North-South Asymmetries in the Thermosphere During the Last Maximum of the Solar Cycle F

Home , December solstice, June solstice

North-south asymmetries in the thermosphere during the Last Maximum of the solar cycle F. Barlier, Pierre Bauer, C. Jaeck, Gérard Thuillier, G. Kockarts

To cite this version:

F. Barlier, Pierre Bauer, C. Jaeck, Gérard Thuillier, G. Kockarts. North-south asymmetries in the thermosphere during the Last Maximum of the solar cycle. Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 1974, 79, pp. 5273-5285. 10.1029/JA079i034p05273. hal-01627389

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. VOL. 79, NO. 34 JOURNAL OF GEOPHYSICALRESEARCH DECEMBER 1, 1974

North-South Asymmetriesin the ThermosphereDuring the Last Maximum of the Solar Cycle

F. BARLIER,x P. BAUER,9' C. JAECK,x G. THUILLIER,a AND G. KOCKARTS4

A large volume of data (temperatures,densities, concentrations, winds, etc.) has been accumulated showingthat in additionto seasonalchanges in the thermosphere,annual variations are presentand have a componentthat is a function of latitude. It appearsthat the helium concentrationshave much larger, variations in the southernhemisphere than in the northern hemisphere;the same holdstrue for the exospherictemperatures deduced from Ogo 6 data. Similarly, satellitedrag data in the 250- to 400-km range indicatethat the bulgeof densitytends to stayover the southernhemisphere, whereas winds deduced from Ogo 4 and 6 data show a tendencyto blow northward acrossthe equator. If part of the explanationof theseasymmetries can be found in a latitude independentcomponent induced by the changingsun-earth distancebetween solstices (Volland et al., 1972;Ching and Chiu, 1972, 1973),the fact that an asymmetry is •till presentat the equinoxessuggests that this is not the solecause: more energyseems to be available for the thermospherein the southernhemisphere during the equinoxes;this may be the result of an asymmetry in the geomagneticfield or an asymmetricaldissipation of tidal waves induced by an asymmetricalworldwide ozone distribution.

An asymmetry between the two hemispheresbecame ap- August 1970. The orbital characteristics were such that 24 parent very early in the study of the semiannualvariations hoursof localtime was covered over a periodof 90 days. of the thermospheric density. The density minimum is The data availablefor this studywere limited to low and systematicallydeeper in July than in January [Paetzoldand middlelatitudes (up to 60ø).These data are in a sensea natural Zschbrner, 196I; Roeruer, 1963; Jacchia, 1965]. For the same choice,since a previousstudy [Blamont and Luton, 1972] has local season the mean density is not the same in both shownthat the high-latitude behavior is largely determined by hemispheres.New evidence of asymmetriesin the thermo- geomagneticactivity. The data were averagedover latitudinal sphericparameters between the two hemisphereshas recently strips 30ø wide. _.beenpointed out by Keating et al. [1973], Barlier et al. Figure I gives variationsof the temperaturedifferences [1973],and Blamont and Luton [1972]. A majorfeature of these betweennorthern and southern mid-latitudes (45øN-45øS and asymmetriesis the nonpermutability of the values of ther- 25øN-25øS)as a functionof the calendarday. A 90-dayrun- mospheric parameters of the northern hemisphere for a ning mean of the data was performedto eliminate diurnal solsticewith those of the southern hemispherefor the other variations.The reason for consideringthe differencein solstice.Furthermore, systematicdifferences exist betweenthe temperaturefor correspondinggeographical latitudes in the two hemispheresat the time of equinox,and thesedifferences two hemispheresis to eliminate the effectsof solar flux and have the same sign for spring and fall conditions. Neutral geomagneticactivity. The observedresiduals should therefore temperature,density, concentration,and wind data have been characterizeessentially the seasonalvariations. Two different gatheredin order to study their hemisphericalasymmetries. featurescan be seenin Figure 1: The first part of the presentpaper is devotedto an analysis 1. For 25ø latitudethe temperature difference is practically of experimentaldata characterizedby asymmetricalproperties alwaysnegative or equalto zero;i.e., the southern hemisphere during the last maximum of solar activity. The secondpart is in thislatitude range appears generally to be warmerthan the designedto show that most of the observedasymmetries can northernhemisphere. This is a clearindication of thepresence be consideredas resultingfrom a unique cause, namely, an of an asymmetryof the temperaturebehavior in thesetwo asymmetricalthermospheric heating of the two hemispheres regions,since a purelyseasonal effect would tend to givean os- for identical solar conditions. Finally, possible energetic cillation centered around zero. It must be noted that at mechanisms are considered, namely, heating linked to equinoxthe southerntemperature is still significantlyhigher geomagneticactivity and heating due to dissipationof tidal thanthe northernone: August 26, 1969,to April 1, 1970,is a waves. periodof continuouslyhigher southern temperatures. 2. The situationis differentat 45ø, where a seasonaleffect DATA ANALYSIS can be clearlyseen. It will be noticed,however, that equal Thermospherictemperatures. Temperatures near 270 km temperaturesare observedaround September16, 1969, and have been deduced from Fabry-Perot interferometric March26, 1970,which tends to enhancethe periodwhen the measurementsof the 6300-,/•red line on board the Ogo 6 southernhemisphere is warmer:the temperaturedifference satellite [Blamont and Luton, 1971]. Reliable data have been seemsto be larger for the Decembersolstice than for the June obtained during the daytime over the period June 1969 to solstice.Although the seasonal variation dominates at 45ø, it is still possibleto detectthe asymmetrynoted for 25ø. • Groupe'deRecherche de G6od6sieSpatiale, Centre d'Etudes et de In order to ascertainthe reality of the effectit is usefulto RecherchesG6odynamiques et Astronomiques,Grasse, France. :Centre National d'Etudesdes T616communications,Issy-les- look at the variationof the 80-daymean solar flux over the Moulineaux, France. periodof interest(Figure 2). Rathersmall variations (+7%) :Centre National de la RechercheScientifique, Service d'A6ro- aroundthe meanvalue are observed,and their residualeffect nomie, Verri•res le Buisson, France. on the temperaturedifferences should be very small. 4Institut d'A6ronomie SpatiMe de Belgique,Brussels, Belgium. A comparisonhas beenmade with WaMteufel's[1971] Copyright ¸ 1974 by the American GeophysicalUnion. model of temperature based on incoherent scatter data

5273 5274 BARLIERET AL.' NORTH-SOUTHASYMMETRIES IN THE THERMOSPHERE

2OO-

1OO-

e i JUL.... • -lOO-

-200- 1969 1970J

200

lee,

e- ß T'"SEP. NOV. JAN. MAY • -lOO-

-200-

Fig. 1. Difference between northern hemisphereand southernhemisphere temperatures deduced from Ogo 6 measurementsaveraged over one revolution of the node for the zone of geographicallatitudes centered on +45 ø and +25 ø gatheredat Arecibo (18øN) and St. Santin (45øN). The ratio of of the 'semiannualvariation' deduced by Jacchia[1971a] from the observedtemperature Togo6 to the value given by this an analysis of satellite drag data, and it constitutes an temperaturemodel, Twa•,is plotted in Figure 3 as a function of asymmetry in the thermosphericbehavior. time for three latitudinal regions.Our purposehere is not to Waldteufel's model can be adjusted to the Ogo 6 discuss the different details, but the main feature must be temperaturedata by varying only the 'seasonal'coefficient Ca

_ pointed out. Although no significanttrend seemsto be present for each range of latitudesconsidered. In this way, annual in the behavior of the temperature ratio at 45øN, the effectsmay appearas seasonalones. The resultsare presented equatorial, and particularly the southern, data indicate that togetherin Figure 4. If only seasonalvariations were present, the observedtemperatures are lower than the model valuesin positivevalues would be observedin the north, and negative July and Augustand higherbetween September and February. valuesin the south. It appears,however, that the reversaloc- Since Waldteufel's model is based only on temperature cursnear 30øN insteadof near the equator.The absolutevalue observationsmade in the northern hemisphere,it is normal to is much larger at 45øS than at 45øN; this confirms that an an- find a rough agreementin the northern hemisphere.However, nual effect exists. This annual effect is in phase with the the systematiceffect observed at 45øS,when it is takeninto ac- seasonaleffect in the southand is out of phasewith that in the count that the model is symmetricalby constructionfor the north. The region where the coefficientCa vanishesis shifted two hemispheres,indicates that the amplitude of the winter to northward. summervariation in the south is larger than that in the north. The nature of the asymmetryis further establishedby the To a lesserextent the samefeature is observedat the equator, latitudinal variation of the temperaturefor a given local time. and this shows that an annual, rather than a seasonal,effect is Figure 5 shows the Ogo 6 temperaturesversus latitude for

involved. This annual. effectis similar to the annual component differentsolstices and equinoxesand for local time varying

170-

150- iJUL. Is,SEP.I ,INOV'I LJAN [MAn. 1969 1970

130-

Fig. 2. Mean solar flux over the period of Ogo 6 measurements. BARLIER ET AL.: NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE 5275

• =+45 ø TOGO-6/ TWAL

1.2

1.0 ß v' Nv, ,ov.,¾ ' ' 'V TM" 0'81

•=0 ø

1.2

1.0 0'81

1.2

1.0 . SEP. NOV. JAN. MAR. 0'81 Fig.3. Ratioof theobserved temperatures Togo 6 and the temperatures obtained by usingthe Waldtcufel model, twal-

between 1000 and 1700 hours. The data were, however, nor- By takingadvantage of the complementarityof the orbitsof malized to a 10.7-cm solar flux of 150 X 10-•"' W m -•' HzT • and these satellitesKeating et al. [1973] evaluatedthe helium a geomagneticactivity corresponding to a meanKp of 2 by use seasonalvariation in the northern and southern hemispheres. of the Jacchia 1971 model formulation [Jacchia, 1971b].The Sucha statisticalanalysis is, however,not availablefrom the solstice data indicate that the amplitude of the variation massspectrometric data presentedby Hedinet al. [1974]. between the June and the December solsticeis much larger in The asymmetricalbehavior of heliumdeduced by Keatinget the souththan in the north. The globalaverage temperature is al. [1973]is presentedin Figure6. Althoughthe heliumcon- thereforehigher in Decemberthan in June. This kind of centrationgenerally seems to be higherin thenorth than in the asymmetryis alsoobserved with the equinoxdata, which in- south,it appearsthat the mostsignificant feature is a much dicate that irrespectiveof the equinox considered,the largerseasonal variation in thesouth than in thenorth, the net temperaturetends to be higher in the south. resultbeing equivalent concentrations for local winter in the Concentrationsand total densities. The exospherichelium north and the south and much lower concentrations for the concentrationswere deducedby Keating et al. [1973] from southern summer than for the northern summer. This kind of dragdata of Explorer9, 19,24, and39 between1961 and 1971. asymmetryis not detectablein the Ogo 6 data [Hedinet al., 1974]:the measurements can be made only when the satellite is nearperigee, and therefore it is impossibleto observedensities in the southernand northern hemispheressimultaneously. Satellitedrag observations compiled by Barliefet al. [1973] and Jaeck-Berger[1973] have been used in severalways. In a .051(•3 ! firststep a statisticaltreatment of dragdata coveringthe last

,4- - solar maximum(1967-1970) was performedaround 280 km. 45 •5 Each observationfor a givenlatitude and local time was nor- I i i ! i malized to the worldwide minimum densitypredicted on the basis of the Jacchia 1971 model for the same conditions (solar flux,magnetic activity, and time of the year).The ratiosof the WALDTEuFEL S MODEL VALUES observeddensities to thecomputed. minimum densities were ...... CORRECTED VALUES / thengathered into mapsin Figure7 correspondingto solstices ! ! and equinoxes. ! It is well known[Jacchia, 1971b] that the latitudeof the sub- maximumdensity point undergoes a migration correlated with the declinationof the sun. However, if the latitude of the max- Fig.4. Seasonalcoefficient Ca of theWaldteufel temperature modelas a functionof latitude,and adjustedvalues with Ogo6 imum is clearlyin the southduring the Decembersolstice, it measurements. tendsto stayclose to theequator during the June solstice. The 5276 BARLIERET AL.' NORTH-SOUTHASYMMETRIES IN THE THERMOSPHERE

local time 10 .--• 17 h local time 16h Tc', ' , , I I I I I

_ 1200 • • •E.•QUiNov _O.•F__SEPTEM B_.E R_6,9

1100 , _

,ooo 1000 | /,,' /C," , , , , _45ø .25 ø 00 2.5ø 45ø _45' ø _215ø 0' ø 25, ø 45, ø GEOGRAPHIC LATITUDE GEOGRAPHIC LATITUDE

Fig. 5. Means for five zonesof geographicallatitude of observedtemperatures corrected for the solar flux and geomagneticactivity by the Jacchia1971 model for (left) 90 days around the solsticesand (right) a few days around the equinoxes.

minimum of the June solsticeis much more elongatedthan vationshave alsobeen averaged over 90 daysfor the purpose that of the December solstice.During the December solstice, of comparisonwith the temperatures.The densitydifferences densitygradients across the equatorare present during the sec- betweenconjugate geographical latitudes have been plotted in ond part of the night; during the June solsticethis is not true. Figure 8. The densitiesin the northernhemisphere appear to The north-southasymmetry is not a phenomenonlimited to be systematicallysmaller than or equal to the corresponding solstices,since an asymmetryis still presentat the equinox values in the southernhemisphere. This is observedat 25ø as when the solar illuminationof both hemispheresis the same well as at 45ø . Such behavior is consistentwith generally (Figure 7). The densityis generallyhigher in the souththan in higher temperaturesobserved over the south. the north: indeed,on the onehand, the densitymaximum stays In order to separatethermal expansion effects from changes in the southernhemisphere, whereas, on the other, there are in the lower boundary densitiesthe 480-km densities,which two densityminimums (one in eachhemisphere), the one in the correspondto atomicoxygen, have been reduced to 200 km by northern hemispherebeing more pronouncedthan the one in making use of the temperaturedetermined by Ogo 6 and by the southernhemisphere. This fact has alsobeen demonstrated assumingthat diffusive equilibrium prevails in this altitude by Harper [ 1971]. range.The choiceof 200 km rather than a lower heightis due The densitiesat 480 km deducedby Barlier et al. [1973]from to the uncertainties concerning both the assumption of satellitedrag data for the period coveringthe Ogo 6 obser- diffusiveequilibrium and the temperatureprofile below 200 km [Alcaydbet al., 1974]. Figure 9 showsthe result of this EXPLORER 9 1924 39 [2/61-9/71) procedurein terms of the north-southdifference-in oxygen concentration at 200 km. It is clear that the results at 45 ø a.re dominatedby seasonaleffects essentially showing that the ox-

WINTER ygen tends to be accumulated over the cold hemisphere o! (winter). This is in agreementwith the results of previous studies[e.g., Evans et al., 1970;Barlier et al., 1971; Hedin et al., 1974; Alcaydb et al., 1974]. The 25ø resultsare quite different and definitelyexhibit an asymmetry.Indeed, the 25øN concentrationsare alwayslarger / than or equal to the 25øS concentrations.This is to be com- / O2 / pared with the asymmetryin temperatures(Figure 1) and also / dec. / tends to show that the atomic oxygen accumulatesover the 0.3 june _ SUMMER colder region. A secondfeature can be deducedfrom the previouscom- putations:a higherseasonal variability of the atomicoxygen -90 s 0 N +90 concentrationin the northern hemispherethan in the southern hemisphere. For this purpose the concentrationsin local LATITUDE, DEGREES winter and local summer have been computed. Figure 10 Fig. 6. Seasonalvariation of exospherichelium in the northern shows the ratio of the above concentrations at 1700 hours local andsouthern hemispheres based on all availablehelium drag data time for differentlatitudes. A greatervariability appearsin the (Explorer9, 19,24, and 39) from February 1961 through September 1971evaluated in separate10ø latitudeincrements [Keating et al., northern hemispherethan in the southernhemisphere, con- 1973]. trary to the helium behavior. BARLIERET AL.: NORTH-SOUTHASYMMETRIES IN THE THERMOSPHERE 5277

wind the hmF9.distribution is symmetricalwith respectto the magnetic dip equator. An asymmetry arises when a wind is blowing: in the summer hemispherethe F layer risesin height, and in the winter hemispherethe F layer falls. The result is the appearanceof a maximum of hmF9.in the summerhemisphere $.:-2o* anda minimumin thewinter hemisphere. The Comitfi Consultatif International des Radiotfilficom- munications(CCIR) predictions[Union Internationale des Tklk- communications,1967] contain an important volume of ionosphericdata. When the correctionspointed out by J. W. King and G. Thuillier (private communication, 1974) are

I 3 5 7 9 11 1:5 15 17 19 21 23 COCALTIME taken into account,the ionosphericbehavior is obtainedfor differentlocal times and seasons.Figure 12 showsthe two belts of extreme hmF•. obtained for a constant local time from the data of the CCIR predictions.The complete set of those maps leads to the behavior of the minimum and maximum hmF•., designatedas hmm(hmF•)and hmax(hmF•.),respectively. Figure 13 compares the results for June and December 1967 and exhibits the following characteristics: 1. In Figure 13a, hmax(hmF9)is greaterin Decemberthan in June. 2. In Figure 13b, hmm(hmF9)is comparable in June and December. The two local time variations are, however, not similar.

1 3 5 7 9 11 13 15 17 19 21 23 LOCALTIMIt 3. In Figure13c the differenceA = [hmax(hmF9)- hm,•(hmFO]/2is plotted versuslocal time. It is seen that A is higher in December than in June and is different in phase. 4. In Figure 13dthe sumS = [hmax(hmFO+ hm•,•(hmFO]/2 is plotted versuslocal time; S representsa mean altitude of the F layer that is higher in December than in June. According to the theoretical results of Bramley and Young •© = + 20ø [1968] it appearsthat the meridional wind blowing from south to north in December is characterizedby a higher speedthan 30 X the wind blowing from north to south in June (property 3). At the beginning of the night the F layer is higher in 70•) December than in June, and this situation remains throughout 5090l-' O0 .... , , • , • LOCAL the night.Therefore during the daytimethe eastwardelectric I 3 5 7 9 '1113 15 1719 2', 13 T•Mœ field, which causes the F layer to rise, must be larger in

Fig. 7. Valuesof the ratio of observeddensities and the night- December than in June (property 4). , time minimum density of the Jacchia 1971 model as a function of Thus the behavior of the F region is not symmetricalin June local solartime and latitudefor time periodcentered around bs - and December and exhibits an annual component. - 20 ø, 0 ø, and + 20 ø. Lower thermosphere sodium concentrations. It is well known that the sodium concentration at 85 km undergoes a Neutral windsin theF region. Thuillier[1973] and Thuillier seasonal variation [Donahue and Blamont, 1961; Gadsden, and Blamont[1973] have shownthat the nighttime6300-A 1964; Gault and Rundle, 1969]. Maximums of concentration emissionobserved on board Ogo 4 and 6 in the tropical have been observed during the local winter for both regionscan be interpretedin termsof thermosphericwinds. hemispheres. Unfortunately, it is difficult to compare the Owingto the role of the magneticfield declination it appears values obtained by different authors becauseof intercalibra- that the region 0ø-150øE is convenientfor the study of tion problems of the photometers. It seems,however, that the meridionalwinds across the equator.The smalldeclination of concentration of sodium is higher in December over the the magneticfield in this regionrenders negligible the effectof northern hemisphereand that the variability of the concentra- zonal wind in the behavior of the 6300-A emission. tion is higher in the north (Table 1). It is still an asymmetrical The tropicalemission north of the magneticequator in characteristic. The seasonal behavior is similar to that of January has been comparedwith the southerndata in June atomic oxygen at 200 km (Figure 9): in particular, the (Figure 11). The intensity variation is due to the altitude variability seemshigher in the northernhemisphere than in the changeof the F layer, and it is possibleto concludethat for southern hemisphere. part of the night the meridionalwind blowingfrom the south towardthe north in Januaryis greaterthan the one blowing TENTATIVE INTERPRETATION from the northtoward the southin June(Figure 11). This ex- The precedingdata show that annual variations in addition perimentalresult is compatiblewith the hypothesesused by to seasonalchanges are presentin the thermosphere.Two an- Keating et al. [1973] to interpret the helium distribution. nual componentsmust be considered:the first component A transequatorialwind blowing from the summer to the is independentof latitude, and the second component re- winter hemispheremodifies the altitudeof the F layer,as has sponsiblefor the asymmetry is a function of latitude. been describedby Bramleyand Young[1968]. Without the Possible sources of latitude independent components. 5278 BARLIERET AL.' NORTH-SOUTHASYMMETRIES IN THE THERMOSPHERE

g/cm•] 1(•14' •E480](4õøN-- 4õø$•

0.1--

0 • I I I I I I •-"'•---,_I MAY I I 1969JUL.I ISEP.'-•,,,• I NOV.•MAR. I •""'•-,.-._•.-----"rJUL

0.1--

10-14

0.1

+• 0- i . . - I F

o.1.-i

Fig. 8. Differencebetween northern hemisphere and southern hemisphere densities at 480km, deducedfrom statistical analysis,for the sameperiod' and geometrical position as the Ogo 6 temperaturemeasurements.

Several studies of possible sources of latitude independent 2. Tidal and gravitywave dissipationat thermospheric componentshave been made, and severalprocesses can be, heightswhose efficiency is also a function of the sun-earth involved' ß geometryand of the absorptionof solar radiation within the 1. Extreme ultraviolet heating in the thermospherelinked ozonelayer [Vollandet al., 1972]. to the changing sun-earth distance [Ching and Chiu, 1972, 3. Jouleheating depending on the geomagneticactivity 1973]. [Chingand Chiu, 1973;Volland et al., 1972].

0200[45'N--45'S ] g/cm•j•o-•3

1970

ß I JUL.

Fig.9. D!fferencebetween northern hemisphere and southern hemisphere atomic oxygen densities at 200km deduced from observeddensities and temperatures. BARLIER ET AL.: NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE 5279

stay in the southern hemisphere. This tendency is actually observed(Figure 7). It is also expectedthat the larger annual temperature variation of the southern middle-latitude ther- mospherewith respectto the northern one also causeslarger annual changesin density, as is observed(Figure 7). Effect on the circulation. The temperature and density buildup in the southern hemisphereleads to pressuremaps capable of causing,in addition to the normal seasonalcircula- . / tion, a wind system blowing from south to north that correspondsto the optical observations(Figure 11) and to the ionospheric data (Figure 12). 0.5- Effect on the composition.Johnson and Gottlieb [1970,. 1973], Johnson[1973], and Reber and Hays [1973] have shown that a meridional circulation tends to accumulate the light Fig. 10. Ratio R = [O]•.00December 1969/[O]•.00June 1969 for different latitudes. constituents(as comparedwith molecular nitrogen) in the region of converging winds (pressure minimums) and to Searchfor a sourceof asymmetryyielding a latitude depen- deplete the heavy constituents(as compared with N•) in the dentcomponent. It is importantto notethat in calculatingthe same region. In addition to the seasonalchanges resulting for action of the sun on the two hemispheres,account must be such a mechanismthere will be an accumulationof the light taken of the fact that the asymmetriespersist at the equinox. gasesover the northern hemisphereas a result of the south to The following hypothesiscan be put forward: for comparable north wind system. Such behavior is observed in the atomic solar conditions (equal and opposite declinationsof the sun oxygen concentration reduced at 200 km (Figure 9) as well as and the samesolar activity) the southernhemisphere is able to in the helium concentration (Figure 6). There is a puzzling gathermore energythan the northernhemisphere (particularly feature that is difficult to describe with the previous at equin6x). mechanism;i.e., the largestseasonal variability appearsin the The effectsof the precedinghypothesis are now analyzedin southern hemispherefor helium (Figure 6), whereas it occurs more detail. in the northern hemispherefor atomic oxygen (Figure 10). Effect of the temperature. The first consequenceof an un- This point can be clarified by noting that a winter bulgecan balanced efficiency of the solar effect in the thermosphere also be explainedby a latitudinal variation of the eddy diffu- betweenthe two hemispheresis a generallyhigher worldwide sion coefficient [Kockarts, 1972]. Furthermore, it has been exospherictemperature for the solsticethat correspondsto shown [Kockarts, 1973] that the effect of a wind systemon a the highest sun-atmosphereinteraction (i.e., the December vertical distributioncan be representedby an appropriateeddy solstice). diffusion coefficient or vice versa. Circulation and turbulence Thus an annual variation must be added to the seasonal shouldtherefore play a simultaneousand complementaryrole variation:in phasein the southand out of phasein the north. in the structureof the upper atmosphere.An illustration of the In this instancethe small temperature variation observedin possibleeffect of turbulence is given in Figure 14, where the the north and the large one observedin the south should be helium concentrationat 400 km is plotted as a function of an noted (Figure 5). Furthermore,the fact that Waldteufeland altitude independenteddy diffusion coefficientadopted above Cogger[1971] did not observeany significantseasonal varia- 90-km height. When the entire variation of helium deducedby tion of the temperatureabove Arecibo (18øN) proves that Keating et al. [1973] is assumedto result from tu'rbulence there is an exact cancellation of the seasonal and annual effects between 90 and 120 km, the required latitudinal and variations at that latitude. The proposedmechanism suggests seasonaldependence of the eddy diffusion coefficientis shown mainly.that for both equinoxesthe southernhemisphere must in Figure 15. These values should be consideredas orders of be warmer than the northern hemisphere.This is in agreement magnitude, since the vertical structure of the eddy diffusion with the features shown in Figure 1. coefficient has been neglected. Effect on the thermosphericdensity. Owing to the thermal With the available data from chemical tracers on turbulence expansionthe density bulge is expectedto have a tendencyto above 90 km [Lloyd et al., 1972; Philbrick et al., 1973a] it is

1 x SOUTHERN ARC IN LOCAL WINTER ß,NORTHERN ARC IN LOCAL WINTER INDIANZONE 200.

100-

;4 ' ;6' ]8' ;0 ' 42' ' 44' ' 4'6' • days 151 156 161 166

local time 0.30 0.00 23.30 23.00 22.30 Fig. 11. Comparison of the intensity of the northern arc in December (local winter) with that of the southern arc in June (local winter). 5280 ]•ARLIERET AL.: NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE

Magnetic equator

.... Line of Minimum h F2 (mean value 306 km) , m

...... Line of Maximum h F2 (mean value 422 km) m + 90

- 9O - 180 W 0 h +180 E december |[}õ] 20 Fig. 12. The two belts of maximum and minimum h,•F2 for December 1967. very difficultto build a global modelof the latitudinaland where it is recombined. The two effects seem to lead to a situa- seasonalvariations [Roper, 1973]. Radio meteortrails are also tion in which for an increasedturbulence the atomic oxygen used to measurethe atmosphericturbulence [Zimmerman, concentration above the peak remains unchanged, whereas 1973]. Although the experimental data do not cover that below it is smaller. The experimental data of Philbrick et sufficientlywell all latitudesand seasons,the theoreticalcom- al. [1973b] seem to confirm the above theoretical work. Thus putationsof Figure15 indicate that theworld average value of the variability is linked more closelyto the winds than to the the eddydiffusion coefficient is higherduring December than turbulenceand will be higher in the north than in the south, as during June. has been indicated in Figure 10. It should be mentioned that The similarityin the northernand southernwinter helium any theoreticalstudy of the latitudinaland seasonalvariation of bulgescan now be explained.The windsystem (south-north) is atomic oxygenshould take into accountthe variations of the more intense in the northern winter (December) than in the molecular oxygenphotodissociation coefficient with the solar southern winter (June) becauseof the asymmetryin the zenith distance.An explanation of this kind may be valuable meridional circulation. Therefore the winter helium bulge for sodium but has not yet been developed. shouldbe largerin the north than in the south.But the world In conclusion,by assumingthat greater energyis kept in the averageturbulence is higherin Decemberthan in June(Figure southernthermosphere than in the northern thermospherefor 15), so that the northernwinter bulgeis not as large as is the same solar conditions a satisfactory understanding of the predictedfrom the meridionalcirculation and finally, the asymmetriesdescribed previously can be obtained. We had to winter bulgesare similar in the two hemispheres. assume,however, that the world average turbulence is higher The situation is not the same for the summerhelium deple- in December than in June. tion. For the southernsummer both the higher turbulencein POSSIBLE NATURE OF SOURCES December and the south-northwind produce a reduction of the helium concentration.Therefore the depletionis larger in The physical characteristicsanalyzed in the precedingsec- the south than in the north, explainingthe differencein the tions should be related to thermosphericheat sourcesthat are winter-summervariability betweenthe two hemispheres. asymmetricaleven during the equinoxes.Therefore the effects One may wonderthen why the samefeature is not observed linked to the geomagneticactivity will be considered,because in the behavior of the atomic oxygenconcentration. Keneshea the geomagnetic field of the earth exhibits a permanent and Zimmerman [1970] showedthat an increasein the tur- asymmetryat high latitudes as well as at low latitudes (South bulencehas two oppositeeffects on the atomicoxygen concen- Atlantic anomaly). The tidal wave dissipation will also be tration: the first is to increasethe productionof O following studied, since the atmospherebelow the thermospherepre- the increase of O•. around 120 km and the second is to in- sentssome asymmetricalaspects. Finally, other sourcesalready crease the rate of downward diffusion of O into the region known must be consideredfor a global understanding;in par- BARLIER ET AL.' NORTH-SOUTH ASYMMETRIESIN THE THERMOSPHERE 5281

hmax (h, f•) hmax (h.,,,f•)

krr JUNE 67

400- 400--

l, ß. ß ß

350- 350-- i I I I I 20 ;,2 o ;, 4 6 20 22 0 hmin(h•.f•) km

300-

250---I i i I • I • 250-- 20 22 0 2 4 6 20 22 0 2 4

A A km krr•

50--

ø--i I i I I 20 22 0 2 4 6 20 22 0 2 4 s

350 , 350-

300"- LOCAL TIME 300] LOCAL TIME I i I i i I ----'• I I I I k 20 22 o 2 4 6 20 22 0 2 4 TYPICALVAtUE OFTHE DISPERSION DUETO THE LONGITUDINAL EFFECT

Fig. 13. Comparisonof the maximum and minimum altitude of the F?.layer for June and December 1967. 5282 BARLIERET AL.' NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE

TABLE 1. Seasonal Variation oœ Sodium Concentrations in the Lower Thermosphere

Na Concentrations, cm-3

Geographical Local Local Author Year Location Winter Summer

Gadsden 1962 to 1963 Lauder 3 x 10 9 1 x 10 9 -45øS, 169øE Donahue and Blamont 1958 to 1959 Haute-Provence 14 x 10 9 1.5 x 10 9 Observatory +44øN, 6øE ticular, the changingearth-sun distance must be taken into ac- sun distancemust be considered,introducing an annual com- count. ponent. Effectslinked to the geomagneticactivity. The importance Unbalancedexcitation of tides. Sincethe needfor a higher of heating processesconnected to geomagneticactivity has worldwide turbulence in December has been noticed and since been pointed out even for quiet periods [Blamontand Luton, turbulenceis linkedto energydeposited in the mesosphereand 1972].In a recentpaper, Tort and Tort [1973]have also shown propagating upward, possible tidal effects should be con- the importanceof the effectsin latitudeand longitudelinked to sidered. geomagneticactivity for an understandingof the winter F It has been suggestedthat the energy dissipation of region anomaly, which is another aspect of the general semidiurnaltides in the thermosphereis sufficientto account asymmetries.These authors conclude that globally, the for the differencebetween the observedtemperatures and southern hemispherereceives more energythan the northern thosecomputed theoretically [Robie and Dickinson,1973] on hemisphere,and they point out a particulargeomagnetic effect the basisof EUV solarfluxes measured by Hinteregger[1970]. in the south. It therefore appearsthat asymmetriescan be in- Lindzenand Blake [ 1970]give an order of magnitudeof 0.3 erg troduced through the geomagneticactivity even during the cm-•' s-• depositedabove 150 km, comparedwith the daily equinoxes.However, this sole sourcecould not explain the averageof 0.5 erg cm-•' s-• for the EUV heating.Some support greater variability of the exospheric temperature in the for this theoreticalwork is givenby recentwind measurements southernthermosphere. Other heat sourcesare important; for in the 100- to 200-km range, exhibiting semidiurnal tides, example,EUV heatingvariations linked to the changingearth- madeby Bernardand Spizzichino[1971], Amayenc [1974], Bet-

0 7• 6--

5---

4--

,,, 3•

2-.--

I I 9• 8-- 7--'

6'-

5--

4--

3•

6 lO5 •o •o7 K I I i I I I I I I I, II I '1 I I I I I IIII I •1 I_,•i----- I 2 3 4 5 6 7 891 2 3 4 5 6 7 891 CFR sec Fig. 14. Heliumconcentration at 400 km as a functionof an altitudeindependent eddy diffusion coefficient adopted above 90-km height. BARLIERET AL.:NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE 5283

ß 2

I "--'

june december

\ \

10• I I I I I I I I I I I I I I I 6• 30ø 0 30ø 60ø - S N latitude

Fig. 15. Latitudinaland seasonal dependence of the eddydiffusion coefficient when the entirevariation of helium deducedby Keatinget al. [1973]is assumedto resultfrom the turbulenceeffect between 90 and 120km. nard [1974]at St. Santin, Wand[1972], and Salahand Evans mesosphere.From these computations it appears that the tidal [1973]. energyinput increaseswith the ozoneconcentration. More For the St. Santin results it should be noted that the specifically,an increaseof thetotal ozone content leads to an amplitudesobserved for the semidiurnaltide seem to indicate increaseof the tidal energydissipation when the absolutevalue an annual trend with a maximum centered around December. of the verticalozone gradient is increasedin the mesosphere. One.might wonder whether this characterizes a local seasonal An analysisof the worldwideeffect based on realisticozone variationor a larger worldwideexcitation of the semidiurnal distributionshas not yet beenmade. Such a studywould re- tidesduring the Decembersolstice. Although all the equip- quire,however, a knowledgeof globalozone distributions up ment allowing this kind of observationis located in the to mesosphericlevels. northern hemisphere,making it impossibleto distinguish Numerousstudies of troposphericand stratosphericozone between annual and seasonalvariations, the convergenceof have been based on the indirect 'Umkehr' m,ethodand on the evidenceis sufficientlystrong to consideran asymmetryin directsoundings [Diitsch, 1970, 1971]. There exists a definite the semidiurnal tide excitation betweenthe solsticesas a possi- latitudinal and seasonal asymmetry between both ble mechanism. hemispheres,although some details are not completely Beforeexamining the sourceof the semidiurnaltide it is in- doc•umentedowing to the smallnumber of stationsin the terestingto indicatethat the equatorialelectrojet [Mayaud, network in the SOuthernhemisphere. From these data it 1967] as observedin Jarvis(160øW, 0ø), where the magnetic appears, however, that the seasonalvariation is less and geographicalequators coincide, shows a definiteannual pronouncedin the southand that the maximum of thetotal variation with higher intensitiesin Decemberthan in June. content occurscloser to mid-latitudes in the south than in the Sincethe electrojetis drivenby tidal motions,this seemsalso north.During late local summer, autumn, and early winter to confirm the higher tidal intensityin December. there is more ozone over mid-latitudes in the southern Atmosphericozone. It is known [Butlerand Small, 1963; hemispherethan in thenorthern hemisphere [Kulkarni, 1962]. Lindzen,1966, 1967; Chapmanand Lindzen,1970] that the Satellitemeasurements of the backscattered ultraviolet earth source of semidiurnal tides is to be found in the ozone layer. radiance[Krueger et al., 1973] as well as infrared in• Thereforean asymmetricaltidal dissipationshould be cor- terferometricdata [Prabhakaraet al., 1973] shouldgive a relatedwith an asymmetryin the ozonedistribution between betterworld coverage between 80øN and 800S'forthe total the two hemispheres.Glass [1973] studied the effectof ozone ozone content and for the vertical distribution up to ap- distribution on the energy deposited by tides in the proximately50 km. Thereare, however, not yet enough data 5284 BARLIER ET AL.: NORTH-SOUTH ASYMMETRIES IN THE THERMOSPHERE to obtain a global picture of mesosphericozone, although Gadsden, M., On the twilight sodium emission,2, A theoretical model nighttime profiles can now be obtained from satellite of sodium abundance, Ann. Geophys.,20, 383, 1964. measurementsof the intensity of ultraviolet stars during oc- Gault, W. A., and H. N. Rundle, Twilight observationsof upper at- mosphericsodium, potassiumand lithium, Can. J. Phys., 47, 85, cultation by the earth's atmosphere[Hays and Roble, 1973]. 1969. Although the asymmetricalbehavior of ozoneis rather com- Glass, M., Etude des limites temporelies des ondes de gravit6 plicated, it is worthwhile to investigateits effecton the tidal observ6esdans la bassethermosph•re, Ph.D. thesis,Univ. de Paris, energy input in the upper atmosphere. Paris, 1973. Harper, R. M., Ph.D. thesis,Rice Univ., Houston, Tex., 1971. CONCLUSION Hays, P. B., and R. G. Roble, Observationof mesosphericozone at low latitudes, Planet. Space Sci., 21, 273, 1973. An asymmetrical behavior of the northern and southern Hedin, A. E., H. G. Mayr, C. A. Reber, N. W. Spencer,and G. R. thermosphereshas beendemonstrated with the aid of several Carignan,Empirical model of globalthermospheric t,emperature typesof experimentaldata. It seemswell establishedthat more and compositionbased on data from the Ogo 6 quadrupole spec- trometer, J. Geophys.Res., 79, 215, 1974. ' energymust be depositedin the southernthermosphere than in Hinteregger, H. E., The extreme ultra violet solar spectrum and its the northern thermosphere,even during the equinoxes.The variation during.a solar cycle, Ann. Geophys.,26, 547, 1970. origin of this fact is not completelyclear, but two mechanisms Jacchia,L. G. The temperatureabove the thermopause,Space Res., 5, can be considered, namely, the geomagneticfield asymmetry 1152, 1965. Jacchia, L. G., Semiannual variation in the heterosphere,A reap- and the tidal wave dissipation, linked to the asymmetrical praisal, J. Geophys.Res., 76, 4602, 19.71a. worldwide ozone distribution. Jacchia, L. G., Revised static models of the thermosphere and ex- ospherewith empiricalte.mPerature profiles, Spec. Rep. 332, 115 Acknowledgments. We would like to thank J. E. Blamont and J. pp., Smithson. Astrophys. Observ., Cambridge, Mass., 1971b. M. Luton for giving us temperaturedata and P. Waldteufel for giving Jaeck-Berger,C., ModUle statisticluede densit• globale entre 180 et us his temperaturemodel. We are very grateful to M. Glassfor several 500 km, Ann. Geophys.,29, 547, 1973. discussionsand to Diane Perret, who started this study and whose Johnson, F. S., Horizontal variations in thermosphericcomposition, results were largely used in this work. Rev. Geophys.Space Phys., 3, 741, 1973. Johnson, F. S., and B. Gottlieb, Eddy mixing and circulation at ionosphericlevels, Planet. Space Sci., 18, 1707, 1970. The Editor thanks L. G. Jacchia and another referee for their Johnson, F. S., and B. Gottlieb, Atomic oxygen transport in the ther- assistancein evaluating this paper. mosphere,Planet. 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