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ON THE MORPHOLOGY OF EQUATORIAL ELECTROJET OVER INDIAN SECTOR

ON THE MORPHOLOGY OF EQUATORIAL ELECTROJET OVER INDIAN SECTOR. A. Babatunde Rabiu 1 , and Nandini Nagarajan 2 1 Department of Physics, Federal University of Technology, Akure, NIGERIA 2 National Geophysical Research Institute, Hyderabad 500 007, INDIA.

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ON THE MORPHOLOGY OF EQUATORIAL ELECTROJET OVER INDIAN SECTOR

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  1. ON THE MORPHOLOGY OF EQUATORIAL ELECTROJET OVER INDIAN SECTOR A. Babatunde Rabiu1, and Nandini Nagarajan2 1Department of Physics, Federal University of Technology, Akure, NIGERIA 2National Geophysical Research Institute, Hyderabad 500 007, INDIA. 2tunderabiu@yahoo.com, 2nandini@ngri.res.in International Advanced School on Space Weather, 2-19 May, 2006, ICTP,Trieste

  2. Outline • Introduction • Presentation of Model • Results • Discussions • Conclusions

  3. Introduction • The E region of the equatorial ionosphere consists of two layers of currents responsible for the quiet solar daily variations in Earth’s magnetic field: the worldwide Sq, (altitude 118 +7 km), & the equatorial electrojet, EEJ, (altitude 106 + 2 km) • Equatorial electrojet EEJ- the intense ionospheric current flowing eastwards within the narrow strip flanking the dip equator, responsible for the observed enhanced horizontal magnetic field intensity at the magnetic equatorial neighbourhood. (Chapman, 1951) • Most of studies on electrojet focused on noon time period as the ranges of the magnetic field intensities were fitted into models in order to evaluate the electrojet characteristics. • Increasing interest in modeling the geomagnetic observations necessitates the need for examination of every aspect of the variation field for accurate formulation and evaluation of model parameters

  4. Fig. 1. Relative positions of the magnetic and geographic equators

  5. Manifestations of EEJ • Spatial structures of its intense current density • configurations & regular temporal variations of its current system • magnetic fields of its current system • the ionospheric plasma density irregularities generated by the turbulent flow of the EEJ current • the electric fields and ionospheric plasma drifts in the dip equatorial zone • the quiet counter equatorial electrojet CEJ • temporal variabilities of the above phenomenon.

  6. Central objectives • To explore the thick shell format of a continuous current distribution model to evaluate the morphology of equatorial electrojet at the Indian sector • To study the transient variations of the landmark parameters of the EEJ

  7. Model Evaluation Onwumechili (1966a, b, c; 1967) presented a two dimensional empirical model of the continuous current distribution responsible for EEJ as: j = jo a2(a2 + x2)b2(b2 + z2) / (a2 + x2)2 (b2 + z2)2 (1) Where j (µA m-2) is the eastward current density at the point (x, z). The origin is at the centre of the current, x is northwards, and z is downwards. The model is extensible to three dimension by introducing the coordinate y or longitude Ø or eastwards local time t. j0 is the current density at the centre, a and b are constant latitudinal and vertical scale lengths respectively,  and  are dimensionless parameters controlling the current distribution latitudinally and vertically respectively. It is a meridional plane model, which in this simple form has to be applied to specific longitudes or local times. The model is a realistic model having both width and thickness.

  8. Model Evaluation contd. Onwumechili (1966c) used the Biot-Savart law to obtain the northwards X and vertical Z components of the magnetic field variation with latitude on the horizontal plane (v = constant) as a result of the current distribution in (1) as follows: (sg. z) P4 X = ½ k [(1+)(v + v +2a)(u + b)2 + 2(1- )(v + v + 4a -2a)(u + b) + (1+ )(v + v + 2a)(v + a)2] (2) - (sg.x) P4 Z = ½ k [(1+ )(1+ )(u + b)3 + ((1+ )(1+ )(u + b)2 + (1+ )(v + v + 3a - a) (v + a) (u + b) - (1- ) b (v + v + 3a - a) (v + a)] (3) Where P2 = (u + b)2 + (v + a)2(4) k = 0.1π2abj0 (5) u = /x/ and v= /z/ (6) sg.x = sign of (x/u) and is ± 1 when x = 0 (7) sg.z = sign of (z/v) and is ± 1 when z= 0 (8) Equations 2 and 3 give the horizontal and vertical magnetic field variations respectively, due to thick current shell format.

  9. Model Evaluation contd. 0° dip latitude does not coincide with the center of the EEJ (Oko, et al., 1996). Therefore we chose to write an expression for the electrojet axis x0, in terms of the dip latitude, , as: u =  - x0 (9) Where x0 is the dip latitude of the current center. Introducing equation 9 in equations 2 and 3 results in a set of pair of non-linear equations. The non-linear model was applied to four data points, each with a pair of simultaneously measured horizontal H and vertical Z variation field components.

  10. Model Evaluation contd Simultaneously recorded hourly horizontal H and vertical Z field values were obtained from 5 stations, in the solar minimum year 1986. (Sunspot number R = 13.4). These hourly horizontal and vertical field values were treated for hourly departures, non-cyclic and Dst variations to ensure absolute quiet condition as required. The electrojet index was obtained by subtracting the hourly values of worldwide Sq as obtained at Hyderabad, a station just outside of electrojet, from other four stations that fall within the electrojet influence. The resultant system of eight non-linear equations with five unknown model parameters and one unknown physical parameter (jo, a, , b, , x0)were subjected to non-linear least square optimisation method. (Rabiu and Nagarajan, 2005).

  11. Coordinates of the geomagnetic observatories Station Code Geog. Dip latitude Lat. N° long °E (°N) Trivandrum TRD 8.29 76.57 0.20 Ettaiyapuram ETT 9.10 78.00 0.50 Kodaikanal KOD 10.23 77.47 2.14 Annamalainagar ANN 11.4 79.7 3.28 Hyderabad HYB 17.42 78.55 9.33

  12. Fig. 2. Geographical distributions of the geomagnetic observatories

  13. the half thickness pkm or degree at half of the peak current density: p2 = b2 [( -1) + {1+ ( -1)2}½ ] (2) • Half of the latitudinal width or the focal distance of the current w km or degree: w2 = -a2/ (3)

  14. Fig.3. Diurnal variation of the electrojet center for E –season

  15. EEJ Thickness demonstrates • a consistent diurnal variation across the seasons • decrease from about 0.06642º at dawn to the minimum at about 1100 hr LT and then begin to increase towards the dusk Fig 4. Diurnal variation of half Thickness of EEJ

  16. Fig. 5. Diurnal variation of Half width of EEJ

  17. Fig. 6. seasonal and annual means of Half Width and Thickness

  18. Comparison of our Half thickness value with literatures • Our result: mean annual 6.94 + 0.41 km • Rocket-borne magnetometers (Sastry, 1970) bottom half: 7 km • Wind model (electrodynamic ) Anandarao & Ragharavao (1987) bottom half: 8 km relative consistency !

  19. Comparison of our Half width values with literatures at 1100 LThr degrees km relative consistency !

  20. Discussion • The variation of the dip latitude of the center of EEJ, x0, clearly demonstrates a consistent diurnal pattern, which described a northwards shift towards the dip equator from the rising of the jet at dawn and becoming closer to the dip equator at the peak intensity period of the jet after which it begins to recede southwards towards the dusk. • Magnitude wise this diurnal observation is in consistency with the Orsted satellite observational result of Jadhav et al (2002a) and contradicts Oko, et al. (1986) (-0.29 + 0.02°) result obtained from thin shell format. • With mean value of -0.1911 + 0.0031°, it is obvious that the center of EEJ is not necessarily at the dip equator in agreement with results of Srivastava (1992) and Onwumechili (1997) among others. This further implies that the equatorial electrojet axis does not coincide with the dip equator. • Obviously the center of the jet is, however, close to the dip equator at about local noon (1000 LT) and always coincides with the hour of occurrence of the maximum peak forward current intensity, peculiar to the region of study, on any day.

  21. Discussion contd. • Richmond (1973) showed that a meridional wind of 10 ms-1 shifts the jet center by 0.8 km. • Anandarao and Raghavarao (1987) revealed that meridional winds shift the center of the jet either southwards or northwards depending upon whether the wind is northwards or southwards. • Anandarao and Raghavarao (1987) have found that a steady northward wind of 100 ms-1 is capable of shifting the center of EEJ southwards by 0.5º. • Forbes (1981) concluded that shifting of electrojet axis can be responsible for day-today variability of electrojet intensity.

  22. Discussion contd. • Diurnal variation of the jet center, x0, follow the satellite observation of Jadhav (2002a), contradicts results reported in literatures based on thin shell format for EEJ current, and confirm the long term assertion of Forbes and Lindzen (1976) that thin shell approximation is only a representation of approximate noontime equatorial magnetic variations, and fail to take into account local time variations of the electrojet. This is further stressed by the fact that our electrojet center, x0, is minimum and closer to the dip equator at about local noon. • Forbes and Lindzen (1976) have demonstrated the defects and inconsistencies in using a thin shell approximation in the vicinity of the magnetic equator.

  23. Discussion contd. Anandarao and Raghavarao (1987) .. • showed that a positive (negative) wind shear decreases (increases) the width (thickness) of the jet. • noted that the zonal wind shears can decrease or increase the width of jet by as much as 100% depending upon their direction, strength and altitude, and concluded that “if the width of the jet is increased, then the thickness would decrease and vice versa.

  24. Conclusions • With mean value of -0.1911 + 0.0031°, it is obvious that the center of equatorial electrojet is not necessarily at the dip equator and implies that the equatorial electrojet axis does not coincide with the dip equator. • The equatorial electrojet center is observed to migrate northwards towards the dip equator from the dawn such that it is closer to the dip equator at about local noon and then reclined southwards towards the dusk. • The model result wholly confirmed the satellite observational results and partly contradicted the results hitherto obtained from approximate thin shell model.

  25. Conclusions contd • The mean annual half thickness and half width for the solar minimum year 1986 (Sunspot number R = 13.4) is 0.0625 + 0.0037º (6.93 + 0.41 km) and 2.68 + 0.23º ()respectively. • The thickness and width of equatorial electrojet EEJ exhibit consistent diurnal variations. • The thickness decreases from about 0.06642º at dawn to the minimum at about 1100 hr LT and then begin to increase towards the dusk.

  26. Conclusions contd • The width increases with the sunrise, reaches maximum at about 1100 hr LT and then begin to decrease towards the dusk • The dynamics of the variation of electrojet intensity and thickness shows that electrojet shrinks as its intensity increases • The thin current shell model best fits only the near local noon jet observation, as the electrojet is thinnest at period of maximum intensity.

  27. Acknowledgements • World Data Centre-C2, Kyoto University, Kyoto, Japan. • National Geophysical Research Institute (NGRI), Hyderabad, India. • Third World Academy of Sciences TWAS, Trieste, Italy, • CSIR (Government of India) for awarding Research Fellowships. • Organisers of, and the co-participants at, the International Advanced School on Space Weather, ICTP, Trieste

  28. THANK YOU

  29. References Anandarao, B. G., and Raghavarao, R., 1979. Effects of vertical shears in the zonal winds on the electrojet. Space Res., 19, 283-286. Anandarao, B. G., and Raghavarao, R., 1987. Structural changes in the currents and fields of the equatorial electrojet due to zonal and meridional winds. J. Geophysical Research, 92. 2514-2526. Chapman, S., 1951. The equatorial electrojet as deduced from the abnormal current distribution above Huancayo and elsewhere. Archiv Fuer Meteorologie, Geophysik und Bioklimatologie, Serie A 4, 368-390. Forbes, J. M., 1981. The equatorial electrojet. Rev. Geophys., 19, 469-504. Forbes J. M. and R. S. Lindzen, 1976. Atmospheric and solar tides and their electrodynamic effects-II, The equatorial electrojet. JoumaI of Atmospheric and Terrestrial Physics, Vol. 38, pp. 911-920.

  30. Holme, R., James, M. A., and Luhr, H., 2004, Magnetic field modelling from scalar-only data: Resolving the Backus effect with the equatorial electrojet. Earth Planets Space, 57, 1-8. Jadhav, G., M. Rajaram, and R. Rajaram, 2002. A detailed study of equatorial electrojet phenomenon using Ørsted satellite observations, J. Geophys. Res., 107(A8), 1175, doi: 10.1029/2001JA000183. Luhr, H., S. Maus, and M. Rother, 2004. Noon time equatorial electrojet: Its spatial feautures as determined by the CHAMP satellite, J. Geophys. Res., 109, A01306,, doi: 10:1029/2002JA009856. Oko, S. O., and Ezema, P. O., Onwumechili, C. A., (1996), Geomagnetically quiet day ionospheric currents over the Indian sector – II. Equatorial electrojet currents, J. Atmos. Terr. Phys., 58, 555-564. Onwumechili, C. A., 1966a. A new model of the equatorial electrojet current, Nigerian J. Sci., 1, 11-19 Onwumechili, C. A., 1966b. A three dimensional model of density distribution in ionospheric currents causing part of quiet day geomagnetic variations. IIe Symposium d'aeronomie equatoriale, Special publication of Annales de Geophysique, 157-162.

  31. Onwumechili, C. A., 1966c. The magnetic field of a current model for part of geomagnetic Sq variations. IIe Symposium d'aeronomie equatoriale, special publication of Annales de Geophysique, 163- 170. Onwumechili, C. A., 1967. Geomagnetic variations in the equatorial zone. In Physics of Geomagnetic Phenomena. Eds. Matsushita S. and Campbell, W. H., Academic press, New York. , Vol. 1,pp 425- 507. Onwumechili, C. A., 1997. The Equatorial Electrojet. Gordon and Breach Science Publishers, Netherlands. 627pp Onwumechili, C. A. and Ezema, P. O., 1992. Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set. J. Atmos. Terr. Phys. 54. 1535-1544. Onwumechili, C. A., and Ogbuehi, P. O. , 1967. Preliminary results on the equatorial electrojet current density. J. Geomag. Geoelectr. 19. 15-22. Onwumechili, C. A., and P.O. Ogbuehi, 1967. Preliminary results on the equatorial electrojet in India. J. Geomag. Geolectr., 19(1), 15-22

  32. Rabiu, A. B. and N. Nagarajan, 2005. Diurnal variation of the center of equatorial electrojet from a thick current shell. Ife Journal of Science, 7(2). Reddy, C. A., and Devasia, C. V., 1981. Height and latitude structure of electric fields and currents due to local east-west winds in the equatorial electrojet, J. Geophys. Res., 86, 5751-5767. Richmond, A. D., 1973. Equatorial electrojet I. Development of a model including winds and instabilities, J. Atmos. Terr. Phys., 35, 1083-1103 Rigoti, A., Chamalaun, F. H. Trivedi, N. B., and Padilha, A. L., 1999. Characteristics of the Equatorial Electrojet determined from an array of magnetometers in N-NE Brazil. Earth Planets Space, 51, 115-128. Sastry, T. S. G. , 1970. Diurnal change in the parameters of the euqtorial electrojet as observed by rocket-borne magnetometers, Space Res., 10. 778-785. Yacob, A., and Khana, K. B., 1963. Geomagnetioc Sq variations and the parameters of the Indian electrojet for 1958, 1959. Indian Meteorol. Geophys., 14, 470-477.

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