High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 405

Northward
IMF

12

12

608 60 8

708 70 8

808

18 06

18 06

B Y , 0 0

B Y .

12

608

708

18 06

B Y ¯ 0

Figure 8.12b The main feature of the dayside convection geometry when the IMF has a
northward component is the existence of four convection cells. [After Heelis et al. (1986).
Reproduced with permission of the American Geophysical Union.]

convection pattern is on closed field lines. When the sunward flow of the auroral
zone crosses the polar cap and becomes antisunward, we use the term merging
to describe the process by which closed field lines become open and connected
to the interplanetary magnetic field (Dungey, 1961). The term “reconnection”
describes the process by which open magnetic field lines are joined in the mag-
netotail to produce a closed field line. The reconnection region must correspond
in the ionosphere to the region where antisunward flow crosses the polar cap
boundary and becomes sunward. With these two processes in mind, Fig. 8.14a
shows that a southward-pointing IMF field line (A-B) breaks and merges with
the earth's field at point N1. Still attached to the solar wind, that field line moves
across the polar cap and eventually rejoins a field line from the other hemisphere
at point N2. During this time the interplanetary electric field penetrates down
into the ionosphere, where it drives an antisunward flow. This picture is not to
scale since the solar wind velocity is so large that, by the time the foot point
in the ionosphere traverses the polar cap, the other end of the field line is very
far away from the earth. Hence, a long magnetic tail forms on the nightside.
The time history of convecting plasma or flux tubes in the magnetosphere and
ionosphere during the circulation of plasma in one cell of the two-cell convection
pattern is shown in Fig. 8.14b. As just noted, on the dayside at point 1, a closed
magnetic field line of the earth “breaks” and connects with the interplanetary
magnetic field to produce an open field line. Not shown is the magnetic field
line from the south polar region that connects to the “other half” of the inter-
planetary field line. This point may alternatively be viewed as the location at