Neutral Winds in the Lower Thermosphere from Dynamics Explorer 2

GEOPHYSICALRESEARCH LETTERS, VOL. 19, NO. 11, PAGES1093-1096, JUNE 2, 1992

NEUTRAL WINDS IN THE LOWER THERMOSPHERE FROM DYNAMICS EXPLORER 2

T.L.Killeenlj, T.. B.Fuller-RowellNardi 1, P.•I. o, andPurcell D. Re,1,esR•G. Roble 2,

Abstract. Doppler line profile measurementsof the OI severalground-based optical measurements of winds using the •557.7nm "greenline" emission,made by the Fabry-Perot lower greenline •,557.7 nm emissionat ~97 km [Coggeret interferometeron DynamicsExplorer 2, haveprovided altitude a!., 1985; Wiens et al., 1988; Lloyd et al., 1990] and profilesof the meridionalcomponent of the lower- individual OH emission lines at-86 km [Hernandez and thermosphericneutral wind. The windinversion technique of Smith, 1984; Rees et al., 1990]. The ground-based Nardi[1991] has been used to extractthe neutral wind profiles measurements made at auroral latitudes have indicated that the fromthe line-of-sight measurements.Individual •.557.7 nm dynamicalstructure is controlledby upward-propagatingfides Dopplerline profiles and inverted volume-emission-rate and and gravity waves [e.g., Manson et al., 1988], and that neutral-wind profiles are presented. Neutral wind magnetosphericeffects are alsosignificant at times[Cogget et measurementsfrom the-120 km altitude level, obtained on al., 1985]. There have been no lower-thermosphericwind multipleorbital passes over the summerhemisphere polar measurementsfrom deepwithin the geomagneticpolar cap. region,have been merged to producea synthesizedaveraged The DE-2 Fabry-Perot interferometer,FPI [Hays et al., "vector"wind field in geomagneticcoordinates. The wind 1981], was designedto measureprimarily the meridional patternexhibits a regionof anticyclonicvorticity in thedaytime componentof theupper-thermospheric neutral wind usingthe sectorof the magneticpolar cap. Averagedwinds of-300 OI 3•630.0nm emission,peaIcing at -240 km. In order to m/secin the equatorialdirection are observedin the early measurewind profilesat lower altitudes,however, a 557.7 nm morning sector. The satellite winds are in reasonable filter was also usedroutinely in a background,t/me-shared agreementwith the predictions of the NCAR and UCL modeand a large database of many hundredsof orbitsexists. thermospheregeneral circulation models, with significant These data have now been reduced to provide detailed regionaldiscrepancies evident in bothmagnitude and direction. Doppler-line-profilemeasurements of the daytime, twilight, and auroral OI•,557.7 nm emissions.The Doppler line Introduction profiles, in turn, have been inverted to provide volume emissionrate (VER) andmeridional wind altitudeprofiles. Observationsmade from the low-altitude,polar-orbiting, We present our first results of the DE-2 lower- DynamicsExplorer-2 (DE-2) spacecraftduring solar-cycle- thermosphericmeridional wind measurementsand compare the maximumconditions have shown that magnetosphere- binned and averageddata from the high-latitude 120 km ionosphere-thermospherecoupling processes profoundly altituderegion with the windscalculated by two thermosphere influencethe upper-thermospheric neutral wind pattern at high general circulation models (TGCMs). The instrumental latitudes.Due to ion drag, thesewinds developvortex techniqueand experimentaldata are f•rst presented,followed structuresthat resemble typical F-region ion convectioncells. by modelcomparisons and, lastly, a summaryof conclusions. Thegeometrical characteristics of the upper-thermospheric high-latitudewind patternand its dependenceon local time, Neutral Wind Observations season,solar cycle, geomagneticactivity, and interplanetary The DE-2 FPI was a plane-etaloninterferometer, with a magneticfield orientationhave been the subjectof numerous 1.26 cm etaIongap, a 3.5 cm diameteraperture, an 8-position recentexperimental studies [e.g., Killeen andRobie, 1988]. filter wheel, a 12-channelphoton-counting detector, and The lower thermosphericwind structureis not as well horizon-scanningoptics. The radiusof theroughly conical FPI understood,owing to the difficulty of making in-situ field of view (at the tangentpoint to the Earth'slimb) was in observationsin a regionwhere spacecraft lifetimes are short therange 7-25 km, dependingon the zenithangle of the view androcket flights infrequent. Measurementsof lower direction.The Doppler-line-profileanalysis technique used thermosphericwinds from variousground-based radars, hasbeen previously described by Killeen andHays [1984]. however,have providedextensive data setsfor specific Figure1 showsindividual dayside •557.7 nm spectrograms installationlocations [e.g., Johnsonand Luhmann,1985; from threedifferent tangent-point altitudes, obtained during Mansonet al., 1989; Johnson,1990]. There have also been orbit 7193. Each 12~channel spectrogram shows the instrumentalcounts accumulated in 0.88 s. The free spectral 1Departmen t ofAtmospheric, Oceanic, and Space Sciences, range of theFPI at 557.7nm is 0.0123nm andthe wavelength TheUniversity of Michigan interval correspondingto an individual channel width is 2High Altitude Observatory, National Center for 0.0014 nm. Successivepeaks, correspondingto different A_tinGspheric Research 3CIRES University ofColorado/NOAA SpaceEnvironment integral orders of interference,are separatedby 8.85 channels. Laboratory The 557.7nm peakappears in channel8, with a portionof the 4Atmospheric PhysicsLaboratory, University College adjacentorder appearing in channels1 and2. The width of the London peakis governedby the ambienttemperature. Figurel c showsthe spectrogramobtained for a tangent Copyright1992 by the American Geophysical Union. point altitude of 82 kin. At and below these altitudeson the Papernumber 92GL01023 daysideof theorbit, Rayleigh-scattered sunlight contaminates 0094-8534/92/92GL-01023 $03.00 the Dopplerline profile,as is evidentby the characteristic

1093 1094 K•leen et al.:lower thermospheric winds from space

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Channel Fig. 2. (a) Measuredsurface brightness for a singlelimb scan of the DE-2 FPI viewingthe %557.7nm emission.The boxes 4000 , representthe direct measurements and the crosses represent the 104Km calculatedbrightnesses based on the derived VER profile.(b) DerivedVER profile.The experimentalerrors are smallerthan or commensuratewith the sizeof theplot symbols. b) peakingnear 170 km altitude.These results are consistent with the soundingrocket data of Wallaceand McElroy [ 1966]. ,oooo BI Figure 3 showsthree examplesof neutral wind prof'res 1 • 3 4 5 6 7 8 9 lO 11 12 derivedfrom the X557.7 nm emissionDoppler line profiles, Channel usingthe wind inversiontechnique of Nardi [1991]. The wind inversiontechnique is considerablymore complicated than that 10000• 82km __FY'/J for VERs. Briefly, it involves developing a systemof ooo equationswhich characterizethe measurementand include explicit terms for the effects of i) instrumentalspectral 2 6000 broadening,ii) weighting by local VERs, iii) field-of-view

o,:, 4000 C ) blurringdue to optical aberrationsand viewing geometries, and iv) variationsin brightnesswithin the field of view. The equationsare linearized using a Taylor-seriesexpansion about 2000 • the apparent (or line-of-sight) winds and solved usinga 1 2 3 4 5 G 7 8 9 10 ll 12 singular-value-decompositionmethod. Channel The profilesshown in Figure3 are for daytimelow-latitude Fig. 1. Examplesof 12-channelFP! spectrogra_msobtained conditions,with positive winds representingthe northward on the daysideof DE-2 orbit 7193. direction. The error bars incorporate uncertaintiesdue to countingstatistics and the wind inversiontechnique. Owing to the large conical field of view (~20 km radius at the lowest "wedge" shape of the spectrogram.The FPI provides altitudes) and the relatively long horizontal path lengths uncontaminateddaytime Doppler measurements down through throughthe emittingatmosphere along the line of sight(~200- the altitudeof the lower greenline emissionat ~97 km. Such 600 km), the remoteFPI measurementstend to smearout large low-altitudedaytime measurements of greenline emissions spatial shear structuresseen in publishedrocket profiles havenever before been available and are possible here only [Heppnerand Miller, 1982].These and many other DE-2 wind becauseof thehigh-spectral resolution of theFPI. profries,however, do depictwavelike structures in thevertical Many thousandsof suchspectra from hundredsof orbital with amplitudesthat increasein magnitudewith altitude,as passeshave been analyzedto providewind and VER data. wouldbe expectedfrom gravitywaves propagating vertically. Figure2 showsa typicaldaytime surface brightness profile The wind magnitudesare generally<100 m/sec,with error andthe corresponding inverted VER profilefor a singleFPI bars of •-_7m/sec at the lowest altitudes and ~:!:50 m/sec at limb scan.The VER profilewas derived using the inversion altitudes >250 km where the emissionrates drop to low techniqueof Nardi[1991]. Briefly, this technique is basedon magnitudes. a constrained"onion-peel" matrix inversion.The constraint usedin theinversion involves the assumption of a multiple- AveragedData-Model Comparisons Chapman-layerform for theVER profile.The data points denotedby rectangles in Figure 2a represent the line-of-sight Wind measurements from the %557.7 nm observations brightnessmeasurements and the crossesrepresent the madeat high southern(summer) latitudes for tangentpoint calculated(or reconstituted)brightnesses that are consistent altitudes between 100 and 140 km have been binned and with the inverted VER profile of Figure 2b. The close averagedin geomagneticpolar coordinatesfor two separate agreementbetween the measured and reconstituted profiles in 40-dayperiods of the DE mission.The bin dimensionswere Figure2a indicatesthat the derived VER profileis consistent 10ø latitude and 1 hour local time. The two data-collection with the experimentalmeasurements. It can be seenthat the periodswere centeredon November 12, 1982 and on January daytimebrightness profile (Figure 2a) increases monotonically 27, 1983.The total number of orbitalpasses used in thisstudy with decreasingaltitude, and the VER profile (Figure2b) was ~100. depictstwo broadpeaks, corresponding to the lower green line In the first datacollection period, the DE-2 spacecraft emissionnear 97 km andthe higheraltitude emission layer crossedthe southern hemisphere polar region in the19:00- Killeenetal.: lower therrnospheric windsfrom space 1095

300 MERIDIONAL WIND DE-2 [DE2-FPiI 250 12

200 Orbit 6681.0 Date 82294 LST 8.9 hrs 150 - Zsat 277 'k.m SZA 45 ø 100 - - - 1o Latitude ,1 o o lOO 200 Wind [mJs] NCAR-TGCM UCL-TGCM 12 24 300 - 300 -

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10(1- < lOO- -9 ø Latitude 4 ø Latitude X•..%•_•_• .•__•..•so. 500 m/see 24 24 5?2OO 'I'00 •{1 •O0 200 Wind [m/s] Wind [nds] Fig. 4. (a) "Synthesized"winds at-120 km overthe southern Fig.3. Examples of daysidelower-thermospheric meridional (summer)hemisphere polar region in geomagneticpolar windprofries. Errors shown include the effects ofthe counting coordinates (outer circle at 50øS).(b) Predictedneutral wind statisticsand the wind inversion. fieldat 120km from the NCAR TGCM. (c) sameas (b), exceptthat the UCL model has been used. 07:00local time plane while, in thesecond period, the orbit hadprecessed to the 14:00-2:00 local time plane. Due to the directionin thedawn sector. This vorticity resembles that relativelylarge field of view at the limb and the natureof the observedat higher altitudes. It is also consistentwith VERprofiles at high-latitude,these averaged measurements incoherent scatter radar observations of lower-thermospheric areconsidered to be representativeof a meanaltitude of ~120 vorticityreported byJohnson and Virdi [ 1991 ]. Weinterpret kin.The mean altitude ascribed tothe binned data is not very boththe vorticity and the strong morning-sector equatorial sensitiveto thepresence or absenceof aurorawithin the field wind evident in our data to be due to the effects of ofview, since the auroral %557.7 nm emission peaks in this magnetosphericcoupling via the ion-drag momentum forcing samealtitude region [Cogger et al., 1985].The single- mechanism.Future investigations will useDE-2 datafrom the component(meridional) wind averagesfrom the two data- northernhemisphere for whichground-based measurements collectionperiods provide separate, near-orthogonal may be available for comparison componentsof the vector wind field that may be combined in The synthesizedexperimental wind vectorshave been thegeomagnetic coordinate frame to providelimited vector comparedwith the calculations of twoseparate TGCMs: the coverageatlatitudes greater than 70 ø S magnetic. The averaged NationalCenter for AtmosphericResearch (NCAR) TGCM geomagnetic-polar-capvector wind field obtainedin this [Robieet al., 1988] and the University College London (UCL) fashionis shownin Figure4a andis referredto hereas the TGCM [Fuller-RowelIet al., 1990].The two modelswere "synthesized"wind field. eachrun twice for geophysical conditions appropriate to the Thisdata handling procedure uses single component twoDE-2 data sets (solar maximum, moderate geomagnetic averagesfor all geomagneticconditions, separated by ,.-2 activity,November 1982 and January, 1983). The TGCM- months,and necessarilyassumes that the seasonalwind calculatedmeridional winds at the120 km altitudelevel were variationover this period is minorcompared with spatial (vectorially)combined in geomagnetic polar coordinates, in variations.While this assumption isquestionable, thedetailed exactlythe same manner as was done for theFPI data,to modelcomparisons (wherein the model predictions were producethe results shown in Figure4b for theNCAR TGCM treatedinexactly the same way) indicate that the synthesized and Figure 4c for the UCL TGCM. As a testof the validity of high-latitudevector wind field providesa reasonablethis procedure a comparison was made between the two model descriptionofthe polar vector wind field for this time period. synthesizedvector wind fields and the "normal" vector wind Thesynthesized vector wind field, shown in Figure4a, fieldscalculated for the individualmodel runs. These representsthefirst large-scale experimental characterization of comparisonsshowed that the synthesized theoretical wind field thelower-thermospheric neutralwind pattern deep within the wassimilar (though not identical in all respects)to the geomagneticpolar cap. Two strikingfeatures are evident. normally-calculatedwind field. First,there is a regionof high-speed, equatorially-directed The comparison between the TGCM predictionsand the windsinthe early-morning sector of thepolar cap. These DE-2data indicates alevel of agreement thatis encouraging. In averagewinds approach -300 m/secin magnitude.They are bothcases the models show an early-morning-sectorcross- consistentin both speed and magnitude with the equatorward polar wind of ~200m/sec magnitude directed along the 03:00 windsobserved from the Sondrestromradar at 74.5ø hrsMLT meridian. The FPI data show an average wind speed geomagneticlatitude during a periodof moderate geomagnetic of greatermagnitude (up to -300 m/sec)in thesame direction. activityin 1987 [Johnson and Virdi, 1991]. Second, there is The experimentaldata and the theoreticalmodel results all evidenceofanticyclonic vorticity inthe dayside sector ofthe showthe presence of anticyclonic vorticity in thedaytime geomagneticpolar cap, with the wind field rotating from a sector,although there are significant discrepancies in both sunwarddirection in the dusk sectorto an antisunward windmagnitude anddirection. On the dayside, for example, 1096 Killeenet al.' lower thermospheric winds from space thesatellite data indicate a clockwiserotation through noon, duringLTCS 1, J. Geophys.Res., 96, A2, 1099-1116, whileboth models show a convergingwind field. 1991. Killeen,T.L., andP.B. Hays, Doppler line profile analysis Conclusions for a multi-channelFabry-Perot interferometer, A_gg[. O_p_k,23, 612, 1984. We havepresented first lower-thermosphericwind and Killeen, T.L., and R.G. Robie, Thermospheredynamics: volumeemission rate data from measurements made by the Contributionsfrom the first 5 years of the Dynamics DE-2 FPI. Theaveraged winds at highgeomagnetic latitudes ExplorerProgram, Rev. Geoph.¾•.,26, 329-367, 1988. in thesummer hemisphere show the presence of significant Lloyd,N., A. H. Manson,D. J. McEwen,and C. E. Meek,A anticyclonicvorticity in thedaytime sector of thepolar cap and Comparison of Middle Atmospheric Dynamics at the presenceof a large magnitude(-300 m/sec)equatorial Saskatoon(52øN, 107øW) as Measuredby a Medium- wind jet in the early morning sector. These results are FrequencyRadar and a Fabry-PerotInterferometer, J• consistent with the the ground-basedmeasurements of G.eo.phys. Res,.,9_•5, D6, 7653-7660, 1990. Johnsonand Virdi [1991] in the northernhemisphere. The Manson,A.H., C.E. Meek,S.K. Avery,and D. Tetenbaum, high-latitudeDE-2 resultsare alsoin encouragingagreement Comparisonof mean wind and tidal fields at Saskatoon with the calculationsof two thermosphericgeneral circulation andPoker Flat during1983/1984, Phys. Scripta, 3_27, 169- models,with significantregional discrepancies. 177, 1988. Manson, A.H., C.E. Meek, H. Teitelbaum, F. Vial, R. Acknowledgements. This work was supportedby NASA Schminder,D. Kurschner,M.J. Smith, G.J. Fraser,and grantsNAG5-465, NAGW-1535, and NGT-50193 and by R.R. Clark, Climatologiesof semidiurnaland diurnal tides NSF grantATM-8918476 to the Universityof Michigan.The in the middleatmosphere (70-110 km) at middlelatitudes authors wish to thank Drs. A. G. Bums, R. M. Johnson,and (40-50ø N), J•_.At_mos. Terr. Phy_s.,5__.1,579-594, 1989. R. J. Niciejewskifor helpful comments. Nardi, B., An Inversion Technique to Recover Lower ThermosphericWinds from Space-BorneMeasurements of References [OI]5577 J•., PhD Thesis, University ofMichigan, 1991. Rees,D., A. Aruliah,T.J. Fuller-Rowell,V.B. Wickwar,and Cogger, L.L., J.S. Murphree, C.A. Tepley, and J.W. R.J. Sica,Winds in the uppermesosphere at midlatitude: Meriwether Jr., Measurementsof the E-regionneutral Firstresults using an imagingFPI, G.oophys. Res. Lett., wind field, Planet. SpaceSci., • 373-379, 1985. 17, 1259-1262, 1990. Fuller-Rowell, T.J., D. Rees, B.A. Tinsley, H. Rishbeth, Robie, R.G., E.C. Ridley, A.D. Richmond, and R.E. A.S. Rodger,and S. Quegan,Modelling the response of Dickinson,A coupledthermosphere/ionosphere general the thermosphereand ionosphereto geomagneticstorms: circulationmodel, Geophys,R•$, Le•t,, 15, 1325-1328, Effectsof a mid-latitudeheat source, Adv. Spac•Res,., 10, 1988. 6, 215-224, 1990. Wallace,L., and M. B. McElroy, The Visual Dayglow, Hays, P.B., T.L. Killeen, and B.C. Kennedy,The Fabry- Planet, SpaceSci., 1_!4,677, 1966. Perotinterferometer on DynamicsExplorer, Space Sci, Wiens, R.H., G.G. Shepherd, W.A. Gault, and P.R. Instrum., 5, 395-416, 1981. Kosteniuk,Optical measurementsof winds in the lower Heppner,J.P., and M.L. Miller,Thermospheric winds at high thermosphere,LGeophys. Res.,93, 5973-5980,1988. latitudesfrom chemical release observations, J. Geophy$• Res., 8•7, 1633-1647, 1982. T. J. Fuller-Rowell, Space EnvironmentalResearch Hernandez, G., and J.L. Smith, Mesosphericwind Lab.,NOAA, 25 Broadway,Boulder, CO 80303. determinationsandthe Pl(2)c,d lines of the X2p OH (8-3) T. L. Killeen, B. Nardi, and P. N. Purcell, Space band, Geophys.Res. Lett., 11, 534-537, 1984. PhysicsResearch Laboratory, The Universityof Michigan, Johnson,R.M., Lower-thermosphericneutral winds at high 2455 HaywardStreet, Ann Arbor,M148109-2143. latitude determinedfrom incoherentscatter measurements: D. Rees,Atmospheric Physics Laboratory, Universky a reviewof techniquesand observations,Adv .... Space CollegeLondon, 67-73 RidingHorse Street, London, U.K. Res., 1._QO,6, 261-275, 1990. W1P 6PP Johnson,R.M., andJ.G. Luhmann, High-latitude mesopause R. G. Robie, High Altitude Observatory,National neutral winds and geomagneticactivity: A cross- Centerfor AtmosphericResearch, Boulder, CO 80303 correlationanalysis, J. Ge.oph_vs.Res., 90, 8501-8506, 1985. Johnson, R.M., and T.S. Virdi, High-latitude lower (Received:March 11, 1992; thermosphericneutral winds at EISCAT andSondrestrom accepted:April 3, 1992)