An intense SFE and SSC event in geomagnetic H, Y and Z ®eldsat the Indian chain of observatories

R. G. Rastogi1*, D. R. K. Rao2, S. Alex2, B. M. Pathan2, T. S. Sastry3

1 Department of Physics, Gujarat University, Ahmedabad 380 009, India2 Indian Institute of Geomagnetism, Mumbai 400 005, India3 National Geophysical Research Institute, Hyderabad 500 007, India

Received: 3 June 1996 / Revised: 27 March 1997 /Accepted: 1 April 1997

Abstract. Changes in the three components of geomag-netic ®eld are reported at the chain of ten geomagneticobservatories in India during an intense solar crochetthat occurred at 1311 h 75° EMT on 15 June 1991 andthe subsequent sudden commencement (SSC) of geo-magnetic storm at 1518 h on 17 June 1991. The solar¯are e�ects (SFE) registered on the magnetogramsappear to be an augmentation of the ionospheric currentsystem existing at the start time of the ¯are. Anequatorial enhancement in DH due to SFE is observedto be similar in nature to the latitudinal variation of SQ(H) at low latitude. DY registered the largest e�ect at3.6° dip latitude at the fringe region of the electrojet. DZhad positive amplitudes at the equatorial stations andnegative at stations north of Hyderabad. The SSCamplitude in the H component is fairly constant withlatitude, whereas the Z component again showed largerpositive excursions at stations within the electrojet belt.These results are discussed in terms of possible currentsof internal and external origin. The changes in the Y®eld strongly support the idea that meridional current atan equatorial electrojet station ¯ows in the ionosphericdynamo, E.

1 Introduction

As is well known, solar ¯are e�ects (SFE) in thegeomagnetic ®eld at ground level represent changes inthe ionospheric currents associated with the arrival ofenhanced electromagnetic radiations from the Sun.Sudden commencements (SSC) of geomagnetic stormsrepresent e�ects due to the ionised plasma cloud from

the Sun following solar ¯ares or from solar coronalholes.

The ®rst observations of the simultaneous occurrenceof a solar ¯are and the crochet in magnetic ®eld relatedto the event of 1 September 1859 were reported byCarrington (1859) and Hodgson (1859). A comprehen-sive study of the SFE was made by McNish (1937), whoexamined the magnetograms from a large number ofobservatories to show that the e�ect of the ¯are on themagnetograms was the augmentation of the ®eld valuesat the time of the ¯are. Nagata (1952) found anabnormally large e�ect of the solar ¯ares on thegeomagnetic H ®eld at Huancayo compared to twoother stations at Kakioka and Watheroo. Forbush andCasaverde (1961) showed that the enhancement of SFEin H at Peruvian stations varied in a manner similar tothe enhancement of the SQ range of H at equatoriallatitudes. Rastogi et al. (1965) showed that the equato-rial enhancement of SFE in H was more pronounced inthe American than in Indian longitudes, correspondingto the longitudinal variation of equatorial electrojetcurrent itself. Srivastava (1974) seems to have been the®rst to describe the e�ect of solar ¯are on D, H and Z®elds at Indian observatories. Rastogi et al. (1975)showed that some of the abnormal features of SFE in Hwere due to a partial counter electrojet at the start timeof the solar ¯are. Rastogi (1996a) described the resultsof an extensive study of SFE in H and D at theequatorial station Annamalainagar over the period1967±1976. During the normal electrojet period a solar¯are produced a positive change in H, a negative changein Y and a negative change in Z. The e�ect on DY(negative) increased linearly with increasing value ofDH. This suggested that the SFE on all the threecomponents of the geomagnetic ®eld were plainly theaugmentation of the ionospheric current over thestation.

The SSC of a magnetic storm is very clearly seen attemperate and low latitudes all over the world simulta-neously to within a few seconds, and signi®es an abruptincrease worldwide in the horizontal geomagnetic ®eld

Correspondence to: D. R. K. Rao*Presently at: School of Physics, University of New South Wales,Sydney, Australia

Ann. Geophysicae 15, 1301±1308 (1997) Ó EGS ± Springer-Verlag 1997

H and with associated sudden changes in declination Dand vertical intensity Z. The amplitude of SSC in Ddepends on local time and season, whereas SSC (Z)depends on the location of the observing station.Latitudinal and longitudinal characteristics of the SSC(H) have been dealt with by many researchers (e.g.Sugiura, 1953; Forbush and Casaverde, 1961; Rastogiet al., 1964, etc.).

These two phenomena, having distinctively di�erentcharacteristics, can be studied from registrations in asuitable network of geomagnetic observatories to un-derstand the ionospheric current systems during theevents. India has a unique set of geomagnetic observa-tories spanning the latitudes of the magnetic equatorand the SQ focus in a region of the world where thegeomagnetic and geographic meridian planes are leastseparated. Taking advantage of this, we aim here tofocus on some of the aspects of an SFE and associatedSSC and their related changes in the ionosphericconductivities and electric ®elds.

2 Event selection and presentation

2.1 SFE on 15 June 1991

The Holloman observatory reported a type-3E opticalsolar ¯are on 15 June 1991 with a duration of 29 min

starting at 0813 UT. The Wroclaw observatory reporteda Ha surge between 0820±0831 UT. The Ondrejovobservatory reported a burst of decimeter and meterradio waves starting at 0813.5 UT and ending at 1032.1UT. Type-II radio bursts were reported at the Potsdam,San Vito and Wissenau observatories. The GOESsatellite recorded beautiful bursts of solar X-ray radia-tions in the 0.5±4 and 1±8 AÊ ranges on 15 June 1991,reproduced in Fig. 1 (Solar Geophysical Data bulletin578 Pt. II). It is to be noted that the X-ray ¯ux startedincreasing slowly after about 01 UT, and after 02 UT itchanged into a strong burst, increasing the ¯ux after 08UT by over two orders of magnitude. The ¯ux hadreturned to its pre-¯are value only by about 16 UT.

An abnormally large geomagnetic crochet (SFE) wasregistered at the Indian geomagnetic observatories at1311 h 75° EMT (0811 UT) on 15 June 1991. A largeSSC followed this crochet on 17 June 1991 at 1518 h,almost 50 h later. The geographic and geomagnetic co-ordinates of the Indian geomagnetic observatories,operational during June 1991, are given in Table 1;station codes are also included. The chain of stationswhose magnetograms are used for the study all lie inmore or less the same longitudinal sectors, barringShillong, which is about 15° further east of this sector.

Figure 2 shows the daily variations in H, Y (Y is theEastward component of the ®eld computed from H andD) at di�erent stations (1) on the ¯are day 15 June 1991

Fig. 1. Variation in solar X-ray ¯ux within 0.5±4 and 1±8 AÊ ranges during 14±16 June 1991 by instruments aboard the GOES satellite

Table 1. Geographic and geo-magnetic coordinates of Indiangeomagnetic observatories op-erating during 1991

station code geographic geomagnetic dipangle

lat. N long. E lat. N long. E

Trivandrum TRD 8.48 76.97 )1.2 146.4 0.6Ettaiyapuram ETT 9.17 78.00 0.6 147.5 2.2Kodaikanal KOD 10.23 77.47 0.6 147.1 4.7Annamalainagar ANN 11.37 79.68 1.4 149.4 7.1Hyderabad HYB 17.42 78.55 7.6 148.4 21.8Alibag ABG 18.62 72.87 9.5 143.6 25.4Ujjain UJJ 23.18 75.78 13.5 147.0 33.9Shillong SHL 25.57 91.88 14.6 162.4 45.9Gulmarg GUL 34.08 74.40 24.5 147.2 50.1

1302 R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories

and (2) averaged over the ®ve international quiet days ofJune 1991 (henceforth denoted SQ) corrected for anydaily inequalities. These curves are drawn through thehourly mean data points.

The variations in H, Y and Z ®elds at the chain ofgeomagnetic observatories along the India-Russia sectorhave been described by Patil et al. (1983) and by Rastogi(1996b). It is to be noted from the Fig. 2 that on quietdays the amplitude of DH decreased progressively fromTrivandrum (TRD) to Gulmarg (GUL). Absence of aclear diurnal variation pattern at GUL and near equal

magnitudes of the pre-noon and post-noon amplitudesuggest that the SQ focus on average may be located inthe vicinity of GUL during this month. The dailyvariation in DY shows a positive peak in the morningand minimum at noon at all stations, including theelectrojet stations, suggesting a well-developed vortexstructure of the current system typical of local summermonths. The daily variation in DZ shows a minimumaround noon, a characteristic of northern-hemispherestations. The daily range of DZ shows a maximum atAnnamalainagar (ANN), which is situated at a latitudeof largest gradient of DH within the equatorial electrojetbelt.

Referring to the variations in the H ®eld on 15 June1991, drawn with full lines, it is to be noted that DH atall low-latitude stations TRD to Alibag (ABG) startedincreasing earlier in the morning on the ¯are day andthat throughout this day DH was larger than the averageSQ values. The value of D at 1130 h at Ettaiyapuram(ETT) was 110 nT on SQ days and 140 nT on 15 June1991. Similarly, at Hyderabad (HYB) DH was 65 nT onSQ days and 70 nT on 15 June 1991. Even the hourlymean value of DH at 13.5 h shot up to a value of 206 nTfrom its 12.5-h value of 140 nT. A similarly largeincrease in hourly mean DH at 13.5 h is seen at allstations up to Ujjain (UJJ). No large increase in DH at13.5 h is evidenced at Sabha wala, (SAB) and GUL dueto its proximity to the SQ focus, where the currentsnorth and south of the station cancel the e�ect on DH.

Regarding the variations in the Z ®eld ignoring theindividual value at 13.5 h at TRD and ANN, one wouldconclude a large increase in electrojet current on 15 June1991 compared to that on SQ days; these e�ects areagain largest at ANN and HYB. What seems to be anabnormal feature is the sudden large positive increase at13.5 h at TRD and ETT [Kodaikanal (KOD) data on Zwere not available on 15 June 1991].

The magnetogram traces at di�erent stations werereorganised to study the e�ect of the solar ¯are on theindividual components H, Y and Z at stations extendingfrom the equatorial location of TRD to a station in thevicinity of SQ focus, GUL, and are shown in a, b and cof Fig. 3, respectively. These are the tracings of actualmagnetograms, and these have di�erent scale values.The numbers indicate the magnitude of SFE on theparticular component.

DH at TRD shows a sudden increase in H at 1311 hEMT due to SFE. The amplitude of 187 nT is unusuallylarge and is one of the largest ¯are e�ects recorded atTRD. The H recordings at ETT went o� the scale, butby comparison with the time-pro®le of DH at neigh-bouring station TRD an amplitude of 176 nT wasestimated. The amplitude of SFE in H decreasedprogressively with increasing latitude of the stationand became quite small ()6 nT) at GUL.

The DY trace at all stations showed a negativeexcursion due to SFE. Examining the daily variations ofDY, it can be seen that the ¯are occurred immediatelyafter noon (1311 h 75° EMT) when DY reached itsdiurnal minimum value. An abnormally large Y, equalto )67 nT, was recorded at ANN. The amplitude of DY

Fig. 2. The variations in the geomagnetic ®eld H (left panel), Y(middle panel) and Z (right panel) at the Indian observatoriesaveraged on ®ve international quiet days of June 1991 (dashed lines);individual day 15 June 1991 ( full lines). All curves constructed fromhourly mean values

R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories 1303

at ETT ()19 nT) was nearly the same as that at TRD()14 nT), which is closest to the dip equator.

The magnetograms showing Z at di�erent stationspresent unexpected registrations. The SFE was verylarge in amplitude, equal to +180 nT at TRD, and thisis found to decrease rapidly with increasing latitude,decreasing to )4 nT at HYB. At stations north of HYB,SFE (Z) was signi®cantly negative, being around)30 nT. This positive SFE (Z) with a strong equatorialenhancement within the electrojet belt is a phenomenonwhich requires further examination.

In order to understand the SFE in H, Y and Z thedeviations of these components just before the start ofthe ¯are with respect to the 0000 h value on the sameday were also noted and referred to as the pre-¯arevalues DH0, DY0 and DZ0.

Figure 4 shows the latitudinal variations of (1) pre-¯are values, DH0, DY0 and DZ0, (2) enhancements due to¯are on the components DHSFE, DYSFE and DZSFE and(3) the ratios DHSFE/DH0 and DYSFE/DY0 for the solar¯are at 1311 h (75° EMT) on 15 June 1991.

The SFE in H is seen to increase in a fashion verysimilar to that of pre-¯are DH0. The ratio of DHSFE/DH0

at all stations except GUL, a station very close to SQfocus, varied from about 1.5 to 2.0.

The eastward ®eld DY0 at the time of ¯are onsetvaried between )15 and )40 nT at di�erent stations,with a general increase in the negative amplitude withlatitude. The solar ¯are e�ect, DYSFE varied between)14 and )70 nT, showing a peak around 10° dip.

The latitudinal variation of DZ0 showed a very lowvalue at the equator and a negative value around)40 nT at other latitudes. There is a suggestion of a

maximum around ANN, a station near the fringe of theelectrojet belt. The DZ due to SFE showed almostsimilar values at stations north of HYB, but abnormallylarge positive deviations are noted at the equatorialstations TRD, ETT and ANN.

Figure 5 shows the current vectors which are com-puted as the resultant magnitudes of the two orthogonalhorizontal ®eld vectors (DH and DY amplitudes of SFEwith respect to pre-¯are values) and rotating the same90° clockwise. It is to be noted that the current vectorsdue to both the SQ as well as to the crochet arepractically in the same direction at low latitudes of theIndian subcontinent, although the magnitudes of thevectors are di�erent, as expected.

2.2 SSC on 17 June 1991

Figure 6 reproduces the magnetograms of the di�erentstations in India showing the SSC e�ects on H, Y and Zcomponents at 1518 h 75° EMT on 17 June 1991. Again,these are tracings of magnetograms with di�erent scalevalues. The magnitude of SSC in nT is indicated on thetracings. The latitudinal variations in the pre-SSC valuesof the three components, viz. DH0, DY0 and DZ0 as wellas SSC amplitudes in these components are shown inFig. 7.

The pre-SSC DH0 showed a monotonous increasewith decreasing latitude. No signi®cant equatorialenhancement of DH0 was evident, as the SSC occurredin the late afternoon hours when the electrojet currentsare greatly reduced. The SSC amplitude in H was 60 nTat TRD and almost of the same amplitude at other

Fig. 3a±c. The magnetogramtracings at Indian Observatoriesshowing the SFEs on a Y, bH andc Z components due to the ¯are at1311 h 75° EMT on 15 June 1991.The amplitudes of SFE at each ofthe stations for the three compo-nents of geomagnetic ®eld areindicated

1304 R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories

stations, indicating again the non-existence of equatorialenhancement which is the characteristic feature ofmidday SSC events at low latitudes (Rastogi, 1993a).The SSC amplitude in DY showed a uniform latitudinalvariation with an indication of larger deviation at ANNsituated at the periphery of the electrojet. The SSCsignature of the DZ ®eld showed a positive excursion atthe equatorial stations and a negative deviation atstations north of HYB. This feature in DZ is nearly thesame as that of the SFE.

3 Discussion

Before summarising and discussing the observationsof solar ¯are and sudden commencements on the threecomponents of the geomagnetic ®eld, it is essential to

seek the origins of these phenomena. The SFE areassociated with the arrival of a sudden increase ofelectromagnetic radiations from the Sun, which, whiletraversing the ionosphere, generate additional ionisat-ions mainly in the E, partially in the D and sometimes inthe F region. The SSC of a magnetic storm indicates thearrival of a dense plasma cloud consisting of chargedparticles from the Sun, generally a few tens of hoursafter the occurrence of the solar ¯are. These clouds arestopped at the magnetopause when the dynamic pres-sure of the charged particles is balanced by the magneticpressure of the Earth's magnetic ®eld and the magne-tosphere is compressed, causing a sudden increase in Hat stations around the world on the dayside as well asthe nightside.

The results of SFE at the Indian stations indicate thatthe disturbance vectors of the solar ¯are were almost inthe same direction as the pre-SFE (SQ) vector justbefore the start of the event. Volland and Taubenheim(1958), while examining SFE at the European stationsbetween latitudes of 10° and 60° North have shown anangular di�erence of 15°±30° between the SQ and SFEvectors. Veldkamp and Sabben (1960) also detecteddi�erences between SFE and SQ vectors at Europeanstations during the ¯are on 23 March 1958. Sabben(1968), after re-examining the current systems due toSFE during the period 1959±1961, concluded that theSFE can be considered as a temporary augmentation ofthe daily variation of the magnetic ®eld based on thecomparison of the disturbance vectors of the SFE and

Fig. 4. Latitudinal variations in the amplitudes in the threecomponents of the geomagnetic ®eld during the ¯are (DYSFE, DHSFE

and DZSFE) and pre-¯are SQ (DY0, DH0 and DZ0). Also the ratios ofthe amplitudes in Y and H components at SFE and pre-SFE timeswith latitudes are shown

Fig. 5. Equivalent ionospheric current vectors (1) solar ¯are e�ect(continuous line) and (2) pre-¯are SQ (dashed line), at Indianobservatories

R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories 1305

SQ currents at individual stations and their distributionover the world. The horizontal equivalent currentvectors for both SFE and pre-SFE (SQ) disturbancesover the Indian stations situated within a narrowlongitude sector, shown in Fig. 4, have nearly the samedirections, thereby indicating (within the limits of timeresolution) the augmentation of the integrated e�ects onthe ground in the north±south and east±west ®elds.

One of the interesting observations of the ¯are eventhere is the existence of signi®cant eastward magnetic®eld within the electrojet region on a normal day and itse�ect on the SFE that is noticed to be proportional tothe pre-¯are condition over wide range of latitudes(Fig. 3). The close similarities of the latitudinal pro®lesof SFE and SQ amplitudes (although substantialenhancement of the ¯are vector at ANN is noticed overthat of SQ) at low-latitude stations in India suggest thatthe SQ variation in the Y ®eld at equatorial stations aredue to currents in the E region of the ionosphere, as inthe case of the H ®eld.

The present result of enhanced amplitudes of DHSFE

at stations under the in¯uence of equatorial electrojet ,and more importantly the large positive excursion of DZat stations TRD, ETT, KOD and ANN is a markedfeature. The ionospheric current models of the equato-rial electrojet envisage that the enhanced eastwardelectric currents are expected to give rise to a minimumof the vertical component of the geomagnetic ®eld atlatitudes in the vicinity of the central axis of the

Fig. 6. Magnetogram tracingsfrom Indian Observatories show-ing signatures of storm suddencommencement (SSC) at 1518 h75° EMT on 17 June 1991. Theamplitudes of SSC at each of thestations for the three componentsof the geomagnetic ®eld in nT areindicated

Fig. 7. Latitudinal variations in the amplitudes of SSC (DHSSC,DYSSC, and DZSSC) at 1518 h 75° EMT on 17 June 1991 and pre-SSC(DH0, DY0 and DZ0) in the three components of the ®eld

1306 R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories

electrojet and should have maxima at the periphery.Thus, the observational fact of large positive excursionsin DZ at electrojet stations may not be the direct e�ect ofthe equatorial electrojet current.

Srivastava (1975) reported that the solar ¯are of 3May 1973 at 1330 h 75° EMT had produced positive DZat all stations in India, except that at ABG DZ wasnegative. The author suggested that DZ at ABG wasapparently contaminated by a coastal e�ect due tooceanic-induced electric currents.

Veldekamp and Sabben (1960) suggested that therewere di�erential movements of the vortices in thecurrent systems due to SQ and SFE. Rikitake andYukutake (1962) and Green®eld and Venkateswaran(1967) suggested the phenomenon of `self induction' toexplain the observations. Roy (1977, 1979) introducedthe idea of mutual induction between the E- and D-layers during solar ¯ares. However, the observations ofabnormal signatures of DZ at the chain of equatorialstations in India, probably described for the ®rst timehere, elude any clear explanation.

It is even more di�cult to explain the latitudinalvariation of SSC amplitude in DZ (shown as DZ SSC inFig. 6). The variations of DZ (SSC) closely follow thatof DZ (SFE) in the sense that positive amplitudes at thestations under the in¯uence of the equatorial electrojetnear zero amplitude at HYB and negative signatures atstations north of this station.

Obayashi and Jacobs (1957) studied the SSC in X, Yand Z components at a large number of stations roundthe world, and showed that the amplitudes of SSC in Xand Y vary in a regular fashion with geomagneticlatitude. The SSC in Z showed very irregular distribu-tion from place to place. They suggested this irregularbehaviour was due to the e�ect of the induced currentsinside the Earth, where the distribution of electricalconductivity may be considerably heterogeneous.

Ivanov (1964) has displayed a map of geomagneticstations that have registered positive and negative signsof DZ (SSC) employing International Geophysical Year(IGY) data of northern-hemisphere observatories. Theauthor also reported several stations (from both thehemispheres) where the sign of SSC in DZ is sometimespositive, and negative other times. As per the theoreticalconsideration of Parker (1962), the sign of DZ (SSC) isnegative (positive) in the northern (southern) hemi-sphere. If it were otherwise, it could be considered asanomalous and this behaviour would be the result ofpeculiarities of the current system induced in the Earth'supper mantle (Ivanov, 1964). If the induced currents areresponsible for the sign changes of the DZ (SSC), it isreasonable to assume that all the short-period variations(SSCs, SIs, SFEs and bays of comparable period) mustshow the same anomalous behaviour at the Indianobservatories consistently. Srivastava and Prasad (1974)have noted that during solar ¯are of 3 May 1973 thee�ects in H and D at ABG and HYB were comparable,whereas in Z there was negative amplitude at the formerand positive at the latter station. They have attributedthe negative signature at ABG to the abnormal inductione�ects associated with Deccan traps and also to the

oceanic-induced electric currents. Nityananda et al.(1977) have studied electromagnetic induction e�ectswithin the Earth at short-period ¯uctuations (SSC andbay events) at the Indian observatories. They haveexplained the variations in theZ component as due to thee�ects of channelling of induced currents through thePalk Straits at the Southern tip of the Indian Peninsula.

It may be recalled that the ®rst model by Chapman(1951) envisaged a maximum of DH at the equator and amaximum negative excursion of DZ at the northernfringe region of the electrojet. The observed DZ at anystation represents the gradient of the east±west currentaround the station. The complex structures in DZ havebeen well demonstrated by the changes in DH duringpartial counter electrojet events (Rastogi et al., 1992;Rastogi, 1993b), which are produced as a resultant oftwo currents at the height of peak E-region ionisation(106 km) and at the height of the largest Hall polariza-tion ®eld (100 km). It is suggested here that theabnormal features of DZ at equatorial stations mayalso possibly be due to the distortions produced in thealtitude pro®le of the ionospheric current at lowlatitudes, depending upon the spectrum of the solar-¯are radiations. Even if the solar-¯are currents producedare uniform in altitude, a di�erent latitude gradient ofthe integrated currents may be generated which woulddistort the amplitude and sign of the variations in thevertical ®eld. Some of these features can be checked withSFE during counter electrojet conditions and by study-ing SFE at a latitudinal chain of stations north andsouth of the magnetic equator.Acknowledgements. The authors are thankful to B. P. Singh, MitaRajaram and M. Roy of the Indian Institute of Geomagnetism,Colaba, Mumbai, for discussions and suggestions. Sincere thanksare also due to R. J. Stening for valuable advice and suggestionsduring the course of revision of the original manuscript.

Topical Editor D. Alcayde thanks T. Araki and R. J. Steningfor their help in evaluating this paper.

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1308 R. G. Rastogi et al.: An intense SFE and SSC event in geomagnetic H, Y and Z ®elds at the Indian chain of observatories