Indian Journal of Radio & Space PhysicsVol. 19, August 1990, pp. 225-240

Equatorial anomaly in F-region-A reviewJ Hanumath Sastri,

Indian Institute of Astrophysics, Bangalore 560034

The current knowledge of the equatorial anomaly in the electron density distribution of the ionosphericF-region is reviewed with emphasis on issues relevant to the quantitative modelling of the phenomena. Thebasic physical mechanisms responsible for the formation, development and decay of the anomaly and theirmutually interactive nature are first outlined, followed by a brief discussion of the relevance of the anomaly toother characteristics of the equatorial low latitude ionosphere. Salient features of the schemes adopted fornumerical simulation of the anomaly and the inputs required for the same are discussed. The inadequacy ofthe currently available global models of the input parameters is pointed out through some of the recentexperimental results. The need for developing reliable data bases of the key parameters of the ionosphere­thermosphere system from systematic, extensive and simultaneous multi-technique/multi-stationobservations on a regional/global basis for a comprehensive understanding of the anomaly is highlighted.

1 Introduction

Equatorial anomaly refers to the double-humpedstructure in the latitudinal profile of the F-regionionization density at low geomagnetic latitudes with atrough centered around the dip equator and twocrests on either side of it around 18°diplatitude. It isone of the many anomalies of the terrestrialionospheric F-region and is a characteristic daytimephenomenon of the equatorial ionosphere. Since thetime of its discovery by Appletonl the anomaly hasbeen the theme of extensive experimental as welltheoretical investigations. The efforts helpedestablish the origin of the anomaly as essentially dueto plasma transport processes in the low latitudeionosphere. They also helped to gain a qualitativeunderstanding of the physical mechanisms that canaccount for the diurnal cycle of formation,

. development and decay of the anomaly and itsvariability with season, phase of the solar activitycycle, longitude and level of geomagnetic activity.Quantitative modelling or simulation of the anomalyto test the theory and the input parameters used forthe simulation, through a comparative study of thepredicted and observed features of the anomaly is,however, in an infancy state. In this paper we present areview of the current status of knowledge of theanomaly with an emphasis on the prevailingdeterrents for extensive quantitative modelling. Thedirections to further work to overcome the deterrents

and to develop a comprehensive understanding and,eventually, predictive capabilities of the anomalywhich has important practical applications areoutlined.

2 MorphologyThere is a wealth of information in the literature on

the morphology of the anomaly derived fromobservations with ground-based ionosondes to startwith, and later from topside sounders, airglowphotometers (ground-based and satellite-borne)and total electron content (TEe) measurementsusing radio beacon transmissions aboard orbitingand geostationary satellites. Review articlessummarizing the observational knowledge of theanomaly have appeared in the literature from time totime2 - 5. In this section we present the salient featuresof the gross morphology of the anomaly to providethe background for a discussion of the theoreticalstudies and their achievements and shortcomings.

2.1 Height structure

The anomaly extends throughout the F-regionstarting from about 200 km but with a characteristicaltitude dependent manifestation. At lower altitudesbelow the height of peak electron density (hmF2), theanomaly crests are shallower and lie farthest from thedip equator. With increase in altitude the crests gainprominence and move closer to the dip equator suchthat the depth of the anomaly (defined as the ratio ofthe electron density at the crests to that at trough) ismaximum around hmF2. In the topside F-regionabove hmF2, the crest amplitudes again decrease andthe crests lie closer to the dip equator such that theanomaly is not usually seen above 1000km. Thelocusof the crests at different altitudes follows ageomagnetic field line called the anomaly field line.The anomaly also manifests in the latitudinal

225

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

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Fig. I---Changes in the latitudinal width of the equatorialanomaly (at noontime} between solar maximum and minimumperiods (upper panel) and variation of the time of maximumdevelopment of the anomaly (at 7YW) with the phase of the solarcycle (1954-64) as a function of seasOn .(bottom panel) (after

Rastogi2 and Rush £'( a/.6)

asymmetric with a larger winter crest than thesummer one. The sense of asymmetry reverses duringthe later part of the night, particularly at solarmaximum when the anomaly persists for a major partof the night 10- 12. An asymmetric anomaly alsomanifests during the equinoxes though with a markedday-to-day variability in the sense of theasymmetry7.12.13. The asymmetry of the anomalyextends well into the topside F-region with the loci ofthe anomaly peaks at different altitudes in the twohemispheres following two different field lines6.

2.4. Geomagnetic storm effects

The structure of the equatorial anomaly under­goes significant changes during geomagneticdisturbances. The pattern commonly noticedirrespective of season and epoch of the solar cycle is aweakening of the anomaly both in the bottomside andtopside F-region3. In bottomside, the response ischaracterized by an enhancement of ionization in thetrough region around.the dip equator and a depletionaround the crests, while in the topside, depletion ofionization in a dome-shaped region over the equatorand at low latitudes is seen. Deviations from thisgeneral pattern do occur. particularly with severegeomagnetic storlns 14 - 16. Examples of such eventsarc the observations. during some storms. of marked

2.3 Seasonal effects

An established feature of the anomaly is theasymmetry of its two crests as regards to theiramplitude and latitude positions and its dependenceon local time, season and longitude. At solarmaximum as well as solar minimum epochs. thedaytime anomaly inf,F2 during the solstice months is

2.2 Diurnal pattern

The development of the anomaly exhibits a diurnalpattern that is dependent on the phase of the solaractivity cycle. During equinoxes, at solar minimum,the crests begin to form in the bottomside F-region(foF2) around 0900 hrs LT and move polewards untilthey reach about 16° dip latitude by 1600 hrs LT. Thecrests then move equatorwards disappearing by 2100hrs LT. The duration of the anomaly is thus limited to0900-2100 hrs LT with maximum development at1600 hrs LT (the time of maximum developmentrefers to the time when either the depth of the anomalyor the latitudinal separation of the crests ismaximum). At solar maximum, though theformation of the crests takes place around 0900 hrsLT as at solar minimum, the crests continue todevelop and move polewards throughout the day tillaround 2000 hrs LT when maximum developmentoccurs. The crests then retrace their motion

disappearing eventually in the later part of the night.The anomaly thus prevails for an extended period(0900-0200 hrs LT) at solar maximum and the time ofmaximum development is delayed by abO'Ut 4 hrrelative to solar minimum. Besides the shift in the timeof maximum development which is evident in allseasons6, the latitudinal separation of the anomalycrests (the anomaly width) is also dependent on thesolar cycle phase(it is higher by about 6° at solarmaximum than at solar minimum2). These twoprominent features of the solar cycle variation of theanomaly are illustrated in Fig. I. The diurnaldevelopment of the anomaly outlined above showspronounced longitudinal differences3 - 5.7. Anextreme form of the longitudinal variability of theanomaly is its strikingly different form in zonesseparated in longitude by as low as 30-40° on certainindividual days as well as in its average pattern forcertain periods4-9. An example of this behaviour asobserved in the topside F-region is presented in Fig.2.

distribution of the ionospheric total electron content(TEe) at low latitudes, and this should be expectedbecause a major contribution to TEe comes fromionization within a few hundred kilometres aroundhmF2 though mainly on the topside3.

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SASTRI: EQUATORIAL ANOMALY IN F-REGION

2.5 Variability

It is a common ohservation that at any givenlongitude the diurnal cycle of formation.

3 Theory3.1 Fountain effect

The daytime F-region is governed by a delicatebalance between production of ionization throughphoto-ionization and loss due to chemical andplasma transport processes. In the vicinity of the dipequat&. vertical diffusion of plasma is not feasiblewithout the aid of an applied electric field because ofthe unique eonflguration of the geomagnetic fieldthere. Such an electric field, nevertheless, prevails dueto the global E-region dynamo driven by tidalwinds5• The E-region east-west electric field getsmapped to the F-region along the highly conductingfield lines, where it causes vertical Ex B plasma driftdue to interaction with the north-south magneticfield. Besides, as the ion gyro-frequency is muchhigher than the ion-neutral collision frequency atF-region altitudes, the plasma once lifted upwardscan diffuse along field lines under the influence of

development and decay of the anomaly exhibitsconsiderable short-term (day-to-day) as well aslong-term (from year to year) variability besides theseasonal and solar cycle trends mentioned above.Part of this ubiquitous variability is obviously due tothe frequency of occurrence of geomagneticdisturbances and their severity. An example insupport of this point of view is the recently evidencedmodulation of characteristics of the anomaly (in theIndian sector) by the polarity of interplanetarymagnetic field (IMF) during equinoxes of high solaractivity epochsl7. Due to changes in the relativeorientation ofGSE and GSM coordinates during thecourse of the year, the level of geomagnetic activity ishigher on negative (sunward) IMF days as comparedto positive (anti-sunward) IMF days in vernalequinox (March-April), and opposite is the situationduring autumnal equinox (September-October)18.In response to these geomagnetic conditions broughtabout by nature, the depth of the anomaly duringdaytime is reduced in vernal equinox and enhanced inautumnal equinox on days with negative IMFpolarity as compared to days with positive IMFpolarity (Fig. 3). The nature of the modulation, whenvie'Yed in the light of the known average pattern ofdependence of the anomaly depth on geomagneticactivity, indicates that it is a second-order effectbrought about by the IMF polarity-related changesin the semi-annual variation of geomagnetic activity.Further concerted efforts are needed to comprehendthe causative mechanisms of the short-term as well aslong-term variahility of the anomaly characteristicsthrough innovative studies of the substantial databases that exist now.

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depressions inf~F2 around the dip equator withoutany corresponding changes at the crests (i.e.enhancements of the anomaly depth) during thestorm recovery phasel5 and enhancements of theanomaly in the topside at low latitudesl6.

227

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

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obtained from numerical solution of plasma continuity equation

with (a) F-region vertical plasma drift neglected and (b) verticalplasma drift included [the latter ease represents a realistic drift

model for the formation of the anomaly (after Anderson"]

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(b)

It was found that field-aligned plasma diffusion,though important, cannot produce a large enoughanomaly by itselfand vertical upward plasma drift isessential for the formation of the anomaly, thusconfirming the 'fountain' theory of Martyn.(Ref. 21and references therein). This result which constitutesa milestone in the theoretical understanding of theanomaly is illustrated in Fig. 4 for equinoctial solarmaximum conditions. It is quite evident thatinclusion of the upward plasma drift leads to theformation and maintenance of the anomaly tillaround 2200 hrs LT, and is in good agreement with theobservations outlined earlier.

The theoretical studies were greatly aided by thedirect measurements of the F-region vertical plasmadrift with the VHF radar technique at Jicamarca,Peru (12.00S, 76.9°W, dip 2°N) which provided notonly the experimental evidence for the vertical driftrequired by the fountain theory, but also realisticinputs for theoretical studies22. Experimentalsupport for the dominant role of the fountain processin anomaly formation was provided by the statisticalcorrelative studies of the strength of the anomaly (asevidenced in the bottomside and topside ionosphere

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Indian equatorial region on days of negative (sunward) and

positive (anti-sunward) polaritv ofintcrplanctary magnetic field(1M F). illustrating the modulation of the anomaly depth during

eq uinoxes of the period 1957-5') [The oppositely directed relative

changes in the anomaly depth \\ith reference to IMF polaritybetween the vernal (March-April) and autumnal

(September-October) eqUinoxes indicate the IMF

polarity-related modulation of the anomaly which is primarily

through changes in geomagnetic activity (after Sastri 17)]

gravitational and pressure gradient forces. Thisimportant and unique facet of the dynamics ofequatorial F-region and its relevance to theformation of the anomaly were recognized andadvocated by Mitra19 and Martyn2o. While Mitrasuggested that field-aligned diffusion of ionizationfrom equator could be responsible for the formationof the anomaly, Martyn advanced the theory that theanomaly is due to the combined effects of verticalupward drift of ionization near the dip equator and itssubsequent diffusion to higher latitudes, amechanism that has come to be known as the'fountain effect'. The availability of high-speedcomputers facilitated numerical studies of theinfluence of the various plasma transport processeson the latitudinal distribution of F-region at lowlatitudes through solution of the plasma continuityequation.

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and in TEe) and the strength of the equatorialelectrojet (which is driven by the same E-regionelectrostatic electric field that is responsible for theF-region vertical plasma drift)23 - 27.In particular, anear 100 % correlation between the anomaly strengthand the time-integrated strength of the electrojet isfound for daytime hours (Fig. 5) lending firm supportto Martyn's theory26. The high correlation alsosuggests a role of perturbations in the E-regionelectric field in the day-to-day variability of theanomaly characteristics. In fact, case studies of thechanges in the electron density profiles in thebottomside F-region on strong counter-electrojetdays at stations near the trough and crest latitudes ofthe anomaly demonstrated the expected response ofthe anomaly parameters to changes in E-regionelectric field through the 'fountain' process28. Themaximum development of the anomaly around 2000hrs LT, which is a characteristic feature of the diurnalpattern of the anomaly at solar maximum, is widelyconsidered to be due to a 'renewal' of the fountain

effect, resulting from a post-sunset increase of theeastward electric field (upward plasma drift) beforeits reversal to westward direction29.3o. Themorphology oflunar tides (lunar semi-monthly andlunar semi-diurnal tides) in NmF2, hmF2 and TEC atlocations in the anomaly belt suggests the origin of the

tides as primarily due to lunar modulation of the'fountain' effect (Refs 31 and 32 and papers citedtherein). The smaller amplitudes of the lunar tides inTEC than in Nm observed close to the dip equator is,however, not consistent with the fountain theory andneeds further investigation32.

3.2 Neutral wind effects

The theoretical studies also demonstrated that,although the formation of the anomaly is primarilydue to the fountain mechanism, many intricatefeatures of the anomaly morphology can beunderstood only when the effects of horizontalneutral winds at F-region altitudes (particularly themeridional component) set up by the global pressuredistribution are considered (see review of Moffett33and references therein). The neutral winds systeminfluences the F-region ionization through its abilityto move the ions and electrons along the magneticfield lines. The associated vertical component ofmotion affects the ion and electron densities becauseof the height-dependent nature of the F-regioneffective .chemical recombination rate. A strikingexample of the wind effects is the asymmetry of theanomaly which commonly prevails during thesolstice months of solar maximum epochs and thelocal time changes in the sense of asymmetry. Under

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Fig. 5--&atter plots of correlation between anomaly strength( s) and the strength of equatorial electro jetrepresented by areas (a and b) and by instantaneous values (c and d) estimated from magnetograms at1000 hrs LT and during 1000-1200 hrs LT [The correlation coefficients are also indicated inside the boxes

(after Raghava Rao 1'( al.26).]

229

INDIAN] RADIO & SPACE PHYS, AUGUST 1990

Fig. 6---Electron density contours (log NO> for December solsticeconditions as a function of altitude and dip latitude at (a) 2000 hrsLT and (b) 2400 hrs LT, illustrating the ability of transequatorialwinds (south to north) to produce an asymmetric anomaly [Tooelectron densities are obtained from solution of the F-regionplasma continuity equation with inputs on neutral temperatureand winds from NCAR TGCM (after Anderson and RobleJ4).]

solstice conditions, transequatorial winds prevaildue to the highly asymmetric pressure distributionsabout the equator, and these can transport plasmafrom the summer hemisphere to the winterhemisphere. In doing so, the layer is raised in thesummer hemisphere (up-wind side) and lowered inthe winter hemisphere (down-wind side) leading toan asymmetric latitudinal distribution of ionization.A theoretical simulation of the effect of

transequatorial winds on the anomaly is illustrated inFig. 6 for December solstice conditions near solarmaximum (F!(17 = 160 x 10- 22 Wm - 2 HZ-I) (Ref.34). At 2000 hrs LT, due to the post-sunsetenhancement in the upward E x B drift the F-Iayer israised to about 600 km at the dip equator, while theionization transport due to transequatorial wind

leads to north-south asymmetries in both the heightand magnitude of the F-region peak electron density(hmF2 and NmF2). The north-to-south density ratioat the crests of the anomaly at this time is about 1.7(upper panel). Subsequently, due to the reversal in theupward E x B drift and associated weakening of thefountain mechanism, the layer is lowered to about 400km at the magnetic equator accompanied by ashifting of the anomaly crests closer to the equator.Resides, as the northern crest is maintained at a loweraltitude, it decays faster than the southern crest,reducing the north-to-south density ratio from 1.7 to1.25 by 2400 hrs LT (lower panel). Eventually, thesense of the crest asymmetry reverses with thesouthern crest becoming greater than the northerncrest (by 0400 hrs LT), and is in good agreement withthe observations. Another important aspect of theanomaly morphology that seems to result fromneutral wind effects is the pronounced longitudinaldifferences in the diurnal pattern of the anomaly fromday to day and from season to season. These arecurrently understood in terms of meridional windeffects due to the regional differences in (i) the realtivelocations of the geographic and geomagneticequators and (ii) the geomagnetic declinations4.

A great deal is learnt from theoretical work andanalyses of data about the origin of the anomaly andits v<lriability under varied geophysical conditionsand for different longitudinal zones. The variability isgenerally interpreted in terms of changes in thephysical mechanisms that govern the anomaly,namely (i) vertical drift of F-region plasma aroundthe dip equator which depends on the local values ofelectric field (E) and the geomagnetic field (B) and (ii)neutral wind field (Vn) particularly the meridionalcomponent whose magnitude and direction dependon the global/regional pressure distribution andambipolar diffusion that depends on the neutraldensity and scale height. We thus have now a soundqualitative understanding of the various aspects ofthe anomaly and there are no basic physics questionsabout its formation. In other words, the anomaly is nomore an enigma or mystery. But, we do not have aquantitative understanding of the anomaly, muchless predictive capabilities. This piquant situationcan be gauged from the following facts: (i) Most of thetheoretical studies done so far were aimed primarilyat explaining the gross features of the anomaly such asits diurnal life cycle, asymmetry, etc. Even then thesolar cycle variation of the anomaly remained to beproperly understood (see Ref. 5 for a discussion of theproblem), and (ii) Although the storm-timemorphology of the anomaly has been extensivelystudied using the varied data bases that become

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SASTRI: EQUATORIAL ANOMALY IN F-REGION

available from time to time, complementarymodelling work to interpret the observations onquantitative terms had not been done on the scale

needed. The only exception is the study of Woodmanet al.35 wherein the diurnal pattern of the electrondensity distribution near the dip equator is simulatedand compared with the observations. The very firstquantitative study has just been reported by Fesen etal.36 for the magnetic storm of 22 Mar. 1979, payingparticular attention to the storm-time perturbationsin the neutral atmosphere and ignoring the possibleelectric field perturbations. While remarkably goodagreement was found between the predicted andobserved changes in the ionospheric parameters atcertain longitudes, poorer agreement was apparent atthe others which is attributed partly to the neglect ofelectric field perturbations. The glaring absence oflarge-scale quantitative modelling efforts so far isprimarily due to the non-availability of the requiredinput data from actual observations. This majordeterrent and the issues related to it are dealt with indetail in Sec. 5.

4 Plasma-neutral interactionsThe bulk motions of the ions and electrons in the

ionosphere are greatly influenced by the ion-dragwhich arises as a result of collisions between them. At

F-region heights the ion-drag which acts as aresistance to the neutral gas flow is proportional to theelectron density and, hence, a variation in the electrondensity leads to a perturbation in the neutral gasdynamics. A strong coupling thus exists between theneutral and the ionized constituents in the

ionosphere, because the neutral wind affects theionization density which, in turn, modifies the windsystem through ion-drag. In this section we describe,in brief, a few physical situations in which ion-dragplays a prominent role to highlight the interactivenature of the physical mechanisms underlying theanomaly.

A poignant indicator of ion-drag effects in theequatorial ionosphere is the neutral anomaly whichrefers to the anomalous latitudinal distribution of

neutral densities (N2) at 450 km at low latitudes, with atrough close to the dip equator and crests about 200 tothe south and north of it37. Neutral anomaly bears a

resemblance to the equatorial ionization anomaly,and has been explained in terms of ion-drag effect bythe ionization anomaly crests impeding the zonalneutral gas flow from the noon meridian' to themidnight meridian. The reduced zonal wind speedwould result in a less efficient equalization of heatfrom the noon to the midnight meridian, and largerdiurnal variations of thermospheric temperatures

(and hence neutral density) at the crests than at thetrough. A phenomenon which arises due tointeractions between the neutral and the ionization

anomalies is the occurrence of 'ledges' or 'cusps' in theequatorial topside ionosphere3~-41. The enhanceddensities at the crests of the neutral anomaly partiallyinhibit the fkld-aligned plasma diffusion from thedip equator to higher latitudes (the diffusioncoefficient is inversely proportional to the neutraldensity) that normally leads to the formation of theionization anomaly. The fountain mechanism is thusso effected that the ionization lifted by Ex B driftaround the dip equator accrues at the top of the fieldline passing through the crests of the neutral andionization anomalies leading to the formation of theledge. Yet another good example of the dynamicinteraction between the neuttal and the ionized

constituents at equatorial latitudes is the nocturnalvariation of the zonal neutral winds. The post-sunsetenhancement of the F-region upward plasma driftnear the dip equator22 raises the F-region to highaltitudes leading to a significant depletion ofionization in the lower ionosphere. The zonal neutralgas accelerates as a result of the reduced ion-drag,and as the F-region descends in height later in theevening due to the reversal in the upward plasma drift,the neutral gas gets decelerated42. Observationsindeed show a substantial acceleration and

subsequent deceleration of the zonal winds in thepost-sunset period as predicted by the theory42.43.

Further evidence of a strong coupling between theneutral and plasma dynamics is the recentobservation44 of a similarity in the tidal patterns inthe zonal ion drift observed at Jicamarca45 and zonal

neutral gas glow measurements from the Wind andTemperature Experiment (W ATE) aboard DE-2satellite46.

The above results demonstrate the close couplingbetween the physical processes underlying theionization anomaly. This helps us to realise that theanomaly is only one of the many facets of theinteractive ionosphere-thermosphere system thatprevails at equatorial latitudes and, as such, it shouldbe viewed and investigated not as an isolatedphenomenon but with a broader perspective to theentire range of processes operative in the system for acomprehensive understanding of the same. The veryrecent findings of a close link between the anomalycharacteristics and the post-sunset onset ofequatorial spread-F (another unique phenomenonof the equatorial ionosphere) substantiates this point

of view4 7.48. The results of Raghavarao et al.48 inparticular showed that, in contrast to days withoutpost-sunset spread-F, on days with spread-F the

231

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

10 12 14 16 18 20 10 12 14 16 18 20

LOCAL TIME(hrs)

Fig. 7-- Local time variation of the electron density ratio (NA/ Nw)between Ahmedabad (NA) and Waltair (Nw) on days with (a)spread-F and (b) no spread-F on bottomside ionograms n<>arthe

dip equator (after Raghava Rao el al'<")

ratio of the electron density between Ahmedabad

(dip. Jat. 18.6°) and Waltair (dip. lat.,106°N)undergoes a sudden enhancement starting at 1700 hrsLT by a factor of 8-30 (at 1900 hrs LT) from anear-constant value of2 during the daytime (Fig. 7).The enhancement of the electron density ratio is amanifestation of the intensification of the ionization

anomaly which, in turn, engenders a similar responsein the neutral anomaly as outlined earlier. As a result,a localized cell of altitude dependent neutral windsgets created which will aid further intensification ofthe crests of both the anomalies and preferential massloading or filling up of the magnetic field tube passingthrough the height of maximum plasma density of theF--region on the dip equator. The net effect of thesecoupling processes is a weakening of the ambienttransequatorial wind (the observations pertain tonorthern winter months) and reduction of the

(b) NO ESF

... (1)aN/at + V (N V) = P - LI I I I I

5 Modelling of the anomaly-Approach and inputsTheoretical modelling or simulation of the

anomaly essentially consists of numerically solvingthe following time-dependent plasma (0 + ion)

continuity equation of the F-region.

north-south asymmetry of the ionization anomalycrests, a condition favourable for the onset ofspread-P7.

where, Nj ( = Ne) is the ion density, Pi and Li are the ionproduction and loss rates, and Vi is the ion transportvelocity. As the plasma in the equatorial F-region istransported perpendicular to the geomagnetic fieldby E x B drift and parallel to the field lines by diffusionand neutral winds, Eq. (1) is solved by transformingthe variables from the spherical polar coordinatesystem to a coordinate system parallel andperpendicular to the magnetic field (B). Thetechnique of solution is numerical integration of thetransformed ion continuity equation for a set ofmagnetic flux tubes. The integration for each fluxtube is carried through an appropriate number ofdiurnal cycles to provide a stable solution. Thetwo-dimensional (altitude and latitude) distributionof the ion (electron) density is built up at any local timefrom the solutions for a set offlux tubes (see review ofMoffett33 and references therein for details oftransformations, numerical solutions and boundaryconditions). The coefficients of the transformed ioncontinuity equation depend on the followingquantities: (i) ion production and loss rates anddiffusion coefficients, (ii) neutral winds (iii)ion-neutral collison frequency, (iv) electron and iontemperatures, (v) electric fields or Ex B drift, and (vi)geomagnetic field (B). It has been a common practiceto seek solution of the ion continuity equationignoring the coupling of the ion and neutral motions.A comprehensive approach is to seek aself-consistent solution of the coupled ion andneutral interaction equations49. The inputparameters required for such calculations (which areof very recent origin) are illustrated schematically inFig. 8. It is to be noted that irrespective of whetherplasma-neutral interactions are taken into accountor not, the input parameters (a majority of which arecommon to both the approaches) required formodelling the anomaly are usually derived frommodels. The veracity of the results of the theoreticalsimulations thus depends on the validity of the modelinputs. used. In the following, a critical but briefdiscussion has been made of the models currently in

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SASTRI: EQUATORIAL ANOMALY IN F-REGION

use for some of the important inputs required formodelling the anomaly.

5.1 Neutral atmosphere

The neutral atmosphere models provide numericaldata on the altitude profiles of the densities of majoratmospheric species (N2, O2 and 0) as a function oflatitude and local time, and for specified levels ofgeomagnetic and solar activities. The models that areusually relied upon are the Jacchia50,51 and MSISmodels52 - 54which are static and semi-empirical innature. The neutral atmosphere models are used inconjunction with appropriate empirical global

models of the ionospheric plasma density, such as ofChiu55 to specify the ion-drag and neutral gasbackground properties, and to derive thetemperature and neutral circulation pattern (windfield) as is done, for example, in the NCARThermospheric General Circulation Model(TGCM)56. The choice of the neutral atmosphericdata plays an important part in the modelling of theanomaly, as the data are used to define not only theneutral temperature (Tn), ion temperature (Tj),

electron temperature (Te) (the assumption Tj = Te =Tn is usually made), and neutral composition andwinds, but also the ion production and loss rates and

Collision

frequency

(Stubbe 1968)

Te' T i(Brace & Theis

1978,Titheridge1976)

Ionosphericelectricfield

(Richmond

et a!. 1980)

Geomagneticfield

(Cain et a!.

1974)

Tn ' neutralcomposition

(Hedin et al.

1977a, b,1979)

Solare.u.v.flux

(Torr & Torr

1979)

Noctura1

ionizationsource

(Strobei et al.

1974, 1980)

Absorpt ion &

ionizat'ion

cross section,branchingratio(Torr & Torr

1979)

Reactionratecoefficients

(Torr & Torr

1979)

Fig. 8-Schematic illustration of major input parameters required for modelling the equatorial anomalythrough numerical solutions of the coupled meridional continuity equation for ionization and

momentum equations for horizontal winds (after Chan & Walker49)

233

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

diffusion coetlicients. Reliability of the models topredict the neutral atmospheric properties atequatorial latitudes, however, had not beenadequately tested in the past due to lack of directexperimental data. The situation has improved, tosome extent, in recent times with the initiation of

measurements of the neutral temperature and windfield at equatorial locations using optical techniques(see recent review of Sridharan57). The currentlyavailable data base, though very limited in size andpertaining mostly to nighttime, has proved to be veryimportant as it revealed marked disagreement, onoccasions. between observations and theoretical

predictions, indicating the complex nature of thephysical processes in the equatorial thermosphere. Abrief discussion of significant results relating to thisissue is presented below with specific reference to theneutral temperature, composition and winds.

5.1.1 Nell/ral /el11pe/'{J/u/'e--Information onneutral temperatures in the nocturnal equatorialthermosphere is currently available frommeasurements at a few locations scattered round the

globe5s-61• The temperatures estimated fromspectroscopic observations 01'630 mm night airglowemissions were found, in general, to be in reasonableagreement with the model values. These observationsat widely separatcd locations, however, also showed,quite consistently, significant positive departures ofthe measured tenweratures (enhancements) from themodel predictions usually under disturbedgeomagnetic conditions and, sometimes, even forquiet conditions. A few examples of these abnormaltemperature enhancements observed at Mt. Abu andArequipa are shown in Fig. 9. The elevatedtemperatures at Mt. Abu located in the vicinity of thenorthern crest of the anomaly in the Indian sector,were found to bear a close relationship with thepresence of spread-F conditions on bottomsideionograms near the dip equator59. Joule heating bythe irregular electric fields engendered by the plasmainstabilities associated with spread-F along themagnetic field line passing through the region ofairglow emission is believed to be responsible for theexcess temperature. Whether the observedtemperature enhancements arc highly localized andindependent events or associated with the equatorialmidnight temperature bulge62 - 65, which is aquasi-permanent feature with highly variable spatialextent and strength. has not been addressed so far andremained an open question. Answers to thisimportant question necessitate temperaturemeasurements at a network of stations covering thelatitude range from the dip equator to well beyond thecrests of the anomaly. The abnormal temperature

234

enhancements are not just limited to the nighttime butoccur in the daytime as weIl66•67. The results of Guptaet al.67 from a rocket chemical-release experimentconducted from SHAR, India (dip lat., 5SN) merit aspecial mention here, as it provided, for the first time,the height profile of the temperature enhancementthat was seen in the twiligh' period just two hoursafter a geomagnetic storm sudden commencement(SSe). The origin of this apparently localizedanomalous heating in the lower thermosphere (Fig10) is obscure, as none of the hitherto knownmechanisms could account for it. Extensive

monitoring and understanding of the myriadtemperature enhancements are important not onlyfor an in-depth understanding of the equatorialthermosphere as such, but also of the anomaly. This isbecause of the fact that the localized circulation

pattern induced by the elevated temperaturesinteracts with the global circulation patternestablished by solar forcing (and also by geomagneticforcing at times and the highly variable tidal r,rcing),and can affect the characteristics of the anomalythrough the associated neutral winds and chemicalcomposition changes as will be discussed in the nextsection.

5.1.2 Neutral composition -The neutral atmos­pheric composition is relevant to the anomalyas it determines the ion production and loss ratesbesides other things. Of particular importance is the

ratio of atomic to molecular species (O/N2) which getsperturbed during disturbed geomagnetic conditions.Global models"'S] predict very modu'iile increasesof temperature (10 to 150 K) and enrichment orJighterconstituents at low latitudes due to the energy input athigh latitudes and consequent modifications in thethermospheric circulation at the time of severegeomagnetic activity. There is now increasingevidence, both direct and indirect, for strongerenhancement of temperature and unexpectedincreases in the degree of molecular enrichment(decrease of O/N2) of neutral species at equatoriallatitudes in association with severe geomagneticstorms. An example of the indirect evidence is theobservation of significant and persistent depressionsinf)F2 during daytime near the dip equator in theIndian sector during recovery phase of the severestorm of 4-6 Dec. 1958 as reported by Sastri 15. Theanomalous reductions inf;)F2 which swamped eventhe normal sunrise build-up of f;)F2 occurred inassociation with a remarkable increase in F-regionheight (indicative of heating effects) and were notaccompanied by any corresponding increases inf;)F2around the crest location of the anomaly(Ahmedabad) as shown in Fig. II. Study of the

r':'"

1'1111'111 i HI '11jI klltiU I I III ,;1'1

SASTRI: EQUATORIAL ANOMALY IN F-REGION

+

+

900+

800

700

6001900 2000

---SPREAO-F"

2100 2200

TIME (1ST)

13-12-77

~KII:21

2300 2400

.000 [ TtM' ,•••goo

---100r

..++rOOr

NO SPREAO-F~ -12-77IK,::14I 100 1900

2000210022002]002400

TIME

liST)

12 (UT)7 (L T)

94

3 622 I

15 18 010 13 19

12

194

03619 22 I

18 21

Kpf.3.1~-I.7.I~~I.~~I~-151~-1 ,1,1-,2,2--5

5/ 1 -Dst

12 lIT (nT ) 1

2000'- June " ./ - i 150~ Aug. -; 2000,/

Tn ~ I ; t j Tn(K) ~ I~ 1~ (K)

T I•

1500'- I i 5; -;501- -f I 500/ •• 13 June~ 1

o

f~~1000

Fig. 9-Examples of abnormal enhancements of neutral temperature at equatoriallatitudes on geomagnetically quiet as well as disturbed days from measurementsmade at Mt.Ahu. India (upper panels) and Arequipa. Peru (bottom panel) [Thestraight solid lines in the figures represent the model predictions. The temperatureenhancements at Mt.Ahumanifest only on days with spread-F (ESF) near the dip

equator (after Rajaram £'1 (/f.5" and Biondi & Meriwether(0).)

235

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

300

AL

DE

15.~_ •• ~ •••••5 ••• '

AH

~1 ~ •••• ~ •••••:I. •••• ~ •• _- •

~ 5 •• " '. MA

II. 15 • _"". ~ •. _ rc:-•••0 r-~ ~'::7 -:-y ~~ •• '. TI

1~ ,-, ~

~~--::.. ~C ..~ ~., KO

5t· '.15 _ 1'--"', ••••.-,-. -V \. ~ ~ "\ TR5' •

00 00 00 00 00 hrsUT3 DEC. 4 DEC. 5 DEC. 6 DEC. 1956

Fig. II-Diurnal profiles off~F2 at stations from the dip equatorto high altitudes in the Asian sector during the severe geomagneticstorm of 4-6 Dec. 1958 [The marked and sustained reductions indaytimej;,F2 near the dip equator on 5 December that occurredwith prominent increases in hmF2 are considered as insignia oflocalized neutral composition changes (decrease in OjN2) causedby the severe storm (after Sastri 15); TR = Trivandrum. KD =Kodaikanal. TI = Tiruchirapalli. MA = Madras. AH =Ahmedabad, DE = Delhi, AL= Alma-Ata, TO = Tomsk,

SV =Sverdlovsk, SA = Salekhard.]

5.1.3 Neutral winds-Data on neutral winds play animportant role in the theoretical studies of theequatorial anomaly because of the ability of winds tomodify the plasma transport processes responsiblefor the anomaly. Due to the absence of directmeasurements. it has been a common practice,particularly in the past, to use simplistic modelswherein the latitudinal variation of winds isrepresented by different functional forms, and thedaily variation is assumed to be simple harmtmic withpoleward winds by day and equatorward winds bynight. Though this practice has helped in gaining an

t _

,. o~~~\-- --~~ _--100 V•..••

°_200

2000

LAUNc~t1300

I1200

100o

140

260

E~ 220wo:::J,...

,...

-;J lBO

500 1000 1500

NEUTRAL TEMPERATURE (K)

Fig. ID--Altitude profile of neutral temperature estimated fromground-based spectroscopic observations of a rocketchemical-release over SHAR. India.just after the occurrence ofastorm sudden commencement [The solid line represents the profile

predicted by Jacchia 77 model (after Gupta eI aI67).]

diurnal profiles of the equatorial electrojet strength(and hence of the electric field and Ex B drift)indicated that the decrease infoF2 could not be due toaugmentation of the fountain process. The logicalinference is that the prolonged negative storm effect isdue to enhanced loss rate brought about by localizedchanges in the chemical composition (decrease ofO/N2). The source and mechanism of the neutralatmospheric perturbations are not quite apparent.That such unpredicted composition changes doindeed occur in the daytime equatorial thermospherein response to severe storms is amply demonstratedby the very recent studies of the neutral atmospheremeasurements at low latitudes by the CACTUSaccelerometer aboard CASTOR satellite by Berger etat. 68,69 The results which are based on a substantialdata base showed in particular that prominentincrease in temperature and decreases in O/N2 ratioprevail by day, both at constant pressure level and atfixed height, under highly disturbed geomagneticconditions as can be seen from the numerical datapn~sented in Table I. This behaviour which is notpredicted by any of the global thermospheric modelsis interpreted in terms of heat transport (from highlatitudes) through thermal conductivity as well aswave dissipation. These results indicate theinadequacy of the global models to predict theequatorial neutral atmospheric response to severestorms, and also raise the question of localized lowlatitude sources of energy which is gainingprominence57,68,

236

I· !"

SASTRI: EQUATORIAL ANOMALY IN F-REGION

(b)

(e)

24

Kp=4-. , .

26 AUQust 1982

22

rN

o

Or~I

______ .....j (0)

i" -----···)no"hwo"~ T 5/55-1- I

2021

50

20

100

CI

•.. 200

100

E

-40

o

40

800

200

~ 1000

~ 500

oz~

......1200~a:~ 1000ILl

I-

oz~ 100~c(zoN

~c(UI- -20a:ILl>

a: -100ILl

~

>l-V)ZILlI­Z

~c(zoo

III-

oz~

III-E

23

TIME(UT)

Fig. I 2-Nocturnal variations of the components of neutral windflow (meridional, zonal and vertical), neutral temperature and 630nrn airglow intensity observed at Natal, Brazil (35.2°W) on 26

Aug. 1982 (L Kp = 27), illustrating the unexplained feature(during 2100-2300 hrs UT) of a persistent convergence inmeridional flow accompanied by a downward wind and increase

in temperature (after Biondi & Sipler61)

LST (hrs) !:!KpDTMMSIS-77MSIS-86OBS

Daytime

2.54 61313

10-20

5.04121835

Nighttime

2.54 61315

20-10

5.04121820

O/N2atP=cst.Daytime

211.015.015.012.5

10-20

4-514.017.015.513.5

7

17.525.016.010.0

Nighttime

211.517.016.013.0

20-104-514.519.018.517.0

718.028.021.536.5

Nighttime

21.051.151.051.05

to daytime

4-51.051.101.201.25

ratio

71.051.101.353.65

insight into the effects of winds on the anomalybehaviour, it remained a major weakness of theearlier theoretical studies because of the ad hocnature of the wind models used. The situation hasimproved to some extent in recent times with thedevelopment of dynamic thermo spheric models likeTGCM. It would, however, be improper to expectglobal TGCM model to correctly predict thecirculation patterns at low latitudes in all situations,as it takes into consideration only the solar forcing forquiet conditions and the additional high latitudeforcing for storm-time conditions. The-neutral windpatterns could deviate fromtnaael predictions due tothe effects of the ever-present and highly variableequatorial midnight temperature bulge and themyriad, rather than localized heating eventsdiscussed earlier. The AE-E satellite measurementsof the meridional winds at low latitudes, indeed,showed the presence of equatorward winds in thepremidnight local time sector, which seems to berelated to the dynamics associated with the equa­torial midnight temperature bulge70• Recentground-based measurements of the wind field,though very scanty, did show reasonable agreementwith model predictions43.61.71; the agreement isparticularly good for the zonal component of theneutral flow. They, however, also revealed severalunexpected features even under quiet conditions, anexample of which is shown in Fig. 12.The prevalence

Table I-Mean temperature increase per Kp unit, when O/N2ratio at a constant pressure level (cst =0.20 x 10-5 Nm-2) withrespect to local solar time and geomagnetic activity level forDTM and MSIS models as well as when deduced from the set of

data observed (after Ref. 68)

237

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

of a net convergence in the meridional flow anddivergence in the zonal flow with an attendantdownward vertical wind and an increase intemperature during 2100-2300 hrs UT is quiteevident from the observations. In addition,practically very little is known about the storm-timewind patterns at equatorial latitudes. The obviousneed of the hour is to make regular and sustainedmeasurements of neutral winds and temperature withnetworks of stations in different longitude sectors notonly to characterize the wind patterns for variedgeophysical conditions, but also to infer the sourcesand sinks oflocalized energy sources indicated by therecent neutral temperature measurements.

5.2 Vertical plasma drift

The pattern ofF-region vertical plasma drift in thelow latitude ionosphere is a crucial input forquantitative modelling of the anomaly. The verticalplasma drift is related to the zonal electric fieldgenerated by the E-region dynamo (F-regiondynamo contribution becomes significant aftersunset and is considered as primarily responsible forthe post-sunset enhancement of the vertical upwarddrift before reversing direction 72) aided by the uniquemagnetic field configuration at the dip equator.Although a simplistic sinusoidal model of upwarddrift by day and downward drift by night was used insome of the very early studies, the availability ofaccurate experimental data from Jicamarca, Peru,beginning in the sixties (see review of Fejer22)facilitated the use of realistic inputs. There is,nevertheless, a glaring lack of data on vertical plasmadrift pattetns in longitude sectors Q.ther than theAmerican sector (75°W). This is a major hurdle forattempting modelling work for the variouslongitudes and validate the logical but qualitativeunderstanding reached so far, as for example, theorigin of the pronounced longitudinal differences inthe development of the anomaly is partly due toregional differences in the electric field pat:terns4. Apromising line of attack to overcome the problem, atleast to some extent, is to undertake large scalereductions of the VHF coherent back scatter radardata (available, for example, from Thumba, India)with the methodology recently devek)ped by Reddyet at.73 to estimate the daytime electric fields. Theother alternative is to use global electric field models,but there is only one model as of now and that ofRichmond et at.74 which is based on F-region driftobservations at four radar stations (Jicamarca,Arecibo, St. Santin and Millstone Hill) on quiet daysof solar minimum epochs. The model has thus onlylimited applicability, and the need for development of

238

comprehensive electric field models from furthercoordinated multistation observations need not be

overemphasized.

The zonal electric fields exhibit considerable

day-to-day variability even on quiet days, the originof which is yet to be understood. An extreme fOIl11ofthis quiet day variability is the occurrenc~ ofcounter-electrojet events 75 and associated dis­turbances as also anomalous reversals of the

daytime zonal electric field76• These quiet time slowand smooth perturbations in the electric field whichpreferentially manifest in the early morning orafternoon hours seem to result from large lunar andsolar tidal effects77• There are algo some evidenceswhich indicate that the equatorial zonal electric fieldspossess a significant non-dynamo componentdue to solar wind-magnetQ6phere-ionosphereinteractions, particularly, during disturbedgeomagnetic conditions76• The non-dynamo electricfield perturbations may be broadly classified into twobroad groups. The first group is of the transient (1-2hr duration) disturbances which often occur in closetemporal association with sudden and prominenttransitions in the north-south (Bz) component ofIMF and onset of substorm recovery phases, and witha characteristic local time dependence as regards totheir amplitude and frequency of occurrence. Theseare now regarded as signatures of direct penetrationof high latitude electric fields to low latitudes, thedegree of penetration being dependent on severalelectrodynamical processes78 -82. The other group ofevents is of the persistent disturbances whichmanifest long after the onset of geomagnetic storms(delay> 10hr) and whose direction is opposite to thatof the normal quiet day pattern at all local times83 -85.Most of these occasional electric field disturbances,which can drastically influence the anomaly pattern,seem to result from modifications in the globalthermo spheric circulation (winds, temperature andcomposition) due to storm-time energy input to highlatitudes, i.e. through the ionospheric 'disturbancedynamo' mechanism86. An example of the'disturbance dynamo' electric fields as evidenced inthe magnetometer/ionosonde data is presented inFig. 13. Many other types of electric fieldperturbations do also occur during disturbedconditions which cannot be accounted for by the twoabove mentioned physical processes. Examples arethe complex electric field patterns during periods ofstrong substorm activity and asymmetric ring currentdevelopment1o, and preferential forenoon reduc­tions in average electric fields on disturbed daysas compared to those on quiet days87.88 (Fig. 14). Anin-depth understanding of the equatorial electric

III Iii

SASTRI:EQUATORIAL ANOMALY IN F-REGION

I15

~09

0·5

0·6

measurements of the parameters of the ionosphere aswell as those of the thermosphere using themulti-stationjmulti-technique approach to developreliable data bases and to improve models of the vitalinputs needed for quantitative modelling of theanomaly. Concerted experimental efforts will alsohelp address outstanding issues concerning otherphenomena like equatorial spread-F and equatorialelectrojet and develop a comprehensive understand­ing of the equatorial ionosphere-thermospheresystem.

0·4 ~

-~ OJ i[

0·2

0·1

07I . I . I

11 13

TIME, hrs (1ST)

Fig. l4-A verage pattern of daytime electric fields ongeomagnetically disturbed days (solid curve) and quiet days(dashed curve) derived from VHF coherent backscatter radardata at Thumba. India(afterViswanathanet al.87)[Thedotted lineshows the smoothed variation of the daytime electric field on

geomagnetically disturbed days.]

00 12 00 References

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(1981) 589... . •8 Raghava Rao R & Sharma P, Can J Phys (Canada). 1989,in

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~ 8~~~4 1614

N12 :I: !'"l>...!286

"IIt..II... c-:<40~I

0...

:f\I

~80 'C<='-40L II

IVI<I

If"\ I I

40 7...o :I:

~~: I I,

,. I400

AU0 AL400

800

239

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

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