Indian Journal of Radio & Space Physics

Vol. 28, December 1999, pp. 253-263

Electromagnetic induction due to sse at equatorial electrojet stations

R G Rastogi* Department of Physics, Gujarat University, Ahmedabad 380009

Received 6 May 1999; revised received 26 July 1999

The effects of storm sudden commencements (SSe) in the horizontal (H) , vertical (2) and eastward (Y) geomagnetic fields at the five equatorial electrojet stations operating during IGY-IGC period have been discussed. The amplitude of sse (H) at any of the stations was maximum around midday hours. The amplitude of sse (2) showed an abnormally large posi­ti ve value at Tri vandrum, the ratio D.Z/ Ml exceeding 1.0 at any time of the day or night. These phenomena are di scussed in terms of the currents in the· ionosphere/magnetosphere and their induced effects in subsurface conducting layers. The most promising source for large sse effects in Z at Trivandrum seems to be due to the concentration of normal induced currents over a wide latitude zone north and south of the magnetic equator through the conducting graben in the Palk Strait, besides the channelling of induced ocean currents in the Bay of Bengal and Indian Ocean physical ly th rough the Palk Strait between Indi a and Sri Lanka.

1 Introduction The quiet-day solar daily variation Sq, of geomag­

netic fie ld has been attributed to currents in the iono­spheric E-region. These currents are due .to the elec­tric field generated by the thermal and/or gravitational tidal winds act ing across the vertical component of the main geomagnetic field. The enhancement of the dai ly variation of the horizontal field H, within ± 3° magnetic latitude' was attributed to a band of intense eastward current over the magnetic equator, named 'Equatorial Electrojet,2. This band of intense east­ward current was assoc iated with the abnormal in­crease of electrical conductivi ties in the latitude re­gion where the geomagnetic field is normal to the electric fi eld' . This current, which is primarily along the magnet ic east-west di rection, is not expected to produce any solar dail y variation of the declination (D) or the eastward fi eld ( Y) . The dai ly variation of the vert ical fie ld (Z) is a re fl ect ion of the latitudinal grad ient of the current over the station and is zero at the magneti c equator and maximum pos itive (or negative) at the southern (northern) peripheral region of the electrojeLThe equator ial e lectrojet is, gener­a ll y, symbolized by the large daily range of H field and consequentl y the studi es of Y and Z fields have been pract icall y ignored so far. The daily variation of Y field at equatorial region is small and the instru-

*Also at: Physical Research Laboratory. Ahmedabad 380009-

ments at most of the observatories use a rather insen­siti ve Y magnetometers, and thus the effects of solar flare or storm sudden commencement (SeC) at many stations cannot be reliably scaled due to small deflec­ti ons on the charts. The signatures of sse in Z fie lds were found to .be random at different stations, and attributed to the induction effects caused by the local inhomogeneity of subsurface conducting layers4. The

study of SSCCZ) at low latitude has been practically neglected, except the study by Forbush and e a­saverde5 for equatorial e lectrojet stations in American longitudes.

During the IGY period, number of magnetic obser­vatories at equatorial latitudes were established in Peru, India and Pacifics. Rastogi6 had. shown signifi­cant longitudinal variation in the daily range of H with maximum in American and minimum in Indian ' longitudes. A latitudinal enhancement of the ampli­tude of SSCCH) over the equator similar to the en­hancement of the range of Sq(H) , was observed by Rastogi et a/.7 . Later, longitudinal variations in sse (H) were identified8 which relate to the mean values of the magnetic field itself. A sudden increase of io­nospheric electric field over the magnetic equator during sse were suggested by Rastogi9 in 1976 and later by Reddy et al. '0 in 1981 . It was suggested by

. ' 9 Rastogi that the sudden commencement of the storms is associated with the compression of the magneto­sphere, increasing till at all stations around the world,

254 INDIAN J RADIO & SPACE PHYS, DECEMBER 1999

as well as by the transmiss ion of magnetopause elec­tric fi eld through the polar region to the equator, in­tensifying the amplitude of SSC(H) within a narrow latitudes over the magnetic equator.

An increase of equatorial electric fi eld during sse shou ld be associated with the increase of the electro­jet current , resulting in an increase of /)J{ at stations close to the equator and a decrease of !1Z at stations near the northern periphery of the e1ectrojet belt. A study of the sse events at Indian geomagnetic sta­tions Trivandrum (TRD), Kodaikanal (KOD), An­namalainagar (ANN) and Alibag (ABG) showed an enhancement of /)J{ during the daytime, but the sse effects on Z field too was a pos iti ve impulse at elec­trojet stations north of the magnetic equator, i.e. at

KOD and AN N. An abnormally large positi ve im­pu lse in Z exceeding the corresponding impulse in H was noti ced at TRD, a station very close to the equa­tor". Thi s is an unexpected result , when !1Z exceeded ;').H du ring sse and has not been reported at any equatoria l or low and midl atitude station.

In this paper. the study of the amplitude of sse in H. Y and Z field at stat ions within the electrojet belt has been described for the year 1958. The stations chosen are Huancayo (HUA) in American longitudes, Addis-Ababa (AAE) in east African longitudes, TRD in India and Korol' (KOR) and Jarvis (J AR) in Pacific

longitudes. Some other stations ' data could not be utilized due to non-availability of the magnetogram films, or due to the poor quality and insufficient se­ries of magnetograms at the stations. The coordinates of the stati ons are given in Table. 1 and the location of stations are shown in the map (Fig. 1). It is to be noted that Huancayo is about ION, while other sta­tions are less than one degree away from the magnetic equator.

2 Results First of all , the yearl y mean solar quiet day varia­

ti ons of H, Y and Z fie lds at each of these stations are examined. In Fig. 2 are shown the solar quiet day var iati ons of hori zontal (H) , eastward (Y) and vertical (2) fields at these stati ons averaged over the year 1958. At any station close to the magnetic equator, the ionospheric current would not produce any change in Y and Z components, and /)J{ would have a max imum shortl y before noon. These vari ations are definitely mod ified by cu rrents induced in conducting layers inside the earth or in ocean or by spatial distri­buti on of the e lectrojet current itself. It is seen from the diagram that the daily variations at Addis Ababa are c lose to the c lass ical expectations, except for the large negati ve excursion of ;').H in the early morning hours. At Huancayo, ;').H shows well-known extraor-

Table I-Coordi nates and other parameters of the stati ons whose data are ut ili zed fo r the present study

Station

Huancayo (HUA)

Addis-Abaha (AAE)

Trivancl;'um (TRD)

Koror (KOR)

Kodaikanal (KOD)

Annamalain agar (AN

Alibag (A BG)

Jarvis (JAR)

Geogr. Geomagn. H Y Lat. Long. Lat. Long. nT nT

12 .1"5 75 .3"W 0.8"S 355.2 27225 985

90.0"N 38.8°E 5.2"N 11 0.5 36075 407

8.5"N nO"E 1. 1 "s 147.8 39896 232

7.3"N 134.5"E 3.00 S 204.7 37959 404

10.2"N 77.5"E 0.6"N 148.4 39265 2444

11 .4"N 79.T'E 1.5°N 151).7 40309 42 13

18.6"N 72.9"E 9.4°N 144.9 38282 17576

O.4"N 160. I"W 0.3"5 270.4 34462 5629

GEOGRAPHIC LONGITUDE

Fig. I-Map showi ng locations of equatorial electro'et stations operating during IGY period

Z Dip.lat. nT

1290 LOON

367 0.3"N

1928 0.3°S

966 0. 1°5

1636 1.7°N

1832 2.8°N

5 18 12.8°N

778 l.l "N

RASTOGI : EM INDUCTION DUE TO SSC AT EQUATORIAL ELECTROJET STATIONS 255

80

40

r I I I /AH

I JAR '.Av

\ \ \ \ \

o ~,,::"':..:"':':-4.-"':'''~=iZ:::::~+=~-I I I I fAH I I

40 KOR/f.Z

N od > .:r: g, 80

~ 40

n: « > I a: I :5 80 'AH o I Vl 40 I

\ \

\ \ ,

.... -~ ADB I fY. ~ 0 ~r-: .... ~ ... -.tzr.~=t=~;;;::..r--' t-~ :J o

60

40

I I I I I I I I ,i'

"

-4~LO--~OL6--~'2~--'~6~

TIME, h rs LT

24

Fig. 2-Yearly mean solar quiet daily variations of horizontal (H), vertical (2) and eastward (Y) field . at equatorial electrojet stations Huanca'yo (HUA) , Addis-Ababa (AAE), Trivandrum ('fRO) and

Koror (KOR) fo r 1958

dinarily large amplitude, but !lH is seen to start in­creasing since hours well before the sunrise. The IlZ shows a minimum in afternoon hours and can be rec­onciled, as Huancayo is ION of the equator. The large positive excursion of ~Y is definitely anomalous and is the characteristic of that longitude sector only l2. At Koror and Trivandrum, AY shows a significant mini­mum around midday; but the forenoon maximum and afternoon minimum of !YZ are definitely anomalous features which seem to be associated with some fea­tures of the currents induced inside the solid earth. The ratio of daily ranges in Y and H, i.e. ~Y/Mf was around 0.15 at KOR, TRD and AAE, but it was as

large as 0.24 at Huancayo. The ratio !1ZIMf was smallest at AAE being only 0.07, it was 0.20 at HUA, 0.27 at K9~ and 0.36 at TRD. Thus, Trivandrum seems to have extraordinarily large DZ in spite of its being very close to the equator.

Next, the signatures of sse on H, Y and Z fields at these stations are illustrated in Fig. 3. It may be men­tioned that the sensitivities of these traces are very different from one station to another and the impulses cannot be compared by their linear dimensions in the diagram.

At Huancayo, analogous to large Sq(H) variation, the amplitude of SSC(H) was also abnormally large, being 245 nT for the event shown here. The ampli­tude of sse in Y is also large, being 40 nT in this case. It may be noted that the sensitivity of D­magnetometers at Huancayo is very small and the excursions of D appear small. The SSC(Z) is positive even though the Sq(Z) at Huancayo is negative during the daytime.

At Addis-Ababa, the sse at 1103 hrs LT on 15 July 1959 produced the excursion of 127 nT in Hand -26nT in Z, while the excursion on Y was slow and uncertain. It is to be noted that even during the early phases of the storm, the fluctuations in H and Z are of opposite sense.

At Trivandrum, the sse at 1159 hrs LT on 5 Dec. 1958 produced an excursion of 88 nT in H and even larger excursion of 100 nT in Z field. The excursion in Y field was as small as 7 nT only.

At Koror, the storm at 1203 hrs LT on 29 July 1958 had produced large excursions in all the three components H, Y and Z.

At Jarvis, the sse at 1626 hrs L T on 21 Aug. 1958 had produced Mf of 84 nT and IlZ of only 13 nT.

Thus, one finds the anomalies of sse large in Y at

256 INDIAN J RADIO & SPACE PHYS, DECEMBER 1999

I 6 Z=OnT

~ r---r---~~~~ > ~ ~~~~~1--j <{ ,

~ o ~ o ~

~ ;:J ~ o z <{

> ~ rl0~~~~~~~~~

<{ ro <{ ro <{ I ~

o o <{

o >­<{

U Z <{ 6H=245nT :::> I

Fig. 3-Tracings of storm sudden commencement effects on H. Y and Z field s at Huancayo. Addis·Ababa. Trivandrum and Koror

Huancayo, large in Z but small in Y at Trivandrum and large in H, Y and Z at Koror.

In Fig. 4 are shown mass plots of the amplitudes of SSC in H, Y and Z fi e lds at these station s against local time for all the events during 1958-59. For Jarvis , we had magnetograms for 1958 only.

At Huancayo, SSC(H) shows well known dayt ime enhancement. The amplitudes of SSC(Y) range from -IOta + 30 nT with a doubtful amplification during the daytime hours . The amplitudes of SSC(Z) vary between 0 and + 20 nT with no variation with the

time of the day. The rati o tYZ/!1H varied between 0.2 and 0.4 with s ligh tl y lower va lues during the day than

during ni ght. The yea rly average value of the ampli­tude ratio of Sq( Y) /Sq (H) was 0 .24 and that of Sq(Z)/Sq(H) was 0 .20 for 1958.

At Addis-Ababa, SSC(H) shows daytime en­hancement and the indi vidual amplitudes are compa­rable to those at Huancayo. The SSC(Y) varied be­tween 0 and 30 nT with no apparent variation with so lar time. The amplitude of SSC in Z varied from 0 to - 40 nT with the larges t impul ses occurring around

midday hours corresponding to large impulses in L1H. The rati o L1Z/L1H varied between -0.1 and -0.4 with no apparent va riati on on local time . The average va lue of the amplitude ratio of Sq( Y)/Sq(H) was 0.15, whi le Sq(Z)/Sq(H) was 0 .07 .

At Trivandrum t.he amplitude of SSC(H) did show day t i me enhancement , but few large SSC(H) had oc­curred during local ni ghttime hours. The individual va lues of L1H are smallest as compared to those at any other equatoria l stati ons . The amplitudes of L1Y are sma ll ly ing between 0 and JO nT wi th no variatioll on loca l time. The amplitudes of SSC(Z) are exception­a ll y large and duplicating the corresponding varia­ti ons o r SSC(H). The ratio L1ZI!1H is generally larger than J .0, reaching a value of 1.4 in some cases . The rati o of yea rl y average amplitudes of Sq(Y)/Sq(H) was 0. 14 and that of Sq(Z)/Sq(H) was 0 .36.

At Koror, SSC(H) showed usual enhancement around midday. The amplitudes of SSC( Y) were ve(1) small at any time of the day. The amplitude of SSC(Z) varied from IOta 80 nT with a definite en­hancement around midday hours. The ratio WM! varied from 0.3 to 0 .9. The yearly average ratio of the amplitude of Sq(Y)/Sq(H) was 0. 15 and that of Sq(Z)/Sq(H) was 0 .27 .

*

RASTOGI : EM INDUcnON DUE TO SSC AT EQUATORIAL ELECTROJET STATIONS 257

HUA AAE I TRD KOR JAR '6 1 '2 II Zlll H : • . , £.\ • 1

oe o

~\" .- . ~ ~-0 '4 .-0

n I 6

To:

1 40: 20:

~

~ ~\,~ ."( ~

"

. \ ',f r" 3:. " .

SSC(Z)

,

. . , .

.' ~ . ~ .. ~. .. . "t : -.. :.: i.e. , ~\ · ,

! · · '. · .

" :. 60 . ,

40: . . . ,

. , . ' 2~ ;.J..,

. ., , , !'::- t. ~ , , ~':..: .. ' :~. ~ ~t: · !l . . ' . . .

v. '7: .' ::-: 20 .. , 'f; .

'n T .. ,.l "S·iC(Y? 0 . .. , .. , .. .... ... . :J . ,., .!t'!,. . ~, If. · 0

2

T n 2 00

00

n.

..... :'!::

, " .. ..

~ , .

06 12

" . ,

~ . . . . ~ , t:r, .

," 16 06

I I . SSC(H) . ' . ~. . . , . . I . . , ' . . " . -.: . 12 18 06

, ,

· --= , ' . . , .' . . . , . · . . ,

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

12 18 06 12 16 06 12 18

TIME, hrs LT

Fig. 4-Mass plots of the amplitudes of SSC in H. Y and Z fields and the ratio of 1lZlt:.H at each of the equatorial electrojet stations during \958-59

At Jarvis SSC(H) did show enhancement around midday, but the amplitudes of SSC impulses in Y and Z were small. The tlZ/ Ml values were small around 0.2 only.

Addis-Ababa (AAE) and Trivandrum (TRD) are separated by only 37.3° in geomagnetic longitudes, and hence, the signatures of SSC component pro­duced by magnetospheric currents should not differ very much , unless modified by local abnormalities in the ionosphere or subsurface conducting layers. In Fig. 5 are hown the mass plot of individual ampli­tudes of SSC in H, Y and Z fields at Addis-Ababa and Trivandrum against UT. It is seen that between 0700 and 1800 hrs UT the SSC(H) is consistently larger at AAE than at TRD. This suggests a decrease of

SSC(H) at Indian electrojet station, probably due to electromagnetic induction in the earth' " crust. The amplitudes of SSc(y) are, in each case, uf opposite sign at the two stations and the modulus value is larger at TRD than at AAE. T he amplitudes of SSC in Z are negative at AAE and positi ve at TRD. Again the modulus values of SSC in Z is much larger at TRD than at AAE for anytime of the day or night. T hese

differences in the signature of the SSCs at AAE and TRD cannot be due to the source in the ionosphere or in the magnetosphere and have to be due to regional anomalies in the subsurface conductivity of the earth.

In order to find if the abnormal LlZ(SSC) at Trivandrum is the characteristic of the station or of Indian longitude sector, the ratio of tlZ/f:.H during SSC has been examined at all the geomagnetic sta­tions in India during IGY, e .g. Trivandrum (TRD), Kodaikanal (KOD), Annamalainagar (ANN) and Ali­bag (ABG). It may be mentioned that Trivandrum is a coastal station almost at the centre of the electrojet belt, experienc ing the largest daily range of H field in India. Kodaikanal is an inland station only 2°N of the magnetic equator and experiences high positive daily range of H and high negative daily range of Z field. Annamalainagar is situated close to the edge of e\ec­trojef belt and experiences largest daily range (nega­tive) of Z fie ld . Alibag is outside the e lectroj et belt and experiences moderate Ml and low ~ Z. In Fig. 6 one sees that the ratios of IiZ/~H due to SSCs at TRD were around 1.3- 1.5 during midnight and around 0.8-1.1 during the midday hours with the whole day mean

258 INDIAN J RADIO & SPACE PHYS .. DECEMBER 1999

1958

+100

+ 80 I 0

.- + 60

J---c

"' +40 N <l + 20

0

- 20 )(

)(

.-+ 20 x l< )( )(

c 0 >-<1_ 20

x AAE - x

nOr 1 TRD-o

.- ~Ulr! II c . :r: <l ~Jh

~O 06

Fig. 5-Universal time variations of the amplitudes of SSC in H. Y and Z fields at Addis-Ababa (x) and Trivandrum (0) for the year

1958

value of 1.17 . At other equatorial electrojet stations KOD and ANN. t:;Z//lH values were large and posi­tive with a maximum around midnight and with whole day mean value of 0.36-0.38. At Alibag the signatures of SSCs were always negative for Z field and the ratio t:;Z/ /lH did not show any variation with the time of the day. Thus, the abnormality in SSC(Z) seems to be experienced at all the electrojet stations in India with largest value at Trivandru01.

3 Discussion Chapman2 gave the first model of the equatorial

electrojet describing the equations to compute the latitudinal variations of surface magnetic field H and Z due to the e lectrojet as a function of height (h) and semi width (w) and di stance from the magnetic equa­

tor (x) as follows. C _\ 2wh

H =- - tan h2 + X2 _ w2 w

(1)

1958-1959

0-• ABG

• 51-

• • • • l- ... • •

• • • • -.. • l- • .... • • • • . - • ••• • • • •• 1-. • •

0·3

• • • ••• If- • • •

MEAN =0-27 o· •

lJ ANN •

It • • l- • • •• •• • •• i- • • • • .. • • • • 06

i- • • • • • • · - -• 0-4

- • .:. • • MEAN=0-36 • • - , 0-2 I . . - .- •

I-- .. - KOD •• ~Ie.

• I·· • • • I-- • •• • -.. • I.

u

~O'5

I--r­<!O'3 ••

•• .. • • MEAN=0-38 • I

~O'l N <l

1·4

1·2

-l-

• f-

f-

~.

l-

-• • • •

• •• •• •• •

•

TRD • • • • • • •••• • -.. •• • • •• • • • •• • • • •• • • MEAN=1-17 •

i- •• 4 1 ·0 • I • • l- • I I I I I.- I I I I

06 12 18 TIME,hrs 75

0 EMT

24

Fig. 6-Daily variations of tUlMi due to SSC at Indian stations Trivandrum (TRD). Kodaikanal (KOD). Annamalainagar (ANN) and Alibag (ABG) during 1958-59

C 2 2

Z _ I (x+ w) +h -- og 2 2

2w (x-w) +h . .. (2)

where, C is the uniform electrojet current. According to Eqs . ( I ) and (2), /lH is maximum at

the centre of the electrojet belt and t:.;Z is maximum .(negative or positive) at the (northern or southern) peripheral region of the electrojet belt . The induced currents wou ld be in the direction opposite to the source current. If the conductivity of the earth is as-

RASTOGI : EM INDUCTION DUE TO SSC AT EQUATORIAL ELECTROJET STATIONS 259

sumed to be infinite below a depth d the surface mag­ne.tic field effects can be calculated using the same equations, assuming the inducing current to be at a height of h + 2d.

Using the data from a close chain of observatories lying between ± 5° of dip equator in the Ethiopian region, Carl o et al .13 separated the external and inter­nal parts of total magnetic field components . It was found that the average induced effects as measured from the rati o of intern al to external component (Hi 1He) was pronou nced fo r substorm (Hi tHe = 0.35) rather than for Sq variations (Hi 1He = 0.28). Figure 7 shows the latitudinal vari ation of surface magnetic fields H and Z as a function of dip latitude for the internal and external components of the electrojet and planetary parts of the total ionospheric current during the substorm event between 0936 and 1030 hrs UT on 9 Apr. 197 1. As fa r as the horizontal field H is con­cerned, the effects due to the source as well as in­duced currents are of the same sign, adding them up to give the total observed effect with the maximum over the magnetic equator. The ratio of internal to

external effects in H, i.e.( MJ/Mle) was about 0.39 for the electrojet component and about 0.45 for the total current. The effects on the Z field were opposite for the source and the induced current. In the northern latitudes, external current produced a minimum at about 3° and the internal current produced a maximum at the same latitudes . The total IlZ was a reduced minimum at northern latitudes for either the electrojet or planetary or for total components of the current effects llZi IMJe and was about 0.29 for the total cur­rent. Thus, the bas ic effects of the induction of the electroj et current are an increase of surface MJ at the magnetic equator and a decrease of IlZ maximum or minimum at the peripheral regions of the electrojet.

Yacob and Khanna l4 showed that both the daily range of H and Z fi e lds at Indian observatories during IGY were largest at station closest to the magnetic

equator. The value of Sq(Z) was found to be 58 nT at TRD, 54 at ANN and -46 nT at ABG. The largest amplitude of Z occurred at around 1000 hrs LT at TRD and at around 1130 hrs LT at ABG. Srivastava

120 ~H APRI L -9J971

SUBSTORM EVENT ~z 100 80 60 40 20

TOTAL

O~----~------~

tc 9 40 !:!J 20 _ .......... , lJ... 0 . ...... .

80 60 40

AHi =0 '39 AHe .....

TOTAL EXTERNAL INTERNAL

nT 20 o

-20

20 o

-20

/' 20 ELECTROJET./ 20 O l--~~-"':""='::=~ --.-- 0 ....... -. ---. --

•••• : /--ExTERNAL - 20 f.Zi/AHe=O·29 ' ..,

- 30°-20. _10° 0° 10° 20° 30° DIP LATITUDE

Fig. 7- Surface magnetic fi eld vari ations of the external and internal parts of the planetary and electrojet components together with total magnetic field components (H and Z) as a function of dip latitude for substorm event from 0936 hrs UT to '1030 hrs UT on 9 Apr. ! 971 (after Carlo et al.13 )

260 INDIAN I RADIO & SPACE PHYS, DECEMBER 1999

and Sankarnarayan 15 suggested the anomaly in ~ at Trivandrum to be due to the coastal effects and due to sub-surface conductivity anomalies. However, no comparison was made between the observations and any sort of computations.

Gettmyl6 showed that ~ at Koror was positive at th~ time of maximum 1lH. Knapp and Gettmyl7 showed that the daily variation of H field simulated the time derivative of the H curve. Fukushimal8 sug­gested the effect as due ~o induced currents at the sea surface under the development and decay of the over­head electrojet cnrrent.

Definitely the closest chains of equatorial obser­vatories in India was operated during the International Equatorial Electrojet Year (IEEY). Numbers of tem­porary observatories were established besides the permanent ones. Arora et al.19 have described pre­

liminary results of Indian IEEY geomagnetic projects. Figure 8.is a modified version of one of the figures in their paper. It shows the dip latitudinal variation of the Sq range of H and Z fields as well as of the ratio of!lZJ MI.

It is clear from the variation of the H field that the equatorial electrojet is superimposed over the global current system. This is in confonnity with the sug­gestion by Rastogi 20

. It is interesting to note that the IlH over the equator due to total current was t 25 nTI out of which the global current component was only 57 nT and hence the electrojet component was 68 nT which was greater than the global current component. Rastogi 21 has shown, from ionospheric drift meas­urements at Thumba and the geomagnetic H field at Trivandrum, that the total ionospheric current over the magnetic equator is composed of a planetary component of the eastward current flowing at an al­titude of 107 km together with the equatorial electro­jet component flowing, at an altitude of 100 km, ei­ther e.astward or westward. At surface, this could re­sult in strong electrojet, partial counter electrojet or a full counter electrojet depending upon the relative strengths and directions of the two current compo­nents .

The latitudinal variation of the range of Z shows expected minimum around 30 dip latitude at Koror (KOR) and Annamalainagar (ANN) . But the large

pos iti ve LlZ at lower lat itude station is anomalous at Kan yakumari (KAN), Trivandrum (TRD) and Ettaiy­apura'm (ETT). It is to be noted that !1ZJ1lH is about

llZ/llH +0·1 O·OJ..-+..l~----------I

-01 -02

Fig. 8-Solar quiet daily range of H and Z field at the chain of Indian observatories during February 1992 as a funct ion of dip latitude [The ratios of daily ranges in Z and H fields (6.ZJMf) are also shown as function of dip latitude (after Arora and KalraI9

) .)

0 .3 both at TRD and ANN. Thus, induced anomalous effect on ~ due to the daily variation is only about 30 % of the Sq(H) similar to the results shown in Fig. 2 ..

Computations were made of surface effect in H and Z fi e lds due to electrojet of varying thickness and due to induced currents wi th varying depth of the conductor. The equatorial electrojet height is assumed to be 100 km and the conductivity of the conductor is assumed to be infinite .

The latitudinal variations of Hx, HxlHo and ZxIHo due to the electrojet having semi-thickness of 200, 300 and 400 km are shown in Fig. 9(a) . Increase of the thickness results in an increase of magn itude of !:1H at the equator, i.e. Ho and a greater distance from the equator being affec ted by the CUlTenl. Examjni ng the cu rve. for HJ H" one can notice that at the periph­eral region of the e lec trojet the !:1Hx reduces to half

,.

•

RASTOGI : EM INDUcnON DUE TO SSC AT EQUATORIAL ELECTROJET STATIONS 261

EQUATORIAL ELECTROJET AT 100 km

)(

SOURCE CURRENT EFFECT

{a} 3-0 r------~-.........., 2-5 2-0

J: 1-5

005 00()

1·0 0 o-a J: - 0-6 )(

J: 0-4

0-2 0-0

0-0-2 J: oK ~0'4 N -006

... 0'8 -1-0 o~---"~:-=-L~-=-'--=±-:::-'-~:-'-::-:!.

INDUCTION CURRENT EFFECT

(b) r---------....., 30()

2-5

2'0 1-5

1·0

L.I-..I...-~:S:§~ 0-5 0·0

DEPTH OF CONDUCTOR km

~~::::::~~ 0·4 _ 0·2

1000

Fig. 9-(a) Surface geomagnetic field H .. ratios H.tHo and Z.tHo due to an electrojet current at 100 km with semi.thickness 200, 300 and 400 km as a function of distance from the centre of the elec­trojet belt for the northern hemisphere, and (b) Surface geomagnetic field Hx, ratios H.tHo and 2.,!H" due to the current induced at an infinite conductivity layer at the depth of 0, 50, 100 and 250 Icrn by an electrojet current at 100 km and semi-width 300 km plotted as a function of the distance from the centre of the electrojet band

tbe value at the equator (Ho). Increasing of semi­thickness increases the magnitude as well as range of distance for effect on !1Z. The minimum of !1Z occurs at a distance equal to the semi-thickness of the elec­trojet.

In Fig. 9 (b) are shown latitudinal variations of Hx,

HxlHo and 2,.IHo due to an eiectrojet current of semi­width 300 km flowing at an altitude of IOQ km.

Regarding the induction effects, decreasing of the depth of conductor increases. the equatorial value of MI. The HxlHo versus equatorial distance curve gets flatter with the increasing of depth of conductor.

Regarding 2,.IHo, the magnitude of the maximum !1Z increases with the decreasing depth of conductor. With the conductor almost at surface level, !1Z reaches the largest value of 0.7 times Ho. Thus, it can be seen that no combination of the depth of conductor can produce any positive !1Z at the equator.

Rajaram et al?2 postulated the channelling of the

internal currents through a conductor in the upper mantle or in the lower crust between India and Sri

. Lanka. Rajaram et ai?" estimated the depth of the subsurface conductor to be 522 km for se storms. It can be seen from Fig. 9 (b) that even if the induced current is assumed to be in south of India and as­sumed to be at zero depth, even then !1Z would in­crease to a value of only about 0.7 times Ho. It is not possible to get a value of !::.ZIMI equal to 1.2 as ob­served and definitely not so with a deep-seated con­ductor at 500 km below the surface.

Srivastava and Abbas24 suggested the anomalies in sse amplitudes at equatorial stations in India due to (i) the concentration of oceanic induced currents along the coastline, (ii) its flow southward along the east coast and (iii) its concentration, as they pass through the Palk Strait.

During nighttime, there is no electrojet, i.e. iono­spheric currents are too weak to be detected. The

262 INDIAN J RADIO & SPACE PHYS, DECEMBER 1999

110nT TRIVADRUM 19-March 1980

sse at 1118hr75°E

L 09 10 11 12 13 14 15 16 17 18 19 20 21HR75°E

TIME

Fig. IO--H and Z magnetogram traces at T rivandrum showing a strong SC (-, +) at 111 8 hrs LT on 19 Mar. 1980. [The ratio of t.ZI/lH for both the reversal impulse and the main impulse is abnor­mally large. )

source field producing SCs are distant and uniform. The increase of the !:1ZJ MI ratio at equatorial stations and the decrease of the ratio with increasing distance from the equator may be due to the diversion of in­duced current from non-equatorial latitudes and its concentration in the conducting channel between In­dia and Sri Lanka.

The low value of !:1ZJ MI during the midday hours is due to off-setting effect of the electrojet when large induced positive effect is seen in MI.

In Fig. 10 are reproduced the Hand Z traces of the magnetogram at Trivandrum on 19 Mar. 1980 when a SSC was recorded at 111 8 hrs LT. This event was rather a rare example when large preliminary reversed impulses were recorded in H as well as in Z traces before the main impulse. It is seen that the !:1ZJMI for the main impulse was 0 .89 and for the preliminary impulse it was 0 .84. It can also be noted that fo llow­ing the SSC, micropulsations were observed in which the indi vidual impul se in Z was almost of the same amplitude as the corresponding impul se in H.

4 Conclusions The subsurface induction effect in the equatorial

ek ctrojet currents in rndian longitude (75°E) sector seem to be a very complex phenomenon having num­ber of different mechani sms . The most plausible source seems to be the concentration of the induced

currents over a very large latitude sector through the conducting graben in the Palk Strait, thereby gener­ating a strong band of subsurface westward current belt south of the dip equator in the region. A more quantitative study of the induction effects to decipher this complex phenomenon is required . Any geomag­netic observations south of Palk Strait, i.e. in Sri Lanka would be of tremendous help in understanding the phenomenon.

Acknowledgements The author is thankful to Indian National Science

Academy, New Delhi , for selecting him as one for the Senior Scientist Schemes, to Gujarat Univers ity and Physical Research Laboratory, Ahmedabad, for the

intrast ructura l facilities provided to him. Thanks are due to Shri B J Srivastava, Profs . D R K Rao, R N Singh, R Raj aram, B R Arora and A C Das for their stimulating discu ssion and suggestions duri ng the course of inves tigations.

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