Geoscience Reference
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night, there is every reason to expect that the layer must be tilted with respect to
the vertical. In such a case, the linear growth rate may be written in the following
form:
E
x
−
wB
LB
g
ν
in
L
cos
E
z
+
uB
LB
γ
RT
=
cos
+
+
sin
(4.17)
where
L
is the gradient scale length on the bottomside,
is the tilt angle, the
z
-axis is upward, the
x
-axis is eastward, and we ignore any dip angle of the
magnetic field. The tilt angle is defined such that if it represented an inclined
plane a ball would roll down and west for positive
and down and east for
negative
. This is the growth rate of the generalized Rayleigh-Taylor instability
as it is applied to the equatorial ionosphere. Kelley et al. (1981), for example,
used it to interpret Fig. 4.1 and concluded that the preference for plumes to
be generated during negative slopes of the Jicamarca radar profile was due to
the contribution of an eastward neutral wind
to the instability growth
rate. That is, if the ionosphere is tilted such that the neutral wind blows toward
the east into a region of increasing plasma density so that
(
u
ˆ
a
x
)
is
antiparallel to the density gradient, a stable configuration occurs. However, if
the wind blows antiparallel to the plasma gradient, which is believed to occur
during the downward slopes in Fig. 4.1, the wind is destabilizing. Tsunoda (1981,
1983; see also Fig. 4.14), using the scanning radar at Kwajalein, has also shown
that wind effects are important. Note, however, that if a perfect polarization
field develops due to the F-region dynamo,
E
z
+
((
u
a
x
×
ˆ
B
)
×
B
)
0, and there would be no
effect of the third term in (4.17). This is a reminder that these instabilities are
current driven, not electric field driven. However, due to a finite E-region loading
of the current,
u
usually exceeds
E
z
/
uB
=
B
by 20% or so, and a vertical current does
flow. Since the electric field is generated by both local and remote wind fields (see
the previous chapter) the generalized growth rate represents quite a complicated
set of phenomena (we have not yet mentioned the role of shear in the plasma
flow).
Vertical neutral winds do occur when gravity waves propagate upward
through the thermosphere-ionosphere system. In addition, there is some evi-
dence that larger-scale, downward winds may occur near the sunset terminator.
Raghavarao et al. (1987), for example, have reported just such conditions from
rocket-borne chemical release experiments. Of course, it could have been the
case that even these winds were due to a long-period gravity wave. For refer-
ence, a vertical density gradient pointed upward is unstable to a downward wind
velocity (Sekar and Raghavarao, 1987). Hysell et al. (1990) conjectured that the
stripes in Fig. 4.8 were due to the vertical velocity phase of a gravity wave.
Linear instability theory may also be used to explain the day-night asymme-
try of CEIS, since the instability is influenced by the effects of the E region on
the charge buildup that leads to the instability. The equatorial ionosphere is not
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