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High Electron Temperature Associated with the Prereversal Enhancement JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A1, PAGES 417-424 JANUARY 1, 1997 High electron temperature associatedwith the prereversal enhancementin the equatorial ionosphere K.-I. Oyama,1 M. A. Abdu,2 N. Balan,1'3 G. J. Bailey,4 S. Watanabe,5 T. Takahashi,5 E. R. de Paula,2 I. S. Batista,2 F. Isoda,6 and H. Oya5 Abstract. High electrontemperature in the equatorialionization anomaly region detectedfirst by Kyokkosatellite and later observedfrequently by Hinotori satelliteare foundto be closelyassociated with the ionizationcrests of the anomaly.This phenomenon,called the equatorialelectron temperature anomaly, is found to occur predominantlyin the equinoctialmonths and to becomeenhanced with the increasein solaractivity. It is mainlya nighttimephenomenon and showsmaximum temperature enhancementat around2100 LT. Using a theoreticalmodel, a mechanismfor its occurrenceis presented.The mechanismis basedon the plasmatransport in the evening equatorialionosphere resulting from the sunsetelectrodynamical processes. 1. Introduction equatorialionosphere sunset electrodynamics is suggestedfor the observedanomalous Te enhancement. The Japanesesatellite Kyokko, launchedin 1978, encoun- tered regionsof high electron temperatureTe over the low- latitudeionosphere during several of its eccentrictrajectories. 2. Morphology of the Hot Region In somecases the temperatureenhancement, with respectto Figure la showsthe electrondensity, and Figure lb shows the expectednighttime backgroundtemperature, exceeded the electrontemperature measured by Hinotori on pass3599. 2000 K [Oyamaand Schlegel,1984]. The region of the en- The two shadedregions in Figure lb representenhancements hancedelectron temperature extended from magneticlatitude in T• of •600 K and •1000 K at magneticlatitudes 8 ø and -9 ø, -30 ø to 7ø . The temperatureenhancement showed maximum respectively,with respectto the backgroundT• values(repre- valuesin the 300ø-310ø geographiclongitude and -30 ø to -8 ø sentedby a dashedcurve). These regionscoincide with the geographiclatitude region.Further, it may be noted that this electrondensity enhancements of the equatorialanomaly (Fig- electrontemperature enhancement occurred in the region ure la). The latitudinalstructure of the equatorialanomaly is wherethe electrondensity was also high. About 16 casesof T e characterizedby a troughat the magneticequator with crests enhancementwere observedout of all the Kyokkopasses. It on eitherside. This feature is reflectedclearly in the latitudinal was also noted that all these Te enhancementsoccurred in variationof T•. The equatorialelectron temperature anomaly (EETA) is thus associatedwith the equatorial ionization March 1978.Although this was a new finding,a detailedin- anomaly(EIA). For a detailed descriptionof the EIA, see vestigationcould not be carried out due to the difficultyin Hanson and Moffet [1966], Moffet [1979], Walkerand Chan identifyingits mechanismand the availabilityof only a small number of observations. [1989],and Balan and Bailey[1995]. The T• enhancementof 1500 K with respectto the backgroundat 0500 LT is a sunrise In 1981the Hinotori satellitewas placed into a nearlyequa- associatedphenomenon. torial (with inclinationof 31ø ) circularorbit at an altitude of about600 km. This satellitealso recorded the phenomenonof 2.1. Global Distribution of the Hot Region the Te enhancement.During its 17 monthsof operation,Hi- The locationsof enhancedT e with respectto the normally notorifrequently observed this phenomenon. The observations expectedbackground were identified in a large number of describedin this paper clearlydemonstrate the associationof Hinotori passesin the evening-premidnightlocal time sector. the Te enhancementwith the electrondensity enhancement of The locationsso identifiedare marked by triangleson the the evening-nighttimeequatorial ionization anomaly. Using a trajectorysegments of the differentpasses shown in Figure2. theoretical model, a mechanism that invokes the role of the The magneticequator is shownby a dashedcurve. As Figure2 showsthe hot regionsare distributedpredominantly at mag- qnstituteof Spaceand Astronautical Science, Sagamihara, Japan. neticlatitudes away and on eitherside of the magneticequator, 2NationalInstitute for SpaceResearch, Sao Jose dos Campos, Bra- not on it. Though not shownseparately, these locationsare zil. 3Onleave from the Departmentof Physics,University of Kerala, alsothe locationsof the electrondensity enhancements of the Trivandrum, India. EIA crests.The reasonfor the relativelypoor data statisticsin 4AppliedMathematics Section, School of Mathematics,University the longituderegion of 130ø-180øis due to the fact that during of Sheffield,Sheffield, England. the timewhen the satellitewas located in thislongitude region SFacultyof Science,Tohoku University, Sendai, Japan. the satellitetracking center (Kagoshima) was recoveringthe 6Facultyof Education,Yokohama National University, Yokohama, Japan. data storedfrom other longitudes. Copyright1997 by the AmericanGeophysical Union. 2.2. SeasonalDependence Paper number 96JA02705. The Te enhancementswhich exceed 200 K were separated 0148-0227/97/96JA-02705509.00 and are plotted in Figure 3a at the latitudesof their occur- 417 418 OYAMA ET AL.: EQUATORIAL ELECTRON TEMPERATURE ANOMALY DATE ----81/10/20 REV"- 3599 1 3000- 2000- LU LU!-- 1001 UT 3:50 4:0 4:10 4:20 4:30 4:40 4:50 5:0 5:10 5:20 5 30 LT 14:12 16:44 18:55 22:9 23:51 2:43 5:11 7:20 9:38 12:21 15:9 LON 155.5 191.0 221.2 252.3 290.0 330.7 5.2 35.0 66.8 105.2 144.6 I_AT 29.3 16.5 -2.2 -20.2 -30.7 -27.6 -12.9 6.2 23.1 31.2 25.8 MLAT 21.3 14.6 2.0 -11.2 -19.5 -19.0 -10.0 3.0 14.7 20.1 16.5 ALT 624.7 611.5 592.5 575.5 567.4 571.6 586.4 605.6 621.4 627.4 621.2 Figure1. (top) The electrondensity. (bottom) Electron temperature measured by Hinotoriduring pass 3599.The twoequatorial ionization anomaly (EIA) peakscentered around the magnetic equator at 0410UT coveringthe localtime interval1700-2300 LT are associatedwith the enhancedelectron temperatures (shadedregions) with respect to the backgroundtemperature and the IRI representation(dashed curve) [Bilitza,1990]. The T e enhancementaround 0511 LT is the sunriseeffect. rencesagainst the dayof the year(top) togetherwith the daily the largerenhancements are concentratedin the equinoctial solar flux at 10.7 cm (bottom).The daysare countedfrom monthsof March and April (aroundday 90), Septemberand January1, 1981; the Hinotori data startedon February22, October(around day 270), and againin March and April 1981, and continued until June 11, 1982. The sizes of the (aroundday 450) of thefollowing year. To seethe solar activity trianglescorrespond to the degreeof departureof the Te value dependenceon the Te enhancement,•Te wasplotted against from its nighttimebackground value (•T e); the larger the thesolar radio flux at 10.7cm in Figure3b. A faintdependence triangle,the largerthe T e enhancement.It is clearlyseen that of the Te enhancementappears to existin dayswhen the solar mo ø _ ! I I I I I I I II I I I I I I ! I I 10.00 36.00 72.00 108.00 144.00 180.00 216.00 252.00 288.00 324.00 360.00 LONGITUDE Figure2. Locationsmarked in solidlines of enhancedelectron temperature along Hinotori trajectory segments,the maximum value of Te beingindicated by triangles.The dashedcurve is themagnetic equator. OYAMA ET AL.: EQUATORIAL ELECTRON TEMPERATURE ANOMALY 419 0•1 I I I I I I I I I I I I I I I I I I 0 60 120 180 240 300 360 420 480 540 600 DAY NUMBER Figure 3a. (top) Occurrenceof hot plasmaregions marked at geomagneticlatitude versus day number startingfrom January 1, 1981(1 monthand 20 daysbefore the Hinotorimission), till the endof the Hinotori dataon June 11, 1982. Increasing sizes of theplotted triangles indicate increasing Te deviation (up to 1200K). (bottom)The solarF•o.7 cm flux diurnalaverages for the sameperiod. flux exceedsabout 150 units. In fact, the seasonaldistribution 2.3. AssociationWith Ground-BasedEIA Diagnostics of the A Te shownin Figure3a is somewhatmodulated by solar fluxvalues greater than 150 unitswith AT e generallyincreas- CachoeiraPaulista (45øW, 22.5øS) in Brazil is situatedvery ing as the solarflux increases.This modulationis to be spe- closeto the EIA crestlocation. Ionograms obtained from rou- ciallynoted for the casesof increasingflux at arounddays 345 tine soundingsover this station were studied when Hinotori and 400, whichproduced enhanced T e valuesduring Decem- flew over Brazil. Although the region surveyedby the ion- ber and February,exceptions from the equinoctialoccurrence osondeis not exactlythe sameas that of the satellitepasses, pattern. their proximity(in somecases by field line connection)could 1400 12.00 1 ooo 800 . 600 : : ß • ß ß • e• 400 ....................................!................... ';'..... ','"'i";'"," .............................. ß.............. ':.......... !................................... -: ,..': 'i."l '. ,. ' ! : 2OO ...................i .........;",&'"'"•'"'""i ......' ........ .t ......,--- ..................... ß........ !.................................. -: : 0 .... ' .... ' .... ' .... J .... ; 1 O0 1 50 2.00 2.50 300 350 F10.7 Figure 3b. Departureof Te fromits nighttime background (/•Te) versussolar F•o.7 cm flUX are plotted in the time period 1900-2100 LT. 420 OYAMA ET AL.: EQUATORIAL ELECTRON TEMPERATURE ANOMALY F- r- --40 _ _ -- ß ß 600 500E_ ß . 400• -.1500 ' •_ ,1 ooo 200:300 ¾ ' ' r • II"' ••500 :- I I ' II1:] •oo- I , , • ,,,,, I , , I,,,I I I I I .,I I I I I iI, I I I I I I I I I I o 0 12 00 12 00 12 00 12 00 12 00 12 00
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