Geoscience Reference
In-Depth Information
∇·
0 and also with a downward-phase propagation of the wave (see Chapter
6). The wave frequency is
U
=
ω
=
(
2
π/
1200
)
(rad/s). It is important to note that no
initial plasma perturbation was included.
The results are shown in Fig. 4.17, where in Fig. 4.17a, the plasma instability
has been turned off by setting
g
0, and only a small perturbation occurs. In
the upper panel of Fig. 4.17b gravity is included, and two well-developed uplifts
with apogee plumes are created within 40 minutes. In the lower panel the waves'
eastward phase velocity was exactly matched to the plasma velocity at 100m
=
s
in the simulation. The structures grow only about 15% faster with the spatial
resonance condition met in which these two velocities are equal. As conjectured
by Kelley et al. (1981), Fig. 4.17b shows that spatial resonance is not required
for gravity wave seeding of CEIS.
These results show that gravity wave winds alone can seed the instability,
not just plasma density perturbations. However, the plasma instability clearly
amplifies the plasma drift perturbations, yielding much larger
/
B
drifts than
the initial neutral wind. For example, between the times in the upper panels of
Fig. 4.17b the average plasma uplift velocity was over 300m
δ
E
×
/
s compared to
w
s for the initial wind speed. These simulations show that nonlinear
processes amplify the initial fluctuations.
Initially it was thought that shear flowwould only lead to stabilization of CEIS
instabilities (see Section 4.2.4). This was borne out by early nonlinear studies.
For example, Sekar and Kelley (1998) studied the effect of a shear in the zonal
drift coupled with the reversal of the vertical drift velocity on development of
CEIS. Due to the shear, the structures tilted toward the east but still were able
to penetrate into the topside. This suggests that the stabilizing effect of the shear
is not very effective in suppressing CEIS all by itself. But when they reversed
the vertical drift part way into the simulation, there was no penetration of the
perturbation above the F peak.
However, these calculations are not self-consistent with respect to the origin
of the shear. Hysell and Kudeki (2004) pointed out that strong shear flow in the
bottomside equatorial F region implies a region of retrograde plasma motion,
where the plasma drifts rapidly westward in strata where the neutral wind is
eastward (see Fig. 4.2 and Chapter 3). By extending the analysis of the elec-
trostatic Kelvin-Helmholtz instability (KHI) of Keskinen et al. (1988) into the
strongly collisional, inhomogeneous limit, they showed that this configuration
is inherently unstable. The growth rate of the instability they found was large
enough to compete with that of the ionospheric interchange instability, and the
range of preferred wavelengths was a better match to the depletion scale-sizes
observed during equatorial spread F. They surmised that this shear-driven insta-
bility could precondition the ionosphere for interchange instabilities, explaining
the relatively early appearance of fully developed irregularity structures after
sunset during spread F events.
Figure 4.18 shows the results of the numerical simulation of the instability.
The initial density profile follows a hyperbolic tangent law below the peak and
an exponential decay law above it. The initial profile of the electrostatic potential
=
4m
/
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