Neutral Winds in the Lower Thermosphere from Dynamics Explorer 2
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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 29O DE-FPI 557.7 nm •'1'" ooo - •240 --•.. I• UT(sec)119•1 J zozkm - I•1•1, Zsat(krn)Glat 30524 • - ', S.Z.A. S'/ø •196--' Date82294 _ 600 _ 14(; _ h 400 a) .. - 9,'uZ , •, 1,,,I •,, I,, p,'- 20 40 60 80 0 250 $00 750 1000 Brightness(kRay) VolumeEmission Rate (ph/cc-s) o ß , a) b) 6 7 8 9 10 11 1:7 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