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of two localized field-aligned current regions symmetrically located about local
noon in both hemispheres. The current directions are opposite to the adjacent
Region 1 currents, being downward into the ionosphere in the afternoon and
upward away from the ionosphere in the morning. Change in the sign of
B
y
significantly affects this distribution. Observations made in both the Northern
and Southern Hemispheres show that when
B
y
>
0, the cusp region current is
extremely asymmetric and is predominantly upward in the NorthernHemisphere
and downward in the Southern Hemisphere. When
B
y
<
0, the current is pre-
dominantly downward in the north and upward in the south.
8.5.2 Current Patterns for a Northward IMF
When the IMF has a northward component, the distribution of field-aligned
currents changes quite dramatically (as does the electric field distribution). The
two-ring pattern of field-aligned currents is retained, but they are seen to move
to higher latitudes and are significantly reduced in intensity. Perhaps more signif-
icant is a reorganization of the cusp currents in such a way that they expand over
most of the high-latitude area enclosed by the Region 1 currents and become as
intense as the Region 1 and Region 2 currents themselves. This new cusp cur-
rent system has been called the “NBZ” current system to denote the northward-
directed
B
z
component that exists during its occurrence. The NBZ current system
again shows a strong dependence on the sign of
B
y
, and Fig. 8.25 shows schemat-
ically its location and distribution in the Southern Hemisphere. Except for their
dramatic redistribution in area, the NBZ currents show essentially the same
behavior as the cusp currents, being predominantly downward in the Southern
Hemisphere when
B
y
is positive and upward when
B
y
is negative. Conversely,
we expect the NBZ current to be upward in the Northern Hemisphere when
B
y
is positive and downward when
B
y
is negative. Because the currents are out of
12
B
y
. 0
12
10
14
10
14
B
y
,
0
NBZ
R1
R1
8
16
16
8
6
75
18
6
75
18
80
85
Pole
85
80
75
80
85
Pole
85
80
75
Current into ionosphere
Current away from ionosphere
Figure 8.25
Cusp currents in the southern hemisphere, shown for the case when the IMF
has a northward component. The cusp currents expand to fill the polar cusp and have
been called NBZ currents. Their dominant polarity is determined by the
y
component
of the IMF. [After Iijima et al. (1984). Reproduced with permission of the American
Geophysical Union.]
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