International Journal of Marine, Atmospheric & Earth Sciences, 213, 1(1): 8-16 International Journal of Marine, Atmospheric & Earth Sciences Journal homepage: www.modernscientificpress.com/journals/ijmaes.aspx ISSN: 2327-3356 Florida, USA Article Longitudinal and Seasonal Variation along the Magnetic Equator Using MAGDAS/CPMN Data Abbas M. 1, *, Joshua B. 1 D. Bonde 1 M. Gwani 1 1 Department of Physics, Kebbi State University of Science and Technology, Aliero, Nigeria * Author to whom correspondence should be addressed; E-Mail: maphysik@yahoo.com Article history: Received 2 December 212, Received in revised form 12 January 213, Accepted 13 January 213, Published 14 January 213. Abstract: Magnetic records from 3 stations along the equatorial chain were examined for solar quiet daily Sq variations of the horizontal component of the earth magnetic field. The result shows there could be substantial day-to-day variability in the electrojet (EEJ) strength. Longitudinal variations of Sq and seasonal variation in the geomagnetic components were also examined. Diurnal variations of the geomagnetic variations along the magnetic equator were also discussed in detail. Ancon in the eastern direction shows maximum diurnal variation and Ilorin in the west display the minimum variation. This shows that the EEJ appear stronger in East than West. This suggests that there could be a process of reinjection of energy as Jet flows eastward. The seasons of peak in H, in each station were also noticed to be different. Quiet day day-to-day variability has consistent and explicable diurnal and seasonal variation. Seasonal change in the Sq variation is explicable in terms of seasonal shift in the mean position of the Sq current system of the ionospheric electrojet, seasonal movement of electrojet current focal latitude and width, and the electrodynamics effects of local winds. Key words: Solar Quiet Daily Sq Variations, Equatorial Electrojet, day-to-day variability. 1. Introduction The first explanation for the regular solar daily variation of the geomagnetic field was proposed by Stewart (1882), who suggested the existence of currents in the upper atmosphere due to the movement of conductive air across the lines of force of earth s magnetic field, caused by the solar Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 9 heating. These daily variations in the geomagnetic fields at the earth s surface during geomagnetically quiet conditions are known to be associated with the dynamo currents which are driven by winds and thermal tidal motions in the E-region of the ionosphere Chapman (1919). At the magnetic dip equator the midday eastward polarization field generated by global scale dynamo action gives rise to a downward Hall current. A strong vertical polarization field is set up which opposes the downward flow of current due to the presence of non-conducting boundaries. This field in turn gives rise to the intense Hall current which Chapman (1951) named the equatorial electrojet (EEJ). When a geomagnetic observatory was established in 1992 at Huancayo in Peru, an abnormally large daily range of geomagnetic horizontal field, H, was noticed by Mc Nish (1937). Plotting the latitudinal variations of the daily range of H at six observatories that existed at low latitudes, three of these in India Egedal (1947) found an intensification of the daily range of the H field over the dip equator. This phenomenon was described by Chapman (1951) as due to an eastward band of electric currents in the ionosphere. According to the model suggested by him, the geomagnetic field H due to the electrojet current should be maximum and the geomagnetic vertical ( Z) due to the current should be zero over the dip equator. The results of geomagnetic survey in India by Pramanic and Yegna Narayanan (1952) and by Pramanik and Hariharan (1953) showed the daily peak of H at 11: LT at stations close to the equator. Doumouya et al., (23) studied the longitudinal variation of geomagnetic field intensities at equatorial zone using surface magnetic data. The nature of the longitudinal inequalities in the EEJ strength indicates that the equatorial electrojet was strongest in South America (8-1 w) and weakest in the Indian sector (75 E) with a secondary minimum and a maximum centered, respectively in the Atlantic Ocean (3 w) and in western Africa (1 E). The primary goal of this paper is to obtain an accurate description of the diurnal and seasonal variation of some selected stations along the equatorial chain. It is intended to set forth the main features of the EEJ on the basis of longitudinal and seasonal variations. 2. Data Analysis The geomagnetic data set consists of published hourly values of H, recorded at a network of three geomagnetic electrojet observatories along the dip equator as shown in table 1. The international quiet days (IQD s) which are by definition the sets of five most quiet days per month on the disturbance Kp index were selected and the hourly values of H analyzed for Sq. The IQD s were selected so as to ensure that only quiet conditions are examined. Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 1 Table 1: Parameters of the stations used in the study Station Code Geog Lat. Geog long. Geomag lat. Geomag long. Dip lat. Ilorin ILR 8.5 4.68-1.82 76.82-2.96 Davao DAV 7. 125.4-1.2 195.54 -.65 Ancon ANC -11.77-77.15.77 354.33.74 2.1. Mean Hourly Values of Sq The concept of local time is used throughout the analysis. The variation baseline is obtained from the 2 hours flanking local midnight, that is, 24 hr LT and 1 hr LT. The daily baseline values (H o ) for the geomagnetic element H are the mean values of the hourly values at these 2 hours: -------------------------------------------------------------------------- (1) where H 24, H 1 represent the values of the geomagnetic element H respectively. The midnight values were subtracted from the hourly values to get the hourly departures from the midnight for the particular day, the i th day. That is; ------------------------------------------------------------------------- (2) where i = 1 to 365; t = 1 to 24; H t is the hourly values of the magnetic element H. 3. Results and Discussion 3.1. Hourly Variations of Day-to-day Variability From Figs. 1 and 2, it is clear that a diurnal variation of day-to-day variability exists in the horizontal component of the earth magnetic field on quiet days throughout the year. These variations maintain a regular pattern and consistent variation. The day-to-day variability peaks during the daytime mostly around the local noon within the range (1 13 hours) for all the months in the H component. This agrees with the diurnal variation pattern of Sq in the earlier works of Onwumechili (196) and Matsushita (1969), which showed that the maximum intensity of Sq occurs around the local noon. Emilia and Last (1977) reported a similar diurnal variation pattern of Sq in H, Sq (H) for 1958 1973 in Addis Ababa. Throughout the year Ancon maintains a maximum Sq variation, it is to be noted that the strength of EEJ is stronger in Ancon (South America) than in Ilorin and Davao, which agrees with Patil et al., (199a, b) that the geomagnetic field intensity vary from one sector to another within the equatorial zone. Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 11 The diurnal variation of day-to-day variability, which followed the variation pattern of Sq, can be attributed to the variability of the ionospheric processes and physical structure such as conductivity and winds structure, which are responsible for the Sq variation. Studying the variabilities in Indian equatorial electrojet sector, Okeke et al, (1998) noted that changes in the electric field control the phase and randomness of the variabilities, while the magnitude of the ionospheric conductivity controls the magnitude of the variabilities. 1 JAN/8 15 FEB/8 15 MAR/8 5 1 5 1 5-5 15 APR/8-5 1 MAY/8-5 1 JUN 1 5 5 5-5 -5-5 1 JUL/8 1 AUG/8 1 SEP/8 5 5 5-5 -5 15 1 5 OCT/8-5 1 5 NOV/8-1 1 5 DEC/8 ILR DAV ANC -5-5 -5 Fig. 1: Diurnal variation of some selected stations along the magnetic equator from July to Dec 28 Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 12 12 Quiet H 1 8 DV(Hq) (nt) 6 4 2 E J D Fig. 2a: Seasonal variation of monthly variability at ILR 28 7 Quiet H 6 5 DV(Hq) (nt) 4 3 2 1 E J D Fig. 2b: Seasonal variation of monthly variability at DAV 28 Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 13 18 Quiet H 16 14 12 DV(Hq) (nt) 1 8 6 4 2 E J D Fig. 2c: Seasonal variation of monthly variability at ANC 28 Fig. 3 shows the annual mean diurnal variation of dh plotted in a quest for greater insight into the Sq variation, with Ancon in the eastern direction showing the maximum diurnal variation and Ilorin in the west is observe to display a minimum variation. This shows that the EEJ appear stronger in East than the West. We suggest that there could be a process of reinjection of energy as Jet flows eastward. A minimum nighttime variation is also observed in Ancon in fig. 1, which is more noticeable in the annual mean plot which may be attributed to a distant current of non ionospheric region. 12 1 Annual Mean Plot ILR ANC DAV 8 6 4 2-2 Fig. 3: Annual mean mass plot Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 14 3.2. Seasonal Variation of Day-to-day Variability The monthly variation in the H elements was evaluated for every month for quiet days (Qd). This was done by using the international quiet days for every month, as is illustrated in fig. 1. Following Lloyd s seasons (Eleman, 1973) the months of the year are classified into Equinox (March, April, September, and October); June Solstice or J-season (May, June, July, and August): and December solstice or D- Season (November December, January, and February). Mean seasonal values of solar variation are estimated by averaging values for all the months in a particular season, and the result is displayed in Fig 2a, 2b and 2c. The results show there is a clear indication that day-to-day variability has seasonal variation. On quiet conditions, the day-to-day variability is observed to be maximum during the Equinox season in Ilorin (West Africa), Maximum in D solstice in Davao and also maximum in June solstice in Ancon (South America). Forbes (1981) noted that seasonal day-today variability could be partially explained by the seasonal variation of the lunar semi-diurnal tide. This June solstitial maximum was also observed at Muntilupa by Rabiu (1992). Onwumechili (1997) and Okeke et al., (1998) reported similar June solstitial maximum in the magnitude of day-to- day variability due to electrojet in horizontal component of quiet conditions in Indian sector. Obviously the seasonal variations of (H) and on quiet conditions with peak at June solstice are annual unlike the semiannual variations of Sq (H) and (Onwumechili, 1997). The seasons of peak in H in quiet condition in each station were also noticed to be different. Seasonal change in the Sq variation is explicable in terms of seasonal shift in the mean position of the Sq current system of the ionospheric electrojet, seasonal movement of electrojet current focal latitude and width, and the electrodynamics effects of local winds. The electrodynamics effects of local winds can also account for seasonal variability since the winds are subject to day-to-day and seasonal variability. Forbes (1981) and Onwumechili (1992b) independently extended further the work of Reddy and Devasia (1981) and demonstrated that the width and intensity of the electrojet are measurably modified by wind structures. Onwumechili (1997) proposed that the seasonal variability of semi-diurnal tides may contribute to the seasonal variability of Sq (H). 4. Conclusions From this study, we can conclude that the diurnal variations of day-to-day variability exist in the horizontal component of the earth magnetic field on quiet days throughout the year. The daytime (1 13 hrs) magnitudes are greater than the nighttime (16 18 hrs) magnitudes for all the months. The diurnal variation of day-to-day variability, which followed the variation pattern of Sq, Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 15 can be attributed to the variability of the ionospheric processes and physical structure such as conductivity and winds structure, which are responsible for the Sq variation. The variability of the nighttime field may be as a result of the variability of the nighttime distant currents. The day-to-day variability has seasonal variation. On quiet conditions, the day-to-day variability is observed to be maximum during the June solstice in Ancon with Ilorin having maximum seasonal variation in Equinox and Dav in June solstice. The electrodynamics effects of local winds can also account for seasonal variability since the winds are subject to day-to-day and seasonal variability. Forbes (1981) and Onwumechili (1992b) independently extended further the work of Reddy and Devasia (1981) and demonstrated that the width and intensity of the electrojet are measurably modified by wind structures. Onwumechili (1997) proposed that the seasonal variability of semi-diurnal tides may contribute to the seasonal variability of Sq (H). Ancon in the eastern direction shows maximum diurnal variation and Ilorin in the west display the minimum variation. This shows that the EEJ appear stronger in East than West. We suggest that there could be a process of reinjection of energy as Jet flows eastward. References Balfour, S. (1882). Hypothetical views regarding the connection between the state of the sun and terrestrial magnetism, Encyclopedia Britanica, 9 th ed., 16: 181-184 Chapman, S. (1951). The equatorial electrojet as detected from the abnormal electric current distribution above Huancayo and elsewhere, Arch. Meteorl. Geophys. Bioclimatol, A 4: 368-392. Chapman, S. (1919). The solar and lunar diurnal variations of terrestrial magnetism, Philos. Trans. Roy. Soc., London, A 218: 1-118. Doumouya, V., Cohen, Y., Arora, B. R., and Yumoto, K. (23). Local time and longitude dependence of the equatorial electrojet magnetic effects, J. Atoms. Terr. Phys., 65: 1265 1282. Eleman, F. (1973). The geomagnetic field in Cosmical Geophysics, ed. Egeland, A., Holter, O, and Omholt, A., Scandinavian University Books, Oslo. Chapter 3, p45-62. Emilia, D. A. and Last, B. J. (1977). A study of Sq variation at Addis Ababa, J. Atoms. Terr. Phys., 39: 375-381. Forbes, J. M. (1981). The equatorial electrojet, Rev. Geophys Space Phys., 19: 469-54. Matsushita, S. (1969). Dynamo Currents, Winds, and Electric Fields, Radio Sci., 4: 771. Mc Nish, A. G. (1937). Bull 1. Int. Assoc. Magn. Transaction of Edin- burgh meeting, Copenhagan, p271 28. Okeke, F. N., Onwumechili, C. A., Rabiu, A. B., (1998). Day-to-day variability of geomagnetic hourly amplitudes at low latitudes, Geophys. J. Intl., 134: 484-5. Copyright 213 by Modern Scientific Press Company, Florida, USA
Int. J. Mar. Atmos. & Earth Sci. 213, 1(1): 8-16 16 Onwumechili, C.A. (1992b). Study of the return current of the equatorial electrojet, J. Geomagn. Geoelect., 44: 1-42 Onwumechili, C. A. (1997). The Equatorial Electrojet, Gordon and Breach Science Publishers, Netherlands, p627. Onwumechili, C.A. (196). Fluctuations in the geomagnetic filed near the magnetic equator. J. Atmos. Terr. Phys., 17: 286-294. Patil, A. R., Rao, D. R. K. and Rastogi, R. G. (199a). Equatorial electrojet strengths in the Indian and American sectors. Part 1. During low solar activity, J. Geom. Geoelectr., 42: 89-811. Patil, A. R., Rao, D. R. K. and Rastogi, R. G. (199b). Equatorial electrojet strengths in the Indian and American sectors. Part II. During high solar activity, J. Geom. Geoelectr., 42: 813-823. Pramanik S. K., Narayanan, S. Y. (1952). Diurnal magnetic variations in equatorial regions, Ind. J. Meteor. Geophys. 3: 212 216. Pramanik, S. K. and Hariharan, P. S. (1953). Diurnal magnetic variations near the magnetic equtor, Ind. J. Meteor. Geophys., 4(4): 353 388. Rabiu, A. B.,(1992) Day-to-day variability in the geomagnetic variations at the low latitude observatory of Muntilupa in the Philippines, M.Sc. Thesis, University of Nigeria, Nsukka. Reddy, C.A., and Devasia, C.V. (1981). Height and latitudinal structures of electric fields and currents due to local east-west winds in the equatorial electrojet, J. Geophys. Res., 86: 5751-5767. Copyright 213 by Modern Scientific Press Company, Florida, USA