High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 410

the plasma flow at the magnetopause and in the ionosphere will be directed
toward dawn when B y is negative. Similar arguments can be applied to the
situation when B y is positive. In this case a dawnward flow component will be
seen in the northern ionospheric polar cap flow. This rather simple idea can
account for the B y dependence of the ionospheric flow direction seen near local
noon in Fig. 8.9 and for the dependence of the flow speed at the dawn and dusk
edges of the polar cap on B y (Heppner, 1972). This dependence is such that in
the Northern Hemisphere a larger flow speed is seen near the dawn side of the
polar cap when B y is positive and a larger flow speed is seen at the dusk side
of the polar cap when B y is negative. Also note that this simple model predicts
that the B y dependences seen in the Northern Hemisphere appear in the opposite
sense for the Southern Hemispheres, a fact borne out by observation.

8.3.2 Models for Northward IMF
The pattern of the high-latitude circulation cells during northward IMF can again
be understood by the rather simple concepts of merging and frozen-in flux used in
the case of southward IMF. As we shall see, a number of different magnetic field
topologies might exist during times of northward IMF. Experimental and theo-
retical investigations in this area are still being undertaken and we will describe
here only some of the basic concepts involved.
One way inwhich the earth's magnetospheric fieldmay connect to a northward
IMF is shown in Fig. 8.17. Here magnetic field lines that extend into the tail of the
magnetosphere have the right orientation for merging to occur with a northward
IMF at position 1. Subsequent antisunward motion of open field lines to points 2
and 3 produces antisunward motion in the ionosphere from 1 to 3 . At this

_

IMF IMF

1

0

SW

49

19

1 9 3 9

4

29

3

2

1

SW

Figure 8.17 Time history of convecting flux tubes (connecting to a northward IMF) that
produce a dominant high-latitude convection cell. This must be combined with the pattern
for viscous interaction shown in Fig. 8.15 to produce the observed four-cell pattern.