Radio Science,Volume 20, Number 4, Pages795-802, July-August 1985 The D regionwinter anomaly at highand middlelatitudes induced by planetarywaves Kohji Kawahira GeophysicalInstitute, Kyoto University,Japan (ReceivedNovember 16, 1984; revisedFebruary 4, 1985; acceptedMarch 20, 1985.) An observationalstudy of the D region winter anomaly at high and middle latitudeshas beenmade duringthe periodof a suddenstratospheric warming of the 1967/1968winter. From isoplethanalysis of the absorptionindex, fmin, systematiclarge-scale distributions of the absorptionare found at even higherlatitudes than 60ø geomagneticlatitude as well as at middlelatitudes. On the basisof evidence that geostrophicwinds induced by well-developedplanetary waves extend to the upper D region,it is found that when southward winds are dominant, the absorptionsbecome weaker at high latitude but strongerat middle latitude, indicatingthat abundant NO could be carried away from high-latitude sourceregions to middle-latitudesink regions.Thus large amplitudeplanetary wavesin the D region could inducethe winter anomaly through NO transportnot only at middle but also at high latitudes. 1. INTRODUCTION transport of NO by enhanced eddy diffusion. A The winter anomaly of radio wave absorption in recent numerical study by Jones and Avery [1984], the D region (60-85 km), which is directly due to on the other hand, demonstratesthe role of planetary remarkable enhancementin the electron density, has wave wind transport of NO from high latitudes in been an important unresolvedphenomenon in the producing the irregular components.There are some lower ionosphere [Offermann, 1979]. According to inconsistenciesin theoretical studies of irregular Arnold and Krankowsky[1977], necessaryconditions components. for the observed electron density increase are an An observationalstudy by Kawahira [1982] of the about tenfold increase of ionization sources as NO winter anomaly at middle latitudes at sudden warm- (andO2(xAg)) and a temperaturerise of a fewtens of ings has given observational evidences that a NO degreesKelvin due to a strongtemperature depen- increase by well-developed planetary wave wind dence of the conversion rate of molecular ion to clus- transports from the polar region, where NO is most ter ion. Thus it is important to identify what pro- abundant [Cravens and Stewart, 1978; Iwagami and cessescould bring about these conditionsin the Ogawa, 1980], could produce irregular components winter D region. of the winter anomaly. Recent observationalstudies With the use of a two-dimensionaldynamical and by Pakhomov et al. [1984] have also verified the chemicalmodel, Solomonet al. [1982] simulatedthe effect of large-scalewinds on the winter anomaly at middle latitudes. winter anomaly at middle and high latitudes, re- sultingfrom a NO increasedue to verticaltransport These observational studies are confined to south from the lower thermospheresource region by eddy of 60ø geomagneticlatitude, south of aurora region. diffusion and meridional circulation. Their results are However, well-developed planetary waves in the consistentwith the regularcomponent of the winter mesospherecould induce a NO change by wind anomaly [Schwentek,1971]. Furthermore, they transport effectsalso at high latitudes, as shown by a pointed out the importantcontribution of aurora numerical study by Roble and Gray [1979] in the eventsto the high-latitudeNO increaseassociated case of NO transport in the lower thermospheric with energeticelectron precipitations [Kondo and polar region. Indeed, recent LIMS satellite observa- Ogawa,1976]. Concerning the mechanismof the ir- tions of NO2 in winter high latitudesby Russelet al. regularcomponent of the winteranomaly, Offermann [1984] find the large-scale distribution at polar et al. [1982] stressedthe importantrole of vertical winter night latitude above 70-km height. This first finding verifies a planetary wave effect on the D Copyright 1985 by the American GeophysicalUnion. region chemical species,not only at middle but also Paper number 5S0256. at high latitudes. Although the high latitude winter 0048-6604/85/005S-0256508.00 anomaly has been studied mainly in relation to 795 796 KAWAHIRA' WINTER ANOMALY INDUCED BY PLANETARY WAVES dencefor the mechanismof this winter anomaly as a NO increase due to southward transport from high latitudes by well-developedplanetary wave winds. A -4O ,/"•Z•••(A)TE MPERATURE schematic picture of the mechanism is shown in Figure 2 rKawahira, 1985]. These large-amplitude -8O • 35KM, FT. CHURCHILL planetary waves in the mesospherehave been identi- - ---o-- 50KM, W. GEIRINISH fied by satellite observations of temperatures [e.g., Hirota and Barnett, 1977]. Thus the figure shows a realistic pattern of geopotential height, especiallyits large variation along latitude circle, in the winter D region at the time of stratosphericsudden warmings. 10-2 Then strong southward geostrophic winds could transport abundant NO from high to middle lati- 10'3 tudes and here bring about an enhancementin NO concentration, resulting in the winter anomaly. On D-4 the other hand, northward winds could cause a de- A .., .... L, UNOAU crease at middle latitudes. As suggestedin Figure 2, ;(u•• •*•:'t.,tu,O•_or '150 •'. WANTE.D SIGNAL. / • ;: U•v[RSITY PARK / even at higher latitudes than 60ø geomagneticlati- ;; ,, ', •x a 75') J o tude, there would be a large-scalewind effect on NO variations in addition to geomagneticstorm effects. •00 In order to elucidate the effect of planetary wave winds on the absorption, the isopleth analysis of z•, : , Afrei., which is the deviation from monthly median 2000 value of frei., is made at high and middle latitudes 1 including aurora region. The Afain is a good indica- 3OOO , ! , I , I I , I • I •,/, • ! Jlo I0 •0 ::• I0 •0 tor of the absorption [Sinno and Higashimura,1969], DECEMOER 1967 J•.NUARY 1968 the isopleth of Afmincorresponds to that of the elec- Fig. 1. Variationsof geophysicaland ionosphericparameters tron density in D region produced by ionization of duringDecember 1967 and January 1968 [after Rowe et al., 1969]. NO due to Lyman • solar radiation, because the analysis is confined to the south of the polar night dominantgeomagnetic effects such as energeticelec- latitude (70øN). tron precipitation[see review of Thomas,1980], the presentstudy aims to elucidatethe dynamicaleffects of planetary waveson the winter anomaly at high [NO]increase latitudesfrom an observationalanalysis. ß 2. RESULTS The present study concentrates on the winter ß anomalywhich occurred during a suddenwarming of the 1967/1968 winter from late December 1967 to early January 1968. Variations of geophysicaland ionosphericparameters are shownin Figure 1 [Rowe et al., 1969]. It is apparentthat the absorptionsen- hanceas the warmingsdevelop, as seenin Figuresl a and ld. Accordingto Roweet al., theelectron density observedat Pennsylvania(40øN) reachedits maxi- mum at 75-80 km height during the period from December25, 1967,to January1, 1968.They further noticedthat the winter anomaly has an intimate re- nc reuse lationshipto suddenwarmings, but not to geomag- neticstorms from the long-termcomparison between Fig. 2. A hemisphericchart of geopotcntialheight in the •intcr D region at the period of suddenwarmings, and schematic ,4p index and the absorption. illustrationfor the mechanismof the •inter anomaly[after Kawahira [1982] proposed an observationalevi- hira, 1985]. KAWAHIRA' WINTER ANOMALY INDUCED BY PLANETARY WAVES 797 • '1:.•!•0.1mHz) (c•) 3 JAN. 196• I i .• (b) 1 0 JAN. 1968 Fig. 3. Large-scaleisopleth analysis of the absorptionindex, Afmin,in the form of 3-day runningmeans using the 3-hour averageat 11, 12 and 13 local time of the deviationfrom the monthly median value of frei. (a) on January3, 1968,and (b) on January 10, 1968. Positivearea is an increasein absorption,and negativearea is a decrease.Dotted line indicates60 ø geomagneticlatitude (A - 60ø). The results of January 3 and 10, 1968, are shown tudesis clearly detected;the scaleis similar as that of in Figure 3, when the warming reachedits peak and planetarywaves. It is noted that the distributionof the absorption enhancementwas remarkable [Rowe Afminalong 60øN is not uniform but showsa "wavy" et al., 1969; Kawahira, 1982]. A systematicand large- pattern.Furthermore, there is a negativecorrelation scaledistribution of the absorptioneven at high lati- of the absorptiondistributions between middle and 798 KAWAHIRA' WINTER ANOMALY INDUCED BY PLANETARY WAVES Geopotentic[i Height 60"1'4 3 JAN.196 6 (a) 10 JAh[1966 (b) (College)Fort Churchill West Gelrinish I I I I I I i - Height 180 90W 0 180 90W 0 --.,,. (km) ß1 km F t 'x O.056mb --70 68km--•-- • -1 kmm-- • ß ß1 km F 62kmT '' -• ''' x..0.15rob -- 60 -1 krn •'-- -- 50 ß1 km F -1 km L-- --40 -- 30 Fig. 4. Vertical profile of geopotentialheight along 60øN from 0øW to 180øW. Real height is shown in the right-hand side,and the scaleof the geopotentialheight in the left-hand sideis twice the real height. The thick line is after the Upper Air SynopticMap (NOAA), and the dotted lines are estimatedfrom hydrostaticbalance with use of observedtemperature at Fort Churchill and West Geirinish, (a) on January 3 and (b) on January 10, 1968 [after Kawahira, 1982]. high latitudes;e.g., along 90øW positiveAfmin is lo- by planetary waves as shown in Figure 4. Thus the cated south of 60øN and negative Afmin, north of planetary wave induced winds would be dominant 60øN. even up to the upper D region, becausethe data at Then the evidence of large amplitude planetary