Variability of Electrojet Strength Along The

7/27/2019 Variability of Electrojet Strength Along The

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JOURNAL OF INFORMATION AND DATA MANAGEMENT VOL.1, NO.1 FEBURARY 2012 10

AbstractOne year data of hourly values of H component of

the earth magnetic field were used to study the magnetic strength

of some selected stations along the 96O

Magnetic Meridian (MM).

The results revealed that the amplitude of dH has diurnal

variation which peaks during the day at about local noon. This

diurnal variation in H component is attributed to the

enhancement dynamo action at this region. The diurnal variation

along the 960 MM reveals a clear nocturnal minimum variation

which could be attributed to distant current of non-ionospheric

origin. The observed minimum variation could be as a result of a

partial ring current. The electrojet strength at Addis Ababa with

respect to Khartoum are 44.05 nT,60 nT and 12.88 nT forJanuary, February and August around local noon, which is

stronger than the electrojet strength observed with respect to

Nairobi, 40.86 nT,42.41 nT and 19.12 nT .However, the pre-noon

and post-noon minimum variation may be attributed to distant

magnetospheric current.

I ndex TermsEquatorial electrojet, Sq variation, solar daily

variation magnetosphere

I. INTRODUCTIONHE large solar daily variations of the Earths magnetic field

at the ground stations were suggested by [1] as due to themovement of the conducting upper atmosphere across the

vertical component of the Earths magnetic field arising due to

the solar heating influence of the atmosphere.

These daily variations in the geomagnetic fields at the

earths surface during geomagnetically quiet conditions are

known to be associated with the dynamo currents which are

driven by winds and thermal tidal motions in the E- region of

the ionosphere [2]. At the magnetic dip equator the midday

eastward polarization field generated by global scale dynamo

action gives rise to a downward Hall current. A strong vertical

polarization field is set up which opposes the downward flow of

current due to the presence of non-conducting boundaries. This

field in turn gives rise to the intense Hall current which [3]

named the equatorial electrojet (EEJ). The phenomenon hasbeen given various attentions and has attracted several research

workers both in the past and recent times.

Manuscript received January 18, 2012. Revised on January 25, 2012;

accepted on February 10, 2012.Abbas M., Joshua B., and D. Bondel are with the Department of Physics,

Kebbi State University of Science and Technology, Aliero, Nigeria.

Adimula I. A. is with Department of Physics, University of Ilorin, Ilorin,

Nigeria.

A. B. Rabiu and O. R. Bello are with the Department of Physics, Federal

University of Technology, Akure, Nigeria.E-mail: [email protected]

Many authors have studied various aspects of the

variabilities of geomagnetic regular variations. The variability

of the solar cycle variations of the geomagnetic solar daily

variation S and lunar daily variation L are evident in [4]. [5]

studied the longitudinal variation of geomagnetic field

intensities at equatorial zone using surface magnetic data

recorded at 26 stations located in six different longitude sectors

that were set up or augmented during the international

Equatorial Electrojet Year IEEY; the nature of the longitudinal

inequalities in the EEJ strength indicates that the equatorial

electrojet was strongest in South America (80100W) and

weakest in the Indian sector (75E) with a secondary minimumand a maximum centered, respectively, in the Atlantic Ocean

(30W) and in western Africa (10E) [5].

During the International Geophysical Year (195758),

geomagnetic observatories within the equatorial electrojet

(EEJ) belt were operated at Addis-Ababa in Ethiopia,

Thiruvananthapuram in India, Koror and Jarvis in the Pacific.

Rastogi showed a significant longitudinal variation in the

strength of the electrojet current, being largest at Huancayo

whereH= 0.28 G and weakest at Thiruvananthapuram, where

H= 0.39 G. He suggested that ionospheric conductivity may be

inversely proportional to the strength of the background mean

geomagnetic field. No correlation in the strength of EEJ

between a pair of stations on day-to-day basis or even for longerperiods has been detected due to the limited number of

electrojet observatories. Later, [6] showed that the average

range ofHat Thiruvananthapuram was slightly larger than the

corresponding range at Addis-Ababa, whereH= 0.36 G.

It should be noted that, although much work has been done

on the variabilities of equatorial electrojet stations but little or

no attention was paid to variability strength of equatorial

electrojet stations.

Thus, the main aim of this study is to investigate the strength

of the equatorial electrojet stations within the same longitude

sector, and to compare the inter- relationship in the solar quiet

(sq) variability along the 960 Magnetic Meridian (MM) of

MagDAS/CPMN data.

II. OBSERVATORY DATAGeomagnetic hourly means of the horizontal intensity from 7

observatories were used in this study. The arrows in Figure 1

shows the global geographical distribution of the geomagnetic

observatories used and Table 1 gives the locations and other

relevant data of the stations whose data were used in the present

analysis.

Variability of Electrojet Strength along the

Magnetic Equator Using MAGDAS/CPMN Data

Abbas M., Joshua B. D. Bonde, Adimula I. A., A. B. Rabiu and O.R. Bello

T


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4 8 12 16 20 24-50

0

50

100

LT (HRS)

HORIZONTALINTENSITYdH(nT)

4 8 12 16 20 24-40

-20

0

20

40

60

LT (HRS)


4 8 12 16 20 24-20

0

20

40

60

80

LT (HRS)


4 8 12 16 20 24-100

-50

0

50

100

150

LT (HRS)


4 8 12 16 20 24-20

0

20

40

60

80

LT (HRS)


LSK

MPT

HER

DES

AAB

0 5 10 15 20 25-20

-10

0

10

20

30

40

50

60

70Annual mean diurnal Variation of dH [nT]

Local Time [Hrs]

dH

[nT]

KRT

AAB

NAB

DES

LSK

MPT

HER

Figure 1. Distribution of geomagnetic observatories used for

the study

Three observatories were grouped into two pairs, with one

of the observatories in the immediate proximity to the dip

equator Addis Ababa (AAB) and others outside of the EEJ

footprint area Khartoum (KRT) and Nairobi (NAB). Each

observatory pair consists of one observatory close to the EEJ

footprint and a second one outside of this area but both within

the same longitude sector. Data were available for January,

February and August 2009. The equatorial electrojet strength

for an observatory pair is computed from the horizontal

intensity, H, as DHEEJ _ DHNonEEJ [see, e.g., 7, 8, 9], where DH is

the variation of H from the mean midnight level for that

observatory. Ideally, this differencing removes the core and

large-scale magnetospheric fields from the data, and on

magnetically quiet days, DH describes the daily variation of Sq

and EEJ plus their induced component.

Table 1. Coordinate of stations along the 960 Magnetic

Meridian used.

III. MIDNIGHT DEPARTURESThe local time of each station is used throughout the analysis.

The variation baseline is obtained from the 2 hours flanking

local midnight that is 24hr LT and 1hr LT. The daily baseline

values (Ho) for the geomagnetic element H are the mean values

of the hourly values at these 2 hours given as;

H0 = 1/2 (H24+H1) (1)

Where H24, H1 represents the values of the geomagnetic

element (H) at 24hr LT and 1hr LT respectively. The midnightbaseline values were subtracted from the hourly values to get

the hourly departures from the midnight for a particular day.

That is:

dH=Ht -Ho (2)

Where t=1 to 24; Ht is hourly values of magnetic element H

[10].

Hence, dH gives the measure of the hourly amplitude of

variation of horizontal component of the earth magnetic field

H.

IV. RESULTSFigure 2 shows the geomagnetic daily variation obtained fromthe hourly mean variation of horizontal component of the earth

magnetic field on five quietest day of the month of February

2009 at Maputo (MPT), Lusaka (LSK), Dares salaam (DES),

Addis Ababa (AAB), Hermanus (HER). Morning depression is

clearly seen in Addis Ababa but it does not seem to alter the

trend of the daily variation at any of the day.The daily variation

of earth magnetic field H at HER shows a maximum variation

around 0900hr LT followed by a dip at 1300hr LT-1500hr LT.

Figure 2. Daily variation of H at LSK, MPT, HER, DES and

AAB on the five quiet days of February 2009.

stations geographical

lat (o)

geomagnetic

lat (0)

khartoum 15.33 5.69

Addis Ababa 9.04 0.18

Nairobi -1.16 -10.65

Daras salam -6.47 -16.26

Lusaka -15.25 -26.06

Maputo -25.58 -35.98

Hermanus -34.34 -42.29


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4 8 12 16 20 24-20

0

20

40

60

Local Time [Hrs]

dH(

nT)

VAR AAB -KRT 01/09

4 8 12 16 20 2 4-50

0

50

100

Local Time [Hrs]

dH(

nT)

VAR AAB-KRT 02/09

4 8 12 16 20 24-40

-20

0

20

Local Time [Hrs]

dH(

nT)

VAR AAB -KRT 08/09

4 8 12 16 20 24

-20

0

20

40

60

Local Time [Hrs]

dH(

nT)

VAR AAB-NAB 01/09

4 8 12 16 20 2 4

-50

0

50

Local Time [Hrs]

dH(

nT)

VAR NAB-KRT 02/09

4 8 12 16 20 24

-40

-20

0

20

Local Time [Hrs]

dH(

nT)

VAR AAB-NAB 08/09

Figure 3. Annual mean diurnal variation of dH (nT)

Figure 3 above displays the annual mean diurnal sq variationof some selected stations along the 960 Magnetic Meridian

(MM). Maputo and Hermanus having minimum variation, and

a post-noon minimum variation is also observed in Hermanus.

Addis Ababa on the magnetic equator shows the maximum

variation. A minimum nocturnal variation is also observed in

Addis Ababa.

Figure 4. Average daily variation of the horizontal component

of the geomagnetic field observed at AAB with respect to the

station KRT and NAB.

Figure 4 above, shows the average daily variation of the

electrojet strength observed at Addis Ababa with respect to

Khartoum and Nairobi for January, February and August. The

EEJ strength reaches a maximum of 44.05 nT, 60.02 nT and

12.88 nT just before noon at about 1100 LT, except at January

where it occurred around 1000 LT. However, Addis Ababa

with respect to Nairobi the strength reaches a maximum of

40.86 nT, 42.41 nT and 19.12 nT for January, February andAugust respectively.

V. DISCUSSION OF RESULTSSolar quiet (Sq) H is expectedly consistently maximum within

the electrojet zone as a result of equatorial electrojet EEJ

phenomena. Sq H at about local noon, when the sun is vertically

overhead and solar activity is maximum on any day at any

location, as shown in Figure 2 has one outstanding peak almost

at magnetic equator. Enhanced Cowling conductivity along the

dip equator has been described as a major cause of the EEJ [11,

12, and 13].Also horizontal component of the earth magnetic

field H, maintains regular pattern and consistent variation. It

increases from night time level and maximizes around local

noon. This is in agreement with [14], concluded that the daily

variation of sq of H arises in day time in consistency with the

atmospheric dynamo theory of the geomagnetic daily variation.

This daily variation so observed is also in agreement with

the daily variation pattern of sq in the earlier works of [15] and

[16], which showed that the maximum intensity of solar quiet (sq) variation occur around the local noon.

Throughout February 2009, large H is noticed at Addis

Ababa (AAB) as it maintains a maximum variation in all the

stations. It is to be noted that the equatorial electrojet activity is

stronger in AAB as it is closer to the magnetic dip equator to

stations south of the magnetic equator along the 96 magnetic

meridian. Also with increasing latitude, H decreases

indicating latitudinal dependence of the geomagnetic

observatory along the 960 Magnetic Meridian (MM).

Also, Addis Ababa (AAB) exhibit a significant negative

excursion during morning hours, subsequently changing over a

definite positive excursion in the afternoon hours with a

maximum observed around 1200hr LT. Hermanus (HER) and

Maputo (MPT) shows minimum variation throughout the

month which agrees with [17] that changes in magnitude of the

ionospheric conductivity controls the magnitude of the

variabilities. [18], pointed out that the day- to-day changes in

the position of the focal latitude during quiet magnetic

condition would influence the pattern of the sq current system.

From Figure 2, one remarkable feature to note is that H

continues to vary from the expected variation at Hermanus

(HER) and Maputo (MPT) throughout the month having

morning crest and afternoon trough. This is an abnormal

feature. It is suggested that it could be cancellation of EEJ.

Equatorial electrojet (EEJ) is enhanced by localized

ionospheric currents and physical structure, flowing at the dip

equator with higher current intensities during the day timewhich are responsible for sq variation. The cancellation of the

EEJ might then be due to the current which flows in opposite

direction. It is clear that some gaps or breaking points are

observed in some of the plots. This is an abnormal feature and

not a common phenomenon. This is as a result of some

unwanted data which are filtered out.

There was a clear minimum nocturnal variation in sq (H) on

about 95% of the geomagnetic observatories considered along

the 960 Magnetic Meridian (MM).

Generally the night-time variation could be attributed to

distant current of non- ionospheric origin such as suggested by

earlier investigators. The observed minimum variation could be

as a result of a partial ring current [19].Figure 3, the annual mean diurnal variation of dH is plotted

in a quest for greater insight into the sq variation with Addis

Ababa on the magnetic equator, which agrees with [3], that

latitudinal variation is expected to be maximum at 00 dip

latitude and a continuous decrease both on the northern and

southern hemisphere of the magnetic equator until the latitude

that defines the edge of the electrojet belt. It also gives the

variation pattern of sq which agrees with the diurnal variation

pattern of sq in the earlier works of [15] and [16], which

showed that the maximum intensity of sq occur around local

noon. Emilia and Last (1977) reported a similar diurnal


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variation pattern of sq (H) for 1958-1973 in Addis Ababa. This

variation can be attributed to the variabilities of the ionospheric

processes and physical structure such as conductivity and

winds structure, which are responsible for the sq variation.

Studying the variabilities in Indian equatorial electrojet sector,

Okeke et al, (1998) noted that changes in the electric field

control the phase and randomness of the variabilities, while the

magnitude of the conductivity controls the magnitude of the

variabilities. The minimum nocturnal variation observed inAddis Ababa could be attributed to distant current of

non-ionospheric origin. The observed minimum variation could

be as a result of a partial ring current [19].

In Figure 4, Khartoum (geomagnetic latitude 5.69) which is

north of addis Ababa and Nairobi (geomagnetic latitude -10.65)

south of Addis Ababa are both outside the EEJ influence. The

average daily variation of the electrojet strength observed at

AddisAbaba with respect to Khartoum for January, Februaryand August are 44.05 nT, 60.02 nT and 12.88 nT just before

noon at about 1100 LT, except at January where it occurred

around 1000 LT. However, the pre-noon and post-noon

minimum variation observed may be attributed to distant

magnetospheric current. Also the electrojet strength at Addis

Ababa with respect to Nairobi shortly before local noon at

about 1100 LT for the month of January, February and August

are 40.86 nT, 42.41 nT 19.12 nT respectively. This confirms the

strength of the equatorial electrojet is controlled by the

eastward electric field, conductivity and also by the latitudinal

separation between the paired stations. The increase in

variation at Nairobi in August suggest that there could be

additional sources of ionization in Nairobi other than solar daily

radiation and that the sources are moving with time. Also it is

observed that an increase in the strength of the sq current

system is accompanied by an equatorward shift of the sq focus

(on the average) and also an increase in the strength of the

electrojet. [20] also showed that the strength of the electrojet

current increased when the sq focus moved towards theequator.

VI. CONCLUSIONSThe following conclusions may be drawn from the present

study: Diurnal variations of day-to-day variability, monthly

variability exist in H element on quiet days throughout

February 2009. The daytime (0700-2000hr) magnitudes are

greater than the nighttime (2000hr-0700hr) in the Horizontal

(H) element. The diurnal variation of day-to-day variability,

which followed the variation pattern of sq, can be attributed to:

1)The atmospheric dynamo current at ionospheric E-region2)The variability of the ionospheric processes and physical

structure such as conductivity and wind structure which

are responsible for the sq variation.

3)The variability of the nighttime field may be as a result ofthe variability of nighttime distant currents.

The solar daily variation observed in some selected stations

along the 960 Magnetic Meridian (MM), follow the latitudinal

profile with Addis Ababa on the magnetic equator exhibiting

maximum variation. Hermanus with GM Lat -42.290 south of

the magnetic equator has the minimum variation. This could be

attributed to changes in magnitude of the ionospheric

conductivity which controls the magnitude of the variabilities.

AAB-KRT with the minimum latitudinal separation

between them display maximum equatorial electrojet strength

for the months considered except at August where it shows

minimum variation which could be attributed to localized

effect. However, AAB-NAB with maximum latitudinal

separation shows weak equatorial electrojet strength. This

reveals that equatorial electrojet strength within the samelongitude sector depends on latitudinal separation between the

stations.

REFERENCES

[1] Stewart, B. Hypothetical views regarding the connection between state ofthe sun and terrestrial magnetism in Encyclopedia Brittanica, 9 th

edition, vol. 16, 181-184.[2] Chapman, S., The solar and lunar diurnal variations of terrestrial

magnetism, Philos. Trans. Roy. Soc., London, A 218, 1-118.

[3] Chapman S., (1951). The equatorial electrojet as detected from theabnormal electric current distribution above Huancayo and elsewhere,

Arch. Meteorl. Geophys. Bioclimatol, A 4, 368-392.

[4] Chapman, S., J. C Gupta, and S. R. C. Malin (1971). The sunspot cycleinfluence on the solar and lunar daily geomagnetic variations, Proc.

Roy.Soc. A324, 1-15.[5] Doumouya, V., Cohen, Y., Arora, B. R., and Yumoto, K.., (2003), J.

Atoms. Terr. Phys., 65,1265.

[6] Chandra, H., H. S. S. Sinha, and R. G. Rastogi, (2000). Equatorialelectrojet studies from rocket and ground measurements,Earth Planet

Space, 52, 111120.

[7] Yacob, A. (1977). Internal induction by the equatorial electrojet in Indiaexamined with surface and satellite geomagnetic observations, J. Atmos.

Terr. Phys., 39, 601606.

[8] Alex, S., and S. Mukherjee (2001), Local time dependence of theequatorial counter electrojet effect in a narrow longitudinal belt, Earth

Planets Space, 53, 11511161.

[9] Anderson, D., A. Anghel, K. Yumoto, M. Ishitsuka, and E. Kudeki(2002), Estimating daytime vertical E x B drift velocities in the equatorial

F-region using ground-based magnetometer observations, Geophys. Res.

Lett., 29(12), 1596, doi:10.1029/2001GL014562.[10] Rabiu, A. B., Nagarajan, N., Okeke, F. N., Anyibi. E. A., (2007). A study

of day-to-day variability in geomagnetic field variations at the electrojetzone of Addis Ababa, East Africa, AJST. Vol.8 pp 54-63.

[11] Onwumechili, C. A., (1967), In: Eds. Matsushita S. and Campbell, W. H.Physics of Geomagnetic Phenomena. 1: 425-507. Academic press, New

York.[12] Onwumechili, C. A., The Equatorial Electrojet, Gordon and Breach

Science Publishers, Netherlands.

[13] Forbes, J. M., (1981), Rev. Geoph. Space Phys., 19, 469.[14] Onwumechili, C.A. and Ezema, P.O., (1977). On course of the

geomagnetic daily variation in low latitudes. Atoms. Terr: Phys., 39.

[15] Onwumechili, C.A., (1960). Fluctuations in the geomagnetic field nearthe magnetic equator.J. Amos. Terr. Phys., 17,286-294.

[16] Matsushita, S., (1969). Dynamo Currents, Winds, and Electric Fields,Radio Sci., 4, 771.

[17] Okeke, F. N., C. A. Onwumechili, and A. B. Rabiu (1998), Day to dayvaraibility of geomagnetic hourly amplitudes in low latitude, Geophys. J.

Int., 134, 484500.

[18] Hasegawa, M., (1960). On the position of the focus of the geomagnetic sqcurrent system, J. Geophys. Res. 65. 1437-1447.

[19] Bhargava, B. N and Yacob, A.,( 1971). Solar wind associated componentin the low-latitude magnetic daily variation, Journ. Geomagnetic geo

elect. 23, p. 249-253.

[20] Rajaram, M., (1983). Determination of the latitude of sq focus and itsrelation to the electrojet variations, J .atmos. Terr. phys., 45, 573-578.

Variability of Electrojet Strength Along The

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