Indian Journal of Radio & Space PhysicsVol. 30, October 2001, pp. 221-232

Geomagnetic field variations at the equatorial electrojet observatory, Etaiyapuram

R G Rastogi & M E JamesDepartment of Physics and Space Sciences, Gujarat University, Ahmedabad 380 009

Received 9 October 2000. revised 26 March 2001; accepted 8 June 2001

The extent of the effect of electromagnetic induction in sub-surface has been investigated using geomagnetic data forthe period 1980-1986 from a station Etaiyapuram situated between Trivandrum and Kodaikanal and away from the sea-coast, but close to the magnetic equator. The station Etaiyapuram exhibits forenoon maximum and afternoon minimum in Zcomponent for all seasons, similar to that for Trivandrum, contrary to the expectations from Chapman's theory of equatorialelectrojet. Daily variation of Z shows characteristic features for Trivandrum during the years when sun is active, butcharacteristic features for Kodaikanal when sun is calm suggesting that the latitudinal extent of anomalous induction in PalkStrait decrease with decreasing solar activity. This abnormal induction effect at Etaiyapuram is found to be enhanced duringperiods of strong electrojet. current. The daily variation of disturbance in geomagnetic Z field does not indicate any abnormaleffect due to induction, but storm time Dst(Z) variation shows clearly the induction effects. Similarly, the amplitude of stormsudden commencement (SSC) in l'>Z at Etaiyapuram is found to be affected by electrojet as well as by the sub-surfacecurrents. I

1 IntroductionThe first systematic equatorial geomagnetic

observations in the world were started at Trivandrum(TRD) in 184l, by the then Maharaja of Travancore,His Highness Rama Verma. Broun) had analyzed thedata from Trivandrum, produced a remarkable set offindings on geomagnetism, and published amonograph in 1874. The most significant result of his

'analysis was the isolation of systematic daily,I seasonal and solar cycle variations in the magneticdeclination field. Broun was the first to identify theeffect of solar disturbances on geomagnetism. Thefirst standard equatorial observatory in the world wasestablished at Kodaikanal '(KOD) in 1902 as the basestation for magnetic survey for Survey of India, butunfortunately . it was closed in, 1923. IndiaMeteorological Department (IMD) re-established astandard geomagnetic observatory at Kodaikanal in1948, which is still operational and has been regularlypublishing its. geomagnetic data. A detaileddescription of the magnetic field variations atKodaikanal on geomagnetic quiet days has beendescribed by Rastogi", Egedae discovered anintensification of the daily range (defined asmaximum - minimum) of horizontal magnetic field(H) within ±3° from the magnetic equator on the basis

, of data from six observatories, out of which three'(Kodaikanal, Madras and Alibag) were from India.Chapman" explained this anomalous intensification ofrange of H over the equator as due to a band of

eastward current in the E-region of ionosphere, whichhe named as equatorial electrojet (EEJ). It wasdecided that the study of equatorial electrojet to be animportant constituent of Indian programme ofInternational Geophysical Year (IGY, 1957-58) and inorder to augment the database, two new geomagneticobservatories were established at Trivandrum (-0.6°

. dip) and at Annamalainagar (5.4° dip). Variousdiscoveries were made on quiet and disturbedvariations of electrojet based on data from theseobservatories'. One of the most important discoveriesmade using IGY data was that the strength of theequatorial electrojet is much smaller over Indiansector than that at American sector".

It 'was soon realized by Yacob and Khanna? thatlarge daily' variations of vertical component ofgeomagnetic field (Z) occur at Trivandrum, situatedvery close to the magnetic equator, which was notexpected on the basis of the Chapman's theory ofequatorial' electrojet. Model calculations of verticalfield by Yacob8 and later by Thakur and Ra09 alsorevealed that the observed Z field at Trivandrum isappreciably different from the theoretically computedvalues. Srivastava and Sankar Narayan'" suggestedthis anomalous Zvariation at Trivandrum to be due tocoastal effects and electromagnetic induction inanomalous sub-surface. conductivity structures. Lateron, anomalous' large Zvariation was noted for shortperiod variations like sudden storm commencement(SSC) and geomagnetic bays at Trivandrum. Rajaram

222 INDIAN J RADIO & SPACE PHYS, OCTOBER 2001

et al." postulated that the channelling of internalcurrents through a' sub-surface conductor betweenIndia and Sri Lanka is responsible for the anomalousZvariation.

Most of the studies about equatorial electrojet havebeen made using only the horizontal field data and toa lesser extent vertical component (2). Practically noserious study was made of the declination (D) oreastward component (Y) data from any equatorialelectrojetstation. Coincidentally the instruments thatrecord declination have been rather insensitive,making the scaling of D unreliably small. Rastogi?analyzed H, Y and Z field data from Kodaikanal for aperiod of 1954-1968. Very consistent daily, seasonal

. and solar cycle variations of Y were noted and it wasconcluded that the meridional current causing changesin Y is closely associated with the zonal component ofequatorial electrojet current. Since 1980, NationalGeophysical Research Institute (NGRI) has beenoperating a standard geomagnetic observatory atEtaiyapuram (EIT) situated between Trivandrum andKodaikanal in the central part of southern Indianpeninsula. It is to be noted that this observatory isfairly away from the sea-coast, and at the same time,it is very close to the magnetic equator, only next toTrivandrum. It was realized that a study of all

components of geomagnetic field at Etaiyapuramwould give more realistic picture of ionosphericcurrents and its image currents below the earthsurface. With courtesy of Director, NGRI, thegeomagnetic data for the period 1980-86 atEtaiyapuram were made available and the presentpaper gives the result of such a study.

2 Results and discussion/

2.1 Geomagnetic field during quiet conditionsThe details of geomagnetic observatory,

Etaiyapuram are given in Table 1. Details of Indiangeomagnetic stations along 145° geomagneticlongitude sector (75°E geographic longitude) for theyear 1980 are given in Table 2. It may be noted thatIndian geomagnetic observatories lie almost on alongitudinal chain extending from magnetic equator(TRD) up to a point close to the focus of Sq currentsystem (GUL). Figure 1 shows the geographicalpositions of four equatorial electrojet stations, whichhave been operational during the period of study. Itmay be noted that the dip equator in 1980 was northof Trivandrum and south of Etaiyapuram. In 1983, thedip equator had crossed TrivaMlrum' movingsouthward.

Table I-Details of the geomagnetic observatory Etaiyapuram

Geographic latitude :09° 10'NGeographic longitude :78° Oi'EDipole latitude :0.6°S

Dipole longitude :147.5°Horizontal field .:39906 nT (1980);

. Vertical field :757 nT (1980);Declination (W) :3° 2.0' (1980);Inclination :1° 05.3'N (1980);

Inclination at Trivandrum : 0019.9'S (1980);

, I

39&31 nT (1986);1365 nT (1986);2°55.3' (1986);

1° 57.8'N (1986);

0° 23.7'N (1986);

39912 nT (1995)1576 nT (1995)2° 42.4' (1995)

2° 15.7'N (1995)

0° 46.3'N (1995)

Table 2-Geomagnetic parameters of all Indian stations (1980)

Observatory Code Geogr. Geogr. Dip Declina- H Z t{'lat.,oN long.,? E lat., ON tion,oE nT nTTrivandrum TRD 8.5 77.0 -0.2 -2°46.2' 39896 -231 fEtaiyapuram ETT 9.2 78.0 0.5 -3°20.0' 39906 757

Kodaikana1 KOD 10.2 77.5 1.8 -2°23.3' 39265 2444

Annarnalainagar ANN 11.4 79.5 3.0 -2°36.3' 40309 4213

Hyderabad HYB 17.4 78.6 10.8 -I °38.6' 39714 15195

Alibag ABG 18.6 72.9 12.9 -0°46.5' 38282 17576. Ujjain U11 23.2 75.8 18.2 - 0°30.1' 37031 24388

Jaipur JAI 26.9 75.8 22.5 -0°45.9' 35737 29594

Sabhuwala SAB 30.4 77.8 26.9 0°23.0' 33863 34446

Gulmarg GUL 34.1 74.6 31.4 1°35.7' 31676 38919

1

i [I

I-

i

RASTOGI & JAMES: GEOMAGNETIC FIELD VARIATIONS AT ETAIY APURAM

12

KODAIKANALo

z-,Wo2 11>--

224 INDIAN J RADIO & SPACE PHYS. OCTOBER 2001'

1980 1986

O~=-+-~~~'+~~~~+'~-r~~J~~~-+~~~ __~~~~~~--1-10-20

I- -30c>"' ·10~ O~~~--~--L-~--~~~~~~~~--oc1 -10 .N~r." .100~ 80

604020O~L-~~~~~~~~~~~-+~-+~-+~-r-+-r~-+~~~~

Fig. 3- Yearly mean quiet day variations of H, Y and Z fields at Etaiyapuram

A comparison of the yearly mean Sq variation of H,Y and Z fields at the Indian observatories fromTrivandrum (TRD) at the magnetic equator toGulmarg (GUL) near the northern focus of the Sqcurrent system for the year 1980 is shown in FigA. Alarge peak of !Vl exceeding 120 nT during middaywas observed at the equatorial station Trivandrum andthe range of t3.H at other stations progressivelydecreased with increasing distance from equator. Thestation, Gulmarg has shown a different characteristicdue to the proximity to the focus of Sq current system.The !3.Z at Trivandrum shows a large amplitude dailyvariation with a maximum around 1000 hrs and aminimum around 1500 hrs, but according toChapman's theory of equatorialelectrojet thereshould not be any daily variation of Z field over themagnetic' equator. At Etaiyapuram, similar variationon !3.Z, but with lesser amplitude was observed. AtKodaikanal and Annamalainagar, a large singleminimum of !3.Z was observed due to the location ofthese stations near the fringe of equatorial electrojet.At all other stations, t3.Z is minimum around noon.Daily variation of t3.Y is very small at Trivandrurn, butcomparatively large daily variations of t3.Y wereobserved at Etaiyapuram with a very prominentminimum around noon. At Kodaikanal again theminimum occurred around noon but with lessamplitude. At Annamalainagar and further northerlystations, the 6Y was' maximum around 0900 hrs andminimum at around l300-1400 hrs, suggesting thatthe meridional current is southward in the forenoonand northward in the afternoon as expected of Sqcurrent system for northern hemisphere.

The Sq variations of H, Y and Z components duringthree seasons, the D-months (Jan., Feb., Nov. andDec.), the E-months (Mar., Apr., Sep. and Oct.) andthe J-months (May, June, July and Aug.) averagedover the entire period 1980-1986 are shown in Fig. 5.The Sq(H) was found to have I.same, diurnalcharacteristic for all seasons, but the amplitude waslargest' for Esmonths. The Sq(Z) was also similar forall seasons but the amplitude was slightly smaller in J-months. The Sq(Y) showed a clear midday minimumduring E-months, during f-rnonths (summer) when theglobal Sq current in the northern hemisphere islargest':', a small positive peak of t3.Y was seen in theearly morning. The Sq(Y) during D-months wasdifferent from that of other seasons with a broad butminimum in the forenoon hours. This characteristic oft3.Y is similar to that observed at the nearby stationKodaikanal'".

The daily variations of H, Y and Z fields atEtaiyapuram on a monthly basis averaged over theyears 1980-1986 are shown in Fig. 6. The t3.H curvesshows large values of nearly same range OCCUlTedduring April and September months when theafternoon decrease was extended up to late nighthours but during summer months the t3.H valuereached the base value by the evening. The variationof Z field curve revealed a large morning peak duringFebruary-April and October and afternoon dipduring March-April and September months.Abnormal induction effects too, seem to be tied withthe variation of H field. The Y field showspredominant midday minimum in all of the monthsespecially' from March to October. Morning

II, 'fI

r,

RASTOGI & JAMES: GEOMAGNETIC FIELD VARIATIONS AT ETAIY APURAM 225

1980llH !:.z6.Y

GUL

I J AII\

1UJJ

1, I

nTnT +20O~--~~~~~~~ O~--~--~---+~~

-10~LW~~~~LU~~

00 06 ,12 1824 00 06 12 18TIME, hrs 75 0 E

2412 1806

Fig. 4-Yearly mean SQvariations of H. Yand Z fields at all Indian observatories

D-MONTHS J-MONTHSE-MONTHSnT+120 ~ - ~ r

~~H

- I- -I- - - -I- - I-- - l-I- I-- - l- r -,l- I I\H - - \ - 7 ~H -/I-- - r- - -I- - f- - I- -- Jb.Z \ - - lb;{ \ - I- {6Z \ -- , - - ..: -~ "'::-- I-- ~- I- /~- ,--, .- -- -..... JI'(""''' ---- -- \ , ,-~ \ ' -',-- ,-- - , I.: " - - / -- --- '-' b.V\ -b.V - - 6V\ I - l-I- - I-- / - - -

+80

+40

o

-40 06 12 181206 18 06 12 18TIME? hrs 750 E

Fig, 5 --Curves showing SQvariations of H. Y and Z fields at Etaiyapuram during three seasons

',.i

.~' .-,,:~~-.-. -..:.

.-. '. . . ~. :' z ... '.~ ': '. ",-"'....:-- ~ -~'" ~~-,:~~,.~.:,- .. ~" ":,:

:~":" ~. ..

j;J!£t-::~·~·;."Zf~:~·....,>~):;;;.>,>.~~;i:{',i;:'" ,,1J}~:';;-;;.,.~",:.""~ ...,.'

. ~~'- . ~-::

226 INDIAN J RADIO & SPACE PHYS, .OCTOBER 2001

O~~--~~4r--~~~~~~--~~~~~~+-~rL~~~~~--,~~+--,~~-- __~~~~~~~-10

-20 ~r*~~~~~,&~+,-~~~-L~*,~~~~~rn~~~~~-k~~~~~~~~~~~-h~

JAN FEB APR' MAYMAR

120

80

06 18 06 12 18

AUG SEP DEeOCT NOV

TIME, hrs 750 E1 1 0 1 18 1 18

Fig. 6- Mean daily variations of H, Y and Z fields at Etaiyapuram on ,a monthly basis

maximum is also visible from April to September, butduring winter months a single broad minimumoccurred in the forenoon hours. Thus, the boundarybetween the northern and southern hemisphericalcurrent system moves across the dip equator in Indiawith time of the day as well as with season.

The contours of /),}{, L\Y and t:;Z averaged for theperiod 1980-1986 on month versus local timecoordinates are plotted in Fig. 7. Contours of /),}{show very clearly two maxima at about 1000-1100 hrs during equinoctial months. The Y fieldexhibits a morning maximum and afternoon minimumduring March-September, but during October-February only a flat forenoon minimum. is observed.This suggests that Etaiyapuram lies within northernhemispherical current system during summer monthsand is in the influence of southern current systemduring winter months. The contours of t:;Z indicateforenoon peak and afternoon minimum for all themonths except June-August, when the equatorialelectrojet is a minimum. Thus, the abnormalelectromagnetic induction effects at Etaiyapuramseem to be strong during days with intense electrojetcurrent and are absent when the electrojet current isweak.

In order to' check whether the dai ly variations ofgeomagnetic field components at Etaiyapuram are dueto daytime or nighttime sources, midday and midnightvalues of horizontal field and declination for the year1980 are grouped based on the range values and thenthe mean values of each group are plotted againstcorresponding range values in Fig. 8. In this analysis,

oN 6.Z (nT)o5A

" ~4-2"-I I

/ '/. 'I

//

)I

III

oN 6.H(10nT)o5AJJMAMFJ

o \N ~5a LW(nT)5 0.AJJMAMFJ

00 06 09 12 15 18

TIME, hrs 75° E2103

Fig. 7-Averaged contours of t'1H, t'1Y and t'1Z at Etaiyapurarn-

·.f

21.

..,

RASTOGI & JAMES: GEOMAGNETIC FIELD VARIATIONS AT ETAIYAPURAM

highly disturbed days have been avoided by choosingdays with Ap < 10. The numbers of observations ineach class are also marked in the Fig. 8. It is evidentfrom Fig. 8 that the midnight values of both Hand Ddo not change with range, but a linear increase ofmidday values with increase in the range of both Hand D components is clearly visible. The largevariations of the daily ranges of Hand Yarecontributed by the corresponding variations of themidday values. So it may be concluded from Fig. 8that the day-to-day variablities of both horizontal andeastward fields of the geomagnetic field atEtaiyapuram are daytime phenomena.

The relationship between day-to-day variation ofnoon values of 6H and flY has been analyzed. In orderto remove the effect of seasonal variation of theseparameters, the monthly mean values were subtractedfrom individual day values. Figure 9 shows therelation between the mean deviations of the noon flYand tlll from its monthly means. It is seen that withincreasing value of range H, there is a systematicincrease of the range of negative Y. Thus, the

227

eastward field is directly related with the horizontalfield.

2.2 Geomagnetic field during disturbed conditionsRastogi 15 has shown that the daily mean values of

H field at Kodaikanal and declination decreasedlinearly with the increase of the equatorial Dst(H)index, as an indication of equatorial ring currentstrength.

The variations of flH and flY in relation to Dst(H)index which represents storminess has beenexamined. The monthly mean values of tlll and flYfields were subtracted from the individual day values

. and these deviations are grouped based on Dst(H)values. Figure 10 represents the relationship betweenthe deviations of flH and flY against Dst(H) index.With increasing negative value of Dst the tlll shows amonotonous decrease, but flY shows an increase. Thisindicates that, with increasing storminess thehorizontal field decreases, while the eastwardcomponent increases. The average flYltlll = -0.21 =sin 12.4°. This indicates that the plane of the

\ I

ETAIYAPURAMHORIZONTAL FIELD 1980 DECLINATION

40050

40000 MIDDAYVALUES

.-'

39900

50 100 nTMIDDAY

o ,15a 200 3 °0L---L---L-.l......J.-21-L---L-.l......J.~.J........I---L......1.....!6min.

MINUS MIDNIGHT VALUES

Fig. 8 -Midday and midnight values of Hand D fields at Etaiyapuram (grouped based on range of values)

• J'.

· :.....! .---:-C;":':'--..- -,--'~, ..;

{- ~ . ~-'-.=

, ' ..

228 INDIAN J RADIO & SPACE PHYS, OCTOBER 2001

disturbance ring current, on the average, is tiltedabout 12° westward from the geographic meridian atthe place.

The yearly average solar variations ofH, Yand Zfields on international quiet days (IQ) and oninternational disturbed days (ID) have been compared

+15I- ETAIYAPURAMc: 1980,>- +10

4 9 No.

RASTOGI & JAMES: GEOMAGNETIC FIELD VARIATIONS AT ETAIY APURAM

,.l

I'If

midday maximum is related inversely to the square ofthe mean magnetic field intensity If (Ref. 14).Amplitude of SSC(H) at any equatorial station isassociated with a temporary increase of eastwardelectric field besides the effect of magnetosphericcompression". A comprehensive study of SSC in H, Yand Z at Indian stations Trivandrum (TRD),Kodaikanal (KOD), Annamalainagar (ANN) andAlibag (ABG) has been described by Rastogi". Theamplitude of SSC in Z at Trivandrum during middayhours was similar to that of SSC in H. The amplitudeof SSC' in Z at other stations had not shown anysignificant daily variation. Nityananda et aZ.17

postulated the presence of conductors in the uppermantel or lower crust between India and Sri Lanka forthe large daily variation of Z at Trivandrum. Theabnormally large SSC(Z) was suggested as due tosimilar induction effects 16. It is important to study thelatitudinal extent up to which these induction effectsare seen by analyzing SSC amplitudes atEtaiyapuram. I

Magnetograms of Etaiyapuram during two SSCevents at 1042 hrs (75eE) on 14 Nov. 1981 and at1236 hrs (75eE) on 1 Mar. 1981 are shown in Fig. 12.On 14 Nov. 1981, the SSC amplitudes were 58 nT forH field, -15 nT for Y field and 39 nT for Z field, theratio til-/I:!,.H being 0.67. On 1 Mar. 1981, I:!.H was 48nT, I:!.Y was -22 nT and I:!.Zwas 59 nT, the ratioI:!.Z//)Jf being 1.23. It is to be noted that large I:!.Z/Wwas associated with a very fast rise of Zfieldassociated with SSC, such that the trace was too weakto make impression on the photographic paper.

Traces of magnetograms at Trivandrum,Etaiyapuram, Kodaikanal, Annamalainagar andHyderabad during SSC at 1220 hrs (75eE) on 30 Mar.1990 are shown in Fig. 13. It is to be noted that thesensitivity of the magnetograms for different stationsand for different parameters is yery much different.To avoid this confusion, scales for differentparameters and different stations are indicated in rightbottom part of the Fig. 13. Amplitudes of the suddencommencements in H, Y and Z fields for each stationare also indicated in the magnetograms. The SSCatTrivandrum, the station close to the equator, producesan impulse of 149 nT in H field and 175 nT in Z field,the ratio I:!.Z/Wbeing 1.42. At Etaiyapuram, W was140 nT andl:!.Z was 77 'nT;whieh ;gives the ratio

. I:!.Z/W=0.55. 'Further;' for'riorthedy StationsKodaikanal and Anriamalainagar, -the ratios were 0:27and 0.29, respectively .Fdr 'the Station Hyderabad,

I.I

J

229

sse AT 1035h ON NOV.14,1981

'/

M=-15nT

09 10 11 12 13h75°E

sse AT 1236h ON MAR.1,1981

I:!,.Z=59nT

~'

11 12 13Fig. 12-Magnetograms at Etaiyapuram during storms

which is situated beyond the equatorial electrojet, theratio was -0.14. This indicates that the effects ofinduction in Palk Strait effects are very significant atTrivandrum and to a lesser extent at Etaiyapuram andprobably absent at higher latitudes.

The solar diurnal variation of Hand Z fields due toSSC at Etaiyapuram is shown in Fig. 14. A middayenhancement of SSC(I:!.H) is very clearly seen. Theratio SSC(I:!.Z/I:!.H) show values of 0.5-0.7 at middayand 1.0-1.2 around midnight. These characteristicsSSC at Etaiyapuram are very similar to thosedescribed for Trivandrum earlier by Rastogi".

The intensified flow of solar plasma enters theearth's magnetosphere through the open field linesand the particles get trapped in earth's magnetic fieldlines and intensify the equatorial ring current. The

. first effect of this westward ring cu~ent is to reduce, -the geomagnetic H field on ground. The development

of ring current is best estimated by the equatorial

";'0.

. \

230 INDIAN J RADIO & SPACE PHYS, OCTOBER 2001

SSC AT 1220h 75°E ON 30 -M ARC H,1990ll.H(nT) ll.Z(nT) ll.Y(nT)

50

36

i#t1-31 I

~. ;1

10 11 12 13 14 h75°E' iI

HlnT

TRD ETT KOD ANN HYB

45 '74 40 40 63

10 11 Y}T 30 66 65 62 I 4812 13 14h7SO£z!nT 68 44 255 104 54

I10 " 12 13 14h7soE

Fig. l3-Magnetograms at equatorial and low latitude stations in India during storms

Dst(H) index derived using data from six midlatitudegeomagnetic stations and is regularly published inSolar Geophysical Bulletins. The effect of storm isgenerally studied as follows. First the Sq variations areremoved from hourly mean values of H, D and Zduring the disturbed period. These deviations of H, Dand Z are arranged during the storm period and areindicated as Dst or disturbance time variations of theparameters.

The disturbance time variations of H, D and Zfields at Etaiyapuram during two storms, one startingat 0457 hrs UT on 19 Dec. 1980 and the other startingat 0532 hrs UT on 5 Mar. 1981 are compared with thevariations of corresponding Dst(H) index. Figure 15(a)

shows that the H field at Etaiyapuram starteddecreasing at 1230 hrs UT on 19 Dec. 1980corresponding to the starting of main phase of thestorm indicated by the start of the decrease of Dst(H)index, reaching the minimum values at around1800 hrs UT. The storm time decrease of H alsoreached minimum around 1800 hrs UT, but theamplitude of I'1H at Etaiyapuram was around 350 nTcompared to the corresponding Dst index by 250 nTonly. The storm time variation of declination wassimilar to that of Dst(H) index. This confirms theearlier result of the study of storm time variations ofHand D at Kodaikanal"'. The storm time variations ofZ field started decreasing at the same time as that of

T

V\I

RASTOGI & JAMES: GEOMAGNETIC FIELD VARIATIONS AT ETAIYAPURAM

ET AIYAPURAM1. 4 ,-_----=-=-=...-=---'19:....:8-=.0--=- 8:.=2-'rl'-"-9:::....94~--=-9.:::...8_-,-- --,

SSC 6Z1!::.Hf-'

60SSC-!::.Z(nT)

··.' .-,~.. :... \ .• •• I.·.

'·2 ~f-'

'·01-0,' •• ,f- ''..0·8 f- "f-

0·6

. ,.. ..: ...••• e•• .. ..

'..

.• 0 .... . .- . .

2 a h:- e. • ->: e. • .: •• • •r:::.".. . . . ... .\. .~... : :. . ..t-"'- •• _:_ • -1\-,,_ •••• e.Or--------r--------~------~------~100~ SSC-!::.H(nT)

80

80

40 ••••

. '.40 0

••. 0

20.. •• -.: .:,.: -. .. .".r- I I I. I',' I I

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.. . ..';.~.-:,.- . ..I I I t I

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