LETTER

Earth Planets Space, 60, 785–790, 2008

A new index to monitor temporal and long-term variations of the equatorial
electrojet by MAGDAS/CPMN real-time data: EE-Index
T. Uozumi1 , K. Yumoto1 , K. Kitamura2 , S. Abe1 , Y. Kakinami1 , M. Shinohara1 , A. Yoshikawa1 , H. Kawano1 ,T. Ueno3 , T. Tokunaga3 , D. McNamara4 , J. K. Ishituka5 , S. L. G. Dutra6 , B. Damtie7 ,V. Doumbia*8 , O. Obrou8 , A. B. Rabiu9 , I. A. Adimula10 , M. Othman11 ,M. Fairos11 , R. E. S. Otadoy12 , and MAGDAS Group1
1 Space Environment Research Center, Kyushu University, 6-10-1 Hakozaki, Fukuoka, Japan
2 Tokuyama College of Technology, Gakuendai, Shunan, Yamaguchi 745-8585, Japan
3 Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka, Japan
4 Manila Observatory, Bldg. at Ateneo de Manila University Campus, Quezon City, Philippines
5 Observatorio de Ancón, Instituto Geofísico del Perú, Carretera Panamericana Norte Km. 43.5, Ancón, Peru
6 National Institute for Space Research (INPE), 12227-010 Sao Jose dos Campos, SP, Brazil
7 Department of Physics, Bahir Dar University, Bahir Dar, PB 79, Ethiopia
8 Laboratorire de Physique de l’Atmosphere, University of Cocody, BPV34 Abidijan, Cote d’Ivoire
9 Department of Physics, Federal University of Technology, PMB 704, Akure, Ondo State, Nigeria
10 Department of Physics, University of Ilorin, Ilorin, Kwara State, Nigeria
11 National Space Agency, Ministry of Science, Technology and Innovation, 62100 Putrajaya, Malaysia
12 Department of Physics, University of San Carlos, Cebu City, Philippines(Received August 27, 2007; Revised February 20, 2008; Accepted March 28, 2008; Online published August 4, 2008)

A new index, EE-index (E Dst, EU, and EL), is proposed to monitor temporal and long-term variations of theequatorial electrojet by using the MAGDAS/CPMN real-time data. The mean value of the H component magnetic
variations observed at the nightside (LT = 18–06) MAGDAS/CPMN stations along the magnetic equatorial
region is found to show variations similar to those of Dst; we defined this quantity as E Dst. The E Dst canbe used as a proxy of Dst for the real-time and long-term geospace monitoring. By subtracting E Dst fromthe H component data of each equatorial station, it is possible to extract the Equatorial Electrojet and Counter
Electrojet components, which are defined as EU and EL, respectively.
Key words: EE-index, equatorial electrojet, counter electrojet, MAGDAS/CPMN, real-time monitoring, Dst
index, space weather.

1.

Introduction

The “equatorial electrojet” (EEJ) is observed near thedayside magnetic dip equator as an enhanced magnetic
variation superposed on the low-latitude Sq (e.g., Forbes,
1981). The EEJ field is caused by the ionospheric current flowing along the narrow channel (∼±3◦ in latitudinal range) of the enhanced ionospheric conductivity (Cowling conductivity), which is formed along the dayside dip
equator. Many investigations have been made on this phenomenon from the early stage of geomagnetism (e.g., Chapman, 1951; Onwumechilli, 1997), because it is the mosttypical and dominant phenomenon showing on ordinary
magnetograms obtained at stations in the vicinity of themagnetic dip equator. Despite the wealth of information
obtained from these investigations, many open questions regarding EEJ still remain such as (1) the occurrence mechanism of day-to-day and long-term variations of EEJ amplitude, (2) penetration mechanisms of DP2-ULF range disturbances from the solar wind region to the magnetic equatorand its relation to the EEJ disturbances, and (3) mechanismsc The Society of Geomagnetism and Earth, Planetary and Space SciCopyright ences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

of ionosphere-thermosphere coupling and its influence on
EEJ variations (e.g., Rastogi, 1992; Kobea et al., 1998; Yumoto and the MAGDAS Group, 2006). In recent years, theimportance of the study of equatorial phenomena is increasing from the aspect of the space weather.
Quasi-periodic magnetic perturbations of EEJ with a period of about half- to a few-hours are commonly recognized
as DP 2 (Nishida, 1968). DP 2 is basically understood asa magnetic perturbation that is closely correlated with thevariation of the interplanetary magnetic field (IMF), especially in its north-south component. One of the uniquefeatures of the equatorial phenomena is the enhancementof magnetic perturbations caused by the Cowling conductivity. This effect appears on various period ranges, including not only that of DP 2 (Kikuchi et al., 1996) but
also that of ULF (Shinohara et al., 1997). The monitoring of EEJ is important for the study of the solar windmagnetosphere-ionosphere-thermosphere coupling system(e.g., Yumoto and the MAGDAS Group, 2006).
The equatorial “counter-electrojet” (CEJ) is also a wellknown phenomenon, which is observed as a depression in
the northward magnetic component while a regular EEJ
shows an increase (Gouin, 1962). The occurrence mechanism of the CEJ has not yet been clarified sufficiently,

785

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T. UOZUMI et al.: EQUATORIAL ELECTROJET (EE-) INDEX BY MAGDAS/CPMN

though its morphological aspects have been accumulated
by many researchers (e.g., Mayaud, 1977 and references
therein). Rastogi (1974) reported that the occurrence of CEJ
is at a maximum when the lunar time is 2.3 and 14.3 h (2.2
and 14.2 h) and the local time is around 7 h (between 15 and17 h). These lunar times correspond to those of maximum
negative effect due to the lunar tidal wave, and these CEJ
maxima are suggested to be due to the superposition of thelunar semi-diurnal wave over the solar daily wave of H . Recently, Kikuchi et al. (2003) proposed that CEJs most likely
occur during the substorm recovery phase and that they arecaused by the rapid change of the ionospheric electric field
generated and transmitted from the IMF, through the mechanism called the over-shielding effect.
The magnetic equatorial region is considered as the terminator of the energy flow associated with the sun-earth
coupling system (e.g., Kobea et al., 1998). Thus, it is vital to monitor equatorial geomagnetic phenomena not only
to clarify their fundamental occurrence mechanisms but
also for space weather studies. In particular, investigationof temporal and long-term variations of equatorial magnetic disturbances associated with equatorial electrojet andcounter electrojet requires the construction of a real-timemonitoring system. To our knowledge, there is no monitoring system as extensive as the MAGDAS/CPMN, which
has a sufficient number of magnetic stations located along
the dip equator. In this paper, we propose a new algorithm to achieve the above goals and introduce a new index,
the EE-Index (E Dst, EU and EL), to quantify the scale ofmagnetic disturbances in the equatorial region using MAGDAS/CPMN real-time data.

2.

Methodology

EU and EL [nT] EDst and Dst [nT] ER and Dst [nT]

ER [nT]

The MAGnetic Data Acquisition System of the Circumpan Pacific Magnetometer Network (MAGDAS/CPMN)
evolved from the present CPMN for real-time monitoring of the electromagnetic and plasma environment in
geospace (Yumoto and the MAGDAS Group, 2006). MAGDAS/CPMN consists of two unique chains of magnetic sta-

200
100

tions; one is installed densely along the 210◦ magnetic
meridian, and the other is installed at whole local time
sectors along the magnetic equator. In this study, MAGDAS/CPMN 1-min resolution data from four equatorial stations were analyzed to construct a basic algorithm for realtime monitoring of long-term variations of the EEJ and the
CEJ.
Figure 1(a) shows a superposed plot of the relative magnetic variation of the H component data E R S (m) (definition given just below) obtained for 1 month (Dec. 1–31,
2006), at the MAGDAS equatorial stations: Addis Ababa(AAB: Dip Lat. = 0.57◦ ; GMLON = 110.47◦ ), Davao(DAV: −0.65◦ ; 196.54◦ ), Ancon (ANC: 0.74◦ ; 354.33◦ )and Eusebio (EUS: −7.03◦ ; 34.21◦ ). The “S” and “m” in“E R S (m)” refer to a station and a point of time in UT, respectively. In order to obtain E R S (m), the median valueof the H component data, which is determined for the period from the start time of the observation to the end timeof E R S (m), was subtracted from the original magnetic data
for each station. It was found (not shown) that the medianvalue, which was calculated from all the data in the abovestated interval, could estimate the non-disturbed nighttime
ambient level. The determination by shorter-period data
might cause some jumps in the base level among different
periods (it might occur in particular during disturbed times
when large magnetic storms occurred frequently). Thus themedian value was calculated for the period from the start
time of the observation to the end of the E R S (m). In the future the subtracting offset value will be corrected by taking
into account the secular variation of the base level, though
the value is currently determined as a constant.
Figure 1(b) plots the superposed H component perturbation field data that was observed during the night time (LT= 18–06) at each station: E R S (m) |LT=18–06 . The solid
thick curve (orange) in Fig. 1(b) is the Dst value for thesame plot interval. This Dst was provided in real-time on
the website of the World Data Center for Geomagnetism,
Kyoto (http://swdc234.kugi.kyoto-u.ac.jp/dst realtime/). In
this figure, Dst was graphically shifted downward so as to

(a)

0
-100
-200
-300
50

0

-50
-100
-150
-200
-250
50

0
-50
-100
-150
-200
200
100

ERS|LT=00-24
AAB

ANC

DAV

EUS

(b)
ERS|LT=18-06
AAB

DAV

ANC

EUS

Dst

(c)
EDst1hEDst6h

Dst

(d)

0
-100
-200

AAB

1

DAV

6

ANC

EUS

11

16
Date (Dec. 2006)

21

26

31

Fig. 1. One month plot of (a) relative magnetic variation of the H component (E R S |LT=00–24 ) observed by MAGDAS/CPMN equatorial network
stations, (b) relative night time (LT=18–06) variation of the H component (E R S |LT=18–06 ) and the real-time Dst, (c) E Dst 1h , E Dst 6h and thereal-time Dst, and (d) EU and E L for MAGDAS/CPMN equatorial network stations, during one month of December 2006.

T. UOZUMI et al.: EQUATORIAL ELECTROJET (EE-) INDEX BY MAGDAS/CPMN

avoid overlap with MAGDAS/CPMN data. It is noticed thatthe nighttime variation of each station was almost alignedwith each other and formed a common base magnetic variation. On the other hand, by comparing Fig. 1(a) and 1(b), it
can be confirmed that the daytime EEJ component of eachstation bulged out independently from the base magnetic
variation. With this presentation, it is evident that the basemagnetic variation E R S (m) |LT=18–06 varies closely withthe traditional Dst (Sugiura, 1963). This similarity will befurther examined later in this section.
Here we define “E Dst (Equatorial Disturbance in stormtime)” as follows; at first, the relative nighttime (LT = 18–
06) variation of the H component data is determined foreach station: E R S (m) |LT=18–06 is calculated by the methodthat was defined above. For the second step, the mean valueof the magnetic variations among stations, when they arelocated in the nighttime sector (LT = 18–06), are calculated
at each minute:E Dst 1m (m) =E R S (m) |LT=18–06

S|LT =18–06

N (m) |LT=18–06(1)

where the sum operation is carried out for the stations
which are located in the nighttime sector (LT = 18–06):
S |LT=18–06 . N (m) |LT=18–06 represents the number of stations which are located in the nighttime sector (LT = 18–06) at a specified time point of m. In case none of the stations was located in the nighttime sector, E Dst 1m (m) takes
a blank value (for the period of the plot, at least one station was located in nighttime sector throughout the interval). 18–06 LT is adopted as the nighttime in the present algorithm. It is noted that the seasonal variation of the nighttime duration takes a minimum value at the equatorial region compared with other latitudes. This means that diurnal variation of the EEJ was almost confined within the 06–
18 LT independently of any seasons, that the effect of the
daytime ionospheric current is not significant for the periodof 18–06 LT, and that the nighttime magnetic variation canbe utilized as the base level for the EEJ and CEJ. This situation has been confirmed by the DAV data observed fromthe period of July 2005 to March 2006 (not shown here).
“E Dst 1h ” is one-hour average of E Dst 1m . In Fig. 1(c),E Dst 1h is plotted the green curve. The blue and orange
curves in Fig. 1(c) are 6-h running averages of E Dst 1h (labeled E Dst 6h ) and the real-time Dst, respectively. Dst
is shifted +42 nT to adjust the base level. It is confirmedthat the E Dst and Dst exhibited the similar temporal variation. It is noticed that E Dst 6h is less deviated from Dst
than E Dst 1h . Correlation coefficients between E Dst 6h andDst, and that between E Dst 1h and Dst, which were calculated for the 1-month period of December 2006, were 0.93
and 0.88, respectively. We have also found that the correlation coefficient between the low-pass-filtered E Dst 6h andthe low-pass-filtered Dst with the cut-off-period of 2 days
was 0.98 is larger than the above-mentioned 0.93.
In order to examine how the number of used nighttime
stations affects the quality of E Dst 1h , here we try all available combinations of the nighttime stations to calculate
E Dst 1h , calculate the correlation between the thus obtained

787

E Dst 1h and Dst, and calculate the standard deviation of(E Dst 1h − Dst). When E Dst 1h is calculated from onenighttime station, the correlation coefficient and the standard deviation are in the range of 0.83–0.92 and 7.12–
12.2 nT, and their mean values are 0.88 and 10.6 nT, respectively. For the case of two nighttime stations, the coefficient and the standard deviation are in the range of 0.91–
0.99 and 3.16–11.1 nT, and their mean values are 0.95 and6.71 nT, respectively. For the case of three nighttime stations (in the present analysis, the AAB, ANC, and EUS
group was the available combination), the coefficient andthe standard deviation are 0.98 and 6.60 nT, respectively.
This examination shows that the correlation coefficient between E Dst 1h and Dst tends to increase and the standard
deviation of (E Dst 1h − Dst) tends to decrease, as the number of nighttime stations increases.
Figure 1(d) shows the relative magnetic perturbation ofeach station, which was calculated by subtracting E Dst 6hfrom the H component data. In this calculation, E Dst 6h
was converted to 1-min resolution data by interpolating
the original E Dst 6h through application of the FIR (finiteduration impulse response) low-pass interpolating method.
It is considered that the positive and negative deviations represent the EEJ and CEJ components, respectively. We define those two component as EU S (m) and E L S (m), respectively. For example, when the EU and E L components
are derived for the DAV station, those values are expressed
as EUDAV (m) and E L DAV (m), respectively. Inspection ofthese data reveals that the EEJ intensity varies from day today; moreover, the peaks and dips of each station are notin-phase with other stations. For example, the intensity of
EEJ at the DAV station (blue line) and at the ANC station(red line) attained local maxima on Dec. 6 and 8, respectively. The mechanism of the 2-day separation is one of theopen questions concerning EEJ. In some cases, it will benecessary to correct the secular variation of a base level todetermine E Dst 6h for properly extracting the EU and E L
component.
In order to examine whether the above-stated procedure
properly extracts the EEJ/CEJ component from the original magnetic perturbation and to examine if the obtained
EU /E L indices can be utilized for the monitoring of thelong-term variation of EEJ/CEJ, we have analyzed the spectral characteristics of the long-term variation of EU /E L.
The data observed at the DAV station during the period from
July 1, 2005 to March 4, 2006 were analyzed. In this period,
because MAGDAS/CPMN equatorial network had not been
established and data from other stations were not available,
Dst, instead of E Dst, was subtracted from the DAV data.
Figure 2(a) and 2(b) shows the Dst and DAV H component data for the above interval, respectively. From thisplot it can be seen that the EEJ component is occasionallydrowned out by geomagnetic storms, and by looking at theraw data it is hard to see the day-to-day variation of the EEJ
during certain periods. However, as shown in Fig. 2(c), thisvisibility problem can be corrected if Dst is subtracted fromthe H component data. This is plotted as (DAV H )−(Dst).
Despite the disturbance of a geomagnetic storm, the dayto-day variations and oscillations of EEJ intensity can beeasily seen in Fig. 2(c).

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T. UOZUMI et al.: EQUATORIAL ELECTROJET (EE-) INDEX BY MAGDAS/CPMN

Dst [nT]

100
0
-100
-200
-300

(a)

(DAV H) - (Dst) DAV H [nT]

(b)
39200
39000
38800
[nT]
39300

(c)

39200
39100
39000
Jul. 1
2005

Aug. 1

Sep. 1

Oct. 1

Nov. 1

Dec. 1

Date

Jan. 1
2006

Feb. 1Mar. 1

Fig. 2. Long-term variation of (a) Dst, (b) H component observed at DAV (DAV H ), and (c) (DAV H )−(Dst), during the period from July 1, 2005 to
March 4, 2006.

10

FFT Analysis: Jul.1,2005-Mar.4, 2006
Table 1. Dominant peak periods detected in the power spectrum forvarious parameters during the period from July 1, 2005 to March 4,
2006.

(DAV H)-(Dst)
8

14.5

35.3

Power [nT2]

Variable

6(DAV H) - (Dst)(DAV H)
Dst
Kp
IMF Bx,y,z,T
Vsw
Tsw
Nsw
PD

7.5

4
2
0

2 5

F10.7

10 15 20 25 30 35 40 45 50
Period [day]Fig. 3. Power spectrum of the subtracted H component data at DAV[(DAV H )−(Dst)] during the period from July 1, 2005 to March 4,
2006.

Figure 3 shows the power spectrum of the subtracted H
component data at DAV; (DAV H )−(Dst) during the period
from July 1, 2005 to March 4, 2006. It is recognized that
there are at least three dominant peaks at 7.5, 14.5, and35.3 day (these dominant peaks were visually identified in
the figure). Table 1 shows the list of dominant period peaksof various parameters. In the tables, (DAV H ) refers to
the original (without Dst subtraction) H component data
at DAV. IMF (B X , BY , B Z and BT ), VSW , TSW , and NSW
are the interplanetary magnetic field, solar wind velocity,temperature, and number density, which were observed by
the ACE spacecraft. PD and  are the dynamic pressure andthe epsilon-parameter, respectively (Perreault and Akasofu,
1978) calculated by using the ACE solar wind data. F10.7
represents the flux of the solar radio emission at 10.7 cm
wavelength. The data of F10.7 was provided at the websiteof the Herzberg Institute of Astrophysics (http://www.draoofr.hia-iha.nrc-cnrc.gc.ca/icarus/www/archive.html). It isnoteworthy that the above-stated dominant period peaks of
7.5 and 14.5 days, which appeared in the spectrum of (DAVJul. 1, 2005 - Mar. 4, 2006
Peak Period [day]
7.5
14.5
35.3
7.7 9.2 11.2 14.5 17.6 22.5 27.4
7.7 9.2 11.2
17.6 22.5 27.4
9.2
27.4
9.2
13.7
27.4
9.2 13.0
27.4
9.2 13.0
27.4
9.2 11.2
27.4 35.3
9.2 11.2
27.4 35.3
9.2
22.5
22.5
35.3

H )−(Dst), did not have their roots in the geomagnetic
activity indices of Dst and K p nor any parameters such as
IMF, VSW , TSW , NSW , PD , , and F10.7 , although the peak at
35.3 days was also identified in NSW , PD , and F10.7 . On theother hand, the original H component data contained thesame spectrum component as geomagnetic activity indicesand solar wind parameters, except 35.3 day.

3.

Discussion

We have constructed a basic algorithm to monitor in realtime the long-term variations of the (EEJ) and the (CEJ) byanalyzing the MAGDAS/CPMN data. We also proposed
new EE-indices, which consist of E Dst, EU , and E L, forthe monitoring of the equatorial geomagnetic phenomena.
The real-time monitoring system of the MAGDAS/CPMN
equatorial network stations, which are spaced longitudinally along the dip-equator, will provide a fundamental reference of various geomagnetic phenomena observed in themagnetic equatorial region.
As shown in Fig. 1(c), the mean nighttime variation ateach moment (named as E Dst), which were observed at theMAGDAS/CPMN equatorial network stations, was highly
correlated with the traditional Dst (Sugiura, 1963). That is,in spite of the smallness of the total number of the nighttime stations (1–3 stations depending on UT) for the ex-

T. UOZUMI et al.: EQUATORIAL ELECTROJET (EE-) INDEX BY MAGDAS/CPMN

amination period, E Dst was derived in sufficient quality,
as shown in the previous section. The correlation between
Dst and E Dst tends to increase, and the standard deviation of (E Dst 1h − Dst) tends to decrease with increasing
number of nighttime stations. When the number of nighttime stations is more than one, the coefficient is larger than
∼0.95, and the standard deviation is smaller than ∼7 nT.
This value corresponds to ∼5% of the normal EEJ amplitude (e.g., Doumouya et al., 2003). This result suggeststhat E Dst has sufficient quality if more than one nighttime
station available for E Dst calculation. When the EE-index
published, the number of nighttime stations used to calculate E Dst will also be published as index for the validity of
E Dst. It is noted that more stations on the magnetic equatorpromise a better accuracy; the MAGDAS/CPMN equatorial
network will extend the number of stations in the future.
Using the obtained E Dst, the EEJ and CEJ component
were derived—and named as EU S and E L S for each equatorial stations (S)—by using only the MAGDAS/CPMN
data without referring to provisional/real-time Dst. E Dst
has consistency with Dst, and concurrently derived EU S
and E L S are possibly utilized as real-time monitoring of
the temporal and long-term variation of the EEJ and CEJ
phenomena. These are useful not only for clarifying theirfundamental mechanism but also for studying the space
weather. The statistical consistency between Dst and E Dst
should be verified successively as long-term data will be
accumulated. The realtime EE-index will be published on
the website at “http://www.serc.kyushu-u.ac.jp/”, and therules for public usage of the EE-index will also be informed
there.
Although E Dst is similar to Dst (as stated above), they
are not identical, and it is important to construct the monitoring system, which is independent of provisional/realtime Dst, as stated below. The behavior of the magnetic
variations which are observed at the equatorial stations aredifferent from those of the low-latitude Dst stations (forexample, the effect of Sq are different). Some portion of
the dissimilarity between E Dst and Dst could be causedby the different behavior of the magnetic variations (andothers could be caused by the day-night asymmetry; E Dst
is derived only from nighttime data). The Sq component,
which was deduced from the quiet-day magnetic variationfor each Dst station, was subtracted in deriving Dst; however, the subtraction might not be perfect, especially in theprovisional/real-time data. That is, it is possible that there
exists some remnant of the Sq component on Dst, and theremnant could affect the EE-index if we use Dst in deriving the EE-index. In addition to the above, the independent
system has advantage to react flexibly to the change in theobservational situation of each station (offset level, secularvariation of the base level, etc.).
In order to derive the EEJ and CEJ component, the nighttime level in the neighboring days has been commonly referred to as the base level. For example, Fambitakoye andMayaud (1976) used linearly interpolated values connecting neighboring days’ midnight values as the basis in deriving the EEJ and CEJ components. Rigoti et al. (1999)
defined the reference (base) level as follows: “At midnight,
the traces are close to a straight line, showing no signifi-

789

cant magnetic variation. Hence, the midnight value or themean between the preceding and subsequent midnight values can be taken as the zero reference level for the diurnal
variation”. The presented method is an extension of suchtraditional algorithms and is basically consistent with them.
The improvement of our algorithm is that it takes into account the temporal variation of the nighttime level affectedby external magnetic variations. The traditional base levelin the literature did not include the magnetic variation that
occurred between both ends of the interpolated period. It
is noted that our new algorithm is not just a mathematical extension of the traditional procedure but that it has ageophysical meaning in that it subtracts Dst. The spectrum analysis (Table 1) revealed that subtraction of Dst(or E Dst) from the H component data removed external
magnetic disturbances. This procedure has an advantage
of enabling the extraction of the EEJ and CEJ components
even under disturbed geomagnetic conditions and to realizing continuous monitoring of the EEJ and CEJ.
As shown in Table 1, subtracted H component data at
an equatorial station: (DAV H )−(Dst), showed 7.5-, 14.5-,
and 35.3-day periodicities and did not contain any spectrum
components for periods longer than the 2 days included in
the geomagnetic indices and solar wind parameters, except
for the 35.3 days (it is a topic of future research to examine
if the periods 7.5 and 7.7 days have a common source ornot). It is reasonable that the same result could be obtained
if we adopted E Dst instead of Dst on deriving the spectrum pattern of (DAV H )−(Dst), because, as examined in
the previous section, magnetic variation with periods longer
than 2 days was highly correlated between E Dst and Dst.
This suggests that by subtracting E Dst from the H component data at each equatorial station, local effects can be deduced, which are interpreted in this paper as the long-term
variation of EEJ. The specifics of the local effects have notbeen fully understood. This is a topic for future research.
One possible cause of those local effects is the interaction
between the upper atmosphere and the ionosphere throughthe planetary or the Rossby normal mode wave (Forbes etal., 1995). The power spectrum shown in Fig. 3 exhibited similar pattern compared with the results presented by
Forbes and Leveroni (1992). They found spectrum peaks at
16 and 34 days. In the present analysis, the spectrum peakof the 35.3-day period appearing in the variation of (DAVH )−(Dst) corresponded to those of NSW , PD ; however, thisresult might not directly point to NSW or PD as a cause of
the ground 35.3-day variations. As suggested by Forbes andLeveroni (1992), this could be the effect of the solar radiation (F10.7 ) on the ionosphere and, consequently, on EEJ.
The spectral analysis for F10.7 radiation represented the existence of 22.5- and 35.3-day peaks. It supports the above
suggestion. We note that the EE-index may include the effect of long-term oscillations in the latitudinal location of
EEJ (e.g., Osborne, 1962). The frequencies of such oscillations would show themselves in the FFT results, but it is
a topic of future research to identify the cause of each FFT
component related to EEJ found in this paper. The realtime monitoring of EEJ by MAGDAS/CPMN will enable
and promote the investigation of mechanisms that result inthe long-term variations within the coupled system of the790

T. UOZUMI et al.: EQUATORIAL ELECTROJET (EE-) INDEX BY MAGDAS/CPMN

atmosphere-ionosphere.
In this paper, we have proposed a new geomagnetic index, the EE-index, which is derived from the data of theMAGDAS/CPMN equatorial network. This network allows for the first time real-time monitoring of geomagnetic
phenomena in the magnetic-equatorial region. This index
will provide information that should clarify the situation ofsolar-geospace coupling and atmosphere-ionosphere coupling along the magnetic equatorial region. Long-term andcontinuous study of the EE-index should greatly expand theunderstanding of the fundamental mechanisms pertaining tovarious equatorial geomagnetic phenomena.
Acknowledgments. We thank World Data Center for Geomagnetism Kyoto for providing geomagnetic indices: Dst and K p.
We thank the ACE MAG and SWEPAM instrument team and theACE Science Center for providing the ACE data. We thank thethe Herzberg Institute of Astrophysics for providing F10.7 solar radiation data. The PI of MAGDAS/CPMN project, K. Yumoto,
SERC, Kyushu Univ. very much appreciates 30 organizations andCo-investigators around the world for their ceaseless cooperation and contribution to the MAGDAS/CPMN project. Financial supports were provided by Japan Society for the Promotionof Science (JSPS) as Grant-in-Aid for Overseas Scientific Survey(15253005, 18253005) and for publication of scientific research
results (188068, 198055).

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