ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-535-2017Longitudinal variation of equatorial electrojet and the occurrence of its
counter electrojetRabiuA. Babatundetunderabiu2@gmail.comhttps://orcid.org/0000-0002-2734-5389FolarinOlanike OlufunmilayoUozumiTeijiAbdul HamidNurul ShazanaYoshikawaAkimasaCentre for Atmospheric Research, National Space Research and
Development Agency, Anyigba, NigeriaIonospheric and Space Physics Laboratory, Department of Physics,
University of Lagos, Akoka, Lagos State, NigeriaInternational Center for Space Weather Science and Education ICSWSE,
Kyushu University, 744, Motooka, Nishi-ku, Fukuoka, 819-0395, JapanNational University of Malaysia, Faculty of Science &
Technology, Selangor,
MalaysiaA. Babatunde Rabiu (tunderabiu2@gmail.com)7April20173535355451August201628February20177March2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/535/2017/angeo-35-535-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/535/2017/angeo-35-535-2017.pdf
We examined the longitudinal variability of the equatorial electrojet (EEJ) and the
occurrence of its counter electrojet (CEJ) using the available records of the
horizontal component H of the geomagnetic field simultaneously recorded
in the year 2009 (mean annual sunspot number Rz= 3.1) along the magnetic equator in the South American, African, and
Philippine sectors. Our results indicate that the EEJ undergoes variability
from one longitudinal representative station to another, with the strongest EEJ of
about 192.5 nT at the South American axis at Huancayo and a minimum peak of
40.7 nT at Ilorin in western Africa. Obtained longitudinal inequality in the EEJ
was explicable in terms of the effects of local winds, dynamics of migratory
tides, propagating diurnal tide, and meridional winds. The African stations of
Ilorin and Addis Ababa registered the greatest % of CEJ occurrence. Huancayo in South America, with the strongest electrojet strength,
was found to have the least occurrence of the CEJ. It is suggested that
activities that support strong EEJ inhibits the occurrence of the CEJ. Percentage
of occurrence of the CEJ varied with seasons across the longitudes. The order of
seasonal variation of morning occurrence does not tally with the evening
occurrence order at any station. A semiannual equinoctial maximum in
percentage of morning occurrence of the CEJ was obtained at Huancayo and Addis
Ababa. Only Addis Ababa recorded equal equinoctial maxima in percentage of
evening occurrence of the CEJ. The seasonal distribution of the occurrences of
the CEJ at different time regimes implies a seasonal variability of causative
mechanisms responsible for the occurrence of the CEJ.
Atmospheric composition and structure (instruments and techniques)Introduction
Space-based technology and applications have become an essential tool in many
countries, especially developing ones, making the world a
“global village”. Earth satellite communication and other applications of
satellite technology have made the study of space environment, including the
ionosphere, more popular in recent times. As a result, the study of
the equatorial ionosphere and its current systems has continued to gain much
interest due to the complexities associated with the region (Rabiu et al.,
2007).
The equatorial ionosphere over the dip equator is characterized by a number
of distinctive electrodynamic processes, which include an eastward current
named the equatorial electrojet (EEJ) (Chapman, 1951). The EEJ is considered as a
band of non-uniform intense eastward ionospheric current flowing within a
latitudinal extent of ±3∘ on either side of the dip equator at a
lower altitude E region centred at around 106 ± 2 km (Richmond, 1973;
Fambitakoye and Mayaud, 1976a, b; Forbes, 1981; Reddy, 1981; Stening, 1985;
Onwumechili, 1997; Rabiu et al., 2013). This is associated with the fact that
the magnetic lines of force around the dip equator are horizontal; thus, a
large vertical, electrical polarization field, which is primarily responsible for
the enhanced eastward currents, can be set up (Onwumechili, 1997).
On occasion during quiet periods, at certain hours of the day
particularly in the early morning and evening, the daytime EEJ strength
is weakened and reverses direction for a short period, after which it later
returns to its normal afternoon value before it disappears around sunset
(Gouin, 1962; Gouin and Mayaud, 1967). The counter electrojet (CEJ) is the EEJ reversal during magnetically quiet periods (Gouin
and Mayaud, 1967). This event was first observed during the study of the
magnetic records at Addis Ababa, Ethiopia, in 1962 by Gouin (1962). It manifests in
the magnetograms as a depression in plot of H versus time. Hanuise et
al. (1983) showed a consistent electrical connection between the CEJ and the
Sq current system, with two horizontal current vortices of opposite directions
that flow on each side of the noon sector, anticlockwise in the morning and
clockwise in the afternoon. These current vortices produce a poleward current
flow at low latitudes at noon (Gurubaran, 2002).
Since the discovery of the EEJ at Huancayo (Peru) as well as the CEJ at Addis Ababa
(Ethiopia) near the dip equator, studies across various longitudinal sectors
have shown that both phenomena display significant diurnal, seasonal,
day-to-day, and solar cycle and longitudinal variabilities (Richmond, 1973; Kane,
1976; Mayaud, 1977; Forbes, 1981; Reddy, 1981, 1989; Stening, 1995; Rastogi,
1989; Amory-Mazaudier et al., 1993; Mazaudier et al., 2005; Doumouya et al., 1998; Doumouya and Cohen,
2004; Luhr et al., 2004; Rabiu et al., 2011; Yizengaw
et al., 2014).
The strength of the EEJ and its width have been established to change with
longitude (Onwumechili, 1997; Rastogi, 1962). Jadhav et al. (2002) studied
the EEJ strength along the Indian and American sectors and showed that the
EEJ strength varies with longitude. Rabiu et al. (2011) clearly revealed that
along the African sector, the EEJ at the western sector appears weaker than the
EEJ at the eastern sector. This west–east asymmetrical behaviour in EEJ strength
in the African sector is further confirmed by Yizengaw et al. (2014) using
data from an array of different magnetometers. In addition, Yizengaw et
al. (2014) reported higher values of EEJ and E×B drift
distribution in the western American sector and decrement towards the eastern
longitudes all the way to the eastern African sector.
Chandrasekhar et al. (2014) noticed that a few studies have reported that
there is large day-to-day variability of the CEJ phenomena over a 45∘
longitude separation, which sometimes occurs over a large longitudinal
difference (Onwumechili and Akasofu, 1972; Mayaud, 1977). Kane and
Trivedi (1980) demonstrated the characteristics of CEJ events at two
locations in the Brazilian region across the east and west coasts of the
South American sector, separated by less than 30∘ longitude. Patil et
al. (1990a, b) reported more CEJ occurrence during low solar activity periods
than high solar activity periods in the Indian and American sectors.
Rangarajan and Rastogi (1993) investigated the afternoon CEJ events at
equatorial stations Addis Ababa and Kodaikanal and concluded that the
afternoon CEJ is localized in longitude, and on some occasions, the events
may not occur on the same day, even at locations separated by a narrow
longitude of 2 h.
Onwumechili (1997) concluded that the properties of S(2,2),
S(2,4) tidal modes suggest that they can contribute to the cause of the CEJ.
However, the contributions of vertical winds and local east–west neutral
winds with vertical shear are more obvious, whether or not they are of tidal
origins. He stated that the very high occurrence frequency of the CEJ near dawn
and near dusk appears to be partly due to late morning reversal and early
evening reversal of the vertical electric field.
Doumouya et al. (1998) reported CEJ occurrences along the western African
longitudes and observed that morning as well as afternoon occurrences of the CEJ
show seasonal variability. Alex and Mukherjee (2001) compared CEJ events
along two equatorial stations, Trivandrum and Addis Ababa, which are
40∘ apart in longitude, and showed that there are some differences
in CEJ events when observed in different longitudinal sectors. Bolaji et
al. (2014) reported more frequent occurrence of morning CEJ events at Ilorin
in 2009 and attributed this to the late reversal of westward to eastward
currents. Rabiu et al. (2017) investigated the simultaneity and asymmetry in
the occurrence of the CEJ along African longitudes and most frequently found
(about 77 %) simultaneous occurrence of CEJ in the morning at two
equatorial stations, one in the west of the continent and the other in the
east.
Stening et al. (1996) suggested an association of sudden stratospheric
warming (SSW) with the occurrence of CEJ events during their study of the
occurrence of the CEJ during the winter months. A distinctive pattern in the
equatorial daytime E×B drift during the SSW years was
also reported by Chau et al. (2009). Balan et al. (2012) also showed that the
low-latitude thermosphere and ionosphere record very clear effects of the
SSW taking place in the polar stratosphere during
an unusual solar minimum. The clear effects at the unusual minimum are
interpreted in terms of the 8 h tide during the warming events.
The need to do more studies on the CEJ, in order to understand its features and
the morphology of both the EEJ and the CEJ, along various
longitudinal sectors has been emphasised by some authors such as Bolaji et
al. (2014) and Chandrasekhar et al. (2014), among others. The main purpose of
the present study is to investigate the longitudinal variability of the
EEJ and the occurrence of its CEJ along the geomagnetic equatorial belt using well-spaced
stations with sufficient data availability from the South American, African, and
Philippine magnetic equatorial sectors. This study also compares the spatial
and temporal morphology of EEJ and CEJ occurrence along various
equatorial sectors.
Geomagnetic observatories and their corresponding
coordinates.
Obtained from ground-based magnetometer observatories of the Magnetic Data Acquisition System
(MAGDAS) of the International Centre for Space Weather Science and Education,
Kyushu University Japan, as well as the INTERMAGNET global network, were 1 year of data of the horizontal components (H) of the geomagnetic field for
the year 2009 (mean annual sunspot number Rz= 3.1). These
observatories are located along the South American, African, and Philippine
longitudes within the magnetic equatorial zone. The details of the geographic
and geomagnetic coordinates of the selected stations and their distributions
are shown in Table 1.
The EEJ produces a strong enhancement in the
H component magnetic field, measured by magnetometers located within
±3∘ of the magnetic equator; therefore, measuring this
perturbation in equatorial magnetometers could provide a direct measure of
the EEJ (Onwumechili, 1997; Yizengaw et al., 2014). Ilorin, Addis Ababa,
Huancayo, Davao, Yap, and Langkawi are located within the equatorial
electrojet belt, which lies around ±3∘ latitude of the magnetic
equator, while Lagos, Nairobi, Trelew, and Hualien are outside the electrojet
belt.
International quiet days engaged in this work and their
equivalent Ap indices.
Since EEJ can be accurately considered during quiet conditions, this research
employed the data obtained on magnetically quiet days (Ap ≤ 3). Table 2 present the dates of the quiet
days engaged in this work with their corresponding Ap indices. The concept of
local time was used throughout this analysis to ensure a measure of accuracy
as the stations vary from one local time to another.
The H components of geomagnetic field stations located within and outside
of the EEJ region are compared and the difference of
the H component within and outside the EEJ, ΔH, was calculated. This
difference is the only part of the H component field that is related to the
EEJ current contribution (Anderson et al., 2004). Thus, hourly equatorial
electrojet strength at any EEJ station (EEJs) is given by the difference
between the H field at that location and another station outside the EEJ zone
(EEJo) but close in longitude to it. Therefore hourly EEJ at various EEJ
stations are
EEJ at Huancayo:EEJtHUA=HtHUA-HtTRWEEJ at Ilorin:EEJtILR=HtILR-HtLAGEEJ at Addis Ababa:EEJtADD=HtADD-HtNABEEJ at Langkawi:EEJtLKW=HtLKW-HtHLNEEJ at Davao:EEJtDAV=HtDAV-HtHLNEEJ at Yap:EEJtYAP=HtYAP-HtHLN,
where Ht(HUA), Ht(TRW), Ht(ILR),
Ht(LAG), Ht(ADD), Ht(NAB),
Ht(LKW), Ht(HLN), Ht(DAV), and
Ht(YAP) are hourly values of horizontal geomagnetic field
component at local time t hours at Huancayo, Trelew, Ilorin, Lagos, Addis
Ababa, Nairobi, Langkawi, Hualien, Davao, and Yap respectively;
EEJt(HUA), EEJt(ILR), EEJt(ADD), EEJt(LKW),
EEJt(DAV), and EEJt(YAP) are hourly EEJ strength at local time t
hours at Huancayo, Ilorin, Addis Ababa, Langkawi, Davao, and Yap
respectively.
It is pertinent to mention that LAG station at -3.04∘ geomagnetic
latitude is 1.22∘ south of ILR, barely outside the EEJ strip and may
still have some EEJ magnetic effects. However, we were constrained to stick
to the choice of the LAG–ILR station pair as LAG is the farthest station we
could get along the same longitude as ILR within the observational network
that we engaged in this work. Therefore, our choice of the LAG–ILR station pair,
may have consequences for the accuracy of our representation of the EEJ at
ILR.
These hourly values were further corrected for non-cyclic variation following
Price and Stone (1964) in a method well explained in Rabiu et al. (2007, 2011).
Furthermore, since the CEJ is the reversed
EEJ current, Gouin (1962), CEJ in daytime at any time t is
estimated when EEJ at that particular time is negative, that is
CEJt=(EEJt<0).
In addition to using simple computer code to detect occurrence of the CEJ,
physical inspection of plots of daily hourly profiles of electrojet strength
EEJ (H) was also carried out for all the available days. Due to lack of
data at some stations, monthly total number of days analysed vary from one
station to another. Table 3 shows the monthly total number of days on which
data were available for all the stations.
Days of data availability (NA means “data not available”).
MonthNumber of days with data at different stations HUAILRADDLKWDAVYAPJan1038101010Feb1081010108Mar10810959Apr106109109May1061010109Jun1078NA1010Jul1066299Aug1086NA1010Sep109910105Oct101086107Nov10NA9NA1010Dec10NA8NA1010Total1207110266114106
Variation of the equatorial electrojet (EEJ) among the stations.
Percentage occurrence of CEJ in a particular batch was estimated as
%occurrence of CEJ=number of occurrence of CEJTotal number of days for which data were available×100.
Occurrence of morning CEJ (MCEJ) in % was considered for the duration between
06:00 and 11:00 LT, while % occurrence of afternoon CEJ (ACEJ) was
considered for the duration between 13:00 and 18:00 LT.
Results and discussionEEJ characteristics at each station
The variability of EEJ across the stations between 06:00 and 18:00 LT is
presented in Fig. 1. The contour plot evidently shows the diurnal
distribution of EEJ at each of the stations.
It is obvious from Fig. 1 that the EEJ undergoes variability from one
longitudinal representative station to another. Figure 2 shows the variation
of the maximum EEJ attained in any location with longitude. This puts the
values at the stations as 192.5 nT on 6 October 2009 at HUA
(-75.22∘), 40.7 nT on 21 October 2009 at ILR (4.68∘),
75.1 nT on 11 December 2009 at ADD (38.77∘), 100.9 nT on
19 September 2009 at LKW (99.78∘), 144.2 nT on 19 September 2009 at
DAV (125.0∘), and 103.6 nT on 4 November 2009 at YAP
(138.08∘). These figures imply that we obtained the strongest EEJ of
about 192.5 nT in the South American axis at Huancayo and a minimum peak of
40.7 nT at Ilorin in western Africa. As noted earlier, the EEJ strength at
Ilorin might be significantly underestimated due to the choice of the LAG–ILR
station pair as the two stations are barely separated by 1.22∘ and
LAG is just at the edge of the EEJ strip. However, lack of adequate
station pairs in this longitude sector makes the LAG–ILR choice unavoidable in
the present analysis. It is hoped that this would be corrected in future
work. Furthermore, Fig. 3 illustrates the variability of the amplitude of
the maximum CEJ strength in the morning and afternoon from one station to
another.
Variation of the maximum EEJ attained in any location with
longitude.
Longitudinal inequality of EEJ, which is well displayed in Figs. 1 and 2,
underscores the effects of local winds in driving the EEJ according to
Stening (1985, 1995). Doumouya et al. (1998, 2003), Jadhav et al. (2002),
Rabiu et al. (2011), and Alken and Maus (2007), among others, had reported
longitudinal variation of EEJ using different data sources and discussed the
longitudinal inequality in terms of dynamics of migratory tides, propagating
diurnal tide, and meridional winds. Jadhav et al. (2002) attributed the
longitudinal inequality found in EEJ strength to non-migratory tides using Ørsted satellite magnetic
field data. Luhr et al. (2004) engaged CHAMP satellite
data and deduced that the longitude dependence of the EEJ intensity can be
explained by varying cross-sectional area of the Cowling channel. Lühr et
al. (2008) discussed the influence of non-migrating tides on the longitudinal
variation of the EEJ by using the climatological model of
EEJ derived earlier from Ørsted, CHAMP, and SAC-C satellite measurements by
Alken and Maus (2007). Firstly Rabiu et al. (2011) and later Yizengaw et
al. (2014) showed that EEJ strength is higher in eastern Africa than western
Africa,
with about 30∘ longitudinal difference. The longitudinal inequality
in the maximum strength of the CEJ either in the morning or afternoon across the
stations, as displayed in Fig. 3, is a reflection of the longitudinal
dependence of the EEJ. South American stations at Huancayo recorded the maximum
CEJ strength just as it is known for its strong EEJ. The longitudinal
variability in the strength of the EEJ obtained from our ground-based results
here confirmed the reproducibility of the satellite-based results from other
sources.
Variation of the maximum CEJ strength attained at any location
with longitude.
Percentage occurrence of morning CEJ.
Percentage occurrence of afternoon CEJ.
Occurrence of CEJ
Figures 4 and 5 presented the % occurrence of MCEJ and %
occurrence of afternoon CEJ in different months along different longitudes.
Figure 6 shows the annual % occurrence of MCEJ and ACEJ at all
locations. Outstandingly clear in all the figures is the greater % of
occurrence of the CEJ at the African stations of ILR and ADD than elsewhere. The
greatest % occurrence of MCEJ was found at the eastern African station
of ADD, while the greatest % occurrence of afternoon CEJ was found at
the western
African station of ILR. ADD recorded 100 % of days with MCEJ for
almost all the days of availability of data, except in the months of June,
November, and December. Meanwhile, ILR in western Africa registered 100 %
occurrence of Afternoon CEJ in all the months of January, April, July,
August, October and November. Figure 4 supported the fact that CEJ
occurrence is more predominant in the African sector than other sectors
considered in this work. In the overall analysis, the station with the
strongest electrojet strength was found to have the least occurrence of the CEJ.
It has often been reported by Onwumechili (1997), and some other authors,
that the CEJ rarely occur during periods when the EEJ is strong. It may be right to
assert that activities that support strong EEJ do inhibit occurrence of the CEJ.
Yearly percentage of occurrence of the CEJ.
The seasonal dependence of occurrence of the CEJ is presented in Figs. 7 and 8,
where it is clear that percentage of occurrence of the CEJ varies with seasons
across the stations. The order of seasonal variation of morning occurrence
does not tally with the evening occurrence order at any station. For
instance, there are equinoctial maxima at the two equinoxes when considering
morning CEJ at Huancayo and Addis Ababa. These maxima have equal values at
the two stations: 100 % at Addis and 75 % at Huancayo. No other
stations had these double equinoctial maxima. However, Ilorin has March
equinoctial maxima and recorded minimum morning occurrence of the CEJ at the
September equinox. LKW, DAV, and YAP all recorded minimum morning occurrence
at the March equinox. Minimum morning occurrence occurred at the December
and June solstices at Addis Ababa and Huancayo.
Seasonal variation of morning CEJ, MCEJ (D_solstice
means December solstice, Equinox_m means March equinox,
J_solstice means June solstice, Equinox_S
means September equinox).
Seasonal variation of afternoon CEJ, ACEJ (D_solstice means December solstice, Equinox_m means March
equinox, J_solstice means June solstice,
Equinox_S means September equinox).
Considering evening occurrence of the CEJ as shown in Fig. 8, only Addis Ababa
recorded equal equinoctial maxima at the two equinoxes. Langkawi and Yap
have maximum evening occurrences at the September equinox, followed by
the December solstice. Ilorin recorded a March equinoctial maximum, while Huancayo
and Davao have December solstitial maxima. Minimum evening occurrence was
found in Davao in September equinox.
Figures 9 and 10 display the seasonal variation of the strength of the
morning and ACEJ respectively across the stations. It is obvious
that the seasonal variation of the occurrence of the CEJ really has no direct
correlation with the seasonal variation of its strength. Seasonal variability
exists in the strength of the CEJ at the two dispensations and any station and
does not have uniform order of seasonal dependence, unlike EEJ, as reported in
various literatures such as Onwumechili (1997) and the references therein.
Seasonal variation of the strength of the morning CEJ across the
stations. D_solstice means December solstice,
Equinox_m means March equinox, J_solstice
means June solstice, and Equinox_S means September equinox.
Seasonal variation of the strength of the afternoon CEJ across
the stations. D_solstice means December solstice,
Equinox_m means March equinox, J_solstice
means June solstice, and Equinox_S means September equinox.
Rabiu et al. (2017) noted that the seasonal distribution of the occurrences
of the CEJ at different time regimes implies a seasonal variability of anyone or
a combination of the following mechanisms responsible for the occurrence of the CEJ
itself: height-varying local winds capable of causing the reversal in the
current system (Richmond, 1973), gravity wave-associated vertical winds
(Raghavarao and Anandarao, 1980), global-scale tidal winds with appropriate
phase combination (Forbes and Lindzen, 1976; Marriott et al., 1979; Forbes,
1981; Singh and Cole, 1987; Stening, 1989; Somayajulu et al., 1993),
high-latitude stratospheric warming events (Stening et al., 1996; Vineeth et
al., 2009), daytime vertically driven downward electric field (negative Ez)
(Onwumechili, 1997), and semidiurnal tide (Gurubaran, 2002; Sridharan et al.,
2002, 2009).
The variances in order of seasonal variation of percentages of occurrence of
the CEJ at different longitudes could be related to local effects such as seasonal
variability of local and neutral winds. In addition, Chandrasekhar et
al. (2014) in their study of occurrence of the CEJ at two new remote electrojet
sites in India separated by 15∘ longitude presented evidence of
the CEJ being caused by local perturbations in their results. They considered the
following as possible factors of CEJ occurrence not being simultaneously
universal: non-migrating eastward- and westward-propagating diurnal tides and
local meteorological phenomena associated with upper mesospheric temperature,
wind, and density variations.
Rabiu et al. (2017) found a situation where 41.3 % of the afternoon CEJ
in western Africa has counterpart occurrence in the morning on the same day in
eastern Africa instead of in the afternoon, a scenario they described as Asymmetry-2;
they attributed this longitudinal variability in the local time of occurrence
of the CEJ along these longitudes to the differences in meridional currents
across different longitudes. Chandrasekhar et al. (2014) found
non-simultaneous occurrence of the CEJ between two Indian stations separated by
15∘ longitude and thus threatened the CEJ longitudinal extent of
30∘ earlier reported by Kane (1973), Kane and Trivedi (1980), and
Rastogi (1974). Rastogi and Yumoto (2006) reported that the direction of
meridional current varies significantly with longitude and, possibly, with
the local time and season. Rabiu et al. (2017) ascribed the variation in
percentage of occurrence of the CEJ with months to the longitudinal variability
of thermospheric winds that drive the ionospheric dynamo even on magnetic
quiet days, as mentioned by Vichare and Richmond (2005), which could be due to
atmospheric solar tides that are not sun synchronous (Hagan and Forbes, 2002,
2003). Other factors that have been reported to be responsible for variation
in percentage of occurrence of the CEJ along different longitudes include
eastward zonal winds (Ramkumar et al., 2002); gravity wave–tidal
interactions and vertical coupling process in the mesosphere – lower thermosphere
– ionosphere (MLTI), resulting in neutral winds (Vineeth et al., 2007; Liu and
Watanabe, 2008) via DE3 tidal modulation of the E layer dynamo, changes in
temperature, wind and density variation in the upper mesospheric region due
to the influence of tides, gravity, and planetary waves from the stratosphere
and troposphere (Vineeth et al., 2012), which strongly influence ionospheric
conductivity over short spatial scales of approximately 1600 km; differences in local
wind shears that modify jet fields (especially during the noon hours) (Reddy
and Devasia, 1981); and changes in neutral winds (Sridharan et al., 2002;
Vineeth et al., 2007).
Conclusions
This work investigated the longitudinal variability of the equatorial
electrojet (EEJ) and the occurrence of its counter equatorial electrojet
(CEJ) along the geomagnetic equatorial belt and made use of data from the
following electrojet stations whose longitudes are given in parentheses:
Huancayo HUA (-75.22∘), Ilorin ILR (4.68∘), Addis Ababa ADD
(38.77∘), Langkawi LKW (99.78∘), Davao DAV (125.0∘),
and Yap YAP (138.08∘). The study compared the spatial and
temporal morphology of EEJ and CEJ occurrence along various equatorial
sectors.
Our results indicated that the EEJ undergoes variability from one
longitudinal representative station to another, with the strongest EEJ of about
192.5 nT in the South American axis at Huancayo and a minimum peak of
40.7 nT at Ilorin in western Africa. Obtained longitudinal inequality in the EEJ
was explicable in terms of the effects of local winds (Stening, 1985, 1995),
dynamics of migratory tides, propagating diurnal tide, and meridional winds
(Doumouya et al., 1998, 2003; Jadhav et al., 2002; Rabiu et al., 2011; Alken
and Maus, 2007).
Of all the sectors considered, the African stations of ILR and ADD registered the
greatest % of occurrence of the CEJ than elsewhere. The greatest %
occurrence of MCEJ was found at Addis Ababa (eastern Africa), while
the greatest % occurrence of afternoon CEJ was found at Ilorin (western
Africa). Thus, CEJ occurrence is more predominant in the African sector than
other sectors considered in this work. Huancayo in South America, with the
strongest electrojet strength, was found to have the least occurrence of
the CEJ. It may be right to assert that activities that support strong EEJ do
inhibit the occurrence of the CEJ.
We reported a variation of percentage of occurrence of the CEJ with seasons
across the longitudes. The order of seasonal variation of morning occurrence
does not tally with the evening occurrence order at any station. Semi-annual
equinoctial maxima in percentage of morning occurrence of the CEJ were obtained
at Huancayo and Addis Ababa. No other stations had these double equinoctial
maxima. However, Ilorin has March equinoctial maxima and recorded minimum
morning occurrence of the CEJ at the September equinox. LKW, DAV, and YAP all
recorded minimum morning occurrence at the March equinox. Minimum morning
occurrence occurred at the December and June solstices at Addis Ababa
and Huancayo respectively. Only Addis Ababa recorded equal equinoctial
maxima in percentage of evening occurrence of the CEJ. The seasonal distribution
of the occurrences of the CEJ at different time regimes implies a seasonal
variability of causative mechanisms responsible for occurrence of the CEJ.
The data collected at Huancayo, Trelew, and Addis Ababa can be obtained from www.intermagnet.org.
While those of Lagos, Ilorin, Nairobi, Langkawi, Davao, Yap, and Hualien are obtainable at http://magdas.serc.kyushu-u.ac.jp/.
The authors declare that they have no conflict of
interest.
Acknowledgements
The results presented in this paper rely on the data collected at Huancayo,
Trelew, and Addis Ababa. We thank the host institutes for supporting their
operation and INTERMAGNET for promoting high standards of magnetic
observatory practice (www.intermagnet.org). The Magnetic Data
Acquisition System (MAGDAS) data used for this paper were obtained from the
International Center for Space Weather Science and Education (ICSWSE), Kyushu
University, Fukuoka, Japan. MAGDAS was supported by the Japan Society for the
Promotion of Science (JSPS) JSPS KAKENHI grant no. 268022. All the hosts
and management of the MAGDAS facilities at the stations whose data were
engaged in this research are also acknowledged. Nurul Shazana Abdul Hamid was
supported by grants GGPM-2015-020 and FRGS/1/2015/ST02/UKM/02/1.
Akimasa Yoshikawa was supported in part by the JSPS Core-to-Core Program (B.
Asia-Africa Science Platforms), Formation of Preliminary Center for Capacity
Building for Space Weather Research and JSPS KAKENHI grants 15H05815. The
authors remain grateful to the anonymous reviewers for the positive
criticism. The topical editor, J.
Makela, thanks the two anonymous referees for help in evaluating this paper.
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