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F region and is called the midlatitude trough. Examination of the plasma drift
characteristics within the premidnight trough in Fig. 9.5 shows that the plasma
depletion lies within a region in which there is a transition from either eastward
or corotating plasma drift equatorward of the auroral zone to westward drift in
the auroral zone. At such a transition zone the plasma flows very slowly, and
chemical recombination will have more time to reduce the ion concentration
than in regions where the plasma moves more quickly or where auroral zone
ionization exists.
Figure 9.6 shows a schematic diagram of a modeled set of convection paths
in the evening trough region that are similar to the data shown in Fig. 9.7. On
the right-hand side the results of a simple chemical decay model are given for
the plasma content, taking into account its motion along these paths. The results
confirm that plasma stagnation in the nighttime region just equatorward of the
auroral zone is a likely candidate for the quiet-time trough formation. We discuss
the storm-time trough in Chapter 10.
The convection paths giving rise to the trough in the premidnight sector result
quite naturally from adding the essentially eastward corotation, dominating the
flow characteristics at low latitudes, and the essentially westward flow from
the magnetospheric field, dominating the flow characteristics at high latitudes.
Figure 9.7 shows the results of adding these two convection sources in a magnetic
coordinate frame. In this figure the premidnight trough results from the very
slow flow in the nighttime region of the trajectory labeled II. The illustrated
flow pattern, of course, has considerable variations because of the universal time
effects mentioned earlier and the temporal changes in the magnetospheric electric
field. These variations can explain the variety of observed latitudinal profiles of
Figure 9.6 Model convection paths having the same properties as those observed in
Fig. 9.5 are shown in (a). The resulting number density profile due to the almost stag-
nant flow is shown in (b). [After Spiro et al. (1978). Reproduced with permission of the
American Geophysical Union.]
 
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