INVESTIGATING THE VARIATIONS OF HORIZONTAL (H) AND VERTICAL (Z) COMPONENTS OF THE GEOMAGNETIC FIELD AT SOME EQUATORIAL ELECTROJET STATIONS

This research is monitoring equatorial geomagnetic current which causes atmospheric instabilities and affects high frequency and satellite communication. It presents the variations of Horizontal (H) and vertical (Z) component of the geomagnetic field at some Equatorial Electrojet (EEJ) Stations during quiet days. Data from five (5) observatories along the magnetic equator were used for the study. Daily baseline values for each of the geomagnetic element 𝐻 and Z were obtained. The monthly average of the diurnal variation and the seasonal variations were found. Results showed that the variations of the geomagnetic element of both H and Z differ in magnitudes from one stations to another along the geomagnetic Equator due to the differences of their geomagnetic latitude. The Amplitude curves for S_q (Z) are seen to be conspicuously opposite to that of S_q (H), and there is absence of CEJ in Z-Component but present in H-Components. The S_q values during the pre-sunrise hours are low compare to daytime hours. Minimum variations of dH was observed during June solstice and maximum variations was observed during Equinox season. This study shows that daily variations of (H) and (Z) occur in all the stations. The enhancement in H is as a result of EEJ current.


INTRODUCTION
examined the variability of equatorial ionosphere by using ground based geomagnetic field data of H and Z obtained at the equatorial station of Ibadan.The results showed that values of Sq daily variation rises from the early morning period to maximum at about local noon and falls to lower values towards evening, hence the ionospheric current responsible for the magnetic field variations was inferred to build up at the early morning periods and attain maximum intensities at about local noon .Previous studies on 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.The day to day variability of the geomagnetic field elements at the African longitudes has been studied for the year 1987 using geomagnetic data obtained from four different African observatories.The analysis was carried out on solar quiet days using hourly values of the H and Z, geomagnetic field values.The results of this study confirm that is a very changeable phenomenon, with a strong day-to-day variation.This day-to-day variation is seen to be superimposed on magnetic disturbances of a magnetospheric origin (Obiekezie, et al., 2013).Under quiet geomagnetic conditions the variations of the three geomagnetic elements H, Z, and D from one day to the next in June, October, and December 1986 at eight Indian observatories from about 0° to 22° dip latitude was studied.The day to day variability was also measured by sequential Variability, (SV).In all the three months, the magnitude of day to day variability in H, Z, and D had a diurnal variation with maximum around local noon and minimum in the night.This is most likely Controlled by the diurnal variation of ionospheric conductivity.In the worldwide part of S_q (WS_q) zone, the SV in H, Z, and D was smaller in October than in June and December 1986, but SV(Z) is greater than SV(H) and SV(D) in all the three months.In the equatorial electrojet (EEJ) zone, the SV due to the EEJ alone in alone in H, Z, and D is greater in June than in October and December contrary to the seasonal variation of S_q (H) and S_q (Z) in the EEJ zone.The SV(D) due to the EEJ alone had a surprising large magnitude.There is evidence that the day to day variability of EEJ and the 〖WS〗_q are not in phase and consequently combine somewhat destructive within the EEJ zone (Okeke and Onwumechili., 1995).

Data and Method of Analysis
Magnetic data for 2008 was used for this study.Magnetic data Acquisition System (MAGDAS) of some stations within the magnetic equatorial belt which the University of Ilorin, Ilorin (geographic latitude: 8.470 N, geographic longitude: 4680 E, geomagnetic latitude :1.820S, geomagnetic longitude: 78.600 W), Nigeria is one of them.The geomagnetic field horizontal (H) and vertical (Z) components data in minutes are converted to hourly values using the MATLAB program.Hence hourly average of horizontal and vertical components was obtained.Stations along the geomagnetic equator were chosen because the aim is to investigate the variation of H and Z components of the geomagnetic field under quiet conditions at some selected geomagnetic observation along the magnetic equator.For each month of the year, there are ten (10) International quietest days (IQDs) and five (5) most disturbed (IDDs).Magnetic data for five (5) quietest days of each month for the year 2008 were used in this study (available at www.ga.gov.au).These days were considered based on magnetic activity index k_p .The k_p index for quietest days is within the range of 0-4.The concept of local time was used throughout the analysis as the stations might be few hours ahead of the Greenwich Meridian Time (GMT).Stations considered were indicated by white star color Figure 1.Table 1 shows the coordinates of the stations used in the study.
Where  1 and  2 are values of geomagnetic element Z at 1hr LT and 24hr LT respectively and  0 is the daily baseline for the geomagnetic element Z which is the mean values of the hourly values at 24hr LT and 1hr LT.The daily baseline  0 for each station on each quiet day was subtracted from the hourly values   to get the hourly departure from the midnight for a particular day.That is;  =   −  0 (2) Where  = 124,  gives the measure of the hourly amplitude of the variation of Z, which is also the solar daily variation in Z ; and   () is  during quiet times.The monthly mean values were derived from the mean of the diurnal variation for the five quietest days in each month.Similarly for the horizontal component (H);  0 = 1 2 ( 24 +  1  =   −  0 Where  = 124, dH gives the measure of the hourly amplitude of the variation of H, which is also the solar daily variation in H and   () is dH during quiet times.

RESULTS AND DISCUSSION Daily Variation 𝑺 𝒒 of H for 2008
The result shows a steady increase in dH at pre-sunrise reaching a peak around local noon almost in a regular pattern and decrease at post-sunset.These features are in conformity with the works of Rastogi and Iyer (1976).The   () variation pattern agrees with the earlier works of Onwumechilli (1960) and Mtshushita (1969) and can be attributed to the variabilities of ionospheric processes and physical structure such as conductivity and wind structure, which are generally responsibility for   () variation.The variation during the daytime which is always higher than nighttime for all the stations is attributed to the afore mentioned ionospheric process and as well as enhancement dynamo action at their respective regions (Onwumechili, 1997).Figure 2 shows the hourly plots of   () of the stations.The buildup flank in the morning hours is steeper than that of the decay phase in the night hours.This was also observed and documented by Rabiu et al., (2009) and Akpaneno & Adimula, (2015) in their magnetic field measurement using MAGDAS.The daily variations of are characterized by maximum day time (0700 to 1700 LT) magnitude, minimum pre-sunrise (0500 to 0600 LT) and night time (1700 to 2400 LT) magnitude, from all the plots in Figure 2. The variational trends is observed between Stations may be attributed to the difference in their latitudinal location (Obiekezie et al;2013).This result is quite in agreement with the work of Bolaji et al (2013) and that of Akpaneno and Adimula (2015).In Figure 4 LKW shows the maximum of dH 110 nT on 9 th October and peak around 1200 LT, minimum at 70 nT on 25 th October and peaks around 1100 LT.In this Figure the amplitude of LKW is seen to be higher than EUS, ILR, DAV and YAP (H) Amplitudes.This high amplitude could be as a result of influence of Equatorial electrojet current (EEJ).The EEJ current is an east-west current which is seen flowing positive in the morning thus, causing an enhancement in the (H) values of station within the EEJ region.This work is quite in agreement with the work of Obiekezie et al (2013).Apart from this maximum and minimum value of (H) observed from the pre sun-rise towards the sun rise period, they are characterized with counter electrojet (CEJ).The CEJ is the local time variation of (H) below zero.Around this period, the CEJ normally occur in both pre sunrise and pre sunset which is 0700 LT to 0800 LT for pre sunrise and 1600 LT to 1700 LT for pre sunset.The CEJ magnitude of   was ranged between -1 to -31 nT.The highest value -31 nT was seen on 02/08 at 0700 LT.Generally, the magnitudes of variation 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 the solar thermal heating is not present.This absence of solar thermal heating causes the ionospheric conductivity to be weaker during pre-sunrise hours over all the days throughout the year 2008.These observations have been previously reported by Bartel andJohnson (1940), Onwumechilli (1967), Rastogi (1974), Okeke and Hamano (2000), Rabiu et al (2007Rabiu et al ( ,2009) ) and Akpaneno and Adimula (2015) that values during the pre-sunrise hours are lower compare to daytime hours.The CEJ occurred on 09/10, 18/10 and 25/10 respectively.On 09/10 CEJ events occurred in the pre sun rise period at LKW and DAV.LKW show the minimum CEJ magnitude of -8 nT and peaks around 0800 LT while DAV show the maximum CEJ at -10 nT and Peak around 0700 LT.On 18/10, the CEJ occur in both pre sun rise and pre sunset.At the pre sun rise DAV show the maximum values of dH at -8 nT and peak around 0700 LT, YAP show the minimum at -3 nT and peak around 0700 LT.But at pre-sunset both of the stations take the maximum at -19 nT, peaks around 1600 LT.On 25/10 LKW take its maximum at -2 nT and peaks around 0800 LT in the pre sun rise.On 24/10 the during the pre-sunrise hour range from 1 to 5 nT which has a maximum values.It is observed in this Figure that LKW has the highest magnitude of 110 nT on 09/10 while ILR lowest magnitude of 48nT on 27/10 and peaks around 1200 LT and 1000 LT respectively.Figure 2 the range is about 37 nT.On 29/09, the variation is between 88 nT to 100 nT; the range is about 12 nT.It is observed that dH between two consecutive quite days could be very large and sometimes very small and occasionally there could be no contrast at all, so dH show remarkable day to day variability of Equinox season.These observations have been previously reported by Akpaneno and Adimula (2015).The amplitude curves for   ( Z) are seen to be conspicuously opposite to that of   ( H), and there is absent of CEJ. Figure 3 shows the hourly plots of   ( Z) for some of the stations.The daily variation of are characterized by maximum day time 0800 to 1400 LT magnitude, and minimum pre sunrise 0700 to 0900 LT and night time 1700 to 2400 LT magnitudes for all the plots.In Figure 3 (d), it was observed that dZ during the Equinox Months is also higher than the solstice Months.It was also observed that, latitudinal positions of the station also affected the Magnetic field variability.

Monthly Variation of 𝑺 𝒒 𝑯
The Monthly mean presented in Figure 4

Figure 1 :
Figure 1: Distribution of the geomagnetic observatories used for the study (Source: Yumoto and MAGDAS Group, 2001) Figure 2(c) LKW show the maximum dH at 80 nT, on 2 nd August, and peak around 1200 LT, while ILR show the minimum dH at 30 nT on 30 th August, and peak around 1000 LT.It was observed that the dH during the Equinox Months is higher than solstice Months.For instance, on 09/10, DAV with maximum of 110 nT for Equinox Months while on 02/08 LKW with amplitude of 80 nT.It was observed that, latitudinal positions of the station also affected the magnetic field variability.Daily Variation   of Z for 2008

Figure 4 :Figure 5 :Figure 6 :
Figure 4: monthly variation for H components of geomagnetic field under quiet condition