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Geomagnetically quiet day ionospheric currents over the Indian sector—II. Equatorial electrojet currents
Journal
of Almaspheric
and Terrestrial
Pergamon 0021-9169(95)00056-9
Physics, Vol. 58, No. 5, pp. 555564, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All riehts reserved OOZI-9169/96 bS.OO+O.OO
Geomagnetically quiet day ionospheric currents over the Indian sector-II. Equatorial electrojet currents S. 0. Oko,* C. A. Onwumechilit
and P. 0. Ezema$
*Department of Computer Science, University of Nigeria, Nsukka, Nigeria; t69 Lansdowne ney, London
E8 3EP, U.K.; SDepartment of Physics, Enugu State University Technology, Enugu, Nigeria
Drive, Hackof Science and
(Received 29 November 1994; accepted in revisedform 8 December 1994) Abstract-With 1986 quiet days data of Indian observatories, the equatorial electrojet (EEJ) has been studied in terms of 8 landmark parameters that reveal the stuctures of hourly latitudinal profiles of EEJ current from 0700 to 1700 h local time. The landmark distances suggest that near dawn, the EEJ is widest and its centre and focus are most northerly before it contracts towards local noon with its centre and focus moving southwards. The seasonal means of peak current intensity and the total forward current seem to peak earlier when the intensity of EEJ is higher than when it is lower. The seasonal order of EEJ intensity is found not to be the same at all hours of the day. This implies that seasonal variation of EEJ is not semiannual at certain daytime hours. The landmark distances of EEJ current have semi-annual variations with minima in the vernal and autumnal equinoxes and maxima at June and December solstices, but annual variations of the measures of EEJ current are just the reverse. Certain properties of the worldwide part of Sq are found to be markedly different from those of EEJ including some key features of diurnal and seasonal variations.
equatorial counter electrojet. This is designed to facilitate their intercomparisons.
1. I’NTRODUCTION
The earliest studies of latitudinal profiles of the Sq magnetic fields were based on daily ranges (Onwumechili, 1959a; Onwumechili, 1959b; Forbush and CasaVerde, 1961; Yacob
2. DATA ANALYSIS It has been demonstrated that the continuous distribution of current density model of ionospheric currents introduced by Onwumechili (1966a); Onwumechili (1966b) fits extremely well the altitude profiles of ionospheric currents observed by rockets (Onwumechili, 1992a), as well as the horizontal X or H and the vertical 2 geomagnetic fields of both the worldwide part of Sq (WSq) and equatorial electrojet (EEJ) (Onwumechili, 1966~; Onwumechili, 1967). Onwumechili and Ezema (1992) have derived the thick current shell and the thin current shell formats of the X and Z magnetic field components from the continuous distribution of current density model. Here we shall use the thin current shell equations of the magnetic fields, namely:
(sg.z)p4X = $a[@+
uv + 2rXa)(u+b)2 +(v+ctv+24(v+4*]
-(sg.x)p’Z
= $u(u+b)[(l
+c()(u+b)2
+(v+av+3u-ctu)(v+u)] 555
(1)
(2)
S. 0. Oko et al
556
P* = (u+b)Z+(u+a)2
(3)
K = 0.27~5,
J,, = K/0.271
(5)
c? Jm = Jo4(cc- 1)
IF = aJ,
Lx2
= __ 4(a-
L, a
M-l = ~ a+1
(10)
(4)
u = (xl and v = 121sg.x = sign of x = x/u and sg.z = sign of z = z/v. The origin is at the centre of the current, x is northward latitudinal distance, y is eastwards, and z is vertically downward distance, a and b are latitudinal and vertical scale lengths, respectively, CLis dimensionless constant controlling latitudinal distribution of current, Jo is the peak current intensity or the heightintegrated current density at the current centre and K is the magnetic field constant. Since the centre of the equatorial electrojet is not necessarily at the dip equator (Onwumechili, 1967; Onwumechili and Ogbuehi, 1967a; Srivastava, 1992) we introduce x,, the dip latitude of the centre of EEJ. Then in equations (1) and (2), u = 6-x,, where 6 is the dip latitude of the observatory. The two equations now contain four parameters of the model: x,, K, a and CC. These parameters may be determined by modelling observed latitudinal profiles with equation (1) or (2) or both. Once these parameters of the model have been determined, we can obtain many physical parameters of the current and its magnetic field. Here we are interested in the following landmark parameters of the latitudinal profile of the current: the peak intensity of the forward current Jo Akm-’ at its centre, the peak intensity of the return current J,,, Akm-‘, the percentage of the peak return to the peak forward current intensity lOOJ,/J,,, the total forward current IF kA flowing between the current foci, the focal distance w degrees from the current centre, the distance x, degrees of the peak return current location from the current centre, and the latitudinal extent of the current L, degrees from its centre. The expressions of these parameters are as follows:
J,/J,
(L,/a+a/L,)tan-‘-
(6b)
1)
(-rx)“2+(l+a)tan-‘(_tL),,2
1
1
(7)
w2 = a’/( - a)
(8)
x2 = a’(a - 2)/a
(9)
We now describe how these parameters have been determined for the equatorial electrojet (EEJ). The latitude L = tan’ (Z/2H) distorts distances on the globe and makes conversion to geographic latitude difficult. We therefore used the true dip latitude 6 which is the actual latitudinal distance from the dip equator. The coordinates of the eight Indian observatories whose 1986 data were used in the analysis are: Trivandrum (8.29”N, 7657”E, 6 = 0.20”), Kodaikanal(10,23”N, 77.47”E, 6 = 2.14”), Annamalainagar (11.37”N, 79.68”E, 6 = 3.28”) Hyderabad (17.42”N, 7855”E, 6 = 9.33”) Alibag (18,63”N, 72.87”E, 6 = 10.54”), Ujjain (23.18”N, 75.78”E, 6 = 15.09”) Jaipur (26,92”N, 75.80”E, 6 = 18.83”) and Sabhawala (30.37”N, 77.80”E, 6 = 22.28”). The dip equator crossed their mean longitude of 76.82 t_ 2”E at 8.09”N. For conversion in the region L = -0.2872 + 1.2387 6. Being a solar activity minimum year, most days of 1986 were quiet. Nevertheless, in each month we used the five quietest among the ten internationaly selected quiet days for the month. Of the 60 days, the Ap index ranged from 2 to 6 with the mean of 3.9 and only 2 days had Ap = 6. See Onwumechili et al. (1995) for more details. It is well known that the observed field variation components X0 and Z, relate to their external parts X, and Z,, and their internally induced parts X, and Z, by X0 = X,+X,,
(1 la)
z, = z,-z,.
(lib)
The last five observatories: Hyderabad, Alibag, Ujjain, Jaipur and Sabhawala, are outside the influence of EEJ and therefore their data represent WSq field. Onwumechili et al. (1995) have described in Paper 1 of this study how the data of these five observatories have been used to determine the model parameters x,, K, a and CI of WSq, using equation (1). Thereafter, the parameters are used in equation (1) and (2) to extrapolate X and Z components of WSq into the EEJ zone, by computing their values at Trivandrum, Kodaikanal and Annamalainagar. These computed values are subtracted from the observed X and Z at the three observatories to obtain X and 2 components of the electrojet field at Trivandrum, Kodaikanal and Annamalanagar. We now describe how the X and Z components of the EEJ field at Kodaikanal and Annamalainagar together with the X component only at Trivandrum have been used to determine the parameters of the
Geomagnetically quiet day ionospheric currents over the Indian sector-II EEJ. Unfortunately, the ratio of internal to external fields of the EEJ is not as clear cut as that of WSq; indeed the ratio is found to depend on the distance from the centre of the EEJ (Davis et al., 1967; Onwumechili, 199213). We represent the internal field by the field of the image EEJ induced in the conducting earth, especially as this has produced results that agreed with observations (Onwumechili, 199213). For this purpose, the earth is modelled as nonconducting to a depth of D, = 200 km and thereafter it becomes perfectly ‘conducting. The image EEJ is as far behind the conducting surface as the overhead EEJ is in front of it. Since the overhead EEJ is at a height of u, = 106 km (Onwumechili, 1992b), the image EEJ is at a depth of vi = 506 km. In equation (1) for the external field X, we substitute u, = 106 km = 0.96” but for the internal field X, we substitute v, = 506 km = 4.6”. Similarly, we substitute the same v, and vi into equation (2) for Z, and Z,, respectively. From rocket-observed altitude profiles of EEJ current density Onwumechili (1992a) determined b = 8 km = 0.07”. This value is substituted for b in the equation (1) and (2) for X,, X,, Z, and Z,. Since u in the equations is measured from the centre of the current, then for each observatory u = 6 -x0, where 6 is its dip latitude. We take Kodaikanal as an example. Following equation (1 la) the observed EEJ X.,, is equated to the sum of the expressions for X, and Xi with u appropriate for Kodaikanal. In like manner following equation (1 lb) the observed 2, of EEJ is equated to the difference of the expressions for Z, from Z, with u appropriate for Kodaikanal. This is repeated for X,, and Z, at Annamalainagar, and X0 at Trivandrum. Thus we have five equations. In view of the substitutions described above, the five equations contain only four unknowns, namely: x,, K, a and CC. The equations are solved by the method of least squares to determine the unknown parameters for the EEJ. Table 1 gives the mean values of the four parameters for each hour 070&l 700 h LT.
3. RESULTS AND DISCUSSIONS
3.1. Diurnal variations The eight parameters selected for study here epitomize the major attributes and structures of the latitudinal variation of the current on a meridional plane. Therefore, Onwumechili et al. (1989b) called them the landmark values of the current. It is convenient to group them into landmark values measuring the current, and landmark values measuring the distances. Figure 1 gives the diurnal variations of the annual
557
Table 1. The parameters of the continuous distribution of current density model for the magnetic field of the equatorial electrojet from 0700 to 1700 h LT: the dip latitude of the current centre x,, the magnetic field constant K, the latitudinal scale length a, and the latitudinal current distribution parameter cr; determined from 1986 data of three Indian observatories (Trivandrum, Kodaikanal and Annamalainagar) using the thin current shell equations. Local Time, h 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 Mean
x0 degrees
KnT
a degrees
0.23 0.05 -0.01 -0.11 -0.18 -0.24 -0.27 -0.27 -0.27 -0.30 -0.30 -0.15
7.7953 19.5276 35.0537 45.5849 57.7163 51.4880 41.5805 33.9021 23.1943 15.7725 8.9801 30.9632
3.8007 3.7396 3.7260 3.6739 3.6154 3.5696 3.5201 3.5215 3.5139 3.5000 3.5060 3.6079
a -1.5154 - 1.5219 - 1.5156 -1.5266 -1.5499 - 1.5708 - 1.5874 - 1.6000 -1.6000 - 1.6000 - 1.5972 -1.5623
means of the landmark measures of the equatorial electrojet current from 0700 to 1700 h LT. The peak forward current intensity J,, the peak return current intensity -.I,,, and the total forward current IF rise from dawn to a peak very close to 1100 h LT before decreasing towards dusk. The percentage ratio of the peak return to the peak forward current intensity - 100J,/JO is lower before noon than afternoon. The variations of J,, --.I,,, and 1, are similar because
32 4
28 -
H!
24 -
IF
-lOOJ,,,/J, _ 25 - 24 +? B
20 -
-23
JO
- 22 60
740 1 a
I-?
-J, +/%+.
:fi: +’
20 0
+’
+’
07 08 09
20 +\
+\ +\
9 ‘i
i!
10 4 %
0
B
7
10 11 12 13 14 15 16 17 Local
time hours
Fig. 1. Diurnal variations of annual means of the landmark measures of equatorial electrojet current: the peak forward current intensity J, Akm-‘, the peak return current intensity -J,,, Akm-‘, the total forward current IF kA, and the percentage
ratio of the peak return to the peak forward intensities - lOOJ,/.T,.
current
S. 0. Oko et al.
558
3.2 s $3.0 ~ d 3 2.8 -
- 5.6
g 12% 4 ll-
8
- 5.4
5
-5.2
,E
7E
SlOC,, , , , , , , ,
(
,_I
07 08 09 10 11 12 13 14 15 16 17
Local time hours Fig. 2. Diurnal variations of the annual means of the landmark distances of the equatorial electrojet current: the dip latitude of the current centre X, degree, the distance of the focus from the current centre w degree, the distance of the peak return current intensity from the current centre x, degrees, and the latitudinal extent of the current L, degrees from the current centre.
increase in Jo is expected to increase both -J,,, and ZF. Figure 2 shows the diurnal variations of the annual means of landmark distances of equatorial electrojet current. The focal distance w degrees from the current centre, the distance x,,, degrees of the peak return current intensity from the current centre, and the latitudinal extent LI degrees of the current decrease from their largest value around dawn to about 1300 h LT and then level up almost to a plateau towards dusk. Their similarity suggests that the EEJ over India as a whole contracts towards local noon from its broadest extent near dawn. Despite its day to day variability, the mean dip latitude of the current centre x, degrees also varies more or less like the other landmark distances. Thus the current centre tends to be most northerly around dawn and moves southwards towards local noon. The mean location of the centre varies little in the afternoon. Similarly, Kane and Trivedi (1980) found in same Indian sector that for the average of quiet days the EEJ seemed to be centered near the dip equator in the morning, but shifted steadily southwards till evening. Comparison of Figs 1 and 2 shows that the intensity ratio - 100Jm/Jo varies inversely as the landmark distances w, x, and L,. This implies that as the EEJ current contracts, its intersity increases. However, the intensification is relatively greater at the more distant location of -J,,, at x, than at the current centre location of Jo. This is a direct demonstration of the conclusion reached by Onwumechili and Agu (198 l),
120-
~loocp x SO.-;: z e, 606 .; 2 2 Li
aI
4020-
ot_, , ,
(
,
,
,
,
,
,
,
07 08 09 10 11 12 13 14 15 16 17
Local time hours Fig. 3. Diurnal variation of seasonal means of two landmark measures of equatorial electrojet current: the peak forward current intensity .Z,,Akm-‘, aid the total forward current IF kA. The Lloyd seasons D. E. J are shown as follows: crosses for D (November, Decembe;, January and February), filled circles E (March, April, September and October), and open circles for J (May, June, July and August).
Onwumechili et al. (1989b) and Onwumechili (1992a). It is seen in Fig. 3 that the seasonal means of the peak forward current intensity Jo Akm-’ and the total forward current Z, kA do not peak at the same time in all seasons. The EEJ seems to peak earlier when its intensity is higher than when it is lower. They peaked at about 1045 h in E season, at about 1130 h in D season and at about 1200 h LT in J season. The peak values are in the following order of decreasing magnitude: E, D and J, but the classical expectation for Sq currents in the region studied is J, E, D. However, the result here is consistent with the seasonal variation of Sq(AH) in the EEJ zone (Chapman and Raja Rao, 1965). The surprising result here is that the seasonal order of magnitude of EEJ intensity is not the same at all hours of the day. For example, the EEJ intensity in D season is lower than the intensities in E and J seasons in the morning before 1000 h LT, but greater than the intensities in E and J seasons in the afternoon after 1300 h LT. This implies that the seasonal variation of EEJ is not semi-annual in the afternoon. Therefore the study of seasonal variations should not be limited to noontime values. Table 2 gives new results on EEJ current. It is
Geomagnetically quiet day ionospheric currents over the Indian sector-II Table 2. Daily mean parameters of equatorial electrojet current: peak forward current intensity J,, peak return current intensity J,,,, percentagl: of peak return to peak forward intensity lOOJ,,,/J,, total forward current I,, dip latitude of current centre x,, focal distance w from current centre, distance of peak return intensity _c, from centre, and latitudinal extent of current L, from its centre. The D, E, J are Lloyd seasons .and A is annual Parameter
D
E
J
A
J, AKm-’ SD J,,, Akm-’ SD
46.97 26.;!1 - 11.08 6.18 -23.60
60.42 40.58 - 14.40 9.67
40.45 20.72 -9.72 4.98
49.28 21.24 -11.74 6.49
- 24.02 0.54 13.92 7.13 -0.16 0.13 2.87 0.08 5.41 0.14 11.72 0.71
- 23.82 0.78 16.89 9.08 -0.15 0.17 2.89 0.12 5.45 0.20 11.96 1.03
lOOJ,,,/J,, SD IF kA SD x, degree SD w degree SD x, degree SD L, degree SD
0.92 15.92 8.61 - 0. II 1 O.L!2 2.92 0:14 5.49 OZ!3 12.20 1.18
-23.83 0.95 20.85 13.43 -0.19 0.20 2.89 0.16 5.45 0.26 11.96 1.28
II
I I I I JFMAMJJASOND
I
I
l
l
3.2. Annual variations at 1100 and 1200 h LT The seasonal variations of the landmark measures of equatorial electrojet (EEJ) current in Fig. 4 at 1100 and 1200 h LT are very clear. The peak forward cur-
IJ
Fig. 4. Annual variations of the landmark measures of the equatorial electrojet current: the peak forward current intensity Jo Akn-‘, the peak return current intensity -J,,, Akm-‘, the total forward current IF kA, and the percentage ratio of the intensities - lOOJ,,,/J,,at 1100 and 1200 h LT. The points are S-month running means.
rent intensity J, Akm-‘, the peak return current intensity -.Z,,, Akm-‘, the total forward current IF kA, and the percentage of the peak return to the peak forward current intensity - lOOJ,,,/J, are highest in equinoxes and lowest in June solstice. Indeed, they are semiannual with maxima in March-April and SeptemberOctober, and minima in JuneJuly and DecemberJanuary. However, the second maximum of - 100Jm/Jo is shifted a month earlier. This semiannual variation of EEJ is in accord with Chapman and Raja Rao (1965) and others after them. The interesting similarity of the annual variation of - 100J,,,/Jo to the others is a new result and will be explained later. Again the seasonal variations of the landmark distances of the equatorial electrojet current in Fig. 5 at 1100 and 1200 h LT are very clear. The dip latitude of the current centre x, degrees, the focal distance w
Table 3. Comparison of some day time mean parameters of the equatorial electrojet found in this study with those of Onwumechili and Ezema (1992) found from satellite observations
Our Tables 1 and 2 Onwumechili et al. (1992)
I
Months 1986
difficult to find results comparable with them in the literature. Onwumechili and Ezema (1992) have given similar parameters of EEJ current. However, their results are for the solar activity maximum period of 1967 to 1969. Besides, their daytime means are from 0900 h to 1500 h LT and our daily means here are from 0700 h to 1700 h LT. Thus even the ratio of active to quiet intensities based on them would be too high because the very low intensities usually found at 0700,0800, 1600 and 1700 h LT would have reduced our means and make them incomparable with theirs. We may however compare other parameters not dependent on currl:nt intensity. Table 3 shows that our daytime mean parameters: a, LX,100J,,,/JO, w, x, and L, compare quite well with those of Onwumechili and Ezema (1992) found from satellite observations.
Source
I
a0
a
3.61 3.40
-1.56 - 1.53
J,,,/J,%. -23.80 -23.13
w”
X0,
L”,
2.89 2.75
5.45 5.17
11.96 12.28
560
a
S.0. Oko et al
10.7- 0.0
1100
8
12.4 -
b %
12.0 -
s
11.6-
\
+I’\+
\+/
Ii
+;+‘+-
5.2
‘+_+’
I I I II JFMAMJJASOND
11
I
I
I
II
Months 1986 Fig. 5. Annual variations of the landmark distances of the equatorial electrojet current: the dip latitude of the current centre x, degrees, the distance of the focus from the current
centre w degrees, the distance of the peak return current intensity from current centre X, degrees, and the latitudinal extent of the current L, degrees from the current centre, at 1100 and 1200 h local time. The points are 3-month running means.
degrees from the current centre, the distance x, degrees of the peak return current intensity from the centre, and the latitudinal extent L, degrees of the current at 1100 and 1200 h LT have maxima at June solstice and December solstice and minima at vernal and autumnal equinoxes. It is interesting that despite the day to day variability and the small magnitude of X o, it achieves a smooth semi-annual variation in accord with other landmark distances. It indicates that on the average the centre of EEJ slightly adjusts southwards as the focus moves southwards. To the best of our knowledge, these results are new. It is remarkable that the semi-annual variation of the landmark distances is the inversion of the semiannual variation of the landmark measures of the EEJ current. However, this is understandable from previous knowledge. Onwumechili and Agu (1981); Onwumechili et al. (1989a) and Onwumechili (1992b) found that the EEJ contracts at times, and as it contracts it intensifies. They also found that the intensification is relatively greater at a greater distance than at a smaller distance from the current centre. The smaller landmark distances at the equinoxes indicate contraction of the EEJ in response to which its current
intensifies at the equinoxes. The reverse happens at the June and December solstices. This explains the inverse variations of the landmark measures of EEJ current in Fig. 4 to the variations of the landmark distances of EEJ current in Fig. 5. This contrast in the annual variation was first apprehended by Onwumechili and Ogbuehi (1967b). We now explain why the annual variation of the percentage ratio of intensities, - 100J,/JO, is inverse to the variations of the landmark measures of EEJ current. As already mentioned, as the EEJ current contracts its intensification is relatively greater at a greater distance than at a smaller distance from the current centre. The peak return current intensity -J,,, is located at x, about 5’ away from the centre while the peak forward current intensity is located right at the centre. Thus as the EEJ current contracts, the intensification of -.I, is relatively greater than the intensification of J,, and therefore - 100J,,,/JO increases. Consequently, as contraction increases - lOOJ,/J,, increases along with the other landmark measures of EEJ current as found in Fig. 4. Indeed, the similarity of the variation of - lOOJ,/J,, to the variations of J,, -J, and I, and its being inverse to the variations of w, x, and L, is a direct demonstration of the relative intensification during contraction of the EEJ current.
Table 4. Noontime mean parameters of equatorial electrojet current: peak forward current intensity J,, peak return current intensity J,,,, percentage of peak return to peak forward intensity 100J,/JO, total forward current I,, dip latitude of current centre x,, focal distance w from current centre, distance of peak return intensity X, from centre, and latitudinal extent of current L, from its centre. The D, E, J are Lloyd seasons and A is annual. These are means of 1100 and 1200 LT Parameters
D
J, Akm-’ SD J,,, Akm-’ SD lOOJ,,,/J, SD IF kA SD X, degree SD w degree SD x, degree SD L, degree SD
75.13 17.95 - 17.66 4.57 - 23.22 0.68 26.39 5.71 -0.17 0.16 2.94
0.10 5.53 0.16 12.55 0.87
E 112.75 15.41 -21.34 4.21 -24.21 0.53 31.17 6.29 -0.29 0.02 2.84 0.06 5.32 0.10 11.37 0.63
J 55.57 10.55 -13.32 2.61 -23.91 0.36 18.90 2.93 -0.16 0.15 2.88 0.09 5.43 0.15 11.84 0.61
A 81.35 14.95 - 19.44 3.90 -23.78 0.59 27.49 5.19 -0.21 0.13 2.89 0.08 5.43 0.14 11.92 0.71
Geomagnetically quiet day ionospheric currents over the Indian sector-II
561
Table 5. Comparison of certain noontime parameters of equatorial electrojet from our present study, from Onwumechili and Ogbuehi (1967a) and from Onwumechili and Ezema (1992): the latitudinal scale length a degree, the latitudinal current distribution parameter G(,the peak forward current intensity J,, the peak return current intensity .Z,, the percentage of peak return to peak forward current intensity lOOJ,,,/J,,,the total forward current IF, the latitudinal distance of the focus w from the current centre, the distance of the peak return current intensity x,,, from the centre, and the latitudinal extent of the current L, from its centre
Parameter a degree SD :D J, Akm-’ SD active/quiet J,,, Akm-’ SD active/quiet lOOJ,,,/J,, SD Z, kA SD active/quiet w degree SD X, degree SD L, degree SD
From our Tables 1 and 4 Equinox
From our Tables 1 and 4 Annual
3.53 0.04 - 1.58 0.01 112.75 15.41
3.59 0.05 - 1.56 0.02 81.35 14.95
- 27.34 4.21
- 19.44 3.90
-24.21 0.53 37.17 6.29
-23.78 0.59 27.49 5.19
2.84 0.06 5.32 0.10 11.37 0.63
2.89 0.01 5.43 0.14 11.92 0.71
The earlier publications of Onwumechili and Ogbuehi (1967a) and Onwumechili and Ezema (1992) provide some material for comparison with Table 4. It is convenient to use Table 5 for a quick comparison. Onwumechili and Ogbuehi (1967a) studied latitudinal profiles of EEJ current with data from some observations in the Indian sector for 21 days of September 1958. The objective was day to day variability and consequently not all the parameters studied here were presented. What we have recovered from their work is given in column 4 of Table 5. The values for c( and 100J,,,/Jo have no standard deviation because they are medians. Since the1.r results in column 4 are for September, they should be compared with our Equinox result in column 2 of Table 5. With the exception of Jo, J,,, and IF, the two columns are in good agreement. These three parameters measure the intensity of EEJ current which was obviously greater in the solar activity maximum year of 1958 than in the solar activity minimum year of 1986. Consequently, we have given the ratio of their values in the row for “active/quiet“. Our mean ratio of 1.75 is in reasonable agreement wi1.h Patil et al. (1990). Using their indicator of EEJ strength devised from Sq(H) they
Onwumechili and Ogbuehi (1967a) 3.91 1.04 -1.59 187 64 1.66 -45.63 16.62 1.67 - 24.40 71.33 38.71 1.92 3.10 0.83 5.87 1.57 12.34 3.38
Onwumechili and Ezema (1992) 3.35 0.12 -1.51 0.08 210.5 18.8 2.59
-47.80 4.27 2.46 -22.71 1.69 69.5 6.7 2.53 2.73 0.06 5.11 0.14 12.40 1.07
found for the Indian sector the active/quiet ratios of 1.9 for D season, 1.4 for E, 1.5 for J and 1.5 for the year. From satellite data covering all sectors and all seasons Onwumechili and Ezema (1992) got the EEJ parameters given in column 5 of Table 5. These may be compared with the annual means of our present study given in column 3 of Table 5. Again, with the exception of the intensity related parameters, .Z,, J,,, and IF, the two columns are in good agreement. Onwumechili and Ezema (1992) worked on the data of solar activity maximum period of 1967-1969 and our study is for solar activity minimum year of 1986. We have calculated the ratio of “active/quiet“ from Jo, J,,, and IF. The mean ratio is 2.53 f 0.07. Marriot et al. (1979) give the global ratio of active/quiet for EEJ as varying from 1.7 to 2.2 depending on the particular solar cycle. Campbell (1989) and Patil et al. (1990) found that this ratio depends on the sector and the season. The difference in sector can account for the difference of this ratio in columns 4 and 5 of Table 5. It should be mentioned that Campbell (1989) found 2.85 f 0.1 for the northern himisphere of all sectors but his data combined WSq and EEJ currents.
562
S. 0. Oko et al.
5.3. Comparison of equatorial electrojet and worldwide part of Sq currents We now compare the main features of the worldwide part of Sq (WSq) in Onwumechili et al. (1995) with those of the equatorial electrojet (EEJ) here. Their diurnal variations of the annual means of .Z,, - .Z,,,and Zr are similar except that in EEJ they peaked at about 1100 hr but in WSq they peaked about l/2 h later at about 1130 h LT. On the other hand the variations of their - 100.Z,,,/.ZO contrast. It was lower before than after 1200 hr in EEJ but higher before than after 1200 h in WSq. The diurnal variations of the annual means of the landmark distances x,, w, x, and L, of WSq are very different from those of the EEJ. These distances for WSq decreased from dawn to a minimum at about 1100 h before increasing towards dusk. On the contrary, the distances for EEJ generally decreased from about dawn towards dusk without any minimum. The diurnal variations of the seasonal meams of .Z,, and IF showed that the order of magnitude of the seasonal means for both WSq and EEJ depend on the hour. There are however, significant differences between the two. The WSq peaked around 1100 h LT in all the seasons. The EEJ peaked in the order of intensity. The most intense E season peaked earliest at about 1045 h, followed by the D season at about 1130 h, and the least intense J season peaked at about 1200 h LT. The tables of daily mean parameters show several differences between WSq and EEJ among which we select the following. There is hardly any difference between the intensities of WSq in three seasons. On the other hand the EEJ is decisively most intense in the E season and least intense in the J season, the intensity ratio being about 1.5. The centre of WSq is about 4” south of the dip equator and lies half way between the geographic and dip equators. On the contrary, the centre of the EEJ is extremely close to the dip equator. The focus of WSq current is at about 40”N geographic latitude (about 35.5” from its centre), but the focus of EEJ current is at about 11 “N geographic latitude (about 2.9” from its centre). The latitudinal extent of WSq is about 86” latitude but the latitudinal extent of EEJ is about 12” latitude from its centre. The annual variations of WSq and EEJ are markedly different. The .Z,, -J,, Z, and - 100J,,,/JOof WSq have no clearly consistent annual variation. The J,, -J,,,, Z, and - 100J,/JO of EEJ have clearly semiannual variation with maxima in vernal equinox (March-April) and autumnal equinox (SeptemberOctober), and minima in June solstice (June-July) and
December solstice (December-January). The x,, w, x, and L, of WSq have no clearly consistent annual variation. On the other hand the x,, w, x, and L1 of EEJ have clearly semi-annual variations with minima in vernal equinox (April) and autumnal equinox (September), and maxima in June solstice (June-July) and December solstice (December-January). Both the WSq and EEJ exhibit the phenomenon of contracting at times, and intensifying while contracting. For both currents the intensification in response to the contraction is relatively greater at a farther than at a nearer distance to the centre of the current.
4. CONCLUSIONS
Some 498 hourly latitudinal profiles of equatorial electrojet (EEJ) from 0700 to 1700 h LT have been analyzed from five quietest days of each month of 1986. They yield interesting diurnal and annual variations of eight landmark parameters depicting the major features and structures of the latitudinal profiles of EEJ current. The eight parameters are: the peak forward current intensity J, Akm-’ at the current centre, the peak return current intensity J,,, Akm-‘, the percentage ratio of the peak return to the peak forward current intensity 100J,,,/JO,the total forward current Z, kA, the dip latitude of the current centre x, degrees, the distance of the focus w degrees from the current centre, the distance of the peak return current intensity x, degrees from the current centre, and the latitudinal extent of the current L, degrees from its centre. The main conclusions include the following. 1. The diurnal
variations
of the annual
means of
J,, -J,,, and Z, peak close to 1100 h LT. 2. The centre of the EEJ varies from day to day but on the average it remains close to the dip equator. Despite these, its mean exhibits smooth diurnal and annual variations similar to other landmark distances. 3. The diurnal variations of x,, w, x, and L, suggest that EEJ is widest near dawn with its centre and focus most northerly. It then contracts as its centre and focus move southwards towards noon. 4. The diurnal variations of the seasonal means of J,, and Z, did not peak at the same time in all seasons. The J, and Z, of EEJ seem to peak earlier when the intensity is higher than when it is lower. 5. The seasonal order of magnitude of the intensity of EEJ is not the same at all hours of the day. The D season is the least intense before 1000 h LT but the most intense after 1300 h LT. Around noontime the E season was most intense and the J season least intense. The seasonal variation is not semi-annual in the afternoon.
Geomagnetically
quiet day ionospheric
6. The landmark measures of EEJ current: J,, -.Z,, IF and - 100J,,,/JO have semi-annual variations with maxima in vernal equinox (March-April) and autumnal equinox (September-October), and minima in June solstice (June-July) and December solstice (December-January). 7. The landmark distances of EEJ current: x,, W, x, and L, have semi-annual variations with minima in vernal equinox (April) and autumnal equinox (September), and maxima in June solstice (JuneJuly) and December solstice (December-January). 8. Thus the landmark measures of EEJ current vary inversely with the landmark distances of EEJ current. As the EEJ current contracts, it intensifies. 9. The increase Iof - lOOJ,/J, as the EEJ current contracts is a direct demonstration of the finding that the intensification in response to contraction is rela-
currents
over the Indian
sector-II
563
tively greater at a farther than at a nearer distance to the current centre. 10. Tables of daily means and noontime means of the eight landmark parameters of EEJ have been provided. Some noontime annual means are: x0 = -0.21 f0.13 degree, w = 2.89f0.08 degree, x, = 5.43fO.l4degree, and& = 11.92kO.71 degree. The ratio of the peak forward current intensity J, in the three seasons is D: E: J = 1.36:2.03:1. 11. Certain properties of the worldwide part of Sq are found to be remarkably different from those of the equatorial electrojet including some key features of diurnal and seasonal variations. Acknowledgements-The authors are very grateful to Dr R. Rajaram and the Director of Indian Institute of Geomagnetism, Bombay, for the supply of the data studied.
REFERENCES
Campbell
W. H
1989
Chapman
S. and Raja Rao K. S.
1965
Davis T. N.. Burrows
Fambitakoye
Forbush
K. and Stolarik
1967
P. N.
1976
M.
1961
0. and Mayaud
S. E. and Casaverde
J. D.
Hesse D.
1982
Hutton
R.
1967a
Hutton
R.
1967b
Kane R. P. and Trivelii N. B.
1980
Marriot R. T., Richmond Venkateswaran S. V. Matsushita S.
1979
A. D. and
1967
Ogbuehi
P. 0. and Onwumechili
C. A.
1963
Ogbuehi
P. 0. and Onwumechili
C. A.
1965
Onwumechili
C. A.
1959a
The regular geomagnetic field variations during quiet solar conditions, Chapter 6, vol. 3, pp. 285460, in Geomagnetism, ed. J. A. Jacobs, Academic Press. The H and Z variations along and near the equatorial electrojet in India, Africa and the Pacific, J. atmos. terr. Phys. 21, 559-58 1. Altitude Survey of the equatorial electrojet with rocketborne magnetometers, J. geophys. Res. 72, 184% 1861. Equatorial electrojet and regular daily variation &-I. A determination of the equatorial electrojet parameters, J. atmos. terr. Phys. 38, l-17. The equatorial electrojet in Peru, Carnegie Institution of Washington Publication No. 620. An investigation of the equatorial electroiet by means of ground-based magnetic measurements in Brazil, Ann. GeoDhvs. 38.315-320. Sq currents in the ‘American equatorial zone during the IGY-I. Seasonal effects, J. atmos. terr. Phys. 29, 141 l-1427. Sq currents in the American equatorial zone during the IGY-II. Day to day variability, J. atmos. terr. Phys., 29, 1429-1442. Implication of Z variations during a near noon equatorial counter electrojet on March 6, 1967 in the Indian zone J. geophys. Res. 85,47054710. The quiet time equatorial electrojet and counter electrojet, J. Geomag. Geoelectr. 31, 3 1 l-340. Solar quiet and lunar daily variation fields, in Physics ofGeomagnetic Phenomena, Chapter III-l, pp. 301424, ed. Matsushita S. and W.H. Campbell, Academic Press, New York. Recent measurements of the magnetic field of the equatorial electrojet in Nigeria, J. geophys. Res., 68,24212424. Seasonal studies of the equatorial electrojet during low solar activity, J. geophys. Res. 70,4909-4919. A study of the equatorial electrojet-I. An experimental study, J. atmos. terr. Phys. 13, 222-234.
S. 0. Oko et al.
564 Onwumechili
C. A.
1959b
Onwumechili
C. A.
1966a
Onwumechili
C. A.
196613
Onwumechili
C. A.
1966~
Onwumechili
C. A.
1967
Onwumechili
C. A.
1992a
Onwumechili
C. A.
Onwumechili
C. A. and Agu C. E.
1981
Onwumechili
C. A. and Ezema P. 0.
1992
Onwumechili
C. A. and Ogbuehi
P. 0.
1967a
Onwumechili
C. A. and Ogbuehi
P. 0.
1967b
Onwumechili
C. A., Agu C. E. and Ozoemena
Onwumechili
C. A., Ozoemena
Onwumechili
C. A., Oko S. 0. and Ezema P. 0.
P. C.
1989a
P. C. and Agu C. E.
1989b
Patil A. R., Rao D. R. K. and Rastogi
Srivastava
1990
1992
B. J.
Yacob A. and Khanna
R. G.
1995
K. B
1963
A study of the equatorial electrojet-II. A model electrojet that fits H observations, J. atmos. terr. Phys. 13,2355257. A new model of the equatorial electrojet current, Nigerian J. Sci. 1, 1 l-19. A three dimensional model of density distribution in ionospheric currents causing part of quiet day geomagnetic variations. II’ Symposium d’aeronomie equatoriale, 1577162. A special publication of Ann. Geophys. The magnetic field of a current model for part of geomagnetic Sq variations. II” Symposium d’aeronomie equatoriale, 1633170. A special publication of Annales de Geophysique. Geomagnetic variations in the equatorial zone. Physics of Geomagnetic Phenomena, chapter 111-2, pp. 425507, ed. Matsushita S. and W.H. Campbell, Academic Press, New York. A study of rocket measurements of ionospheric currents-v. Modelling rocket profiles of low latitude ionospheric currents, Geophys. J. Int. 108,673-682. A study of rocket measurements of ionospheric currents-111. Ionospheric currents at the magnetic dip equator, Geophys. J. Int. 108,647-659. The relationship between the current and the width of the equatorial electrojet, J. atmos. terr. Phys. 43,573578. Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set, J. atmos. terr. Phys. 54, 1535-l 544. Preliminary results on the equatorial electrojet in India, J. Geomag. Geoelectr. 19, 15-22. Analysis of the magnetic field of the equatorial electrojet, J. atmos. terr. Phys. 29, 5533566. Effect of equatorial electrojet intensity on its landmark distances, J. Geoelectr. 41,461467. Landmark values of equatorial electrojet current and magnetic field along a meridian near noon, J. Geomag. Geoelectr. 41,443459. Geomagnetically quiet day ionospheric currents over the Indian sector-I. Worldwide part of Sq currents, Paper I of this study. Equatorial electrojet strengths in the Indian and American sectors-II. During high solar activity, J. Geomag. Geoelectr. 42,813-823. New results on the dip equator and equatorial electrojet in India, J. atmos. terr. Phys. 54, 871-880. Geomagnetic Sq variations and parameters of the Indian electrojet for 1958 and 1959, Indian J. Meteoroi. Geophys. 14,470-477.
of Almaspheric
and Terrestrial
Pergamon 0021-9169(95)00056-9
Physics, Vol. 58, No. 5, pp. 555564, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All riehts reserved OOZI-9169/96 bS.OO+O.OO
Geomagnetically quiet day ionospheric currents over the Indian sector-II. Equatorial electrojet currents S. 0. Oko,* C. A. Onwumechilit
and P. 0. Ezema$
*Department of Computer Science, University of Nigeria, Nsukka, Nigeria; t69 Lansdowne ney, London
E8 3EP, U.K.; SDepartment of Physics, Enugu State University Technology, Enugu, Nigeria
Drive, Hackof Science and
(Received 29 November 1994; accepted in revisedform 8 December 1994) Abstract-With 1986 quiet days data of Indian observatories, the equatorial electrojet (EEJ) has been studied in terms of 8 landmark parameters that reveal the stuctures of hourly latitudinal profiles of EEJ current from 0700 to 1700 h local time. The landmark distances suggest that near dawn, the EEJ is widest and its centre and focus are most northerly before it contracts towards local noon with its centre and focus moving southwards. The seasonal means of peak current intensity and the total forward current seem to peak earlier when the intensity of EEJ is higher than when it is lower. The seasonal order of EEJ intensity is found not to be the same at all hours of the day. This implies that seasonal variation of EEJ is not semiannual at certain daytime hours. The landmark distances of EEJ current have semi-annual variations with minima in the vernal and autumnal equinoxes and maxima at June and December solstices, but annual variations of the measures of EEJ current are just the reverse. Certain properties of the worldwide part of Sq are found to be markedly different from those of EEJ including some key features of diurnal and seasonal variations.
equatorial counter electrojet. This is designed to facilitate their intercomparisons.
1. I’NTRODUCTION
The earliest studies of latitudinal profiles of the Sq magnetic fields were based on daily ranges (Onwumechili, 1959a; Onwumechili, 1959b; Forbush and CasaVerde, 1961; Yacob
2. DATA ANALYSIS It has been demonstrated that the continuous distribution of current density model of ionospheric currents introduced by Onwumechili (1966a); Onwumechili (1966b) fits extremely well the altitude profiles of ionospheric currents observed by rockets (Onwumechili, 1992a), as well as the horizontal X or H and the vertical 2 geomagnetic fields of both the worldwide part of Sq (WSq) and equatorial electrojet (EEJ) (Onwumechili, 1966~; Onwumechili, 1967). Onwumechili and Ezema (1992) have derived the thick current shell and the thin current shell formats of the X and Z magnetic field components from the continuous distribution of current density model. Here we shall use the thin current shell equations of the magnetic fields, namely:
(sg.z)p4X = $a[@+
uv + 2rXa)(u+b)2 +(v+ctv+24(v+4*]
-(sg.x)p’Z
= $u(u+b)[(l
+c()(u+b)2
+(v+av+3u-ctu)(v+u)] 555
(1)
(2)
S. 0. Oko et al
556
P* = (u+b)Z+(u+a)2
(3)
K = 0.27~5,
J,, = K/0.271
(5)
c? Jm = Jo4(cc- 1)
IF = aJ,
Lx2
= __ 4(a-
L, a
M-l = ~ a+1
(10)
(4)
u = (xl and v = 121sg.x = sign of x = x/u and sg.z = sign of z = z/v. The origin is at the centre of the current, x is northward latitudinal distance, y is eastwards, and z is vertically downward distance, a and b are latitudinal and vertical scale lengths, respectively, CLis dimensionless constant controlling latitudinal distribution of current, Jo is the peak current intensity or the heightintegrated current density at the current centre and K is the magnetic field constant. Since the centre of the equatorial electrojet is not necessarily at the dip equator (Onwumechili, 1967; Onwumechili and Ogbuehi, 1967a; Srivastava, 1992) we introduce x,, the dip latitude of the centre of EEJ. Then in equations (1) and (2), u = 6-x,, where 6 is the dip latitude of the observatory. The two equations now contain four parameters of the model: x,, K, a and CC. These parameters may be determined by modelling observed latitudinal profiles with equation (1) or (2) or both. Once these parameters of the model have been determined, we can obtain many physical parameters of the current and its magnetic field. Here we are interested in the following landmark parameters of the latitudinal profile of the current: the peak intensity of the forward current Jo Akm-’ at its centre, the peak intensity of the return current J,,, Akm-‘, the percentage of the peak return to the peak forward current intensity lOOJ,/J,,, the total forward current IF kA flowing between the current foci, the focal distance w degrees from the current centre, the distance x, degrees of the peak return current location from the current centre, and the latitudinal extent of the current L, degrees from its centre. The expressions of these parameters are as follows:
J,/J,
(L,/a+a/L,)tan-‘-
(6b)
1)
(-rx)“2+(l+a)tan-‘(_tL),,2
1
1
(7)
w2 = a’/( - a)
(8)
x2 = a’(a - 2)/a
(9)
We now describe how these parameters have been determined for the equatorial electrojet (EEJ). The latitude L = tan’ (Z/2H) distorts distances on the globe and makes conversion to geographic latitude difficult. We therefore used the true dip latitude 6 which is the actual latitudinal distance from the dip equator. The coordinates of the eight Indian observatories whose 1986 data were used in the analysis are: Trivandrum (8.29”N, 7657”E, 6 = 0.20”), Kodaikanal(10,23”N, 77.47”E, 6 = 2.14”), Annamalainagar (11.37”N, 79.68”E, 6 = 3.28”) Hyderabad (17.42”N, 7855”E, 6 = 9.33”) Alibag (18,63”N, 72.87”E, 6 = 10.54”), Ujjain (23.18”N, 75.78”E, 6 = 15.09”) Jaipur (26,92”N, 75.80”E, 6 = 18.83”) and Sabhawala (30.37”N, 77.80”E, 6 = 22.28”). The dip equator crossed their mean longitude of 76.82 t_ 2”E at 8.09”N. For conversion in the region L = -0.2872 + 1.2387 6. Being a solar activity minimum year, most days of 1986 were quiet. Nevertheless, in each month we used the five quietest among the ten internationaly selected quiet days for the month. Of the 60 days, the Ap index ranged from 2 to 6 with the mean of 3.9 and only 2 days had Ap = 6. See Onwumechili et al. (1995) for more details. It is well known that the observed field variation components X0 and Z, relate to their external parts X, and Z,, and their internally induced parts X, and Z, by X0 = X,+X,,
(1 la)
z, = z,-z,.
(lib)
The last five observatories: Hyderabad, Alibag, Ujjain, Jaipur and Sabhawala, are outside the influence of EEJ and therefore their data represent WSq field. Onwumechili et al. (1995) have described in Paper 1 of this study how the data of these five observatories have been used to determine the model parameters x,, K, a and CI of WSq, using equation (1). Thereafter, the parameters are used in equation (1) and (2) to extrapolate X and Z components of WSq into the EEJ zone, by computing their values at Trivandrum, Kodaikanal and Annamalainagar. These computed values are subtracted from the observed X and Z at the three observatories to obtain X and 2 components of the electrojet field at Trivandrum, Kodaikanal and Annamalanagar. We now describe how the X and Z components of the EEJ field at Kodaikanal and Annamalainagar together with the X component only at Trivandrum have been used to determine the parameters of the
Geomagnetically quiet day ionospheric currents over the Indian sector-II EEJ. Unfortunately, the ratio of internal to external fields of the EEJ is not as clear cut as that of WSq; indeed the ratio is found to depend on the distance from the centre of the EEJ (Davis et al., 1967; Onwumechili, 199213). We represent the internal field by the field of the image EEJ induced in the conducting earth, especially as this has produced results that agreed with observations (Onwumechili, 199213). For this purpose, the earth is modelled as nonconducting to a depth of D, = 200 km and thereafter it becomes perfectly ‘conducting. The image EEJ is as far behind the conducting surface as the overhead EEJ is in front of it. Since the overhead EEJ is at a height of u, = 106 km (Onwumechili, 1992b), the image EEJ is at a depth of vi = 506 km. In equation (1) for the external field X, we substitute u, = 106 km = 0.96” but for the internal field X, we substitute v, = 506 km = 4.6”. Similarly, we substitute the same v, and vi into equation (2) for Z, and Z,, respectively. From rocket-observed altitude profiles of EEJ current density Onwumechili (1992a) determined b = 8 km = 0.07”. This value is substituted for b in the equation (1) and (2) for X,, X,, Z, and Z,. Since u in the equations is measured from the centre of the current, then for each observatory u = 6 -x0, where 6 is its dip latitude. We take Kodaikanal as an example. Following equation (1 la) the observed EEJ X.,, is equated to the sum of the expressions for X, and Xi with u appropriate for Kodaikanal. In like manner following equation (1 lb) the observed 2, of EEJ is equated to the difference of the expressions for Z, from Z, with u appropriate for Kodaikanal. This is repeated for X,, and Z, at Annamalainagar, and X0 at Trivandrum. Thus we have five equations. In view of the substitutions described above, the five equations contain only four unknowns, namely: x,, K, a and CC. The equations are solved by the method of least squares to determine the unknown parameters for the EEJ. Table 1 gives the mean values of the four parameters for each hour 070&l 700 h LT.
3. RESULTS AND DISCUSSIONS
3.1. Diurnal variations The eight parameters selected for study here epitomize the major attributes and structures of the latitudinal variation of the current on a meridional plane. Therefore, Onwumechili et al. (1989b) called them the landmark values of the current. It is convenient to group them into landmark values measuring the current, and landmark values measuring the distances. Figure 1 gives the diurnal variations of the annual
557
Table 1. The parameters of the continuous distribution of current density model for the magnetic field of the equatorial electrojet from 0700 to 1700 h LT: the dip latitude of the current centre x,, the magnetic field constant K, the latitudinal scale length a, and the latitudinal current distribution parameter cr; determined from 1986 data of three Indian observatories (Trivandrum, Kodaikanal and Annamalainagar) using the thin current shell equations. Local Time, h 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 Mean
x0 degrees
KnT
a degrees
0.23 0.05 -0.01 -0.11 -0.18 -0.24 -0.27 -0.27 -0.27 -0.30 -0.30 -0.15
7.7953 19.5276 35.0537 45.5849 57.7163 51.4880 41.5805 33.9021 23.1943 15.7725 8.9801 30.9632
3.8007 3.7396 3.7260 3.6739 3.6154 3.5696 3.5201 3.5215 3.5139 3.5000 3.5060 3.6079
a -1.5154 - 1.5219 - 1.5156 -1.5266 -1.5499 - 1.5708 - 1.5874 - 1.6000 -1.6000 - 1.6000 - 1.5972 -1.5623
means of the landmark measures of the equatorial electrojet current from 0700 to 1700 h LT. The peak forward current intensity J,, the peak return current intensity -.I,,, and the total forward current IF rise from dawn to a peak very close to 1100 h LT before decreasing towards dusk. The percentage ratio of the peak return to the peak forward current intensity - 100J,/JO is lower before noon than afternoon. The variations of J,, --.I,,, and 1, are similar because
32 4
28 -
H!
24 -
IF
-lOOJ,,,/J, _ 25 - 24 +? B
20 -
-23
JO
- 22 60
740 1 a
I-?
-J, +/%+.
:fi: +’
20 0
+’
+’
07 08 09
20 +\
+\ +\
9 ‘i
i!
10 4 %
0
B
7
10 11 12 13 14 15 16 17 Local
time hours
Fig. 1. Diurnal variations of annual means of the landmark measures of equatorial electrojet current: the peak forward current intensity J, Akm-‘, the peak return current intensity -J,,, Akm-‘, the total forward current IF kA, and the percentage
ratio of the peak return to the peak forward intensities - lOOJ,/.T,.
current
S. 0. Oko et al.
558
3.2 s $3.0 ~ d 3 2.8 -
- 5.6
g 12% 4 ll-
8
- 5.4
5
-5.2
,E
7E
SlOC,, , , , , , , ,
(
,_I
07 08 09 10 11 12 13 14 15 16 17
Local time hours Fig. 2. Diurnal variations of the annual means of the landmark distances of the equatorial electrojet current: the dip latitude of the current centre X, degree, the distance of the focus from the current centre w degree, the distance of the peak return current intensity from the current centre x, degrees, and the latitudinal extent of the current L, degrees from the current centre.
increase in Jo is expected to increase both -J,,, and ZF. Figure 2 shows the diurnal variations of the annual means of landmark distances of equatorial electrojet current. The focal distance w degrees from the current centre, the distance x,,, degrees of the peak return current intensity from the current centre, and the latitudinal extent LI degrees of the current decrease from their largest value around dawn to about 1300 h LT and then level up almost to a plateau towards dusk. Their similarity suggests that the EEJ over India as a whole contracts towards local noon from its broadest extent near dawn. Despite its day to day variability, the mean dip latitude of the current centre x, degrees also varies more or less like the other landmark distances. Thus the current centre tends to be most northerly around dawn and moves southwards towards local noon. The mean location of the centre varies little in the afternoon. Similarly, Kane and Trivedi (1980) found in same Indian sector that for the average of quiet days the EEJ seemed to be centered near the dip equator in the morning, but shifted steadily southwards till evening. Comparison of Figs 1 and 2 shows that the intensity ratio - 100Jm/Jo varies inversely as the landmark distances w, x, and L,. This implies that as the EEJ current contracts, its intersity increases. However, the intensification is relatively greater at the more distant location of -J,,, at x, than at the current centre location of Jo. This is a direct demonstration of the conclusion reached by Onwumechili and Agu (198 l),
120-
~loocp x SO.-;: z e, 606 .; 2 2 Li
aI
4020-
ot_, , ,
(
,
,
,
,
,
,
,
07 08 09 10 11 12 13 14 15 16 17
Local time hours Fig. 3. Diurnal variation of seasonal means of two landmark measures of equatorial electrojet current: the peak forward current intensity .Z,,Akm-‘, aid the total forward current IF kA. The Lloyd seasons D. E. J are shown as follows: crosses for D (November, Decembe;, January and February), filled circles E (March, April, September and October), and open circles for J (May, June, July and August).
Onwumechili et al. (1989b) and Onwumechili (1992a). It is seen in Fig. 3 that the seasonal means of the peak forward current intensity Jo Akm-’ and the total forward current Z, kA do not peak at the same time in all seasons. The EEJ seems to peak earlier when its intensity is higher than when it is lower. They peaked at about 1045 h in E season, at about 1130 h in D season and at about 1200 h LT in J season. The peak values are in the following order of decreasing magnitude: E, D and J, but the classical expectation for Sq currents in the region studied is J, E, D. However, the result here is consistent with the seasonal variation of Sq(AH) in the EEJ zone (Chapman and Raja Rao, 1965). The surprising result here is that the seasonal order of magnitude of EEJ intensity is not the same at all hours of the day. For example, the EEJ intensity in D season is lower than the intensities in E and J seasons in the morning before 1000 h LT, but greater than the intensities in E and J seasons in the afternoon after 1300 h LT. This implies that the seasonal variation of EEJ is not semi-annual in the afternoon. Therefore the study of seasonal variations should not be limited to noontime values. Table 2 gives new results on EEJ current. It is
Geomagnetically quiet day ionospheric currents over the Indian sector-II Table 2. Daily mean parameters of equatorial electrojet current: peak forward current intensity J,, peak return current intensity J,,,, percentagl: of peak return to peak forward intensity lOOJ,,,/J,, total forward current I,, dip latitude of current centre x,, focal distance w from current centre, distance of peak return intensity _c, from centre, and latitudinal extent of current L, from its centre. The D, E, J are Lloyd seasons .and A is annual Parameter
D
E
J
A
J, AKm-’ SD J,,, Akm-’ SD
46.97 26.;!1 - 11.08 6.18 -23.60
60.42 40.58 - 14.40 9.67
40.45 20.72 -9.72 4.98
49.28 21.24 -11.74 6.49
- 24.02 0.54 13.92 7.13 -0.16 0.13 2.87 0.08 5.41 0.14 11.72 0.71
- 23.82 0.78 16.89 9.08 -0.15 0.17 2.89 0.12 5.45 0.20 11.96 1.03
lOOJ,,,/J,, SD IF kA SD x, degree SD w degree SD x, degree SD L, degree SD
0.92 15.92 8.61 - 0. II 1 O.L!2 2.92 0:14 5.49 OZ!3 12.20 1.18
-23.83 0.95 20.85 13.43 -0.19 0.20 2.89 0.16 5.45 0.26 11.96 1.28
II
I I I I JFMAMJJASOND
I
I
l
l
3.2. Annual variations at 1100 and 1200 h LT The seasonal variations of the landmark measures of equatorial electrojet (EEJ) current in Fig. 4 at 1100 and 1200 h LT are very clear. The peak forward cur-
IJ
Fig. 4. Annual variations of the landmark measures of the equatorial electrojet current: the peak forward current intensity Jo Akn-‘, the peak return current intensity -J,,, Akm-‘, the total forward current IF kA, and the percentage ratio of the intensities - lOOJ,,,/J,,at 1100 and 1200 h LT. The points are S-month running means.
rent intensity J, Akm-‘, the peak return current intensity -.Z,,, Akm-‘, the total forward current IF kA, and the percentage of the peak return to the peak forward current intensity - lOOJ,,,/J, are highest in equinoxes and lowest in June solstice. Indeed, they are semiannual with maxima in March-April and SeptemberOctober, and minima in JuneJuly and DecemberJanuary. However, the second maximum of - 100Jm/Jo is shifted a month earlier. This semiannual variation of EEJ is in accord with Chapman and Raja Rao (1965) and others after them. The interesting similarity of the annual variation of - 100J,,,/Jo to the others is a new result and will be explained later. Again the seasonal variations of the landmark distances of the equatorial electrojet current in Fig. 5 at 1100 and 1200 h LT are very clear. The dip latitude of the current centre x, degrees, the focal distance w
Table 3. Comparison of some day time mean parameters of the equatorial electrojet found in this study with those of Onwumechili and Ezema (1992) found from satellite observations
Our Tables 1 and 2 Onwumechili et al. (1992)
I
Months 1986
difficult to find results comparable with them in the literature. Onwumechili and Ezema (1992) have given similar parameters of EEJ current. However, their results are for the solar activity maximum period of 1967 to 1969. Besides, their daytime means are from 0900 h to 1500 h LT and our daily means here are from 0700 h to 1700 h LT. Thus even the ratio of active to quiet intensities based on them would be too high because the very low intensities usually found at 0700,0800, 1600 and 1700 h LT would have reduced our means and make them incomparable with theirs. We may however compare other parameters not dependent on currl:nt intensity. Table 3 shows that our daytime mean parameters: a, LX,100J,,,/JO, w, x, and L, compare quite well with those of Onwumechili and Ezema (1992) found from satellite observations.
Source
I
a0
a
3.61 3.40
-1.56 - 1.53
J,,,/J,%. -23.80 -23.13
w”
X0,
L”,
2.89 2.75
5.45 5.17
11.96 12.28
560
a
S.0. Oko et al
10.7- 0.0
1100
8
12.4 -
b %
12.0 -
s
11.6-
\
+I’\+
\+/
Ii
+;+‘+-
5.2
‘+_+’
I I I II JFMAMJJASOND
11
I
I
I
II
Months 1986 Fig. 5. Annual variations of the landmark distances of the equatorial electrojet current: the dip latitude of the current centre x, degrees, the distance of the focus from the current
centre w degrees, the distance of the peak return current intensity from current centre X, degrees, and the latitudinal extent of the current L, degrees from the current centre, at 1100 and 1200 h local time. The points are 3-month running means.
degrees from the current centre, the distance x, degrees of the peak return current intensity from the centre, and the latitudinal extent L, degrees of the current at 1100 and 1200 h LT have maxima at June solstice and December solstice and minima at vernal and autumnal equinoxes. It is interesting that despite the day to day variability and the small magnitude of X o, it achieves a smooth semi-annual variation in accord with other landmark distances. It indicates that on the average the centre of EEJ slightly adjusts southwards as the focus moves southwards. To the best of our knowledge, these results are new. It is remarkable that the semi-annual variation of the landmark distances is the inversion of the semiannual variation of the landmark measures of the EEJ current. However, this is understandable from previous knowledge. Onwumechili and Agu (1981); Onwumechili et al. (1989a) and Onwumechili (1992b) found that the EEJ contracts at times, and as it contracts it intensifies. They also found that the intensification is relatively greater at a greater distance than at a smaller distance from the current centre. The smaller landmark distances at the equinoxes indicate contraction of the EEJ in response to which its current
intensifies at the equinoxes. The reverse happens at the June and December solstices. This explains the inverse variations of the landmark measures of EEJ current in Fig. 4 to the variations of the landmark distances of EEJ current in Fig. 5. This contrast in the annual variation was first apprehended by Onwumechili and Ogbuehi (1967b). We now explain why the annual variation of the percentage ratio of intensities, - 100J,/JO, is inverse to the variations of the landmark measures of EEJ current. As already mentioned, as the EEJ current contracts its intensification is relatively greater at a greater distance than at a smaller distance from the current centre. The peak return current intensity -J,,, is located at x, about 5’ away from the centre while the peak forward current intensity is located right at the centre. Thus as the EEJ current contracts, the intensification of -.I, is relatively greater than the intensification of J,, and therefore - 100J,,,/JO increases. Consequently, as contraction increases - lOOJ,/J,, increases along with the other landmark measures of EEJ current as found in Fig. 4. Indeed, the similarity of the variation of - lOOJ,/J,, to the variations of J,, -J, and I, and its being inverse to the variations of w, x, and L, is a direct demonstration of the relative intensification during contraction of the EEJ current.
Table 4. Noontime mean parameters of equatorial electrojet current: peak forward current intensity J,, peak return current intensity J,,,, percentage of peak return to peak forward intensity 100J,/JO, total forward current I,, dip latitude of current centre x,, focal distance w from current centre, distance of peak return intensity X, from centre, and latitudinal extent of current L, from its centre. The D, E, J are Lloyd seasons and A is annual. These are means of 1100 and 1200 LT Parameters
D
J, Akm-’ SD J,,, Akm-’ SD lOOJ,,,/J, SD IF kA SD X, degree SD w degree SD x, degree SD L, degree SD
75.13 17.95 - 17.66 4.57 - 23.22 0.68 26.39 5.71 -0.17 0.16 2.94
0.10 5.53 0.16 12.55 0.87
E 112.75 15.41 -21.34 4.21 -24.21 0.53 31.17 6.29 -0.29 0.02 2.84 0.06 5.32 0.10 11.37 0.63
J 55.57 10.55 -13.32 2.61 -23.91 0.36 18.90 2.93 -0.16 0.15 2.88 0.09 5.43 0.15 11.84 0.61
A 81.35 14.95 - 19.44 3.90 -23.78 0.59 27.49 5.19 -0.21 0.13 2.89 0.08 5.43 0.14 11.92 0.71
Geomagnetically quiet day ionospheric currents over the Indian sector-II
561
Table 5. Comparison of certain noontime parameters of equatorial electrojet from our present study, from Onwumechili and Ogbuehi (1967a) and from Onwumechili and Ezema (1992): the latitudinal scale length a degree, the latitudinal current distribution parameter G(,the peak forward current intensity J,, the peak return current intensity .Z,, the percentage of peak return to peak forward current intensity lOOJ,,,/J,,,the total forward current IF, the latitudinal distance of the focus w from the current centre, the distance of the peak return current intensity x,,, from the centre, and the latitudinal extent of the current L, from its centre
Parameter a degree SD :D J, Akm-’ SD active/quiet J,,, Akm-’ SD active/quiet lOOJ,,,/J,, SD Z, kA SD active/quiet w degree SD X, degree SD L, degree SD
From our Tables 1 and 4 Equinox
From our Tables 1 and 4 Annual
3.53 0.04 - 1.58 0.01 112.75 15.41
3.59 0.05 - 1.56 0.02 81.35 14.95
- 27.34 4.21
- 19.44 3.90
-24.21 0.53 37.17 6.29
-23.78 0.59 27.49 5.19
2.84 0.06 5.32 0.10 11.37 0.63
2.89 0.01 5.43 0.14 11.92 0.71
The earlier publications of Onwumechili and Ogbuehi (1967a) and Onwumechili and Ezema (1992) provide some material for comparison with Table 4. It is convenient to use Table 5 for a quick comparison. Onwumechili and Ogbuehi (1967a) studied latitudinal profiles of EEJ current with data from some observations in the Indian sector for 21 days of September 1958. The objective was day to day variability and consequently not all the parameters studied here were presented. What we have recovered from their work is given in column 4 of Table 5. The values for c( and 100J,,,/Jo have no standard deviation because they are medians. Since the1.r results in column 4 are for September, they should be compared with our Equinox result in column 2 of Table 5. With the exception of Jo, J,,, and IF, the two columns are in good agreement. These three parameters measure the intensity of EEJ current which was obviously greater in the solar activity maximum year of 1958 than in the solar activity minimum year of 1986. Consequently, we have given the ratio of their values in the row for “active/quiet“. Our mean ratio of 1.75 is in reasonable agreement wi1.h Patil et al. (1990). Using their indicator of EEJ strength devised from Sq(H) they
Onwumechili and Ogbuehi (1967a) 3.91 1.04 -1.59 187 64 1.66 -45.63 16.62 1.67 - 24.40 71.33 38.71 1.92 3.10 0.83 5.87 1.57 12.34 3.38
Onwumechili and Ezema (1992) 3.35 0.12 -1.51 0.08 210.5 18.8 2.59
-47.80 4.27 2.46 -22.71 1.69 69.5 6.7 2.53 2.73 0.06 5.11 0.14 12.40 1.07
found for the Indian sector the active/quiet ratios of 1.9 for D season, 1.4 for E, 1.5 for J and 1.5 for the year. From satellite data covering all sectors and all seasons Onwumechili and Ezema (1992) got the EEJ parameters given in column 5 of Table 5. These may be compared with the annual means of our present study given in column 3 of Table 5. Again, with the exception of the intensity related parameters, .Z,, J,,, and IF, the two columns are in good agreement. Onwumechili and Ezema (1992) worked on the data of solar activity maximum period of 1967-1969 and our study is for solar activity minimum year of 1986. We have calculated the ratio of “active/quiet“ from Jo, J,,, and IF. The mean ratio is 2.53 f 0.07. Marriot et al. (1979) give the global ratio of active/quiet for EEJ as varying from 1.7 to 2.2 depending on the particular solar cycle. Campbell (1989) and Patil et al. (1990) found that this ratio depends on the sector and the season. The difference in sector can account for the difference of this ratio in columns 4 and 5 of Table 5. It should be mentioned that Campbell (1989) found 2.85 f 0.1 for the northern himisphere of all sectors but his data combined WSq and EEJ currents.
562
S. 0. Oko et al.
5.3. Comparison of equatorial electrojet and worldwide part of Sq currents We now compare the main features of the worldwide part of Sq (WSq) in Onwumechili et al. (1995) with those of the equatorial electrojet (EEJ) here. Their diurnal variations of the annual means of .Z,, - .Z,,,and Zr are similar except that in EEJ they peaked at about 1100 hr but in WSq they peaked about l/2 h later at about 1130 h LT. On the other hand the variations of their - 100.Z,,,/.ZO contrast. It was lower before than after 1200 hr in EEJ but higher before than after 1200 h in WSq. The diurnal variations of the annual means of the landmark distances x,, w, x, and L, of WSq are very different from those of the EEJ. These distances for WSq decreased from dawn to a minimum at about 1100 h before increasing towards dusk. On the contrary, the distances for EEJ generally decreased from about dawn towards dusk without any minimum. The diurnal variations of the seasonal meams of .Z,, and IF showed that the order of magnitude of the seasonal means for both WSq and EEJ depend on the hour. There are however, significant differences between the two. The WSq peaked around 1100 h LT in all the seasons. The EEJ peaked in the order of intensity. The most intense E season peaked earliest at about 1045 h, followed by the D season at about 1130 h, and the least intense J season peaked at about 1200 h LT. The tables of daily mean parameters show several differences between WSq and EEJ among which we select the following. There is hardly any difference between the intensities of WSq in three seasons. On the other hand the EEJ is decisively most intense in the E season and least intense in the J season, the intensity ratio being about 1.5. The centre of WSq is about 4” south of the dip equator and lies half way between the geographic and dip equators. On the contrary, the centre of the EEJ is extremely close to the dip equator. The focus of WSq current is at about 40”N geographic latitude (about 35.5” from its centre), but the focus of EEJ current is at about 11 “N geographic latitude (about 2.9” from its centre). The latitudinal extent of WSq is about 86” latitude but the latitudinal extent of EEJ is about 12” latitude from its centre. The annual variations of WSq and EEJ are markedly different. The .Z,, -J,, Z, and - 100J,,,/JOof WSq have no clearly consistent annual variation. The J,, -J,,,, Z, and - 100J,/JO of EEJ have clearly semiannual variation with maxima in vernal equinox (March-April) and autumnal equinox (SeptemberOctober), and minima in June solstice (June-July) and
December solstice (December-January). The x,, w, x, and L, of WSq have no clearly consistent annual variation. On the other hand the x,, w, x, and L1 of EEJ have clearly semi-annual variations with minima in vernal equinox (April) and autumnal equinox (September), and maxima in June solstice (June-July) and December solstice (December-January). Both the WSq and EEJ exhibit the phenomenon of contracting at times, and intensifying while contracting. For both currents the intensification in response to the contraction is relatively greater at a farther than at a nearer distance to the centre of the current.
4. CONCLUSIONS
Some 498 hourly latitudinal profiles of equatorial electrojet (EEJ) from 0700 to 1700 h LT have been analyzed from five quietest days of each month of 1986. They yield interesting diurnal and annual variations of eight landmark parameters depicting the major features and structures of the latitudinal profiles of EEJ current. The eight parameters are: the peak forward current intensity J, Akm-’ at the current centre, the peak return current intensity J,,, Akm-‘, the percentage ratio of the peak return to the peak forward current intensity 100J,,,/JO,the total forward current Z, kA, the dip latitude of the current centre x, degrees, the distance of the focus w degrees from the current centre, the distance of the peak return current intensity x, degrees from the current centre, and the latitudinal extent of the current L, degrees from its centre. The main conclusions include the following. 1. The diurnal
variations
of the annual
means of
J,, -J,,, and Z, peak close to 1100 h LT. 2. The centre of the EEJ varies from day to day but on the average it remains close to the dip equator. Despite these, its mean exhibits smooth diurnal and annual variations similar to other landmark distances. 3. The diurnal variations of x,, w, x, and L, suggest that EEJ is widest near dawn with its centre and focus most northerly. It then contracts as its centre and focus move southwards towards noon. 4. The diurnal variations of the seasonal means of J,, and Z, did not peak at the same time in all seasons. The J, and Z, of EEJ seem to peak earlier when the intensity is higher than when it is lower. 5. The seasonal order of magnitude of the intensity of EEJ is not the same at all hours of the day. The D season is the least intense before 1000 h LT but the most intense after 1300 h LT. Around noontime the E season was most intense and the J season least intense. The seasonal variation is not semi-annual in the afternoon.
Geomagnetically
quiet day ionospheric
6. The landmark measures of EEJ current: J,, -.Z,, IF and - 100J,,,/JO have semi-annual variations with maxima in vernal equinox (March-April) and autumnal equinox (September-October), and minima in June solstice (June-July) and December solstice (December-January). 7. The landmark distances of EEJ current: x,, W, x, and L, have semi-annual variations with minima in vernal equinox (April) and autumnal equinox (September), and maxima in June solstice (JuneJuly) and December solstice (December-January). 8. Thus the landmark measures of EEJ current vary inversely with the landmark distances of EEJ current. As the EEJ current contracts, it intensifies. 9. The increase Iof - lOOJ,/J, as the EEJ current contracts is a direct demonstration of the finding that the intensification in response to contraction is rela-
currents
over the Indian
sector-II
563
tively greater at a farther than at a nearer distance to the current centre. 10. Tables of daily means and noontime means of the eight landmark parameters of EEJ have been provided. Some noontime annual means are: x0 = -0.21 f0.13 degree, w = 2.89f0.08 degree, x, = 5.43fO.l4degree, and& = 11.92kO.71 degree. The ratio of the peak forward current intensity J, in the three seasons is D: E: J = 1.36:2.03:1. 11. Certain properties of the worldwide part of Sq are found to be remarkably different from those of the equatorial electrojet including some key features of diurnal and seasonal variations. Acknowledgements-The authors are very grateful to Dr R. Rajaram and the Director of Indian Institute of Geomagnetism, Bombay, for the supply of the data studied.
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W. H
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