the absenceof plasmathermalequilibrium Thetidalanalysis in thispaperisperformed ontheassumption andvariationsin ion composition. In addition,spectral' analysisof of a dominant semidiurnal wave. As the radar-derived wind and the data, as will be discussedin section3, confirmsthe dominance temperature uncertainties in theE regionarelarge(10-20m/s)and of the semidiurnal tide at altitudes below 130 km and indicates disthenumberof datapointsislimited,it isdifficulttoanalyzethedaysipationof semidiurnaltide in the vicinity 130 km. Thuswe have to-dayvariabilityof the semidiurnal tide withineachcampaign. focusedour studieson the semidiurnal oscillation.Someattempts Althoughthereareindications of largeday-to-day variability, the
GONCHARENKO AND SALAH: CLIMATOLOGY OF SEMIDIURNAL TIDE Eostword wind
97kin
20,717
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10
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30
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20
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109km
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100 tI•{ • +t{{i•• t 1• 15 -oo I+{{{ -200
0
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20
40
60
80
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10
20
50
Figure 1. Horizontaleastward wind(left) andLomb-Scargle periodograms of eastward wind(right)duringOctober 8-12, 1996.In theperiodograms, 50% (dottedlines),90% (dashedlines),and99% (dot-dashed lines)significance levels are shown.
uncertainty of the 12-hourfit for eachseparate dayis notsufficient to deriveconclusions aboutday-to-daytidalvariability.We obtain the tidal characteristics by fittinga semidiurnal waveto the data averaged overeachcampaign period.Therearethreedifferentaveragesdescribed in thisreport.First,we calculatetheaveragewinds (or temperatures) for all daysof eachcampaign by sortingthedata into 1-hourbins,andthenwe fit theaverageddatain eachbinwith 12-hourwavesto determine the tidalcharacteristics for eachspe-
cificcampaign. An exampleof thisprocedure is shownin Figure2. The dataareseento bewell-fittedby the 12-hourwave,anda downwardphaseprogression withaltitudeis clearlydiscernible. Second, seasonal averages are obtainedby averaging the datafor all campaignsin a particularseason asdenotedin Table2 andby fittingthe averageswith semidiurnalwaves.Finally,for the climatological averagetide,we usedatafromall 15 LTCS campaigns asa single data set.
20,718
GONCHARENKO AND SALAH: CLIMATOLOGY OF SEMIDIURNAL TIDE 100km
•
103km ,
2OO
.
,
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o
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10
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EST (hr)
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106km
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20
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Figure2. Hourlyaveraged horizontal eastward windduringOctober8-12, 1996.Thesolidlineis theleastsquares fit to thesemidiurnal component.
4. Semidiurnal
Winds
andfall. For spring,maximumamplitudes of 80-95 m/speakin a narrowaltituderegion,butthealtitudeof themaximumchanges in 4.1. Seasonal Variations in Winds a wide rangefrom 109 to 115 km. The differencein phaseoften exceeds1 hour,especiallyfor altitudesabove115 km. Fall camThe individual tidal characteristicsof the semidiurnal wave for paigns havesomewhat smallermaximumamplitudes, ~70-80m/s, all 15 LTCScampaigns, separated according to season,arepresented in Figure3 fortheeastward windcomponent andin Figure with the heightof maximumvaryingfrom 112to 122km, andthe 4 forthenorthward windcomponent. Fortheeastward component,differencein phasesare~1-2 hours.Thelargestvariationin thetidal
a largevariability in amplitude is seendepending onseason. Large eastward windamplitudes areobtained in spring,summer, andfall, and muchsmalleramplitudesare foundin winter.Summereastwardwindsarecharacterized bymaximumamplitudes of 60-80m/ s, andthesamepatternin amplitude andphaseis foundin all four summercampaigns. Maximumsummeramplitudes showa narrow peakat altitudesof 106-109km, although,in thecaseof LTCS-3 (June2-5, 1989),maximumamplitudes of~80 m/swereobserved in the 103-118km altituderange.Summereastwardwind data
revealveryconsistent phases withdifferences typicallyLTCS-11 .LTC s-10 100
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Figure 4. SameasFigure3, butfor northwardhorizontalneutralwind.
considered all datafor thesecampaigns asa singledatasetin order to obtainthe equinoctial tides.As wintercampaigns arecharacterizedby a largevariabilityin winds,it is difficultto obtainseasonally averaged semidiurnal components for boththeeastward andnorthward winds. However, we were able to obtain the average semidiurnaltide for winter by excludingthe mostdifferentcampaign(LTCS-6,December4-11, 1991,seeFigures3 and4) from the analysis. As shownin Figure5, semidiurnaleastwardandnorthward tidal windsattainmaximumat altitudesof 103-112km. The largest valuesin the eastwardwind occurin summer(-80 m/s) ratherthan
equinox(-70 m/s),butequinoxeastward windshavelargeramplitudes above the maximum
altitude. The winter
values in the
amplitudeof theeastward windaresignificantly smallerfor all alti-
tude range and do not exceed30 m/s. The northwardwind amplitudesaregenerallysmallerthantheeastwardamplitudes for all seasons andreach-65 m/s at equinox,-45 m/s in summer,and -30 m/s in winter.The semidiurnalphaseexhibitsseasonal invari-
ancein equinoxandsummerandchanges generally
