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Equivalent current system
B
z
. 0
12
12
2
160
280
60
8
2
160
2
2
2
0
80
0
280
18
06
0
0
2
80
40
80
1
1
1
B
y
,
0
B
y
5
0
B
y
.
0
B
z
5
0
12
12
60
8
2
80
80
2
2
160
2
2
2
160
18
06
0
280
0
80
0
1
1
1
80
0
B
y
,
0
B
y
5
0
B
y
.
0
B
z
, 0
Eastward
electrojet
12
12
2160
2
80
0
608
2
2
280
2
2160
0
2
80
06
18
2
80
1
1
1
0
80
0
Westward
electrojet
20 kA
B
y
, 0
B
y
5 0
B
y
. 0
Figure 8.28
Contours showing the horizontal equivalent currents for different orienta-
tions of
B
z
and
B
y
. The horizontal equivalent current shows many of the features of the
high-latitude convection pattern. In fact, the features differ only where substantial field-
aligned currents and conductivity gradients exist. [After Friis-Christensen et al. (1985).
Reproduced with permission of the American Geophysical Union.]
systems, which are strongly dependent on the
y
and
z
components of the IMF,
respectively. The features of the IMF-dependent DPY current system dominate
the horizontal current distribution and can be clearly seen if we subtract the
IMF-independent contribution from equivalent current systems measured when
B
y
has a substantial positive component and when it has a negative component.
Figure 8.29 shows the result of such an exercise, in which it can be seen that the
IMF-dependent system is approximately zonal and flows across the local noon
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