1 Introduction

Stewart (1882) suggested that geomagnetic daily variations are produced by electric currents in the upper atmosphere. This upper atmosphere (ionosphere) is electrically conducting as a result of the partly ionized plasma, which is generated by photo-ionization as a result of solar radiation and is the major source for solar-quiet (S q ) current. Schuster (1889); Chapman and Bartels (1940); Alex et al. (1992) observed that the prevailing wind in the upper atmosphere causes currents to flow at altitudes between 100 and 130 km, which is the S q wind dynamo current systems within the E region. This prevailing wind system will transport conducting plasma across the Earth’s magnetic field and this process generates a current which then results in geomagnetic perturbations at the ground. On magnetic quiet or less-disturbed days, a regular variation of the geomagnetic field, known as the solar quiet daily variation, S q , is recorded at mid-low latitudes (Chapman 1919; Matsushita 1967; Campbell 1989, 1997). Challinor (1968) interpreted the prevailing wind as a wind system fixed with respect to the position of the Sun, but varying in direction. Doumbia et al. (2007) inferred that the neutral wind that is mainly caused by migrating tides changes the structure of the equatorial electrojet (EEJ). Siebert (1961), Smaller and Butler (1961), Butler and Small (1963) have shown that the semi-diurnal oscillation as well as the diurnal component are largely accounted for in terms of the thermal forcing of the atmosphere arising from the absorption of solar radiation by atmospheric constituents and the rotation of the Earth within the radiation field of the Sun. Stening (1969, 1970) suggested that the driving source of the S q current system is a combination of migrating and non-migrating components. Brown and Williams (1969) demonstrated that the average diurnal features of the observed S q pattern are produced by a diurnal wind and the day-to-day variability is assumed to be generated by the semi-diurnal component of the winds.

Chapman (1951), Richmond (1973), Onwumechili (1997), Rabiu and Nagarajan (2008) suggested that two interactive layers of currents flow during daytime hours (0600–1800 LT) within the equatorial low latitude of the E-region. They observed that the daily variation of the geomagnetic field component at the equatorial station is due to the superimposition of the global S q current that is flowing eastward at a lower altitude of 113 ± 7 km and the EEJ current, which is flowing either eastward or westward at lower altitude of 107 ± 8 km. This indicates that if the S q and the EEJ currents are flowing eastward, the S q magnitude will be enhanced. Chapman (1956) explained that this increase in S q is due to a belt of stronger eastward current named the equatorial electrojet (EEJ), which is flowing within the equatorial ionosphere at E-region heights due to the enhanced conductivity created there by the inhibition of the vertical Hall polarization field. Also, if S q and EEJ currents are flowing opposite to one another such that the westward EEJ current exceeds the global eastward S q current, then, counter electrojet (CEJ) named by Gouin and Mayaud (1967) will be produced. This decrease on the horizontal (H) component value that resulted in a depression of the daily variation of S q H beyond the base or nighttime level was as well detected at equatorial stations by Hutton and Oyinloye (1970), Fambitakoye (1971), Onwumechilli and Akasofu (1972), Rastogi (1973). The flow of S q that will be observe at both the northern and southern hemispheres of the equatorial region will either increase or decrease in value during the daytime hours (abnormal flow). These interactive layers are the reason for the abnormal flow of global S q as two currents whorls at both the northern and southern hemispheres of the equatorial region. Increase in S q H value during daytime hours is known to occur at stations within the range of 3°N and 3°S from the magnetic equator (Egedal 1947; Onwumechili 1967).

Bartels and Johnston (1940), Onwumechili (1967), Rastogi (1974), Okeke and Hamano (2000), Rabiu et al. (2007, 2009) suggested that the magnitude of S q was sensitively dependent on local time and that the strength of S q current is enhanced at the dip equator. They observed that the time of maximum S q H around the equatorial region varies from 1000 to 1200 LT. Onwumechili (1967) concluded that the maximum variability of S q H at low latitudes are most frequent at 1200 LT during high solar active period because the conducting plasma of the upper atmosphere is active for a longer period and reaches its peak at the zenith (1200 LT) during high solar activity. The maximum variability of S q H are most frequent at 1100 LT during the low solar active period (Rastogi 1974). The day-to-day variability of S q can be attributed to the variations in the dynamo driving force that varies the total ionization, which results in changes in the ionospheric conductivity (Schlapp 1968). Campbell 1979 found that the ionospheric E-region dynamo during the daytime hours is absent during the nighttime hours (1900–0500 LT) and despite that, there are persistent smaller variations during nighttime hours. Onwumechili and Ezema (1977) and Rabiu (1996) reported that this nighttime variability is independent on solar local time but comes from a source that is different from the ionospheric daytime sources. They ascribed this apparent nighttime variability of S q H at equatorial low latitudes to the distant magnetospheric currents (decay of ring currents) during quiet periods. Chapman and Raja Rao (1965); Chandra et al. (1971); Campbell (1982); Rastogi et al. (1994); Okeke and Hamano (2000); Rabiu et al. (2007) reported that at the equatorial station, a clearer semi-annual and equinoctial maxima are observed from the annual and seasonal variations respectively. Chandra et al. (1971) attributed equinoctial maximum to the more intense EEJ, which is narrower at the equinoxes. The S q H semi-annual changes followed the seasonal ionization pattern (Campbell 1982). This shows that the semi-annual component is in the range of the S q H seasonal variation that depicts characteristics of the EEJ (Rastogi et al. 1994).

From the reports above, efforts of space physicists regarding the variability of S q H over the equatorial ionosphere has been mentioned and highlighted. To the best of our knowledge, the scarcities of geomagnetic data over the African equatorial sector have limited scientific reports on the geomagnetic activity in the zone for the past 50 years. Recently, Bolaji et al. (2011) employed geomagnetic field intensity data of declination (D) component for the year 2008 from the Magnetic Data Acquisition System (MAGDAS) that was installed at Ilorin, Nigeria in August, 2006. They suggested for the first time the existence of inter-hemispheric field-aligned currents (IHFACS) flow over the Nigerian ionosphere. From the same MAGDAS at Ilorin, Rabiu et al. (2009) had previously reported preliminary results on S q variability of the H, D and vertical (Z) components immediately after MAGDAS installation, which is based on one day of data and gave ideas about the current patterns of these geomagnetic components over Nigeria. This paper presents comprehensive morphology about the S q H variability over Ilorin, Nigeria during low solar active period. In view of this, the results from this work will update older results of S q H variability over the African equatorial regions.

2 Materials and Methodology

The year 2009 magnetic data that was employed to achieve the set targets of this work were obtained from the Magnetic Data Acquisition System (MAGDAS) facilities at the University of Ilorin, Ilorin (geographic latitude: 8.47°N, geographic longitude: 4.68°E, geomagnetic latitude: 1.82°S, geomagnetic longitude: 78.6°S), Nigeria. The data set from the MAGDAS consists of 1 min records of H, the geomagnetic north–south component that were converted to hourly values.

The averages of the minutes records of the geomagnetic elements of the H were taken at 1 h centered on the local time (LT). Nigeria is 1 h ahead of Greenwich Mean Time (GMT). Therefore, 1,200 UT is 1,300 LT in Nigeria.

The baseline (BL) value was deduced by calculating the hourly average value of the 4 h flanking the local midnight (0100, 0200, 2300 and 2400 LT). Therefore, the BL value for the horizontal (H) component is expressed as follows:

$$ BL = \frac{{H_{0100} + H_{0200} + H_{2300} + H_{2400} }}{4} $$
(1)

The BL value was further corrected to the nearest whole number in nano-Tesla unit. H 0100, H 0200, H 2300 and H 2400 are the hourly values of the H-components at 0100, 0200, 2300 and 2400 LT respectively. The hourly departure of H-component (Hhd) that is approximately equal to hourly solar quiet of H-component (S q H t ) was obtained by subtracting the BL value for a particular day from each of the hourly values for that particular day. Therefore, for a particular t hour,

$$ S_{q} H_{t} = H_{t} - BL $$
(2)

where t is the time in hours and ranged from 0100 to 2400 LT, S q H t is the solar quiet value for a particular hour and H t is the hourly H-component value for a particular hour.

The S q H t is further corrected for non-cyclic variation, an estimate such that the value at 0100 LT is the same as the value at 2400 LT (Vestine 1947; Rabiu 2000). This non-cyclic variation is achieved by making linear adjustment on the daily hourly values such that the S q H t value at 0100 LT, 0200 LT … 2400 LT are consider as M 1, M 2 … M 24 and taking

$$ \Updelta_{K} = \frac{{M_{1} - M_{24} }}{23} $$
(3)

The linearly adjusted values at these hours will be

$$ M_{1} + 0\Updelta_{K} ,M_{2} + 1\Updelta_{K,} M_{3} + 2\Updelta_{K} \ldots M_{23} + 22\Updelta_{K} ,M_{24} + 23\Updelta_{K} $$
(4)

Equation 3 and 4 can be re-presented as follow;

$$ S_{q} H_{t} (M) = M_{t} + (t - 1)\Updelta_{K} $$
(5)

This corrected hourly value of S q H t for a whole day will give the solar quiet daily variation of the H-component (S q H). This calculated diurnal variation was extended to all days in each month throughout the year 2009. The monthly median values (MS q H) and standard deviation (STD) values for each month are deduced from the S q H values.

The annual variation of the monthly median values of S q H (MS q H) is shown by a contour diagram plotted as a function of local time and months for the year 2009 over Ilorin. The seasonal variation is grouped into three seasons; December season (November, December, January and February), Equinox season (March, April, September and October), June season (May, June, July and August). Each season is estimated by averaging the MS q H values on each hour under a particular season. From the World Data Centre catalogue (2011) at http://wdc.kugi.kyoto-u.ac.jp/, on looking at the Disturbance storm time (Dst) and Planetary magnetic (Kp) indices for the year 2009, we found that this year is a low solar activity year with an average sunspot number of 3.1 and no major storm was produced except a moderate storm observed on 22nd July, 2009 (Dst = −78 nT and Kp = 4.3). Using a 3-hourly planetary magnetic indices (aa) from International Service of Geomagnetic Indices (2011) at http://www.cetp.ipsl.fr/des_aa.htm/, which determines the geomagnetic activity of the year 2009, quiet days with aa < 20 nT were selected from each month for this study.

3 Results and Discussions

3.1 Diurnal, Standard Deviation and Monthly Median Variations of S q H

The S q H values that are derived from the measured geomagnetic element of the horizontal intensity (H) (as described in the previous section) have been computed for all the days during the year 2009. There were a number of data gaps as a result of interruptions in the data recording, mostly due to power interruption. The available days, with magnitudes that aa < 20 nT (quiet days) in each month is shown with histogram graph in Fig. 1. From Fig. 1, month of February had fifteen quiet days. Twenty-one and twenty-two quiet days are available in the months of January and August respectively. For the month of April and May, twenty quiet days are available and the remaining months are above twenty-two quiet days.

Fig. 1
figure 1

Histogram of the number of days less than 20 nT (quiet days) in each month

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Time series line plots of the diurnal variations of S q H for the entire months of the year 2009 along with their corresponding standard deviation (STD) and monthly median variations (MS q H) are presented in Fig. 2 of panel (a) to (l). The entire plots showed the S q H variation during all days and MS q H variation; both on the left-handed side of the y-axis. The STD is plotted on the right-handed side of the y-axis, the base-line (zero) values are shown with a black line on each plot and they are further plotted together with local time (LT) on the x-axis. The S q H variations are given in green lines, MS q H variations are given in dotted black lines with asterisks and STD variations are given in red lines with downward-pointing triangles. The contour diagram showing annual variation of MS q H for all the month over all hours of the day for the year 2009 at Ilorin is presented in Figs. 3 and 4 shows the diurnal variation for different seasons. As can be observed from these plots (Fig. 2 panel a–l), variations of S q H, STD and MS q H are characterized by maximum daytime (0700–1700 LT) magnitudes, minimum pre-sunrise (0500–0600 LT) and nighttime (1800–0400 LT) magnitudes. From all the graphs in Fig. 2.0 panel a–l, the S q H have consistent minimum values during pre-sunrise hours between 0500 and 0600 LT. They rise steeply during the sunrise period (0700–0900 LT), peaks during the daytime mostly around 1000–1200 LT and subsides afterward.

Fig. 2
figure 2figure 2

Diurnal, standard deviation and monthly-median variation of \( S_{q} H \) in each month during a–d January–April, e–g May–August, i–l September–December, 2009

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Fig. 3
figure 3

Contour diagram of \( S_{q} H \) plotted as a function of local time and months for the year 2009 over Ilorin

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Fig. 4
figure 4

Daily variations of the seasonal mean of \( S_{q} H \) for different seasons over Ilorin

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The S q H during the pre-sunrise hour ranges from 1 to 20 nT with the maximum value observed in September at 0600 LT. The MS q H value is in the range of 1–4 nT between pre-sunrise hours of 0500 and 0600 LT. Maximum value of MS q H variation during the pre-sunrise hour of 0600 LT was observed in the month of September and have a value of 4 nT while the minimum values of MS q H at the same pre-sunrise hour was seen in March with a value of 1 nT.

Apart from these maxima and minima values of S q H and MS q H observed from the pre-sunrise towards the sunrise period, they are characterized with counter electrojet (CEJ).

This CEJ could be clearly identified from Fig. 2 panel a-l by a black line drawn on the base-line (zero) value of each plot. The LT variation of S q H below zero value depicts CEJ. Around this period; 0500–0700 LT in all the months, CEJ magnitude of S q H is ranged between −1 and −16 nT. The highest value (−16 nT) was seen in March at 0700 LT. CEJ were also observed from MS q H variations in April, May and June around 0500–0600 LT and ranged between −0.1 and −2.0 nT. The CEJ value of −2.0 nT was observed in April at 0500 LT. In general, the magnitudes of S q H and MS q H variations were found to be larger before midnight than before sunrise hours. This depicts that the conductivities are still low before sunrise and the neutral wind pattern due to solar thermal heating is not present. This absence of solar thermal heating causes the ionospheric conductivities to be weakest during pre-sunrise hours over all the days throughout the year 2009. These observations have been previously reported by Bartels and Johnston (1940), Onwumechili and Ogbuechi (1962), Onwumechili (1967), Rastogi (1974), Okeke and Hamano (2000), Rabiu et al. (2007, 2009) that S q H values during pre-sunrise hours are lowest compare to during daytime hours.

Figure 2 panel a–l shows that the hours at which S q H reached their peak values varies from 1 day to another and that its daytime magnitude is greater than its dusk, post sunset and pre-sunrise magnitudes. From all the months, maximum S q H daytime magnitude range is between 20 and 91 nT in July and March respectively while during the dusk through post sunset to pre-sunrise period, magnitude range is between 2 and 14 nT in July and February respectively. The daytime MS q H maximum value occurs near noon hours (1100 LT) in all the months. Exceptions to these maximum values of MS q H near noon hours were observed at noon (1200 LT) in January and December. The MS q H daytime magnitudes were found to range between ~35 nT in December and ~61 nT in October. It is clearer that the range in maximums during daytime hours of S q H and MS q H in all the months is greater than their range in minimums observed at nighttime (post-sunset and pre-sunrise) hours. Their greater magnitudes during daytime hours are attributed to continuous solar heating of the ionosphere as a result of prevailing wind system, which are maxima around noon. This highest solar heating intensity during the daytime hours with minimal loss rates contribute to their greater magnitudes. Our results are in closer agreement with the works of Bartels and Johnston (1940), Onwumechili and Ogbuechi (1962), Onwumechili (1967), Rastogi (1974), Okeke and Hamano (2000), Rabiu et al. (2007, 2009).

The percentage ratio of the number of days that CEJ events did not occur to the total number of the days between the hours of 1000 and 1200 LT in the entire months showed statistically that 100 % of their daily S q H variations showed no CEJ events (Fig. 2 panel a–l and 3). The CEJ characteristics that were observed in MS q H variation around 0500 and 0600 LT in the month of April, May and June were also seen in February, March, April, May, June, July, August, November and December around 1600–1800 LT. These CEJ seen during the morning and evening hours have different magnitudes and occurred at different times for all the days of the aforementioned months (Fig. 2.0 panel a–l). Around 1800 LT, highest CEJ magnitude of −4 nT was observed in July. This reveals that around 1800 LT, when the magnitudes of S q H and MS q H variation with LT are smaller, CEJ occur more easily because a small variation in S q H and MS q H will bring the value to negative range. Our results further show that these minimum magnitudes were seen during pre-sunrise and pre-sunset periods. The magnitudes and occurrences of CEJ events are greater during the pre-sunset than the pre-sunrise in all the months. Therefore, the CEJ events were observed when there are weaker S q currents. Such similar CEJ events at equatorial stations have been extensively reported in the earlier works of Gouin and Mayaud (1967), Hutton and Oyinloye (1970), Fambitakoye (1971), Onwumechilli and Akasofu (1972), Rastogi (1973, 1974)). They attributed CEJ phenomenon to the stronger westward current that exceeds the global eastward S q current. Rastogi (1962), Maynard (1967), Alex and Mukherjee (2001) reported that the CEJ events were more frequent with highest magnitude during the evening as compared to the morning hours during years of minimum solar activity. Rastogi (1974) reported that CEJ events were not observed in their results around noon hours. This kind of characteristic could be that the year under investigation, being a low solar activity year was close to or following years of maximum solar activity. These periods when there are absences of CEJ events in each month demonstrated that a global eastward S q current that is intense are dominant. Apart from these, it is clearer from our results (Fig. 2 panel a–l) that late reversals of nighttime westward to daytime eastward currents during sunrise period from most of the day-to-day variations in all the months resulted in CEJ. Exceptions to these late reversals were observed in May and June, where there are almost complete reversals of the nighttime westward currents of the day-to-day variations to daytime eastward currents around 0700–0800 LT.

The indexes of variability of S q H (STD) that were observed from the day-to-day variations ranged between ~1.5 and 13.5 nT. From this study, the STD was found to closely respond to all the diurnal variations of S q H in each month (Fig. 2 panel a–l). The STD variability showed minima pre-sunrise, maxima daytime and post-afternoon decrease till nighttime. The exception to these characteristics that were observed from STD variability during daytime hours was seen in April and July with a slight daytime depression around 1100 and 1200 LT. On the STD, this variation intersects around noon, such that the diurnal component of the daytime peak experiences a large phase shift caused by the migrating semidiurnal tidal mode and resulted in a peak before and after noon. Fesen et al. (2000) and Eccles et al. (2011) suggested that such a decrease in the strength of the noontime SqH varies approximately with a 14.75 day period, which is associated with a migrating semidiurnal tidal mode. From the dusk to the nighttime hours in all the months, our results revealed that a greater STD variation responded to greater magnitudes of the day-to-day westward currents. There are highest magnitudes of the westward currents in the range of −33 and −31 nT, which was observed in February around 1900 and 2000 LT respectively and resulted to highest nighttime enhancement of STD (~14 nT) at the same hours (Fig. 2 panel b). Similar behaviour of STD in February from the dusk till nighttime hours was as well observed in January and March, but, with lesser values (LOOK AT Stratospheric Sudden Warning [SSW]). The day-to-day variations have been discussed extensively by Schlapp (1968), Rabiu et al. (2007). They attributed their responses to the variations of the ionospheric processes and physical structures such as conductivity and winds structure, which are responsible for the S q variation. Brown and Williams (1969) demonstrated that the day-to-day variation of S q was generated by the semi-diurnal component of the winds. Campbell (1979), Onwumechili and Ezema (1977), Rabiu (1996, 2000), Rabiu et al. (2007) and Hibberd (1981) reported on S q H local time (nighttime) variation and suggested that it is independent of solar local time, but comes from a distant magnetospheric source during quiet times. These greater magnitudes of STD from the dusk to nighttime hours were found to likely have connection with the greater magnitudes of the nighttime westward currents from 1 day to another because they simultaneously occurred around the same period (dusk till nighttime hours). However, this could be indicator to suppression of onset of equatorial spread-F (ESF), scintillations and plasma bubbles build-ups. Haerendel and Eccles (1992) suggested that to sustain the F-region dynamo around sunset, additional EEJ current, which is generated from a westward electric field that will strengthen the F-region pre-reversal enhancement (PRE) is required. However, when CEJ occur during this pre-sunset period, a westward current is considered to flow in the pre-sunset E-region, in which case, the strengthening PRE suggested by Haerendel and Eccles (1992) would not occur. Similar results have been obtained and reported by Uemoto et al. (2010). They found that the PRE strengths and ESF onsets were found to be suppressed when the pre-sunset integrated EEJ (IEEJ) is negative owing to the evening CEJ. However, the precise forecast of ESF onsets requires the examinations of all the physical processes related to the ESF development and the seeding mechanism of the initial perturbation in detail. Several authors (Alex et al. 1989; McClure et al. 1977; Basu and Basu 1985; Whalen 2003, 2004; Retterer and Gentile 2009) have reported that the onsets of scintillations, plasma bubbles and ESF at low latitude are indicated by larger onset of PRE velocity during the dusk period. Another reason could be that the magnetic field intensity of the horizontal component during the dusk till nighttime periods is controlled by the magnetospheric ring current, which is directed westward and therefore produces negative perturbation likened to the CEJ.

3.2 Annual Variation of MS q H

The contour diagram in Fig. 3 shows the annual variation of the monthly median values of S q H (MS q H) plotted as a function of local time and months for the year 2009 over Ilorin. This annual variation of S q H values is plotted using its median values from January to December, 2009 over all the hours. There are data gaps from the deduced S q H, which did not give good representation of results regarding annual variation. Therefore, contour diagram made from the MS q H values are used. Maximum values of MS q H were observed between 1100 and 1200 LT. As earlier explained, from nighttime till pre-sunrise hours, minimum MS q H values with westward currents are observed on most hours of the entire months. Also, the highest maximum value of MS q H at peak period varies from ~35 to ~61 nT around 1100 and 1200 LT. The highest maximum MS q H value was seen in October (~61 nT), follow by September (~57 nT) and later March (~54 nT). Other months as well have high MS q H values at peak period but not as much as those mentioned. The legend colour-bar beside the contour plot (Fig. 3) shows the level of their differences in magnitudes. Semi-annual variation of MS q H values was observed in Fig. 3, they are two peaks of maximum MS q H values. These two peaks occurred around March and October at 1200 and 1100 LT respectively. From our results, higher values of MS q H are observed in March, April, September and October compare to the lower values observed in January, February, May, June, July, August, November and December, which resulted to semi-annual variation. Such a semi-annual variation was also observed in ionospheric quantities; total electron content (TEC) e.g. Bolaji et al. (2012). Our results reasonably correspond to the previous works of Chapman and Raja Rao (1965), Chandra et al. (1971), Campbell (1982), Rastogi et al. (1994), Okeke and Hamano (2000), Rabiu et al. (2007) at the equatorial stations. They observed similar semi-annual variation in their work. Campbell (1982) found that the S q H semi-annual changes followed the seasonal ionization patterns. This shows that the semi-annual component is in the range of S q H seasonal variation, which depicts characteristics of the EEJ (Rastogi et al. 1994). This indicates that MS q H are enhanced during the months of March, April, September and October compare to the months of January, February, May, June, July, August, November and December.

3.3 Seasonal Variation of S q H

From Fig. 4, all the seasonal mean peaked at 1100 LT with highest magnitude of 54 nT observed during equinoctial months; March, April, September and October. The solstices; December (November, December, January and February) and June (May, June, July and August) magnitudes are lower compare to the equinox. However, December solstice magnitude (48 nT) is greater than that of the June solstice magnitude (47 nT). Thus, the seasonal variation shows a semi-annual pattern, maximum in equinoctials months and minimum in solstice’s months. The possible mechanisms that could be responsible for this pattern have been previously discussed under annual variation. Despite that all the S q H seasonal variation reached their peak values at the same period (1100 LT), the highest magnitude seen during the equinox shows that greater intensity of S q H resulted from greater solar dynamo process.

Chapman and Raja Rao (1965), Chandra et al. (1971), Campbell (1982), Rastogi et al. (1994), Okeke and Hamano (2000), Rabiu et al. (2007) reported greater equinoctial maxima from their observed seasonal variations. Onwumechili and Ogbuechi 1967 and Chandra et al. 1971 attributed equinoctial maximum to the more intense S q H, which is narrower at the equinoxes. The minimum range value of ~−1 to 2 nT around 1700–1800 LT that was observed in June solstice is an evidence of partial seasonal CEJ. This partial seasonal CEJ during daytime resulted from series of strongest CEJ events that were observed from the month of May, June, July and August, which is a sub-division of June solstice.

4 Conclusions

This paper presents the variability of solar quiet of horizontal component (S q H), during low solar activity period of the year 2009. Minimum value of MS q H that were characterized by counter electrojet (CEJ) was observed during sunrise period, while the maximum value of S q H was observed during the daytime period (1100–1200 LT) and ranged from ~20 to 91 nT. STD depicts summary measure of the day-to-day variability of S q H in each month. It also gave clearer representation of their nighttime enhancements. Further study on this nighttime enhancement will be the topic of a future effort. The CEJ events were found to be frequent during the evening than the morning hours. Late reversal of nighttime westward currents to daytime eastward currents during sunrise period resulted to CEJ. The present study has demonstrated that semi-annual variation was revealed by the annual and seasonal studies. Seasonal CEJ was observed during evening daytime period of June solstice. Finally, this work has updated S q H variation over the African sub-region.