www.elsevier.com/locate/asr

Advances in Space Research 39 (2007) 1304–1312

Local time dependant response of Indian equatorial ionosphereto the moderate geomagnetic storms

S. Tulasi Ram a, P.V.S. Rama Rao a,*, D.S.V.V.D. Prasad a, K. Niranjan a,R. Sridharan b, C.V. Devasia b, Sudha Ravindran b

a Space Physics Laboratories, Department of Physics, Andhra University, Visakhapatnam, Indiab Space Physics Laboratory, VSSC, Trivandrum, India

Received 30 August 2006; received in revised form 19 December 2006; accepted 20 December 2006

Abstract

The electrodynamics and neutral dynamics of the equatorial ionosphere undergo significant changes during geo-magnetically dis-turbed periods as a result of the high and low latitude electro-dynamical coupling. These changes may sometimes trigger the plasmainstabilities that lead to the onset of spread-F and scintillations even at the L-band frequencies. The response of the equatorial iono-sphere–thermosphere system over the Indian sector to moderate geomagnetic storms of 11th Feb 2004 and 9th Mar 2004 has been stud-ied using the ground based measurements from the Indian equatorial and low latitude region. A large vertical drift of the equatorialF-layer followed by nearly simultaneous onset of Spread-F on ionograms and scintillations at VHF (244 MHz) and L-band(1.5 GHz) frequencies, have been observed during the early evening periods that correspond to the time of rapid decrease (�25 to�30 nT/h) in Sym-H index due to prompt penetration of eastward electric fields into equatorial and low latitudes. The Spread-F andscintillations continued to exist for longer durations up to post-midnight to pre-dawn hours, possibly due to the combined action ofprompt penetration and long-lived Disturbance Dynamo electric fields. Due to prolonged geomagnetic activity during the subsequentdays (morning-to-noon hours), strong reversal in the Equatorial ElectroJet current is observed and the Equatorial Ionization Anomaly(EIA) is significantly suppressed under the influence of westward disturbance dynamo electric fields. The post-sunset enhancements in thevertical drift of the equatorial F-layer are also reduced significantly from their quiet day patterns and the subsequent occurrence ofSpread-F and scintillations is inhibited.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Geomagnetic storms; Electric fields; Equatorial spread-F; Scintillations; Equatorial ionization anomaly

1. Introduction

The ionospheric electric fields and currents at equatorialand low latitudes during the periods of disturbed geomag-netic activity differs significantly from their quiet-day pat-terns due to variable nature of coupling between highand low latitudes. This high latitude–low latitude couplinghas been thought of in terms of two basic mechanismsnamely, the solar wind-magnetospheric dynamo (Senior

0273-1177/$30 � 2007 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2006.12.031

* Corresponding author.E-mail address: palurirao@yahoo.com (P.V.S. Rama Rao).

and Blanc, 1984; Spiro et al., 1988) and the ionospheric dis-turbance dynamo (Blanc and Richmond, 1980). The form-er is due to dynamic interactions between the solar windand the magnetosphere that leads to changes in the polarcap potential causing prompt penetration of electric fieldsinto low latitudes (Fejer and Scherliess, 1997) giving riseto a short-lived (�2 h) disturbances most commonly inthe zonal electric fields. Whereas, the later is due to globalthermospheric circulation, induced by joule heating atauroral latitudes that generates long-lived (several hours)electric field disturbances at mid and low latitudes by ion-ospheric wind dynamo action (Scherliess and Fejer, 1997).The local time dependence of the polarity and amplitude of

ed.

S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312 1305

electric field disturbances from these two processes, in fact,determines the favorable or unfavorable conditions for thedevelopment of Equatorial Spread-F (ESF) irregularities atany given location.

It was shown by Aarons (1991) and Abdu et al. (1995)that, if the onset of the geomagnetic activity occurs priorto local sunset, then, inhibition of ESF takes place due tothe superposition of westward electric fields of IonosphericDisturbance Dynamo (IDD) on the normal pre-reversalenhancement (PRE) thereby reducing the post-sunsetupward ExB drift. On the other hand, Kelley and Maruy-ama (1992); Hysell and Burcham (1998) have shown thatthe geomagnetic activity in the post-midnight sector caninitiate ESF conditions due to anomalous upward reversalsin the vertical plasma drift under the action of eastwardelectric fields of IDD and prompt penetration mechanisms.

Abdu et al. (1995) have also shown that, the rapidincrease in high latitude convection around local sunsetcauses a prompt penetration of eastward electric field aug-menting the normal PRE, resulting in a dramatic develop-ment of ESF including the formation of plasma bubbles.While, Fejer et al. (1999) have shown that prolonged geo-magnetic activity during daytime, in general, causes a largereduction in the occurrence of post-sunset ESF at Jicamar-ca but only during equinoxes of solar maximum.

Recently, Basu et al. (2001) and Basu et al. (2001, 2005)have reported that there is an abrupt onset of VHF andL-band scintillations in the longitude sector for which theearly evening period corresponds to the time of rapiddecrease in the Sym-H/Dst index. They have also demon-strated that the abrupt onset of these scintillations is dueto the prompt penetration of high latitude electric fields(eastward) into low latitudes even in the presence of back-ground ionospheric disturbance dynamo induced (west-ward) electric fields.

In this paper, an experimental evidence for the develop-ment of ESF due to the prompt penetration of eastwardelectric fields into low latitudes during post-sunset hoursand their extended sustenance under the influence of bothprompt penetration and disturbance dynamo induced elec-tric fields during the post-midnight hours is presented. It isalso presented in this paper that the strong reversalsobserved in the day time ElectroJet current system becauseof disturbed geomagnetic activity during the morning tonoon hours, as a result of which, the equatorial ionizationanomaly as well as the pre-reversal enhancement in ExBdrift are suppressed and the subsequent occurrence ofSpread-F/scintillations is inhibited.

2. Data

The time dependant response of Indian equatorial andlow latitude ionosphere to the moderate geomagneticstorms of Feb 11–12, 2004 and March 9–12, 2004 is studiedusing the data from two ground based ionospheric sound-ers from a magnetic equatorial station, Trivandrum(8.47�N, 76.91�E, 0.5�N dip), and an off-equatorial station,

SHAR (13.7�N, 80.2�E, 10.8�N dip). The simultaneousobservations of VHF (244 MHz) and L-band (1.5 GHz)scintillations from two geo-stationary satellites FLEET-SAT (sub-ionospheric coordinates 16.4 �N, 82.7�E)and INMARSAT (sub-ionospheric coordinates 16.4�N,81.8�E), respectively, over Waltair were also used in thisstudy. The difference between the DH values (H-compo-nent of the magnetic field after subtracting the night timebase level) from the magnetometer data of an equatorialstation, Tirunelveli (8.7�N, 77.7�E, 0.6�N dip) and an off-equatorial station, Alibagh (18.5�N, 72.9�E, 13.0�N dip),i.e., DHT-A (nT) is taken as a measure of Equatorial Elec-troJet (EEJ) strength (Rusch and Richmond, 1973; Rastogiand Klobuchar, 1990).

3. Results

The role of high and low latitude electro-dynamicalcoupling during two moderate geo-magnetic storms ofFeb 11–12, 2004 and March 9–12, 2004, causing magneto-spheric electric field penetration into the equatorialionosphere is examined in the context of the generationof plasma density instabilities and associated Spread-F/scintillations over Indian equatorial and low latitudes.

In order to examine the local time dependence of theionospheric response to various phases of the geomagneticstorm activity and the associated electric field disturbances,all the geophysical parameters and magnetic indices used inthe present study are shown as a function of Indian localtime, which corresponds to 82.5�E meridian time.

3.1. Magnetic storm of Feb 11–12, 2004

On 11th Feb 2004, the interplanetary magnetic field,IMF Bz, turned southward around 1430 LT and showeda strong negative component of ��15 nT at 1830 LT, asmay be seen from Fig. 1(a). The high resolution (1 min)Sym-H index (which closely follows the hourly Dst-index,Iyemori et al., 2000) started decreasing around 1700 LTand reached a minimum value of �107 nT at 2245 LT, witha maximum time rate of change of �30 nT/h between 2000and 2100 LT, as seen from Fig. 1(b). The virtual height ofthe F-layer (h’F) at the equatorial station Trivandrum, hasrisen above 400 km altitude (Fig. 1(e)) during that evening,followed by the onset of strong range Spread-F at 2015 LTwhich continued to exist till the next morning (0600 LT) asmay be seen from Fig. 1(e). The average quiet day diurnalvariation of h’F is shown (with scatter bars) in this figure(Fig. 1(e)) for ready reference. It may be noticed from thisfigure that the h’F on this evening increased markedly fromits quiet day level, followed by the intensification of Equa-torial Ionization Anomaly (EIA) as seen (Fig. 1(d)) fromthe increase of foF2 at an off-equatorial station, SHAR(red line) and decrease of foF2 at the equatorial station Tri-vandrum (blue line). Also, an abrupt onset of VHF scintil-lations at 2020 LT is observed on the same evening over thelow latitude station Waltair, the S4-index of which is

Fig. 1. Time histories of (a) IMF Bz (b) Sym-H index, (c) Equatorial ElectroJet (EEJ) strength, DHT-A (nT), (d) foF2 at SHAR and Trivandrum, (e) h’F atthe equator (Trivandrum) and (f) S4-index of the VHF scintillations during 11th–12th February 2004. The shaded portion indicates the night time period.

1306 S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312

presented in Fig. 1(f). From these results, it is suggestedthat the prompt penetration of eastward electric fields intothe equatorial latitudes drives strong vertical drifts, therebylifting the F-layer to higher altitudes where the growth rateof the Rayleigh-Taylor instability maximizes. Consequent-ly, this resulted in a sudden onset of VHF scintillations atWaltair (Fig. 1(f)) and strong range Spread-F echoes atTrivandrum (Fig. 1(e)), which continued to exist till thepre-dawn hours of the subsequent day. It can be seen fromFig. 1(f) that the VHF scintillations over Waltair also con-tinued to exist for longer duration and extends up to 0345LT of the next morning.

On the next day, i.e., 12th Feb 2004, during the recoveryphase of the storm, the IMF Bz exhibits rapid fluctuations(Fig. 1(a)) and Sym-H index also shows negative excursions(Fig. 1(b)) around noon hours. It is seen from Fig. 1(c) that

there is a strong reversal in the ElectroJet current at theequator around noon hours, and the EIA also gets weak-ened from 1400 LT, which may be seen from the foF2 data(Fig. 1(d)) of Trivandrum and SHAR. Also, the post-sunsetenhancement of h’F at the equator is reduced compared toits average quiet day value (Fig. 1(e)), and consequently, theoccurrence of Spread-F and scintillations is inhibited onthis night.

Fig. 2 shows a series of ionograms from 0830 to 1800 hLT on 12th February 2004 at Trivandrum. It can benoticed from this figure that the equatorial (q-type) Spo-radic-E (denoted as Es-q) disappeared between the succes-sive ionograms of 1030 and 1045 h LT, and reappearedafter several hours at 1715 h LT (the characteristics forthe identification of q-type Es are provided in the Appen-dix). Rastogi (1972b) has explained that Es-q configuration

Fig. 2. Series of ionograms from 0830 to 1800 h LT on 12th February 2004 at Trivandrum. Note the disappearance of Es-q from 1045 h LT, precisely thetime when the reversal in ElectroJet current at the equator (Fig. 1(c)) is observed.

S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312 1307

in the ionograms at equatorial latitudes is due to scatteringby irregularities generated by gradient drift instabilitymechanism when the electron density gradient (dN/dh)and Hall field are in the same direction. On certain occa-sions when the primary horizontal electric field reversesto westward direction, the Hall field becomes downwardand the Es-q irregularities disappear.

3.2. Magnetic storm of March 9–12, 2004

Another moderate geomagnetic storm occurred on 9thMarch 2004, showing similar equatorial ionosphericresponse to that of the 11–12 Feb 2004, storm is presentedin Fig. 3. Following the southward turning of IMF Bz(Fig. 3(a)), the Sym-H index started decreasing from 1648LT of 9th March 2004 and reached a minimum value of�75 nT at 2335 LT, with a maximum time rate of changeof �25 nT/h between 1930 and 2030 LT, as seen from

Fig. 3(b). The h’F at the equatorial station, Trivandrum(Fig. 3(e)) has increased quiet significantly from its quietday value during the same period and intense Spread-Fechoes are observed from 1945 LT, which continued toexist up to 0530 LT of the next morning. Also, a nearlysimultaneous onset of VHF and L-band (Fig. 3(f)) scintil-lations were observed from 1958 LT over Waltair. TheVHF scintillations continued to exist up to 0300 LT ofthe next morning and the L-band scintillations also contin-ued to occur for longer duration extending to the post-mid-night hours which slowly decayed by about 0130 LT.

Further, due to prolonged geomagnetic activity of thisstorm, i.e., up to the noon hours of the next day (10thMarch 2004), a strong reversal in the ElectroJet current isobserved at the equator as may be seen from Fig. 3(c).The foF2 values (Fig. 3(d)) of Trivandrum and SHARare nearly equal to one another during the day time hoursindicating the reduced strength of the EIA. During this

Fig. 3. Time histories of (a) IMF Bz (b) Sym-H index, (c) Equatorial ElectroJet (EEJ) strength, DHT-A (nT), (d) foF2 at SHAR and Trivandrum, (e) h’F atthe equator (Trivandrum) and (f) S4-index of the VHF (red line) and L-band (blue line) scintillations during 9th–10th March 2004. The shaded portionindicates the night time period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

1308 S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312

evening, the post-sunset enhancement in the h’F at theequator is suppressed (Fig. 3(e)) and subsequently theoccurrence of Spread-F and scintillations is inhibited(Fig. 3(f)). Also, it is seen from Fig. 3(d) that, around1800 LT the foF2 at the off-equatorial station SHAR is lessthan that at the equatorial station Trivandrum indicatingthe total absence of equatorial ionization anomaly.

Similar ionospheric response has been observed duringthe subsequent couple of days (11th and 12th March2004), where the geomagnetic storm activity continued topersist. The IMF Bz turned southward at 1800 LT andthe time rate of change of Sym-H index showed a markeddecrease of �25 nT/h between 1945 and 2045 LT on 11thMar 2004, as may be seen from Fig. 4(a) and (b). On thisevening, the h’F at the equator (Fig. 4(e)) raises to a peakaltitude of 378 km at 2100 LT and the EIA is intensified as

observed in the increase of foF2 at SHAR and decrease offoF2 at Trivandrum (Fig. 4(d)) during the post-sunsethours. Strong Spread-F echoes were observed at Trivan-drum from 2115 LT, which continued to exist till 0630LT of the next morning (12th March 2004) as seen fromFig. 4(e). The VHF and L-band scintillations (Fig. 4(f))over Waltair show a nearly simultaneous impulsive onsetat 2114 LT and persisted for about half an hour, followedby another scintillation patch around 2300 LT at bothVHF and L-band frequencies. The L-band scintillationsstart decaying from 0100 LT, whereas the VHF scintilla-tions continued to occur till 0600 LT of the next morning(12th March 2004) but with reduced intensities (Fig. 4(f)).

Further, a strong reversal in the ElectroJet current isobserved at the equator on the subsequent day, i.e., 12thMarch 2004 (Fig. 4(c)) due to the continued geomagnetic

Fig. 4. Time histories of (a) IMF Bz (b) Sym-H index, (c) Equatorial ElectroJet (EEJ) strength, DHT-A (nT), (d) foF2 at SHAR and Trivandrum, (e) h’F atthe equator (Trivandrum) and (f) S4-index of the VHF (red line) and L-band (blue line) scintillations during 11th–12th March 2004. The shaded portionindicates the night time period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312 1309

activity. The day time EIA is weakened and the post-sunsetenhancement of h’F at the equator is also reduced to lessthan its quiet day value, and therefore, the subsequentoccurrence of Spread-F and scintillations is inhibited asseen from Fig. 4(d)–(f). It is also observed from Fig. 4(d)that around sunset, the foF2 at the off-equatorial station,SHAR is reduced significantly than the foF2 at the equato-rial station Trivandrum indicating the inhibition of EIA.

4. Discussion

Basu et al. (2001) and Basu et al. (2001, 2005) haveshown that, the prompt penetration of eastward electricfields from high latitudes to low latitudes, caused the onsetof 250 MHz and L-band scintillations in the longitude sec-tor where the time of rapid decrease in Dst/Sym-H index

(�50 nT/h or above) occurs in the early dusk periods. Theresults in the present study, also consistently showing theprompt penetration of eastward electric fields during theearly evening hours that corresponds to the local times ofrapid decrease in Sym-H index. As a result, the post-sunsetupward drifts of the equatorial F-layer (h’F at Trivandrum)are increased quiet significantly from their quiet day values( Figs. 1(e), 3(e) and 4(e)) and subsequently strong Spread-Fat Trivandrum as well as VHF and L-band scintillations atWaltair have been observed. However, one significant dif-ference is that, the prompt penetration of eastward electricfields into the equatorial ionosphere has occurred even dur-ing the moderate geomagnetic storm activity with lesser(�25 to �30 nT/h) rate of decrease in Sym-H index com-pared to that observed (�50 to �120 nT/h) by Basu et al.(2001) and Basu et al. (2001, 2005).

1310 S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312

Further, the observed Spread-F and scintillations werecontinued to exist for longer durations extending intopost-midnight to pre-dawn hours. This extended suste-nance of VHF and L-band scintillations beyond thepost-midnight hours is quite uncommon, particularly,during the low sunspot activity periods. The long dura-tion scintillation patches at VHF may be observed beyondthe post-midnight hours during the low sunspot activityperiods, but mostly in the summer months, without anysignatures at L-band frequencies, and are generally knownto be due to Bottom Side Sinusoidal (BSS) irregularities(Das Gupta et al., 1983; Basu et al., 1986; Rama Raoet al., 2005). However, no such cases with extended suste-nance of scintillation activity was observed over Waltairin the months of February, March and April, 2004 exceptduring these three storm-time events presented in Figs. 1,3 and 4. Thus, it may be inferred that these scintillationshave occurred as a consequence of the response to thegeomagnetic storm activity and the nature of the electricfields induced into the low latitude ionosphere.

It may be noticed from Figs. 1(b) and 3(b) that theSym-H index started decreasing around the local sunsetand gradually reaches its minimum value around localmidnight with a time rate of change of �20 to �30 nT/h. That means, the main phase of the storm lasts forabout 6–7 h with more or less similar rate of decrease inSym-H index. Thus, the longitudes where the local dusktime overlaps with the time of decrease (�20 to�30 nT/h) in Sym-H index extends over wide longitudinalsector. Hence, it can be understood that the prompt pen-etration of high latitude electric fields into equatorial lat-itudes can occur over a wide longitudinal sector. Thus, itis possible that the ESF irregularities may also developover the longitudes to the west of the present observation-al locations but at a later time. These irregularities, lateron will come over the observational locations as they gen-erally drift towards east with velocities of about 100 m/s(Aarons et al., 1980; Rama Rao et al., 2005), causingSpread-F and scintillations to persist for longer durationsover Trivandrum and Waltair, as may be seen from Figs.1(e),(f) and 3(e),(f). However, since we do not have theobservations to the west of Trivandrum, this hypothesiscould not be supported experimentally.

It is known from both theory and observations that theDisturbance Dynamo induced electric field reverses toeastward around 2200 h LT lifting the F-layer to higheraltitudes where collisions are less and creating conditionsfavorable for the generation of irregularities. The scintilla-tion patch observed at 2300 h of 11th March 2004(Fig. 4(f)) may be attributed to be generated due to theIonospheric Disturbance Dynamo (IDD) induced east-ward electric fields. Thus, due to the combined action ofprompt penetration and Disturbance Dynamo electricfields, the equatorial ionospheric scintillation activity sig-nificantly enhances, posing a major threat to the trans-ionospheric communication signals even during the mod-erate geomagnetic storm periods.

Under quiet geomagnetic conditions, the zonal elec-tric field is eastward during the day, westward duringthe night and exhibits a strong eastward enhancementfor a short period just after sunset known as pre-rever-sal enhancement. But during the disturbed geomagneticperiods, the disturbance electric fields developed due toIDD are westward for most of the day and reversesto eastward around 2200 h LT (Blanc and Richmond,1980; Scherliess and Fejer, 1997; Sastri et al., 2002).Superposition of this westward electric field of Distur-bance Dynamo on the primary eastward electric fieldsduring day time results in reduced eastward or westwardzonal electric field depending upon their relativestrengths. Further, the sudden disappearance of q-typeEs between the successive ionograms of 1030 and1045 h LT (Fig. 2), precisely the time when the Electro-Jet current (DHT-A) becomes negative (Fig. 1(c)), indi-cates the presence of strong westward zonal electricfield. Rastogi (1972a) has shown that the sudden disap-pearance of Es-q is associated with a decrease of the H-field below its nighttime value. He attributed this phe-nomenon is due to westward reversal of zonal electricfield in the E-region as inferred from the reversal ofionospheric drift patterns observed from JicamarcaRadar. On certain occasions when the primary horizon-tal electric field reverses to a westward direction, theHall field becomes downward and Es-q irregularities dis-appear (Rastogi, 1972a).

Further, due to these westward disturbance dynamoelectric fields, the day time EIA and the post-sunsetenhancement in upward ExB drift are reduced and subse-quent occurrence of Spread-F and Scintillations is inhibit-ed as may be seen from Figs. 1, 3 and 4. After sunset, thezonal electric field reverses to westward and the distur-bance dynamo induced electric fields are also westward;which may lead to a total inhibition of fountain processand thus the equatorial ionization anomaly (Rama Raoet al., 1983; Sastri, 1988). This could be the reason forlarger foF2 values observed at the equatorial station Tri-vandrum than at the off-equatorial station SHAR duringthe post-sunset hours of 12th Feb, 10th Mar and 12thMar of 2004 as may be seen from Figs. 1(d), 3(d) and4(d).

5. Summary

The results presented in Figs. 1, 3 and 4, showing theequatorial ionospheric response to the moderate geomag-netic storms of Feb 11–12, 2004 and March 9–12, 2004,have shown some interesting and common features, whichare summarized hereunder.

(i) When the local time at which a rapid decrease(�25 nT/h or above) in the Sym-H index occurs inthe early evening hours, the onset of Spread-F onionograms and scintillations at both VHF and L-band frequencies has taken place in the equatorial

S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312 1311

and low latitude sectors. This could possibly be dueto the prompt penetration of eastward electric fieldsinto equatorial latitudes augmenting the normalPRE ExB drift and thereby lifting the equatorialF-layer to greater altitudes where the growth rateof Rayleigh-Taylor instability maximizes.

(ii) The Spread-F at Trivandrum and SHAR is continuedto exist till the morning hours of the subsequent dayof the geo-magnetic storm.

(iii) Although, the L-band scintillations over Waltair aredecayed after midnight, the VHF scintillations con-tinue to occur till the pre-dawn hours (Figs. 1(f),2(f) and 3(f)) of the subsequent day.

(iv) Because of prolonged geomagnetic disturbance dur-ing morning to noon hours of the subsequent dayof the storm, strong reversal in the electrojet currentis observed under the influence of westward electricfield of Disturbance Dynamo origin. Due to thiswestward disturbance electric field, the EIA is sup-pressed during most of the day time hours and exhib-its a reverse signature with larger ionization over theequator than at the off-equatorial latitudes after sun-set. The post-sunset enhancement in the upward ExBdrift is also suppressed, consequently the occurrenceof Spread-F and scintillations is inhibited.

Acknowledgements

This work has been carried out under the Departmentof Science and Technology (DST), Govt. of India spon-sored Project No. SR/S4/AS: 226/03. One of the authors(STR) wishes to express his sincere thanks to Earth Sys-tem Science (ESS) division of DST for providing him witha JRF in the project to carry out this work. The authorsexpresses their sincere thanks to IIG, Mumbai for provid-ing the ElectroJet Strength data. The authors STR andPVS wish to express their sincere thanks to COSPAR sec-retariat for providing them with partial financial supportto participate in the 36th COSPAR Scientific Assembly,at Beijing, China and enabling them to present this paper.The authors wish to acknowledge the critical commentsmade by both the referees which had certainly improvedthe quality of the paper.

Appendix A

Knecht (1959) described the special features of Esobserved within the equatorial electrojet region and calledit q-type Es, denoted by Es-q. He described the character-istics of Es-q as follows:

(i) It is always transparent to radio waves i.e., it neverblankets reflections from higher layers.

(ii) Multiple echoes from this layer are not observed.(iii) It usually shows a well defined lower edge lying

between 100 and 110 km, with diffused echoes abovethe principal echo.

(iv) It occurs within ±5� dip latitudes on either side of theequator.

References

Aarons, J. The role of the ring current in the generation and inhibition ofequatorial F layer irregularities during magnetic storms. Radio Sci. 26,1131–1149, 1991.

Aarons, J., Mullen, J.P., Whitney, H.E., Mackenzie, E.M. The dynamicsof equatorial irregularity patch formation, motion and decay. J.Geophys. Res. 85, 139–149, 1980.

Abdu, M.A., Batista, I.S., Walker, G.O., Sobral, J.H.A., Trivedi,N.B., de Paula, E.R. Equatorial ionospheric fields duringmagnetospheric disturbances: local time/longitudinal dependenc-es from recent EITS Camaigns. J. Atmos. Solar-Terr. Phys. 57,1065–1083, 1995.

Basu, Su., Basu, S., Valladares, C.E. Ionospheric effects of major magneticstorms during the International Space Weather Period of Septemberand October 1999: GPS observations, VHF/UHF scintillations, andin situ density structures at middle and equatorial latitudes. J.Geophys. Res. 106, 30143–30389, 2001.

Basu, S., Basu, Su., Valladares, C.E., Das Gupta, A., Whitney,H.E. Scintillations associated with bottomside sinusoidal irregu-laritiesin the equatorial F-region. J. Geophys. Res. 91, 270–276, 1986.

Basu, S., Basu, Su., Groves, K.M., Yeh, H.C., Su, S.Y., Rich, F.J., Sultan,P.J., Keskinen, M.J. Response of the equatorial ionosphere in theSouth Atlantic region to the great magnetic storm of July 15, 2000.Geophys. Res. Lett. 28, 3577–3580, 2001.

Basu, S., Basu, Su., Groves, K.M., MacKenzie, E., Keskinen, M.J.,Rich, F.J. Near-simultaneous plasma structuring in the midlatitude andequatorial ionosphere during magnetic superstorms. Geophys. Res.Lett. 32, L12S05, 1–4, doi:10.1029/2004GL021678, 2005.

Blanc, M., Richmond, A.D. The ionospheric disturbance dynamo. J.Geophys. Res. 85, 1669–1688, 1980.

Das Gupta, A., Basu, Santimay, Aarons, J., Klobuchar, J.A., Basu,Sunanda, Bushby, A. VHF amplitude scintillations and associatedelectron content depletions as observed at Arequipa, Peru. J. Atmos.Terr. Phys. 45, 15–26, 1983.

Fejer, B.G., Scherliess, L. Emperical models of storm time equatorialzonal electric fields. J. Geophys. Res. 102, 24047–24056, 1997.

Fejer, B.G., Scherliess, L., de Paula, E.R. Effects of the vertical plasmadrift velocity on the generation and evolution of equatorial spread F. J.Geophys. Res. 104, 19859–19869, 1999.

Hysell, D.L., Burcham, J.D. JULIA radar studies of equatorial spread-F.J. Geophys. Res. 103, 29155–29167, 1998.

Iyemori, T., Araki, T., Kamei, T., Takeda, M. Mid-latitude geomagneticindices ‘‘ASY’’ and ‘‘SYM’’. Available from: <http://swdcdb.kugi.kyoto-u.ac.jp/aeasy/asy.pdf>. World Data Cent. C2 forGeomagn., Kyoto, Japan, 2000.

Kelley, M.C., Maruyama, T. A diagnostic model of equatorial Spread-F 2.The effect of magnetic activity. J. Geophys. Res. 97, 1271–1277, 1992.

Knecht, R.W. An additional lunar influence on equatorial Es atHuancayo. J. Atmos. Terr. Phys. 14, 348–349, 1959.

Rama Rao, P.V.S., Das Gupta, A., Klobuchar, J.A., Rastogi, R.G.Electrojet Control over the equatorial anomaly in Total ElectronContent and equivalent slab thickness, in: Proc. of COSPAR BeaconSatellite Symposium, 261–268, India, 1983.

Rama Rao, P.V.S., Tulasi Ram, S., Niranjan, K., Prasad, D.S.V.V.D.,Gopi Krishna, S., Lakshmi, N.K.M. VHF and L-band scintillationcharacteristics over an Indian low latitude station, Waltair (17.7�N,83.3�E). Ann. Geophys. 23, 2457–2464, 2005.

Rastogi, R.G. Equatorial sporadic E layer during geomagnetic storms. J.Geomagn. Geoelectr. 24, 2429–2440, 1972a.

Rastogi, R.G. Equatorial sporadic E and plasma instabilities. Nature 237,73–75, 1972b.

1312 S. Tulasi Ram et al. / Advances in Space Research 39 (2007) 1304–1312

Rastogi, R.G., Klobuchar, J.A. Ionospheric electron content within theequatorial F2 layer anomaly belt. J. Geophys. Res. 95, 19045–19052,1990.

Rusch, C.M., Richmond, A.D. The relationship between the structure ofthe equatorial anomaly and the strength of the equatorial electrojet. J.Atmos. Terr. Phys. 35, 1171–1180, 1973.

Sastri, J.H. Equatorial electric fields of ionospheric disturbance dynamoorigin. Ann. Geophys. 6, 635–642, 1988.

Sastri, J.H., Niranjan, K., Subbarao, K.S.V. Response of the equatorialionosphere in the Indian (midnight) sector to the severe magnetic

storm of July 15, 2000. Geophys. Res. Lett. 29 (13), 1651–1655,doi:10.1029/2002GL015133, 2002.

Scherliess, L., Fejer, B.G. Storm time dependence of equatorial disturbancedynamo zonal electric fields. J. Geophys. Res. 102, 24037–24046, 1997.

Senior, C., Blanc, M. On the control of magnetospheric convection by thespatial distribution of ionospheric conductivities. J. Geophys. Res. 89,261–284, 1984.

Spiro, R.W., Wolf, R.A., Fejer, B.G. Penetration of high latitude electricfield effects to low latitudes during SUNIDAL 1984. Ann. Geophys. 6(1), 39–50, 1988.