Quiet Time Geomagnetic Field Variations in the Equatorial East African Region During the Inclining Phase of Solar Cycle 24
Omondi George Erick1, *, Baki Paul2, Ndinya Boniface3
1Department of Physics and Materials Science, Maseno University, Maseno, Kenya
2Department of Physics and Space Science, Technical University of Kenya, Nairobi, Kenya
3Department of Physics, Masinde Muliro University of Science and Technology, Kakamega, Kenya
To cite this article:
Omondi George Erick, Baki Paul, Ndinya Boniface. Quiet Time Geomagnetic Field Variations in the Equatorial East African Region During the Inclining Phase of Solar Cycle 24. International Journal of Astrophysics and Space Science. Vol. 4, No. 2, 2016, pp. 21-25. doi: 10.11648/j.ijass.20160402.11
Received: May 23, 2016; Accepted: June 1, 2016; Published: June 20, 2016
Abstract: In the present paper, geomagnetic field data obtained from magnetometer measurements at two ground based stations have been used to study solar activity dependence of the solar quiet variations. The study has focused on the horizontal component of the geomagnetic field. The data used in the current study was obtained for all quiet periods from the solar minimum year (2009) through the solar maximum year (2014) to the start of the declining phase (year 2015) of Solar Cycle 24. The present study uses the magnetic data from International Real-time Magnetic Observatory Network (INTERMAGNET) station at Addis Ababa (geomagnetic latitude 0.18°N, geomagnetic longitude110.47°E) and MAGnetic Data Acquisition System (MAGDAS) station at Nairobi (geomagnetic latitude 10.65°S, geomagnetic longitude 108.18°E). The amplitude of mean Sq(H) has shown a dependence on local time of the day and solar activity, with peak values occurring between 1100 LT and 1200 LT and increasing with increase in solar activity; attaining highest values during the solar maximum year. Further, the amplitude of mean Sq(H) at Nairobi is higher than the corresponding values at Addis Ababa in the morning hours around 0700-0800 LT. The local time dependence is attributed to the variation in solar heating and ionization rates while the solar activity dependence is attributed to the increase in electron density with increase in solar activity. The larger morning hours’ amplitudes at Nairobi than Addis Ababa are possibly due to counter electrojet effects close to the geomagnetic equator.
Keywords: Geomagnetic Field, Solar Quiet, Solar Activity
Ions and to a lesser degree, electrons in the E-region of the ionosphere are coupled to the neutral components of the atmosphere; hence follow their dynamics. Atmospheric winds and tidal oscillations of the atmosphere force the E-region ion component to move across the magnetic field lines, while electrons move much slower at right angles to both the field and neutral wind (Baumjohann and Treumann, 1996). This relative movement constitutes an electric current and the separation of charge produces an electric field which in turn affects the current. The relation between conductivity, electric field, and neutral wind velocity, , is obtained by adding an electric field term to the Ohm’s law equation to write
For mid and low latitude dynamo currents, the dominant driving force is the electric field induced by ion motion across the magnetic field. The current system created by the tidal motion is called solar quiet or. The currents which create a disturbance on the magnetic field can be estimated on the ground by magnetometers and allows the determination of the strength of the currents. In accordance with the contrast between day and night equatorial E-region electron densities, the Sq currents are concentrated in the dayside region. Matsushita and Maeda (1965) explain that Sq can be estimated by selecting quiet days from each month, eliminating the effect caused by lunar variation field and excluding non cyclic variation. The Sq field varies slowly in amplitude and phase through the months of the year (Campbell, 2003).
A number of studies of Sq variations have been carried out within the African sector. For example, Rabiu (2001) did a comprehensive study of the daily Sq. variations in the geomagnetic field for the entire solar cycle 22 (1986-1996) in middle latitude regions in the northern hemisphere. The Sq seasonal variation was maximum in June solstice and minimum in December solstice. Rabiu et al. (2007a) 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 the 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 intensity at about local noon. Rabiu et al. (2007b) analyzed magnetic records obtained at geomagnetic observatory of Addis Ababa in Africa during the sunspot minimum year 1986 for day-to-day variability of the hourly amplitudes of solar daily variation in H and Z geomagnetic field intensities. The variability was studied under quiet and disturbed conditions. Quiet day-to-day variability had consistent and explicable diurnal and seasonal variation and the daytime (0700-2000 hours) magnitudes were greater than the nighttime magnitudes (2000 -0700 hours through 2400 hours) for all the months in the two elements. Rabiu et al. (2011) examined the daily Sq for hourly profiles of H and D taken on 29th December, 2008 at ten MAGDAS stations along 96° Magnetic Meridian (MM) in Africa. The results showed that the H component experienced more variation within the equatorial electrojet zone. ElHawary et al. (2012) studied the daily Sq variations of the geomagnetic field by using the geomagnetic data of the ten International Quietest Days. Geomagnetic data from MAGDAS I and II stations were selected around the 96°MM over Africa and analyzed from September 2008 to August 2009. The work analyzed the H and D components of the geomagnetic field and found that Sq(H) and Sq(D) show a predominantly annual variation. Abbas et al. (2013) used hourly variation of two magnetic elements H and D from Aswan and Nairobi from January to December, 2008 to study the variation pattern of solar quiet and solar disturbed conditions. The variation pattern of Sq obtained showed that the maximum intensity occurred around local noon.
The main objective of the present study is to determine the mean annual variation of the quiet time geomagnetic field from the solar minimum year (2009) through solar maximum year (2014) to the start of the declining phase (year 2015) of Solar Cycle 24 with a view to investigating its solar activity dependence and any unique features. Such a study has not been previously performed in the region of interest.
2. Data and Methods of Analysis
The present study uses the magnetic data from International Real-time Magnetic Observatory Network (INTERMAGNET) station at Addis Ababa (geomagnetic latitude 0.18°N, geomagnetic longitude110.47°E) and MAGnetic Data Acquisition System (MAGDAS) station at Nairobi (geomagnetic latitude 10.65°S, geomagnetic longitude 108.18°E).
The concept of local time was used throughout the analysis. The conversion from universal time (UT) to local time (LT) was done by considering that the Earth makes one complete rotation in 24 hours. It follows, therefore, that a longitudinal difference of 1° is equivalent to a time difference of 4 minutes. In this regard, the conversion from UT to LT was done according to:
where is the geodetic longitude of the station in question.
All the days with maximum were classified as quiet days. Such days were extracted (from 1st January, 2009 to 31st December, 2015) from the list of Kp index posted by the World Data Centre for Geomagnetism, Kyoto on their website (http://wdc.kugi.kyoto-u.ac.jp/cgi-bin/kp-cgi).
The present study analyzed the H-component of the geomagnetic field mainly because the region of study is equatorial where it is well known that the geomagnetic field is nearly horizontally directed North to South (Campbell, 1989) and the daily geomagnetic field variations, especially during quiet times, usually depend on the local tensor conductivity and the orientation of the main geomagnetic field.
A quiet level baseline was defined for each day as the arithmetic mean the geomagnetic field forthe first 300 minutes (5hours) starting from local midnight of the same day in a manner similar to that of (Yamazaki and Kosch, 2015).
In mathematical symbols, this means that;
The usage of these baselines is based on the assumption that the nighttime ionospheric currents are trivial. This assumption is generally accepted based on the fact that the conducting E region of the ionosphere largely disappears at night hours except at the auroral latitudes and currents flowing in the F region of the ionosphere during the night are much smaller in magnitude than the daytime dynamo currents (Takeda and Araki, 1985, Yamazaki et al., 2011).
The baseline value for each day was subtracted from each of the one- minute resolution H-field values to get the variation amplitude from the baseline for the particular day. That is;
where t = 01 to1440, corresponding to the period00.00h to 23.98h. The resulting variation amplitude for any hour is associated with the ionospheric current at that hour (Rastogi et al., 2004).
During quiet times, the geomagnetic field variation is, theoretically, expected to exhibit a variation pattern similar to that of a periodic function diurnally in such, a way that the value at 00h should be equal to the value at 24h. However, this is not always necessarily the case in some data sets. A correction factor, known as non- cyclic variation, should therefore be performed on the data (Matsutisha and Maeda, 1965, Rastogi et al., 2004). The non-cyclic variation factor was defined as;
where 1439 is the number of intervals in the data set.
Equation (5) can be expressed in the form:
Equation (6a) implies that in our new arrangement, the first data point will be given by the term on the right hand side, that is,
We make a generalization by setting
to write a general equation that represents the solar quiet daily variation as
Equation (7) was used to determine the Sq(H) variation for each quiet day in the entire year. The average of these, Sq(H) , variations for each year define the mean annual variation of the H field. The results are presented in Figure 1a, b, c, d, e, f and g.
3. Results and Discussion
Addis Ababa (AAB) is represented by the red colour while Nairobi (NAB) is represented by the black colour as illustrated by the legend. The margins have been removed from the legends. In the year 2015, Nairobi magnetometer recorded no data.
The amplitude of mean annual Sq(H), () shows a clear dependence on local time of the day at both stations: Addis Ababa and Nairobi.
At AAB, increases from about 0 nT in the morning (around 0700 LT) attaining maximum values around 1100LT during low to moderate solar activity years (2009, 2010, 2011 and 2015) and 1200LT during high solar activity years (2012, 2013 and 2014), then decreases attaining values of about 0in the evening at around 1700 LT. Nonetheless, at NAB, generally increases from about 0 nT in the morning (around 0700 LT) attaining maximum values around 1130LT; then decreases attaining values around 0 nT in the evening at around 1700 LT. It is known that the maximum amplitude of Sq(H) appears before local noon in the range between 1030LT to 1130 LT during minimum sunspot years, around dip equatorial regions, and around local noon during maximum sunspot years (Rastogi and Iyer, 1976, El Hawary et al., 2012). One of the explanations suggested by Rastogi and Iyer, (1976) for maximum Sq(H) occurring well before noon is that, since Sq(H) represents currents in the ionosphere and current being a product of electron density, ne, electronic charge, e, and charge drift speed, v, would be maximum at some point between the time of maximum electron density and drift speed. The electron density maximizes close to noon while the charge drift speed (hence the electric field) becomes maximum near 0900 to 1000 LT. It follows that the product (current) should maximize around 1100 LT. However, with increasing solar activity (sunspot number), the electron density is known to increase by 50% or more; whereas the electron drift speed remains constant or decreases. Thus the product of ne and v should be able to shift to a later time near noon in high sunspot years.
Solar heating and ionization rates are important causes of Sq(H) at all latitudes. This accounts for the minimal values (around 0 ) of observed at AAB and NAB in the morning and evening hours, owing to low solar intensity at these hours.
The peak amplitude of at AAB is consistently higher than the peak amplitude of at NAB. This difference is attributed to the existence of the equatorial electrojet that intensifies just before local noon close to the geomagnetic equator (such as at AAB); hence imposing onto the Sq(H) current system. The equatorial electrojet is caused by enhancement of the effective conductivity at the geomagnetic equator. For instance, consider a situation where the magnetic field is horizontal to the surface of the Earth as is the case at the geomagnetic equator. The direction of the magnetic field is from South to North, along the x-axis. The primary Sq Pedersen current flows eastward, in the y direction (orthogonal to the magnetic field), parallel to the primary ionospheric electric field. This primary electric field drives a Hall current which flows vertically downward in the z direction (perpendicular to both the primary electric field and the magnetic field), causing a charge separation in the equatorial ionosphere with negative charges accumulating at the top boundary and positive charges accumulating at the bottom of the highly conducting layer. This space charge distribution forms a secondary polarization electric field vertically directed from the bottom to the top of the conducting ionosphere. The polarization electric field drives a vertical Pedersen current opposing the Hall current until it fully cancels (compensates) it, resulting in an equilibrium condition in which no current flows in the vertical direction. Moreover, the secondary polarization electric field component generates a secondary Hall current component, flowing into the y direction.
The total current flowing in the eastward direction (positive y direction) is the sum of the primary Pedersen and the secondary Hall currents resulting in enhanced conductivity called Cowling conductivity. For typical Hall to Pedersen ratios lying in the range of 3 to 4, the Cowling conductivity is of order of magnitude higher than the usual Pedersen conductivity; hence explaining the amplification and concentration of the equatorial electrojet above the geomagnetic equator.
The amplitude of at NAB is consistently higher than the amplitude of at AAB in the morning hours between 0700-0800 LT. This difference is possibly due to the existence of a counter electrojet, a westward electric field close to the dip equator (such as at AAB) in the morning and/or evening hours; that tends to cancel (reduce) the current system.
The maximum amplitude of increases as solar activity increases. The largest increase occurs between solar minimum year (2009) and solar maximum year (2014), with about 50% and 75% for AAB and NAB respectively.
In the present study, the mean annual variation in Sq(H) at two stations (Addis Ababa and Nairobi) within the geomagnetic equatorial East African region has been investigated from the solar minimum year (2009), through the solar maximum year (2014) to the start of the declining phase (year 2015) of the Solar Cycle 24 with a view to determining its solar activity dependence and identifying any unique features in the variation. The amplitude of mean Sq(H) has shown dependence on local time of the day and solar activity. The mean Sq(H) increases steadily from morning hours attaining peak values around 1100 LT at Addis Ababa during low to moderate solar activity years (2009, 2010, 2011 and 2015) and 1200 LT during high solar activity years (2012, 2013 and 2014), then decreases gradually attaining minimum values, around 0 nT, in the evening hours around 1700 LT. In Nairobi, the peak in mean Sq(H) is achieved around 1130 LT. This local time dependence is attributed to the variation in solar heating and ionization rates. Peak values of the mean Sq(H) are highest in solar maximum year. In addition, the morning mean Sq(H) at Nairobi is consistently higher than the corresponding values at Addis Ababa; attributed to the morning counter electrojet that acts to depress the Sq(H) current system closer to the geomagnetic equator.
The results presented in this paper rely on the data collected at <Addis Ababa>. We thank <Bahir Dar University>, for supporting its operation and INTERMAGNET for promoting high standards of magnetic observatory practice. The results presented for Nairobi rely on data obtained from Nairobi MAGDAS station. We, therefore, thank the MAGDAS team for installing the magnetometer at University of Nairobi.
The corresponding author is grateful to Dr. John Bosco Habarulema of South African National Space Agency, Hermanus, South Africa for useful comments during preparation of this paper. We are also grateful to the anonymous reviewer for evaluating this paper.
Baki Paul (ORCID:0000-0001-5813-3576).