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215
,
h
255 wind vectors
and model a
ion convection
,
12
h
12
h
15
h
9
h
9
h
15
h
80°
80°
18
h
6
h
18
h
6
h
21
h
3
h
3
h
21
h
400
m/s
0
h
0
h
(a) Actual wind locations
(b) Wind locations displaced 2 hours
Figure 9.17
(a) Compilation of barium neutral cloud velocities for a number of local
times superimposed on a plasma convection model. In (b) the same data are plotted 2
hours earlier in local time. [After Heppner andMiller (1982). Reproducedwith permission
of the American Geophysical Union.]
Fig. 9.17b. The wind locations were arbitrarily shifted by two hours to take into
account the sluggish response of the neutrals (e.g., see the preceding calculation
of
t
). The vectors show a much clearer relationship to the ion flow in Fig. 9.17b.
One obvious difference between the plasma flow and the measured winds, even
in Fig. 9.17b, is that the strong neutral flow seems to continue at latitudes well
below the auroral oval. This occurs because there is no comparable force to
“turn” the neutral wind when the flow leaves the sunward convection part of
the two-celled plasma flow pattern. Notice that something of a flywheel effect
will occur in this case since the wind in the subauroral region can create electric
fields via the F region dynamo process discussed in Chapters 3 and 5. This effect
has been suggested by Gonzales et al. (1978) to explain some of the anomalous
midlatitude electric field observations in the evening sector (see Chapter 5).
Although the primary electrodynamic effect seems to be momentum transfer
via the
J
δ
B
force, changes in pressure due to Joule heating of the neutral gas
discussed in Section 9.2 also affect the neutral atmospheric dynamics. This effect
may be more important in producing vertical upwelling and neutral composition
changes than in generating strong neutral winds.
Three-dimensional models of neutral wind forcing due to realistic electric field
patterns are now available that include all of the important physical processes.
An example of such a calculation is given in the series of plots in Fig. 9.18,
which are polar diagrams of the wind velocity at 240-km altitude for various
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