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The equatorial electrojet and the profile parameters B0 and B1 around midday
Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
www.elsevier.com/locate/jastp
The equatorial electrojet and the pro(le parameters B0 and B1 around midday O.K. Obroua; b;∗ , S.M. Radicellaa , J.O. Adeniyia; c a International Center for Theoretical Physics, Trieste, Italy de Physique de l’Atmosph ere U.F.R. SSMT, Universit$e de Cocody, Ivory Coast c Physics Department, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria
b Laboratoire
Received 28 June 2001; accepted 3 December 2002
Abstract This study presents the results of the comparison of B0, B1 and hmF2 with 4H . B0 and B1 are parameters used in the international reference ionosphere model for the calculation of the F region bottom side pro(les. The parameter 4H obtained from the magnetic data recorded during the International Equatorial Electrojet Year (IEEY) in West Africa is used to describe the strength of the equatorial electrojet. This work covers the years 1993 and 1994, two years of low and moderate solar activity. The result shows that the electric (eld (E), which drives the equatorial electrojet, plays a major role in the variation of the thickness and the height of the F2 layer. However, the variation of the shape of the bottomside F2 is not sensitive to the electric (eld. c 2003 Elsevier Science Ltd. All rights reserved. Keywords: Equatorial ionosphere; Equatorial electrojet; Electron density pro(les
1. Introduction The bottomside F region of the ionosphere is represented in the international reference ionosphere model (IRI) by the analytic function N (h) = NmF2
exp(−X B1 ) ; Cosh(X )
where X=
hmF2 − h : B0
B0 measures the thickness of the bottom side pro(le while B1 determines the shape of the pro(le between hmF2 and h0:24 , which is the height where the electron density drops to 0.24NmF2. There are two options for the calculation of B0 ∗ Corresponding author. NASA, Goddard Space Flight Center, Code 632, Greenbelt, MD 20771, USA. E-mail address: [email protected] (O.K. Obrou).
in the IRI. The standard model is based on a table of values deduced from mid-latitude ionosonde stations. The second option, which is the newer one, is based on the model of Gulaevas (1987) that gives a relationship between the F2 peak and the half-density height (h0:5) (Bilitza, 1990). Adeniyi (1997) showed that for an equatorial station, the newer option gives a better result than the older one. The newer one however still has considerable shortcomings at equatorial latitudes (Adeniyi and Radicella, 1998). Most of the main features of the equatorial F2 layers have been explained in terms of the movement of ionization caused by the cross product of the electric (E) and magnetic (elds (B). The east–west electric (eld in conjunction with the earth’s north–south magnetic (eld lines, which is near horizontal around the equator, causes a force to act on ionization in the vertical direction (Balan and Bailey, 1996). The electric (eld E causes the intense current How around the equator usually referred to as the equatorial electrojet (EEJ). The intensity of the EEJ is characterized by its magnetic eIects, which are recorded by ground based magnetic network (Hesse, 1982;
c 2003 Elsevier Science Ltd. All rights reserved. 1364-6826/03/$ - see front matter doi:10.1016/S1364-6826(02)00336-X
300
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
Fambitakoye and Mayaud, 1976a, b, c). This explains why the ground based magnetic (eld parameter is usually employed as an indicator of the EEJ strength (Vikramkumar et al., 1987). In this study, we present the results of the comparison of B0, B1, and hmF2 with an equatorial electrojet parameter to be de(ned later. The study is for an equatorial station in the African sector.
Fambitakoye and Mayaud (1976a), and Doumouya et al. (1998) have considered the EEJ as a current ribbon Howing eastward along the geomagnetic equator at an altitude h = 105 km. They assumed that the electric current in the EEJ has a quadratic distribution. This assumption allows having the best (t to the latitudinal pro(les of the magnetic (eld components. Fig. 1 shows a sketch of the variations of the H and Z components with geographic latitude. The amplitude of H (4H ), which is used as an indicator of the strength of the EEJ, is determined using the latitudinal pro(les of the H component, diurnal variations. It is estimated as the maximum value of the H latitudinal pro(le at the location of the center of the EEJ. The method of obtaining 4H is indicated on Fig. 1. In this way, seasonal variation of 4H can be obtained for any time of the day. The seasonal variation of the EEJ parameter 4H estimated in this way by Doumouya et al. (1998) during the IEEY in West Africa is shown in Fig. 2. These are the monthly means of 4H , at 1200 local time and this is what is used for this study. The ionospheric data used are those from Korhogo in Cˆote-d’Ivoire (Latitude 9:3◦ N, Longitude 5:4◦ W, Dip 0:67◦ S), for the years 1993 and 1994 with average sunspot number (R12) of 56 and 30, respectively. Ionograms on 4 – 6 days without magnetic storms, in the months corresponding to those used in the analysis of 4H were selected. These include months of the whole year of 1993 and from January to August of 1994. The new POLAN code (Titheridge, 1995), which allows the computation of B0 and B1, was used for the ionogram inversion. Ionospheric pro(les from individual ionograms along with the corresponding B0 and B1 were obtained for each of the hours (1000 –1400) considered. For each of the months, averages of B0 and B1 were computed for the period centered around 1200 LT. This noontime averages of B0 and B1 are the average over a period of 5 h (1000 –1400 LT). The noontime hmF2 was analyzed by the same method used for B0 and B1.
2. Data and method of analysis The magnetic data are those of the IEEY campaign in the African continent. From December 1992 to December 1994; a chain of 10 magnetotelluric stations were installed along the 5◦ W meridian, within a band of 3◦ of longitude.
Fig. 1. Theoretical magnetic eIect of a parabolic electric current (center C, width 2a and amplitude 4H ) (after Doumouya et al., 1998).
160
∆H (nT)
120
80
40
0 J
F M A M
J
J
1993
A
S
O N D
J
months
F
M A M
J
J
A
S O N D
1994
Fig. 2. Seasonal variation of amplitude of the horizontal component 4H based on the monthly means, with corresponding rms error-bars (after Doumouya et al., 1998).
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
301
220
125 ∆Η
B0 210
112.50 200 100
180
87.50
B0 / km
∆Η (γ)
190
170 75 160 62.50 150 1994
1993
140
50 J
F M A M J
J
A S O N D J Months
F M A M J
J
A
Fig. 3. Monthly average variation of B0 and 4H . 125
3.0 ∆Η
B1 2.75
112.50 2.5 100
2.0
87.50
B1
∆Η (γ)
2.25
1.75 75 1.5 62.50 1.25 1993
1994 1.0
50 J
F M A M J
J
A S O N D J F Months
M A M J
J
A
Fig. 4. Monthly average variation of B1 and 4H .
3. Results Fig. 3 show the variations of the monthly noontime average of B0 and the corresponding average of 4H for the 2 years considered. The variation of B0 follows very closely
that of 4H . B0 is positively correlated with 4H . Both parameters exhibit a distinct peak in April 1993, which is the March equinox, season. Minima that are not as well de(ned as the peak occur during the June and December solstices. The correlation between B1 and 4H (Fig. 4) is negative
302
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304 475
125 ∆Η
hmF2
450
100
425
87.50
400
75
375
62.50
350
1993
hmF2 / km
∆Η (γ)
112.50
1994 325
50 J
F M A M J
J
A S O N D J Months
F M A M J
J
A
Fig. 5. Monthly average variation of hmF2 and 4H .
but is not as good as that of B0. The seasonal eIects on B1 are not well de(ned. The result for hmF2 (Fig. 5) is very similar to that of B0. The seasonal changes in hmF2 are similar to those of B0 and hmF2 is positively correlated with 4H . The regression lines for each of the parameters B0, B1, and hmF2 with respect to 4H are shown in Fig. 6a–c. The correlation coeOcients (r) are indicated on the (gures and are 0.88, 0.57 and 0.79 at 100% con(dence level for B0, B1 and hmF2, respectively. These values con(rm the strong correlation between these ionospheric parameters and 4H although that of B1 is not as high as those of B0 and hmF2. 4. Discussions and conclusions We have mentioned earlier on that 4H is used to indicate the strength of the EEJ. Sampath and Sastry (1979) obtained a linear relationship between EEJ measured directly and the ground measurement of 4H at Thumba another equatorial station. In this study, we have observed that both B0 and hmF2 have high positive correlation with 4H . This indicates the existence of a relationship between these ionospheric parameters and the EEJ around midday. Our result, which is based on data from the African sector, con(rms that reported for the South American station (Abdu et al., 1990). It is well known that the equatorial anomaly aIects the latitudinal variation of hmF2 (Klobuchar et al., 1990). The equatorial anomaly is formed by the action of the force produced by the electric (eld E and the magnetic (eld B (E × B)
(Rush and Richmond, 1973). The direction of the electric (eld is eastwards during the daytime. The E × B therefore acts in the upward direction during the daytime under magnetically quiet condition. Electrons move upwards along the magnetic (eld lines away from the equator. The eIect of this is that there is a depletion of electrons at the equatorial region and the F2 peak is moved to a higher altitude. This movement also aIects the thickness of the F2 layer. It is also known that the strength of the equatorial electrojet is proportional to E. This implies that the peak density height hmF2 and thickness of the F2 layer indicated by B0 is greatly inHuenced by the electric (eld whose strength is indicated by 4H used in this study. The correlation found between 4H and B0 and hmF2 in this study shows that the electric (eld which drives the EEJ (and whose strength is indicated by 4H ) plays a major role in the variation of the thickness and height of the F2 layer. The correlation between B1 and 4H around midday is not as high as those of B0 and hmF2. This suggests that the variation of the shape of the bottomside F2 pro(le is not as sensitive to the electric (eld E as B0 and hmF2. It is obvious that when the upward ionization drift velocity increases the thickness of the F2 layer should increase. The determination of an empirical relationship between B0 and E or vertical drift velocity provides a way of predicting the thickness of the F2 layer. The same thing should also be applicable for hmF2. It should be possible to do such a prediction on a short- or long-term basis because of the availability of vertical drift velocity data and its empirical model (Scherliess and Fejer,
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
303
220 r = 0.88
B0 / km
200 180 160 140 50
60
70
80
90
100
110
120
130
100
110
120
130
100
110
120
130
∆H (γ)
(a) 3.0
r = 0.57
2.75 2.5 B1
2.25 2.0 1.75 1.5 1.25 1.0 50
60
70
80
(b)
90 ∆H (γ)
480
r = 0.79
hmF2 (km)
460 440 420 400 380 360 340 50
60
70
80
90 ∆H (γ)
(c)
Fig. 6. Regression line for B0, B1, and hmF2 with respect to 4H .
1999). An empirical relationship of this nature, is a way of bridging the gap of lack of availability of ionospheric data in the equatorial region of the African sector for the purpose of modeling, when compared to the data from the middle and high latitudes. If such empirical relationship is incorporated into IRI model, it will greatly improve the prediction of equatorial ionospheric pro(les.
Acknowledgements We thank KonQe NahQegan (Station ionosphQerique de Korhogo) who supplied the ionograms for this work. O.K. Obrou thanks the Atmospheric Physics Laboratory of UFR
SSMT at University of Cocody, (Cˆote-d’Ivoire) for granting him sabbatical leave during the period this work was carried out. J.O.Adeniyi would like to thank the University of Ilorin, Nigeria for granting him a Special leave for External Award during the period this work was carried out. References Abdu, M.A., Walker, G.O., Reddy, B.M., Sobral, J.H.A., Fejer, B.G., Kikuchi, T., Trivedi, N.B., Szuszczewicz, E.P., 1990. Electric (eld versus neutral wind control of the equatorial anomaly under quiet and disturbed condition: a global perspective from SUNDIAL 86. Annales de Geophysicae 8 (6), 419–430.
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O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
Adeniyi, J.O., 1997. Experimental equatorial ionosphere pro(les and IRI pro(les. Journal of Atmospheric and Solar-Terrestrial Physics 59 (10), 1205–1208. Adeniyi, J.O., Radicella, S.M., 1998. Diurnal variation of ionospheric pro(le parameters; B0 and B1 for an equatorial station at low solar activity. Journal of Atmospheric and Solar-Terrestrial Physics 60, 381–385. Balan, N., Bailey, G.J., 1996. Modelling studies of equatorial plasma fountain and equatorial anomaly. Advances in Space Research 18 (3), 107–116. Bilitza, D. (Ed.), 1990. International reference ionosphere, Report NSSDC 90-2. World Data Center. A for Rockets and Satellites, Greenbelt, MD. Doumouya, V., Vassal, J., Cohen, Y., Fambitakoye, O., Menvielle, M., 1998. Equatorial electrojet at African longitudes: (rst results from magnetic measurements. Annales de Geophysicae 16, 658–676. Fambitakoye, O., Mayaud, P.N., 1976a. The equatorial electrojet and regular daily variation SR : I A determination of the equatorial electrojet parameters. Journal of Atmospheric and Solar-Terrestrial Physics 38, 1–18. Fambitakoye, O., Mayaud, P.N., 1976b. The equatorial electrojet and regular daily variation SR : II The center of the equatorial electrojet. Journal of Atmospheric and Solar-Terrestrial Physics 38, 19–26. Fambitakoye, O., Mayaud, P.N., 1976c. The equatorial electrojet and regular daily variation SR : IV Special features in particular days. Journal of Atmospheric and Solar-Terrestrial Physics 38, 123–134.
Gulaevas, T., 1987. Progress in ionospheric informatics based on electron density pro(les analysis of ionograms. Advances in Space Research 7 (6), 39. Hesse, D., 1982. An investigation of the equatorial electrojet by means of ground-based magnetic measurements in Brazil. Annales de Geophysicae 38, 315–320. Klobuchar, J.A., Anderson, D.N., Doherty, P.H., 1990. The latitudinal extent of the equatorial anomaly ionospheric eIects. Proceedings of the Symposium IES-90, Washington, DC, May. Rush, C.M., Richmond, A.D., 1973. The relationship between the structure of the equatorial anomaly and the strength of the equatorial electrojet. Journal of Atmospheric and Terrestrial Physics 35, 1171–1180. Sampath, S., Sastry, T.S.G., 1979. Results in situ measurements of ionospheric currents in the equatorial region—I. Journal of Geomagnetism and Geoelectricity 31, 373–379. Scherliess, L., Fejer, B.G., 1999. Radar and satellite global equatorial F region vertical drift model. Journal of Geophysical Research 104, A4, 6829–6842. Titheridge, J.E., 1995. Ionogram analysis with the generalized program New POLAN. Private communication. Vikramkumar, B.T., Rao, P.B., Viswanathan, K.S., Reddy, C.A., 1987. Electric (elds and currents in the equatorial electrojet deduced from VHF radar observation—III. Comparison of observed 4H values with those estimated from measured electric (elds. Journal of Atmospheric and Terrestrial Physics 49 (2), 201–207.
www.elsevier.com/locate/jastp
The equatorial electrojet and the pro(le parameters B0 and B1 around midday O.K. Obroua; b;∗ , S.M. Radicellaa , J.O. Adeniyia; c a International Center for Theoretical Physics, Trieste, Italy de Physique de l’Atmosph ere U.F.R. SSMT, Universit$e de Cocody, Ivory Coast c Physics Department, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria
b Laboratoire
Received 28 June 2001; accepted 3 December 2002
Abstract This study presents the results of the comparison of B0, B1 and hmF2 with 4H . B0 and B1 are parameters used in the international reference ionosphere model for the calculation of the F region bottom side pro(les. The parameter 4H obtained from the magnetic data recorded during the International Equatorial Electrojet Year (IEEY) in West Africa is used to describe the strength of the equatorial electrojet. This work covers the years 1993 and 1994, two years of low and moderate solar activity. The result shows that the electric (eld (E), which drives the equatorial electrojet, plays a major role in the variation of the thickness and the height of the F2 layer. However, the variation of the shape of the bottomside F2 is not sensitive to the electric (eld. c 2003 Elsevier Science Ltd. All rights reserved. Keywords: Equatorial ionosphere; Equatorial electrojet; Electron density pro(les
1. Introduction The bottomside F region of the ionosphere is represented in the international reference ionosphere model (IRI) by the analytic function N (h) = NmF2
exp(−X B1 ) ; Cosh(X )
where X=
hmF2 − h : B0
B0 measures the thickness of the bottom side pro(le while B1 determines the shape of the pro(le between hmF2 and h0:24 , which is the height where the electron density drops to 0.24NmF2. There are two options for the calculation of B0 ∗ Corresponding author. NASA, Goddard Space Flight Center, Code 632, Greenbelt, MD 20771, USA. E-mail address: [email protected] (O.K. Obrou).
in the IRI. The standard model is based on a table of values deduced from mid-latitude ionosonde stations. The second option, which is the newer one, is based on the model of Gulaevas (1987) that gives a relationship between the F2 peak and the half-density height (h0:5) (Bilitza, 1990). Adeniyi (1997) showed that for an equatorial station, the newer option gives a better result than the older one. The newer one however still has considerable shortcomings at equatorial latitudes (Adeniyi and Radicella, 1998). Most of the main features of the equatorial F2 layers have been explained in terms of the movement of ionization caused by the cross product of the electric (E) and magnetic (elds (B). The east–west electric (eld in conjunction with the earth’s north–south magnetic (eld lines, which is near horizontal around the equator, causes a force to act on ionization in the vertical direction (Balan and Bailey, 1996). The electric (eld E causes the intense current How around the equator usually referred to as the equatorial electrojet (EEJ). The intensity of the EEJ is characterized by its magnetic eIects, which are recorded by ground based magnetic network (Hesse, 1982;
c 2003 Elsevier Science Ltd. All rights reserved. 1364-6826/03/$ - see front matter doi:10.1016/S1364-6826(02)00336-X
300
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
Fambitakoye and Mayaud, 1976a, b, c). This explains why the ground based magnetic (eld parameter is usually employed as an indicator of the EEJ strength (Vikramkumar et al., 1987). In this study, we present the results of the comparison of B0, B1, and hmF2 with an equatorial electrojet parameter to be de(ned later. The study is for an equatorial station in the African sector.
Fambitakoye and Mayaud (1976a), and Doumouya et al. (1998) have considered the EEJ as a current ribbon Howing eastward along the geomagnetic equator at an altitude h = 105 km. They assumed that the electric current in the EEJ has a quadratic distribution. This assumption allows having the best (t to the latitudinal pro(les of the magnetic (eld components. Fig. 1 shows a sketch of the variations of the H and Z components with geographic latitude. The amplitude of H (4H ), which is used as an indicator of the strength of the EEJ, is determined using the latitudinal pro(les of the H component, diurnal variations. It is estimated as the maximum value of the H latitudinal pro(le at the location of the center of the EEJ. The method of obtaining 4H is indicated on Fig. 1. In this way, seasonal variation of 4H can be obtained for any time of the day. The seasonal variation of the EEJ parameter 4H estimated in this way by Doumouya et al. (1998) during the IEEY in West Africa is shown in Fig. 2. These are the monthly means of 4H , at 1200 local time and this is what is used for this study. The ionospheric data used are those from Korhogo in Cˆote-d’Ivoire (Latitude 9:3◦ N, Longitude 5:4◦ W, Dip 0:67◦ S), for the years 1993 and 1994 with average sunspot number (R12) of 56 and 30, respectively. Ionograms on 4 – 6 days without magnetic storms, in the months corresponding to those used in the analysis of 4H were selected. These include months of the whole year of 1993 and from January to August of 1994. The new POLAN code (Titheridge, 1995), which allows the computation of B0 and B1, was used for the ionogram inversion. Ionospheric pro(les from individual ionograms along with the corresponding B0 and B1 were obtained for each of the hours (1000 –1400) considered. For each of the months, averages of B0 and B1 were computed for the period centered around 1200 LT. This noontime averages of B0 and B1 are the average over a period of 5 h (1000 –1400 LT). The noontime hmF2 was analyzed by the same method used for B0 and B1.
2. Data and method of analysis The magnetic data are those of the IEEY campaign in the African continent. From December 1992 to December 1994; a chain of 10 magnetotelluric stations were installed along the 5◦ W meridian, within a band of 3◦ of longitude.
Fig. 1. Theoretical magnetic eIect of a parabolic electric current (center C, width 2a and amplitude 4H ) (after Doumouya et al., 1998).
160
∆H (nT)
120
80
40
0 J
F M A M
J
J
1993
A
S
O N D
J
months
F
M A M
J
J
A
S O N D
1994
Fig. 2. Seasonal variation of amplitude of the horizontal component 4H based on the monthly means, with corresponding rms error-bars (after Doumouya et al., 1998).
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
301
220
125 ∆Η
B0 210
112.50 200 100
180
87.50
B0 / km
∆Η (γ)
190
170 75 160 62.50 150 1994
1993
140
50 J
F M A M J
J
A S O N D J Months
F M A M J
J
A
Fig. 3. Monthly average variation of B0 and 4H . 125
3.0 ∆Η
B1 2.75
112.50 2.5 100
2.0
87.50
B1
∆Η (γ)
2.25
1.75 75 1.5 62.50 1.25 1993
1994 1.0
50 J
F M A M J
J
A S O N D J F Months
M A M J
J
A
Fig. 4. Monthly average variation of B1 and 4H .
3. Results Fig. 3 show the variations of the monthly noontime average of B0 and the corresponding average of 4H for the 2 years considered. The variation of B0 follows very closely
that of 4H . B0 is positively correlated with 4H . Both parameters exhibit a distinct peak in April 1993, which is the March equinox, season. Minima that are not as well de(ned as the peak occur during the June and December solstices. The correlation between B1 and 4H (Fig. 4) is negative
302
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304 475
125 ∆Η
hmF2
450
100
425
87.50
400
75
375
62.50
350
1993
hmF2 / km
∆Η (γ)
112.50
1994 325
50 J
F M A M J
J
A S O N D J Months
F M A M J
J
A
Fig. 5. Monthly average variation of hmF2 and 4H .
but is not as good as that of B0. The seasonal eIects on B1 are not well de(ned. The result for hmF2 (Fig. 5) is very similar to that of B0. The seasonal changes in hmF2 are similar to those of B0 and hmF2 is positively correlated with 4H . The regression lines for each of the parameters B0, B1, and hmF2 with respect to 4H are shown in Fig. 6a–c. The correlation coeOcients (r) are indicated on the (gures and are 0.88, 0.57 and 0.79 at 100% con(dence level for B0, B1 and hmF2, respectively. These values con(rm the strong correlation between these ionospheric parameters and 4H although that of B1 is not as high as those of B0 and hmF2. 4. Discussions and conclusions We have mentioned earlier on that 4H is used to indicate the strength of the EEJ. Sampath and Sastry (1979) obtained a linear relationship between EEJ measured directly and the ground measurement of 4H at Thumba another equatorial station. In this study, we have observed that both B0 and hmF2 have high positive correlation with 4H . This indicates the existence of a relationship between these ionospheric parameters and the EEJ around midday. Our result, which is based on data from the African sector, con(rms that reported for the South American station (Abdu et al., 1990). It is well known that the equatorial anomaly aIects the latitudinal variation of hmF2 (Klobuchar et al., 1990). The equatorial anomaly is formed by the action of the force produced by the electric (eld E and the magnetic (eld B (E × B)
(Rush and Richmond, 1973). The direction of the electric (eld is eastwards during the daytime. The E × B therefore acts in the upward direction during the daytime under magnetically quiet condition. Electrons move upwards along the magnetic (eld lines away from the equator. The eIect of this is that there is a depletion of electrons at the equatorial region and the F2 peak is moved to a higher altitude. This movement also aIects the thickness of the F2 layer. It is also known that the strength of the equatorial electrojet is proportional to E. This implies that the peak density height hmF2 and thickness of the F2 layer indicated by B0 is greatly inHuenced by the electric (eld whose strength is indicated by 4H used in this study. The correlation found between 4H and B0 and hmF2 in this study shows that the electric (eld which drives the EEJ (and whose strength is indicated by 4H ) plays a major role in the variation of the thickness and height of the F2 layer. The correlation between B1 and 4H around midday is not as high as those of B0 and hmF2. This suggests that the variation of the shape of the bottomside F2 pro(le is not as sensitive to the electric (eld E as B0 and hmF2. It is obvious that when the upward ionization drift velocity increases the thickness of the F2 layer should increase. The determination of an empirical relationship between B0 and E or vertical drift velocity provides a way of predicting the thickness of the F2 layer. The same thing should also be applicable for hmF2. It should be possible to do such a prediction on a short- or long-term basis because of the availability of vertical drift velocity data and its empirical model (Scherliess and Fejer,
O.K. Obrou et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 299 – 304
303
220 r = 0.88
B0 / km
200 180 160 140 50
60
70
80
90
100
110
120
130
100
110
120
130
100
110
120
130
∆H (γ)
(a) 3.0
r = 0.57
2.75 2.5 B1
2.25 2.0 1.75 1.5 1.25 1.0 50
60
70
80
(b)
90 ∆H (γ)
480
r = 0.79
hmF2 (km)
460 440 420 400 380 360 340 50
60
70
80
90 ∆H (γ)
(c)
Fig. 6. Regression line for B0, B1, and hmF2 with respect to 4H .
1999). An empirical relationship of this nature, is a way of bridging the gap of lack of availability of ionospheric data in the equatorial region of the African sector for the purpose of modeling, when compared to the data from the middle and high latitudes. If such empirical relationship is incorporated into IRI model, it will greatly improve the prediction of equatorial ionospheric pro(les.
Acknowledgements We thank KonQe NahQegan (Station ionosphQerique de Korhogo) who supplied the ionograms for this work. O.K. Obrou thanks the Atmospheric Physics Laboratory of UFR
SSMT at University of Cocody, (Cˆote-d’Ivoire) for granting him sabbatical leave during the period this work was carried out. J.O.Adeniyi would like to thank the University of Ilorin, Nigeria for granting him a Special leave for External Award during the period this work was carried out. References Abdu, M.A., Walker, G.O., Reddy, B.M., Sobral, J.H.A., Fejer, B.G., Kikuchi, T., Trivedi, N.B., Szuszczewicz, E.P., 1990. Electric (eld versus neutral wind control of the equatorial anomaly under quiet and disturbed condition: a global perspective from SUNDIAL 86. Annales de Geophysicae 8 (6), 419–430.
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