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of rotation around the sun once per year, is negligible compared to the con-
vective motion of the plasma during a one-day cycle. In this reference frame
the plasma has a corotation component that is independent of local time and a
magnetospheric component that is dependent on local time and universal time
as the magnetic pole rotates around the geographic pole. Figure 9.4 shows sev-
eral different plasma convection paths resulting from this complex addition of
corotation and magnetospheric convection. Each path is traced for a 24-hour
period to illustrate that the paths do not necessarily close and are certainly quite
different from the convection paths illustrated in Chapter 8, in which the motion
of the coordinate system was removed. The starting points for these three trajec-
tories may be roughly summarized as follows: (a) dusk convection cell, (b) polar
cap, and (c) dawn convection cell. Also shown in each portion of the figure is
the position of the solar terminator in the Northern Hemisphere during winter,
summer, and equinox. Notice in this inertial frame that the plasma can move in
and out of sunlight as well as in and out of the auroral zone. A complex distribu-
tion of plasma production, transport, and loss, having local time and universal
time dependences, results from this motion.
With the goal of understanding some of the consequences of this plasma
motion, we will consider some large-scale features observed in the high-latitude
F region. Figure 9.5 shows the total plasma concentration and the perpendicular
components of the plasma drift velocity observed by a satellite traversing the
high-latitude ionosphere. The plot is made in a rotating frame such that a coro-
tating plasma would register zero velocity. In the time interval labeled B, the
zonal flow velocity indicated by the upper curve is weak and to the west. At
the point where the curve crosses the dashed line, the plasma is flowing toward
the west with the same velocity at which the earth is rotating toward the east
at that latitude. This means that the dusk terminator is moving westward at the
same velocity as the plasma, and thus, the plasma sees solar conditions that are
independent of time.
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(a)
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Figure 9.4
Several complex trajectories that plasma may undergo in a 24-hour period
due to displacement of the geomagnetic and geographic poles. [After Sojka et al. (1979).
Reproduced with permission of the American Geophysical Union.]
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