Geoscience Reference
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E
x
0
/
of an electric field by replacing
g
B
in the growth rate,
where
E
x
0
is the zonal component of the electric field in the neutral frame of
reference. A zonally eastward electric field drives a Pedersen current to the east.
Any undulation of the boundary will intercept charge just as in the gravitational
case and cause the perturbation to grow. Hence, an eastward
E
x
0
(eastward
E
x
and/or downward wind) is destabilizing. A zonally westward field will be
stabilizing on the bottomside. The general condition for instability is that the
E
x
0
×
/ν
in
with
g
/ν
in
+
B
direction be parallel to the plasma density gradient. As discussed in
the previous chapter, the zonal electric field component at the equator often
increases to a large eastward value just after sunset, driving the F layer to very
high altitudes. This uplift contributes in two ways to the destabilization of the
plasma. Not only is the electric field in the right direction for instability but
also the gravitational term becomes large due to the high altitude of the layer.
The growth rates of the gravitational and electric field-driven processes are
plotted as a function of height in Fig. 4.11. For a 0
m eastward elec-
tric field, the two sources of instability are equal at an altitude of 375 km. The
gravitational term dominates above this height and increases exponentially with
altitude.
Since a large-scale neutral wind is usually horizontal,
.
5mV
/
(
U
×
B
)
×
B
is also hor-
izontal and thus usually has no component parallel to
∇
n
if the ionosphere is
E
x
0
×
vertically stratified. Hence,
n
above is due entirely to the zonal
electric field. However, other terms can be added to the linear growth rate by
considering the possibility of a horizontal component of
(
B
)
·∇
n
and/or a vertical
wind. In fact, since the layer does change height during the course of any given
∇
600
GRT
(electric
field)
500
GRT
(gravitational)
400
300
200
10
24
10
23
10
22
Growth rate (S
21
)
Figure 4.11
Linear growth rates for the gravitational and electric field-driven Rayleigh-
Taylor instabilities in the equatorial ionosphere for typical conditions. [After Kelley et al.
(1979). Reproduced with permission of the American Geophysical Union.]
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