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Southward
IMF
12
12
60°
70°
60°
70°
80°
80°
18
06
18
06
12
B
Y
,,
0
B
Y
,
0
60°
70°
80°
18
06
12
12
B
Y
¯
0
60°
60°
70°
70°
80°
80°
18
06
06
18
0
B
Y
.
B
Y
..
0
Figure 8.9
Schematic representation of the day-side high-latitude convection pattern
showing its dependence on the
y
component of the IMF when
B
z
is south. [After Heelis
(1984). Reproduced with permission of the American Geophysical Union.]
high velocity, the convection changed from antisunward to sunward (Berg et al.,
1994). Thus, the polar cap/auroral oval boundary seems to shrink in discrete
steps as arcs propagate across the polar cap.
8.3 Simple Models of Convection in the Magnetosphere
In our simple considerations of magnetosphere-solar wind coupling, the iono-
spheric plasmamotion reflects the motion of the magnetospheric plasma towhich
it is magnetically connected. At the highest latitudes, antisunward convection is
associated with similar flow in the magnetosheath, while sunward convection
at lower latitudes is associated with flow in the plasma sheet. This relationship
between plasma flows in different regions makes the concept of moving magnetic
field lines or frozen-in flux extremely useful. However, if, as we suggested, the
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