Geoscience Reference
In-Depth Information
12
12
18
6
18
6
24
24
(a)
(b)
12
12
18
6
18
6
24
24
(c)
(d)
Figure 8.10 The nightside convection pattern can have a variety of geometries. Patterns
a and d are most frequently observed. [After Heelis and Hanson (1980). Reproduced with
permission of the American Geophysical Union.]
magnetic field lines associated with plasma flow in the polar cap are open and
those associated with plasma flow in the auroral zones are closed, then this con-
cept must break down wherever the plasma flows across the boundary between
these two regions. When this breakdown occurs, the term “merging” or “recon-
nection” is frequently used to describe the phenomenon. A description of the
plasma processes involved in a merging or reconnection region is beyond the
scope of this topic. It is convenient, however, to use these terms when describing
moving magnetic field lines that change their identity either from an open to a
closed topology or from one open topology to another.
8.3.1 Models for Southward IMF
We have seen that many ionospheric observations indicate that the IMF exer-
cises significant control over the plasma convection pattern. From examining the
energetic electron precipitation patterns at high latitudes and the direct detection
of solar cosmic rays in the polar ionosphere, it is generally believed that the anti-
sunward flow in the polar cap during a southward IMF exists on open magnetic
field lines. The sunward flow in the auroral zone that completes the two-cell
Search WWH ::




Custom Search