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westward electric field must build up such that
E
a
x
. The net plasma
velocity will again be due to the combined effect of the electric field and the
neutral velocity. In this case the total plasma velocity due to the
E
=
ν
B
sin
I
ˆ
B
drift and
the component of
U
parallel to
B
, when projected into geographic coordinates,
becomes
V
y
×
sin
2
I
cos
2
I
0.
Again, the plasma does not move vertically at all and matches the horizontal
neutral speed. Here,
J
=
ν
+
ν
=
ν
and
V
z
=
ν
sin
I
cos
I
−
ν
cos
I
sin
I
=
0 initially, since electrical energy is created from a
mechanical source as in a dynamo. Note that
·
E
≤
ν
is negative in Fig. 5.15b (south-
ward winds).
Finally, sedimentation can also act to create horizontal ion motion. Since the
downward velocity due to gravity is proportional to (
ν
in
)
−
1
, which increases
rapidly with height, there is a natural limitation on the altitude to which
either an equatorward wind (which pushes plasma up along
B
) or an east-
ward electric field (which
E
B
drifts the plasma upward and northward)
can drive the plasma. Eventually the material falls along
B
as fast as it is
pushed up and no net vertical motion occurs. This is the equilibrium dis-
cussed in the previous section. Note that for the equatorial neutral wind
case (and if no F-region dynamo electric field is generated), the net hori-
zontal plasma flow is actually zero, whereas for the applied eastward elec-
tric field case, the net horizontal plasma flow is poleward. Curiously, this
gravitational effect on the plasma flow only acts for equatorward wind
and/or eastward electric field. The opposite signs merely act in consort with
gravity to drive the plasma deep into the atmosphere, where it is lost to
recombination—the postmidnight collapse effect. In the next chapter we show,
after Perkins (1973), that in the former case the equilibrium represented by
(5.22) is stable, provided that no meridional electric field and no zonal wind
exist.
We now turn to some experimental data aimed at trying to sort out these
various processes. All these effects (and more!) seem to compete for control of
the midlatitude F-layer plasma. An example in which the local dynamo effect
seems to explain everything is illustrated in Fig. 5.16a and b. Two components
of the plasma drift, the height of the F peak (
h
max
)
×
and the maximum value of
the electron density, are plotted in Fig. 5.16a. The standard coordinate system
in which the Arecibo data are presented differs from ours. In Fig. 5.14 and
Figs. 5.16-5.18, the Arecibo-measured
V
is positive for drifts anti-parallel to
||
a
z
a
y
direction.
In Fig. 5.16a, the two middle curves are anticorrelated until about 0340 LT,
when sunrise occurred in the
conjugate
hemisphere, which is a nonlocal effect.
The plasma motion is therefore nearly horizontal all night long. Indeed,
h
max
,
only slowly changes from 2300 until 0300, which also shows that the vertical
velocity was very small, averaging less than 1 m/s (downward) during this time.
Fabry-Perot wind measurements are available for this night, and the measured
northward neutral wind (
ˆ
(
V
is positive in the
−
B
direction) and
V
N
is positive along the
ˆ
||
⊥
ν
) and the northward horizontal component of the
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