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It can be seen from a comparison of the primed paths in Figs. 8.14 and 8.15
that observations of just the ionospheric convection signature cannot distinguish
between these two convectionmechanisms. However, simultaneous observations
of the convection signature and the energetic particle environment indicate that
the magnetic field topology is often open. Likewise, theoretical considerations
of the physics involved in a viscous interaction indicate that the potential asso-
ciated with this mechanism is usually less than 10 kV. Observed cross-polar cap
potentials in excess of 60 kV and the dependence of the convection geometry on
the y component of the solar wind magnetic field strongly suggest that the merg-
ing process dominates over viscous interaction when the IMF has a southward
component.
Although a rigorous discussion of the way in which the solar wind and earth's
magnetic fields interconnect is beyond the scope or requirements of this topic, it
is possible to understand the basic properties of the convection geometry from
rather simple considerations. We start by assuming that field lines interconnect
when they have antiparallel components in a plane perpendicular to the earth-
sun line (the y - z plane). Figure 8.16 shows views from above the north pole of
one of the earth's magnetic field lines that is connected to the IMF when the
IMF y component is nonzero. In the situation for negative B y , once merging
has taken place the newly open field lines have extremely large curvature near
the merging point. The resulting tension in the Northern Hemisphere open field
lines will produce a duskward component in the plasma flow in the dayside
magnetosphere. A corresponding duskward plasma flow will exist in the dayside
polar cap of the northern hemispheric ionosphere. In the Southern Hemisphere
X
V sw
Y
V pc
V pc
V sw
B y , 0
B y . 0
Figure 8.16 Schematic illustration of the magnetic field line orientation producing dif-
ferent flow directions in the polar cap when B y is positive versus the situation where B y
is negative.
 
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