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persist at all times. In addition, the interplanetary magnetic field magnitude and
orientation introduce changes in the location and configuration of the current
systems. This should not be surprising, considering the intimate relationships
between currents and electric fields that were derived in Section 8.1. Since we
have established the basic magnetic field topologies associated with the electric
field configuration and given a qualitative description of the drivers that pro-
duce a dependence on the IMF orientation, we will confine ourselves here to a
description of the field-aligned current morphology. In the final section we will
describe the behavior of the horizontal currents.
8.5.1 Current Patterns for a Southward IMF
When the IMF has a southward component, Fig. 8.23 shows the most stable
features of the field-aligned current system seen during times (a) when the inter-
nal state of the magnetosphere is quiet and (b) when it is more disturbed. The
following features should be noted:
1. There exist essentially two concentric field-aligned current rings approximately the
same as those predicted from the simple considerations of Section 8.1.
2. The inner ring, which may be considered to contain the driving currents for iono-
spheric convection, has current directed into the ionosphere in the morning hemi-
sphere and away from the ionosphere in the evening hemisphere. These are the
Region 1 currents, identified in Section 8.1.
|
AL
|
, 100
g
|
AL
|
> 100
g
12
12
60°
60°
14
10
14
10
70°
7
0°
8
16
8
16
80°
80°
18
6
18
6
20
4
20
4
2
22
22
2
0
0
Current into ionosphere
Current away from ionosphere
(a)
(b)
Figure 8.23
Two overlapping rings of field-aligned currents always exist at high lat-
itudes. They occur at higher latitudes and have smaller latitudinal extent during quiet
times. The inner ring is termed Region 1 current and the outer ring Region 2. [After Iijima
and Potemra (1978). Reproduced with permission of the American Geophysical Union.]
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