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

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North-South Asymmetries in the Thermosphere During the Last Maximum of the Solar Cycle F 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 HAL Id: hal-01627389 https://hal.archives-ouvertes.fr/hal-01627389 Submitted on 1 Nov 2017 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 holds true for the ex- ospherictemperatures 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 different at 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. 4 Institutd'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
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