Midday reversal of equatorial ionospheric electric ®eld

R. G. Rastogi

INSA Senior Scientist, Department of Physics and Space Sciences, Gujarat University, Ahmedabad 380 009, India

Received: 7 June 1996 /Revised: 27 March 1997 /Accepted: 1 April 1997

Abstract. A comparative study of the geomagnetic andionospheric data at equatorial and low-latitude stationsin India over the 20 year period 1956±1975 is described.The reversal of the electric ®eld in the ionosphere overthe magnetic equator during the midday hours indicatedby the disappearance of the equatorial sporadic E regionechoes on the ionograms is a rare phenomenon occur-ring on about 1% of time. Most of these events areassociated with geomagnetically active periods. Bycomparing the simultaneous geomagnetic H ®eld atKodaikanal and at Alibag during the geomagneticstorms it is shown that ring current decreases areobserved at both stations. However, an additionalwestward electric ®eld is superimposed in the ionosphereduring the main phase of the storm which can be strongenough to temporarily reverse the normally eastwardelectric ®eld in the dayside ionosphere. It is suggestedthat these electric ®elds associated with the V � Bzelectric ®elds originate at the magnetopause due to theinteraction of the solar wind and the interplanetarymagnetic ®eld.

Introduction

The establishment of geomagnetic and ionosphericobservatories at low magnetic latitudes in Peru andColumbia during IGY-IGC period have provided veryvaluable data on the equatorial electrojet and equatorialionosphere. Knecht and McDu�e (1962) have describedthe special properties of the equatorial sporadic E layer,Es-q, which was shown to be closely related to theequatorial electrojet both in daily and latitudinalvariations.

BartelsandJohnston(1970a, b)hadshownthatthegeo-magnetic horizontal ®eld,H , atHuancayoduring the day-time hours on certain occasions decrease below thecorresponding nighttime base value. Gouin and Mayaud

(1967) called such events counter electrojets and sug-gested they may be due to a westward current in theequatorial ionosphere. Rastogi et al. (1971) showed, forthe ®rst time, that the counter electrojet events observedin geomagnetic H ®eld, the absence of Es-q re¯ections inthe ionograms and the reversal of the ionospheric driftdirection during the daytime hours at an equatorialstation were concurrent phenomena. Fambitakoye et al.(1973) showed that the reversal of the latitudinal pro®lesof DH and DZ are the main criterions for the disap-pearance of Es-q rather than the decrease DH below itsnight time level at the equatorial station. Comparing thehigh resolution modi®ed range time intensity (MRTI)records of VHF backscatter radar echoes at Jicamarcaand the magnetograms at Huancayo and Fuquene,Rastogi et al. (1977) showed that the E region irregu-larities disappear precisely (with an accuracy of less than2 min) when DH at Huancayo minus DH at Fuquenedecreases below the corresponding night time value.Similar results were obtained by Rastogi and Patil(1986) by comparing Doppler shift data of VHFbackscatter radar echoes at Thumba and DH atKodaikanal minus DH at Alibag.

Rastogi (1974a) described the ®rst global morphol-ogy of the occurrence of counter electrojet based on thevisual examination of the magnetograms of a number ofequatorial stations. The counter electrojets are observedmostly either in the morning hours around 0700 LT orduring the afternoon hours around 1600 LT. Very fewevents were observed around noon hours. The occur-rence of counter electrojet events are inversely related tothe sunspot activity. During geomagnetic disturbedperiods, the presence of counter electrojet events cannotbe precisely estimated by simple visual examination ofthe ionograms.

Rastogi (1975) suggested that the observed dailyvariation of the H ®eld at an equatorial station is due tothe superimposition of the global Sq current ¯owingeastward at 106 km and the electrojet current ¯owingeither eastward or westward at 100 km altitude. Alexet al. (1986) have discussed the two prominent sources

Ann. Geophysicae 15, 1309±1315 (1997) Ó EGS ± Springer-Verlag 1997

of westward electrojet current, one due to certainabnormal modes of luni-solar semi-diurnal tides, andanother due to the interaction of solar plasma on thesunward side of the magnetosphere. Thus, the westwardcurrent has to exceed the global eastward Sq current atthat time to generate counter electrojet event. Since theSq current maximizes around 1100±1200 LT, the occur-rence of the counter electrojet should be least probablearound noon hours. The partial counter electrojetcannot be identi®ed by visual examination of themagnetograms and one has to compare DH at anequatorial and another low-latitude station along thesame longitude before deriving the index of counterelectrojet. The ionograms depict the conditions of theregion of the ionosphere from where the radio waves arere¯ected or scattered back. Thus, the disappearance ofthe equatorial type of sporadic E re¯ections on equato-rial ionograms would be a more reliable and sensitiveindex of the counter electrojet events.

Rastogi (1974b) has described the characteristics ofthe sporadic E layer at Kodaikanal. It was shown thatthe time of the ®rst disappearance of the Es-q in themorning varied from day to day depending upon growthof the normal electrojet. Similarly the occurrence of Es-qin the evening hours was greatly a�ected by the counterelectrojet. However during the midday hours, Es-q hadoccurred over 90±99% of times. Rastogi (1972a) hadshown that during a geomagnetic storm, Es-q at anequatorial station does not indicate any decrease of themaximum Es-q frequency, but it does disappear com-pletely during very large decreases of DH at theparticular station.

Data

The ionospheric observatory at Kodaikanal in India hasbeen in continuous operation since September 1952 andhas regularly published its ionospheric data. It wasdecided to study the counter electrojet in Indianlongitudes using the data of the absence of Es-q atKodaikanal. The period of study chosen was January1956 to December 1975 which covered the period ofmaximum �sunspots � 200� to minimum �sunspots � 0�solar activity. The list of stations whose data is used,with their coordinates, is given in Table 1.

The whole data covering a period of twenty yearswere divided into two groups: (1) 10 years of highsunspots activity consisting of years 1956 to 1961 and1968 to 1971; (2) 10 years of low sunspots activityconsisting of the years 1962±1967 and 1972±1975. The

data for each epoch were further grouped into thefollowing Lloyd seasons: (1) D-months consisting ofNovember, December, January and February represent-ing local winter season; (2) E-months consisting ofMarch, April, September and October; and (3) J-monthsconsisting of May and June, July and August monthsrepresenting local summer season.

Results of the analysis

Figure 1 shows the percent occurrence of the disappear-ance of Esq, referred to as No-Es-q events, for each of theseasons of high and low sunspot epochs. Referring toannual average curves, there is some shift in the curvetowards higher local time during high sunspot yearssuggesting that the forenoon No-Es-q events are morecommon during the high sunspot than during the lowsunspot years and the case is reversed for afternoonNo-Es-q events. The same trend is seen for any individualseason. This is in conformity with the results of Patilet al. (1991a, b) that the counter electrojet events in theIndian sector were seen more often in the afternoon thanin the forenoon hours during 1964±65 but the eventswere more often secn in the forenoon hours of highsunspot years of 1958±59. It is to be noted that theNo-Es-q events were least frequent (about 1±2% of time)around 1100 LT which happens to be the time of thepeak electrojet current during the day. Regarding theseasonal e�ects, No-Es-q events during midday hourswere most frequent during D-months at about 3% andleast frequent during E-months being only 0.3±0.5% oftime. It is to be noted that the peak altitude of theelectrojet in the Indian sector is weakest during D-months and strongest during E months (Rastogi andPatil 1992).

Figure 2a shows the year to year variation of the totalnumber of days with No-Es-q events at Kodaikanalduring any of the hours 1000±1200 LT. In Fig. 2b areshown the number of No-Es-q events at 1100 LT duringdi�erent months of the year. Referring to Fig. 2a, noapparent solar cycle variation of No-Es-q events can bederived. It may be noted that the occurrence ofafternoon counter electrojet events are inversely relatedto the sunspot numbers (Rastogi, 1974a) while themidday counter electrojet, observed less frequently, doesnot show any clear dependence on sunspot numbers.

Referring to Fig. 2b, No-Es-q events occur mostfrequently during D-months and least frequently duringE-months with a secondary peak during J-months. Atotal of 96 events were observed at 1100 LT during the

Table 1. List of stations whosedata is utilized in the presentstudy

Station Code Geographic Dipole Dip

Latitudes �E Longitude �N Latitude Latitude

Trivandrum TRD 8.5 77.0 1:1�S 0:6�SKodaikanal KOD 10.2 77.5 0:6�N 1:5�NAnnamalainagar ANN 11.6 79.7 1:4�N 2:5�NAlibag ABG 18.4 72.5 9:5�N 12:0�N

1310 R. G. Rastogi: Midday reversal of equatorial ionospheric electric ®eld

20 year period of 1956±75, out of which 23 events wereobserved in January, 6 in June and only 2 in March.

Next we examined the geomagnetic disturbance indexon the days when Es-q were absent at any hour between1000±1200 LT. Figure 3 shows the percent occurrence ofNo-Es-q events as a function of daily sum Kp index�RKp� for low and high sunspot years. During highsunspot years, the RKp were high during No-Es-q days.The peak of the histogram shows a value around 30 forRKp. Out of 101 events during the ten year period, 59events were observed during the main phase of ageomagnetic storm. Extremely few cases were observedduring undisturbed days. During low sunspots, out of 79

events, the storm was in progress on 22 days. Some ofthe events were observed on the days immediatelyfollowing the storm. Of course, during the low sunspotyears a signi®cant number of cases of No-Es-q wasobserved on very quiet days when the normal electrojetintensity was very small. Such quiet time No-Es-q eventswere fairly common during the years 1964, 1965. We arenot combining here the midday occurrence of No-Es-qwith afternoon counter electrojet occurring between1400 and 1800 hours. Thus, it may be concluded that thereversal of electrojet current during the noon hour isbasically a disturbed-day phenomenon.

The geomagnetic data were next examined during thestorms when Es-q was observed to be absent during themidday hours. During normal eastward electrojet cur-rent conditions a pronounced peak of DH is observed atTrivandrum. The deviation of the geomagnetic vertical®eld Z from its midnight value would be practically zeronear the center of the electrojet. At Annamalainagar(ANN) which lies near the northern fringe of theelectrojet, DZ would experience the largest negativedeviation. During a counter electrojet event, identi®edby a negative value of DH (TRD-ABG) or of DH(KOD-ABG), the deviation of vertical ®eld Z would bepositive at ANN and KOD.

The ®rst No-Es-q event around noon hours occurredon 4 March 1964. A gradual commencement type ofstorm was observed at Kodaikanal at 1818 h on 3March 1964 and the storm range in H ®eld was 162 nT.Figure 4a shows the variations of the Dst (H ) index,disturbance daily variations of DH at ABG and KOD,DZ (KOD-ABG) and the maximum Es-q frequency at 3,4 and 5 March 1964. It is seen that the Dst (H ) at Alibag

Fig. 1. Daily variation of No-Es-q events at Kodaikanal averaged forthe D, E and J-months of low and high sunspot years

Fig. 2. a Year to year variations of No-Es-q events at Kodaikanalduring the interval of 1000±1200 LT; 2 bmonth to month variation ofthe total number of No-Es-q events at Kodaikanal averaged for theperiod of study 1956±75

R. G. Rastogi: Midday reversal of equatorial ionospheric electric ®eld 1311

varied with time in a similar fashion as the internationalequatorial Dst (H ) index, but the Dst (H ) at Kodaik-anal showed larger ¯uctuations with deep decreases ofthe H ®eld around 1200 and 1700 h on 4 March 1964.The large decrease H at KOD around 1200 h wasassociated with a positive peak at DZ at KOD and anegative peak of DH (KOD-ABG). The ionograms atKodaikanal on 4 March 1964 showed clear Es-qre¯ections at 1015 h and its sudden disappearance at1030 h (Fig. 4b). The Es-q echoes were absent atKodaikanal till 1130 h. Thus, the Es-q at Kadaikanalhad disappeared only when DH (KOD-ABG) wasnegative and DZ (KOD) positive, suggesting a westward(counter) electrojet current at that time.

The second No-Es-q event discussed refers to the SCtype of geomagnetic storm which had started at 0008 h75�E on 1 December 1960. Figure 5 shows the variationsof various parameters of magnetic and Es-q data for theperiod 30 November to 2 December 1960. Based on thevariation of the Dst index, the main phase had startedon 1 December 1960 at 0500 hours, reached its largestdepression of about ÿ130 nT at 1700 h and at 2300 hand started recovering thereafter. The midday depres-sion of DH was ÿ65 nT at ABG, ÿ200 nT at KOD andat TRD, thereby DH (TRD-ABG) or DH (KOD-ABG)

Fig. 3. Distribution of daily sum Kp index �Rkp� on days of No-Es-qevent during midday hours for low and high sunspot years

Fig. 4. The storm time variations ofDst index, DH at ABG and KOD,DZ at KOD, DH (KOD-ABG) and the maximum frequency re¯ectedfrom the ionosphere ( fEsq) over Kodaikanal during the GC typegeomagnetic storm starting at 1820 h on 3 March 1964; b ionogramsat Kodaikanal on 4 March 1964 showing the presence of Es-q echoesat 1015 h, 75�E and complete absence of Es-q echoes at 1030 h

1312 R. G. Rastogi: Midday reversal of equatorial ionospheric electric ®eld

showed a very large decrease around 1100 h. The valueof DZ at KOD or at ANN was strongly positive. Thesefeature suggest the existence of a strong counterelectrojet around midday hours on 1 December 1960,further con®rmed by the absence of Es-q 1100±1400 h. Itshould be noted that a large decrease of Dst during thelate evening hours on 1 December 1960 was associatedwith almost similar decreases of DH at ABG, KOD and

Fig. 5. The storm time variations of Dst index, H ®elds at ABG,KOD, TRD, DH (K-A), DH (T-A), KOD Z, ANN Z and foEsq atKodaikanal during the sudden commencement storm starting at 75�hEMT on 1 December 1960

Fig. 6. The storm time variation of Dst index, H ®elds at ABG,KOD, TRD, DH (T-A), KOD-Z, ANN-Z and foEs at Kodaikanalduring sudden commencement storm starting at 0214 h 75�E on 22October 1958

R. G. Rastogi: Midday reversal of equatorial ionospheric electric ®eld 1313

ABG and did not show any evidence of a counterelectrojet at that time.

The next event discussed refers to the SC stormsstarting at 0214 h 75�E on 22 October 1958, which wasfollowed by another storm at 1218 h 75�E on 24October 1958. Figure 6 shows the storm time variationsof the various parameters for the period 22 to 25October 1958. This period showed major depressions ofstorm time H ®eld at ABG with a magnitude of ÿ100 nTat 1400 h on 22 October, another of ÿ150 nT at 2200 hon 23 October and the third one of )200 nT at 1900 hon 24 October 1958. The ®rst depression on 22 Octoberwas associated with similar depressions at KOD andTRD and no counter electrojet e�ects were observed.The second depression on 23 October around middayhours was associated with much larger depressions atKOD and TRD of magnitudes about ÿ200 nT whichproduced a large decrease of DH (TRD-ABG) and inDH (KOD-ABG) and a major positive peak of DZ atKOD and ANN resulting in the disappearance of theEs-q at Kodaikanal. The third depression, although thelargest one, was associated with similar magnitudes ofDH � ÿ250 NT at all the Indian stations and did notproduce any changes in the sporadic e layer at Kodaik-anal.

Thus, it can be concluded that some large geomag-netic storms during its main phase produce an abnor-mally large depression of DH at the stations close to themagnetic equator and in the local noon meridian. Thesedepressions at the equator ofDH aremuch larger than thecorresponding depression of DH at mid latitudes orthe corresponding Dst index.

Discussion

The analyses of single station geomagnetic data havebeen hampered by the di�culties of isolating theionospheric and magnetospheric sources of the changesof geomagnetic ®eld components observed at groundlevel. The simultaneous observations of the geomagnetic®eld and the ionospheric electric ®eld through VHFbackscatter radar would isolate the two sources of thedisturbance electric currents. However the scarcity ofradar facilities at low latitudes and the few radaravailable from Jicarmaca data makes this combinationimpracticable. It had been shown that the uniquecombination of the network of geomagnetic and iono-spheric observatories in India and the easy availabilityof these data make it a very useful source of studyingmagnetospheric and ionospheric interactions.

Rastogi (1972b) had shown the e�ect of geomagneticstorm of fEs at equatorial latitudes has been not todecrease the value of foEs, but to inhibit the occurrenceof Es-q at certain moments when a large depression ofthe H ®eld is observed at the station. Rastogi and Patel(1975) were ®rst to show that during a compressed stateof the magnetosphere as a result of intense plasmabubble from the Sun, a sudden change of the componentof the interplanetary magnetic ®eld Bz from southwardto northward produces a )V ´ Bz electric ®eld at the

magnetopause which when imposed on the ionospherecaused a reversal or decrease of equatorial ionosphericelectric ®eld evidenced by the Doppler shift of equatorialVHF backscatter radar echoes or with the disappear-ance of sporadic echoes from the equatorial E region ofthe ionosphere by an ionosonde. These Es echoes wereshown to be the scattered re¯ections of HF and VHFradio waves from the cross ®eld instabilities generated at100 km level due to the interaction of vertically upwardHall polarization ®eld with the vertical gradient of theelectron density (Rastogi, 1972b). Any reversal ofhorizontal electric ®eld at 100km altitude in the equa-torial ionosphere would reverse the direction of the Hallpolarization ®eld, inhibiting the cross-®eld irregularitiesin less than one minute. Kikuchi et al. (1978) andKikuchi and Araki (1979) have shown that the electric®eld imposed on the polar latitudes originating from themagnetopause can be instantly transmitted to equatoriallatitudes through the zeroth order transverse magnetic(TMo) waveguide mode by the Earth-ionosphere wave-guide system.

Rastogi (1973) has described the data from theionosonde at Huancayo and VHF back-scatter radar atJicamarca during the major geomagnetic storm of 7±9March 1970. The storm was associated with largedepressions of the H ®eld at Huancayo during thedaytime hours on these days. These decreases of DHwere found to be remarkably simultaneous with thedisappearance of Es-q echoes on the ionograms and withthe downward reversal of the vertical F region electrondrifts. These observations con®rmed that the absence ofEs-q during geomagnetic storms are due to the tempo-rary superimposition of westward electric ®eld over theequatorial ionosphere.

The main phase of the storm besides the gradualdecrease of the H ®eld at low latitudes is marked withthe number of substorms associated with the passages ofsolar wind discontinuities. The ¯uctuations of the Bzcomponent of IMF during these substorms have verydi�erent e�ects on the H ®eld at equatorial and otherlow latitudes. Rastogi (1977) had shown that during thegeomagnetic storm of 16 October 1970, a sudden changeof the latitude h of IMF form ÿ90� to �90� at 1900 UTwas associated with a decrease of H ®eld at Huancayoby more than 200 nT but no change was observed in Hat low-latitude Fuquene or in the equatorial Dst index.

The present analyses clearly shows that the stormtime variation of the geomagnetic ®eld at an equatorialstation has two components, one due to the equatorialring current and another due to changes of the electric®eld of magnetospheric origin penetrating from highlatitudes.

A coordinated study of all three components of thegeomagnetic ®eld at geomagnetic observatories in theIndo-Russian sector together with various magneto-sphere and interplanetary data would lead to newinsight into the low-latitude geomagnetic storm phe-nomena.

Acknowledgements. The author thanks the Indian National ScienceAcademy, New Delhi for o�ering him the Senior Scientist's

1314 R. G. Rastogi: Midday reversal of equatorial ionospheric electric ®eld

position, and the Department of Physics, Gujarat UniversityAhmedabad and Physical Research Laboratory, Ahmedabad forproviding the facilities for this research. The author also thanks thereferee whose remarks have improved the presentation of the paper.The ®nal version of the paper was prepared during the author'svisit to the School of Physics, La Trobe University, Bundoora,Australia 3083.

Topical Editor D. Alcayde thanks B. Reinisch for his help inevaluating this paper.

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